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Embedded C
8322 Prelims (i-xvi) 25/2/02 3:04 pm Page ii
:322 Prelims (i-xvi) 25/2/02 3:04 pm Page iii
Embedded C
Michael J. Pont
Addison -Wesley
An imprint of Pearson Education
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:322 Prelims (i-xvi) 25/2/02 3:04 pm Page iv
PEARSON EDUCATION LIMITED
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First published in Great Britain in 2002
© Pearson Education Limited 2002
The right of Michael J. Pont to be identified as Author
of this Work has been asserted by him in accordance
with the Copyright, Designs and Patents Act 1988.
ISBN 201 79523 X
British Library Cataloguing in Publication Data
A CIP catalogue record for this book can be obtained from the British Library
Library of Congress-in-Publication Data
Pont, Michael J.
Embedded C/MichaelJ. Pont.
p. cm.
Includes bibliographical references and index.
ISBN 0-201-79523-X (pbx. : alk. paper)
l.C (Computer program language) 2. Embedded computer systems-Design and
construction. I. Title.
Q QA76.73.C15 P65 2002
005.265--dc21
2001056731
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The programs in this book have been included for their instructional value.
The publisher does not offer any warranties or representations in respect of
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Many of the designations used by manufacturers and sellers to distinguish
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The publishers wish to thank Infineon Technologies for permission to
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:322 Prelims (i-xvi) 25/2/02 3:04 pm Page v
This book is dedicated to Sarah
:322 Prelims (i-xvi) 25/2/02 3:04 pm Page vi
About
the author
Michael J. Pont is an experienced software engineer who began his first embedded
project in 1986. Since then he has lectured and carried out research at the
University of Sheffield and the University of Leicester, and has provided consul-
tancy and training services to a range of international companies. Michael is the
author of two previous books Patterns for Time-Triggered Embedded Systems and
Software Engineering with C++ and CASE tools.
:322 Prelims (i-xvi) 25/2/02 3:04 pm Page vii
Contents
Preface xi
1 Programming embedded systems in C 1
1.1 Introduction 1
1 .2 What is an embedded system? 1
1 .3 Which processor should you use? 2
1.4 Which programming language should you use? 7
1 .5 Which operating system should you use? 9
1 .6 How do you develop embedded software? 1 2
1 .7 Conclusions 1 5
2 Introducing the 8051 microcontroller family 17
2.1 Introduction 1 7
2.2 What's in a name? 1 7
2.3 The external interface of the Standard 8051 1 8
2.4 Reset requirements 20
2.5 Clock frequency and performance 21
2.6 Memory issues 23
2.7 I/O pins 29
2.8 Timers 29
2.9 Interrupts 30
2.10 Serial interface 32
2.11 Power consumption 32
2.12 Conclusions 34
3 Hello, embedded world 35
3.1 Introduction 35
3.2 Installing the Keil software and loading the project 36
J322 Prelims (i-xvi) 25/2/02 3:04 pm Page viii
vlij Contents
3.3 Configuring the simulator 37
3.4 Building the target 39
3.5 Running the simulation 39
3.6 Dissecting the program 43
3.7 Aside: Building the hardware 55
3.8 Conclusions 56
4 Reading switches 57
4.1 Introduction 57
4.2 Basic techniques for reading from port pins 58
4.3 Example: Reading and writing bytes 60
4.4 Example: Reading and writing bits (simple version) 61
4.5 Example: Reading and writing bits (generic version) 62
4.6 The need for pull-up resistors 67
4.7 Dealing with switch bounce 69
4.8 Example: Reading switch inputs (basic code) 70
4.9 Example: Counting goats 75
4.10 Conclusions 80
5 Adding structure to your code 81
5.1 Introduction 81
5.2 Object-oriented programming with C 82
5.3 The Project Header (Main . H) 88
5.4 The Port Header (Port . H) 94
5.5 Example: Restructuring the 'Hello Embedded World' example 96
5.6 Example: Restructuring the goat-counting example 103
5.7 Further examples 111
5.8 Conclusions 111
6 Meeting real-time constraints 113
6.1 Introduction 113
6.2 Creating 'hardware delays' using Timer and Timer 1 116
6.3 Example: Generating a precise 50 ms delay 120
:322 Prelims (i-xvi) 25/2/02 3:04 pm Page ix
Contents ix
6.4 Example: Creating a portable hardware delay 1 24
6.5 Why not use Timer 2? 129
6.6 The need for 'timeout' mechanisms 1 29
6.7 Creating loop timeouts 1 30
6.8 Example: Testing loop timeouts 1 33
6.9 Example: A more reliable switch interface 1 34
6.10 Creating hardware timeouts 136
6.1 1 Example: Testing a hardware timeout 140
6.12 Conclusions 142
7 Creating an embedded operating system 143
7.1 Introduction 143
7.2 The basis of a simple embedded OS 147
7.3 Introducing sEOS 152
7.4 Using Timer or Timer 1 161
7.5 Is this approach portable? 1 66
7.6 Alternative system architectures 1 66
7.7 Important design considerations when using sEOS 1 72
7.8 Example: Milk pasteurization 1 74
7.9 Conclusions 1 87
8 Multi-state systems and function sequences 189
8.1 Introduction 189
8.2 Implementing a Multi-State (Timed) system 192
8.3 Example: Traffic light sequencing 1 92
8.4 Example: Animatronic dinosaur 198
8.5 Implementing a Multi-State (Input/Timed) system 204
8.6 Example: Controller for a washing machine 205
8.7 Conclusions 21 5
9 Using the serial interface 21 7
9.1 Introduction 217
9.2 What is RS-232? 21 7
9.3 Does RS-232 still matter? 21 8
:322 Prelims (i-xvi) 25/2/02 3:04 pm Page x
x Contents
9.4 The basic RS-232 protocol 21 8
9.5 Asynchronous data transmission and baud rates 219
9.6 Flow control 220
9.7 The software architecture 220
9.8 Using the on-chip UART for RS-232 communications 222
9.9 Memory requirements 224
9.10 Example: Displaying elapsed time on a PC 225
9.11 The Serial-Menu architecture 237
9.12 Example: Data acquisition 237
9.13 Example: Remote-control robot 252
9.14 Conclusions 253
10 Case study: Intruder alarm system 255
10.1 Introduction 255
10.2 The software architecture 257
10.3 Key software components used in this example 257
10.4 Running the program 258
10.5 The software 258
10.6 Conclusions 283
1 1 Where do we go from here 285
11.1 Introduction 285
1 1 .2 Have we achieved our aims? 285
11.3 Suggestions for further study 286
1 1 .4 Patterns for Time-Triggered Embedded Systems 288
1 1 .5 Embedded Operating Systems 288
11.6 Conclusions 289
Index 291
Licensing Agreement 295
:322 Prelims (i-xvi) 25/2/02 3:04 pm Page xi
Preface
This book provides a 'hardware-free' introduction to embedded software for
people who:
• Already know how to write software for 'desktop' computer systems.
• Are familiar with a C-based language (Java, C++ or C).
• Want to learn how C is used in practical embedded systems.
The remainder of this preface attempts to answer some questions which prospec-
tive readers may have about the contents.
I What is an embedded system?
As far as this book is concerned:
An embedded system is an application that contains at least one programmable
computer (typically in the form of a microcontroller, a microprocessor or digital
signal processor chip) and which is used by individuals who are, in the main,
unaware that the system is computer-based.
This type of embedded system is all around us. Use of embedded processors in pas-
senger cars, mobile phones, medical equipment, aerospace systems and defence
systems is widespread, and even everyday domestic appliances such as dishwash-
ers, televisions, washing machines and video recorders now include at least one
such device.
II What type of processor is discussed?
This book focuses on the embedded systems based on the 8051 family of microcon-
trollers. Prices for 8051 devices start at less than $1.00 (US). At this price, you get a
performance of around 1 million instructions per second, and 256 bytes (not
megabytes!) of on-chip RAM. The 805 l's profile (price, performance, available
memory) matches the needs of many embedded systems very well. As a result, the
J322 Prelims (i-xvi) 25/2/02 3:04 pm Page xii
xjj Preface
8051 architecture - originally developed by Intel - is now implemented in more than
400 chips; these are produced by a diverse range of companies including Philips,
Infineon, Atmel and Dallas. Sales of this vast family are estimated to have the largest
share (around 60%) of the microcontroller market as a whole, and to make up more
than 50% of the 8-bit microcontroller market. Versions of the 8051 are currently used
in a long list of embedded products, from automotive systems to children's toys.
The low cost, huge range, easy availability and widespread use of the 8051
family makes it an excellent platform for developing embedded systems: these
same factors also make it an ideal platform for learning about embedded systems.
Whether you will subsequently use 8-, 16- or 32-bit embedded processors, learning
to work within the performance and memory limits of devices such as the 8051 is
a crucial requirement in the cost-conscious embedded market. You simply cannot
acquire these skills by developing code for a Pentium (or similar) processor.
Which operating system is used?
The 256 bytes of memory in the 8051 are - of course - insufficient to support any ver-
sion of Windows, Linux or similar desktop operating systems. Instead, we will describe
how to create your own simple 'embedded operating system' (see Chapter 7). This 'do-
it-yourself approach is typical in small embedded applications, where the memory
requirements and expense of a desktop operating system (like Windows or Linux) or of
a so-called 'real-time operating system' simply cannot be justified. However, the
approach is also in widespread use in large embedded systems (for example, aerospace
applications or X-by-wire systems in the automotive industry), where conventional
operating systems are generally considered to be too unpredictable.
Learning to work on a 'naked' processor and create your own operating system are
key requirements for software developers wishing to work with embedded systems.
IV What type of system is discussed?
This book presents a number of examples adapted from working embedded sys-
tems. These include:
• A remotely-controlled robot.
• A traffic-light sequencer.
• A system for monitoring liquid flow rates.
• A controller for a domestic washing machine.
8322 Prelims (i-xvi) 25/2/02 3:04 pm Page xiii
Preface xiii
• An animatronic dinosaur.
• A general-purpose data acquisition system.
These and other examples are used to illustrate key software architectures that are
in widespread use in embedded designs; the examples may be adapted and
extended to match the needs of your own applications.
The book concludes with a final case study: this brings together all of the fea-
tures discussed in earlier chapters in order to create an intruder alarm system. This
case study includes the following key components:
• A suitable embedded operating system.
• A multi-state system framework.
• Software to process the inputs from door and window sensors.
• A simple 'keypad' library to process passwords entered by the user.
• Software to control external port pins (to activate the external bell).
• An 'RS-232' library to assist with debugging.
V Do I need a degree in electronics in order to use this book?
Please consider the following statement:
'I'd like to learn about embedded software, but I don't know enough about electronics. '
This is a concern which is commonly expressed by desktop programmers who - if
they ever learned anything about electronics at school, college or university -
have probably forgotten it.
If you don't know the difference between a MOSFET and a BJT, or even the dif-
ference between a resistor and a capacitor, please relax. You don't need to have
any knowledge of electronics in order to make full use of this book. Neither
will you need a soldering iron, breadboard or any electronic components. In short,
this book is (99%) hardware free.
To write software for the 8051 devices considered in this book, we will use an
industry-standard (Keil) compiler. To test this software, we will use a hardware
simulator. Copies of both compiler tools and the simulator are included on the
enclosed CD. Using these tools, all of the examples in the book may be run, mod-
ified and recompiled and tested, using a standard Windows PC.
This approach allows experienced desktop programmers to quickly understand
the key features of embedded systems before they need to 'get their hands dirty'
and build some hardware.
J322 Prelims (i-xvi) 25/2/02 3:04 pm Page xiv
xiv Preface
VI What's on the CD?
In addition to the Keil compiler and hardware simulator (discussed in the previous
section), the CD also includes source code files for all the examples and the case
study: this code is in the 'C programming language and is compatible with the
Keil compiler.
The CD also contains useful information about the 8051 microcontroller
family, including a large number of relevant data sheets and application notes.
VII What's the link between this book and your other 8051 book
(Patterns for Time-Triggered Embedded Systems)?
Embedded C provides an introduction to the use of C in embedded projects. If you
want to learn more about embedded systems after you finish this book, then
Patterns for Time-Triggered Embedded Systems (PTTES) may be of interest. 1
PTTES is a large (1000-page) book which includes a comprehensive set of
'design patterns' to support the development of embedded systems based on the
8051 family of microcontrollers. In total, details of more than 70 useful patterns
are provided, complete with guidelines to help you apply these techniques in your
own projects: full source code for all of the patterns is included on the PTTES CD.
The book includes: patterns for embedded operating systems (for both single-
processor and multi-processor applications); patterns for user-interface designs
using switches, keypads, LED and liquid crystal displays; patterns for PID control-
patterns for PWM; patterns for analogue-to-digital and digital-to-analogue conver-
sion; patterns for RS-232, RS-485, CAN, SPI and I 2 C serial networks; hardware
patterns describing reset, oscillator and memory circuits.
VIII Is the code 'free ware'?
The code included in this book took many years to produce. It is not 'free ware',
and is subject to some simple copyright restrictions. These are as follows:
• If you have purchased a copy of this book, you are entitled to use the code
listed in the text (and included on the CD) in your projects, should you choose
to do so. If you use the code in this way, then no run-time royalties are due.
1. Pont, M.J. (2001) Patterns for time-triggered embedded systems: Building reliable applications with the
8051 family of microcontroller, Addison-Wesley / ACM Press.
:322 Prelims (i-xvi) 25/2/02 3:04 pm Page xv
Preface xv
If you are using the code in a company, and (for example) ten people are using
the code, the company should own ten copies of this book.
If you are teaching in a university or college, you may freely distribute this code
to your students without requiring a licence, as long as the code is used for
teaching purposes and no commercial application is involved. Please note that
teaching (by university or college staff, or anyone else) of 'short courses' for
industry or for purposes of 'continuing professional development' does not fall
into this category: if in doubt, please contact me for clarification. 2
You may not, under any circumstances , publish any of the source code
included in the book or on the CD, in any form or by any means, without
explicit written authorization from me. If you wish to publish limited code frag-
ments then, in most circumstances, I will grant this permission, subject only to
an appropriate acknowledgment accompanying the published material. If you
wish to publish more substantial code listings, then payment of a fee may be
required. Please contact me for further details.
IX How should this book be read?
This short book is intended to be read from cover to cover.
Access to a Windows PC while reading will be useful in later chapters, as this
will allow you to try out the examples for yourself: however, this is not essential.
X What about bug reports and code updates?
There is fair amount of code involved in this project, both in the book itself and
on the associated CD. I have personally tested all of the code that appears here.
Nonetheless, errors can creep in.
If you think you have found a bug, please send me an e-mail (the address is at
the end of this preface), and I will do my best to help.
XI What about other reader comments?
I began my first embedded project in 1986. When writing Embedded C, I wanted to, try
and provide the kind of information that I needed (but could not find) at that time.
2. I can be contacted either by post (via the publishers, please), or much more efficiently by e-mail
at the address given at the end of this preface.
J322 Prelims (i-xvi) 25/2/02 3:04 pm Page xvi
xvi Preface
I would appreciate your comments and feedback. For example, should the book
be longer? Shorter? What other areas should I cover? What should I miss out?
Would you like to see a future edition focusing on a different family of microcon-
trollers? If so, which one?
To ensure that any future editions continue to provide the information
you need, I would be delighted to hear of your experiences (good or bad) using
the book.
XII Credit where credit is due
The publication of this book would not have been possible without the help and
support of a number of people.
In particular, I would like to thank:
• The 'Electronic and Software Engineering' students at the University of
Leicester who have provided useful feedback on this material as they attended
my introductory courses in embedded systems in recent years.
• Simon Plumtree at Pearson Education, who responded positively to my sugges-
tion that this material was suitable for wider publication.
• Karen Sellwood at Pearson, who helped to keep the project on the rails.
• Reinhard Keil and his colleagues, for reviewing the first draft of this book and -
again - providing the core of the CD.
• Jim Cooling, for his review of the first draft of this book.
© Chris Stephens, for his review of the first draft of this book.
• Penelope Allport for managing the project.
• Sara Barnes for copy editing; Claire Brodmann for the design; Barbara Archer for
proof reading and David Worthington for the index.
• Barbara and Gordon Pont for proof reading.
• Sarah, for convincing me that 'No More Shall We Part' was worth listening
to again.
Michael J. Pont
Great Dalby, February 2002
Michael .Pont@tesco.net
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 1
chapter
Programming embedded
systems in C
1.1 Introduction
This is a short book for people who already know how to program desktop
computers and now wish to develop software for embedded systems.
In this introductory chapter, we consider some important decisions that must
be made at the start of any embedded project:
• The choice of processor.
• The choice of programming language.
• The choice of operating system.
We begin by considering the meaning of the phrase 'embedded system'.
1.2 What is an embedded system?
When we talk about 'embedded systems', what do we mean? Opinions vary.
Throughout this book, we will use the following loose definition:
An embedded system is an application that contains at least one programmable
computer (typically in the form of a microcontroller, a microprocessor or digital
signal processor chip) and which is used by individuals who are, in the main,
unaware that the system is computer-based.
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 2
Embedded C
Typical examples of embedded applications that are constructed using the tech-
niques discussed in this book include:
• Mobile phone systems (including both customer handsets and base stations).
• Automotive applications (including braking systems, traction control, airbag
release systems, engine-management units, steer-by-wire systems and cruise-
control applications).
• Domestic appliances (including dishwashers, televisions, washing machines,
microwave ovens, video recorders, security systems, garage door controllers).
• Aerospace applications (including flight control systems, engine controllers,
autopilots and passenger in-flight entertainment systems).
• Medical equipment (including anaesthesia monitoring systems, ECG moni-
tors, drug delivery systems and MRI scanners).
• Defence systems (including radar systems, fighter aircraft flight control sys-
tems, radio systems and missile guidance systems).
Please note that our definition of embedded systems excludes applications such as
'personal digital assistants' (PDAs) running versions of Windows or similar operating
systems: from a developer's perspective, these are best viewed as a cut-down version
of a desktop computer system. This type of application makes up a very small per-
centage of the overall 'embedded' market and is not considered in this book.
1.3 Which processor should you use?
When desktop developers first think about working with embedded systems, there
is a natural inclination to stick with what they know and look for a book which
uses Pentium processors or other devices from this family (such as the 80486, or
the Intel 188). However, if you open up the engine management unit or the airbag
release system in your car, or take the back off your dishwasher, you will not find
any of these processors sitting inside, nor will there be anywhere to plug in a key-
board, graphics display or mouse.
Typical desktop processors cost more than US $100.00 a piece (often much
more). This cost puts them out of reach of all but the most expensive embedded
application. (Who would pay more than US $100 for a TV remote-control unit?)
In addition, a desktop processor requires numerous external support chips in order
to function: this further increases the cost. The additional components also
increase the physical size of the system, and the power consumption: both of
these factors are major problems for battery-powered embedded devices. (Who
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 3
Programming embedded systems in C 3
would buy a portable music player that requires ten large batteries to run, and
needs a trolley to transport it?)
Overall, the state-of-the art technology used in desktop processors matches the
needs of the PC user very well: however, their key features - an ability to execute
industry-standard code at a rate of more than 1000 million instructions per second -
come with a heavy price tag and are simply not required in most embedded systems.
The 8051 device is very different. It is a well-tested design, introduced in its
original form by Intel in 1980 (Figure 1.1). The development costs of this device
have now been fully recovered, and prices of modern 8051 devices now start at
less than US $1.00. At this price, you get a performance of around 1 million
instructions per second, and 256 bytes (not megabytes!) of on-chip RAM. You also
get 32 port pins and a serial interface. The 805 l's profile (price, performance,
available memory, serial interface) match the needs of many embedded systems
very well. As a result, it is now produced in more than 400 different forms by a
diverse range of companies including Philips, Infineon, Atmel and Dallas. Sales of
this vast family are estimated to have the largest share (around 60%) of the micro-
controller market as a whole, and to make up more than 50% of the 8-bit
microcontroller market. Versions of the 8051 are currently used in a long list of
embedded products, from children's toys to automotive systems.
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FIGURE 1.1 The external interface of a 'Standard' '8051' microcontroller (40-pin DIP package).
Standard 8051s have four ports, and are pin compatible with the original 8051/8052 from Intel.
Further information about the pin functions is given in Chapter 2
Building a desktop PC from an 8051 would not be a practical proposition, but it is
an excellent device for building many embedded systems. One important factor is
that the 8051 requires a minimum number of external components in order to
operate. For example, Figure 1.2 shows the circuit diagram for a complete 8051-
based application.
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 4
Embedded C
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FIGURE 1.2 An example of an 8051 microcontroller in use. In this example, the microcontroller is
intended to flash an LED connected to Pin 6. In addition to this LED, only a simple 'reset' circuit is
required (the capacitor and resistor connected to Pin 9), plus an external oscillator (in this case, a
3-pin ceramic resonator). We will consider some software that could be used to control such an
application in Chapter 3
The different nature of the embedded and desktop markets is emphasized by the
fact that some of the more recent 8051 devices - far from being more powerful
and having more features than the 1980 original - actually have fewer features. For
example, as we will discuss in Chapter 2, the original 8051 (Figure 1.1) had 32 I/O
pins and could - if necessary - be connected to up to 128 kbytes of external
memory. By contrast, the more recent 'Small 8051' devices typically have only
some 15 I/O pins, and do not support external memory. These devices are finding
their way into applications that would have involved a small number of discrete
components (transistors, diodes, resistors, capacitors) a few years ago, but which
may now be implemented more cheaply using microcontrollers (Figure 1.3).
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 5
Programming embedded systems in C
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FIGURE 1.3 An example of a 'Small 8051' in use. Small 8051s are produced by Atmel and
Philips. They have two ports (or less), and around 20 pins. In this case, an Atmel AT89C2051 is
used, in a 'cupboard light' application. The circuit here is intended for use in a cupboard
(closet), and will be battery powered. When the light is switched on (by pressing the switch),
it will operate for 20 seconds. If, in this time, the user does not press the switch again (to turn
off the light), the power will be removed automatically. This is a typical example of a very
simple product that may now be economically produced using a microcontroller
Both the Standard and Small 8051s are aimed, largely, at low-performance applica-
tion areas, where limited memory is required, and one of the most important
considerations is product cost. This forms a large segment of the embedded market
but - of course - not all projects take this form. To develop applications requiring
additional hardware or larger amounts of memory, we can opt to switch to a 16-bit
(or 32-bit) microcontroller environment - or even consider using a desktop micro-
processor. However, such a move can require a major investment in staff, training
and development tools. An alternative is to use one of the Extended 8051 devices
introduced in recent years by a range of manufacturers (see Figure 1.4).
One important application area for Extended 8051s has been the automotive
sector. Recent economic, legislative and technological developments in this sector
mean that an increasing number of road vehicles contain embedded systems.
Linking these systems together in many recent vehicles is a low-cost, two-wire
Controller Area Network (CAN) computer bus. The CAN bus eliminates the expen-
sive (and heavy) multi-wire looms, shaving around US $600 or more from
production costs: a significant saving.
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 6
Embedded C
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on-chip facilities, and additional port pins. In the case of the Infineon C515C (shown here), the
additional facilities include support for the 'Controller Area Network' (CAN) bus: this bus is
widely used in the automotive sector and in industrial environments. Figure reproduced with
permission from Infineon
This is not quite the end of the story. In order to connect to the CAN bus, the var-
ious devices - from door mirrors to braking systems - each require an embedded
processor. As a consequence, a modern passenger car may typically have 50
processors on board. To use 50 desktop chips in these circumstances, we would
need to spend around US $5000. This cost greatly outweighs any saving achieved
through the use of the CAN bus in the first place: in addition, the desktop proces-
sor would not have on-chip support for CAN, so additional hardware would be
needed in order to provide this, further increasing our costs and design complex-
ity. As an alternative, various Extended 8051s have on-chip hardware support for
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 7
Programming embedded systems in C 7
CAN (see Figure 1.4). The use of 50 such chips in a car design would not generally
cost more than US $200. Using these processors, the switch to CAN may result in
overall savings of around US $400 per vehicle.
The final thing to note about the 8051 architecture is that, if none of the 400 or
so existing chips matches the needs of your application, you can now build your
own device. For example, the Triscend 3 E5 series of devices have 8051 cores, plus
an additional area of field-programmable gate arrays (FPGAs) with which you can
create your own 'on chip' hardware. Alternatively, for even greater flexibility,
Xilinx Foundation 4 provides a comprehensive set of tools for the programming of
'blank' FPGAs or Application-Specific ICs (ASICs). Compatible with these tools are
a small range of 8051 'cores' which can be purchased - for example - from
Dolphin Integration. 5 The use of such techniques allows you to create your own
completely customized 8051 microcontroller, in order to match precisely your par-
ticular requirements.
Overall, the low cost, huge range, easy availability and widespread use of the
8051 architecture makes it an excellent platform for developing embedded sys-
tems: these same factors also make it an ideal platform for learning about
embedded systems. Whether you will subsequently use 8-, 16- or 32-bit embedded
processors, learning to work within the performance and memory limits of devices
such as the 8051 is a crucial requirement in the cost-conscious embedded market.
You simply cannot acquire these skills by developing code for a Pentium (or simi-
lar desktop) processor.
1.4 Which programming language should you use?
Having decided to use an 8051 processor as the basis of your embedded system,
the next key decision that needs to be made is the choice of programming lan-
guage. In order to identify a suitable language for embedded systems, we might
begin by making the following observations:
• Computers (such as microcontroller, microprocessor or DSP chips) only accept
instructions in 'machine code' ('object code'). Machine code is, by definition, in
the language of the computer, rather than that of the programmer. Interpretation
of the code by the programmer is difficult and error prone.
• All software, whether in assembly, C, C++, Java or Ada must ultimately be trans-
lated into machine code in order to be executed by the computer.
3. www.triscend.com 4. www.xilinx.com 5. www.dolphin.fr
:322 Chapter 1 pl-16 21/2/02 9:52 am Page
8 Embedded C
• There is no point in creating 'perfect' source code, if we then make use of a poor
translator program (such as an assembler or compiler) and thereby generate
executable code that does not operate as we intended.
• Embedded processors - like the 8051 - have limited processor power and very
limited memory available: the language used must be efficient.
• To program embedded systems, we need low-level access to the hardware: this
means, at least, being able to read from and write to particular memory loca-
tions (using 'pointers' or an equivalent mechanism).
Of course, not all of the issues involved in language selection are purely technical:
• No software company remains in business for very long if it generates new code,
from scratch, for every project. The language used must support the creation of
flexible libraries, making it easy to re-use (well-tested) code components in a
range of projects. It must also be possible to adapt complete code systems to
work with a new or updated processor with minimal difficulty.
• Staff members change and existing personnel have limited memory spans. At
the same time, systems evolve and processors are updated. As concern over the
'Year 2000' problem in recent years has illustrated, many embedded systems
have a long lifespan. During this time, their code will often have to be main-
tained. Good code must therefore be easy to understand now, and in five years'
time (and not just by those who first wrote it).
• The language chosen should be in common use. This will ensure that you can
continue to recruit experienced developers who have knowledge of the lan-
guage. It will also mean that your existing developers will have access to sources
of information (such as books, training courses, WWW sites) which give exam-
ples of good design and programming practice.
Even this short list immediately raises the paradox of programming language
selection. From one point of view, only machine code is safe, since every other
language involves a translator, and any code you create is only as safe as the code
written by the manufacturers of the translator. On the other hand, real code needs
to be maintained and re-used in new projects, possibly on different hardware: few
people would argue that machine code is easy to understand, debug or to port.
Inevitably, therefore, we need to make compromises; there is no perfect solu-
tion. All we can really say is that we require a language that is efficient, high-level,
gives low-level access to hardware, and is well defined. In addition - of course -
the language must be available for the platforms we wish to use. Against all of
these points, C scores well.
We can summarize C's features as follows:
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 9
Programming embedded systems in C 9
• It is 'mid-lever, with 'high-lever features (such as support for functions and
modules), and 'low-level' features (such as good access to hardware via pointers).
• It is very efficient.
• It is popular and well understood.
• Even desktop developers who have used only Java or C++ can soon understand
C syntax.
• Good, well-proven compilers are available for every embedded processor (8-bit
to 32-bit or more).
• Experienced staff are available.
• Books, training courses, code samples and WWW sites discussing the use of the
language are all widely available.
Overall, C's strengths for embedded system development greatly outweigh its
weaknesses. It may not be an ideal language for developing embedded systems,
but it is unlikely that a 'perfect' language will ever be created.
1.5 Which operating system should you use?
Having opted to create our 8051 -based applications using C, we can now begin to
consider how this language can be used. In doing so, we will begin to probe some of
the differences between software development for desktop and embedded systems.
In the desktop environment, the program the user requires (such as a word
processor program) is usually loaded from disk on demand, along with any
required data (such as a word processor file). Figure 1.5 shows a typical operating
environment for such a word processor. Here the system is well insulated from the
underlying hardware. For example, when the user wishes to save his or her latest
novel on disk, the word processor delegates most of the necessary work to the
operating system, which in turn may delegate many of the hardware-specific com-
mands to the BIOS (basic input/output system).
The desktop PC does not require an operating system (or BIOS). However, for most
users, the main advantage of a personal computer is its flexibility: that is, that the
same piece of equipment has the potential to run many thousands of different pro-
grams. If the PC had no operating system, each of these programs would need to be
able to carry out all the low-level functions for itself. This would be very inefficient
and would tend to make systems more expensive. It would also be likely to lead to
errors, as many simple functions would have to be duplicated in even the smallest of
programs. One way of viewing this is that a desktop PC is used to run multiple pro-
grams, and the operating system provides the 'common code' (for printing, file
storage, graphics, and so forth) that is required by this set of programs.
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 10
10
Embedded C
Word Processor
Operating System
BIOS
Hardware
OS provides 'common code' for:
• Graphics
• Printing
• File storage
• Sound
FIGURE 1.5 A schematic representation of the BIOS/OS sandwich from a desk-bound computer
system running some word processor software
There are two fundamental differences between the embedded systems we are con-
cerned with in this book and desktop computer systems:
1 The vast majority of embedded systems are required to run only one program:
this program will start running when the microcontroller is powered up, and
will stop running when the power is removed.
2 Many of the facilities provided by the modern desktop OS - such as the ability
to display high-resolution graphics, printing facilities and efficient disk access -
are of little value in embedded systems, where sophisticated graphics screens,
printers and disks are generally unavailable.
As a consequence, the simplest architecture in an embedded system is typically a
form of 'Super Loop' (see Listing 1.1).
Listing 1 .1 Part of a simple Super Loop demonstration
void main (void)
{
// Prepare run function X
X_Init() ;
whi1e(1) // 'for ever' (Super Loop)
{
X() ; // Run function X()
}
}
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 11
Programming embedded systems in C 11
It is important to appreciate that there is no operating system in use here. When
power is applied to the system, the function mai n ( ) will be called: having per-
formed the initializations, the function X ( ) will be called, repeatedly, until the
system is disconnected from the power supply (or a serious error occurs).
For example, suppose we wish to develop a microcontroller-based control
system to be used as part of the central-heating system in a building. The simplest
version of this system might consist of a gas-fired boiler (which we wish to con-
trol), a sensor (measuring room temperature), a temperature dial (through which
the desired temperature is specified) and the controller itself (Figure 1.6).
Temperature
sensor
Temperature
dial
4>
Boiler
FIGURE 1.6 An overview of a central heating controller
We assume that the boiler, temperature sensor and temperature dial are connected
to the system via appropriate ports.
Here, precise timing is not required, and a Super Loop framework similar to that
shown in Listing 1.2 may be appropriate.
Listing 1 .2 Part of the code for a simple central-heating system
/ * *
Main.C
Framework for a central heating system using a Super Loop.
[Compiles and runs but does nothing useful]
#include "Cen_Heat . h"
/* */
void main(void)
{
// Init the system
C_HEAT_Init() ;
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 12
12 Embedded C
while(1) // 'for ever' (Super Loop)
{
// Find out what temperature the user requires
// (via the user interface)
C_HEAT_Get_Requi red_Temperature ( ) ;
// Find out what the current room temperature is
// (via temperature sensor)
C_HEAT_Get_Actual_Temperature() ;
// Adjust the gas burner, as required
C_HEAT_Control_Boiler() ;
}
END OF FILE
It should be noted that the Super Loop architecture employed in this central-
heating system is not appropriate for all embedded applications. For more
advanced applications, we will describe how to create your own embedded operat-
ing system in Chapter 7.
1.6 How do you develop embedded software?
The process of compiling, linking and executing the program shown (in part) in
Listing 1.2 on a desktop PC is straightforward. In this environment, the executable
code we create will, in almost all cases, be intended to run on a desktop computer
similar to the one on which the code development is carried out. In the embedded
environment this is rarely the case. For example, the 8051 devices we will use
throughout this book do not have sufficient memory resources to allow them to
be used for compiling programs, and they will not support a keyboard or graphics
display. As a result, the code will be 'cross-compiled' on a desktop PC, generating
machine code that is compatible with the 8051 family: this process will be exam-
ined in greater detail in Chapter 3.
Having created the required executable code, we need to test it and refine it. To
do this, we need to do the following:
1 Build the hardware for the embedded system.
2 Transfer the executable code to the embedded hardware and test the system.
;322 Chapter 1 pl-16 21/2/02 9:52 am Page 13
Programming embedded systems in C 13
For programmers without experience of electronics, the process of building
embedded hardware is a daunting one. A typical approach used to prototype small
embedded applications is the 'breadboard'. This allows the microcontroller and
associated components to be connected together, without soldering, in order to
test and refine the hardware and software design.
For example, consider the breadboard shown in Figure 1.7. This code is for a
simple alarm clock with an LED display and some switches to adjust the time and
alarm settings. As the photograph makes clear, even in this simple application
there are many opportunities for wiring errors. As a result - if the system does not
operate correctly - it may not be clear whether this is as a result of software errors
or hardware problems.
A means of transferring the program code to the microcontroller is also
required. In examples such as the alarm clock shown in Figure 1.7, the microcon-
troller used will typically incorporate some form of 'flash' memory to store the
program code. 6 To transfer the executable code to the microcontroller, we will use
a 'flash' programmer. Flexible programmers (for use with the whole family of 805 1
processors, and other devices) may cost several hundred dollars. Alternatively,
'hobby' developers often choose to save money by developing their own program-
mer at home: however, to do this, experience of electronics construction
techniques is required.
Overall, the process of wiring up a breadboard to test your first simple embedded
programs can be daunting, expensive and rather prone to error. Fortunately, there
is now an alternative approach that can be used by those starting out in this area:
FIGURE 1.7 An example of a simple 8051 -based alarm clock assembled on a breadboard.
The board was assembled by Neeraj Kachhwaha, Bresley Lim, Melvin Lim and Henry Tarn.
Photograph by Michael Pont
6. We will consider the different types of available memory in greater detail in Chapter 2.
:322 Chapter 1 pl-16 21/2/02 9:52 am Page !■
14
Embedded C
1 Create the executable code for the embedded system on a desktop PC using an
appropriate cross-compiler and related tools (as above).
2 Use a software simulator (running on the desktop PC) to test the code.
3 Repeat Step 1 and Step 2, as necessary, until the software operates as required.
Throughout this book we will use such a simulator, produced by Keil Software. This
provides you with a very flexible 'hardware' platform, on which you can gain expe-
rience of embedded software without simultaneously having to learn how to solder.
A copy of this simulator is included on the enclosed CD. Please note that this is
an evaluation version of the simulator and has some restrictions compared with
the full version: nonetheless, it can be used to run all of the examples in this book.
We describe how to use this simulator in Chapter 3.
An example of the simulator in use is given in Figure 1.8. As we will see in sub-
sequent chapters, this simulator accurately reproduces the activity of all key
components in various members of the 805 1 family.
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detail in Chapter 3
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 15
Programming embedded systems in C 15
Please note that these simulators are not simply 'toys' to be used only when learn-
ing how to develop embedded software. Even where hardware will be constructed,
most developers will conduct early software tests on a simulator before using the
hardware. This can greatly speed up the development process. For example, in
applications using a serial interface (see Chapter 9), the simulator can determine
the precise baud rate being generated by your software: this can avoid many sub-
sequent problems. In addition, the simulator provides key facilities for debugging,
such as support for 'profiling' the code, a process that will typically involve meas-
uring the duration of particular functions. As we will see in subsequent chapters,
timing plays a central role in most embedded applications, and the ability to
measure function durations in a straightforward way makes the simulator a key
debugging tool.
1.7 Conclusions
In this introductory chapter, we have considered:
• The type of embedded systems that will be discussed in this book.
• The choice of programming language for embedded systems.
• The choice of operating system for embedded systems.
• The process of creating executable code for an embedded processor on a desktop PC.
• The process of testing the embedded code.
In the next chapter, we will look more closely at the features of the 8051
microcontroller.
:322 Chapter 1 pl-16 21/2/02 9:52 am Page 16
:322 Chapter 2 pl7-34 21/2/02 9:54 am Page 17
chapter
Introducing the 8051
microcontroller family
2.1 Introduction
In Chapter 1, we looked at some of the key features of 'embedded' software. In the
remainder of the book, our focus will be on 8051-based embedded systems.
In this chapter, we consider some of the key features of the 8051 family.
2.2 What's in a name?
Before we look in more detail at the features of the various 8051 devices, we
should note that the names given to the various family members is always a
source of confusion to new developers. For example, the 8031, 8751, 8052, 8032,
C505C, C515C, C509, C868, 80C517, 83C452, 80C390, ADuC812 and MAX7651
are all members of the 8051 family. The names of the devices provide little or no
indication of the family connections.
Particular confusion arises over the labels '8051' and '8052'. The 8052 was
launched (by Intel) shortly after the 8051 appeared. The architecture was the
same, except that the 8052 had more on-chip RAM (256 bytes cf. 128 bytes), and
also had an additional timer (Timer 2).
You should be aware that, despite the fact that they are described as '8051s',
almost all current devices are based on the slightly later 8052 architecture . As
the distinction between '8051' and '8052' is now purely of historical interest, we
will follow this convention and use the label '8051' throughout this book.
If you want more information about the various 8051 devices that are available,
you will find a large number of relevant data sheets on the CD. In addition, Keil
17
;322 Chapter 2 pl7-34 21/2/02 9:54 am Page 1
18
Embedded C
Software have a WWW site 7 with details of current 8051 variants (more than 400
devices are listed at the time of writing). This site is regularly updated. Another
useful source of information about the different members of the 805 1 family is the
'Micro Search' facility provided by Computer Solutions. 8
2.3 The external interface of the Standard 8051
As we saw in Chapter 1, the 400 different devices in the 8051 family can be
divided into three main groups: the Standard 8051s, the Small 8051s and the
Extended 8051s (Figure 2.1).
Small 8051
Low-cost members
of the 8051 family with
reduced number of port
pins, and no support for
off-chip memory.
Typical application:
Low-cost consumer goods
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support for large
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Typical applications:
Industrial and
automotive systems
FIGURE 2.1 The relationship between the various 'clans' in the 8051 family. The Standard
8051s are modern implementations of the original 8051 / 8052 device. Both the Small 8051s
and the Extended 8051s are derived from (and share key architectural features with) the
Standard 8051s. From the developer's perspective, a key feature of this family is that a single
compiler is required in order to generate code for all current devices
We will assume the use of Standard 8051 devices throughout this book. As the
architecture of the Small and Extended 8051s is derived from that of the Standard
8051, the techniques we discuss may be applied with any 8051-based device, and
the C compiler used throughout this book (and introduced in Chapter 3) can be
used to develop code for all 8051 derivatives.
Figure 2.2 shows the external interface of the Standard 8051, and briefly sum-
marizes the function of each of the port pins. Note that, in many cases, the port
pins can serve more than one purpose.
7. www.keil.com 8. www.computer-solutions.co.uk
:322 Chapter 2 pl7-34 21/2/02 9:54 am Page 19
Introducing the 8051 microcontroller family 19
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Pin(s) Function
1-8 Port 1. The bi-directional pins on this port may be used for input and output: each pin may be
individually controlled and - for example - some may be used for input while others on the same
port are used for output. Use of these pins is discussed in detail in Chapter 3 and Chapter 4.
In 8052-based designs, Pin 1 and Pin 2 have alternative functions associated with Timer 2 (see
Section 2.8).
9 The 'Reset' pin. When this pin is held at Logic 0, the chip will run normally. If, while the
oscillator is running, this pin is held at Logic 1 for two (or more) machine cycles, the
microcontroller will be reset. An example of simple reset hardware is given in Section 2.4.
10-17 Port 3. Another bi-directional input port (same operation as Port 1).
Each pin on this port also serves an additional function.
Pin 10 and Pin 11 are used to receive and transmit (respectively) serial data using the 'RS-232'
protocol. See Chapter 9 for details.
Pin 12 and Pin 13 are used to process interrupt inputs. We say more about interrupts in Section 2.9.
Pin 14 and Pin 15 have alternative functions associated with Timer and Timer 1 (see Section 2.8).
Pin 16 and Pin 17 are used when working with external memory (see Section 2.6).
18-19 These pins are used to connect an external crystal, ceramic resonator or oscillator module to the
microcontroller. See Section 2.5 for further details.
20 Vss. This is the 'ground' pin.
21-28 Port 2. Another bi-directional input port (same operation as Port 1).
These pins are also used when working with external memory (see Section 2.6).
29 Program Store Enable (PSEN) is used to control access to external CODE memory (if used). See
Section 2.6.
30 Address Latch Enable (ALE) is used when working with external memory (see Section 2.6). Note
that some devices allow ALE activity to be disabled (if external memory is not used): this can
help reduce the level of electromagnetic interference (EMI) generated by your product.
This pin is also used ( on some devices) as the program pulse input (PROG) during Flash programming.
31 External Access (EA). To execute code from internal memory (e.g. on-chip Flash, where available)
this pin must be connected to Vcc. To execute code from external memory, this pin must be
connected to ground. Forgetting to connect this pin to Vcc is a common error when people first
begin working with the 8051.
32-39 Port 0. Another bi-directional input port (same operation as Port 1). Note that - unlike Port 1, Port 2
and Port 3 - this port does NOT have internal pull-up resistors. See Chapter 4 for further details.
These pins are also used when working with external memory (see Section 2.6).
40 Vcc. This is the '5V pin (on 5V devices; 3V on 3V devices, etc).
FIGURE 2.2 The external interface to the standard '805V microcontroller
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20
Embedded C
2.4 Reset requirements
The process of starting any microcontroller is a non-trivial one. The underlying
hardware is complex and a small, manufacturer-defined, 'reset routine' must be
run to place this hardware into an appropriate state before it can begin executing
the user program. Running this reset routine takes time, and requires that the
microcontroller's oscillator is operating.
Where your system is supplied by a robust power supply, which rapidly reaches
its specified output voltage when switched on, rapidly decreases to OV when
switched off, and - while switched on - cannot 'brown out' (drop in voltage), then
you can safely use low-cost reset hardware based on a capacitor and a resistor to
ensure that your system will be reset correctly: this form of reset circuit is shown
in Figure 2.3a.
Where your power supply is less than perfect, and / or your application is safety
related, the simple RC solution will not be suitable. Several manufacturers provide
more sophisticated reset chips which may be used in these circumstances: Figure
2.3b illustrates one possibility.
Vcc
Vcc
(b)
FIGURE 2.3a and b Two possible reset circuits for 8051 -based designs. We will not consider
hardware issues in detail in this book: please refer to Chapter 1 1 for sources of further
information about this topic
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Introducing the 8051 microcontroller family 21
2.5 Clock frequency and performance
All digital computer systems are driven by some form of oscillator circuit: the 8051
is certainly no exception (see Figure 2.4).
The oscillator circuit is the 'heartbeat' of the system and is crucial to correct
operation. For example, if the oscillator fails, the system will not function at all;
if the oscillator runs irregularly, any timing calculations performed by the system
will be inaccurate.
We consider some important issues linked to oscillator frequency and perform-
ance in this section.
FIGURE 2.4 An example of a simple crystal oscillator circuit. We will not consider hardware
issues in detail in this book: please refer to Chapter 1 1 for sources of further information about
this topic
a) The link between oscillator frequency and machine-cycle period
One of the first questions to be asked when considering a microcontroller for a
project is whether it has the required level of performance.
As a general rule, the speed at which your application runs is directly determined
by the oscillator frequency: in most cases, if you double the oscillator frequency,
the application will run twice as fast. When we want to compare different proces-
sors, we need a way of specifying performance in a quantitative manner. One
popular measure is the number of machine instructions that may be executed in
one second, usually expressed in 'MIPS' (Million Instructions Per Second). For
example, in the original Intel 8051 microcontroller, a minimum of 12 oscillator
cycles was required to execute a machine instruction. The original 8051 had a max-
imum oscillator frequency of 12 MHz and therefore a peak performance of 1 MIP.
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22 Embedded C
A simple way of improving the 8051 performance is to increase the clock fre-
quency. More modern (Standard) 8051 devices allow the use of clock speeds well
beyond the 12 MHz limit of the original devices. For example, the Atmel
AT89C55WD, allow clock speeds up to 33 MHz: this raises the peak performance
to around 3 MIPS.
Another way of improving the performance is to make internal changes to the
microcontroller so that fewer oscillator cycles are required to execute each
machine instruction. The Dallas 'High Speed Microcontroller' devices (87C520,
and similar) use this approach, so that only four oscillator cycles are required to
execute a machine instruction. These Dallas devices also allow faster clock rates
(typically up to 33 MHz). Combined, these changes give a total performance of
around 8 MIPS. Similar changes are made in members of the Winbond family of
Standard 8051 devices (see the Winbond W77E58, for example) resulting in per-
formance figures of up to 10 MIPS.
Clearly, for maximum performance, we would like to execute instructions at a
rate of one machine instruction per oscillator cycle. For example, the Dallas 'Ultra
High Speed' 89C420 operates at this rate: as a result, it runs at 12 times the speed
of the original 8051. In addition, the 89c420 can operate at up to 50 MHz, increas-
ing overall performance to around 40-50 MIPS.
To put all these figures in perspective, a modern desktop PC has a potential per-
formance of around 1000 MIPS. However, a good percentage of this performance
(perhaps 50% or more) will be 'consumed' by the operating system. By contrast,
the embedded operating system we will describe in Chapter 7 consumes less than
1% of the processor resources of the most basic 8051: this leaves sufficient CPU
cycles to run a complex embedded application.
b) Why you should choose a low oscillator frequency
In our experience, many developers select an oscillator frequency that is at or near
the maximum value supported by a particular device. For example, the Infineon
C505/505C will operate with crystal frequency of 2-20 MHz, and many people
automatically choose values at or near the top of this range, in order to gain max-
imum performance.
This can be a mistake, for the following reasons:
• Many applications do not require the levels of performance that a modern 8051
device can provide.
• In most modern (CMOS-based) 8051s, there is an almost linear relationship
between the oscillator frequency and the power supply current. As a result, by
using the lowest frequency necessary it is possible to reduce the power require-
ment: this can be useful, particularly in battery-powered applications.
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Introducing the 8051 microcontroller family 23
• When accessing low-speed peripherals (such as slow memory, or liquid-crystal
displays), programming and hardware design can be greatly simplified - and
the cost of peripheral components, such as memory latches, can be reduced - if
the chip is operating more slowly.
• The electromagnetic interference (EMI) generated by a circuit increases with
clock frequency.
In general, you should operate at the lowest possible oscillator frequency compati-
ble with the performance needs of your application. As we will see in later
chapters, simulating the processor is a good way of determining the required oper-
ating frequency for a particular application.
2.6 Memory Issues
We consider some of the memory issues relating to the 8051 in this section.
a) Types of memory
On the desktop, most designers and programmers can safely ignore the type of
memory they are using. This is seldom the case in embedded environments, and
we therefore briefly review some of the different types of memory below.
First, a short history lesson, to explain the roots of an important acronym. On
early mainframe and desktop computer systems, long-term data storage was car-
ried out using computer tapes. Reading or writing to the tape took varying
amounts of time, depending whether it involved, for example, rewinding the
entire tape, or simply rewinding a couple of centimetres. In this context, new
memory devices appeared that could be used to store data while the computer was
running, but which lost these data when the power was removed. These read-write
memory devices were referred to as 'random access memory' (RAM) devices,
because - unlike tape-based systems - accessing any element of memory 'chosen at
random' took the same amount of time.
Tapes have now largely disappeared, but the acronym RAM has not, and is still
used to refer to memory devices that can be both read from and written to.
However, since RAM was first introduced, new forms of memory devices have
appeared, including various forms of ROM (read-only memory). Since these ROM
devices are also 'random access' in nature, the acronym RAM is now best trans-
lated as 'Read- Write Memory'.
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24 Embedded C
Dynamic RAM (DRAM)
Dynamic RAM is a read-write memory technology that uses a small capacitor to
store information. As the capacitor will discharge quite rapidly, it must be fre-
quently refreshed to maintain the required information: circuitry on the chip
takes care of this refresh activity. Like most current forms of RAM, the information
is lost when power is removed from the chip.
Static RAM (SRAM)
Static RAM is a read-write memory technology that uses a form of electronic flip-
flop to store the information. No refreshing is required, but the circuitry is more
complex and costs can be several times that of the corresponding size of DRAM.
However, access times may be one-third those of DRAM.
Mask Read-Only Memory (ROM)
Mask ROM is - from the software developer's perspective - read only: however,
the manufacturer is able to write to the memory, at the time the chip is created,
according to a 'mask' provided by the company for which the chips are being
produced. Such devices are therefore sometimes referred to as 'factory-
programmed ROM'. Mask programming is not cheap, and is not a low-volume
option: mistakes can be very expensive, and providing code for your first mask
can be a character-building process. Access times are often slower than RAM:
roughly 1.5 times that of DRAM.
Many members of the 8051 family are available with on-chip, mask-
programmed, ROM.
Programmable Read-Only Memory (PROM)
PROM is a form of Write-Once, Read-Many (WORM) or 'One-Time Programmable'
(OTP) memory. Basically, we use a PROM programmer to blow tiny 'fuses' in the
device. Once blown, these fuses cannot be repaired; however, the devices them-
selves are cheap.
Many modern members of the 8051 family are available with OTP ROM.
UV Erasable Programmable Read-Only Memory (UV EPROM)
Like PROMs, UV EPROMs are programmed electrically. Unlike PROMs, they also
have a quartz window which allows the memory to be erased by exposing the
internals of the device to UV light. The erasure process can take several minutes
and, after erasure, the quartz window will be covered with a UV-opaque label. This
form of EPROM can withstand thousands of program / erase cycles.
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Introducing the 8051 microcontroller family 25
More flexible than PROMs and once very common, UV EPROMs now seem
rather primitive compared with EEPROMs (see below). They can be useful for pro-
totyping but are prohibitively expensive for use in production.
Many older members of the 8051 family are available with on-board UV EPROM.
EEPROM and Flash ROM
Electrically-Erasable Programmable Read-Only Memory (EEPROMs) and 'Flash'
ROMs are a more user-friendly form of ROM that can be both programmed and
erased electrically.
EEPROM and Flash ROM are very similar. EEPROMs can usually be re-
programmed on a byte-by-byte basis, and are often used to store passwords or
other 'persistent' user data. Flash ROMs generally require a block-sized 'erase' oper-
ation before they can be programmed: often the size of the block will be several
kilobytes: such ROMs are often used for the storage of program code.
Many members of the 8051 family are available with on-board EEPROM or flash
ROM, and some devices contain both types of memory.
b) Memory organization and 'hex'
As you will recall, all data items are represented in computer memory as binary
codes, each containing a certain number of bits. To simplify the storage and retrieval
of data items, these memory bits are organized into memory locations, each with a
unique memory address. In a common byte-oriented memory (as used in desktop
PCs and - with some exceptions - in the 8051), each memory location contains
eight bits (one byte) of storage, and each byte has a unique address (Figure 2.5).
Address
Contents
...
•
•
0x04
1
1
1
1
1
1
0x03
1
1
1
1
0x02
1
1
1
0x01
1
1
1
0x00
1
1
1
1
1
FIGURE 2.5 A segment of byte-oriented memory. Note that each byte (rather than each bit)
has its own unique address, shown in hexadecimal notation here. See text for details
:322 Chapter 2 pl7-34 21/2/02 9:55 am Page 26
26 Embedded C
In Figure 2.5, the memory addresses are given in hexadecimal notation. This is a
base-16 numbering scheme which provides a compact way of representing large
binary numbers: it is widely used in embedded systems. Note that the prefix 'Ox' is
used in C (and elsewhere) to indicate that a number is in 'hex' notation.
Table 2.1 shows a list of numbers with their hex, binary and 'ordinary' decimal
representations.
TABLE 2.1 Different number representations. See text for details
Hexadecimal
(base
16)
Binary (base
2)
Decimal (base 10)
0x00
00000000
OxFF
11111111
255
OxOF
00001111
15
OxFO
1111 0000
240
OxAA
10101010
170
OxFFFF
1111111111111111
65535
OxFFAA
1111111110101010
65450
c) The 8051 memory architecture
Having considered some of the basic memory types available and some general
features of computer memory organization, we are now in a position to consider
the memory architecture of the 8051.
There are two distinct memory regions in an 8051 device: the DATA area and
the CODE area. We will consider each region in turn here.
DATA memory
DATA memory is used to store variables and the program stack while the program
is running. The DATA area will be implemented using some form of RAM.
Most of the DATA area has a byte-oriented memory organization. However,
within the DATA area is a 16-byte BDATA area which can also be accessed using bit
addresses. This area can be used to store bit-sized variables (see Figure 2.6). The
8051 has machine instructions which allow bit variables to be manipulated very
efficiently. These instructions can be easily utilized from C, as we will demonstrate
in Chapter 3.
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Introducing the 8051 microcontroller family 27
Bit address
Byte
address
0x2 F
0x2E
0x2D
0x2C
0x2B
0x2A
0x29
0x28
0x27
0x26
0x25
0x24
0x23
0x22
0x21
0x20
FIGURE 2.6 On-chip RAM memory in the 8051 : The BDATA area. See text for details
Note that the various locations can be accessed either via their byte addresses
(0x20 to 0x2F) or via their bit addresses (0x00 to 0x7F).
Note also that there is an area where the bit addresses and the byte addresses are
the same. For example, within byte address 0x24, there is a bit location with
address 0x27: there is also a byte with address 0x27 in the BDATA area. It may
appear that this will cause problems for the compiler. However, no conflicts will
arise because the compiler can always determine from the context (that is, the type
of data being manipulated) whether the bit address or byte address should be used.
0x7F
0x7E
0x7D
0x7C
0x7B
0x7A
0x79
0x78
0x77
0x76
0x75
0x74
0x73
0x72
0x71
0x70
0x6F
0x6E
0x6D
0x6C
0x6B
0x6A
0x69
0x68
0x67
0x66
0x65
0x64
0x63
0x62
0x61
0x60
0x5F
0x5E
0x5D
0x5 C
0x5B
0x5A
0x59
0x58
0x57
0x56
0x55
0x54
0x53
0x52
0x51
0x50
0x4F
0x4E
0x4D
0x4C
0x4B
0x4A
0x49
0x48
0x47
0x46
0x45
0x44
0x43
0x42
0x41
0x40
0x3 F
0x3E
0x3D
0x3C
0x3B
0x3A
0x39
0x38
0x37
0x36
0x35
0x34
0x33
0x32
0x31
0x30
0x2F
0x2E
0x2D
0x2C
0x2B
0x2A
0x29
0x28
0x27
0x26
0x25
0x24
0x23
0x22
0x21
0x20
OxlF
OxlE
OxlD
OxlC
OxlB
OxlA
0x19
0x18
0x17
0x16
0x15
0x14
0x13
0x12
Oxll
0x10
OxOF
OxOE
OxOD
OxOC
OxOB
OxOA
0x09
0x08
0x07
0x06
0x05
0x04
0x03
0x02
0x01
0x00
CODE memory
Not surprisingly, the CODE area is used to store the program code, usually in some
form of ROM ('read-only memory') .
Please note:
• The CODE area may also contain read-only variables ('constants'), such as filter
co-efficients or data for speech playback.
• On a desktop PC, code is copied from disk to RAM when you run the program.
It is then executed from RAM. In most embedded systems, like the 8051, code is
'executed in place', from ROM.
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28
Embedded C
• One consequence of the 'execute in place' operation is that, in most applica-
tions, while you may require up to 10 kbytes of CODE memory, you will rarely
need much DATA memory. This is reflected in the fact that many 8051 devices
have 20 kbytes or more of on-chip ROM, but will often have no more than 256
bytes of RAM. This is an appropriate mix for most general applications.
d) 8-bit family, 16-bit address space
The Standard 8051 can be described as an 8-bit microcontroller with a 16-bit
address space.
Here the fact that it is an '8-bit microcontroller' refers to the size of the registers
and data bus. This means that the family will handle 8-bit data very quickly and
process 16-bit or 32-bit data rather less efficiently.
The 16-bit address space means that the device can directly address 2 16 bytes of
memory: that is, 64 kbytes. Note that the (Harvard-like) architecture of the 8051
means that it can access both 64 kbytes of CODE memory and 64 kbytes of DATA
memory. These figures refer to the total amount of memory you can access: to
reach these limits, you will need to connect memory devices to the external inter-
face (Figure 2.7).
P2.7(A15)
P2.6(A14)
P2.5 (A1 3)
P2.4(A12)
P2.3(A11)
P2.2 (Al 0)
P2.1 (A9)
P2.0 (A8)
ALE
O
00
c
■M
P0.7 (AD7)
P0.6 (AD6)
P0.5 (ADS)
P0.4 (AD4)
P0.3 (AD3)
P0.2 (AD2)
P0.1 (AD1)
PO.O (ADO)
P3.7 (/RD)
P3.6 (/WE)
/PSEN
Timing and control lines
FIGURE 2.7 The 8051 external memory interface. Use of this external memory interface
involves the use of the whole of Port and Port 2, plus some of Port 3. We will not consider
hardware issues in detail in this book: please refer to Chapter 1 1 for sources of further
information about this topic
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Introducing the 8051 microcontroller family 29
Where possible, it is better to use a device with all the required memory on the
chip: this can improve reliability, reduce costs, reduce the application size, and
reduce power consumption.
Finally, please note that - while all 8051s are 8-bit microcontrollers - some
more recent devices (like the Dallas 80c390) support an address space greater than
16 bits. This allows access to much larger amounts of memory. Please refer to the
80c390 data sheet (on the CD) for details.
2.7 I/O pins
Much of the activity in embedded systems involves reading pins (which may, for
example, be connected to switches or keypads) and altering the value of other pins
(which may, in turn, control anything, from an LED to a 5 -tonne industrial robot).
The 8051 architecture has a number of features which make it very well suited to
such applications.
Most 8051s have four 8-bit ports, giving a total of 32 pins you can individually
read from or control. All of the ports are bidirectional: that is, they may be used for
both input and output. To limit the size of the device, some of the port pins have
alternate functions. For example, as we saw in the previous section, Ports 0, 2 (and
part of Port 3) together provide the address and data bus used to support access to
external memory. Similarly, two further pins on Port 3 (Pin 16 and Pin 17) also pro-
vide access to the on-chip UART (see Section 2.10). When in their 'alternative
roles', these pins cannot be used for ordinary input or output. For example, if using
a Standard 8051 device with external memory, Port 1 is the only (complete) port
available for general-purpose I/O operations.
The comments above all refer to the Standard 8051: the number of available
ports on 8051 microcontrollers varies enormously: the Small 8051s have the
equivalent of approximately two ports, and the Extended 8051s have up to ten
ports. Despite these differences, the control of ports on all members of the 8051
family is carried out in the same way.
We will consider how to use the port pins for input and output in Chapter 3
and Chapter 4.
2.8 Timers
All members of the 8051 family have at least two timer/counters, known as Timer
and Timer 1: most also have an additional timer (Timer 2). These are 16-bit
timers, which means they can hold values from to 65535 (decimal).
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30 Embedded C
Timers like these are crucial to the development of embedded systems. To see
why, you need to appreciate that, when configured appropriately, the timers are
incremented periodically: specifically, in most 8051 devices, the timers are incre-
mented every 12 oscillator cycles. Thus, assuming we have a 12 MHz oscillator,
the timer will be incremented 1 million times per second.
There are many things we can do with such a timer:
• We can use it to measure intervals of time. For example, we can measure the
duration of a function by noting the value of a timer at the beginning and end
of the function call, and comparing the two results.
• We can use it to generate precise hardware delays, as we discuss in Chapter 6.
• We can use it to generate 'time out' facilities: this is a key requirement in sys-
tems with real-time constraints (see Chapter 6).
• Most important of all, we can use it to generate regular 'ticks', and drive an
operating system: we say a little more about this in the next section.
2.9 Interrupts
If you were to ask developers who are experienced in embedded systems to sum up
in one word the difference between desktop software and embedded software,
many would probably choose the word 'interrupt'.
From a low-level perspective, an interrupt is a hardware mechanism used to
notify a processor that an 'event' has taken place: such events may be 'internal'
events (such as the overflow of a timer) or 'external' events (such as the arrival of a
character through a serial interface).
Viewed from a high-level perspective, interrupts provide a mechanism for creat-
ing multitasking applications: that is applications which, apparently, perform more
than one task at a time using a single processor. To illustrate this, a schematic repre-
sentation of interrupt handling in an embedded system is shown in Figure 2.8.
In Figure 2.8 the system executes two (background) functions, Function 1 and
Function 2. During the execution of Function 1, an interrupt is raised, and an
'interrupt service routine' (ISR1) deals with this event. After the execution of ISR1
is complete, Function 1 resumes its operation. During the execution of Function 2,
another interrupt is raised, this time dealt with by ISR2.
The original '8051' ('8052') architecture supported seven interrupt sources: 9
• Two or three timer/counter interrupts (related to Timer 0, Timer 1 and - in the
8052 -Timer 2).
9. More recent devices support larger numbers of interrupts without altering the core architecture.
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Introducing the 8051 microcontroller family 31
Background
Foreground
Time
tl
t2
t3
t4
t5
Function 1
Function 1
Function 2
ISR1
Function 2
ISR1
FIGURE 2.8 A schematic representation of interrupt handling in an embedded system
• Two UART-related interrupts (note: these share the same interrupt vector, and
can be viewed as a single interrupt source).
• Two external interrupts.
In addition, there is one further interrupt source over which the programmer has
minimal control:
• The 'power-on reset' (POR) interrupt.
When an interrupt is generated, the processor 'jumps' to an address at the bottom
of the CODE memory area. These locations must contain suitable code with which
the microcontroller can respond to the interrupt. In most cases, the locations will
include another 'jump' instruction, giving the address of suitable 'interrupt service
routine' located elsewhere in (CODE) memory.
This process may sound complicated but, from the perspective of the C pro-
grammer, an interrupt service routine is simply a function that is 'called by the
microcontroller', as a result of a particular hardware event. As we will see in later
chapters, use of interrupts in a high-level language is a straightforward process.
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32 Embedded C
As we noted above, three of the '8051 / 8052' interrupt sources are associated
with on-chip timers. This is because such timers are a particularly effective and
widely-used source of interrupts. For example, we can use such timers to gener-
ate an interrupt (a 'tick') at regular and precise intervals of (say) 1 millisecond.
As we will see in Chapter 7, such 'timer ticks' form the basis of all real-time
operating systems.
2.10 Serial interface
The 8051 has a serial port compatible with what is commonly referred to as the
RS-232 communication protocol. This allows you to transfer data between an 8051
microcontroller and some form of personal computer (desktop PC, notebook PC
or similar).
Such an interface is common in embedded processors, and is widely used. Here
are some examples:
• The serial port may be used to debug embedded applications, using a desktop PC.
• The serial port may be used to load code into flash memory for 'in circuit pro-
gramming'. This can be very useful, for example, when code must be updated
'in situ' (for example, when the product is already installed in a vehicle, or on a
production line).
• The serial port may be used to transfer data from embedded data acquisition
systems to a PC, or to other embedded processors.
We will consider the use of the serial interface in detail in Chapter 9.
2.1 1 Power consumption
The final issue we need to address is power consumption.
On a desktop PC, processor power consumption is not generally a major con-
cern. In such an environment, the typical CRT-based screen, DVD/CD drive and
hard disk will consume much more power than the processor itself: efforts at
saving power therefore tend to focus on 'shutting down' peripheral components
rather than optimizing the processor itself.
The embedded environment is very different, particularly where battery-
powered applications are being developed. In such applications, the microcontroller
is often the main drain on our battery power. To achieve longer battery life, all
modern implementations of 8051 processors have at least three operating modes:
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Introducing the 8051 microcontroller family 33
• Normal mode.
• Idle Mode.
• Power-Down Mode.
The 'Idle' and Tower Down' modes are intended to be used to save power at times
when no processing is required. Typical current requirements for the various
modes are shown in Table 2.2. Note that the original 'Intel 8051' figure (for the
1980 processor) is shown for comparative purposes: this did not have power-
saving modes.
TABLE 2.2 Typical current consumption figures for a selection of Standard 8051 devices.
Note that figures vary (approximately linearly) with oscillator frequency: in this case, the clock
frequency is assumed at 12 MHz for each device
Device
Normal
Idle
Power Down
Intel 8051
160 mA
-
-
Atmel 89S53
11 mA
2 mA
60 uA
Dallas 87C520
15 mA
8 mA
50 uA
Intel 80C51
16 mA
4 mA
50 uA
To put these figures in context, bear in mind that - for reasonable battery life - we
generally aim for an average power consumption of less than 10 mA. As the table
makes clear, this means that operating in 'Normal' mode for extended periods is
not a practical option.
The Infineon C501 is an example of a Standard 8051 device, which offers
power-down modes identical to those available in the 8052 and many other
modern devices. The following description of the C501 idle modes, adapted from
the user manual, describes these modes in detail. Please note that this description
applies equally well to most Standard 8051s.
a) Idle mode
In the idle mode the oscillator of the C501 continues to run, but the CPU is gated
off from the clock signal. However, the interrupt system, the serial port and all
timers are connected to the clock. The CPU status is preserved in its entirety.
The reduction of power consumption which can be achieved by this feature
depends on the number of peripherals running. If all timers are stopped and the
serial interface is not running, the maximum power reduction can be achieved:
the developer has to determine which peripheral must continue to run and which
may be stopped.
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34 Embedded C
The idle mode is entered by setting the flag bit IDLE (PCON.O). The easiest way
to set the IDLE bit is with the following 'C statement:
PCON |= 0x01; // Enter idle mode
There are two ways to terminate idle mode:
• Activate any enabled interrupt. This interrupt will be serviced and the program
will continue by executing the instruction following the instruction that sets
the IDLE bit.
• Perform a hardware reset.
b) Power-down mode
In the power-down mode, the on-chip oscillator is stopped. Therefore all func-
tions are stopped; only the contents of the on-chip RAM are maintained.
The power-down mode is entered by setting the flag bit PDE (PCON.l). This is
most easily done in 'C as follows:
PCON |= 0x02; // Enter power-down mode
The only exit from power-down mode is a hardware reset.
2.12 Conclusions
All members of the 8051 family provide the hardware components - multiple port
pins, timers, interrupt handling, serial interface and memory - required in embed-
ded applications. We will describe how to make full use of these components in the
remainder of this book.
In Chapter 3, we describe how the operation of the 8051 can be simulated on a
desktop PC.
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 35
chapter
Hello, Embedded World
3.1 Introduction
^^3
FIGURE 3.1 A desk-bound computer (left) sporting general-purpose input and output facilities
(keyboard and high-resolution graphics screen), compared with an embedded, single-board
computer system which lacks any high-level I/O facilities
When you first started programming a desktop computer, you probably wrote a pro-
gram similar to the example shown in Listing 3.1 for every language you learned.
Listing 3.1 A simple desktop C program
#include <stdio.h>
int main(void)
{
printf ("Hello world\n") ;
return 0;
}
35
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 36
36 Embedded C
In this chapter, we will dissect a 'Hello World' program written for the 805 1 micro-
controller. Like the desktop program, this will be designed to introduce key features
of the development tools and the environment. Unlike the desktop version, we
have no screen on which to display text (Figure 3.1): instead, the code will be
designed to flash an LED on for one second, off for one second, ad infinitum.
The program we will use to do this is shown (in part) in Listing 3.2.
Listing 3.2 Part of the 'Hello, Embedded World' example code
void main (void)
{
LED_FLASH_Init() ;
wh i 1 e ( 1 )
{
// Change the LED state (OFF to ON, or vice versa)
LED_FLASH_Change_State() ;
// Delay for *approx* 1000 ms
DELAY_L00P_Wait(1000) ;
}
}
We begin by considering the Keil tools that will be used to develop this software.
We will then consider the various components of the program in detail.
3.2 Installing the Keil software and loading the project
To make full use of the material in this chapter, you need to install the Keil C51
development tools: these are included on the CD. Please refer to the CD for details.
Note: the example projects for this book are NOT loaded automatically when
you install the Keil compiler. Instead, these files are stored on the CD in a direc-
tory '/Pont'. The files are arranged by chapter: the project discussed here is in the
directory '/Pont/ Ch03_00 - Hello'.
Rather than using the projects on the CD (where changes cannot be saved),
please copy the files from the CD onto an appropriate directory on your hard disk.
Note: you will need to change the file properties after copying: files transferred
from the CD will be 'read only'.
When you have copied the files onto your hard disk, please run the Keil
(i Vision application, and use the 'Open Project' option (from the 'Project' menu)
to load the 'Hello' example. You will then be able to work with this project by fol-
lowing the example described in the remainder of this chapter.
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 37
Hello, Embedded World 37
3.3 Configuring the simulator
Having loaded the 'Hello' project in the Keil u Vision environment, we will begin
by exploring the project settings.
First, using the Project menu, we will look at the 8051 device which we are
intending to use for this application (Figure 3.2).
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FIGURE 3.2 This is how we choose which particular 8051 chip the hardware will simulate. In
this project, we are using a 'generic' 8052 driver. This driver will work with all of the examples
in this book
;322 Chapter 3 p35-56 21/2/02 9:55 am Page 3
38
Embedded C
In this case, we will use a generic '8052' driver: as we discussed in Chapter 2, the
8052 architecture forms the basis of the great majority of current '8051' devices.
This driver can be used with all of the examples in this book.
The next thing we need to check is the oscillator frequency. As we also dis-
cussed in Chapter 2, the choice of oscillator frequency has a large impact on
8051-based applications. In most examples in this book, we will assume that the
oscillator frequency is 12 MHz. Figure 3.3 shows how to inspect and - if required -
alter this frequency.
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FIGURE 3.3 Viewing and - if necessary - changing the oscillator frequency
1322 Chapter 3 p35-56 21/2/02 9:55 am Page 39
Hello, Embedded World 39
Note: one of the key reasons for setting the oscillator frequency in the simulator is
that any attempting at 'profiling' the application (for example, measuring function
durations) will only be successful if the oscillator frequency in the simulator matches
the frequency that will be used in the real system hardware (see Section 3.6e).
3.4 Building the target
We next need to build the 'target', as illustrated in Figure 3.4.
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FIGURE 3.4 Building the target (compiling and linking your source files) in the Keil
environment
3.5 Running the simulation
Having successfully built the target, we are now ready to start the debug session
and run the simulator.
First, start a debug session (Figure 3.5).
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 40
40
Embedded C
m ink -lAiwd
G» fr* Hfc- Ehm b*m
FIGURE 3.5 Starting the simulator (debugger)
The 'flashing LED' we will view will be connected to Port 1. We therefore want to
observe the activity on this port (Figure 3.6).
By default, to speed up the simulation, updates to the various components are
carried out only on demand. For our purposes, we want to ensure that the simula-
tor regularly updates the screen: we do this by ticking the 'Periodic Window
Update' option in the 'View' menu (Figure 3.7).
Finally, we are ready to start running our 'Hello, Embedded World' program in
the simulator (Figure 3.8).
As the program runs, observe that Pin 1.5 flashes 'on' and 'off, as required.
Please note:
The simulator does NOT operate in real time. What this means is that - except
by chance - the port pin will not flash on for one second, off for one second,
when you execute this program.
To determine the speed at which the code will execute on the real system,
you need to use the 'Performance Analyzer' functions provided in the simulator:
we discuss these functions in Section 3.6e.
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 41
Hello, Embedded World 41
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FIGURE 3.6 The flashing LED will be connected to Port 1 . We therefore need to have the
simulator display the activity on this port
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 42
42 Embedded C
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FIGURE 3.7 To ensure that the port activity is visible, we need to set the 'Periodic Window
Update' flag
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FIGURE 3.8 Starting the simulator run
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 43
Hello, Embedded World 43
3.6 Dissecting the program
So far in this chapter we have simply demonstrated the operation of the Keil 805 1
simulator.
We now begin to consider how the code in Listing 3.2 actually works.
a) The complete program
The complete 'Hello, Embedded World' program is shown in Listing 3.3.
Listing 3.3 The complete 'Hello, Embedded World' program
/ * *
Hello. C (v1 .00)
A "Hello Embedded World" test program for 8051.
#include <reg52.h>
// LED is to be connected to this pin
sbit LED_pin = P1 A 5;
// Stores the LED state
bit LED_state_G;
// Function prototypes
void LED_FLASH_Init(void) ;
void LED_FLASH_Change_State(void) ;
void DELAY_L00P_Wait( const unsigned int);
/* */
void main(void)
{
LED_FLASH_Init() ;
whi le(1 )
{
// Change the LED state (OFF to ON, or vice versa)
LED_FLASH_Change_State() ;
:322 Chapter 3 p35-56 21/2/02 9:55 am Page
44 Embedded C
// Delay for *approx* 1000 ms
DELAY_L00P_Wait(1000) ;
}
LED_FLASH_Init()
Prepare for LED_Change_State() function - see below
void LED_FLASH_Init(void)
{
LED_state_G = 0;
}
/*
LED_FLASH_Change_State ( )
Changes the state of an LED (or pulses a buzzer, etc) on a
specified port pin.
Must call at twice the required flash rate: thus, for 1 Hz
flash (on for 0.5 seconds, off for 0.5 seconds),
this function must be called twice a second.
voi d LED_FLASH_Change_State ( voi d )
{
// Change the LED from OFF to ON (or vice versa)
if (LED_state_G == 1)
{
LED_state_G = 0;
LED_pin = 0;
}
else
{
LED_state_G = 1 ;
LED_pin = 1 ;
}
}
/
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 45
Hello, Embedded World 45
DELAY_L00P_Wait()
Delay duration varies with parameter.
Parameter is, *R0UGHLY*, the delay, in milliseconds,
on 12MHz 8051 (12 osc cycles).
You need to adjust the timing for your application!
* *
void DELAY_L00P_Wait( const unsigned int DELAY)
{
unsigned int x, y;
for (x = 0; x <= DELAY; x++)
{
for (y = 0; y <= 120; y++) ;
}
}
/ * *
END OF FILE ■
We will consider the various parts of this program in detail in the sections below.
b) The Super Loop architecture
The main function (shown in Listing 3.4) is a classic example of a Super Loop
architecture introduced in Chapter 1.
Listing 3.4 Part of the 'Hello, Embedded World' example code
void main(void)
{
LED_FLASH_Init() ;
whi le(1 )
{
// Change the LED state (OFF to ON, or vice versa)
LED_FLASH_Change_State() ;
// Delay for *approx* 1000 ms
DELAY_L00P_Wait(1000) ;
}
}
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 46
46 Embedded C
In this function, we perform some initialization operations (considered below),
then we enter the endless loop. In this loop, we alternately call a function to
'flash/ the LED, and a one-second-delay function.
The functions LED_FLASH_Ini t ( ), LED_FLASH_Change_State ( ), and
DELAY_ L00P_Wai t ( ) are discussed below.
c) Controlling the port pins
We next consider the functions used to control the LED.
Listing 3.5 highlights several key features from this part of the program.
Listing 3.5 Part of the 'Hello, Embedded World' example code
#include <reg52.h>
// Port pins
sbit LED_pin = P1 A 5;
// Private variable definitions
bit LED_state_G;
LED_FLASH_Init()
Prepare for LED_Change_State() function - see below.
*
void LED_FLASH_Init(void)
{
LED_state_G = 0;
}
/*
LED_FLASH_Change_State ( )
Changes the state of an LED (or pulses a buzzer, etc) on a
specified port pin.
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 47
Hello, Embedded World 47
Must call at twice the required flash rate: thus, for 1 Hz
flash (on for 0.5 seconds, off for 0.5 seconds) must call
every 0.5 seconds.
voi d LED_FLASH_Change_State ( voi d )
{
// Change the LED from OFF to ON (or vice versa)
if (LED_state_G == 1)
{
LED_state_G = 0;
LED_pin = 0;
}
else
{
LED_state_G = 1 ;
LED_pin = 1 ;
}
}
To understand Listing 3.5, recall that - as we discussed in Chapter 2 - Standard 8051s
have four 8-bit ports. The ports are referred to as Port 0, Port 1, Port 2 and Port 3. All
ports are bidirectional: that is, they may be used for both input and output.
Control of the ports is carried out using what are known as 'special function
registers' (SFRs). The SFRs are 8-bit latches: in practical terms, this means that the
values written to the port are held there until a new value is written or the device
is reset. Each of the four ports is represented by an SFR: these are named, appropri-
ately, PO, PI, P2 and P3. Physically, each SFR is an area of memory in the upper
areas of internal RAM: P0 is at address 0x80, PI at address 0x90, P2 at address
OxAO and P3 at address OxBO.
Table 3.1 shows the representation of the ports in the SFR area of memory.
:322 Chapter 3 p35-56 21/2/02 9:55 am Page
48
Embedded C
TABLE 3.1 On-chip RAM memory in the 8051 : The SFR area. See text for details
Byte
address
Bit address
OxBO
Pin 3.7
0xB7
Pin 3.6
0xB6
Pin 3.5
0xB5
Pin 3.4
0xB4
Pin 3.3
0xB3
Pin 3.2
0xB2
Pin 3.1
OxBI
Pin 3.0
OxBO
Port 3
...
OxAO
Pin 2.7
0xA7
Pin 2.6
0xA6
Pin 2.5
0xA5
Pin 2.4
0xA4
Pin 2.3
0xA3
Pin 2.2
0xA2
Pin 2.1
OxAl
Pin 2.0
OxAO
Port 2
...
0x90
Pin 1.7
0x97
Pin 1.6
0x96
Pin 1.5
0x95
Pin 1.4
0x94
Pin 1.3
0x93
Pin 1.2
0x92
Pin 1.1
0x91
Pin 1 .0
0x90
Port 1
...
0x80
Pin 0.7
0x87
Pin 0.6
0x86
Pin 0.5
0x85
Pin 0.4
0x84
Pin 0.3
0x83
Pin 0.2
0x82
Pin 0.1
0x81
Pin 0.0
0x80
PortO
If we want to write to the ports, we need to write to these addresses. Assuming
that we are using a C compiler, the process of writing to an address is usually
carried out by means of a SFR variable declaration, 10 hidden in a header file.
In Listing 3.5, the first #i ncl ude directive ensures that a copy of the Keil
'reg52' file is inserted into the source file before compilation. Note that this file
matches the '8052' register set, and is compatible with the selection of 8052 as the
target hardware (see Section 3.3).
The key part of this file as far as this program is concerned is as follows:
/* BYTE Registers */
sfr P0 = 0x80
sfr P1 = 0x90
sfr P2 = OxAO
sfr P3 = OxBO
Having declared the SFR variables, we can write to the ports in a straightforward
manner. For example, we can send some data to Port 1 as follows:
10. Reminder. A variable definition consumes memory. A variables declaration does not consume
memory it simply associates a name with an area of memory defined elsewhere in the pro-
gram. Variable definitions are also variable declarations, but the reverse is not true.
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 49
Hello, Embedded World 49
unsigned char Port_data;
// REMEMBER:
// Ox identifies the number as hexadecimal (base 16)
// OxOF (hexadecimal) = 00001111 (binary)
Port_data = OxOF;
P1 = Port_data; // Write 00001111 to Port 1
Please note that, for experienced C programmers, this code may appear to be
incorrect: PI represents an address, and the correct C syntax would appear to be:
*P1 = Port_data; // Surely this is correct?
However, because the compiler knows how to deal with SFRs, the use of the indi-
rection operator is not necessary (and code using it will not work as intended).
The example above assumed that we wished to control the whole of Port 1. In
the 'Hello, Embedded World' program, we simply wish to control an individual
port pin. Control of the Pin 1.5 is achieved through the following variable declara-
tion in Listing 3.3:
// Port pins
sbit LED_pin = P1 A 5;
The variable type here is sbit: this is not ISO / ANSI C, but is a Keil extension. Use
of this keyword allows you to declare bit variables which are part of already-
defined (byte-sized) variables. Note also the use of the ' A ' symbol to access a
particular pin: this is again a Keil-specific operation.
Following this declaration, we can control the status of this individual pin by
writing, for example:
LED_pin = 1 ;
This is exactly what we do later in the program:
// Change the LED from OFF to ON (or vice versa)
if (LED_state_G == 1)
{
LED_state_G = 0;
LED_pin = 0;
}
else
{
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 50
50 Embedded C
LED_state_G = 1 ;
LED_pin = 1 ;
}
d) Creating and using a bit variable
Listing 3.3 also includes the definition of the bit variable LED_state_G:
// Private variable definitions
bit LED_state_G;
As we discussed in Chapter 2, the 8051 device has a BDATA area (16-bytes in size)
in which user-defined bit-sized variables may be stored. (You may like to refer back
to Figure 2.6 for details for this).
In this case, we are using the Keil keyword bit to define LED_state_G, a bit-
sized variable that will be stored in the BDATA area. LED_state_G can (of course)
take on the values of only 1 or 0. It is used in the program to store the current
state of the LED (ON or OFF):
if (LED_state_G == 1)
{
LED_state_G = 0;
LED_pin = 0;
}
e) The delay function
The next key component of the 'Hello, Embedded World' program is a delay function.
The creation of accurate delays is a key requirement in many embedded appli-
cations. One way of creating such delays is by using a loop delay, implemented
as follows:
void Loop_Delay (void)
{
unsigned int x;
for (x=0; x <= 65535; x++) ;
}
If we find that these delays are not long enough, we can easily extend them by
adding additional layers, as shown in Longer_Loop_Del ay ( ) :
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 51
Hello, Embedded World 51
void Longer_Loop_Delay (void)
{
unsigned int x;
unsigned int y;
for (x=0; x <= 65535; x++)
{
for (y=0; y <= 65535; y++) ;
}
}
Listing 3.6 shows how the 1 -second delay required in this application is achieved
using this approach.
Listing 3.6 Part of the 'Hello, Embedded World' example code
DELAY_L00P_Wait()
Delay duration varies with parameter.
Parameter is, *R0UGHLY*, the delay, in milliseconds,
on 12MHz 8051 (12 osc cycles).
You need to adjust the timing for your application!
* * /
void DELAY_L00P_Wait( const unsigned int DELAY)
{
unsigned int x, y;
for (x = 0; x <= DELAY; x++)
{
for (y = 0; y <= 120; y++) ;
}
}
Note: there is nothing miraculous about the values used in this listing: they were
determined by trial and error. If you use this (or similar) code in a real application,
the delays will vary with compiler optimization settings and other factors. They
must always be checked carefully as part of the final pre-release tests. 11
11. Please note that some more robust techniques for generating delays are considered in Chapter 6.
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 52
52 Embedded C
To check the delay durations, run the Keil simulator as shown earlier. While the
simulation is running, view the Performance Analyzer (Figure 3.9).
1 - -I
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FIGURE 3.9 Running the Performance Analyzer in order to measure the duration of the loop
delay function
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 53
Hello, Embedded World 53
At this stage, 100% of the processor time is devoted to 'Unspecified' activities. We
now need to indicate that we wish to measure the duration of the loop delay func-
tions (Figure 3.10).
UN UICII ItMkti-u
iiliijc ft fc"> c— j/75 57
FIGURE 3.10 (Part 1 of 2) Measuring the duration of the loop delay. Note that we can profile
multiple functions simultaneously using this approach
;322 Chapter 3 p35-56 21/2/02 9:55 am Page 5
54 Embedded C
»"■*« -uivn-mZ
■^ ten [m>f Pn#w i Je-w MS i^w- am
*-*i
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.
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FIGURE 3.10 (Part 2 of 2) Measuring the loop delay duration. Note that we can
simultaneously profile multiple functions using this approach
Figure 3.10 (Part 2) reveals that the duration of the loop delay is very close to the
required value.
We can go back and alter the delay code:
void DELAY_L00P_Wait( const unsigned int DELAY)
{
unsigned int x, y;
for (x = 0; x <= DELAY; x++)
{
for (y = 0; y <= 1200; y++) ;
}
}
If we then repeat the simulation, 12 we can see that the loop delay duration has
increased precisely as expected (Figure 3.11).
12. To make this change, you need to: [1] stop the simulation; [2] end the debug session; [3] edit
the file and re-build the target; [4] re-start the debug session.
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 55
Hello, Embedded World 55
m m * m - Ji'Ha'u
fa O ■ - * n *p?« = t>
■Ji - fS" 3LB 1
THE
EaiTig ■■ Inl Hi
Luiri «EtfVu- HAI -.FriiliiiaL
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FIGURE 3.11 The result of altering the loop delay code
3.7 Aside: Building the hardware
This book is largely 'hardware free'. However, for the benefit of those readers who
- having got this far - would like to run their program on 'rear hardware, we pro-
vide some suggestions here.
To make your life easier, you should base your design around an 8051 device with
flash memory: for example, the Atmel AT89C52 is widely available, at low cost.
The required hardware schematic is given in Figure 1.2. This may be assembled
on a breadboard of the type illustrated in Figure 1.7.
You will also require a suitable programmer with which to program your chosen
microcontroller. Various companies produce suitable devices. Alternatively, if you
have some experience in electronic construction (or simply enjoy a challenge),
you will find an Application Note from Atmel on the CD ROM which describes
how to construct a suitable programmer. Software to drive the programmer is also
included on the CD.
:322 Chapter 3 p35-56 21/2/02 9:55 am Page 56
56
Embedded C
3.8 Conclusions
The purpose of this chapter was to show how simple C programs can be developed
and tested using the software tools included with this book.
In the next chapter, we will go on to look at techniques for reading port pins
and working with mechanical switches.
:322 Chapter 4 p57-80 21/2/02 9:55 am Page 57
chapter
Reading switches
4.1 Introduction
In earlier chapters, we have considered some of the fundamental differences
between software development for desktop systems and embedded systems. We've
noted that embedded systems usually execute one program, which begins running
when the device is powered up. We've begun to look at a simple software architec-
ture - the Super Loop - that is used at the heart of many embedded systems.
Another key challenge for desktop programmers moving into the embedded
market is the implementation of the user interface. On the desktop, design of the
user interface means working with a high-resolution graphics screen, some form of
mouse (or equivalent 'pointing' device), and a large keyboard. Design freedom is
restricted by the fact that the user of your application wants to have a similar 'look
and feel' to other applications that he or she uses. To match these design con-
straints - and speed up the development process - developers will typically use
some form of standard code library when building applications, rather than
attempting to create all the code from scratch (Figure 4.1).
In the embedded world, it may appear - at first sight at least - that there are
fewer constraints. Instead, it can seem that there is a 'free for all' where every
developer will implement a different interface to their system. However, there is at
least one common denominator: embedded systems usually use switches as part of
their user interface (see Figure 4.2). This general rule applies from the most basic
remote-control system for opening a garage door, right up to the most sophisti-
cated aircraft autopilot system. Whatever the system you create, you need to be
able to create a reliable switch interface.
57
;322 Chapter 4 p57-80 21/2/02 9:55 am Page 5
58
Embedded C
Use standard libraries for:
• GUI primitives
(circles, etc)
• GUI components
(dialog boxes, etc)
• Keyboard
• Mouse interface
FIGURE 4.1 Developing the user interface for a modern desktop application will almost
invariably mean working with a high-resolution graphics screen, a keyboard and a mouse, using
code libraries written in, say, Java or C++
1
r°"i
J
Off
Start
s
A
1
213
4
5
1
<
>
v
J
Engage AP
Temporary Manual
Disengage AP
Up and Around
FIGURE 4.2 A collection of user-interface components taken from a range of different
embedded systems. Most such interfaces contain at least one switch
In this chapter, we consider how you can read inputs from mechanical switches in
your embedded application. Before considering some important characteristics of
the switches themselves, we will consider the process of reading the state of port
pins to which the switches will be connected.
4.2 Basic techniques for reading from port pins
As we saw in Chapter 3, control of the 8051 ports is carried out using 8-bit latches
(SFRs). We can send some data to Port 1 as follows: 13
sfr P1 = 0x90; // Usually in header file
P1 = OxOF; // Write 00001111 to Port 1
13. Remember: numbers beginning 'Ox...' are in hexadecimal. 'Hex' was reviewed in Chapter 2.
:322 Chapter 4 p57-80 21/2/02 9:55 am Page 59
Reading switches 59
In exactly the same way, we can read from Port 1 as follows:
unsigned char Port_data;
P1 = OxFF; // Set the port to 'read mode'
Port_data = P1 ; // Read from the port
Note: because of the underlying hardware, we can only read from a pin if the
corresponding latch contains a 'V . In practice, this means that - in order to read
from a pin - we need to ensure that the last thing written to the pin was a '1 '.
After the 8051 microcontroller is reset, the port latches all have the value OxFF
(11111111 in binary): that is, all the port-pin latches are set to values of '1'. It is
tempting to assume that writing data to the port is therefore unnecessary, and that
we can get away with the following version:
unsigned char Port_data;
// Assume nothing written to port since reset
// - DANGEROUS! ! !
Port_data = P1 ;
The problem with this code is that, in simple test programs it works: this can lull
the developer into a false sense of security. If, at a later date, someone modifies the
program to include a routine for writing to all or part of the same port, this code
will not generally work as required:
unsigned char Port_data;
P1 = 0x00;
In most cases, initialization functions are used to set the port pins to a known
state at the start of the program. Where this is not possible, it is safer to always
write *V to any port pin before reading from it.
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60
Embedded C
4.3 Example: Reading and writing bytes
Listing 4.1 is a simple example which illustrates how we can read from one port
on an 8051 microcontroller and 'echo' the result on another port.
As with the examples in Chapter 3, the Keil hardware simulator (included on
the CD) will allow you to simulate suitable hardware for use with this program.
Figure 4.3 shows the output from one such simulation.
Listing 4.1
PW1
. ? bfek u
Ftm i.fi" pppprppp
The input port
FIGURE 4.3 The output from the program in Listing 4.1 produced using the Keil hardware
simulator included on the CD. Note the difference between the Port 1 latch value (OxFF) and the
pin value (0xB7)
A simple 'Super Loop' application which copies the values from
PI to P2.
/*
Bytes. C (v1 .00)
Reads from P1 and copies the value to P2.
#include <Reg52.H>
/
void main (void)
{
unsigned char Port1_value;
// Must set up P1 for reading
P1 = OxFF;
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Reading switches 61
whi le(1 )
{
// Read the value of P1
Port1_value = P1 ;
// Copy the value to P2
P2 = Port1_value;
}
}
/
*
*
*
END OF FILE
*/
4.4 Example: Reading and writing bits (simple version)
Listing 4.1 demonstrated how to read from or write to an entire port. However, sup-
pose we have a switch connected to Pin 1.0 and an LED connected to Pin 1.1. We
might also have input and output devices connected to the other pins on Port 1.
These pins may be used by totally different parts of the same system, and the code
to access them may be produced by other team members, or other companies. It is
therefore essential that we are able to read-from or write-to individual port pins
without altering the values of other pins on the same port.
We provided a simple example in which we controlled an individual pin in Chapter
3. Listing 4.2 goes one step further and illustrates how we can read from Pin 1.0, and
write to Pin 1.1, without disrupting any other pins on this (or any other) port.
Listing 4.2 Reading and writing bits (simple version)
Bitsl .C (v1 .00)
Reading and writing individual port pins
NOTE: Both pins on the same port
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62 Embedded C
#include <Reg52.H>
sbit Switch_pin = P1 A 0;
sbit LED_pin = P1 A 1 ;
/
void main (void)
{
bit x;
// Set switch pin for reading
Switch_pin = 1 ;
wh i 1 e ( 1 )
{
x = Switch_pin; // Read Pin 1.0
LED_pin = x; // Write to Pin 1.1
}
}
END OF FILE
Experienced 'C programmers please note these lines:
sbit Switch_pin = P1 A 0;
sbit LED_pin = P1 A 1 ;
Here we gain access to two port pins through the use of an sbit variable declara-
tion. The symbol ' A ' is used, but the XOR bitwise operator is NOT involved.
For programmers with less experience, we say more about the bitwise operators
in the next section.
4.5 Example: Reading and writing bits (generic version)
We can make Listing 4.2 more flexible by making use of the bitwise AND, OR and
'complement' operators.
Even if you have programmed in C before on a desktop computer, you may not
have come across the bitwise operators (&, |, A , «, », ~). These are not widely used
by desktop programmers, but they allow a number of data manipulations that are
:322 Chapter 4 p57-80 21/2/02 9:55 am Page 63
Reading switches 63
very useful in embedded applications, including the introductory examples presented
in this chapter. We will therefore briefly review the use of these operators here.
The six bitwise operators are listed in Table 4.1.
TABLE 4.1 The C bitwise operators
Operator
Description
&
Bitwise AND
| Bitwise OR (inclusive OR)
A
Bitwise XOR (exclusive OR)
«
Left shift
»
Right shift
One's complement
To remind you, a summary of the AND, OR and XOR operations is given in Table 4.2.
TABLE 4.2 The AND, OR and XOR operations
A
B
A AND B
A ORB
A XOR B
1
1
1
1
1
1
1
1
1
1
Some examples of the use of all six of the bitwise operators are given in Listing
4.3. Note that this program is written in 'Desktop C: it cannot be compiled using
the Keil tools.
Listing 4.3 Demonstrating the C bitwise operators.
/ * *
Main.C
Illustrating the use of bitwise operators
* * /
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64
Embedded C
#include <stdio.h>
void Display_Byte(const unsigned char);
int main()
{
unsigned char x
unsigned int y
= OxFE;
= OxOAOB;
printf ("%-35s" ,Y) ;
Display_Byte(x) ;
printf ("S-35S" , "1s complement [~x]");
Display_Byte(~x) ;
printf ("%-35s",' 'Bitwise AND [x & OxOf]");
Display_Byte(x & OxOf ) ;
printf ("%-35s", "Bitwise OR [x | OxOf]");
Display_Byte(x | OxOf);
printf ("%-35s", "Bitwise XOR [x A OxOf]");
Display_Byte(x A OxOf);
printf ("%-35s\ "Left shift, 1 place [x «= 1] ");
Display_Byte(x «= 1);
x = Oxfe; /* Return x to original value */
printf ("*-35sV 'Right shift, 4 places [x »= 4]");
Display_Byte(x »= 4);
printf ("\n\n") ;
printf ("%-35s" , "Display MS byte of unsigned int y");
Display_Byte( (unsigned char) (y » 8));
printf ("%-35s" , "Display LS byte of unsigned int y");
Display_Byte( (unsigned char) (y & OxFF) ) ;
return 0;
}
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Reading switches 65
void Display_Byte (const unsigned char CH)
{
unsigned char i, c = CH;
unsigned char Mask = 1 « 7;
for (i = 1 ; i <= 8; i++)
{
putchar(c & Mask ? *1 ' : '0') ;
c «= 1 ;
}
putchar( ' \n ' ) ;
}
END OF FILE
The output from the program in Listing 4.3 is as follows:
X
11111110
Is complement [~x]
00000001
Bitwise AND [x & OxOf]
00001110
Bitwise OR [x | OxOf]
11111111
Bitwise XOR [x A OxOf]
11110001
Left shift, 1 place [x «= 1]
1 1 1 1 1 1 00
Right shift, 4 places [x »= 4]
00001 1 1 1
Display MS byte of unsigned int y
00001010
Display LS byte of unsigned int y
00001011
The use of some of these operators in an embedded application is illustrated in
Listing 4.4 which echoes the input on Pin X to Pin Y on Port 1.
Note that:
• The function Read_Bit_P1 () allows the programmer to specify the particular
pin (on Port 1) that is to be read.
• The function Wri te_Bi t_P1 ( ) allows the programmer to specify both the pin
to be written to, and the value to be written (1 or 0).
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66 Embedded C
Listing 4.4 Reading and writing bits (generic version). See text for details
Bits2.C (v1 .00)
Reading and writing individual port pins
NOTE: Both pins on the same port
— Generic version
#include <reg52.H>
// Function prototypes
void Write_Bit_P1 (const unsigned char, const bit);
bit Read_Bit_P1 (const unsigned char);
/*
void main (void)
{
bit x;
wh i 1 e ( 1 )
{
x = Read_Bit_P1 (0) ; // Read Port 1, Pin
Write_Bit_P1 (1 ,x) ; // Write to Port 1, Pin 1
}
}
void Write_Bit_P1 (const unsigned char PIN, const bit VALUE)
{
unsigned char p = 0x01; // 00000001
// Left shift appropriate number of places
p «= PIN;
// If we want 1 output at this pin
if (VALUE == 1)
:322 Chapter 4 p57-80 21/2/02 9:55 am Page 67
Reading switches 67
{
P1 |= p; // Bitwise OR
return ;
}
// If we want output at this pin
p = ~p; // Complement
P1 &= p; // Bitwise AND
}
/
bit Read_Bit_P1 (const unsigned char PIN)
{
unsigned char p = 0x01; // 00000001
// Left shift appropriate number of places
p «= PIN;
// Write a 1 to the pin (to set up for reading)
Write_Bit_P1 (PIN, 1);
// Read the pin (bitwise AND) and return
return (P1 & p) ;
}
/*
END OF FILE
4.6 The need for pull-up resistors
In our discussions in this chapter, we have assumed that we were reading from port
pins in the simulator: we have not yet considered how a switch will actually be con-
nected to the 8051 port pin in a real application. Figure 4.4 illustrates one possibility.
This hardware operates as follows:
• When the switch is open, it has no impact on the port pin. An internal resistor
on the port 'pulls up' the pin to the supply voltage of the microcontroller (typi-
cally 5V). If we read the pin, we will see the value '1'. (We say more about
pull-up resistors below).
;322 Chapter 4 p57-80 21/2/02 9:55 am Page 6
68 Embedded C
6
O
To pin on:
^ Port 1,
Port 2,
or
Port 3.
FIGURE 4.4 Connecting a switch to an 8051 port pin. See text for details
• When the switch is closed (pressed), the pin voltage will be OV. If we read the
the pin, we will see the value '0'.
The internal 'pull up' resistors referred to above can be thought of as small springs,
connecting the port pin to a '1' value. To change the port setting, we need to
'push' the pin down to 0, against the force of the spring (Figure 4.5).
Vcc
Vcc
Switch released
Reads T
Switch pressed
Reads '0'
FIGURE 4.5 A schematic representation of a switch connected to a port (with internal pull-up
resistors). See text for details
Where there is no pull-up resistor, changing the state of the input pin is not possi-
ble: not matter how hard we push, the pin will always read '0' (Figure 4.6).
Vcc
Vcc
Switch released
Reads '0'
Switch pressed
Reads '0'
FIGURE 4.6 A schematic representation of a switch connected to a port ( without internal pull-
up resistors). See text for details
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Reading switches 69
Returning to Figure 4.4, please note that the simple switch arrangement shown
only applies to Port 1, Port 2 and Port 3, since these ports all have internal pull-up
resistors. This arrangement does not apply to Port 0, which has no internal pull-
ups. If you need to connect a switch (or similar device) to Port 0, you can do so,
but you need to add an external pull-up resistor: a resistance of 10 KQ, is appropri-
ate here (see Figure 4.7).
Vcc
10 KQ.
To pin on:
PortO
6
O
FIGURE 4.7 An example of a push-button ('normally open') switch input. Where there is no
internal pull-up, this arrangement must be used
4.7 Dealing with switch bounce
In an ideal world, this change in voltage obtained by connecting a switch to the
port pin of an 8051 microcontroller would take the form illustrated in Figure 4.8
(top). In practice, all mechanical switch contacts bounce (that is, turn on and off,
repeatedly, for a short period of time) after the switch is closed or opened. As a
result, the actual input waveform looks more like that shown in Figure 4.8
(bottom). Usually, switches bounce for less than 20 ms: however large mechanical
switches exhibit bounce behaviour for 50 ms or more.
When you turn on the lights in your home or office with a mechanical switch,
the switches will bounce. As far as humans are concerned, this bounce is imper-
ceptible. However, as far as the microcontroller is concerned, each 'bounce' is
equivalent to one press and release of an 'ideal' switch. Without appropriate soft-
ware design, this can give rise to a number of problems, not least:
• Rather than reading 'A' from a keypad, we may read 'AAAAA'.
• Counting the number of times that a switch is pressed becomes extremely difficult.
• If a switch is depressed once, and then released some time later, the 'bounce' may
make it appear as if the switch has been pressed again (at the time of release).
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70
Embedded C
+5v
Voltage
+5v
tl
t2
Time
FIGURE 4.8 The voltage signal resulting from the switch shown in Figure 4.7. [Top] Idealized
waveform resulting from a switch depressed at time t1 and released at time t2. [Bottom] Actual
waveform showing leading edge bounce following switch depression and trailing edge bounce
following switch release
Fortunately, creating the software needed to check for a valid switch input is
straightforward:
1 We read the relevant port pin.
2 If we think we have detected a switch depression, we wait for 20 ms and then
read the pin again.
3 If the second reading confirms the first reading, we assume the switch really has
been depressed.
Note that the figure of '20 ms' will, of course, depend on the switch used: the data
sheet of the switch will provide this information. If you have no data sheet, you can
either experiment with different figures, or measure directly using an oscilloscope.
4.8 Example: Reading switch inputs (basic code)
In this example, we present a simple code library for reading the input from a
mechanical switch. The code implements - directly - the algorithm described in
Section 4.7: that is, it tests the switch state and - if the switch is pressed - executes
a 'debounce delay' before testing the switch again.
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Reading switches 71
This switch-reading code is adequate if we want to perform operations such as:
• Drive a motor while a switch is pressed.
• Switch on a light while a switch is pressed.
• Activate a pump while a switch is pressed.
These operations could be implemented using an electrical switch, without using a
microcontroller; however, use of a microcontroller may well be appropriate if we
require more complex behaviour. For example:
• Drive a motor while a switch is pressed
Condition: If the safety guard is not in place, don't turn the motor. Instead
sound a buzzer for 2 seconds.
• Switch on a light while a switch is pressed
Condition: To save power, ignore requests to turn on the light during daylight
hours.
• Activate a pump while a switch is pressed
Condition: If the main water reservoir is below 300 litres, do not start the main
pump: instead, start the reserve pump and draw the water from the emergency tank.
The key to this library is the function SWITCH_Get_Input ( ), which is shown in
context in Listing 4.5.
Listing 4.5 Reading switch inputs (basic code)
Switch_read.C (v1 .00)
A simple 'switch input' program for the 8051.
- Reads (and debounces) switch input on Pin1 A
- If switch is pressed, changes Port 3 output
#include <Reg52.h>
// Connect switch to this pin
sbit Switch_pin = P1 A 0;
// Display switch status on this port
#define 0utput_port P3
:322 Chapter 4 p57-80 21/2/02 9:55 am Page 72
72 Embedded C
// Return values from Switch_Get_Input ()
#define SWITCH_NOT_PRESSED (bit)
#define SWITCH_PRESSED (bit) 1
// Function prototypes
void SWITCH_Init(void) ;
bit SWITCH_Get_Input (const unsigned char DEBOUNCE_PERIOD) ;
void DISPLAY_SWITCH_STATUS_Init(void) ;
void DISPLAY_SWITCH_STATUS_Update (const bit);
void DELAY_L00P_Wait (const unsigned int DELAY_MS) ;
/*
void main (void)
{
bit Sw_state;
// Init functions
SWITCH_Init() ;
DISPLAY_SWITCH_STATUS_Init() ;
wh i 1 e ( 1 )
{
Sw_state = SWITCH_Get_Input(30) ;
DISPLAY_SWITCH_STATUS_Update(Sw_state) ;
}
}
SWITCH_Init()
Initialisation function for the switch library.
*
void SWITCH_Init(void)
{
Switch_pin = 1; // Use this pin for input
}
/*
SWITCH_Get_Input()
Reads and debounces a mechanical switch as follows
:322 Chapter 4 p57-80 21/2/02 9:55 am Page 73
Reading switches 73
1. If switch is not pressed, return SWITCH_NOT_PRESSED.
2. If switch is pressed, wait for DEBOUNCE_PERIOD (in ms) ,
then :
a. If switch is still pressed, return SWITCH_PRESSED.
b. If switch is not pressed, return SWITCH_NOT_PRESSED
See Switch_Wait .H for details of return values.
* * /
bit SWITCH_Get_Input (const unsigned char DEBOUNCE_PERIOD)
{
bit Return_value = SWITCH_NOT_PRESSED;
if (Switch_pin == 0)
{
// Switch is pressed
// Debounce - just wait...
DELAY_LOOP_Wait(DEBOUNCE_PERIOD) ;
// Check switch again
if (Switch_pin == 0)
{
Return_value = SWITCH_PRESSED;
}
}
// Now return switch value
return Return_val ue;
}
DISPLAY_SWITCH_STATUS_Ini t ( )
Initialization function for the DISPLAY_SWITCH_STATUS library.
* *
void DISPLAY_SWITCH_STATUS_Init(void)
{
Output_port = OxFO;
}
;322 Chapter 4 p57-80 21/2/02 9:55 am Page 7
74 Embedded C
DISPLAY_SWITCH_STATUS_Update ( )
Simple function to display data (SWITCH_STATUS)
on LEDs connected to port (Output_Port)
void DISPLAY_SWITCH_STATUS_Update (const bit SWITCH_STATUS)
{
if (SWITCH_STATUS == SWITCH_PRESSED)
{
Output_port = OxOF;
}
else
{
Output_port = OxFO;
}
}
/*
DELAY_L00P_Wait()
Delay duration varies with parameter.
Parameter is, *R0UGHLY*, the delay, in milliseconds,
on 12MHz 8051 (12 osc cycles).
You need to adjust the timing for your application!
void DELAY_L00P_Wait( const unsigned int DELAY_MS)
{
unsigned int x, y;
for (x = 0; x <= DELAY_MS; x++)
{
for (y = 0; y <= 120; y++) ;
}
}
END OF FILE
:322 Chapter 4 p57-80 21/2/02 9:55 am Page 75
Reading switches 75
Figure 4.9 shows this program running in the simulator.
The output port
dial Pert 3
Pw13
Ft. |*'H-
Varti
The input port
PWl
RM
:F
I
rrrrFFFP
rrrrFFFF
PI: |i ^t
Pttt |wr
7 Br*L. 1
FFFFFFFP
Fppppppr
:
FIGURE 4.9 Running the code from Listing 4.5 in the simulator. Changing the status of the
switch pin (Pin 1 .0) alters the output on Port 3
4.9 Example: Counting goats
Variations of the simple switch-reading code presented in Listing 4.5 are cur-
rently in use in many embedded systems. However, this code is not suitable for
all purposes.
With this simple code, problems can arise whenever a switch is pressed for a
period longer than the debounce interval. This is a concern, because in many
cases, users will press switches for at least 500 ms (or until they receive feedback
that the system has detected the switch press). As a result, a user typing 'Hello' on
a keypad may see 'HHHHHHHHHeeeeeeeeellllllllllllllllooooooooooo' appear
on the screen.
One consequence is that this code is not suitable for applications where we
need to count the number of times that a switch is pressed and then released.
For example, suppose we wish to use this code to count the number of goats
passing into a milking parlour (Figure 4.10). We assume that the optical sensor
arrangement gives a 'Logic 0' output while a goat is passing and a 'Logic V
output at other times (Figure 4.11).
Mechanical sensor
at goat's body
1^
J
m
^
"t_^
n
|£ height /*fc.
w
1
*
W
1
i
.
w
&
1
i
-
i
1
FIGURE 4.10 A system for counting the number of goats passing into a milking parlour
:322 Chapter 4 p57-80 21/2/02 9:55 am Page 76
76 Embedded C
Sensor -
Goat detected
FIGURE 4.1 1 The output of the 'goat sensor'
If we try to use the code in Listing 4.5 to count these goats, we will get a very mis-
leading result. For example, suppose that the debounce period is 20 ms. If a goat
takes - say - around five seconds to pass the sensor, then the SWITCH_Get_Input ( )
function will count a total of around 250 goats every time one goat passes (250 =
5000 ms / 20 ms): in other words, the goat sensor will not allow us to count
the number of goats but will instead provide an indication of the time taken
for the goats to pass the sensor .
A simple way to solve such problems is to wait until a switch is released
before returning from a switch-test function. This approach is illustrated in the
function SWITCH_Get_Input ( ) in Listing 4.6. The code in this listing is used
to do two things:
• Debounce the switch input.
• Count the number of times that a switch is pressed and then released.
This code could be applied - for example - to count the number of (human) visi-
tors entering a museum. It could also be used to measure the speed of rotating
machinery, or the flow of liquid through pipes, assuming that an appropriate
sensor was employed. 14
Listing 4.6 A simple program for counting the number of times that a switch is pressed
and released
Switch_count.C (v1.00)
A 'goat counting' program for the 8051...
#include <Reg52.h>
14. See the 'milk pasteurization' example in Chapter 7 (p. 174) for further details.
:322 Chapter 4 p57-80 21/2/02 9:55 am Page 77
Reading switches 77
II Connect switch to this pin
sbit Switch_pin = P1 A 0;
// Display count (binary) on this port
#define Count_port P3
// Return values from Switch_Get_Input ()
#define SWITCH_NOT_PRESSED (bit)
#define SWITCH_PRESSED (bit) 1
// Function prototypes
void SWITCH_Init(void) ;
bit SWITCH_Get_Input (const unsigned char);
void DISPLAY_COUNT_Init(void) ;
void DISPLAY_COUNT_Update (const unsigned char);
void DELAY_L00P_Wait( const unsigned int);
/*
void main(void)
{
unsigned char Switch_presses = 0;
// Init functions
SWITCH_Init() ;
DISPLAY_COUNT_Init() ;
whi le(1 )
{
if (SWITCH_Get_Input(30) == SWITCH_PRESSED)
{
Swi tch_presses++ ;
}
DISPLAY_COUNT_Update(Switch_presses) ;
}
}
SWITCH_Init()
Initialisation function for the switch library
void SWITCH_Init(void)
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78 Embedded C
{
Switch_pin = 1; // Use this pin for input
}
SWITCH_Get_Input()
Reads and debounces a mechanical switch as follows:
1. If switch is not pressed, return SWITCH_NOT_PRESSED.
2. If switch is pressed, wait for DEBOUNCE_PERIOD (in ms) .
a. If switch is not pressed, return SWITCH_NOT_PRESSED.
b. If switch is pressed, wait (indefinitely) for
switch to be released, then return SWITCH_PRESSED
See Switch_Wait .H for details of return values.
bit SWITCH_Get_Input (const unsigned char DEBOUNCE_PERIOD)
{
bit Return_value = SWITCH_NOT_PRESSED;
if (Switch_pin == 0)
{
// Switch is pressed
// Debounce - just wait...
DELAY_L00P_Wait(DEB0UNCE_PERI0D) ;
// Check switch again
if (Switch_pin == 0)
{
// Wait until the switch is released.
while (Switch_pin == 0);
Return_value = SWITCH_PRESSED;
}
}
// Now (finally) return switch value
return Return_val ue;
}
/ * *
DISPLAY_COUNT_Init()
:322 Chapter 4 p57-80 21/2/02 9:55 am Page 79
Reading switches 79
Initialisation function for the DISPLAY COUNT library.
void DISPLAY_COUNT_Init(void)
{
Count_port = 0x00;
}
/* -
DISPLAY_COUNT_Update ( )
Simple function to display tByte data (COUNT)
on LEDs connected to port (Count_Port)
void DISPLAY_COUNT_Update (const unsigned char COUNT)
{
Count_port = COUNT;
}
/*
DELAY_L00P_Wait()
Delay duration varies with parameter.
Parameter is, *R0UGHLY*, the delay, in milliseconds,
on 12MHz 8051 (12 osc cycles).
You need to adjust the timing for your application!
void DELAY_L00P_Wait( const unsigned int DELAY_MS)
{
unsigned int x, y;
for (x = 0; x <= DELAY_MS; x++)
{
for (y = 0; y <= 120; y++) ;
}
}
END OF FILE
:322 Chapter 4 p57-80 21/2/02 9:55 am Page 80
80 Embedded C
Figure 4.12 shows the program in Listing 4.6 running in the simulator.
I !*■*?.'--.- r*Mi.'- - -r+ 1 1 m 1 1 — ■ ■ ■ ii * *:■ n mi- Wi o« ■ &■.-■? ■ ^ii^-wii -sr-x.Mn--
i ui *tf»M iM fciti i iwHiaU inMi.i lrai:.ri mMfcj!
IMa HTia: IU kiifUr bur I.mi-1
C*Wi
— d^-*— M . t ,—« «-. ra=^=r
LWi
FIGURE 4.12 Counting the number of goats using the hardware simulator. The switch input is
on Pin 1 .0: the count is shown (in binary) on Port 3
4.10 Conclusions
The switch interface code presented and discussed in this chapter has allowed us
to do two things:
• To perform an activity while a switch is depressed.
• To respond to the fact that a user has pressed - and then released - a switch.
In both cases, we have illustrated how the switch may be 'debounced' in software.
In Chapter 5, we turn our attention to techniques that can help you re-use the
code you develop in subsequent projects.
:322 Chapter 5 p81-112 21/2/02 9:57 am Page
chapter
Adding structure to your code
5.1 Introduction
In addition to the key technical issues we have examined in previous chapters
(such as the use of a Super Loop, or the design and implementation of an appro-
priate switch interface), there are other factors which need to be considered by
desktop developers migrating to the desktop area. For example, we made the fol-
lowing observations in Chapter 1:
• No software company remains in business for very long if it generates new code,
from scratch, for every project. The language used must support the creation of
flexible libraries, making it easy to re-use (well-tested) code components in a range
of projects. It must also be possible to adapt complete code systems to work with a
new or updated processor with minimal difficulty.
• Staff members change and existing personnel have limited memory spans. At
the same time, systems evolve and processors are updated. As concern over the
Tear 2000' problem in recent years has illustrated, many embedded systems
have a long lifespan. During this time, their code will often have to be main-
tained. Good code must therefore be easy to understand now, and in five years'
time (and not just by those who first wrote it).
To support these activities, we will do three things in this chapter:
1 We will describe how to use an object-oriented style of programming with C
programs, allowing the creation of libraries of code that can be easily adapted
for use in different embedded projects.
81
:322 Chapter 5 p81-112 21/2/02 9:57 am Page 82
82 Embedded C
2 We will describe how to create and use a 'Project Header' file. This file encapsu-
lates key aspects of the hardware environment, such as the type of processor to be
used, the oscillator frequency and the number of oscillator cycles required to exe-
cute each instruction. This helps to document the system, and makes it easier to
port the code to a different processor.
3 We will describe how to create and use a Tort Header' file. This brings together
all details of the port access from the whole system. Like the Project Header,
this helps during porting and also serves as a means of documenting important
system features.
We will use all three of these techniques in the code examples presented in subse-
quent chapters.
We begin by discussing how to use object-oriented styles of programming with
the C language.
5.2 Object-oriented programming with C
One way in which the different programming languages may be classified is as a
series of generations (see Table 5.1).
TABLE 5.1 The classification of programming languages into different generations. Please note
that some people consider O-O languages to be 5GLs: however, this distinction will not have
an impact on our discussions here
Language generation
Example languages
-
Machine Code
First-Generation Language (1GL)
Assembly Language.
Second-Generation Languages (2GLs)
COBOL, FORTRAN
Third-Generation Languages (3GLs)
C, Pascal, Ada 83
Fourth-Generation Languages (4GLs)
C++, Java, Ada 95
It is often argued that object-oriented (O-O) design - and O-O programming lan-
guages - have advantages when compared with those from earlier generations. For
example, as Graham notes: 15
[The phrase] 'object-oriented' has become almost synonymous with modernity, good-
ness and worth in information technology circles.
15. Graham, I. (1994) Object-Oriented Methods, (2nd edn) Addison-Wesley, Harlow, England, p. 1.
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Adding structure to your code 83
A frequent argument is that the O-O approach is more effective than those previ-
ously used because it represents 'a more natural way' of thinking about problems.
As Jalote notes: 16
One main claimed advantage of using object orientation is that an 00 model closely rep-
resents the problem domain, which makes it easier to produce and understand designs.
You might reasonably ask why this book uses C, rather than a language from a
later generation (such as C++ or Java). The reason is that O-O languages are not
readily available for small embedded systems, primarily because of the overheads
inherent in the O-O approach.
It is easy to see the source of these overheads. Suppose, for example, we have a
C program with a variable Xyz that we wish to set to some value and then display.
We might do so using the following code:
int Xyz;
Xyz = 3;
printf ("fcd" , Xyz) ;
Now, consider the following O-O version, using C++:
class cClass
{
publ ic:
int Get_Xyz(void) const;
void Set_Xyz(const int);
private:
int _Xyz; // Encapsulated data
};
cClass abc;
abc.Set_Xyz(3) ;
cout « abc.Get_Xyz() ;
16. Jalote, P. (1997) An Integrated Approach to Software Engineering, (2nd edn) Springer-Verlag,
New York.
:322 Chapter 5 p81-112 21/2/02 9:57 am Page 84
84 Embedded C
The C++ version has both strengths and weaknesses:
In the C++ code, the data (_Xyz) are encapsulated in the class: access to these
data is controlled because it is possible only via the two member functions.
By contrast, the data (Xyz) in the C version are 'global' variables and can be
altered anywhere in the program. From a design perspective, the C++ code is
more elegant. It may also prove easier to maintain.
There is a CPU time overhead associated with the C++ code: this is, at least, the
cost of two (member) function calls. In real applications, such overheads can be
substantial: even Stroustrup, creator of the C++ programming language, has
acknowledged that a C++ implementation is likely to run 25% more slowly than
an equivalent application in FORTRAN. 17 This can have implications for embed-
ded projects where speed of processing is a primary concern. For example,
Sommerville cites the case of an aircraft system in which an O-O solution was
abandoned due to the impact of these overheads. 18
One solution to such performance problems is to 'in line' the member functions:
this can greatly reduce the CPU overhead, but a penalty will then be paid in terms
of memory usage. 19
Neither the CPU performance load nor the alternative memory load present a sig-
nificant problem on multi-megabyte desktop PCs, but - on the type of embedded
projects considered in this book (where teams may struggle with code to save one
or two bytes of memory) a 'pure' O-O approach is rarely practical.
Does this mean that O-O design principles need (or should) be avoided by C pro-
grammers? Fortunately it does not. For many years before O-O techniques entered
the mainstream, C programmers used a 'modular' style of programming which is
well supported by the language. Using this approach, it is possible to create 'file-
based-classes' in C without imposing a significant memory or CPU load:
// BEGIN: File XYZ.C
static int Xyz;
Xyz = 3;
printf("%d" , Xyz);
// END: File XYZ.C
17. Stroustrup, B. (1994) The Design of C++, University Video Communications, Stanford CA, USA.
Recorded 2 March, 1994.
18. Sommerville, I. (1996) Software Engineering, (5th edn) Addison-Wesley, London, p. 301.
19. Pont, M.J. (1996) Software Engineering with C++ and CASE Tools, (1st edn) Addison-Wesley,
London, pp. 191-93.
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Adding structure to your code 85
The change here is minor: we have simply used the (ISO/ ANSI) C keyword stati c
to ensure that only functions within the file XYZ . c are able to access the data Xyz.
As a consequence, the source file becomes our 'class' and the 'static' data in that
file become private data members of that class. The functions defined within the
file become the member functions in our class, and our whole program may be
split into a number of clearly-defined (file-based) classes (Figure 5.1).
All
program
code
in a
single
source
file
Header file
Serial. C
Header file
Switch. C
Header file
sEOS.C
FIGURE 5.1 Turning a monolithic program into object-oriented C. See text for details
Note that we can also create 'private' member functions (which are accessible only
by functions defined within a particular file) simply by including the prototypes
for the function in the .C file (rather than the .H file) for the particular class.
This process is illustrated in Listing 5.1 and Listing 5.2. These files are part of a
library of code designed to allow an 8051 microcontroller to use a serial interface.
Please note that we will not be concerned with the operation of this code here
(that will be considered in Chapter 9): at this time, we are simply concerned with
the library structure.
As you look at this code, please note the presence of:
• Public 'member' functions, such as PC_LINK_I0_Wri te_String_To_Buf fer().
Such functions have their prototypes in the H file.
• A private 'member' function, PC_LINK_IO_Send_Char ( ). Such static func-
tions have their prototypes in the C file.
• A public constant (PC_LINK_I0_N0_CHAR), with a value which must be
accessed by the rest of the program (see the H file).
• A limited number of public variables (e.g. In_read_i ndex_G), defined in the C
file (without the use of the stati c keyword).
• Numerous private constants and private variables (e.g. RECV_BUFFER_LENGTH),
which are 'invisible' outside the C file.
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86 Embedded C
Listing 5.1 An example of a file-based C class (H file). See text for details
PC_I0.H (v1 .00)
- see PC 10. C for details.
#ifndef _PC_I0_H
#define _PC_I0_H
// Public constants
// Value returned by PC_LINK_Get_Char_From_Buffer if
// no character is available in buffer
#define PC LINK 10 NO CHAR 127
//
Public function prototypes
void PC_LINK_IO_Write_String_To_Buffer (const char* const);
void PC_LINK_IO_Write_Char_To_Buffer (const char);
char PC_LINK_IO_Get_Char_From_Buffer(void) ;
// Must call this function frequently...
void PC_LINK_I0_Update(void) ;
#endif
END OF FILE
Listing 5.2 An example of a file-based C class (C file). See text for details
PC_I0.C (v1 .00)
Core files for simple PC link library for 8051 family
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Adding structure to your code 87
Uses the UART, and Pins 3.1 (Tx) and 3.0 (Rx)
[INCOMPLETE - STRUCTURE ONLY - see Chap 9 for complete library]
/
#include "Main.H"
#include "PC 10. H"
//
Public variable definitions
tByte In_read_index_G; // Data in buffer that has been read
tByte In_waiting_index_G; // Data in buffer not yet read
tByte Out_written_index_G; // Data in buffer that has been written
tByte Out_waiting_index_G; // Data in buffer not yet written
//
Private function prototypes
static void PC_LINK_IO_Send_Char (const char);
//
Private constants
// The receive buffer length
#define RECV_BUFFER_LENGTH 8
// The transmit buffer length
#define TRAN_BUFFER_LENGTH 50
#define XON 0x11
#define X0FF 0x13
//
Private variables
static tByte Recv_buffer [RECV_BUFFER_LENGTH] ;
static tByte Tran_buffer [TRAN_BUFFER_LENGTH] ;
/*
void PC_LINK_I0_Update( . . . )
{
}
/
voi d PC_LINK_I0_Wri te_Char_To_Buf f er (
{
■)
}
:322 Chapter 5
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88
Embedded C
/
void PC_LINK_IO_Write_String_To_Buffer(. . .)
/*-
cha
r PC_LINK_IO_Get_Char_From_Buffer( . . . )
/*-
voi
d PC_LINK_IO_Send_Char( . . . )
END OF FILE
/
Overall, this approach is very common in C programs. If used with care (and,
where necessary, enforced by company coding guidelines) it can provide many of
the benefits of an O-O language without the corresponding performance or
memory costs.
5.3 The Project Header (Main.H)
The 'Project Header' is simply a header file, included in all projects, that groups
the key information about the 8051 device you have used, along with other key
parameters - such as the oscillator frequency - in one file (Figure 5.2). As such, it is
a practical implementation of a standard software design guideline: 'Do not dupli-
cate information in numerous files; place the information in one place, and refer
to it where necessary.' 20
In the case of the great majority of the examples in this book, we use a Project
Header file. This is always called Mai n . H. An example of a typical project header
file is included in Listing 5.3.
20. In a database system (for example) this rule will be expressed more formally by the requirement
that your data should be in 'First Normal Form'. See Date, CJ. (1999) An Introduction to Database
Systems, (7th edn) Addison- Wesley, London.
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Adding structure to your code 89
Project Header (Main.H)
_n
11.0592MHz
#include <AT89S53.H>
#define 0SC_FREQ ( 110 5920 OUL)
typedef unsigned char tByte;
FIGURE 5.2 A schematic representation of the project header file
Listing 5.3 An example of a typical project header file (Mai n . H)
Main.H (vl.OO)
'Project Header' for project HELL02 (see Chap 5)
#ifndef _MAIN_H
#define _MAIN_H
//
// WILL NEED TO EDIT THIS SECTION FOR EVERY PROJECT
//
// Must include the appropriate microcontroller header file here
#include <reg52.h>
// Oscillator / resonator frequency (in Hz) e.g. (11059200UL)
#define 0SC_FREQ (12000000UL)
// Number of oscillations per instruction (12, etc)
// 12 - Original 8051 / 8052 and numerous modern versions
// 6 - Various Infineon and Philips devices, etc.
// 4 - Dallas 320, 520 etc.
// 1 - Dallas 420, etc.
#define 0SC_PER_INST (12)
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90 Embedded C
//
// SHOULD NOT NEED TO EDIT THE SECTIONS BELOW
//
// Typedefs (see Chap 5)
typedef unsigned char tByte;
typedef unsigned int tWord;
typedef unsigned long tLong;
// Interrupts (see Chap 7)
#define INTERRUPT_Timer_0_Overf low 1
#define INTERRUPT_Timer_1 .Overflow 3
#define INTERRUPT_Timer_2_0verf low 5
#endif
END OF FILE
We consider the various components of this file in the sub-sections below.
a) The device header
The first entry in the project header is the link to the appropriate 'device header' file.
We discussed one such file ('reg52.h') in Chapter 3. These files will, in most
cases, have been produced by your compiler manufacturer, and will include the
addresses of the special function registers (SFRs) used for port access, plus similar
details for other on-chip components such as analog-to-digital converters.
For example, Listing 5.4 shows part of the device header for an Extended 8051,
the Infineon C515C. This device has eight ports, a watchdog unit, analog-to-digital
converter and other components, all made accessible through the device header.
/
*
REG515C.H
Header file for the Infineon C515C
Copyright (c) 1995-1999 Keil Elektronik GmbH All rights
reserved .
*/
:322 Chapter 5
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/* A/D Converter */
sfr ADCONO = 0xD8 ;
/* Interrupt System */
sfr IENO = 0xA8;
Adding structure to your code 91
/* Ports */
sfr
PO
= 0x80
sfr
P1
= 0x90
sfr
P2
= OxAO
sfr
P3
= OxBO
sfr
P4
= 0xE8
sfr
P5
= 0xF8
sfr
P6
= OxDB
sfr
P7
= OxFA
/* Serial Channel */
sfr SCON = 0x98;
/* TimerO / Timerl */
sfr TCON = 0x88;
/* CAP/COM Unit / Timer2 */
sfr CCEN = 0xC1 ;
/* Watchdog */
sfr WDTREL = 0x86;
/* Power Save Modes */
sfr PCON = 0x87;
sfr PC0N1 = 0x88;
b) Oscillator frequency and oscillations per instruction
If you create an application using a particular 8051 device operating at a particular
oscillator frequency, with a particular number of oscillations per instruction, this
information will be required when compiling many of the different source files in
:322 Chapter 5 p81-112 21/2/02 9:57 am Page 92
92 Embedded C
your project. For example - in many cases - we can create code for generating
delays (and similar purposes) if we store information about the oscillator fre-
quency and number of oscillations-per-instruction in an appropriate form. This is
done in the Mai n . H file as follows:
// Oscillator / resonator frequency (in Hz) e.g. (11059200UL)
#define 0SC_FREQ (12000000UL)
/ Number of oscillations per instruction (12, etc)
/ 12 - Original 8051 / 8052 and numerous modern versions
/ 6 - Various Infineon and Philips devices, etc.
/ 4 - Dallas 320, 520 etc.
/ 1 - Dallas 420, etc.
#define 0SC_PER_INST (12)
We will demonstrate how to use this information in Chapter 6 (for creating
delays), Chapter 7 (for controlling timing in an operating system) and Chapter 9
(for controlling the baud rate in a serial interface).
c) Common data types
The next part of the Project Header file in Listing 5.3 includes three typedef
statements:
typedef unsigned char tByte;
typedef unsigned int tWord;
typedef unsigned long tLong;
In C, the typedef keyword allows us to provide aliases for data types: we can then
use these aliases in place of the original types. Thus, in the projects you will see
code like this:
tWord Temperature;
Rather than:
unsigned int Temperature;
The main reason for using these typedef statements is to simplify - and promote
- the use of unsigned data types. This is a good idea for two main reasons:
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Adding structure to your code 93
• The 8051 does not support signed arithmetic and extra code is required to
manipulate signed data: this reduces your program speed and increases the pro-
gram size. Wherever possible, it makes sense to use unsigned data, and these
typedef statements make this easier.
• Use of bitwise operators (see Chapter 4) generally makes sense only with
unsigned data types: use of 'typedef' variables reduces the likelihood that pro-
grammers will inadvertently apply these operators to signed data.
Finally, as in desktop programming, use of the typedef keyword in this way can
make it easier to adapt your code for use on a different processor (for example,
when you move your 8051 code to a 32-bit environment). In many circumstances,
you will simply be able to change the typedef statements in Mai n . H, rather than
editing every source file in your project.
d) Interrupts
As we noted in Chapter 2, interrupts are a key component of most embedded systems.
The following lines in the Project Header are intended to make it easier for you
to use (timer-based) interrupts in your projects:
#define INTERRUPT_Timer_0_Overflow 1
#define INTERRUPT_Timer_1 .Overflow 3
#define INTERRUPT_Timer_2_0verf low 5
We discuss how to make use of this facility in Chapter 7.
e) Summary: Use of a Project Header?
Use of Project Header can help to make your code more readable, not least because
anyone using your projects knows where to find key information, such as the model
of microcontroller and the oscillator frequency required to execute the software.
The use of a Project Header can help to make your code more easily portable, by
placing some of the key microcontroller-dependent data in one place: if you
change the processor or the oscillator used then - in many cases - you will need to
make changes only to the Project Header.
Almost every example project on the CD includes a Project Header file. Search
for the file Mai n . H on the CD to see further examples.
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94
Embedded C
5.4 The Port Header (Port . H)
In a typical embedded project, you may have a user interface created using an
LCD, a keypad, and one or more single LEDs. There may be a serial (RS-485) link
to another microcontroller board. There may be one or more high-power devices
(say 3-phase industrial motors) to be controlled.
Each of these (software) components in your application will require exclusive
access to one or more port pins. Following the structure discussed in Section 5.2,
the project may include 10-20 different source files, created - perhaps - by five
different people. How do you ensure that changes to port access in one compo-
nent does not impact on another? How do you ensure that it is easy to adapt the
application to an environment where different port pins must be used?
These issues are addressed through the use of a simple Port Header file (Figure 5.3).
Using a Port Header, you pull together the different port access features for the whole
project into a single (header) file. Use of this technique can ease project develop-
ment, maintenance and porting.
Port Header (Port.H)
// Pins 3.0 and 3.1 used
// for RS-232 interface
// Switches
sbit Sw_up = Pl"2;
sbit Sw_down = Pl"3;
/
\
Up
Down
V
/
FIGURE 5.3 A schematic representation of the port header file
The Port Header file is simple to understand and easy to apply. Consider, for exam-
ple, that we have three C files in a project (A, B, C), each of which require access to
one or more port pins, or to a complete port.
File A may include the following:
// File A
sbit Pin_A = P3 A 2;
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File B may include the following:
// File B
#define Port B P0
File C may include the following:
// File C
sbit Pin_C = P2 A 7;
Adding structure to your code 95
In this version of the code, all of the port access requirements are spread over mul-
tiple files. Instead of this, there are many advantages obtained by integrating all
port access in a single Port . H header file:
//
Port.H
// Port access for File B
#define Port_B P0
// Port access for File A
sbit Pin_A = P3 A 2;
// Port access for File C
sbit Pin_C = P2 A 7;
Each of the remaining project files will then '#i ncl ude' the file Tort . H'.
Listing 5.5 shows a complete example of a Port . H file from a real application.
Listing 5.5 An example of a real Port Header file (Port.H) from a project using an interface
consisting of a keypad and liquid crystal display
Port.H (vl.OO)
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96 Embedded C
'Port Header' (see Chap 5) for project DATA_ACQ (see Chap 9)
#ifndef _P0RT_H
#define _P0RT_H
#include 'Main.H'
// Menu_A.C
// Uses whole of Port 1 and Port 2 for data acquisition
#define Data_Port1 P1
#define Data Port2 P2
// PC_I0.C
// Pins 3.0 and 3.1 used for RS-232 interface
#endif
END OF FILE
Despite its simplicity, use of a Port Header file can improve reliability and safety,
because it avoids potential conflicts between port pins, particularly during the
maintenance phase of the project when developers (who may not have been
involved in the original design) are required to make code changes.
A Port Header is itself portable: it can be used with any microcontroller, and is not
linked to the 8051 family. Use of a Port Header also improves portability, by making
accessible, in one location, all of the port access requirements of the application.
5.5 Example: Restructuring the Hello, Embedded World example
We present here the complete source code listing for the 'Hello, Embedded World'
example introduced in Chapter 3. This time, the code is restructured to match the
layout suggestions given in this chapter.
The complete code for this project is also included on the CD.
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Adding structure to your code 97
Listing 5.6 Part of the 'Hello, Embedded World' example (restructured version)
Main.H (vl.OO)
'Project Header'
#ifndef _MAIN_H
#define _MAIN_H
//
// WILL NEED TO EDIT THIS SECTION FOR EVERY PROJECT
//
// Must include the appropriate microcontroller header file here
#include <reg52.h>
// Oscillator / resonator frequency (in Hz) e.g. (11059200UL)
#define 0SC_FREQ (12000000UL)
// Number of oscillations per instruction (12, etc)
// 12 - Original 8051 / 8052 and numerous modern versions
// 6 - Various Infineon and Philips devices, etc.
// 4 - Dallas 320, 520 etc.
// 1 - Dallas 420, etc.
#define 0SC_PER_INST (12)
//
// SHOULD NOT NEED TO EDIT THE SECTIONS BELOW
//
// Typedefs (see Chap 5)
typedef unsigned char tByte;
typedef unsigned int tWord ;
typedef unsigned long tLong;
// Interrupts (see Chap 7)
#define INTERRUPT Timer Overflow 1
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98 Embedded C
#define INTERRUPT_Timer_1 .Overflow 3
#define INTERRUPT_Timer_2_0verf low 5
#endif
END OF FILE
Listing 5.7 Part of the 'Hello, Embedded World' example (restructured version)
Port.H (v1 .00)
'Port Header' for project HELL02 (see Chap 5)
#ifndef _P0RT_H
#define _P0RT_H
// LED Flash.C
// Connect LED to this pin, via appropriate resistor
sbit LED_pin = P1 A 5;
#endif
END OF FILE
Listing 5.8 Part of the 'Hello, Embedded World' example (restructured version)
Main.C (v1 .00)
A "Hello Embedded World" test program for 8051.
(Re-structured version - multiple source files)
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Adding structure to your code 99
#include "Main.H"
#include "Port.H"
#include "Delay_Loop. h"
#include "LED_Flash . h"
void main (void)
{
LED_FLASH_Init() ;
whi le(1 )
{
// Change the LED state (OFF to ON, or vice versa)
LED_FLASH_Change_State() ;
// Delay for *approx* 1000 ms
DELAY_L00P_Wait(1000) ;
}
END OF FILE
Listing 5.9 Part of the 'Hello, Embedded World' example (restructured version)
/*
LED_flash.H (vl.OO)
- See LED flash.C for details.
#ifndef _LED_FLASH_H
#define LED FLASH H
//
Public function prototypes
void LED_FLASH_Init(void) ;
void LED_FLASH_Change_State(void) ;
#endif
END OF FILE
:322 Chapter 5 p81-112 21/2/02 9:57 am Page 10
100 Embedded C
Listing 5.10 Part of the 'Hello, Embedded World' example (restructured version)
LED_f1ash.C (v1 .00)
Simple 'Flash LED' test function
#indude "Main.H"
#indude "Port.H"
#indude "LED flash. H"
//
Private variable definitions
static bit LED_state_G;
LED_FLASH_Init()
Prepare for LED_Change_State() function - see below
void LED_FLASH_Init(void)
{
LED_state_G = 0;
}
/*
LED_FLASH_Change_State ( )
Changes the state of an LED (or pulses a buzzer, etc) on a
specified port pin.
Must call at twice the required flash rate: thus, for 1 Hz
flash (on for 0.5 seconds, off for 0.5 seconds) must call
every 0.5 seconds.
voi d LED_FLASH_Change_State ( voi d )
:322 Chapter 5 p81-112 21/2/02 9:57 am Page 10:
Adding structure to your code 101
{
// Change the LED from OFF to ON (or vice versa)
if (LED_state_G == 1)
{
LED_state_G = 0;
LED_pin = 0;
}
else
{
LED_state_G = 1 ;
LED_pin = 1 ;
}
}
END OF FILE
Listing 5.1 1 Part of the 'Hello, Embedded World' example (restructured version)
Delay_Loop.H (v1 .00)
- See Delay_Loop.C for details
#ifndef _DELAY_L00P_H
#define DELAY LOOP H
// Public function prototype
void DELAY_L00P_Wait( const tWord) ;
#endif
END OF FILE
;322 Chapter 5 p81-112 21/2/02 9:57 am Page 102
102 Embedded C
Listing 5.12 Part of the 'Hello, Embedded World' example (restructured version)
De1ay_Loop.C (v1 .00)
Create a simple software delay using a loop
#include "Main.H"
#include "Port.H"
#include M Delay_loop. h"
DELAY_L00P_Wait()
Delay duration varies with parameter.
Parameter is, *R0UGHLY*, the delay, in milliseconds,
on 12MHz 8051 (12 osc cycles).
You need to adjust the timing for your application!
void DELAY_L00P_Wait (const tWord DELAY_MS)
{
tWord x, y;
for (x = 0; x <= DELAY_MS; x++)
{
for (y = 0; y <= 120; y++) ;
}
}
END OF FILE
;322 Chapter 5 p81-112 21/2/02 9:57 am Page 103
Adding structure to your code 103
5.6 Example: Restructuring the goat-counting example
In Chapter 4 (Section 4.9), we presented an example in which the number of goats
passing a sensor was measured and displayed on a port. Here we present another ver-
sion of this example, restructured according to the guidelines presented in this chapter.
Listing 5.1 3 Part of the 'Goat' example (restructured version)
Main.H (vl.OO)
'Project Header' (see Chapter 5).
#ifndef _MAIN_H
#define _MAIN_H
//
// WILL NEED TO EDIT THIS SECTION FOR EVERY PROJECT
//
// Must include the appropriate microcontroller header file here
#include <reg52.h>
// Oscillator / resonator frequency (in Hz) e.g. (11059200UL)
#define 0SC_FREQ (12000000UL)
// Number of oscillations per instruction (12, etc)
// 12 - Original 8051 / 8052 and numerous modern versions
// 6 - Various Infineon and Philips devices, etc.
// 4 - Dallas 320, 520 etc.
// 1 - Dallas 420, etc.
#define 0SC_PER_INST (12)
//
// SHOULD NOT NEED TO EDIT THE SECTIONS BELOW
//
// Typedefs (see Chap 5)
typedef unsigned char tByte;
;322 Chapter 5 p81-112 21/2/02 9:57 am Page 104
104 Embedded C
typedef unsigned int tWord;
typedef unsigned long tLong;
// Interrupts (see Chap 7)
#define INTERRUPT_Timer_0_Overf low 1
#define INTERRUPT_Timer_1 .Overflow 3
#define INTERRUPT_Timer_2_0verf low 5
#endif
END OF FILE
Listing 5.14 Part of the 'Goaf example (restructured version)
Port.H (v1 .00)
'Port Header' for project G0ATS2 (see Chap 5)
#ifndef _P0RT_H
#define PORT H
// Switch_Wait.C ■
// Connect switch to this pin
sbit Switch_pin = P1 A 0;
// Display_count .C
// Display count (binary) on this port
#define Count_port P3
#endif
END OF FILE
;322 Chapter 5 p81-112 21/2/02 9:57 am Page 105
Adding structure to your code 105
Listing 5.1 5 Part of the 'goat' example (restructured version)
Main.C (v1 .00)
A 'switch count' program for the 8051
#indude "Main.H"
#indude "Port.H"
#indude "Switch_wait .H"
#indude "Disp1ay_count .H"
/*
void main(void)
{
tByte Switch_presses = 0;
// Init functions
SWITCH_Init() ;
DISPLAY_C0UNT_Init() ;
whi 1e(1 )
{
if (SWITCH_Get_Input(30) == SWITCH_PRESSED)
{
Swi tch_presses++ ;
}
DISPLAY_COUNT_Update(Switch_presses) ;
}
END OF FILE
;322 Chapter 5 p81-112 21/2/02 9:57 am Page 106
106 Embedded C
Listing 5.1 6 Part of the 'goaf example (restructured version)
Switch_wait.H (v1 .00)
- See Switch wait.C for details.
#ifndef _SWITCH_WAIT_H
#define SWITCH WAIT H
// Public constants
// Return values from Switch_Get_Input ()
#define SWITCH_N0T_PRESSED (bit)
#define SWITCH_PRESSED (bit) 1
// Public function prototype
void SWITCH_Init(void) ;
bit SWITCH_Get_Input (const tByte) ;
#endif
END OF FILE
Listing 5.1 7 Part of the 'goaf example (restructured version)
Switch_Wait.C (v1 .00)
Simple library for debouncing a switch input.
NOTE: Duration of function is highly variable!
#indude "Main.H"
#indude "Port.H"
;322 Chapter 5 p81-112 21/2/02 9:57 am Page 107
#include "Switch_wait . h"
#include "Delay_loop.h"
/*
Adding structure to your code 107
SWITCH_Init()
Initialisation function for the switch library
void SWITCH_Init(void)
{
Switch_pin = 1; // Use this pin for input
}
/*
SWITCH_Get_Input()
Reads and debounces a mechanical switch as follows:
1. If switch is not pressed, return SWITCH_NOT_PRESSED.
2. If switch is pressed, wait for DEBOUNCE_PERIOD (in ms)
a. If switch is not pressed, return SWITCH_NOT_PRESSED
b. If switch is pressed, wait (indefinitely) for
switch to be released, then return SWITCH_PRESSED
See Switch Wait.H for details of return values.
bit SWITCH_Get_Input (const tByte DEBOUNCE_PERIOD)
{
bit Return_value = SWITCH_NOT_PRESSED;
if (Switch_pin == 0)
{
// Switch is pressed
// Debounce - just wait...
DELAY_LOOP_Wait(DEBOUNCE_PERIOD) ;
// Check switch again
if (Switch_pin == 0)
{
// Wait until the switch is released.
:322 Chapter 5 p81-112 21/2/02 9:57 am Page 10:
108 Embedded C
while (Switch_pin == 0);
Return_value = SWITCH_PRESSED;
}
}
// Now (finally) return switch value
return Return_val ue;
}
END OF FILE
Listing 5.1 8 Part of the 'goaf example (restructured version)
Di spl ay_count . H ( v1 . 00)
- See Display_count .C for details
#ifndef _DISPLAY_C0UNT_H
#define DISPLAY COUNT H
// Public function prototypes --
void DISPLAY_C0UNT_Init(void) ;
void DISPLAY_C0UNT_Update (const tByte) ;
#endif
END OF FILE
Listing 5.19 Part of the 'goat' example (restructured version)
Display_count .C (v1 .00)
;322 Chapter 5 p81-112 21/2/02 9:57 am Page 109
Adding structure to your code 109
Display an unsigned char on a port
#include "Main.H"
#include "Port.H"
#include "Display_Count .H"
/
DISPLAY_COUNT_Init()
Initialisation function for the DISPLAY COUNT library
void DISPLAY_COUNT_Init(void)
{
Count_port = 0x00;
}
/*
DISPLAY_C0UNT_Update ( )
Simple function to display tByte data (COUNT)
on LEDs connected to port (Count_Port)
void DISPLAY_C0UNT_Update (const tByte COUNT)
{
Count_port = COUNT;
}
/
END OF FILE
Listing 5.20 Part of the 'goaf example (restructured version)
Delay_Loop.H (v1 .00)
- See Delay_Loop.C for details.
:322 Chapter 5 p81-112 21/2/02 9:57 am Page
110 Embedded C
#ifndef _DELAY_L00P_H
#define _DELAY_L00P_H
// Public function prototype
void DELAY_L00P_Wait (const tWord) ;
#endif
/*
END OF FILE
Listing 5.21 Part of the 'goat' example (restructured version)
De1ay_Loop.C (v1 .00)
Create a simple software delay using a loop.
#include "Main.H"
#include "Port.H"
#include "Delay_loop. h"
DELAY_L00P_Wait()
Delay duration varies with parameter.
Parameter is, *R0UGHLY*, the delay, in milliseconds,
on 12MHz 8051 (12 osc cycles).
You need to adjust the timing for your application!
*
void DELAY_L00P_Wait (const tWord DELAY_MS)
{
tWord x, y;
for (x = 0; x <= DELAY_MS; x++)
{
;322 Chapter 5 p81-112 21/2/02 9:57 am Page 111
Adding structure to your code 1 1 1
for (y = 0; y <= 120; y++) ;
}
}
END OF FILE
5.7 Further examples
For further examples of structured code, Port . H files and Mai n . H files throughout
the remaining chapters in this book - please refer to the CD for details.
5.8 Conclusions
Over the course of the first five chapters of this book, we have reached the stage where
we can create a simple - but functional - code framework for an embedded application.
In the remainder of this book, much of what we do will involve examining and
refining different parts of this framework until - by the time we reach the case
study in Chapter 11 - we will be in a position to assemble a range of complete
embedded projects.
;322 Chapter 5 p81-112 21/2/02 9:57 am Page 112
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 113
chapter
Meeting real-time constraints
6.1 Introduction
In this chapter, we begin to consider the issues involved in the accurate measure-
ment of time. These issues are important because many embedded systems must
satisfy real-time constraints.
For example, consider the aircraft autopilot application illustrated in Figure 6.1.
Here we assume that the pilot has entered the required course heading, and that
the system must make regular and frequent changes to the rudder, elevator,
aileron and engine settings (for example) in order to keep the aircraft following
this path.
An important characteristic of this embedded system is the need to process
inputs and generate outputs very rapidly, on a time-scale measured in millisec-
onds. In this case, even a slight delay in making changes to the rudder setting (for
example) may cause the plane to oscillate very unpleasantly or, in extreme circum-
stances, even to crash. However, in order to be able to have such an autopilot
system certified for use, ensuring that the processing was 'as fast as we could make
it' would not be enough to satisfy the relevant authorities: in this situation, as in
many other real-time applications, the key characteristic is deterministic process-
ing. What this means is that in many real-time embedded systems we need to be
able to guarantee that a particular activity will always be completed within (say) 2
ms: if the processing does not match this specification, then the application is not
simply slower than we would like, it is useless.
113
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 11-
114 Embedded C
X,1)
y-Pv
x, y, z = position coordinates
v, (3, 05 = velocity coordinates
p = roll rate
q = pitch rate
r = vaw rate aZ=7 ^ Rudder 5r
Elevator 5e
Pitch
(rate)
sensor
Roll
(rate)
sensor
Main
pilot
controls
Position
sensors
(GPS)
Yaw (rate)
sensor
Velocity
sensors
(3 axes)
Rudder
Elevator
Aileron
Main engine
(fuel)
controllers
FIGURE 6.1 A high-level schematic view of a simple autopilot system
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 115
Meeting real-time constraints 115
To date, the code we have developed certainly does not satisfy the real-time
requirements of many embedded applications. For example, in Chapter 4, we pre-
sented an example in which we counted goats moving into a milking parlour. At
the heart of this system was the function shown in annotated form in Listing 6.1.
Listing 6.1 Identifying problems with some simple switch-interface code
bit SWITCH_Get_Input (const tByte DEBOUNCE_PERIOD)
{
tByte Return_va1ue = SWITCH_NOT_PRESSED;
if (Switch_pin == 0)
{
// Switch is pressed
// Debounce - just wait...
DELAY_LOOP_Wait(DEBOUNCE_PERIOD) ; // POTENTIAL PROBLEM
// Check switch again
if (Switch_pin == 0)
{
// Wait until the switch is released.
while (Switch_pin == 0); // POTENTIAL CATASTROPHE
Return_value = SWITCH_PRESSED;
}
}
// Now (finally) return switch value
return Return_val ue;
}
Listing 6.1 highlights two potential problems with function SWITCH_Get_Input ( ) . 21
The first problem is that we wait for a 'debounce' period in order to confirm
that the switch has been pressed:
DELAY_LOOP_Wait(DEBOUNCE_PERIOD) ;
Because this delay is implemented using a software loop it may not be very pre-
cisely timed. We illustrate how we can set this delay period more accurately using
a hardware timer in Section 6.2.
21. There are, in fact, three problems. The third problem is that - while waiting for the switch to be
released and (to a lesser extent) while executing the 'debounce delay' - we are wasting processor
cycles that could be put to better use. We need to defer consideration of this third problem until
Chapter 7, when we will demonstrate that it may be solved using an embedded operating system.
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 116
116 Embedded C
The second problem is even more serious in a system with real-time characteris-
tics. With this code:
while (Switch_pin == 0);
we cause the system to wait - indefinitely - for the user to release the switch. This
means that a damaged switch connection - or a determined user - can bring the
system to a halt. Such code is simply unacceptable in production code (and cer-
tainly would not achieve certification, in the case of the autopilot system). We will
begin to consider how we can solve this second problem in Section 6.6.
6.2 Creating 'hardware delays' using Timer and Timer 1
We have used simple loop delays (similar to that shown below) in earlier chapters:
void DELAY_L00P_Wait (const unsigned int DELAY)
{
unsigned int x, y;
for (x = 0; x <= DELAY; x++)
{
for (y = 0; y <= 120; y++) ;
}
}
This approach to generating delays is easy to understand and easy to use (on any
embedded or desktop processor). However, it is not easy to produce precisely
timed delays using this approach, and the loops must be re-tuned if you decide to
use a different processor, change the clock frequency, or even change the compiler
optimization settings.
If we want to create more accurate delays, then we can do so using one of the
805 l's on-chip timers. As we noted in Chapter 2, all members of the 8051 family
have at least two 16-bit timer / counters, known as Timer and Timer 1. These
timers can be used to generate accurate delays.
To see how these timers operate, we need to explain the features of:
• The TCON SFR;
• The TMOD SFR; and,
• The THx and TLx registers.
We will first consider - in outline - how hardware timers are used to generate
accurate delays, then consider each of the above timer components in more detail.
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 117
Meeting real-time constraints 117
a) Overview
We noted in Chapter 2 that - when the timer hardware is running and appropri-
ately configured - the timers are incremented periodically. In the original 8051
and 8052, the timers were incremented every 12 oscillator cycles: 22 assuming we
used a 12 MHz oscillator, the timers were incremented 1 million times per second.
In most cases, we will be concerned with 16-bit timers. Assuming the count
starts at 0, then - after 65.535 ms - our 12 MHz 8051 will reach its maximum
value (65535) and the timer will then 'overflow'. When it overflows, a hardware
flag will be set. We can easily read this flag in software.
This is very useful behaviour. For example, if we start the timer with an initial value
of zero and wait until the flag is set, we know that precisely 65.535 ms will have
elapsed. More importantly, we can vary the initial value stored in the timer: this allows
us to generate shorter, precisely-timed, delays of - say - 50ms or 1ms, as required.
Building on the material discussed above, calculations of hardware delays
generally take the following form:
• We calculate the required starting value for the timer.
• We load this value into the timer.
• We start the timer.
• The timer will be incremented, without software intervention, at a rate deter-
mined by the oscillator frequency; we wait for the timer to reach its maximum
value and 'roll over'.
• The timer signals the end of the delay by changing the value of a flag variable.
As noted above, if we are using a '12-osciHations per instruction' 8051, running at
12 MHz, the longest delay that can be produced with a 16-bit timer is approxi-
mately 65 ms. If we need longer delays, we can repeat the steps above.
We will now consider how we can write code to achieve this.
b) The TCON SFR
We first need to introduce the TCON special function register (SFR): see Table 6.1
The various bits in the TCON SFR have the following functions:
TABLE 6.1 The TCON Special Function Register. Note that the grey bits are not connected with either Timer
or Timer 1
Bit
7 (MSB)
6
5
4
3
2
1
(LSB)
NAME
TF1
TR1
TF0
TR0
IE1
IT1
IE0
IT0
22. We consider some more modern 8051 devices in Section 6.4: these require fewer than 12
oscillator cycles per timer increment.
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118 Embedded C
TRO, TR1 Timer run control bits
These bits need to be set to 1 (by the programmer) to run the corresponding timer (TRO
controls Timer 0, TR1 controls Timer 1). If set to 0, the corresponding timer will not run.
Default value (after reset) is 0.
TFO, TF1 Timer overflow flags
These bits are set to 1 (by the microcontroller) when the corresponding timer overflows. They
need to be manually cleared (set to 0) by the programmer. Default value (after reset) is 0.
For completeness we will briefly explain the purpose of the remaining bits in TCON:
IE0, IE1 Interrupt flags
Set by hardware when an (external) interrupt is detected on Pin 12 or Pin 13, respectively.
These features are not related to Timer or Timer 1 and are not used in this book (these bits
can be left at their default value).
IT0, IT1 Interrupt type control bit
Used to determine with the external interrupt flags (above) are set in response to a falling
edge ('edge triggered') or a low-level (level triggered') input on the relevant port pins.
These features are not related to Timer or Timer 1 and are not used in this book (these bits
can be left at their default value).
c) What about interrupts?
The code we use to create delays in this chapter will take the following form:
while (TFO == 0); // Wait until Timer overflows
That is, we wait until the timer overflow flag indicates that the timer has reached
its maximum value. It should be pointed out that the overflow of the timers can
be used to generate an interrupt: this can allow you to perform other activities
while the counting goes on. We will not make use of this facility in the delay code
presented in this chapter, but we will do so when we create an embedded operat-
ing system in Chapter 7.
To disable the generation of interrupts, we can use the C statements:
ET0 = 0; // No interupts (Timer 0)
ET1 =0; // No interupts (Timer 1)
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 11!
Meeting real-time constraints 119
d) The TMOD SFR
We also need to introduce the TMOD SFR (Table 6.2).
TABLE 6.2 The TMOD Special Function Register. See text for details
Bit 7 (MSB) 6 5 4 3 2 1 (LSB)
NAME Gate C/T Ml MO Gate C/T M1 MO
Timer 1 Timer
The main thing to note is that there are three modes of operation (for each timer), set
using the Ml and MO bits. We will only be concerned in this book with Mode 1 and
Mode 2, which operate in the same way for both Timer and Timer 1, as follows:
Mode 1 (Ml = 1; M2 = 0)
16-bit timer/counter (with manual reload) 23
Mode 2 (Ml =0; M2 = 1)
8-bit timer/counter (with 8-bit automatic reload)
The remaining bits in TMOD have the following purpose:
GATE Gating control
We will not use gating control in this book. We will ensure that this bit is cleared, which
means that Timer or Timer 1 will be enabled whenever the corresponding control bit (TRO
or TR1 ) is set in the TCON SFR.
C/T Counter or timer select bit
We will not use counter mode in this book. We will ensure that this bit is cleared (for the
appropriate timer), so that timer mode is used.
e) The THx and TLx registers
The final components you need to be aware of are the two 8-bit registers associ-
ated with each timer: these are known as TLO and THO, and TL1 and TH1. Here, 'L'
and 'H' refer to 'low' and 'high' bytes, as will become clear shortly.
23. The distinction between 'automatic reload' and 'manual reload' timers has no impact on the
hardware delay code we use here: to avoid confusion, we delay a discussion of this topic until
Section 7.2b.
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 12
120 Embedded C
In all of the delay examples we use in this book, we will use the timers in Mode 1:
that is, as 16-bit timers. If we are using Timer 1 to generate the delays, then the
values loaded into TL1 and TH1 at the start of the delay routine will determine the
delay duration.
For example, suppose we wish to generate a 15 ms hardware delay. We will
assume that we are using a 12 MHz 8051, and that this device requires 12 oscilla-
tor cycles to perform each timer increment: the timer is incremented at a 1 MHz
rate. A 15 ms delay therefore requires the following number of timer increments:
1 5ms x 1 000000 = 1 5000 increments.
1 000ms
The timer overflows when it is incremented from its maximum count of 65535
Thus, the initial value we need to load to produce a 15ms delay is:
65536 - 1 5000 = 50536 (decimal) = 0xC568
We can load this initial value into Timer 1 as follows:
TH1 = 0xC5; // Timer 1 initial value (High Byte)
TL1 = 0x68; // Timer 1 initial value (Low Byte)
We give another illustration of this process in the next section.
6.3 Example: Generating a precise 50 ms delay
To see how this all fits together, we will consider a 'flashing LED' example based
on a precise 50 ms hardware delay (Listing 6.2).
Listing 6.2 A complete example using a hardware-based delay. See text for details
Hardware_Del ay_50ms . C ( v1 . 00)
A test program for hardware-based delays.
#include <reg52.h>
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Meeting real-time constraints 121
sbit LED_pin = P1 A 5;
bit LED_state_G;
void LED_FLASH_Init(void) ;
void LED_FLASH_Change_State(void) ;
void DELAY_HARDWARE_One_Second(void) ;
void DELAY_HARDWARE_50ms(void) ;
/*
void main(void)
{
LED_FLASH_Init() ;
while(1)
{
// Change the LED state (OFF to ON, or vice versa)
LED_FLASH_Change_State() ;
// Delay for approx 1000 ms
DELAY_HARDWARE_One_Second() ;
}
LED_FLASH_Init()
Prepare for LED_Change_State() function - see below
void LED_FLASH_Init(void)
{
LED_state_G = 0;
}
/*
LED_FLASH_Change_State ( )
Changes the state of an LED (or pulses a buzzer, etc) on a
specified port pin.
Must call at twice the required flash rate: thus, for 1 Hz
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 122
122 Embedded C
flash (on for 0.5 seconds, off for 0.5 seconds) must call
every 0.5 seconds.
*
voi d LED_FLASH_Change_State ( voi d )
{
// Change the LED from OFF to ON (or vice versa)
if (LED_state_G == 1)
{
LED_state_G = 0;
LED_pin = 0;
}
else
{
LED_state_G = 1 ;
LED_pin = 1 ;
}
}
/* -
DELAY_HARDWARE_One_Second ( )
Hardware delay of 1000 ms .
*** Assumes 12MHz 8051 (12 osc cycles) ***
void DELAY_HARDWARE_One_Second(void)
{
unsigned char d;
// Call DELAY_HARDWARE_50ms() twenty times
for (d = 0; d < 20 ; d++)
{
DELAY_HARDWARE_50ms() ;
}
}
DELAY_HARDWARE_50ms ( )
Hardware delay of 50ms
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 123
Meeting real-time constraints 123
Assumes 12MHz 8051 (12 osc cycles) ***
*
/
void DELAY_HARDWARE_50ms(void)
{
// Configure Timer as a 16-bit timer
TMOD &= OxFO; // Clear all TO bits (T1 left unchanged)
TMOD |= 0x01; // Set required TO bits (T1 left unchanged)
ETO =0; //No interupts
// Values for 50 ms delay
TH0 = 0x3C; // Timer initial value (High Byte)
TL0 = OxBO; // Timer initial value (Low Byte)
TF0 =0; // Clear overflow flag
TR0 = 1 ; // Start timer
while (TFO ==0); // Loop until Timer overflows (TF0 == 1)
TR0 =0; // Stop Timer
}
END OF FILE
* * /
In Listing 6.2, these lines set up Timer O, in Mode 1 (16-bit timer), without gating:
TMOD &= OxFO; // Clear all TO bits (T1 left unchanged)
TMOD |= 0x01; // Set required TO bits (T1 left unchanged)
Note the use of the bitwise operators to change the state of SFR bits while leaving
others unchanged: this is important as a different part of your program may be
using Timer 1 for another purpose.
As we discussed in Section 6.2c the overflow of a timer can be used to generate
an interrupt: we have no need for this in the delay code presented here. We there-
fore disable interrupt generation as follows:
ETO =0; // No interupts
Next, we load the timer registers with the initial timer value:
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 12
124 Embedded C
THO = 0x3C; // Timer initial value (High Byte)
TLO = OxBO; // Timer initial value (Low Byte)
In this case, we assume - again - the standard 12 MHz / 12 oscillations-per-instruc-
tion microcontroller environment. We require a 50 ms delay, so the timer requires
the following number of increments before it overflows:
50ms
1 000ms
x 1000000 = 50000 increments
The timer overflows when it is incremented from its maximum count of 65535
Thus, the initial value we need to load to produce a 50 ms delay is:
65536 - 50000 = 1 5536 (decimal) = 0x3CB0
Then we are ready to clear the timer flag, and start the timer running:
TF0 =0; // Clear overflow flag
TR0 = 1 ; // Start timer
We discussed how to use the Keil simulator to 'profile' code in Chapter 3. In this
case, executing the program in the hardware simulator confirms that the delays
operate as required (Figure 6.2).
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FIGURE 6.2 Executing Listing 6.2 in the hardware simulator
6.4 Example: Creating a portable hardware delay
In previous examples, we have usually assumed that the oscillator frequency will
be 12 MHz and that the processor will require 12 oscillations per instruction. Of
course, this will not always be the situation and - if your delay code assumes
'12 MHz / 12 osc' (or any other combination) - then you may run into problems.
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 125
Meeting real-time constraints 125
Here are some examples:
• If your application is battery powered, you will generally wish to use as low an
operating frequency as possible (refer back to Chapter 2, Section 2.5b, for fur-
ther details). To do this, you will typically work with late prototypes of your
system (first in the simulator and then on hardware) to 'profile' the code and
determine the minimum safe operating frequency. If delay code is 'hard wired'
it is easy to forget to adjust the timing in these circumstances.
• If you add a serial interface then - for reasons we will discuss in Chapter 9 - you
are likely to use an 11.059 MHz crystal. If you assumed a 12 MHz oscillator and
forget to adjust delay timing, then the differences can be difficult to detect in
bench tests, and any problems may only show up after your product has been
released. This can prove very costly.
• System maintenance is always an issue. Operation of your code may be clear to
you, but less experienced developers may subsequently assume that your '50 ms
delay code' always gives a 50 ms delay (no matter what oscillator they use).
To reduce the likelihood of such problems, the code presented in this example is
designed to be as portable as possible. What this means in practice is that the initial
timer values are 'automatically' determined for different processor and oscillator
combinations, by means of the project header file (Mai n . H: see Chapter 5), and
some appropriate use of C pre-processor directives.
The relevant parts of Mai n . H are as follows:
// Oscillator / resonator frequency (in Hz) e.g. (11059200UL)
#define 0SC_FREQ (12000000UL)
// Number of oscillations per instruction (12, etc)
// 12 - Original 8051 / 8052 and numerous modern versions
// 6 - Various Infineon and Philips devices, etc.
// 4 - Dallas 320, 520 etc.
// 1 - Dallas 420, etc.
#define 0SC_PER_INST (12)
The timer reload values are then determined as follows (please refer to the file
Delay_T0.C):
// Timer preload values for use in simple (hardware) delays
// - Timers are 16-bit, manual reload ('one shot').
//
// NOTE: These values are portable but timings are *approximate*
// and *must* be checked by hand if accurate timing is
// required.
//
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 126
126 Embedded C
// Define Timer / Timer 1 reload values for ~1 msec delay
// NOTE:
// Adjustment made to allow for function call overheard etc.
#define PREL0AD01 (65536 - (tWord) (0SC_FREQ / (OSC_PER_INST
1020)))
#define PREL0AD01H (PREL0AD01 / 256)
#define PREL0AD01L (PREL0AD01 % 256)
// Delay value is *approximately* 1 ms per loop
for (ms = 0; ms < N ; ms++)
{
THO = PREL0AD01H;
TLO = PREL0AD01L;
// Clear overflow flag
// Start timer
: = 0) ; // Loop until Timer overflows (TF0 == 1)
// Stop Timer
To illustrate how this all fits together, the two key files required in the project are
shown in Listing 6.3 and Listing 6.4 in their entirety. As usual, a complete set of
files is included on the CD.
TF0
=
0;
TR0
=
1;
whil
e
(TF0
TR0
=
0;
}
Listing 6.3 Part of the generic delay code (Hardware Delay) example
Main.C (v1 .00)
Flashing LED with hardware-based delay (TO)
#include "Main.H"
#include "Port.H"
#include "Delay.TO.h"
#include "LED Flash. h"
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 127
Meeting real-time constraints 127
void main(void)
{
LED_FLASH_Init() ;
while(1)
{
// Change the LED state (OFF to ON, or vice versa)
LED_FLASH_Change_State() ;
// Delay for *approx* 1000 ms
DELAY_T0_Wait(1000) ;
}
END OF FILE
Listing 6.4 Part of the generic delay code (Hardware Delay) example
Delay_T0.C (v1 .00)
Simple hardware delays based on TO
#include "Main.H"
//
Private constants
/ Timer preload values for use in simple (hardware) delays
/ - Timers are 16-bit, manual reload ('one shot').
/
/ NOTE: These values are portable but timings are *approximate*
/ and *must* be checked by hand if accurate timing is
/ required.
/
/ Define Timer / Timer 1 reload values for ~1 msec delay
/ NOTE: Adjustment made to allow for function call overheard etc.
#define PREL0AD01 (65536 - (tWord) (0SC_FREQ / (0SC_PER_INST * 1020)))
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 128
128 Embedded C
#define PRELOAD01H (PRELOAD01 / 256)
#define PRELOAD01L (PRELOAD01 % 256)
DELAY_T0()
Function to generate N millisecond delay (approx) .
Uses Timer (easily adapted to Timer 1).
void DELAY_T0_Wait (const tWord N)
{
tWord ms ;
// Configure Timer as a 16-bit timer
TMOD &= OxFO; // Clear all TO bits (T1 left unchanged)
TMOD |= 0x01; // Set required TO bits (T1 left unchanged)
ETO =0; //No interupts
// Delay value is *approximately* 1 ms per loop
for (ms = 0; ms < N; ms++)
{
TH0 = PREL0AD01H;
TL0 = PREL0AD01L;
TF0 =0; // clear overflow flag
TR0 = 1 ; // start timer
while (TF0 ==0); // Loop until Timer overflows (TF0 == 1)
TR0 =0; // Stop Timer
}
}
/ * *
END OF FILE
* * /
The output from this project is shown running in the Keil hardware simulator in
Figure 6.3. Note that the delay value obtained is very close to the required value.
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 12
Meeting real-time constraints 129
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FIGURE 6.3 The output from the Hardware Delay example project running in the Keil hardware
simulator. Please see Chapter 3 for information about the use of the performance analyzer
6.5 Why not use Timer 2?
In many cases, as we saw in Chapter 2, modern '8051' family devices are based on
the slightly later 8052 architecture: such devices include an extra, more flexible
timer: this is called - logically - Timer 2.
Timer 2 can be used to generate delays (in a manner nearly identical to that
used with Timer and Timer 1 in this chapter). However, this is not an inappro-
priate use for this resource (in most applications). This is because Timer 2 has
features which make it particularly well suited to the creation of an operating
system, and most 'rear applications will reserve Timer 2 for this purpose.
We will describe an operating system based on Timer 2 in Chapter 7.
6.6 The need for 'timeout' mechanisms
In Section 6.1 we considered some of the weaknesses of the software used to
implement the first version of our goat-counting system from Chapter 4. One of
the main problems we noted arose from the use of code like this:
while (Switch_pin == 0);
As we noted above, the problem is that the system will 'hang' if the switch is never
released. In our goat-counting system, this might happen - for example - if one of
the animals got stuck in the entrance to the milking parlour, and held up the
whole herd. In these circumstances, it would be useful if we could have an alarm
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 13
130 Embedded C
sound (to alert the farmer) if any goat took longer than - say - 15 seconds to pass
through the gate. However, with the first simple implementation of this system,
100% of the processor power is wasted in an idle loop, waiting for the switch to be
released. This is clearly not an ideal design.
Such problems are not, of course, limited to switch interfaces. For example, the
Philips 8Xc552 is an Extended 8051 device with a number of on-chip peripherals,
including an 8-channel, 10-bit analog-to-digital converter (ADC). Philips provide
an application note (AN93017) that describes how to use this feature of the micro-
controller. This application note includes the following code:
// Wait until AD conversion finishes (checking ADCI)
while ((ADC0N & ADCI) == 0) ;
Such code is potentially unreliable, because there are circumstances under which our
application may 'hang'. This might occur for one or more of the following reasons:
• If the ADC has been incorrectly initialized, we cannot be sure that a data con-
version will be carried out.
• If the ADC has been subjected to an excessive input voltage, then it may not
operate at all.
• If the variable ADCON or ADCI were not correctly initialized, they may not
operate as required.
The Philips example is not intended to illustrate 'production' code. Unfortunately,
however, code in this form is common in embedded applications. If the systems
you create are to be reliable, you need to be able to guarantee that no function will
hang in this way.
There are several ways we can provide such a guarantee. We will consider two of
the most popular in the sections that follow:
• The loop timeout (see Section 6.7).
• The hardware timeout (see Section 6.10).
6.7 Creating loop timeouts
A loop timeout may be easily created. The basis of the code structure is a form of
loop delay, created as follows:
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 131
Meeting real-time constraints 1 31
tWord Timeout_loop = 0;
while (++Timeout_loop != 0);
This loop will keep running until the variable Timeout_loop reaches its maxi-
mum value (assuming 16-bit integers) of 65535, and then overflows. When this
happens, the program will continue. Note that, without some simulation studies
or prototyping, we cannot easily determine how long this delay will be. However,
we do know that the loop will, eventually, time out.
Such a loop is simply a slightly modified version of the loop-delay code which
we have presented previously. However, if we consider again the ADC example dis-
cussed in the previous section, we can illustrate how such a loop can be used to
provide a timeout facility. Recall that the original code was as follows:
// Wait until AD conversion finishes (checking ADCI)
while ((ADCON & ADCI) == 0);
Here is a modified version of this code, with a loop timeout:
tWord Timeout_loop = 0;
// Take sample from ADC
// Wait until conversion finishes (checking ADCI)
// - simple loop timeout
while (((ADCON & ADCI) == 0) && (++Timeout_loop != 0));
Note that we can vary the duration of the loop timeout by changing the initial
value assigned to the loop variable. The file Timeout L. H, reproduced in Listing
6.5 and included on the CD in the directory associated with this chapter, includes
a set of constants that give - very approximately - the specified timeout values.
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 132
132 Embedded C
Listing 6.5 The file Ti meout L . H. See text for details
TimeoutL.H (v1 .00)
Simple software (loop) timeout delays for the 8051 family.
* THESE VALUES ARE NOT PRECISE - YOU MUST ADAPT TO YOUR SYSTEM
#ifndef _TIME0UTL_H
#define TIMEOUTL H
//
Public constants
// Vary this value to change the loop duration
// THESE ARE APPROX VALUES FOR VARIOUS TIMEOUT DELAYS
// ON 8051, 12 MHz, 12 Osc / cycle
//
//
MUST BE FINE TUNED FOR YOUR APPLICATION ***
Timings vary with compiler optimisation settings
// tWord
#define L00P_TIME0UT_INIT_001ms 65435U
#define L00P_TIME0UT_INIT_010ms 64535U
#define L00P_TIME0UT_INIT_500ms 14535U
// tLong
#define L00P_TIME0UT_INIT_1 0000ms 4294795000UL
#endif
END OF FILE
We give an example of how to use this file below.
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 133
Meeting real-time constraints 1 33
6.8 Example: Testing loop timeouts
We present a simple example here that may be used - in the simulator - to fine-
tune the software timeout loops to match your particular hardware.
Listing 6.6 shows the required code.
Listing 6.6 Testing loop timeouts
Main.C (v1 .00)
Testing timeout loops
#include <reg52.H>
#include "TimeoutL.H"
// Typedefs (see Chap 5)
typedef unsigned char tByte;
typedef unsigned int tWord ;
typedef unsigned long tLong;
// Function prototypes
void Test_Timeout (void) ;
/*
void main (void)
{
wh i 1 e ( 1 )
{
Test_Timeout () ;
}
}
/*
void Test_Timeout (void)
{
tLong Timeout_loop = L00P_TIME0UT_INIT_1 0000ms ;
// Simple loop timeout...
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 13
134 Embedded C
while (++Timeout_loop != 0) ;
}
END OF FILE
The simulator output is shown in Figure 6.4.
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FIGURE 6.4 Testing timeout loops: see text for details
Jd
6.9 Example: A more reliable switch interface
In this example, we apply the loop timeout code to the problem of switch debouncing.
In this case, the user is allowed up to 10 seconds to release the switch: if it is not
released in this period, the function will 'time out' and return.
The key code is shown in Listing 6.7. Complete files for this project will be
found on the CD.
Listing 6.7 Code for a more reliable switch interface. See text for details
Switch_Wait_TimeoutL.C (vl.OO)
Simple library for processing a switch input.
(Made more reliable by means of a ~10-second timeout.)
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 135
Meeting real-time constraints 1 35
#include "Main.H"
#include "Port.H"
#incl ude "Switch_wait_Timeoutl_.H"
#include "Delay_T0.h"
#include "TimeoutL.H"
SWITCH_Init()
Initialisation function for the switch library.
void SWITCH_Init(void)
{
Switch_pin = 1; // Use this pin for input
}
SWITCH_Get_Input()
Reads and debounces a mechanical switch as follows:
1. If switch is not pressed, return SWITCH_NOT_PRESSED.
2. If switch is pressed, wait for DEBOUNCE_PERIOD (in ms) .
a. If switch is not pressed, return SWITCH_NOT_PRESSED.
b. If switch is pressed, wait (with timeout) for
switch to be released. If it times out,
then return SWITCH_NOT_PRESSED: otherwise, return
SWITCH_PRESSED.
See Switch Wait.H for details of return values.
bit SWITCH_Get_Input (const tByte DEBOUNCE_PERIOD)
{
tByte Return_value = SWITCH_NOT_PRESSED;
tLong Timeout_loop = L00P_TIME0UT_INIT_1 0000ms ;
if (Switch_pin == 0)
{
// Switch is pressed
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 136
136 Embedded C
// Debounce - just wait...
DELAY_TO_Wait(DEBOUNCE_PERIOD) ;
// Check switch again
if (Switch_pin == 0)
{
// Wait until the switch is released.
// (WITH TIMEOUT LOOP - 10 seconds)
while ((Switch_pin == 0) && (++Timeout_loop != 0));
// Check for timeout
if (Timeout_loop == 0)
{
Return_value = SWITCH_N0T_PRESSED;
}
el se
{
Return_value = SWITCH_PRESSED;
}
}
}
// Now (finally) return switch value
return Return_val ue;
}
/* *_
END OF FILE
* * /
6.10 Creating hardware timeouts
If we consider the ability to meet real-time requirements by code without timeouts
and code with a loop timeout, it is clear that the loop timeout solution is signifi-
cantly better.
Loop timeouts are particularly well suited to applications involving long time-
out delays (typically measured in seconds). Where we require shorter delays, with
very precise timing, we can often gain a further improvement in performance
through the use of hardware-based timeouts.
As we saw in earlier in this chapter, we can create portable and easy to use delay
code for the 8051 family as follows:
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 137
Meeting real-time constraints 1 37
// Define Timer / Timer 1 preload values for ~1 msec delay
#def ine PREL0AD_01ms (65536- (tWord) (0SC_FREQ/ (0SC_PER_INST*1000) ) )
#define PREL0AD_01ms_H (PREL0AD_01ms / 256)
#define PREL0AD_01ms_L (PREL0AD_01ms % 256)
//
void Hardware_Delay_TO (const tLong MS)
{
tLong ms ;
// Configure Timer as a 16-bit timer
TMOD &= OxFO; // Clear all TO bits (T1 left unchanged)
TMOD |= 0x01; // Set required TO bits (T1 left unchanged)
ETO =0; //No interrupts
// Delay value is *approximately* 1 ms per loop
for (ms =0; ms < MS; ms++)
{
TH0 = PREL0AD_01ms_H;
TL0 = PREL0AD_01ms_L;
TF0 =0; // Clear overflow flag
TR0 = 1 ; // Start Timer
while (TF0 ==0); // Loop until Timer overflows
TR0 =0; // Stop Timer
}
}
Creating a hardware timeout involves a simple variation on this technique, and
allows precise timeout delays to be easily generated.
For example, in Section 6.6 we considered the process of reading from an ADC in
a Philips 8Xc552 microcontroller. This was the original, potentially dangerous, code:
// Wait until AD conversion finishes (checking ADCI)
while ((ADC0N & ADCI) == 0);
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 13
138 Embedded C
Here is a solution with a hardware timeout, providing a delay of 10 ms which will, with
reasonable accuracy, apply across the whole 8051 family (without code modifications):
// Configure Timer as a 16-bit timer
TMOD &= OxFO; // Clear all TO bits (T1 left unchanged)
TMOD |= 0x01; // Set required TO bits (T1 left unchanged)
ET0 =0; //No interrupts
// Simple timeout feature - approx 10 ms
TH0 = PREL0AD_10ms_H; // See Timeout. H for PRELOAD details
TL0 = PREL0AD_10ms_L;
TF0 = 0; // Clear flag
TR0 = 1 ; // Start timer
while (((ADC0N & ADCI) == 0) && !TF0) ;
Various portable PREL0AD_ macros suitable for use in this way are given in the file
Ti meout . H reproduced in Listing 6.8 and included on the CD. Note that the same
PREL0AD_ values may be used with either Timer or Timer 1, as required.
Listing 6.8 The file Ti meoutH . H
TimeoutH.H (v1 .00)
Simple timeout delays for the 8051 family based on T0/T1
Timer must be correctly configured to use these values:
See Chapter 6 for details.
#ifndef _TIME0UTH_H
#define _TIME0UTH_H
// Public constants
// Timer T_ values for use in simple (hardware) timeouts
// - Timers are 16-bit, manual reload ('one shot').
//
// NOTE: These macros are portable but timings are *approximate
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 139
Meeting real-time constraints 1 39
//
//
//
and *must* be checked by hand if accurate timing is
required .
// Define initial Timer / Timer 1 values for -50 us delay
#define T_50micros (65536 -(tWord) ( (0SC_FREQ / 26000) /(0SC_PER_INST)))
#define T_50micros_H (T_50micros / 256)
#define T_50micros_L (T_50micros % 256)
// Define initial Timer / Timer 1 values for -500 us delay
#define T_500micros (65536 - (tWord) (0SC_FREQ / (0SC_PER_INST * 2000)))
#define T_500micros_H (T_500micros / 256)
#define T_500micros_L (T_500micros % 256)
// Define
#define T_
#define T
#define T
//
// Define
#define T_
#define T
#define T
II
II Define
#define T_
#define T
#define T
II
II Define
#define T_
#define T
#define T
II
II Define
#define T_
#define T
#define T
II
II Define
initial Timer / Timer 1 values for ~1 msec delay
01ms (65536 - (tWord) (0SC_FREQ / (0SC_PER_INST * 1000)))
_01ms_H (T_01ms / 256)
_01ms_L (T_01ms % 256)
initial Timer / Timer 1 values for ~5 msec delay
05ms (65536 - (tWord) (0SC_FREQ / (0SC_PER_INST * 200)))
_05ms_H (T_05ms / 256)
_05ms_L (T_05ms % 256)
initial Timer / Timer 1 values for -10 msec delay
10ms (65536 - (tWord) (0SC_FREQ / (0SC_PER_INST * 100)))
_10ms_H (T_10ms / 256)
_10ms_L (T_10ms % 256)
initial Timer / Timer 1 values for -15 msec delay
_15ms (65536 - (tWord) (0SC_FREQ / (OSC_PER_INST * 67)))
_15ms_H (T_15ms / 256)
_15ms_L (T_15ms % 256)
initial Timer / Timer 1 values for -20 msec delay
_20ms (65536 - (tWord) (0SC_FREQ / (OSC_PER_INST * 50)))
_20ms_H (T_20ms / 256)
_20ms_L (T_20ms % 256)
initial Timer / Timer 1 values for -50 msec delay
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 140
140 Embedded C
#define T_50ms (65536 - (tWord) (0SC_FREQ / (OSC_PER_INST * 20)))
#define T_50ms_H (T_50ms / 256)
#define T_50ms_L (T_50ms % 256)
#endif
END OF FILE
6.1 1 Example: Testing a hardware timeout
Listing 6.9 shows a simple program for testing hardware-based timeout mechanisms.
Please note that, to avoid undue repetition, only part of this source file is repro-
duced here: the complete file is included on the CD.
Main.C (v1 .00)
Testing hardware timeouts
#include "Main.H"
#include "TimeoutH.H"
// Function prototypes
void Test_50micros(void) ;
void Test_500micros(void) ;
void Test_1ms(void) ;
void Test_5ms(void) ;
void Test_1 0ms (void)
void Test_1 5ms (void)
void Test_20ms(void)
void Test_50ms(void)
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 141
Meeting real-time constraints 141
// TIMEOUT code variable & TIMEOUT code (dummy here)
#define TIMEOUT OxFF
tByte Error_code_G;
/* -
void main (void)
{
whi 1e(1 )
{
Test_50micros() ;
Test_500micros() ;
Test_1ms() ;
Test_5ms() ;
Test_10ms()
Test_15ms()
Test_20ms()
Test_50ms()
}
}
/* */
void Test_50micros(void)
{
// Configure Timer as a 16-bit timer
TMOD &= OxFO; // Clear all TO bits (T1 left unchanged)
TMOD |= 0x01; // Set required TO bits (T1 left unchanged)
ETO = 0;
// No interrupts
// Simple timeout feature - approx 50 us
TH0 = T_50micros_H; // See TimeoutH.H for T_ details
TL0 = T_50micros_L;
TF0 = 0; // Clear flag
TR0 = 1 ; // Start timer
while (!TF0) ;
TR0 = 0;
// Normally need to report timeout TIMEOUTs
// (this test is for demo purposes here)
if (TF0 == 1)
{
:322 Chapter 6 pll3-142 21/2/02 9:58 am Page 142
142 Embedded C
// Operation timed out
Error_code_G = TIMEOUT;
}
// Other functions very similar [OMITTED HERE]
// see CD for details
/
END OF FILE
The output from Listing 6.9 is shown in Figure 6.5.
FIGURE 6.5 Testing hardware-based timeout mechanisms. See text for details
6.12 Conclusions
The delay and timeout considered in this chapter are widely used in embedded
applications.
In Chapter 7, we go on to consider another key software component in many
embedded applications: the operating system.
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 143
chapter
Creating an embedded
operating system
7.1 Introduction
The Super Loop architecture illustrated in Listing 7.1 is used in many embedded
applications and has formed the basis of all of the example code which we have
considered in previous chapters. From the developer's perspective, the main
advantages of this architecture are that it is easy to understand, and that it con-
sumes virtually no system memory or CPU resources.
Despite these advantages, Super Loops are not an appropriate basis for all
embedded applications. A particular limitation with this architecture is that it is
very difficult to execute function X( ) at precise intervals of time: as we will see,
this is a very significant drawback.
Listing 7.1 In this chapter, we will consider an alternative to this simple Super Loop
architecture
void main (void)
{
// Prepare run function X
X_Init() ;
while(1) // 'for ever' (Super Loop)
{
X() ; // Run function X
}
}
143
:322 Chapter 7 pl43-l
21/2/02 10:02 am Page 1-
144 Embedded C
fp
L
FIGURE 7.1 Using an audio museum guide. Please see text for details
For example, consider an embedded application that is now in widespread use:
an audio guide (Figure 7.1). Such applications are used in museums and galleries
throughout the world, in order to describe the numbered exhibits to visitors. The
guide can typically be used in two ways. First, the visitor can select 'auto pilot':
this will then cause the guide to describe a sequence of exhibits in order, in the
following manner:
Item 345 was painted by Selvio Guaranteen early in the 16th century. At this time,
Guaranteen, who is generally known as a member of the Slafordic School, was ...
Now turn to your left, and locate Item 346, a small painting which was until recently
also thought to have been painted by Guarateen but which is now ....
Alternatively, the visitor can use the device in 'manual' mode. In this case, the
user will be free to wander at leisure around the exhibition and, when he or she
finds an item of interest, they will type in the exhibit number on the electronic
guide: they will then hear the relevant commentary.
No matter what technique we use, the basic processing required in this applica-
tion will be the same: we need to generate a long stream of speech from a store of
data in memory. This will typically involve using a digital-to-analog converter to
generate an analog signal at a rate of at least 5000 samples per second (Figure 7.2).
The need to call the same function repeatedly, at precise intervals, is by no
means restricted to this museum system (or similar systems such as MP3 players).
For example, consider a collection of requirements assembled from a range of dif-
ferent embedded projects (in no particular order):
:322 Chapter 7 pl43-l
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Creating an embedded operating system 145
Signal m
level
Time
Signal a
level
(c)
• ••
• •
Time
(b)
Samples = {0.46, 0.42, 0.1 7, 0.04,
0.00, 0.13, 0.21, 0.53
0.84, 0.89, 1 .00, 1 .00,
0.63,0.42,0.42, 0.21,
0.00, 0.11, 0.00,0.42,
0.42, 0.23, 0.46, 0.42,
0.48, 0.52, 0.54, 0.57 }
(a)
FIGURE 7.2 The generation of speech for the museum guide, (a) The raw speech data, stored
in ROM. (b) The result of playing these data through a digital-to-analog converter, generating
one sample every 0.2 ms. (c) The (low-pass) filtered and amplified version of the converter
output signal
• The current speed of the vehicle must be measured at 0.5 second intervals.
• The display must be refreshed 40 times every second.
• The calculated new throttle setting must be applied every 0.5 seconds.
• A time-frequency transform must be performed 20 times every second.
• The engine vibration data must be sampled 1000 times per second.
• The frequency-domain data must be classified 20 times every second.
• The keypad must be scanned every 200 ms.
• The master (control) node must communicate with all other nodes (sensor
nodes and sounder nodes) once per second.
• The new throttle setting must be calculated every 0.5 seconds.
• The sensors must be sampled once per second.
In practice, many embedded systems must be able to support this type of 'periodic
function': that is, activities that are performed repeatedly, every millisecond or
every ten milliseconds. In most cases, the process must take place at precisely the
specified interval if the device is to operate as required. In the case of the museum
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 146
146 Embedded C
guide, for example, imprecise function timing will result in unpleasant distortions
to the frequency components in the signal. In other cases - such as the aircraft
autopilot system discussed in the introduction to Chapter 6 - lack of precise
timing may mean that the system becomes unstable.
We cannot obtain precise timing for such activities using the Super Loop archi-
tecture shown in Listing 7.1. Suppose, for example, that we need to start function
X( ) every 60 ms, and that the function takes 10 ms to complete. Listing 7.2 illus-
trates one way in which we might adapt the code in Listing 7.1 in order to try and
achieve this.
Listing 7.2 Trying to use the Super Loop architecture to execute functions at regular
intervals
void main (void)
{
Init_System() ;
while(1) // 'for ever' (Super Loop)
{
X(); // Call the function (10 ms duration)
Delay_50ms() ; // Delay for 50 ms
}
}
You may recall that we used this basic architecture in our 'Hello, Embedded World'
program in Chapter 3. For the purposes of an introductory example, or in situa-
tions - such as the simple central-heating controller outlined in Chapter 1 - this
approach is often adequate.
However, in the many circumstances where more accurate timing is needed, the
architecture shown in Listing 7.2 will only prove adequate if the following condi-
tions are satisfied:
• We know the precise duration of function X ( ) , and,
• This duration never varies.
In practical applications, determining the precise function duration is rarely
straightforward. Suppose we have a very simple function that does not interact
with the outside world but, instead, performs some internal calculations. Even
under these rather restricted circumstances, changes to compiler optimization set-
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 147
Creating an embedded operating system 147
tings - even changes to an apparently unrelated part of the program - can alter the
speed at which the function executes. This can make fine-tuning the timing very
tedious and error-prone.
The second condition is even more problematic. Often in an embedded system
functions will be required to interact with the outside world in a complex way. In
these circumstances the function duration will vary according to outside activities
in a manner over which the programmer has very little control.
Finally, it should also be noted that the code shown in Listing 7.2 is very ineffi-
cient, because most of the processor time is wasted in a delay loop.
7.2 The basis of a simple embedded OS
To obtain periodic function executions - and avoid wasting processor cycles - we
can use interrupts.
As we saw in Chapter 2, an interrupt is a hardware mechanism used to notify a
processor that an 'event' has taken place. Timer overflows are a particularly effec-
tive and widely-used source of interrupts in embedded applications. For example,
the 8051 microcontrollers discussed in this book all have an on-chip timer which
can be set to generate an interrupt (a 'tick') at regular and precise intervals of, say, 1
millisecond. This interrupt can be used to call an appropriate function periodically.
While the process of handling interrupts may seem rather complicated, creating
interrupt service routines (ISRs) in a high-level language is a straightforward
process, as illustrated in Listing 7.3.
Listing 7.3 The framework of an application using a timer ISR to call functions on a
periodic basis
/ * *
Main.c
Simple timer ISR demonstration program.
*
#include <Reg52.H>
#define INTERRUPT Timer 2 Overflow 5
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148 Embedded C
// Function prototype
// NOTE:
// ISR is not explictly called and does not require a prototype
void Timer_2_Init (void) ;
/
/
void main (void)
{
Timer_2_Init() ; // Set up Timer 2
EA = 1 ;
wh i 1 e ( 1 ) ;
}
// Globally enable interrupts
// An empty Super Loop
void Timer_2_Init (void)
/ Timer 2 is configured as a 16-bit timer,
/ which is automatically reloaded when it overflows
/
/ This code (generic 8051/52) assumes a 12 MHz system osc.
/ The Timer 2 resolution is then 1.000 us
/
/ Reload value is FC18 (hex) = 64536 (decimal)
/ Timer (16-bit) overflows when it reaches 65536 (decimal)
/ Thus, with these setting, timer will overflow every 1 ms
T2C0N = 0x04; // Load Timer 2 control register
// Load Timer 2 high byte
// Load Timer 2 reload capt . reg . high byte
// Load Timer 2 low byte
// Load Timer 2 reload capt. reg. low byte
// Timer 2 interrupt is enabled, and ISR will be called
// whenever the timer overflows - see below.
ET2 = 1 ;
// Start Timer 2 running
TR2 = 1 ;
}
TH2
= OxFC
RCAP2H
= OxFC
TL2
= 0x18
RCAP2L
= 0x18
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 149
Creating an embedded operating system 149
void X(void) interrupt INTERRUPT_Timer_2_0verflow
{
// This ISR is called every 1 ms
// Place required code here...
}
END OF FILE
/
The result of running the program shown in Listing 7.3 in the Keil hardware simu-
lator is shown in Figure 7.3.
r Pi f d f im + t W 4|H^KCT
m 10 *o
4D 50 t*
ftmWc P4-&K-. Bp^iIK. iMltofc X tfaWt
aowxii offlKBcn aawwi tWMJWfsio smjmw
7t aa « iaifc J
ivei.'I.cuvei 3
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P T J Rn 1 ' EMF2
FIGURE 73 The result of running the program shown in Listing 7.3 in the Keil
hardware simulator
Much of Listing 7.3 should be familiar. The code to set up Timer 2 in the function
Ti mer_2_Ini t ( ) is the same as the delay code discussed in Chapter 6, the two
main differences being that, in this case:
1 The timer will generate an interrupt when it overflows, and
2 The timer will be automatically reloaded, and will immediately begin counting
again.
We discuss both of these differences in the following sub-sections.
a) The interrupt service routine (ISR)
The interrupt generated by the overflow of Timer 2, invokes the ISR called, here, X ( ) .
:322 Chapter 7 pl43-l
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150 Embedded C
/
void X(void) interrupt INTERRUPT_Timer_2_0verf low
{
// This ISR is called every 1 ms
// Place required code here...
}
The link between this function and the timer overflow is made using the Keil key-
word i interrupt (included after the function header in the function definition):
void X(void) interrupt INTERRUPT_Timer_2_0verf low
plus the following #def i ne directive:
#define INTERRUPT_Timer_2_0verf low 5
To understand where the '5' comes from, note that the interrupt numbers used in
ISRs directly correspond to the enable bit index of the interrupt source in the 805 1
IE SFR. That is, bit of the IE register will be linked to a function using 'interrupt 0'.
Table 7.1 shows the link between the interrupt sources and the required interrupt
numbers for the original 8051/8052.
TABLE 7.1 8051 interrupt sources. Please note that many 8051s have further interrupt
sources: refer to the manufacturer's documentation for details of the required
interrupt numbers
Interrupt source
Address
IE Index
Power On Reset
0x00
-
External Interrupt
0x03
Timer Overflow
OxOB
1
External Interrupt 1
0x13
2
Timer 1 Overflow
0x1 B
3
UART Receive/Transmit
0x23
4
Timer 2 Overflow
0x2B
5
Overall, the use of interrupts linked to timer overflows is a safe and powerful
technique.
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Creating an embedded operating system 151
b) Automatic timer reloads
In the hardware delay code we considered in Chapter 6, we used a code structure
like this:
// Preload values for 50 ms delay
THO = 0x3C; // Timer initial value (High Byte)
TLO = OxBO; // Timer initial value (Low Byte)
TFO =0; // Clear overflow flag
TR0 = 1 ; // Start timer
while (TFO ==0); // Loop until Timer overflows (TFO == 1)
TR0 =0; // Stop Timer
In this case, we load the counter registers with an appropriate initial value, run the
timer and wait until it overflows: we then stop the timer. This is appropriate
behaviour for a delay function.
For our operating system, we have slightly different requirements:
• We require a series of interrupts, generated for a long period, at a precisely-
determined intervals.
• We would like to generate these interrupts without imposing a significant load
on the CPU.
Timer 2 matches these requirements precisely. 24 When Timer 2 overflows, it is
automatically reloaded, and immediately begins counting again. In this case, the
timer is reloaded using the contents of the 'capture' registers (note that the names
of these registers vary slightly between chip manufacturers):
RCAP2H = OxFC; // Load Timer 2 reload capt . reg . high byte
RCAP2L = 0x18; // Load Timer 2 reload capt. reg. low byte
This automatic reload facility ensures that the timer keeps generating the required
ticks, at precise 1 ms intervals, with very little software load, and without any
intervention from the user's program.
24. As we discussed in Chapter 2, Timer 2 was a component added (by Intel) when the 8052 archi-
tecture was introduced shortly after the launch of the 8051. Most - but not all - current
'8051s' include this component.
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152 Embedded C
7.3 Introducing sEOS
The techniques used in Listing 7.3 can be adapted very easily to create the simple
embedded operating system that we require. For ease of reference, we will refer to
this operating system as 'sEOS' in this book. We examine the operation and use of
sEOS here.
a) Complete code listing
To illustrate the use and operation of sEOS, we will re-implement the following
example using this operating system:
void main (void)
{
Init_System() ;
while(1) // 'for ever' (Super Loop)
{
X(); // Call the function (10 ms duration)
Delay_50ms() ; // Delay for 50 ms
}
}
In this case, the sEOS code required to provide the same behaviour is given (in
part) in Listing 7.4 to Listing 7.6. (As in previous example, all of the files for this
project are included on the CD: to avoid undue repetition, only the key files are
presented here.)
Listing 7.4 Part of a demonstration of sEOS running a dummy task
Main.c (v1 .00)
Demonstration of sEOS running a dummy task.
#include "Main.H"
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Creating an embedded operating system 153
#include "Port.H"
#include "Simple_EOS.H"
#include "X.H"
/
void main(void)
{
// Prepare for dummy task
X_Init() ;
// Set up simple EOS (60 ms tick interval)
sE0S_Init_Timer2(60) ;
while(1) // Super Loop
{
// Enter idle mode to save power
sE0S_Go_To_Sl eep ( ) ;
}
}
/*
END OF FILE
Listing 7.5 Part of a demonstration of sEOS running a dummy task
Simple_EOS.C (v1 .00)
Main file for Simple Embedded Operating System (sEOS) for 8051
Demonstration version with dummy task X().
#include "Main.H"
#include "Simple_EOS.H"
// Header for dummy task
#include "X.H"
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154 Embedded C
sEOS_ISR()
Invoked periodically by Timer 2 overflow:
see sE0S_Init_Timer2() for timing details
void sE0S_ISR() interrupt INTERRUPT_Timer_2_0verf low
{
// Must manually reset the T2 flag
TF2 = 0;
//
USER CODE - Begin
// Call dummy task here
x();
//
}
USER CODE - End
sE0S_Init_Timer2()
Sets up Timer 2 to drive the simple EOS.
Parameter gives tick interval in MILLISECONDS.
Max tick interval is ~60ms (12 MHz oscillator).
Note: Precise tick intervals are only possible with certain
oscillator / tick interval combinations. If timing is
important, you should check the timing calculations manually.
* * /
void sE0S_Init_Timer2 (const tByte TICK_MS)
{
tLong Inc;
tWord Reload_16;
tByte Reload_08H, Reload_08L;
// Timer 2 is configured as a 16-bit timer,
// which is automatically reloaded when it overflows
T2C0N = 0x04; // Load Timer 2 control register
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Creating an embedded operating system 155
// Number of timer increments required (max 65536)
Inc = ((tl_ong)TICK_MS * (0SC_FREQ/1000) ) / (tl_ong)OSC_PER_INST;
// 16-bit reload value
Re1oad_16 = (tWord) (65536UL - Inc);
// 8-bit reload values (High & Low)
Reload_08H = (tByte) (Reload_16 / 256);
Reload_08L = (tByte) (Reload_16 % 256);
// Used for manually checking timing (in simulator)
//P2 = Reload_08H;
//P3 = Reload_08L;
TH2 = Reload_08H; // Load T2 high byte
RCAP2H = Reload_08H; // Load T2 reload capt. reg . high byte
TL2 = Reload_08L; // Load T2 low byte
RCAP2L = Reload_08L; // Load T2 reload capt. reg. low byte
// Timer 2 interrupt is enabled, and ISR will be called
// whenever the timer overflows.
ET2 = 1 ;
// Start Timer 2 running
TR2 = 1 ;
EA
}
= 1;
// Globally enable interrupts
sEOS_Go_To_Sleep()
This operating system enters 'idle mode' between clock ticks
to save power. The next clock tick will return the
processor to the normal operating state.
/
void sEOS_Go_To_Sleep(void)
{
PCON |= 0x01; // Enter idle mode (generic 8051 version)
}
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 156
156 Embedded C
END OF FILE
Listing 7.6 Part of a demonstration of sEOS running a dummy task
X.C (v1 .00)
Dummy task to introduce sEOS.
#include "X.H"
X_Init()
Dummy task init function
void X_Init(void)
{
/ / Dummy task init . . .
}
/*
X()
Dummy task called from sEOS ISR.
void X(void)
{
// Dummy task
}
/* ■
END OF FILE
/
This program uses all of the features discussed in Section 7.2, but arranges the code in
a slightly different way, in order to make it easy to adapt for use in different projects.
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 157
Creating an embedded operating system 157
b) Tasks, functions and scheduling
Before we consider the implementation of this simple embedded operating system,
we should first say something about the terminology used to describe such systems.
In discussions about embedded systems, you will frequently hear and read
about 'task design', 'task execution times' and 'multi-tasking' systems. In this con-
text, the term 'task' is usually used to refer to a function that is executed on a
periodic basis. In the case of sEOS, we are able to control the execution times of a
single task. This process is often referred to as 'scheduling the task'. The task will
be implemented as (or called from) an interrupt service routine: this ISR will, in
turn, be invoked by the overflow of a timer.
Note that the task will often call (other) functions in order to meet the needs of
the application. For example, in the design of a controller for a domestic washing
machine presented in Chapter 8, we will identify one task required by the system,
and ten functions to be called from this task.
c) Setting the tick interval
Let us now consider the operation of sEOS in more detail. One of the key features
of this OS is the way that the tick interval is set. In the function mai n ( ), we can
see that the control of this interval has been largely automated:
// Set up simple EOS (60 ms tick interval)
sE0S_Init_Timer2(60) ;
In this example, a tick interval of 60 ms is used: this means that the ISR (the
'update' function) at the heart of sEOS will be invoked every 60 ms:
/ * *
sE0S_ISR()
Invoked periodically by Timer 2 overflow:
see sE0S_Init_Timer2() for timing details.
void sE0S_ISR() interrupt INTERRUPT_Timer_2_0verf low
{
}
The 'automatic' tick interval control is achieved using the C pre-processor, and the
information included in the project header file (Mai n . H):
:322 Chapter 7 pl43-l
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158 Embedded C
// Oscillator / resonator frequency (in Hz) e.g. (11059200UL)
#define 0SC_FREQ (12000000UL)
// Number of oscillations per instruction (12, etc)
#define OSC_PER_INST (12)
This information is then used to calculate the required timer reload values in
Si mpl e_E0S . C as follows:
// Number of timer increments required (max 65536)
Inc = ((tLong)TICK_MS * (0SC_FREQ/1000) ) / (tl_ong)OSC_PER_INST;
// 16-bit reload value
Reload_16 = (tWord) (65536UL - Inc);
// 8-bit reload values (High & Low)
Reload_08H = (tByte) (Reload_16 / 256);
Reload_08L = (tByte) (Reload_16 % 256);
TH2 = Reload_08H; // Load T2 high byte
RCAP2H = Reload_08H; // Load T2 reload capt. reg. high byte
TL2 = Reload_08L; // Load T2 low byte
RCAP2L = Reload_08L; // Load T2 reload capt. reg. low byte
It is very important to understand that, if using a 12 MHz oscillator, then accurate
timing can usually be obtained over a range of tick intervals from 1 ms to 60 ms
(approximately). However, if using other clock frequencies (such as the popular
11.0592 MHz), 25 precise timing can only be obtained at a much more limited
range of tick intervals. If you are developing an application where precise timing is
required, you must check the timing calculations by hand. This code in
Si mpl e_E0S . C - used in the simulator - can help you check the timing:
// Used for manually checking timing (in simulator)
P2 = Reload_08H;
P3 = Reload_08L;
25. In applications involving the serial interface, 11.0592 MHz is widely used, as we will see in
Chapter 8. This 'odd' frequency is used because it gives rise to acurate baud rate values (e.g.
9600 baud). If you require both accurate baud rates and asccurate EOS timing, use an 11.0592
MHz crystal and a tick rate of 5, 10, 15, ... 60 or 65 ms. Such 'divide by 5' tick rates are precise
with an 11.0592 MHz crystal. (As an exercise, you might like to confirm this for yourself.)
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 159
Creating an embedded operating system 159
d) Saving power
One of the problems noted with many 'Super Loop' applications is that that the
processor wasted a large number of CPU cycles in a delay loop:
void main (void)
{
Init_System() ;
while(1) // 'for ever' (Super Loop)
{
X(); // Perform the task (10 ms duration)
Delay_50ms() ; // Delay for 50 ms
}
}
We noted in Chapter 2 that 8051 devices have an 'idle' mode where power con-
sumption may be reduced by a factor of 10 (approximately). Using sEOS, we can
reduce the power consumption of the application by having the processor enter
idle mode when it finishes executing the ISR (Figure 7.4).
Idle
Idle
Idle
t System 'ticks'
I
♦
♦
Time
FIGURE 7.4 Saving power with sEOS. See text for details
This is achieved through the function sE0S_Go_To_Sl eep ( ) :
sE0S_Go_To_Sleep()
This operating system enters 'idle mode' between clock ticks
to save power. The next clock tick will return the processor
to the normal operating state.
void sE0S_Go_To_Sl eep (void)
{
PC0N |= 0x01; // Enter idle mode (generic 8051 version)
}
:322 Chapter 7 pl43-l
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160 Embedded C
Note that the processor will automatically return to 'Normal' mode when the
timer next overflows (generating an interrupt).
Overall, this application is 'asleep' for 99.9% of the time, meaning that the total
load imposed by this operating system is around 0.1% of the available CPU capac-
ity (Figure 7.5).
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FIGURE 7.5 The sEOS example is in idle mode for 99.9% of the time, meaning that the
total load imposed by this operating system is around 0.1% of the available CPU capacity
[12 MHz/ 12 osc]
If we assume that the system is implemented using an Atmel AT89S53 chip, then -
referring back to Table 2.1 - we can see that the power consumption figures are
as follows:
Device
Normal
Idle
Power Down
Atmel 89S53
11 mA
2 mA
60 uA
In this case, we would expect the average current consumption to be given by:
2x99.9 + 11 xO.1
Average current consumption = = 2.009 mA
100
If the system runs at 5V, the average power consumption will then be: 10.045 mW.
Note that this is the power consumption of the processor itself: if you add LEDs
or other devices to the system, then the power consumption will inevitably
increase. In these circumstances, theoretical calculations will only be a guide, and
will you need to take some measurements from a prototype system to determine
the true figure.
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Creating an embedded operating system 161
e) Using sEOS in your own projects
When using sEOS in your own applications, you will need to include a copy of the
files Si mpl e_E0S . C and Si mpl e_E0S . H in your project: the .C file will then need
to be edited - in the area indicated below - in order to match your requirements:
void sE0S_ISR() interrupt INTERRUPT_Timer_2_0verf low
{
// Must manually reset the T2 flag
TF2 = 0;
//===== USER CODE - Begin ======
// ADD YOUR FUNCTION CALLS HERE
//===== USER CODE - End
}
7.4 Using Timer or Timer 1
The example code above used Timer 2 as the source of ticks for the operating
system. In most cases, this is good choice: however, if Timer 2 is not available on
your 8051 device, or is in use for some other purpose, Timer or Timer 1 can be
used in its place.
Like Timer 2, Timer and Timer 1 also have an auto-reload capability. However,
Timer 2 has this facility when used in 16-bit mode whereas Timer and Timer 1
can only be reloaded automatically when operating in 8-bit mode. In typical 8051
applications, an 8-bit timer can only be used to generate interrupts at intervals of
around 0.25 ms (or less). This can be useful for some applications where very rapid
processing is required. However, in most cases, this short tick interval will simply
increase the processor load imposed by the operating system.
To illustrate the increased load imposed by short tick intervals, Figure 7.6
repeats the example shown in Figure 7.5, this time using a 0.25 ms tick interval.
Note that by using the shorter tick interval, the percentage of 'sleep' time has
fallen from 99.9% to 74.5%: this reflects the fact that the scheduler ISR is called
more frequently.
Although the auto-reload modes of Timer and Timer 1 are less useful than the
equivalent mode in Timer 2, Timer and Timer 1 can also be used in 'manual
reload' mode. Manual-reload mode means that - when it overflows - the timer
must be stopped, loaded with the required count value and then restarted. With
this approach it is almost impossible to guarantee precise tick intervals. However,
the level of timing accuracy obtained is adequate for many applications.
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 162
162 Embedded C
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flflazJj
M5
FIGURE 7.6 The impact of a 0.25ms tick interval [12 Mhz / 12 osc 8051]. See text for details
Listing 7.7 shows the key source code from an operating system based on Timer 0,
with manual reloads. The code may be easily adapted to work with Timer 1 if required.
Listing 7.7 Driving sEOS using Timer 0. See text for details.
Simple_E0S.C (v1 .00)
Main file for Simple Embedded Operating System (sEOS) for 8051
*** This version uses TO (easily adapted for T1 ) ***
Demonstration version with dummy task X()
#include "Main.H"
#include M Simple_E0S. H"
// Header for dummy task
#include "X.H"
// Private variable definitions
static tByte Reload_08H;
static tByte Reload_08L;
// Private function prototypes
static void sE0S_Manual_Timer0_Reload(void) ;
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Creating an embedded operating system 163
sE0S_ISR()
Invoked periodically by Timer overflow:
see sEOS_Init_TimerO() for timing details
void sE0S_ISR() interrupt INTERRUPT_Timer_0_Overf low
{
// Flag cleared automatically but must reload the timer
sEOS_Manual_TimerO_Reload() ;
//===== USER CODE - Begin ==============================
// Call dummy task here
X();
//===== USER CODE - End
}
sEOS_Init_TimerO()
Sets up Timer to drive the simple EOS.
Parameter gives tick interval in MILLISECONDS.
Max tick interval is ~60ms (12 MHz oscillator).
Note: Precise tick intervals are only possible with certain
oscillator / tick interval combinations. If timing is important,
you should check the timing calculations manually.
void sEOS_Init_TimerO (const tByte TICK_MS)
{
tLong Inc;
tWord Reload_16;
// Using Timer 0, 16-bit *** manual reload ***
TMOD &= OxFO; // Clear all TO bits (T1 left unchanged)
TMOD |= 0x01; // Set required TO bits (T1 left unchanged)
// Number of timer increments required (max 65536)
Inc = ((tLong)TICK_MS * (0SC_FREQ/1000) ) / (tLong)0SC_PER_INST;
:322 Chapter 7 pl43-l
21/2/02 10:02 am Page 16.
164 Embedded C
// 16-bit reload value
Reload_16 = (tWord) (65536UL - Inc) ;
// 8-bit reload values (High & Low)
Reload_08H = (tByte) (Reload_16 / 256);
Reload_08L = (tByte) (Reload_16 % 256);
// Used for manually checking timing (in simulator)
//P2 = Reload_08H;
//P3 = Reload_08L;
TLO = Reload_08L;
THO = Reload_08H;
// Timer interrupt is enabled, and ISR will be called
// whenever the timer overflows.
ETO = 1 ;
// Start Timer running
TRO = 1 ;
EA
}
= 1;
// Globally enable interrupts
sEOS_Manual _Ti merO_Rel oad ( )
This OS uses a (manually reloaded) 16-bit timer.
The manual reload means that all timings are approximate
THIS OS IS NOT SUITABLE FOR APPLICATIONS WHERE
ACCURATE TIMING IS REQUIRED!!!
Timer reload is carried out in this function.
voi d sEOS_Manual_Ti merO_Rel oad ( )
{
// Stop Timer
TRO = 0;
// See 'init' function for calculations
TLO = Reload_08L;
THO = Reload_08H;
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 165
Creating an embedded operating system 165
// Start Timer
TRO = 1 ;
}
sEOS_Go_To_Sleep()
This operating system enters 'idle mode' between clock ticks
to save power. The next clock tick will return the processor
to the normal operating state.
/
void sEOS_Go_To_Sleep(void)
{
PCON |= 0x01; // Enter idle mode (generic 8051 version)
}
/*
END OF FILE
Figure 1.1 shows this example running in the simulator.
=- F^rfarrnantt- AruUyri-r
i* iu ,'u w 49 >.:
■■J Hi III Uliv
*EO&_KR
rfQS-HILTi-Bin
rwimp .i^m pjgtn* Mto* * nriirt
nr.ms nutcrr it'vw, -i-f.^tp? hi Sqq
=
sj
FIGURE 7.7 Driving sEOS using Timer
A complete set of files for this project will be found on the CD.
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 166
166 Embedded C
7.5 Is this approach portable?
The presence of an on-chip timer which can be used to generate interrupts in this
way is by no means restricted to the 8051 family: almost all processors intended
for use in embedded applications have timers which can be used in a manner very
similar to that described in this chapter.
For example, similar timers are included on other 8-bit microcontrollers (e.g.
Microchip PIC family, the Motorola HC08 family), and also on 16-bit devices (e.g.
the Infineon CI 6 7 family) as well as on 32-bit processors (e.g. the ARM family, the
Motorola MPC500 family). 26
7.6 Alternative system architectures
The very simple operating system discussed in this chapter has two key features:
• A time-triggered architecture.
• A co-operative scheduling algorithm.
To explain what these phrases mean, and consider some alternative approaches, we
will examine how tasks in this system are started, and how they come to an end.
a) Starting tasks
The 'time-triggered' nature of this system means that functions are started (or
'triggered') at pre-determined points in time.
The main alternative to this architecture is referred to as 'event-triggered'. In
embedded systems, event-triggered behaviour is often achieved through the use of
interrupts. To support these, event-triggered system architectures often provide
multiple interrupt service routines.
To understand the difference between event- and time-triggered architectures,
we will first consider an analogy. Suppose that a hospital doctor must look after
the needs of ten seriously-ill patients overnight, with the support of some nursing
staff. The doctor might consider two ways of performing this task:
• The doctor might arrange for one of the nursing staff to waken her if there is a
significant problem with one of the patients. This is the 'event triggered' solution.
• The doctor might set her alarm clock to ring every hour. When the alarm goes
off, she will get up and visit each of the patients, in turn, to check that they are
well and, if necessary, prescribe treatment. This is the 'time triggered' solution.
26. For sources of further information about the use of the embedded operating system described
here on other processors, please refer to Chapter 11.
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Creating an embedded operating system 167
For most doctors, the event-triggered approach will seem the most attractive,
because they are likely to get a few hours of sleep during the course of the night.
By contrast, with the time-triggered approach, the doctor will inevitably suffer
sleep deprivation.
However, in the case of many embedded systems - which do not need sleep -
the time-triggered approach has many advantages. Indeed, within industrial sec-
tors where safety is an obvious concern, such as the aerospace industry and the
automotive industry, time-triggered techniques are widely used because it is
accepted, both by the system developers (and certification authorities), that they
help improve reliability and safety.
The main reason that time-triggered approaches are preferred in safety-related
applications is that they result in systems which have very predictable behaviour. If
we revisit the hospital analogy, we can begin to see why this is so.
Suppose that our 'event triggered' doctor is sleeping peacefully. An apparently
minor problem develops with one of the patients, and the nursing staff decide not
to awaken the doctor but to deal with the problem themselves. After another two
hours, when four patients have 'minor' problems, the nurses decide that they will
have to wake the doctor after all. As soon as the doctor sees the patients, she rec-
ognizes that two of them have a severe complications, and she has to begin
surgery. Before she can complete the surgery on the first patient, the second
patient is very close to death.
Consider the same example with the 'time triggered' doctor. In this case, because
the patient visits take place at hourly intervals, the doctor sees each patient before
serious complications arise, and arranges appropriate treatment. Another way of
viewing this is that the workload is spread out evenly throughout the night. As
a result, all of the patients survive the night without difficulty.
In embedded applications, the (rather macabre) hospital situation is mirrored in
the event-driven application by the occurrence of several events (that is, several
interrupts) at the same time. This might indicate, for example, that two different
faults had been detected simultaneously in an aircraft, or simply that two switches
had been pressed at the same time on a keypad.
To see why the simultaneous occurrence of two interrupts causes a problem,
consider what happens in the 8051 architecture in these circumstances. Like many
microcontrollers, the original 8051 architecture supports two different interrupt
priority levels: Low and High. If two interrupts (we will call them Interrupt 1 and
Interrupt 2) occur in rapid succession, the system will behave as follows:
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 16
168 Embedded C
• If Interrupt 1 is a low-priority interrupt and Interrupt 2 is a high-priority interrupt.
The Interrupt service routine (ISR) invoked by a low-priority interrupt can be
interrupted by a high-priority interrupt. In this case, the low-priority ISR will be
paused, to allow the high-priority ISR to be executed, after which the operation
of the low-priority ISR will be completed. In most cases, the system will operate
correctly (provided that the two ISRs do not interfere with one another).
• If Interrupt 1 is a low-priority interrupt and Interrupt 2 is also a low-priority
interrupt.
The ISR invoked by a low-priority interrupt cannot be interrupted by another low-
priority interrupt. As a result the response to the second interrupt will be at the
very least delayed; under some circumstances it will be ignored altogether.
• If Interrupt 1 is a high-priority interrupt and Interrupt 2 is a low-priority interrupt.
The interrupt service routine (ISR) invoked by a high-priority interrupt cannot be
interrupted by a low-priority interrupt. As a result the response to the second
interrupt will be at the very least delayed; under some circumstances it will
be ignored altogether.
• If Interrupt 1 is a high-priority interrupt and Interrupt 2 is also a high-priority
interrupt.
The interrupt service routine (ISR) invoked by a high-priority interrupt cannot be
interrupted by another high-priority interrupt. As a result the response to the
second interrupt will be at the very least delayed; under some circum-
stances it will be ignored altogether.
Note carefully what this means! There is a common misconception among the
developers of embedded applications that interrupt events will never be lost.
This simply is not true. If you have multiple sources of interrupts that may
appear at 'random' time intervals, interrupt responses can be missed: indeed,
where there are several active interrupt sources, it is practically impossible to
create code that will deal correctly with all possible combinations of interrupts.
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Creating an embedded operating system 169
It is the need to deal with the simultaneous occurrence of more than one event
that both adds to the system complexity and reduces the ability to predict the
behaviour of an event-triggered system under all circumstances. By contrast, in a
time-triggered embedded application, the designer is able to ensure that only
single events must be handled at a time, in a carefully controlled sequence.
b) Stopping tasks
As we noted at the start of this section, sEOS has a time-triggered, co-operatively
scheduled, architecture. We have discussed the meaning of 'time triggered'. Now
we turn our attention to the co-operative nature of this system.
When we say that an operating system is co-operative, we mean that a task,
once started, will run until it is complete: that is, the OS will never interrupt an
active task. This is a 'single task' approach to operating system design. The alterna-
tive is a 'pre-emptive' or 'time sliced' approach. In a pre-emptive system, tasks will
typically run for - say - a millisecond. The OS will then pause this task, and run
another task for a millisecond, and so on. From the perspective of the user, the
pre-emptive OS appears to be running multiple tasks at the same time.
Co-operative scheduling is simpler and is generally considered to be more pre-
dictable than pre-emptive scheduling. To understand why this is, consider that we
wish to run two tasks on a pre-emptive system, and that both tasks require access
to the same port. Suppose that one task is reading from this port, and that the
scheduler performs a 'context switch', causing the second task to access the same
port: under these circumstances, unless we take action to prevent it, data may be
lost or corrupted.
This problem arises frequently in multi-tasking environments where we have
what are known as 'critical sections' of code. Such critical sections are code
segments that - once started - must run to completion, without interruption.
Examples of critical sections include:
• Code which modifies or reads variables, particularly global variables used for
inter-task communication. In general, this is the most common form of critical
section, since inter-task communication is often a key requirement.
• Code which interfaces to hardware, such as ports, analog-to-digital converters
(ADCs), and so on. What happens, for example, if the same ADC is used simul-
taneously by more than one task?
• Code which calls common functions. What happens, for example, if the same
function is called simultaneously by more than one task?
In a co-operative system, these problems do not arise, since only one task is ever
active at a time.
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170 Embedded C
To deal with critical sections of code in a pre-emptive system, we have two
main possibilities:
• 'Pause' the scheduling by disabling the scheduler interrupt before beginning
the critical section; re-enable the scheduler interrupt when we leave the critical
section, or;
• Use a 'lock' (or some other form of 'semaphore mechanism') to achieve a simi-
lar result.
The first solution means that, when we start accessing the shared resource (say
Port X), we disable the scheduler. This solves the immediate problem since (say)
Task A will be allowed to run without interruption until it has finished with Port
X. However, this 'solution' is less than perfect. For one thing, by disabling the
scheduler, we will no longer be keeping track of the elapsed time and all timing
functions will begin to drift - in this case by a period up to the duration of Task A
every time we access Port X. This simply is not acceptable.
The use of locks is a better solution and appears, at first inspection, easy to
implement. Before entering the critical section of code, we 'lock' the associated
resource; when we have finished with the resource we 'unlock' it. While locked,
no other process may enter the critical section. 27
This is one way we might try to achieve this:
1 Task A checks the 'lock' for Port X it wishes to access.
2 If the section is locked, Task A waits.
3 When the port is unlocked, Task A sets the lock and then uses the port.
4 When Task A has finished with the port, it leaves the critical section and
unlocks the port.
Implementing this algorithm in code also seems straightforward, as illustrated in
Listing 7.8.
27. Of course, this is only a partial solution to the problem caused by multi-tasking. If the purpose
of Task A is to read from an ADC, and Task B has locked the ADC when the Task A is invoked,
then Task A cannot carry out its required activity. Use of locks, or any other mechanism, will
not solve this problem; however, they may prevent the system from crashing.
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Creating an embedded operating system 1 71
Listing 7.8 Attempting to implement a simple locking mechanism in a pre-emptive
scheduler. See text for details
#define UNLOCKED
#define LOCKED 1
bit Lock; // Global lock flag
// . . .
// Ready to enter critical section
// - wait for lock to become clear
// (FOR SIMPLICITY, NO TIMEOUT CAPABILITY IS SHOWN)
while (Lock == LOCKED) ;
// Lock is clear
// Enter critical section <T A
// Set the lock
Lock = LOCKED;
// CRITICAL CODE HERE //
// Ready to leave critical section
// Release the lock
Lock = UNLOCKED;
// . . .
However, the above code cannot be guaranteed to work correctly under all cir-
cumstances.
Consider the part of the code labelled 'A' in Listing 7.8. If our system is fully
pre-emptive, then Task A may be at this point when the scheduler performs a con-
text switch and allows (say) Task B access to the CPU. If Task B also requires access
the Port X, we can then have a situation as follows:
• Task A has checked the lock for Port X and found that the port is not locked;
Task A has, however, not yet changed the lock flag.
• Task B is then 'switched in'. Task B checks the lock flag and it is still clear. Task B
sets the lock flag and begins to use Port X.
• Task A is 'switched in' again. As far as Task A is concerned, the port is not
locked; this task therefore sets the flag, and starts to use the port, unaware that
Task B is already doing so.
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172 Embedded C
As we can see, this simple lock code violates the principal of mutual exclusion: that
is, it allows more than one task to access a critical code section. The problem arises
because it is possible for the context switch to occur after a task has checked the
lock flag but before the task changes the lock flag. In other words, the lock
'check and set code' (designed to control access to a critical section of code), is
itself a critical section.
This problem can be solved. For example, because it takes little time to 'check
and set' the lock code, we can disable interrupts for this period. However, this is
not in itself a complete solution: because there is a chance that an interrupt may
have occurred even in the short period of 'check and set', we then need to check
the relevant interrupt flag(s) and - if necessary - call the relevant ISR(s). This can
be done, but it adds substantially to the complexity of the operating environment.
c) Reliability matters
As we have discussed, the simple, predictable, nature of time-triggered, co-opera-
tively scheduled, applications makes this approach the usual choice in
safety-related applications, where reliability is a crucial design requirement.
However, the need for reliability is not restricted to systems such as fly-by-wire air-
craft and drive-by-wire passenger cars: even at the lowest level, an alarm clock that
fails to sound on time, or a video recorder that operates intermittently may not
have safety implications but, equally, will not have high sales figures.
In addition to increasing reliability, the use of time-triggered techniques
can help to reduce both CPU loads and memory usage: as a result, even the
smallest of embedded applications can benefit from the use of this form of
system architecture.
1.1 Important design considerations when using sEOS
Following on from the discussions in Section 7.6, we consider here two important
factors which must be taken into account when using sEOS in your own projects.
a) Worst-case task execution time
We noted in Section 7.6 that sEOS has a time-triggered architecture. This has impor-
tant implications for the designer of the system. In particular, the designer must ensure
that the execution time for a task can never exceed the tick interval (Figure 7.8).
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Creating an embedded operating system 1 73
Tick interval
Execution time
T System 'ticks'
!
Time
FIGURE 7.8 The operating system described in the chapter will only be reliable if the execution
time of the task (run from the ISR) never exceeds the system tick interval
For example, suppose the following situation applies:
• The operating system has a 10 ms tick interval.
• A task runs for 22 milliseconds.
In these circumstances, at least one 'tick' will be lost and the system may fail to
operate as required (see Figure 7.9).
T System 'ticks'
Time
FIGURE 7.9 Executing a 22 ms task using an OS with a 1 ms tick interval. See text for details
These problems can be solved with due care at the design stage. For example, we
have shown (first in Chapter 3) how simulations can be used to determine task
durations. We have also considered (in Chapter 6) several different timeout mech-
anisms that can help us to meet 'worst-case execution time' conditions.
We make some suggestions for further improvements to this operating system
in Chapter 11: these can help to greatly improve the performance of this system in
the presence of 'long tasks'.
b) The 'One Interrupt per Microcontroller' rule
As we discussed in Section 7.6a, sEOS has a time-triggered architecture. The sEOS
initialization function enables the generation of interrupts associated with the
overflow of one of the microcontroller timers. To ensure correct operation of the
system, it is essential that - with the exception of the single timer interrupt driv-
ing the OS - all interrupts are disabled.
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 17
174 Embedded C
If you fail to do this, then you are trying to operate sEOS as an 'event-triggered'
system (see Section 7.6 for details of the problems this can cause).
We must issue a clear warning:
IF YOU ATTEMPT TO USE THE OS CODE PRESENTED IN THIS CHAPTER WITH
ADDITIONAL INTERRUPTS ENABLED, YOUR SYSTEM CANNOT BE GUARANTEED
TO OPERATE AT ALL : AT BEST, YOU ARE LIKELY TO OBTAIN VERY UNPRE-
DICTABLE - AND UNRELIABLE - SYSTEM BEHAVIOUR.
7.8 Example: Milk pasteurization
We will now consider the use of sEOS in a practical embedded application.
The system discussed in this example is intended to monitor the rate of liquid
(milk) flow through a pasteurization 28 system. The monitoring is required prima-
rily because too high a flow rate can result in incomplete sterilization of the
product, and a consequent reduction in shelf life.
Note that facilities for the measurement of flow rates will probably already be
included as part of the main pasteurization system: our system will reproduce this
behaviour. This form of 'redundancy' is a common requirement in embedded
applications with safety implications. Here we assume that it is unlikely that one
device for measuring flow rates may fail, and very unlikely that both will fail
simultaneously. As a consequence, adding the redundant sensor from this study
will be likely to improve the overall reliability of the system.
a) Measuring the flow rates
To measure the rate of flow, we will be measuring the frequency of a stream of
pulses. This is a common requirement: for example, many industrial systems
require measurement of rotational speed. This is often carried out using a sensor of
the type illustrated in Figure 7.10.
As the figure illustrates, the rotating shaft gives rise to a pulse train.
In the case of the milk production system, we assume that a similar sensor is
used (Figure 7.11). This (optical) sensor will not exhibit 'switch bounce' (see
Chapter 4). However, some older pasteurization systems still employ mechanical
28. The French chemist Louis Pasteur demonstrated that the spoilage of perishable products could
be prevented by destroying the microbes through a heat treatment and then protecting the
(sterilized) material from subsequent contamination. Pasteur applied this theory to the preser-
vation of beverages and foodstuffs. His process is now most widely associated with milk
production.
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 175
Creating an embedded operating system 1 75
switches: these pulse streams will exhibit switch bounce. To make this system as
flexible as possible, we will incorporate debounce behaviour (in software) in our
system. This will have no impact on the measurement of pulses from the optical
sensor, but will allow our system to be applied more widely.
Optical
sensor
en) o jnjnj~LrLn_rL *>
To 'counter' input
Rotating
shaft
FIGURE 7.10 Counting pulses from an optical encoder in order to measure the speed of
rotation of a shaft
Determine flow rate from pulse stream
Milk pasteurization system
FIGURE 7.11 A schematic representation of the milk pasteurization system considered in the
present example
b) Output from the program
The output from this program will take two forms:
• A bargraph display, giving the operator an instant visual representation of the
flow rate.
• An audible alarm, which will sound when the flow rate falls below an accept-
able level.
Please see Section 7.8d for relevant code listings.
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 176
176 Embedded C
c) Running the program
The code for the project is on the CD, in the directory 7Pont/Ch07_05 - Milk'
(please refer to Chapter 3 for further information about the CD).
Figure 7.12 shows the project running in the Keil hardware simulator.
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FIGURE 7.12 Running the milk-flow monitoring system in the Keil hardware simulator
The key files in this project are listed and described below: a complete set of files is
included on the CD.
d) Software
The key files associated with this project are listed here.
When examining this code, please note in particular:
• The efficient code for counting the pulses: no delay code is required.
• The techniques used to generate the bargraph display.
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 177
Creating an embedded operating system 1 77
Listing 7.9 Part of the software for the milk pasteurization system
Port.H (vl.OO)
Port Header file for the milk pasteurization example (Chapter 7)
//
Pulse Count. C
// Connect pulse input to this pin - debounced in software
sbit Sw_pin = P3 A 0;
// Connect alarm to this pin (set if pulse is below threshold)
sbit Alarm_pin = P3 A 7;
//
Bargraph .C
// Bargraph display on these pins
// The 8 port pins may be distributed over several ports if
requi red
sbit PinO = P1 A
sbit Pin1 = P1 A 1
sbit Pin2 = P1 A 2
sbit Pin3 = P1 A 3
sbit Pin4 = P1 A 4
sbit Pin5 = P1 A 5
sbit Pin6 = P1 A 6
sbit Pin7 = P1 A 7
END OF FILE
Listing 7.1 Part of the software for the milk pasteurization system
Main.c (vl.OO)
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178 Embedded C
Milk pasteurization example
#include "Main.H"
#include "Port.H"
#include "Simple_EOS. H"
#include "Bargraph . H"
#include "Pulse Count. H"
/
void main (void)
{
PULSE_COUNT_Init() ;
BARGRAPH_Init() ;
// Set up simple EOS (30ms tick interval)
sE0S_Init_Timer2(30) ;
while(1) // Super Loop
{
// Enter idle mode to save power
sEOS_Go_To_Sleep() ;
}
}
END OF FILE
Listing 7.1 1 Part of the software for the milk pasteurization system
Simple_EOS.C (v1 .00)
Main file for Simple Embedded Operating System (sEOS) for 8051.
-- This version for milk-flow-rate monitoring.
:322 Chapter 7 pl43-l
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Creating an embedded operating system 1 79
#include "Main.H"
#include "Simple_EOS.H"
#include "Pulse count. H"
sE0S_ISR()
Invoked periodically by Timer 2 overflow:
see sE0S_Init_Timer2() for timing details.
void sE0S_ISR() interrupt INTERRUPT_Timer_2_0verf low
{
// Must manually reset the T2 flag
TF2 = 0;
//
USER CODE - Begin
// Call 'Update' function here
PULSE_COUNT_Update() ;
//===== USER CODE - End
}
sE0S_Init_Timer2()
Sets up Timer 2 to drive the simple EOS.
Parameter gives tick interval in MILLISECONDS.
Max tick interval is ~60ms (12 MHz oscillator).
Note: Precise tick intervals are only possible with certain
oscillator / tick interval combinations. If timing is important,
you should check the timing calculations manually.
* * /
void sE0S_Init_Timer2 (const tByte TICK_MS)
{
tLong Inc;
tWord Reload_16;
:322 Chapter 7 pl43-l
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180 Embedded C
tByte Reload_08H, Reload_08L;
// Timer 2 is configured as a 16-bit timer,
// which is automatically reloaded when it overflows
T2C0N = 0x04; // Load Timer 2 control register
// Number of timer increments required (max 65536)
Inc = ((tl_ong)TICK_MS * (0SC_FREQ/1000) ) / (tl_ong)0SC_PER_INST;
// 16-bit reload value
Reload_16 = (tWord) (65536UL - Inc);
// 8-bit reload values (High & Low)
Reload_08H = (tByte) (Reload_16 / 256);
Reload_08L = (tByte) (Reload_16 % 256);
// Used for manually checking timing (in simulator)
//P2 = Reload_08H;
//P3 = Reload_08L;
TH2
Reload,
_08H
RCAP2H =
Reload,
_08H
TL2
Reload,
_08L
RCAP2L =
Reload
08L
// Load T2 high byte
// Load T2 reload capt .
// Load T2 low byte
// Load T2 reload capt.
reg . high byte
reg . low byte
// Timer 2 interrupt is enabled, and ISR will be called
// whenever the timer overflows.
ET2 = 1 ;
// Start Timer 2 running
TR2 = 1 ;
EA
}
= 1;
// Globally enable interrupts
sEOS_Go_To_Sleep()
This operating system enters 'idle mode' between clock ticks
to save power. The next clock tick will return the processor
to the normal operating state.
/
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Creating an embedded operating system 181
void sE0S_Go_To_S1eep(void)
{
PCON |= 0x01; // Enter idle mode (generic 8051 version)
}
/ * *
END OF FILE
Listing 7.1 2 Part of the software for the milk pasteurization system
Pu1se_Count.C (v1 .00)
Count pulses from a mechanical switch or similar device.
Responds to falling edge of pulse
#include "Main.H"
#include "Port.H"
#include "Bargraph.H"
#include "Pulse Count. H"
// Private function prototypes
void PULSE_COUNT_Check_Below_Threshold (const tByte) ;
// Public variable declarations
// The data to be displayed
extern tBargraph Data_G;
// Public variable definitions
// Set only after falling edge is detected
bit Fal 1 ing_edge_G;
// Private variable definitions
// The results of successive tests of the pulse signal
// (NOTE: Can't have arrays of bits...)
static bit Test4, Test3, Test2, Testl , TestO;
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 182
182 Embedded C
static tByte Total_G = 0;
static tWord Calls_G = 0;
//
Private constants
// Allows changed of logic without hardware changes
#define HI_LEVEL (0)
#define L0J.EVEL (1)
/*
PULSE_COUNT_Init()
Initialisation function for the switch library.
void PULSE_COUNT_Init(void)
{
Sw_pin = 1; // Use this pin for input
// The tests (see text)
Test4 = L0_LEVEL
Test3 = L0_LEVEL
Test2 = L0_LEVEL
Testl = L0J.EVEL
TestO = L0_LEVEL
}
PULSE_C0UNT_Check_Bel ow_Threshol d ( )
Checks to see if pulse count is below a specified
threshold value. If it is, sounds an alarm.
void PULSE_COUNT_Check_Below_Threshold (const tByte THRESHOLD)
{
if (Data_G < THRESHOLD)
{
Alarm_pin = 0;
}
else
{
Alarm_pin = 1 ;
:322 Chapter 7 pl43-l
21/2/02 10:02 am Page 183
Creating an embedded operating system 183
PULSE_COUNT_Update()
This is the main switch function.
It should be called every 30 ms
(to allow for typical 20 ms debounce time)
void PULSE_COUNT_Update(void)
{
// Clear timer flag
TF2 = 0;
// Shuffle the test results
Test4 = Test3
Test3 = Test2
Test2 = Testl
Testl = TestO
// Get latest test result
TestO = Sw_pin;
/ Required result:
/ Test4 == HIJ.EVEL
/ Test3 == HIJ.EVEL
/ Testl == L0J.EVEL
/ TestO == L0J.EVEL
if ((Test4 == HI_LEVEL) &&
(Test3 == HI_LEVEL) &&
(Testl == L0_LEVEL) &&
(TestO == L0_LEVEL))
{
// Falling edge detected
Fal 1 ing_edge_G = 1;
}
else
{
// Default
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page U
184 Embedded C
Fal 1 ing_edge_G = 0;
}
/ Calculate average every 45 calls to this task
/ - maximum count over this period is 9 pulses
/ if (++Calls_G < 45)
/ 450 used here for test purposes (in simulator)
/ [Because there is a limit to how fast you can
/ simulate pulses by hand...]
if (++Calls_G < 450)
{
Total _G += (int) Fal 1 ing_edge_G;
}
else
{
// Update the display
Data_G = Total _G; // Max is 9
Total _G = 0;
Calls_G = 0;
PULSE_COUNT_Check_Bel ow_Threshol d (3) ;
BARGRAPH_Update() ;
}
}
END OF FILE
Listing 7.1 3 Part of the software for the milk pasteurization system
Bargraph.h (v1 .00)
- See Bargraph.c for details
#include "Main.h"
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21/2/02 10:02 am Page 185
Creating an embedded operating system 185
//
Public data type declarations
typedef tByte tBargraph;
//
Public function prototypes
void BARGRAPH_Init(void) ;
void BARGRAPH_Update(void) ;
//
Public constants
#define BARGRAPH_MAX (9)
#define BARGRAPH_MIN (0)
/*
END OF FILE
Listing 7.14 Part of the software for the milk pasteurization system
Bargraph .c (v1 .00)
Simple bargraph library
#include "Main.h"
#include "Port.h"
#include "Bargraph . h"
//
Public variable declarations
// The data to be displayed
tBargraph Data_G;
//
Private constants
#define BARGRAPH_ON (1)
#define BARGRAPH_OFF (0)
//
Private variables
// These variables store the thresholds
:322 Chapter 7 pl43-188 21/2/02 10:02 am Page 186
186 Embedded C
// used to update the display
static tBargraph M9_1_G
static tBargraph M9_2_G
static tBargraph M9_3_G
static tBargraph M9_4_G
static tBargraph M9_5_G
static tBargraph M9_6_G
static tBargraph M9_7_G
static tBargraph M9_8_G
/
BARGRAPH_Init()
Prepare for the bargraph display
void BARGRAPH_Init(void)
{
PinO = BARGRAPH_OFF
Pin1 = BARGRAPH_OFF
Pin2 = BARGRAPH_OFF
Pin3 = BARGRAPH_OFF
Pin4 = BARGRAPH_OFF
Pin5 = BARGRAPH_OFF
Pin6 = BARGRAPH_OFF
Pin7 = BARGRAPH OFF
// Use a linear scale to display data
// Remember: *9* possible output states
// - do all calculations ONCE
M9_1_G = (BARGRAPH_MAX - BARGRAPH_MIN) / 9;
M9_2_G
= M9_1_G * 2
M9_3_G
= M9_1_G * 3
M9_4_G
= M9_1_G * 4
M9_5_G
= M9_1_G * 5
M9_6_G
= M9_1_G * 6
M9_7_G
= M9_1_G * 7
M9_8_G
= M9_1_G * 8
}
:322 Chapter 7 pl43-l
21/2/02 10:02 am Page 187
Creating an embedded operating system 187
BARGRAPH_Update()
Update the bargraph display.
void BARGRAPH_Update(void)
{
tBargraph Data = Data_G - BARGRAPH_MIN ;
PinO
=
(Data
>=
M9_1_G)
— —
BARGRAPH_ON)
Pin1
=
[(Data
>=
M9_2_G)
==
BARGRAPH_ON)
Pin2
=
[(Data
>=
M9_3_G)
==
BARGRAPH_ON)
Pin3
=
[(Data
>=
M9_4_G)
==
BARGRAPH_ON)
Pin4
=
[(Data
>=
M9_5_G)
==
BARGRAPH_ON)
Pin5
=
[(Data
>=
M9_6_G)
==
BARGRAPH_ON)
Pin6
= i
[(Data
>=
M9_7_G)
==
BARGRAPH_ON)
Pin7
}
= (
[(Data
>=
M9_8_G)
==
BARGRAPH_ON)
END OF FILE
7.9 Conclusions
The operating system ('sEOS') introduced in this chapter imposes a very low
processor load but is nonetheless flexible and useful.
The simple nature of sEOS also provides other benefits. For example, it means
that developers themselves can, very rapidly, port the OS onto a new microcon-
troller environment. It also means that the architecture may be readily adapted to
meet the needs of a particular application.
However, perhaps the most important side effect of this form of simple OS is that
- unlike a traditional 'real-time operating system/ - it becomes part of the applica-
tion itself (Figure 7.13). In our experience, this tight integration of OS and
application means that developers quickly understand, and claim ownership of,
the OS code. This is important, since it avoids a 'not invented here' or 'blame it on
the OS' philosophy, which can arise when developers must interface their code to
a large and complex real-time operating system, the features and behaviour of
which they may never fully understand.
:322 Chapter 7 pl43-l
21/2/02 10:02 am Page U
188 Embedded C
System hardware
Application software
Traditional "Real-time
Operating System"
System hardware
FIGURE 7.13 A simple OS (of the type discussed in this book) becomes part of the developer's
application, rather than being seen as a 'separate system that has nothing to do with us'
Finally, as we conclude this chapter, we should note that it is possible to create
other, more flexible, operating systems for the 8051 and other processors by build-
ing on the techniques presented here: we consider some of the possibilities in
Chapter 11.
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 189
chapter
Multi-state systems and function
sequences
8.1 Introduction
In Chapter 6, we considered the design of a simple aircraft autopilot system. We
assumed that the pilot would enter the required course heading and that the
autopilot would then monitor the aircraft position and orientation, and - where
necessary - make changes to the rudder, elevator, aileron and engine settings in
order to keep the aircraft following this path.
Like the various embedded systems we considered in Chapter 7, this system
involves periodic function calls. In the case of the autopilot, the various sensor
inputs would be measured at pre-determined intervals (typically every 10 ms). The
autopilot would then calculate the required system outputs, and make appropriate
adjustments to the relevant actuators (Figure 8.1).
As we saw in Chapter 7, the ability to execute tasks on a periodic basis can be
provided very effectively using a simple operating system like sEOS. On its own,
however, periodic execution of functions would not allow us to meet all the
requirements of the autopilot application. In particular, it would not allow us to
execute the sequence of operations that would be required if we wanted to have
the aircraft follow a preset course, carry out an automated landing or even retract
its undercarriage. This requires us to execute a sequence of functions, in a pre-
determined order.
In most cases, it is helpful to model such systems as a series of states: in each
state, we may read system inputs and / or generate appropriate system outputs.
This architecture is common not only in autopilot systems, but also in auto-
matic car wash systems, industrial robots and traffic lights, all the way through to
domestic dishwashers and microwave ovens.
189
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 19
190 Embedded C
Pitch
(rate)
sensor
Roll
(rate)
sensor
Main
pilot
controls
Position
sensors
(GPS)
Yaw (rate)
sensor
Velocity
sensors
(3 axes)
Rudder
Elevator
Aileron
Main engine
(fuel)
controllers
FIGURE 8.1 An overview of a simple autopilot system showing the main sensors to be
monitored (left) and the actuators to be controlled (right)
FIGURE 8.2 A high-level schematic view of a simple autopilot system in use
The common characteristics of such systems are that:
• They involve a series of system states.
• In each state, one or more functions may be called.
• There will be rules defining the transitions between states.
• As the system moves between states, one or more functions may be called.
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 191
Multi-state systems and function sequences 191
The different nature of the rules used to control the transitions between states
allows us to identify two broad categories of multi-state systems:
• Multi-State (Timed)
In a multi-state (timed) system, the transition between states will depend only
on the passage of time.
For example, the system might begin in State A, repeatedly executing
FunctionA(), for ten seconds. It might then move into State B and remain
there for five seconds, repeatedly executing Functi onB ( ) . It might then move
back into State A, ad infinituum.
A basic traffic-light control system might follow this pattern.
• Multi-State (Input/Timed)
This is a more common form of system, in which the transition between states
(and behaviour in each state) will depend both on the passage of time and on
system inputs.
For example, the system might only move between State A and State B if a
particular input is received within X seconds of a system output being generated.
The autopilot system discussed at the start of this chapter might follow
this pattern, as might a control system for a washing machine, or an intruder
alarm system.
For completeness, we will mention one further possibility:
• Multi-State (Input)
This is a comparatively rare form of system, in which the transition between
states (and behaviour in each state) depends only on the system inputs.
For example, the system might only move between State A and State B if a
particular input is received. It will remain indefinitely in State A if this input is
not received.
Such systems have no concept of time, and - therefore - no way of imple-
menting timeout or similar behaviours. We will not consider such systems in
this book.
In this chapter, we will consider how the Multi-State (Time) and Multi-State
(Input/Time) architectures can be implemented in C.
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192 Embedded C
8.2 Implementing a Multi-State (Timed) system
We can describe the time-driven, Multi-State architecture as follows:
• The system will operate in two or more states.
• Each state may be associated with one or more function calls.
• Transitions between states will be controlled by the passage of time.
• Transitions between states may also involve function calls.
Please note that, in order to ease subsequent maintenance tasks, the system states
should not be arbitrarily named, but should - where possible - reflect a physical
state observable by the user and / or developer. For example, a telephone system
with the set of states {'State 1', 'State 2' ', 'State 3'} may prove more difficult to
maintain than one with states ('Charging', 'In use', 'Ringing'}.
Please also note that the system states will usually be represented by means of a
switch statement in the operating system ISR.
We illustrate this architecture in the two examples that follow.
8.3 Example: Traffic light sequencing
Suppose we wish to create a system for driving three traffic light bulbs in a traffic
system for use in Europe. The conventional 'red', 'amber' (orange) and 'green'
bulbs will be used, with European sequencing (Figure 8.3).
Time
FIGURE 8.3 The required light sequence from red, amber and green bulbs in a traffic-light
application (European sequence)
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Multi-state systems and function sequences 193
a) Basic system architecture
A basic version of the traffic-light sequencer requires no inputs from the environ-
ment and will perform well by executing a sequence of pre-determined
manoeuvres. It is a classic example of a Multi-State (Timed) system.
b) The system states
In this case, the various states are easily identified:
• Red
• Red-Amber
• Green
• Amber
In the code, we will represent these states as follows:
// Possible system states
typedef enum {RED, RED_AND_AMBER, GREEN, AMBER} el_ight_State ;
We will store the time to be spent in each state using appropriate constants:
// Times in each of the (four) possible light states
// (Times are in seconds)
//
#define RED_DURATION 20
#define RED_AND_AMBER_DURATION 5
#define GREEN_DURATION 30
#define AMBER_DURATION 5
In this simple case, we do not require function calls from (or between) system states:
the required behaviour will be implemented directly through control of the (three)
port pins which - in the final system - would be connected to appropriate bulbs.
For example:
case RED:
{
Red_light = ON;
Amber_light = OFF;
Green_light = OFF;
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 19.
194 Embedded C
c) Complete implementation
Listing 8.1 to Listing 8.3 show how we can use sEOS to implement the complete
traffic-light sequencer.
Listing 8.1 Part of an example showing traffic-light sequencing using a simple EOS
Main.c (vl.OO)
Traffic light example
*
#include "Main.H"
#include "Port.H"
#indude M Simp1e_E0S. H"
#indude "TJ.ights.H"
/*
void main (void)
{
// Prepare to run traffic sequence
TRAFFIC_LIGHTS_Init(RED) ;
// Set up simple EOS (50 ms ticks)
sE0S_Init_Timer2(50) ;
while(1) // Super Loop
{
// Enter idle mode to save power
sE0S_Go_To_S1 eep ( ) ;
}
}
END OF FILE
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 195
Multi-state systems and function sequences 195
Listing 8.2 Part of an example showing traffic-light sequencing using a simple EOS
T_Lights.H (vl.OO)
- See T_Lights.C for details
#ifndef _T_LIGHTS_H
#define T LIGHTS H
//
Public data type declarations
// Possible system states
typedef enum {RED, RED_AND_AMBER, GREEN, AMBER} eLight_State ;
//
Public function prototypes
void TRAFFIC_LIGHTS_Init (const eLight_State) ;
void TRAFFIC_LIGHTS_Update(void) ;
#endif
END OF FILE
Listing 8.3 Part of an example showing traffic-light sequencing using a simple EOS
T_lights.C (v1 .00)
Traffic light control program (Test Version 1.0)
#include "Main.H"
#include "Port.H"
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196 Embedded C
#include "T_l ights.H"
// Private constants
// Easy to change logic here
#define ON
#define OFF 1
// Times in each of the (four) possible light states
// (Times are in seconds)
#define RED_DURATION 20
#define RED_AND_AMBER_DURATION 5
#define GREEN_DURATION 30
#define AMBER_DURATION 5
// Private variables
// The state of the system
static el_ight_State Light_state_G;
// The time in that state
static tLong Time_in_state;
// Used by sEOS
static tByte Call_count_G = 0;
TRAFFIC_LIGHTS_Init()
Prepare for traffic light activity.
void TRAFFIC_LIGHTS_Init (const el_ight_State START_STATE)
{
Light_state_G = START_STATE; // Decide on initial state
}
TRAFFIC_LIGHTS_Update ( )
Must be called once per second.
voi d TRAFFIC_LIGHTS_Update ( voi d )
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 197
Multi-state systems and function sequences 197
{
switch (Light_state_G)
{
case RED:
{
RecMight = ON;
Amber_light = OFF;
Green_light = OFF;
if (++Time_in_state == RED_DURATION)
{
Light_state_G = RED_AND_AMBER;
Time_in_state = 0;
}
break;
}
case RED_AND_AMBER:
{
RecMight = ON;
Amber_light = ON;
Green_light = OFF;
if (++Time_in_state == RED_AND_AMBER_DURATION)
{
Light_state_G = GREEN;
Time_in_state = 0;
}
break;
}
case GREEN:
{
RecMight = OFF;
AmberMight = OFF;
Green_light = ON;
if (++Time_in_state == GREEN_DURATION)
{
Light_state_G = AMBER;
Time_in_state = 0;
}
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 198
198 Embedded C
break;
}
case AMBER:
{
Red_light = OFF;
Amber_light = ON;
Green_light = OFF;
if (++Time_in_state == AMBER_DURATION)
{
Light_state_G = RED;
Time_in_state = 0;
}
break;
}
}
}
END OF FILE
8.4 Example: Animatronic dinosaur
The scheduling of traffic lights is a common example. In this second example, we
will consider a more complicated system.
The Natural History Museum in London recently installed a robotic dinosaur
among the fossils in their Dinosaur Gallery. 29 This large exhibit models a tyran-
nosaurus rex guarding a recent kill: the robot is large, very loud and moves
quickly. It has proved to be very popular with visitors.
We will consider here the software architecture that could be used in the imple-
mentation of such a system.
a) Basic system architecture
This system requires no inputs from the environment and will perform well by
executing a sequence of pre-determined manoeuvres. It is a good example of a
Multi-State (Timed) system.
29. There is a video showing the dinosaur in action on the museum website: www.nhm.ac.uk
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 199
Multi-state systems and function sequences 199
FIGURE 8.4 This example considers the software architecture required to control an
animatronic dinosaur. See text for details
We will assume that the sequence of manoeuvres will take a few minutes to exe-
cute, and will run approximately every ten minutes.
To implement a complete system, we would probably prepare a number of
slightly different programs, and execute a program 'at random/ . This would make
the display more interesting for people who choose to watch the show more than
once. This option is not implemented in the example code here: however, the
approach used to implement different washing machine programs could be
employed here if you wish to explore this example further (see Section 8.6).
b) The system states
We will assume that the following system states are to be implemented:
• Sleeping:
The dinosaur will be largely motionless, but will be obviously 'breathing'.
Irregular snoring noises, or slight movements during this time will add interest
for the audience.
• Waking:
The dinosaur will begin to wake up. Eyelids will begin to flicker. Breathing will
become more rapid.
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200 Embedded C
• Growling:
Eyes will suddenly open, and the dinosaur will emit a very loud growl. Some
further movement and growling will follow.
• Attacking:
Rapid 'random' movements towards the audience. Lots of noise (you should be
able to hear this from the next floor in the museum).
In the code, we will represent these states using the en urn and typedef keywords,
as follows:
typedef enum {SLEEPING, WAKING, GROWLING, ATTACKING}
eDinosaur_State;
We will also store the time to be spent in each state using appropriate contants:
// Times in each of the (four) possible states
// (Times are in seconds)
#define SLEEPING_DURATION 255
#define WAKING_DURATION 60
#define GR0WLING_DURATI0N 40
#define ATTACKING_DURATI0N 120
Appropriate functions will also be created to implement the key behaviours:
// Private function prototypes
void DINOSAUR_Perform_Sleep_Movements(void) ;
void DINOSAUR_Perform_Waking_Movements(void) ;
void DIN0SAUR_Growl (void) ;
void DINOSAUR_Perform_Attack_Movements(void) ;
We will not be concerned with the implementation of these functions in this
introductory book (Chapter 11 provides suggestions for further reading if you
want to explore this example in more depth).
c) Complete implementation
Listing 8.4 shows how we can use sEOS to implement the complete dinosaur
behaviour.
Note that we assume the dinosaur 'update' function will be called from the
sEOS ISR once per second: refer to the CD for complete code details.
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 201
Multi-state systems and function sequences 201
Listing 8.4 Part of an example showing control of an animatronic dinosaur
using a simple EOS
Dinosaur. C (v1 .00)
Demonstration of multi -state (timed) architecture
Dinosaur control system.
#include "Main.h"
#include "Port.h"
#include "Dinosaur. h"
// Private data type declarations
// Possible system states
typedef
enum {SLEEPING, WAKING, GROWLING, ATTACKING} eDi nosaur_State;
// Private function prototypes
void DINOSAUR_Perform_Sleep_Movements(void) ;
void DINOSAUR_Perform_Waking_Movements(void) ;
void DINOSAUR_Growl (void) ;
void DINOSAUR_Perform_Attack_Movements(void) ;
// Private constants
// Times in each of the (four) possible states
// (Times are in seconds)
#define SLEEPING_DURATION 255
#define WAKING_DURATION 60
#define GR0WLING_DURATI0N 40
#define ATTACKING DURATION 120
// Private variables
// The current state of the system
static eDinosaur_State Dinosaur_state_G;
// The time in the state
static tByte Time_in_state_G;
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 202
202 Embedded C
// Used by sEOS
static tByte Cal l_count_G = 0;
/*
DINOSAUR_Init()
Prepare for the dinosaur activity
void DINOSAUR_Init(void)
{
// Initial dinosaur state
Dinosaur_state_G = SLEEPING;
}
/*
DINOSAUR_Update()
Must be scheduled once per second (from the sEOS ISR)
*
void DINOSAUR_Update(void)
{
switch (Dinosaur_state_G)
{
case SLEEPING:
{
// Call relevant function
DINOSAUR_Perf orm_Sl eep_Movements ( ) ;
if (++Time_in_state_G == SLEEPING_DURATION)
{
Dinosaur_state_G = WAKING;
Time_in_state_G = 0;
}
break;
}
case WAKING:
{
// Call relevant function
DINOSAUR_Perf orm_Waki ng_Movements ( ) ;
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 203
}
Multi-state systems and function sequences 203
if (++Time_in_state_G == WAKING_DURATION)
{
Dinosaur_state_G = GROWLING;
Time_in_state_G = 0;
}
break;
}
case GROWLING:
{
// Call relevant function
DINOSAUR_Growl () ;
if (++Time_in_state_G == GROWLING_DURATION)
{
Dinosaur_state_G = ATTACKING;
Time_in_state_G = 0;
}
break;
}
case ATTACKING:
{
// Call relevant function
DINOSAUR_Perf orm_Attack_Movements ( ) ;
if (++Time_in_state_G == ATTACKING_DURATION)
{
Dinosaur_state_G = SLEEPING;
Time_in_state_G = 0;
}
break;
}
}
/*
voi d DINOSAUR_Perf orm_Sl eep_Movements ( voi d)
{
/ / Demo only. . .
P1 = (tByte) Dinosaur_state_G;
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 2
204 Embedded C
P2 = Time_in_state_G;
}
/* */
void DINOSAUR_Perform_Waking_Movements(void)
{
// Demo only-
Pi = (tByte) Dinosaur_state_G;
P2 = Time_in_state_G;
}
/* */
void DINOSAUR_Growl (void)
{
// Demo only-
Pi = (tByte) Dinosaur_state_G;
P2 = Time_in_state_G;
}
/* */
void DINOSAUR_Perform_Attack_Movements(void)
{
/ / Demo only. . .
P1 = (tByte) Dinosaur_state_G;
P2 = Time_in_state_G;
}
/ * *
END OF FILE
* * /
8.5 Implementing a Multi-State (Input/Timed) system
As we noted in Section 8.1, the general Multi-State software architecture has two
common forms. Here we consider how we can create a Multi-State system driven
both by time and system inputs.
a) Basic design
We can describe the time-and-inout-driven Multi-State architecture as follows:
:322 Chapter 8 pl89-216 21/2/02 10:03 am Page 205
Multi-state systems and function sequences 205
• The system will operate in two or more states.
• Each state may be associated with one or more function calls.
• Transitions between states may be controlled by the passage of time, by system
inputs or by a combination of time and inputs.
• Transitions between states may also involve function calls.
b) Implementing state timeouts
We introduced timeout mechanisms in Chapter 6. As we noted in Chapter 7, such
mechanisms play a key role in applications using sEOS.
When working with Multi-State systems, timeout mechanisms are still impor-
tant: however, in this case we sometimes require what might be described as a
'state timeout' mechanism.
For example, consider the following - informal - system requirements:
• The pump should be run for 10 seconds. If, during this time, no liquid is
detected in the outflow tank, then the pump should be switched off and 'low
water' warning should be sounded. If liquid is detected, the pump should be
run for a further 45 seconds, or until the 'high water' sensor is activated
(whichever is first).
• After the front door is opened, the correct password must be entered on the
control panel within 30 seconds or the alarm will sound.
• The 'down flap' signal will be issued. If, after 5 ms, no flap movement is
detected, it should be concluded that the flap hydraulics are damaged. The
system should then alert the user and enter manual mode.
To meet this type of requirement, we will do two things:
• Keep track of the time in each system state.
• If the time exceeds a pre-determined error value, then we should move to a dif-
ferent state.
We illustrate this procedure in the next example.
8.6 Example: Controller for a washing machine
In this example we consider the design of a controller for a domestic washing
machine (Figure 8.5).
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206 Embedded C
Start
switch
Selector
dial
Water
level
sensor
Temperature
sensor
Washing
machine
controller
Water
valve
Water
heater
Water
pump
Drum
motor
LED
indicators
Detergent
match
FIGURE 8.5 The outline design - in the form of a context diagram (left) - for the control system to be used in a
domestic washing machine (right)
Here is a brief description of the way in which we expect the system to operate:
1 The user selects a wash program (e.g. 'Wool', 'Cotton') on the selector dial.
2 The user presses the 'Start' switch.
3 The door lock is engaged.
4 The water valve is opened to allow water into the wash drum.
5 If the wash program involves detergent, the detergent hatch is opened. When
the detergent has been released, the detergent hatch is closed.
6 When the 'full water level' is sensed, the water valve is closed.
7 If the wash program involves warm water, the water heater is switched on. When
the water reaches the correct temperature, the water heater is switched off.
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Multi-state systems and function sequences 207
8 The washer motor is turned on to rotate the drum. The motor then goes through
a series of movements, both forward and reverse (at various speeds) to wash the
clothes. (The precise set of movements carried out depends on the wash program
that the user has selected.) At the end of the wash cycle, the motor is stopped.
9 The pump is switched on to drain the drum. When the drum is empty, the
pump is switched off.
The description is simplified for the purposes of this example, but it will be ade-
quate for our purposes here.
a) Functions
Based on the above description we will try to identify some of the functions that
will be required to implement this system. A provisional list might be as shown in
Figure 8.6.
•
Read_
_Selector_Dial ()
•
Control.
_Detergent_Hatch ( )
•
Read_
_Start_Switch()
•
Control.
_Door_Lock()
•
Read.
_Water_Level ()
•
Control.
.Motor ()
•
Read.
_Water_Temperature ( )
•
Control.
_Pump()
•
Control.
_Water_Heater()
•
Control.
_Water_Val ve()
FIGURE 8.6 A provisional list of functions that could be used to develop a washing-machine
control system
b) System architecture
The washing machine software will be a based on a 'Multi-State' task. It will be
driven by both elapsed time and system inputs.
c) Code listing
Listings for key files are given in this section.
When reviewing this code, please note the key differences between this archi-
tecture and that of the earlier traffic-light and dinosaur examples. In the earlier
examples, the system was 'blind': it performed a sequence of actions, without ref-
erence to the system environment. In this case, the system is much more
responsive. Typical behaviour involves starting a pump and waiting - for a finite
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208 Embedded C
time - for the drum to fill. If the behaviour has not completed within this period,
then we assume that an error has occurred.
Please note, again, that this behaviour is by no means unique to 'white goods'
(such as washing machines). For example, the sequence of events used to raise the
landing gear in a passenger aircraft will be controlled in a similar manner. In this
case, basic tests (such as 'WoW - 'Weight on Wheels') will be used to determine
whether the aircraft is on the ground or in the air: these tests will be completed
before the operation begins. Feedback from various door and landing-gear sensors
will then be used to ensure that each phase of the manoeuvre completes correctly.
Listing 8.5 Part of the framework for a simple washing machine controller
Washer. C (v1 .00)
Multi -state framework for washing-machine controller
#include "Main.H"
#include "Port.H"
#include "Washer. H"
// Private data type declarations
// Possible system states
typedef enum {INIT, START, FILL_DRUM, HEAT_WATER,
WASH_01 , WASH_02, ERROR} eSystem_state ;
// Private function prototypes
tByte WASHER_Read_Selector_Dial (void) ;
bit WASHER_Read_Start_Swi ten (void) ;
bit WASHER_Read_Water_Level (void) ;
bit WASHER_Read_Water_Temperature(void) ;
void WASHER_Control_Detergent_Hatch(bit) ;
void WASHER_Control_Door_Lock(bit) ;
void WASHER_Control_Motor(bit) ;
void WASHER_Control_Pump(bit) ;
void WASHER_Control_Water_Heater(bit) ;
void WASHER_Control_Water_Valve(bit) ;
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Multi-state systems and function sequences 209
//
Private constants
#define OFF
#define ON 1
#define MAX_FILL_DURATION (tLong) 1000
#define MAX_WATER_HEAT_DURATION (tLong) 1000
#define WASH 01 DURATION 30000
//
Private variables
static eSystem_state System_state_G;
static tWord Time_in_state_G;
static tByte Program_G;
// Ten different programs are supported
// Each one may or may not use detergent
static tByte Detergent_G[10] = {1,1,1,0,0,1,0,1,1,0};
// Each one may or may not use hot water
static tByte Hot_Water_G[10] = {1,1,1,0,0,1,0,1,1,0};
/*
void WASHER_Init(void)
{
System_state_G = INIT;
}
/*
void WASHER_Update(void)
{
// Call once per second
switch (System_state_G)
{
case INIT:
{
// For demo purposes only
Debug_port = (tByte) System_state_G;
// Set up initial state
// Motor is off
WASHER_Control_Motor(0FF) ;
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210 Embedded C
// Pump is off
WASHER_Control_Pump(OFF) ;
// Heater is off
WASHER_Control_Water_Heater(OFF) ;
// Valve is closed
WASHER_Control_Water_Valve(OFF) ;
// Wait (indefinitely) until START is pressed
if (WASHER_Read_Start_Switch() != 1)
{
return ;
}
// Start switch pressed...
// Read the selector dial
Program_G = WASHER_Read_Selector_Dial () ;
/ / Change state
System_state_G = START;
break;
}
case START:
{
// For demo purposes only
Debug_port = (tByte) System_state_G;
// Lock the door
WASHER_Control_Door_Lock(0N) ;
// Start filling the drum
WASHER_Control_Water_Valve(ON) ;
// Release the detergent (if any)
if (Detergent_G[Program_G] == 1)
{
WASHER_Control_Detergent_Hatch (ON) ;
}
/ / Ready to go to next state
System_state_G = FILL_DRUM;
Time_in_state_G = 0;
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Multi-state systems and function sequences 211
break;
}
case FILL_DRUM:
{
// For demo purposes only
Debug_port = (tByte) System_state_G;
// Remain in this state until drum is full
// NOTE: Timeout facility included here
if (++Time_in_state_G >= MAX_FILL_DURATION)
{
// Should have filled the drum by now...
System_state_G = ERROR;
}
// Check the water level
if (WASHER_Read_Water_Level () == 1)
{
// Drum is full
// Does the program require hot water?
if (Hot_Water_G[Program_G] == 1)
{
WASHER_Control_Water_Heater(ON) ;
// Ready to go to next state
System_state_G = HEAT_WATER;
Time_in_state_G = 0;
}
else
{
// Using cold water only
// Ready to go to next state
System_state_G = WASH_01 ;
Time_in_state_G = 0;
}
}
break;
}
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212 Embedded C
case HEAT_WATER:
{
// For demo purposes only
Debug_port = (tByte) System_state_G;
// Remain in this state until water is hot
// NOTE: Timeout facility included here
if (++Time_in_state_G >= MAX_WATER_HEAT_DURATION)
{
// Should have warmed the water by now...
System_state_G = ERROR;
}
// Check the water temperature
if (WASHER_Read_Water_Temperature() == 1)
{
// Water is at required temperature
// Ready to go to next state
System_state_G = WASH_01 ;
Time_in_state_G = 0;
}
break;
}
case WASH_01 :
{
// For demo purposes only
Debug_port = (tByte) System_state_G;
// All wash program involve WASH_01
// Drum is slowly rotated to ensure clothes are fully wet
WASHER_Control_Motor(ON) ;
if (++Time_in_state_G >= WASH_01_DURATI0N)
{
System_state_G = WASH_02;
Time_in_state_G = 0;
}
break;
}
// REMAINING WASH PHASES OMITTED HERE ...
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Multi-state systems and function sequences 213
case WASH_02:
{
// For demo purposes only
Debug_port = (tByte) System_state_G;
break;
}
case ERROR:
{
// For demo purposes only
Debug_port = (tByte) System_state_G;
break;
}
/
tByte WASHER_Read_Selector_Dial (void)
{
// User code here. . .
return 0;
}
/
bi t WASHER_Read_Start_Swi tch ( voi d )
{
// Simplified for demo ...
if (Start_pin == 0)
{
// Start switch pressed
return 1 ;
}
else
{
return 0;
}
}
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214 Embedded C
bit WASHER_Read_Water_Level (void)
{
/ / User code here. . .
return 1 ;
}
/* */
bit WASHER_Read_Water_Temperature(void)
{
/ / User code here. . .
return 1 ;
}
/* */
void WASHER_Control_Detergent_Hatch(bit State)
{
bit Tmp = State;
// User code here. . .
}
/* */
void WASHER_Control_Door_Lock(bit State)
{
bit Tmp = State;
/ / User code here. . .
}
/* */
void WASHER_Control_Motor(bit State)
{
bit Tmp = State;
/ / User code here. . .
}
/* */
void WASHER_Control_Pump(bit State)
{
bit Tmp = State;
/ / User code here. . .
}
/* */
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Multi-state systems and function sequences 215
void WASHER_Control_Water_Heater(bit State)
{
bit Tmp = State;
// User code here. . .
}
/* */
void WASHER_Control_Water_Valve(bit State)
{
bit Tmp = State;
/ / User code here. . .
}
/ * *
END OF FILE
* * /
8.7 Conclusions
This chapter has discussed the implementation of Multi-State (Tmed) and Multi-
State (Input/Timed) systems. Used in conjunction with an operating system like
that presented in Chapter 7, this flexible system architecture is in widespread use
in embedded applications.
In Chapter 9, we move on to look at the use of the 805 l's serial interface.
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:322 Chapter 9 p217-254 21/2/02 10:04 am Page 217
chapter
Using the serial interface
9.1 Introduction
In Chapter 2, we introduced the 8051 microcontroller and saw that it had the fol-
lowing features:
• 32 I/O pins.
• At least two timers.
• An interrupt system.
• A serial interface supporting the RS-232 standard.
In previous chapters, we have used the I/O pins for input and output, and the
timers for generating accurate delays. We have also used the timers - and the inter-
rupt system - to create a simple embedded operating system. In this chapter, we
complete our discussion of 8051 features by examining the serial interface.
9.2 What is RS-232?
In 1997 the Telecommunications Industry Association released what is formally
known as TIA-232 Version F, a serial communication protocol which has been uni-
versally referred to as 'RS-232' since its first 'Recommended Standard' appeared in
the 1960s. Similar standards (V.28) are published by the International
Telecommunications Union (ITU) and by CCITT (The Consultative Committee
International Telegraph and Telephone).
The 'RS-232' standard includes details of:
217
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218 Embedded C
• The protocol to be used for data transmission.
• The voltages to be used on the signal lines.
• The connectors to be used to link equipment together.
Overall, the standard is comprehensive and widely used, at data transfer rates of
up to around 115 or 330 kbits / second (115 / 330 k baud). Data transfer can be
over distances of 15 metres or more.
Note that RS-232 is a peer-to-peer communication standard. Unlike - for exam-
ple - the RS-485 standard, RS-232 is intended to link only two devices together.
9.3 Does RS-232 still matter?
On the desktop, standards such as USB are rapidly replacing RS-232 as 'desirable'
protocols. Nonetheless - for embedded development - RS-232 remains important.
Some important applications of RS-232 in embedded systems include the following:
• To program on-chip flash memory (for testing or low-volume production), sev-
eral 8051 devices use the serial port. This can also be an important means of
performing code upgrades to devices in the field.
• Many developers use RS-232 during system development, in order to communi-
cate between desktop PCs (on which code is developed) and prototype boards
(on which the code is tested). For example, since most embedded devices have
no screen or keyboard available, it is useful to be able to send data to and from
a desktop PC to clarify program operation.
• Many embedded data acquisition and control systems receive instructions from
and / or transfer data to desktop PCs over an RS-232 link.
• Even very recent components, such as Global Positioning System (GPS) sensors,
have RS-232 interfaces.
9.4 The basic RS-232 protocol
RS-232 is a byte-oriented protocol. That is, it is intended to be used to send single
8-bit blocks of data. To transmit a byte of data over an RS-232 link, we generally
encode the information as follows:
• We send a 'Start' bit.
• We send the data (8 bits).
• We send a 'Stop' bit (or bits).
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Using the serial interface 219
We consider each of these stages below.
Quiescent state
When no data are being sent on an RS-232 'transmit' line, the line is held at a
Logic 1 level.
Start bit
To indicate the start of a data transmission we pull the 'transmit' line low.
Data
Data are often encoded in ASCII (American Standard Code for Information
Interchange), in 7-bit form. The bits are sent least-significant bit first. If we are
sending 7-bit data, the 8th data bit is often used as a simple 'parity check bit', in
order to provide a rudimentary error detection facility.
Note that none of the code presented here uses parity bits: we use all 8 bits for
data transfer.
Stop bit(s)
The stop bits consist of a Logic 1 output. These can be 1 or - less commonly - 1.5
or 2 pulses wide.
Note that we will use a single stop bit in all code examples.
9.5 Asynchronous data transmission and baud rates
RS-232 uses an asynchronous protocol. This means that no clock signal is sent with
the data. Instead, both ends of the communication link have an internal clock,
running at the same rate. The data (in the case of RS-232, the 'Start' bit) is then
used to synchronize the clocks, if necessary, to ensure successful data transfer.
RS-232 generally operates at one of a (restricted) range of baud rates. Typically
these are: 75, 110, 300, 1200, 2400, 4800, 9600, 14400, 19200, 28800, 33600,
56000, 115000 and (rarely) 330000 baud. Of these, 9600 baud is a very 'safe'
choice, as it is very widely supported.
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220 Embedded C
9.6 Flow control
RS-232 is often used with some form of flow control. This is a protocol, imple-
mented through software or hardware, that allows a receiver of data to tell the
transmitter to pause the data flow. This might be necessary, for example, if we
were sending data to a PC, and the PC had filled a RAM buffer: the PC would then
tell our embedded application to pause the data transfer until the buffer contents
had been stored on disk.
Although hardware handshaking can be used, this requires extra signal lines.
The most common flow control technique is 'Xon / Xoff control. This requires a
half- or full-duplex communication link, and can operate as follows:
1 Transmitter sends a byte of data.
2 The receiver is able to receive more data: it does nothing.
3 The transmitter sends another byte of data.
4 Steps 1-3 continue until the receiver cannot accept any more data: it then
sends a 'Control s' (Xoff) character back to the transmitter.
5 The transmitter receives the 'Xoff command and pauses the data transmission.
6 When the receiver node is ready for more data, it sends a 'Control q' (Xon)
character to the transmitter.
7 The transmitter resumes the data transmission.
8 The process continues from Step 1.
We illustrate this process in a code library presented in Section 9.12.
9.7 The software architecture
Suppose we wish to transfer data to a PC at a standard 9600 baud; that is, 9600 bits
per second. As we discussed above, transmitting each byte of data, plus stop and
start bits, involves the transmission of 10 bits of information (assuming a single
stop bit is used). As a result, each byte takes approximately 1 ms to transmit.
This has important implications in all applications, not least those using an
embedded operating system like sEOS. If, for example, we wish to send this infor-
mation to the PC:
Current core temperature is 36.678 degrees
1322 Chapter 9 p217-254 21/2/02 10:04 am Page 221
Using the serial interface 221
then the task sending these 42 characters will take more than 40 milliseconds to
complete. This may be an unacceptably long duration if we need to operate with a
high tick rate of, say 10 ms or less (Figure 9.1).
T System 'ticks'
Time
FIGURE 9.1 A schematic representation of the problems caused by sending a long character
string on an embedded system with a simple operating system. In this case, sending the
message takes 32 ms while the OS tick interval is 10 ms. Please refer back to Chapter 7 for
further details
Perhaps the most obvious way of addressing this issue is to increase the baud rate;
however, this is not always possible, and - even with very high baud rates - long
messages or irregular bursts of data can still cause difficulties.
A complete solution involves a change in the system architecture. Rather than
sending all of the data at once, we can simply store the data we want to send to
the PC in a buffer (Figure 9.2). Every ten milliseconds (say) we check the buffer
and send the next character (if there is one ready to send). In this way, all of the
required 43 characters of data will be sent to the PC within 0.5 seconds. This is
often (more than) adequate. However, if necessary, we can reduce this time by
checking the buffer every millisecond. Note that because we do not have to wait
for each character to be sent, the process of sending data from the buffer will be
very fast (typically a fraction of a millisecond).
Current core temperature
is 36.678 degrees
Buffer
All characters
written immediately
to buffer
(very fast operation)
Scheduler sends one
character to PC
every 1 ms
(for example)
FIGURE 9.2 A schematic representation of the software architecture used in the RS-232 library
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222 Embedded C
9.8 Using the on-chip UART for RS-232 communications
Having decided on the basic architecture for the RS-232 library, we need to con-
sider in more detail how the on-chip serial port is used.
This port is full duplex, meaning it can transmit and receive simultaneously. It
is also receive-buffered, meaning it can commence reception of a second byte
before a previously received byte has been read from the receive register.
(However, if the first byte still has not been read by the time reception of the
second byte is complete, one of the bytes will be lost.)
The serial port can operate in four modes (one synchronous mode, three asyn-
chronous modes). In this chapter, we are primarily interested in Mode 1. In this
mode, 10 bits are transmitted (through TxD) or received (through RxD): a start bit
(0), 8 data bits (least-significant bit first), and a stop bit (1).
Note that the serial interface may also provide interrupt requests when transmission
or reception of a byte has been completed. However, for reasons discussed in Chapter
7, none of the 'time-triggered' code used in this chapter will generate interrupts.
a) Serial port registers
The serial port control and status register is the special function register SCON.
This register contains the mode selection bits (and the serial port flags, TI and RI).
SBUF is the receive and transmit buffer for the serial interface. Writing to SBUF
loads the transmit register and initiates transmission. Reading from SBUF accesses
a physically separate receive register.
b) Baud rate generation using Timer 1
There are several different ways to generate the baud rate clock for the serial port
depending on the mode in which it is operating.
As noted above, we are primarily concerned here with the use of the serial port
in Mode 1. In this mode the baud rate is determined by the overflow rate of Timer
1 or Timer 2. For reasons discussed in Chapter 7, we assume that, if Timer 2 is
available, it will be used to drive the operating system. Therefore we focus on the
use of Timer 1 for baud rate generation.
The baud rate is determined by the Timer 1 overflow rate and the value of
SMOD as follows:
_ . t „. A .. 2 M0D * Frequency mcillator
Baud rate (Mode 1) =
32 x Instructions le x (256 - 7H1)
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Using the serial interface 223
Where:
SMOD is the 'double baud rate' bit in the PCON register;
Frequency oscjllator is the oscillator / resonator frequency (in Hz);
Instructions . is the number of machine instructions per oscillator cycle (e.g. 12 or 6)
THI is the reload value for Timer 1
Note that Timer 1 is used in 8-bit auto-reload mode and that interrupt generation
should be disabled.
It is very important to appreciate that it is not generally possible to produce
standard baud rates (e.g. 9600) using Timer 1 (or Timer 2), unless you use an
1 1 .0592 MHz crystal oscillator.
To see why this is so, we will assume SMOD = (it works equally well with SMOD
= 1), that there are 12 instructions per cycle and that we require a baud rate of
9600. The above equation becomes:
11059200
9600 =
32 x 12 x (256- TH1)
This becomes:
11059200 ■ 256 -7H1
9600 x 384
Or:
256 - TH1 = 3
Thus, if we set TH1 to 253 (OxFD) we get a precise 9600 baud rate.
If - for example - we repeat this with a 12 MHz oscillator, we get:
12000000 ■ 256 -7H1
9600 x 384
Or:
256 - TH1 = 3.255208333333
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224 Embedded C
Thus, the required value of TH1 is 252.7447916667.
The nearest integer value is 253. This means that our actual baud rate will be
approximately 10417 baud - more than 8% higher than the required (9600) rate.
Remember: this is an asynchronous protocol, and relies for correct operation
on the fact that both ends of the connection are working at the same baud
rate. In practice, you can work with a difference in baud variations of ±2.5 %.
Despite this margin, it is always good policy to get the baud rate as close
as possible to the standard value because, in the field, there may be significant
temperature variations between the oscillator in the PC and that in the embed-
ded system. This will lead to a 'drift' in baud rates on PC and microcontroller,
even if they were precisely the same to start with: as a result, if the baud rates
were initially mismatched, then communication with the PC may fail completely
during normal use. This type of 'inexplicable fault' has caused many developers
sleepless nights ('I don't understand it! It works fine in all the tests in the lab!').
c) Other techniques for generating standard baud rates
The frequency 11.0592 MHz may be ideal for generating precise baud rates, but it
is not an ideal frequency for driving an embedded operating system of the type
presented in Chapter 7.
One good solution to this problem is to use an 8051 device with a programma-
ble baud-rate generator. This allows you to use an oscillator frequency compatible
with the operating system (e.g. 24 MHz), and generate precise baud rates.
The Infineon C515C and C517A are examples of 8051 devices with programma-
ble baud-rate generators.
9.9 Memory requirements
The buffered architecture discussed in this chapter is an effective way of managing
the transmission of messages and data to a desktop PC.
The biggest problem caused by this approach is the memory load. The root of
this difficulty is that fact that, as we discussed in above, the architecture used (to
transmit data to the PC) involves a buffer to which the user writes as required; this
buffer is then emptied, one character at a time, by a sEOS task. 30
The buffer itself is very simple (in the code we will describe here):
30. Note that while both the 'transmit' and (where available) 'receive' channels have associated
buffers, the transmit buffer is usually larger than the receive buffer, since - in most cases - the
direction of data flow is from the microcontroller to the PC. As a result, we focus on the transmit
buffer here. However, similar concerns - and solutions - apply to the receive buffer in circum-
stances where the main direction of information flow is from the PC to the microcontroller.
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Using the serial interface 225
// The transmit buffer length (MAX IS 127; MIN is 1)
#define PC_LINK_TRAN_BUFFER_LENGTH 100
static tByte Tran_buffer[PC_LINK_TRAN_BUFFER_LENGTH] ;
If you run out of memory, then there are several options:
• You can reduce the buffer size. This may mean that the functions in your appli-
cation must break down the data they send into smaller blocks. Typically, the
main implication is that shorter strings must be used.
• You can increase the baud rate, and adapt the code so that the system sends
more than one byte of data in every time the RS-232 'update' function is called.
• If using on-chip memory only, you can choose an 8051 device with additional on-
chip RAM: for example, Dallas and Infineon produce a number of such devices.
9.10 Example: Displaying elapsed time on a PC
This example illustrates how to link a Standard 8051 device to a PC.
Please note that the crystal used is 11.0592 MHz, for reasons discussed in
Section 9.8.
The software is 'write only': data is transferred from the microcontroller to the
PC but not vice versa. To illustrate this, the software displays elapsed time on the
PC via a terminal emulator program.
In Figure 9.3, we illustrate this software running on the Keil hardware simulator.
E
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FIGURE 9.3 Output from a simple 'output only' PC-link library, running under the Keil
hardware simulator. See text for details
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 226
226 Embedded C
Listing 9.1 Part of the code for a project displaying elapsed time over an RS-232 link
Port.H (v1 .00)
'Port Header' (see Chap 5) for the project TIME (see Chap 9)
#ifndef _P0RT_H
#define PORT H
//
PC o.c
// Pins 3.0 and 3.1 used for RS-232 interface
#endif
END OF FILE
Listing 9.2 Part of the code for a project displaying elapsed time over an RS-232 link
Main.c (v1 .00)
RS-232 (Elapsed Time) example - sEOS
#include "Main.H"
#include "Port.H"
#include "Simple_E0S. H"
#include "PC_0_T1 .h"
#include "Elap_232.h"
/*
void main (void)
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 227
Using the serial interface 227
{
// Set baud rate to 9600
PC_LINK_0_Init_T1 (9600) ;
// Prepare for elapsed time measurement
El apsed_Ti me_RS232_Ini t ( ) ;
// Set up simple EOS (5ms tick)
sE0S_Init_Timer2(5) ;
while(1) // Super Loop
{
sEOS_Go_To_Sleep() ; // Enter idle mode to save power
}
}
*
END OF FILE
Listing 9.3 Part of the code for a project displaying elapsed time over an RS-232 link
Elap_232.C (v1 .00)
Simple library function for keeping track of elapsed time
Demo version to display time on PC screen via RS232 link.
#include "Main.h"
#include "Elap_232.h"
#include "PC O.h"
//
Public variable definitions
tByte Hou_G
tByte Min_G
tByte Sec_G
//
Public variable declarations
// See Char_Map.c
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 22
228 Embedded C
extern const char code CHAR_MAP_G[10] ;
El apsed_Ti me_RS232_Ini t ( )
Init function for simple library displaying elapsed time on PC
via RS-232 link.
voi d El apsed_Ti me_RS232_Ini t ( voi d )
{
Hou_G =
Min_G =
Sec G =
}
El apsed_Ti me_RS232_Update ( )
Function for displaying elapsed time on PC Screen
*** Must be called once per second ***
voi d El apsed_Ti me_RS232_Update ( voi d )
{
char Time_Str[30] = "\rElapsed time: ";
if
( + +
{
Sec
_G == 60 ;
Sec
_G
= 0;
if
( ++
{
Mir
Min_G ==
_G = 0;
60)
if
(++Hou_G
{
Hou_G =
}
0;
}
24)
}
Time_Str[15] = CHAR_MAP_G[Hou_G / 10];
Time_Str[16] = CHAR_MAP_G[Hou_G % 10];
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 229
Using the serial interface 229
Time_Str[18] = CHAR_MAP_G[Min_G / 10];
Time_Str[19] = CHAR_MAP_G[Min_G % 10];
Time_Str[21] = CHAR_MAP_G[Sec_G / 10];
Time_Str[22] = CHAR_MAP_G[Sec_G * 10];
// We don't display seconds in this version.
// We simply use the seconds data to turn on and off the colon
// (between hours and minutes)
if ((Sec_G % 2) == 0)
{
Time_Str[17] = ' : ' ;
Time_Str[20] = ' : ' ;
}
else
{
Time_Str[17] = ' ' ;
Time_Str[20] = * ' ;
}
PC_LINK_0_Write_String_To_Buffer(Time_Str) ;
}
*
END OF FILE
Listing 9.4 Part of the code for a project displaying elapsed time over an RS-232 link
/*
PC_0_T1 .C (v1 .00)
Simple write-only PC link library Version A (generic)
[Sends data to PC - cannot receive data from PC]
Uses the UART, and Pin 3.1 (Tx)
See text for details (Chapter 9)
#include "Main.h"
#include "PC T1 .h"
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 23
230 Embedded C
//
Public variable declarations
extern tByte Out_written_index_G;
extern tByte Out_waiting_index_G;
/*
PC_LINK_0_Init_T1()
This version uses T1 for baud rate generation
Uses 8051 (internal) UART hardware
/
void PC_LINK_0_Init_T1 (const tWord BAUD_RATE)
{
PCON &= 0x7F; // Set SMOD bit to (don't double baud rates)
// Receiver disabled
// 8-bit data, 1 start bit, 1 stop bit, variable baud rate
// (asynchronous)
SCON = 0x42;
TMOD |= 0x20; // T1 in mode 2, 8-bit auto reload
TH1 = (256 - (tByte) ( ( ( (tl_ong)0SC_FREQ / 100) * 3125)
/ ((tLong) BAUD_RATE * 0SC_PER_INST * 1000)));
TL1 = TH1 ;
TR1 = 1 ; // Run the timer
TI = 1 ; // Send first character (dummy)
// Set up the buffers for reading and writing
0ut_written_index_G = 0;
0ut_waiting_index_G = 0;
// Interrupt *N0T* enabled
ES = 0;
}
END OF FILE
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 231
Using the serial interface 231
Listing 9.5 Part of the code for a project displaying elapsed time over an RS-232 link
PC_0.C (v1 .00)
Core files for simple write-only PC link library for 8051
fami ly
[Sends data to PC - cannot receive data from PC]
Uses the UART, and Pin 3.1 (Tx)
See text for details (Chapter 9).
#include "Main.h"
#include "PC_0.h"
#include "Elap_232. h"
//
Public variable definitions
tByte Out_written_index_G; // Index of data that has been sent
tByte 0ut_waiting_index_G; // Index of data not yet sent
//
Private constants
// The transmit buffer length
#define TRAN BUFFER LENGTH 20
//
Private variables
static tByte Tran_buffer [TRAN_BUFFER_LENGTH] ;
static tByte Time_count_G = 0;
PC_LINK_0_Update()
Sends next character from the software transmit buffer
NOTE: Output-only library (Cannot receive chars)
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 232
232 Embedded C
Uses on-chip UART hardware.
void PC_LINK_0_Update(void)
{
// Deal with transmit bytes here
//
// Are there any data ready to send?
if (Out_written_index_G < Out_waiting_index_G)
{
PC_LINK_0_Send_Char (Tran_buf f er [Out_wri tten_i ndex_G] ) ;
Out_wri tten_i ndex_G++ ;
}
else
{
// No data to send - just reset the buffer index
Out_waiting_index_G = 0;
Out_written_index_G = 0;
}
}
PC_LINK_0_Wri te_Stri ng_To_Buf f er ( )
Copies a (null terminated) string to the character buffer.
(The contents of the buffer are then passed over the serial
link)
void PC_LINK_0_Write_String_To_Buffer (const char* const STR_PTR)
{
tByte i = 0;
while (STR_PTR[i] != ' \0')
{
PC_LINK_0_Write_Char_To_Buffer(STR_PTR[i ] ) ;
i++;
}
}
PC_LINK_0_Write_Cnar_To_Buffer()
/
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 233
Using the serial interface 233
Stores a character in the 'write' buffer, ready for
later transmission
void PC_LINK_0_Write_Char_To_Buffer (const char CHARACTER)
{
// Write to the buffer *only* if there is space
// (No error reporting in this simple library...)
if (Out_waiting_index_G < TRAN_BUFFER_LENGTH)
{
Tran_buffer[Out_waiting_index_G] = CHARACTER;
Out_wai ti ng_i ndex_G++ ;
}
}
PC_LINK_0_Send_Char()
Uses on-chip UART hardware
void PC_LINK_0_Send_Cnar (const char CHARACTER)
{
tLong Timeoutl = 0;
if (CHARACTER == '\n')
{
Timeoutl = 0;
while ((++Timeout1) && (TI == 0));
if (Timeoutl == 0)
{
// UART did not respond - error
// No error reporting in this simple library
return ;
}
TI = 0;
SBUF = OxOD; // Output CR
}
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 23
234 Embedded C
Timeoutl = 0;
while ( (++Timeout1 ) && (TI
■= 0));
if (Timeoutl == 0)
{
// UART did not respond - error
// No error reporting in this simple library
return ;
}
TI = 0;
SBUF = CHARACTER;
}
END OF FILE
Listing 9.6 Part of the code for a project displaying elapsed time over an RS-232 link
Simple_EOS.C (v1 .00)
Main file for Simple Embedded Operating System (sEOS) for 8051.
- This version for project TIME (Chap 9)
#include "Main.H"
#include M Simple_E0S. H"
#include M PC_0.H"
#include ,, Elap_232.H"
// Private variable definitions
static tByte Call_count_G = 0;
/
sE0S_ISR()
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 235
Using the serial interface 235
Invoked periodically by Timer 2 overflow:
see sE0S_Init_Timer2() for timing details.
void sE0S_ISR() interrupt INTERRUPT_Timer_2_0verf low
{
// Must manually reset the T2 flag
TF2 =
//===== USER CODE - Begin ========================
// Call RS-232 update function every 5ms
PC_LINK_0_Update() ;
// This ISR is called every 5 ms
// - only want to update time every second
if (++Call_count_G == 200)
{
// Time to update time
Call_count_G = 0;
// Call time update function
Elapsed_Time_RS232_Update() ;
}
//===== USER CODE - End ===================
}
sE0S_Init_Timer2()
Sets up Timer 2 to drive the simple EOS.
Parameter gives tick interval in MILLISECONDS.
Max tick interval is ~60ms (12 MHz oscillator).
Note: Precise tick intervals are only possible with certain
oscillator / tick interval combinations. If timing is important,
you should check the timing calculations manually.
void sE0S_Init_Timer2 (const tByte TICK_MS)
{
tLong Inc;
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 236
236 Embedded C
tWord Reload_16;
tByte Reload_08H, Reload_08L;
// Timer 2 is configured as a 16-bit timer,
// which is automatically reloaded when it overflows
T2C0N = 0x04; // Load Timer 2 control register
// Number of timer increments required (max 65536)
Inc = ((tl_ong)TICK_MS * (0SC_FREQ/1000) ) /
(tl_ong)0SC_PER_INST;
// 16-bit reload value
Reload_16 = (tWord) (65536UL - Inc);
// 8-bit reload values (High & Low)
Reload_08H = (tByte) (Reload_16 / 256);
Reload_08L = (tByte) (Reload_16 % 256);
// Used for manually checking timing (in simulator)
//P2 = Reload_08H;
//P3 = Reload_08L;
TH2 = Reload_08H;
RCAP2H = Reload_08H;
TL2 = Reload_08L;
RCAP2L = Reload_08L;
// Load Timer 2 high byte
// Load Timer 2 reload capt. reg. high
byte
// Load Timer 2 low byte
// Load Timer 2 reload capt. reg. low
byte
// Timer 2 interrupt is enabled, and ISR will be called
// whenever the timer overflows.
ET2 = 1 ;
// Start Timer 2 running
TR2 = 1 ;
EA
}
= 1;
// Globally enable interrupts
sEOS_Go_To_Sleep()
This operating system enters 'idle mode' between clock ticks
to save power. The next clock tick will return the processor
to the normal operating state.
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 237
Using the serial interface 237
* * /
void sEOS_Go_To_Sleep(void)
{
PCON |= 0x01; // Enter idle mode (generic 8051 version)
}
/ * *
END OF FILE
* * /
9.11 The Serial-Menu architecture
In Chapter 4, we discussed the fact that mechanical switches are a very common
part of the user interface for embedded applications. Another common interface
feature is the Serial Menu.
The Serial Menu architecture involves the use of a desktop PC (or similar) on
which a list of menu options will be displayed. This menu will allow the user to
execute code on an embedded system, which may be located some distance away.
The embedded menu architecture is particularly common in data acquisition
and control applications. We give an example of both types of application in the
sections that follow.
9.12 Example: Data acquisition
In this section, we give an example of a simple data-acquisition system with a
Serial Menu architecture.
In this case, using the menu, the user can determine the state of the input pins
on Port 1 or Port 2, as required (Figure 9.4).
The code required to generate this behaviour is given in Listing 9.7 to
Listing 9.11.
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 23
238 Embedded C
a - lind Pi
b - ffesd F?
1 - DUG
nn :
h - ft#4d PJ
b - fi**<1 F?
- : L-
ipz - ho
4ynu :
a - Bwd PJ
b - RhJ PI
r 7 B*:-
Ftp*' FFPFFCPF
Pta.l r pppprrrr
=MUU1 r HLTJ. T I UJL
■no
p -n
i" -
FIGURE 9.4 Running a data-acquisition example in the simulator. See text for details
Listing 9.7 Part of a data-acquisition example.
Main.c (v1 .00)
Data acquisition example
#include "Main.h"
#include "Simple.EOS.H"
#include "PC 10 T1 .h"
/*
/*
*/
*/
void main (void)
{
// Set baud rate to 9600: generic 8051 version
PC_LINK_I0_Init_T1 (9600) ;
// Set up sEOS (5ms tick)
sE0S_Init_Timer2(5) ;
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 239
Using the serial interface 239
while(1) // Super Loop
{
sEOS_Go_To_Sleep() ; // Enter idle mode to save power
}
}
*
END OF FILE
Listing 9.8 Part of a data-acquisition example
Menu_Data.C (v1 .00)
Simple framework for menu-driven data acquisition.
Use ' Hyperterminal ' (under Windows 95, 98, 2000) or similar
terminal emulator program on other operating systems.
Terminal options:
Data bits
= 8
Parity
= None
Stop bits
= 1
Flow control
= Xon / Xoff
#include "Main.H"
#include "Port.H"
#include "Menu_Data.h"
#include "PC 10. h"
// Public variable declarations
// See Char_Map.c
extern const char code CHAR_MAP_G[10] ;
// Private variables -
static bit Fi rst_time_only_G;
/*
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 240
240 Embedded C
MENU_Command_Processor ( )
This function is the main menu 'command processor' function.
Schedule this (say) once every 10 ms (approx.).
void MENU_Command_Processor(void)
{
char Ch ;
if (Fi rst_time_only_G == 0)
{
Fi rst_time_only_G = 1;
MENU_Show_Menu() ;
}
// Check for user inputs
PC_LINK_IO_Update() ;
Ch = PC_LINK_IO_Get_Char_From_Buffer() ;
if (Ch != PC_LINK_I0_N0_CHAR)
{
MENU_Perform_Task(Ch) ;
MENU_Show_Menu() ;
}
}
MENU_Show_Menu()
Display menu options on PC screen (via serial link)
- edit as required to meet the needs of your application
void MENU_Show_Menu(void)
{
PC_LINK_IO_Write_String_To_Buffer( M Menu:\n M ) ;
PC_LINK_IO_Write_String_To_Buffer( M a - Read P1\n");
PC_LINK_IO_Write_String_To_Buffer( M b - Read P2\n\n");
PC_LINK_IO_Write_String_To_Buffer( M ? : ");
}
/
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 241
Using the serial interface 241
MENU_Perform_Task()
Perform the required user task
- edit as required to match the needs of your application.
*
void MENU_Perform_Task(char c)
{
// Echo the menu option
PC_LINK_IO_Write_Char_To_Buffer(c) ;
PC_LINK_IO_Write_Char_To_Buffer( ' \n' ) ;
// Perform the task
switch (c)
{
case 'a' :
case 'A' :
{
Get_Data_From_Port1 () ;
break;
}
case ' b' :
case 'B' :
{
Get_Data_From_Port2() ;
break;
}
}
Get_Data_From_Port1 ()
void Get_Data_From_Port1 (void)
{
tByte Porti = Data_Port1 ;
char String[11] = "\nP1 = XXX\n\n";
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 242
242 Embedded C
String[6] = CHAR_MAP_G[Port1 / 100];
String [7] = CHAR_MAP_G[ (Portl / 10) % 10] ;
String[8] = CHAR_MAP_G[ Portl % 10];
PC_LINK_IO_Write_String_To_Buffer (String) ;
}
Get_Data_From_Port2 ( )
*
void Get_Data_From_Port2(void)
{
tByte Port2 = Data_Port2;
char String[11] = ' \nP2 = XXX\n\n';
String[6] = CHAR_MAP_G[Port2 / 100];
String[7] = CHAR_MAP_G[ (Port2 / 10) % 10];
String[8] = CHAR_MAP_G[Port2 % 10];
PC_LINK_IO_Write_String_To_Buffer (String) ;
}
/*
END OF FILE
Listing 9.9 Part of a data-acquisition example
PC_I0_T1 .C (v1 .00)
PC link library. Bidirectional . T1 used for baud rate generation
Uses the UART, and Pins 3.1 (Tx) and 3.0 (Rx)
See text for details (Chapter 9)
#include "Main.h"
#include "PC 10 T1 .h"
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 243
Using the serial interface 243
//
Public variable declarations
extern tByte In_read_index_G;
extern tByte In_waiting_index_G;
extern tByte Out_written_index_G;
extern tByte Out_waiting_index_G;
/*
PC_LINK_I0_Init_T1 ()
This (generic) version uses T1 for baud rate generation.
void PC_LINK_I0_Init_T1 (const tWord BAUD_RATE)
{
PCON &= 0x7F; // Set SMOD bit to (don't double baud rates)
// Receiver enabled.
// 8-bit data, 1 start bit, 1 stop bit,
// Variable baud rate (asynchronous)
// Receive flag will only be set if a valid stop bit is received
// Set TI (transmit buffer is empty)
SCON = 0x72;
TMOD |= 0x20; // T1 in mode 2, 8-bit auto reload
TH1 = (256 - (tByte) ( ( ( (tl_ong)0SC_FREQ / 100) * 3125)
/ ((tLong) BAUD_RATE * 0SC_PER_INST * 1000)));
TL1 = TH1 ;
TR1 = 1 ; // Run the timer
TI = 1 ; // Send first character (dummy)
// Set up the buffers for reading and writing
In_read_index_G = 0;
In_waiting_index_G = 0;
0ut_written_index_G = 0;
0ut_waiting_index_G = 0;
// Interrupt *N0T* enabled
ES = 0;
}
/ * *
END OF FILE
/
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 2>
244 Embedded C
Listing 9.1 Part of a data-acquisition example
PC_I0.C (v1 .00)
Core files for simple PC link library for 8051 family
Uses the UART, and Pins 3.1 (Tx) and 3.0 (Rx)
See text for details (Chapter 9).
#include "Main.h"
#include "PC 10. h"
//
Public variable definitions
tByte In_read_index_G; // Data in buffer that has been read
tByte In_waiting_index_G; // Data in buffer not yet read
tByte 0ut_written_index_G; // Data in buffer that has been sent
tByte 0ut_waiting_index_G; // Data in buffer not yet sent
//
Private function prototypes
static void PC_LINK_I0_Send_Char (const char);
//
Private constants
// The receive buffer length
#define RECV_BUFFER_LENGTH 8
// The transmit buffer length
#define TRAN_BUFFER_LENGTH 50
#define X0N 0x11
#define X0FF 0x13
// Private variables -
static tByte Recv_buffer [RECV_BUFFER_LENGTH] ;
static tByte Tran_buffer [TRAN_BUFFER_LENGTH] ;
/*
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 245
Using the serial interface 245
PC_LINK_IO_Update()
Checks for character in the UART (hardware) receive buffer
Sends next character from the software transmit buffer
void PC_LINK_IO_Update(void)
{
// Deal with transmit bytes here
// Is there any data ready to send?
if (Out_written_index_G < Out_waiting_index_G)
{
PC_LINK_IO_Send_Char(Tran_buffer[Out_written_index_G]) ;
Out_wri tten_i ndex_G++ ;
}
else
{
// No data to send - just reset the buffer index
Out_waiting_index_G = 0;
Out_written_index_G = 0;
}
// Only dealing with received bytes here
// -> Just check the RI flag
if (RI == 1)
{
// Flag only set when a valid stop bit is received,
// -> data ready to be read into the received buffer
// Want to read into index 0, if old data has been read
// (simple -circular buffer)
if (In_waiting_index_G == In_read_index_G)
{
In_waiting_index_G = 0;
In_read_index_G = 0;
}
// Read the data from UART buffer
Recv_buffer [In_waiting_index_G] = SBUF;
if (In_waiting_index_G < RECV_BUFFER_LENGTH)
{
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 246
246 Embedded C
// Increment without overflowing buffer
In_wai ti ng_i ndex_G++ ;
}
RI = 0; //Clear RT flag
}
PC_LINK_IO_Write_Char_To_Buffer()
Stores a character in the 'write' buffer, ready for
later transmission
void PC_LINK_IO_Write_Char_To_Buffer (const char CHARACTER)
{
// Write to the buffer *only* if there is space
/ / - No error reporting in this simple library...
if (Out_waiting_index_G < TRAN_BUFFER_LENGTH)
{
Tran_buffer[Out_waiting_index_G] = CHARACTER;
Out_wai ti ng_i ndex_G++ ;
}
}
PC_LINK_IO_Write_String_To_Buffer()
Copies a (null terminated) string to the character buffer.
(The contents of the buffer are then passed over the serial link)
STR_PTR - Pointer to the NULL-TERMINATED string.
void PC_LINK_IO_Write_String_To_Buffer (const char* const STR_PTR)
{
tByte i = 0;
while (STR_PTR[i] != *\0')
{
PC_LINK_IO_Write_Char_To_Buffer(STR_PTR[i ] ) ;
i++;
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 247
Using the serial interface 247
}
}
PC_LINK_IO_Get_Char_From_Buffer()
Retrieves a character from the (software) buffer, if available
The character from the buffer is returned, or - if no
data are available - PC LINK 10 NO CHAR is returned.
char PC_LINK_IO_Get_Char_From_Buffer(void)
{
char Ch = PC_LINK_IO_NO_CHAR;
// If there is new data in the buffer
if (In_read_index_G < In_waiting_index_G)
{
Ch = Recv_buffer [In_read_index_G] ;
if (In_read_index_G < RECV_BUFFER_LENGTH)
{
In_read_i ndex_G++ ;
}
}
return Ch ;
}
PC_LINK_IO_Send_Char()
Based on Keil sample code, with added (loop) timeouts
Implements Xon / Off control .
Uses on-chip UART hardware.
void PC_LINK_IO_Send_Char (const char CHARACTER)
{
tLong Timeoutl = 0;
tLong Timeout2 = 0;
if (CHARACTER == ' \n')
{
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 2>
248 Embedded C
if (RI)
{
if (SBUF == XOFF)
{
Timeout2 = 0;
do {
RI = 0;
// Wait for uart (with simple timeout)
Timeoutl = 0;
while ((++Timeout1) && (RI == 0));
if (Timeoutl == 0)
{
// UART did not respond - error
// No error reporting in this simple library
return ;
}
} while ((++Timeout2) && (SBUF != XON));
if (Timeout2 == 0)
{
// UART did not respond - error
// No error reporting in this simple library...
return;
}
RI = 0;
}
}
Timeoutl = 0;
while ((++Timeout1) && (TI == 0));
if (Timeoutl == 0)
{
// UART did not respond - error
// No error reporting in this simple library
return ;
}
TI = 0;
SBUF = OxOD; // Output CR
}
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 249
Using the serial interface 249
if (RI)
{
if (SBUF == XOFF)
{
Timeout2 = 0;
do {
RI = 0;
// Wait for UART (with simple timeout)
Timeoutl = 0;
while ( (++Timeout1) && (RI == 0));
if (Timeoutl == 0)
{
// UART did not respond - error
// No error reporting in this simple library
return;
}
} while ((++Timeout2) && (SBUF != XON));
RI = 0;
}
}
Timeoutl = 0;
while ((++Timeout1) && (TI == 0));
if (Timeoutl == 0)
{
// UART did not respond - error
// No error reporting in this simple library
return ;
}
TI = 0;
SBUF = CHARACTER;
}
END OF FILE
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 250
250 Embedded C
Listing 9.1 1 Part of a data-acquisition example
Simple_EOS.C (v1 .00)
Main file for Simple Embedded Operating System (sEOS) for 8051.
- This version for project DATA_ACQ (Chapter 9).
#include "Main.H"
#include "Simple_EOS. H"
#include "Menu Data.H"
sE0S_ISR()
Invoked periodically by Timer 2 overflow:
see sE0S_Init_Timer2() for timing details
void sE0S_ISR() interrupt INTERRUPT_Timer_2_0verf low
{
// Must manually reset the T2 flag
TF2 = 0;
//===== USER CODE - Begin ========================
// Call MENU_Command_Processor every 5ms
MENU_Command_Processor() ;
//
USER CODE - End
sE0S_Init_Timer2()
Sets up Timer 2 to drive the simple EOS.
Parameter gives tick interval in MILLISECONDS.
Max tick interval is ~60ms (12 MHz oscillator)
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 251
Using the serial interface 251
Note: Precise tick intervals are only possible with certain
oscillator / tick interval combinations. If timing is important,
you should check the timing calculations manually.
void sE0S_Init_Timer2 (const tByte TICK_MS)
{
tLong Inc;
tWord Reload_16;
tByte Reload_08H, Reload_08L;
// Timer 2 is configured as a 16-bit timer,
// which is automatically reloaded when it overflows
T2C0N = 0x04; // Load Timer 2 control register
// Number of timer increments required (max 65536)
Inc = ((tl_ong)TICK_MS * (0SC_FREQ/1000) ) / (tl_ong)0SC_PER_INST;
// 16-bit reload value
Reload_16 = (tWord) (65536UL - Inc);
// 8-bit reload values (High & Low)
Reload_08H = (tByte) (Reload_16 / 256);
Reload_08L = (tByte) (Reload_16 % 256);
// Used for manually checking timing (in simulator)
//P2 = Reload_08H;
//P3 = Reload_08L;
TH2 = Reload_08H; // Load T2 high byte
RCAP2H= Reload_08H; // Load T2 reload capt . reg . high byte
TL2 = Reload_08L; // Load T2 low byte
RCAP2L= Reload_08L; // Load T2 reload capt. reg. low byte
// Timer 2 interrupt is enabled, and ISR will be called
// whenever the timer overflows.
ET2 = 1 ;
// Start Timer 2 running
TR2 = 1 ;
EA
}
= 1;
// Globally enable interrupts
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 252
252 Embedded C
sEOS_Go_To_Sleep()
This operating system enters 'idle mode' between clock ticks
to save power. The next clock tick will return the processor
to the normal operating state.
/
void sEOS_Go_To_Sleep(void)
{
PCON |= 0x01; // Enter idle mode (generic 8051 version)
}
/*
END OF FILE
9.13 Example: Remote-control robot
In this example, we will use a Serial Menu architecture to control a remote robot.
Such devices are used for inspecting the inside of gas pipes and for landmine
detection or bomb disposal.
In our simplified example, the robot is power by two stepper motors (Figure 9.5).
Such motors rotate a fixed amount (typically 7.5°) in response to an applied voltage
Front roller
(not driven)
©
Rear wheel
driven by stepper
motor
'RS-232"
control link
FIGURE 9.5 A schematic representation of the remote-control robot
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 253
Using the serial interface 253
pulse. In this case, the two motors are driven by pulses from pins on Port 1. By
'stepping' one or both motors, we can control the direction of movement.
Output from the system running in the simulator is shown in Figure 9.6.
Complete code for this example is given on the CD.
FIGURE 9.6 Interface to a remotely-controlled robot, running in the simulator
9.14 Conclusions
In this chapter, we have examined how the serial interface on the 8051 microcon-
troller may be used in practical embedded systems involving data transfers to (or
from) a desktop PC, or similar device.
In the next chapter a case study is presented. This study illustrates how the key
techniques discussed throughout this book may be used, in combination, in order
to create substantial and reliable systems.
:322 Chapter 9 p217-254 21/2/02 10:04 am Page 25.
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 255
chapter
Case study:
Intruder alarm system
10.1 Introduction
In this case study, we will consider the design and implementation of a small
intruder alarm system suitable for detecting attempted thefts in a home or busi-
ness environment.
As an example of a typical application of this type of system, Figure 10.1 shows
a small art gallery containing three statues.
Door
Window
Statue
FIGURE 10.1 A small art gallery containing three statues: we will consider the design of an
intruder alarm system suitable for use in this environment
255
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 256
256 Embedded C
Figure 10.2 shows the same gallery with the alarm system installed. In this figure,
each of the windows has a sensor to detect class breakage. A magnetic sensor is
also attached to the door. In each case, the sensors appear to be simple switches as
far as the alarm system is concerned. Figure 10.2 also shows a 'bell box' outside
the property: this will sound if an intruder is detected.
Bell box
Door
Window
W
Control
panel
W
Statue
W
w
w
FIGURE 10.2 The art gallery with alarm system installed. See text for details
Inside the door (in Figure 10.2), we have the alarm control panel: this consists
mainly of a small keypad, plus an additional 'buzzer' to indicate that the alarm
has sounded (Figure 10.3). The alarm system is designed in such a way that the
user - having set the alarm by entering a four-digit password - has time to open
the door and leave the room before the monitoring process starts. Similarly, if the
user opens the door when the system is armed, he or she will have time to enter
the password before the alarm begins to sound.
000
000
000
000
o o
o o
o
o o
o o
FIGURE 10.3 The main control panel for the alarm system
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 257
Case study: Intruder alarm system 257
Overall, the system is to operate as follows:
• When initially activated, the system is in ' Disarmed ' state.
• In Disarmed state, the sensors are ignored. The alarm does not sound. The
system remains in this state until the user enters a valid password via the
keypad (in our demonstration system, the password is '1234'). When a valid
password is entered, the systems enters ' Arming ' state.
• In Arming state, the system waits for 60 seconds, to allow the user to leave the
area before the monitoring process begins. After 60 seconds, the system enters
'Armed' state.
• In Armed state, the status of the various system sensors is monitored. If a
window sensor is tripped, 31 the system enters ' Intruder ' state. If the door sensor
is tripped, the system enters ' Disarming ' state. The keypad activity is also moni-
tored: if a correct password is typed in, the system enters ' Disarmed ' state.
• In Disarming state, we assume that the door has been opened by someone who
may be an authorized system user. The system remains in this state for up to 60
seconds, after which - by default - it enters Intruder state. If, during the 60-
second period, the user enters the correct password, the system enters
' Disarmed ' state.
• In Intruder state, an alarm will sound. The alarm will keep sounding (indefi-
nitely), until the correct password is entered.
Overall, our demonstration system is somewhat simplified, but the overall system
architecture is correct, and the code may be easily extended to add additional features.
10.2 The software architecture
As the description above makes clear, this system translates naturally into a multi-
state task, of the type discussed in Chapter 8.
10.3 Key software components used in this example
This case study uses the following software components:
• Software to control external port pins (to activate the external bell), as intro-
duced in Chapter 3.
31. In our demonstration system, only a single window sensor is mentioned.
;322 Chapter 10 p255-284 21/2/02 10:04 am Page 25
258 Embedded C
Switch reading, as discussed in Chapter 4, to process the inputs from the door
and window sensors. Note that - in this simple example (intended for use in
the simulator) - no switch debouncing is carried out. This feature can be added,
if required, without difficulty.
The embedded operating system, sEOS, introduced in Chapter 7.
A simple 'keypad' library, based on a bank of switches. Note that - to simplify
the use of the keypad library in the simulator - we have assumed the presence
of only eight keys in the example program (0 - 7). This final system would
probably use at least 10 keys (see Figure 10.3): support for additional keys can
be easily added if required.
The RS-232 library (from Chapter 9) is used to illustrate the operation of the
program. This library would not be necessary in the final system (but it might
be useful to retain it, to support system maintenance).
10.4 Running the program
Figure 10.4 shows this system in operation in the Keil simulator.
■?*r^H
- E K
121*
n
-Pwl
Fl:|w^
FPFFFFFP
ft*|K r
FPFFPFFP
d
-inl *l Far 7
aag. . .
hntd
Zanr npta
l'LHj-mj tg
t'i Eirul
_
Pan .7
F
7 =*i
FPFFPFFP
■inll
FIGURE 10.4 The intruder alarm running in the simulator
10.5 The software
A full listing of all the files associated with this project are given in this section.
These files are also included on the CD.
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 259
Case study: Intruder alarm system 259
Listing 1 0.1 Part of the intruder-alarm code (the Project Header file)
Main.H (vl.OO)
'Project Header' (see Chap 5)
#ifndef
#def ine
//
MAIN_H
MAIN H
// WILL NEED TO EDIT THIS SECTION FOR EVERY PROJECT
//
// Must include the appropriate microcontroller header file here
#include <reg52.h>
// Oscillator / resonator frequency (in Hz) e.g. (11059200UL)
#define 0SC_FREQ (11059200UL)
// Number of oscillations per instruction (12, etc)
// 12 - Original 8051 / 8052 and numerous modern versions
// 6 - Various Infineon and Philips devices, etc.
// 4 - Dallas 320, 520 etc.
// 1 - Dallas 420, etc.
#define OSC_PER_INST (12)
//
// SHOULD NOT NEED TO EDIT THE SECTIONS BELOW
//
// Typedefs (see Chap 5)
typedef unsigned char tByte;
typedef unsigned int tWord ;
typedef unsigned long tLong;
// Interrupts (see Chap 7)
#define INTERRUPT_Timer_0_Overf low 1
#define INTERRUPT_Timer_1 .Overflow 3
#define INTERRUPT Timer 2 Overflow 5
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 260
260 Embedded C
#endif
END OF FILE
Listing 1 0.2 Part of the intruder-alarm code (the Port Header file)
Port.H (v1 .00)
'Port Header' (see Chap 5) for project INTRUDER (see Chap 10)
//
Keypad .C
#define KEYPAD PORT P2
sbit
K0
=
KEYPAD.
_P0RT A
sbit
K1
=
KEYPAD,
_P0RT A 1
sbit
K2
=
KEYPAD.
_P0RT A 2
sbit
K3
=
KEYPAD,
_P0RT A 3
sbit
K4
=
KEYPAD.
_P0RT A 4
sbit
K5
=
KEYPAD.
_P0RT A 5
sbit
K6
=
KEYPAD.
_P0RT A 6
sbit
/ / _
K7
—
KEYPAD.
TntruHc
_P0RT A 7
sbit Sensor_pin = P1 A 0;
sbit Sounder_pin = P1 A 7;
// Lnk O.C
// Pins 3.0 and 3.1 used for RS-232 interface
END OF FILE
;322 Chapter 10 p255-284 21/2/02 10:04 am Page 26:
Case study: Intruder alarm system 261
Listing 10.3 Part of the intruder-alarm code (Main.C)
Main.c (vl.OO)
Simple intruder alarm system
#indude "Main.H"
#indude "Port.H"
#indude "Simp1e_E0S.H"
#indude "PC_0_T1 .h"
#include " Key pad. h"
#indude "Intruder, h"
/*
void main(void)
{
// Set baud rate to 9600
PC_LINK_0_Init_T1 (9600) ;
// Prepare the keypad
KEYPAD_Init() ;
// Prepare the intruder alarm
INTRUDER_Init() ;
// Set up simple EOS (5ms tick)
sE0S_Init_Timer2(5) ;
while(1) // Super Loop
{
sE0S_Go_To_S1eep() ; // Enter idle mode to save power
}
}
/*
END OF FILE ■
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 262
262 Embedded C
Listing 1 0.4 Part of the intruder-alarm code (Intruder.H)
Intruder. H (v1 .00)
- See Intruder. C for details.
#include "Main.H"
//
Public function prototypes
void INTRUDER_Init(void) ;
void INTRUDER_Update(void) ;
/*
END OF FILE
Listing 1 0.5 Part of the intruder-alarm code (Intruder.C)
Intruder. C (v1 .00)
Mul ti -state framework for intruder alarm system
#include "Main.H"
#include "Port.H"
#include "Intruder.H"
#include " Key pad. h"
#include "PC O.h"
//
Private data type declarations
// Possible system states
typedef
enum
{DISARMED, ARMING, ARMED, DISARMING, INTRUDER} eSystem_state;
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 263
Case study: Intruder alarm system 263
// Private function prototypes --
bit INTRUDER_Get_Password_G(void) ;
bit INTRUDER_Check_Window_Sensors(void) ;
bit INTRUDER_Check_Door_Sensor(void) ;
void INTRUDER_Sound_Al arm (void) ;
// Private variables
static eSystem_state System_state_G;
tWord State_cal l_count_G;
char Input_G[4] = { 'X' , 'X' , 'X' , 'X' } ;
char Password_G[4] = {'1','2','3','4'};
tByte Position_G;
bit New_state_G = 0;
bit Alarm_bit = 0;
/*
void INTRUDER_Init(void)
{
// Set the initial system state (DISARMED)
System_state_G = DISARMED;
// Set the 'time in state' variable to
State_cal l_count_G = 0;
// Clear the keypad buffer
KEYPAD_Clear_Buffer() ;
// Set the 'New state' flag
New_state_G = 1 ;
// Set the (two) sensor pins to 'read' mode
Window_sensor_pin = 1;
Sounder_pin = 1 ;
}
/*
void INTRUDER_Update(void)
{
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 264
264 Embedded C
// Incremented every time
if (State_call_count_G < 65534)
{
State_cal l_count_G++ ;
}
// Call every 50 ms
switch (System_state_G)
{
case DISARMED:
{
if (New_state_G)
{
PC_LINK_0_Wri te_St ri ng_To_Buf f er ( " \ nDi sarmed " ) ;
New_state_G = 0;
}
// Make sure alarm is switched off
Sounder_pin = 1 ;
// Wait for correct password ...
if (INTRUDER_Get_Password_G() == 1)
{
System_state_G = ARMING;
New_state_G = 1 ;
State_cal l_count_G = 0;
break;
}
break;
}
case ARMING:
{
if (New_state_G)
{
PC_LINK_0_Write_String_To_Buffer( M \nArming. . .") ;
New_state_G = 0;
}
// Remain here for 60 seconds (50 ms tick assumed)
if (++State_call_count_G > 1200)
{
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Case study: Intruder alarm system 265
System_state_G = ARMED;
New_state_G = 1 ;
State_cal l_count_G = 0;
break;
}
break;
}
case ARMED:
{
if (New_state_G)
{
PC_LINK_0_Write_String_To_Buffer( n \nArmed") ;
New_state_G = 0;
}
// First, check the window sensors
if (INTRUDER_Check_Window_Sensors() == 1)
{
// An intruder detected
System_state_G = INTRUDER;
New_state_G = 1 ;
State_cal l_count_G = 0;
break;
}
// Next, check the door sensors
if (INTRUDER_Check_Door_Sensor() == 1)
{
// May be authorised user - go to 'Disarming' state
System_state_G = DISARMING;
New_state_G = 1 ;
State_cal l_count_G = 0;
break;
}
// Finally, check for correct password
if (INTRUDER_Get_Password_G() == 1)
{
System_state_G = DISARMED;
New_state_G = 1 ;
State_cal l_count_G = 0;
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 266
266 Embedded C
break;
}
break;
}
case DISARMING:
{
if (New_state_G)
{
PC_LINK_0_Write_String_To_Buffer( M \nDisarming. . .");
New_state_G = 0;
}
// Remain here for 60 seconds (50 ms tick assumed)
// to allow user to enter the password
// - after time up, sound alarm
if (++State_call_count_G > 1200)
{
System_state_G = INTRUDER;
New_state_G = 1 ;
State_cal l_count_G = 0;
break;
}
// Still need to check the window sensors
if (INTRUDER_Check_Window_Sensors() == 1)
{
// An intruder detected
System_state_G = INTRUDER;
New_state_G = 1 ;
State_cal l_count_G = 0;
break;
}
// Finally, check for correct password
if (INTRUDER_Get_Password_G() == 1)
{
System_state_G = DISARMED;
New_state_G = 1 ;
State_cal l_count_G = 0;
break;
}
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 267
Case study: Intruder alarm system 267
break;
}
case INTRUDER:
{
if (New_state_G)
{
PC_LINK_0_Write_String_To_Buffer("\n** INTRUDER! **");
New_state_G = 0;
}
// Sound the alarm!
INTRUDER_Sound_Alarm() ;
// Keep sounding alarm until we get correct password
if (INTRUDER_Get_Password_G() == 1)
{
System_state_G = DISARMED;
New_state_G = 1 ;
State_cal l_count_G = 0;
}
break;
}
/
bit INTRUDER_Get_Password_G(void)
{
signed char Key;
tByte Password_G_count = 0;
tByte i ;
// Update the keypad buffer
KEYPAD_Update() ;
// Are there any new data in the keypad buffer?
if (KEYPAD_Get_Data_From_Buffer(&Key) == 0)
{
// No new data - password can't be correct
return 0;
}
;322 Chapter 10 p255-284 21/2/02 10:04 am Page 26
268 Embedded C
// If we are here, a key has been pressed
// How long since last key was pressed?
// Must be pressed within 50 seconds (assume 50 ms 'tick')
if (State_call_count_G > 1000)
{
// More than 50 seconds since last key
// - restart the input process
State_call_count_G = 0;
Position_G = 0;
}
if (Position_G == 0)
{
PC_LINK_0_Write_Cnar_To_Buffer( ' \n' ) ;
}
PC_LINK_0_Write_Cnar_To_Buffer(Key) ;
Input_G[Position_G] = Key;
// Have we got four numbers?
if ( (++Position_G) == 4)
{
Position_G = 0;
Password_G_count = 0;
// Check the password
for (i = 0; i < 4; i++)
{
if (Input_G[i] == Password_G[i] )
{
Password_G_count++ ;
}
}
}
if (Password_G_count ==
{
// Password correct
return 1 ;
}
else
4)
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 269
Case study: Intruder alarm system 269
{
// Password NOT correct
return 0;
}
/
bit INTRUDER_Check_Window_Sensors(void)
{
// Just a single window 'sensor' here
// - easily extended
if (Window_sensor_pin == 0)
{
// Intruder detected...
PC_LINK_0_Write_String_To_Buffer("\nWindow damaged") ;
return 1 ;
}
// Default
return 0;
}
/
bit INTRUDER_Check_Door_Sensor(void)
{
// Single door sensor (access route)
if (Door_sensor_pin == 0)
{
// Someone has opened the door...
PC_LINK_0_Write_String_To_Buffer( ' \nDoor open') ;
return 1 ;
}
// Default
return 0;
}
/
void INTRUDER_Sound_Al arm (void)
{
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 270
270 Embedded C
if (Alarm_bit)
{
// Alarm connected to this pin
Sounder_pin = 0;
Alarm_bit = 0;
}
else
{
Sounder_pin = 1 ;
Alarm_bit = 1 ;
}
}
END OF FILE
Listing 10.6 Part of the intruder-alarm code (Keypad. H)
Keypad. h (v1.00)
#ifndef _KEY_H
#define _KEY_H
// Public function prototypes
void KEYPAD_Init(void) ;
void KEYPAD_Update(void) ;
bit KEYPAD_Get_Data_From_Buffer(char* const);
void KEYPAD_Clear_Buffer(void) ;
#endif
END OF FILE
;322 Chapter 10 p255-284 21/2/02 10:04 am Page 27
Case study: Intruder alarm system 271
Listing 10.7 Part of the intruder-alarm code (Keypad. C)
/*
Keypad. C (v1 .00)
Simple library, for a switch-based "keypad".
#include "Main.h"
#include "Port.h"
#include "Keypad. h"
// Private function prototypes
bit KEYPAD_Scan(char* const);
// Private constants
#define KEYPAD RECV BUFFER LENGTH 6
// Any valid character will do - must not match anything on keypad
#define KEYPAD_NO_NEW_DATA (char)
// Private variables
static char KEYPAD_recv_buf fer [KEYPAD_RECV_BUFFER_LENGTH+1 ] ;
// Data in buffer that has been read
static tByte KEYPAD_in_read_i ndex ;
// Data in buffer not yet read
static tByte KEYPAD_in_waiting_index;
static char Last_val id_key_G = KEYPAD_NO_NEW_DATA;
static data char 01d_key_G;
KEYPAD_Init()
Init the keypad.
void KEYPAD_Init(void)
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 272
272 Embedded C
{
KEYPAD_in_read_index = 0;
KEYPAD_in_waiting_index = 0;
}
KEYPAD_Update()
The main 'update' function for the keypad library.
Must call this function approx every 50 - 200 ms.
*
void KEYPAD_Update(void)
{
char Key;
// Scan keypad here...
if (KEYPAD_Scan(&Key) == 0)
{
// No new key data - just return
return;
}
// Want to read into index 0, if old data has been read
// (simple -circular buffer)
if (KEYPAD_in_waiting_index == KEYPAD_i n_read_i ndex)
{
KEYPAD_in_waiting_index = 0;
KEYPAD_in_read_index = 0;
}
// Load keypad data into buffer
KEYPAD_recv_buf f er [KEYPAD_i n_wai ti ng_i ndex] = Key ;
if (KEYPAD_in_waiting_index < KEYPAD_RECV_BUFFER_LENGTH)
{
// Increment without overflowing buffer
KEYPAD_i n_wai ti ng_i ndex++ ;
}
}
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 273
bit KEYPAD_Get_Data_From_Buffer(char* const pKey)
{
// If there are new data in the buffer
if (KEYPAD_in_read_index < KEYPAD_i n_wai ti ng_i ndex)
{
*pKey = KEYPAD_recv_buffer[KEYPAD_in_read_index] ;
KEYPAD_i n_read_i ndex++ ;
return 1 ;
}
return 0;
}
KEYPAD_C1 ear_Buf f er ( )
*
void KEYPAD_Clear_Buffer(void)
{
KEYPAD_in_waiting_index = 0;
KEYPAD_in_read_index = 0;
}
/*
KEYPAD_Scan ( )
This function is called from scheduled keypad function
Must be edited as required to match your key labels.
Adapt as required!
*
bit KEYPAD_Scan(char* const pKey)
{
Case study: Intruder alarm system 273
KEYPAD_Get_Char_From_Buf f er ( )
The Update function copies data into the keypad buffer.
This function extracts data from the buffer.
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 274
274 Embedded C
char Key = KEYPAD_NO_NEW_DATA;
if
(KO == 0)
{ Key
if
(K1 == 0)
{ Key
if
(K2 == 0)
{ Key
if
(K3 == 0)
{ Key
if
(K4 == 0)
{ Key
if
(K5 == 0)
{ Key
if
(K6 == 0)
{ Key
if
(K7 == 0)
{ Key
'0'
'1 '
*2*
'3'
'4'
'5'
'6'
'7'
if (Key == KEYPAD_NO_NEW_DATA)
{
// No key pressed
01d_key_G = KEYPAD_NO_NEW_DATA;
Last_valid_key_G = KEYPAD_NO_NEW_DATA;
return 0; // No new data
}
// A key has been pressed: debounce by checking twice
if (Key == 01d_key_G)
{
// A valid (debounced) key press has been detected
// Must be a new key to be valid
if (Key != Last_val id_key_G)
{
/ / New key !
*pKey = Key;
Last_val id_key_G = Key;
- no 'auto repeat'
return 1 ;
}
}
// No new data
01d_key_G = Key;
return 0;
}
END OF FILE
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 275
Case study: Intruder alarm system 275
Listing 1 0.8 Part of the intruder-alarm code (PC_0_T1 .H)
PC_0_T1 .h (v1 .00)
- see PC T1.c for details.
#include "PC O.h"
//
Public function prototypes
void PC_LINK_0_Init_T1 (const tWord) ;
END OF FILE
Listing 1 0.9 Part of the intruder-alarm code (PC_0_T1 .C)
PC_0_T1 .C (v1 .00)
Simple write-only PC link library Version A (generic)
[Sends data to PC - cannot receive data from PC]
Uses the UART, and Pin 3.1 (Tx)
See text for details (Chapter 9)
#include "Main.h"
#include "PC T1 .h"
//
Public variable declarations
extern tByte 0ut_written_index_G;
extern tByte 0ut_waiting_index_G;
/*
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 276
276 Embedded C
PC_LINK_0_Init_T1 ()
This version uses T1 for baud rate generation.
Uses 8051 (internal) UART hardware
void PC_LINK_0_Init_T1 (const tWord BAUD_RATE)
{
PCON &= 0x7F; // Set SMOD bit to (don't double baud
rates)
// Receiver disabled
// 8-bit data, 1 start bit, 1 stop bit, variable baud rate
(asynchronous)
SCON = 0x42;
TMOD |= 0x20; // T1 in mode 2, 8-bit auto reload
TH1 = (256 - (tByte) ( ( ( (tl_ong)0SC_FREQ / 100) * 3125)
/ ((tLong) BAUD_RATE * OSC_PER_INST * 1000)));
TL1 = TH1 ;
TR1 = 1 ; // Run the timer
TI = 1 ; // Send first character (dummy)
// Set up the buffers for reading and writing
Out_written_index_G = 0;
Out_waiting_index_G = 0;
// Interrupt *N0T* enabled
ES = 0;
}
/ * *
END OF FILE -
* * /
Listing 1 0.1 Part of the intruder-alarm code (PC_O.H)
PC_0.h (v1.00)
- see PC O.h for details.
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 277
Case study: Intruder alarm system 277
II
Public function prototypes
void PC_LINK_0_Write_String_To_Buffer (const char* const);
void PC_LINK_0_Write_Char_To_Buffer (const char);
void PC_LINK_0_Send_Char (const char);
void PC_LINK_0_Update(void) ;
/*
END OF FILE
Listing 10.1 1 Part of the intruder-alarm code (PC_O.C)
PC_0.C (v1 .00)
Core files for simple write-only PC link library for 8051 family
[Sends data to PC - cannot receive data from PC]
Uses the UART, and Pin 3.1 (Tx)
See text for details (Chapter 9).
#include "Main.h"
#include "PC_0.h H
#include "Elap_232.h"
//
Public variable definitions
tByte 0ut_written_index_G; // Data in buffer that has been written
tByte 0ut_waiting_index_G; // Data in buffer not yet written
//
Private function prototypes
static void PC_LINK_0_Send_Char (const char);
//
Private constants
// The transmit buffer length
#define TRAN BUFFER LENGTH 20
;322 Chapter 10 p255-284 21/2/02 10:04 am Page 27
278 Embedded C
//
Private variables
static tByte Tran_buffer[TRAN_BUFFER_LENGTH] ;
static tByte Time_count_G = 0;
PC_LINK_0_Update()
Sends next character from the software transmit buffer
NOTE: Output-only library (Cannot receive chars)
Uses on-chip UART hardware.
void PC_LINK_0_Update(void)
{
// Deal with transmit bytes here
//
// Are there any data ready to send?
if (Out_written_index_G < Out_waiting_index_G)
{
PC_LINK_0_Send_Char (Tran_buf f er [Out_wri tten_i ndex_G] ) ;
Out_wri tten_i ndex_G++ ;
}
else
{
// No data to send - just reset the buffer index
Out_waiting_index_G = 0;
Out_written_index_G = 0;
}
PC_LINK_0_Wri te_Stri ng_To_Buf f er ( )
Copies a (null terminated) string to the character buffer.
(The contents of the buffer are then passed over the serial link)
/
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 279
Case study: Intruder alarm system 279
void PC_LINK_0_Write_String_To_Buffer (const char* const STR_PTR)
{
tByte i = 0;
while (STR_PTR[i] != '\0')
{
PC_LINK_0_Write_Char_To_Buffer(STR_PTR[i ] ) ;
i++;
}
}
PC_LINK_0_Write_Char_To_Buffer()
Stores a character in the 'write' buffer, ready for
later transmission
void PC_LINK_0_Write_Char_To_Buffer (const char CHARACTER)
{
// Write to the buffer *only* if there is space
// (No error reporting in this simple library...)
if (Out_waiting_index_G < TRAN_BUFFER_LENGTH)
{
Tran_buffer[Out_waiting_index_G] = CHARACTER;
Out_wai ti ng_i ndex_G++ ;
}
}
PC_LINK_0_Send_Char()
Based on Keil sample code, with added (loop) timeouts
Implements Xon / Off control .
Uses on-chip UART hardware.
void PC_LINK_0_Send_Cnar (const char CHARACTER)
{
tLong Timeoutl = 0;
if (CHARACTER == '\n')
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 280
280 Embedded C
{
Timeoutl = 0;
while ((++Timeout1) && (TI =■
0));
if (Timeoutl == 0)
{
// UART did not respond - error
// No error reporting in this simple library
return ;
}
TI = 0;
SBUF = OxOD; // Output CR
}
Timeoutl = 0;
while ((++Timeout1) && (TI == 0));
if (Timeoutl == 0)
{
// UART did not respond - error
// No error reporting in this simple library...
return ;
}
TI = 0;
SBUF = CHARACTER;
}
END OF FILE
Listing 1 0.1 2 Part of the intruder-alarm code (Simple_EOS.H)
Simple_E0S.H (v1 .00)
- see Simple_E0S.C for details.
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 2
Case study: Intruder alarm system 281
void sE0S_Init_Timer2 (const tByte TICK_MS) ;
void sEOS_Go_To_Sleep(void) ;
/*
END OF FILE
Listing 10.1 3 Part of the intruder-alarm code (Simple_EOS.C)
Simp1e_E0S.C (v1 .00)
Main file for Simple Embedded Operating System (sEOS) for 8051.
Part of intruder alarm case study (Chapter 10).
#include "Main.H"
#include "Simple_EOS.H"
#include "PC_0.H"
#include "Intruder. H"
// Private variable definitions
static tByte Cal l_count_G;
/
sE0S_ISR()
Invoked periodically by Timer 2 overflow:
see sE0S_Init_Timer2() for timing details.
void sE0S_ISR() interrupt INTERRUPT_Timer_2_0verf low
{
// Must manually reset the T2 flag
TF2 = 0;
//===== USER CODE - Begin ========================
// Call RS-232 update function every 5ms
PC_LINK_0_Update() ;
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 282
282 Embedded C
// This ISR is called every 5 ms
// - only want to update intruder every 50 ms
if (++Call_count_G == 10)
{
// Time to update intruder alarm
Call_count_G = 0;
// Call intruder update function
INTRUDER_Update() ;
}
//===== USER CODE - End ======================
}
sE0S_Init_Timer2()
Sets up Timer 2 to drive the simple EOS.
Parameter gives tick interval in MILLISECONDS.
Max tick interval is ~60ms (12 MHz oscillator).
Note: Precise tick intervals are only possible with certain
oscillator / tick interval combinations. If timing is important,
you should check the timing calculations manually.
void sE0S_Init_Timer2 (const tByte TICK_MS)
{
tLong Inc;
tWord Reload_16;
tByte Reload_08H, Reload_08L;
// Timer 2 is configured as a 16-bit timer,
// which is automatically reloaded when it overflows
T2C0N = 0x04; // Load Timer 2 control register
// Number of timer increments required (max 65536)
Inc = ((tLong)TICK_MS * (0SC_FREQ/1000) ) / (tLong)0SC_PER_INST;
// 16-bit reload value
Reload_16 = (tWord) (65536UL - Inc);
// 8-bit reload values (High & Low)
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 283
Case study: Intruder alarm system 283
Reload_08H = (tByte) (Reload_16 / 256);
Reload_08L = (tByte) (Reload_16 % 256);
// Used for manually checking timing (in simulator)
//P2 = Reload_08H;
//P3 = Reload_08L;
// Load T2 high byte
// Load T2 reload capt . reg . high byte
// Load T2 low byte
// Load T2 reload capt. reg. low byte
// Timer 2 interrupt is enabled, and ISR will be called
// whenever the timer overflows.
ET2 = 1 ;
// Start Timer 2 running
TR2 = 1 ;
TH2
Reload.
_08H
RCAP2H =
Reload,
_08H
TL2
Reload.
_08L
RCAP2L =
Reload
08L
EA
}
= 1;
// Globally enable interrupts
sEOS_Go_To_Sleep()
This operating system enters 'idle mode' between clock ticks
to save power. The next clock tick will return the processor
to the normal operating state.
/
void sEOS_Go_To_Sleep(void)
{
PCON |= 0x01; // Enter idle mode (generic 8051 version)
}
/ * *
END OF FILE
10.6 Conclusions
This case study has illustrated most of the key features of embedded C, as dis-
cussed throughout the earlier chapters in this book.
:322 Chapter 10 p255-284 21/2/02 10:04 am Page 284
:322 Chapter 11 p285-290 21/2/02 10:04 am Page 285
chapter
Where do we go from here?
11.1 Introduction
In this closing chapter, we review the material that has been presented throughout
this introductory book, and consider some further topics that you will need to
consider if you want to learn more about embedded systems.
1 1 .2 Have we achieved our aims?
As we stated in the preface, this book was intended to provide an introduction to
embedded software for people who:
• Already knew how to write software for 'desktop' computer systems.
• Were familiar with a C-based language (Java, C++ or C).
• Wanted to learn how C is used in practical embedded systems.
In previous chapters, we have covered a lot of essential material. To summarize:
• We have considered the choice of processor and programming language for
embedded systems.
• We have shown how to exploit key object-oriented design and programming
features in embedded C programs, with the aim of making it easier to re-use
code in subsequent projects.
• We have described the Super Loop software architecture used in many simple
embedded applications.
285
:322 Chapter 11 p285-290 21/2/02 10:04 am Page 286
286 Embedded C
We have described a simple embedded operating system, suitable for use in a
wide range of embedded applications. This operating system, driven by timer
interrupts, provides very accurate control over system timing but uses less than
1% of the available processor power.
We have explored techniques for controlling the state of individual port pins.
We have examined the reading of port pins and mechanical switches.
We have described how to use the 805 l's serial port.
We have presented a case study, to illustrate how all of the above material
comes together in a real application.
11.3 Suggestions for further study
If you are still keen to learn more about embedded systems, then some suggestions
for further study are made in this section.
a) Hardware issues
This introductory book has used a hardware simulator to introduce key aspects of
embedded software. This is a good way to begin work in this area, not least (T)
because it allows you to concentrate on getting the software right without having
to worry about possible wiring errors.
However, we have gone as far as we can with a simulator. Some aspects of hard-
ware cannot be ignored if you want to progress further in this area. Some key areas
that you need to address are:
• Designs for oscillator and reset circuits.
• Techniques for connecting external ROM and RAM memory.
• Interface circuits suitable for controlling low- and high-voltage DC and AC loads.
b) LCD and LED displays
Many embedded applications make use of LCD or LED displays. These require par-
ticular programming techniques that we have not been able to consider in this
introductory book.
:322 Chapter 11 p285-290 21/2/02 10:04 am Page 287
Where do we go from here? 287
c) Monitoring and control
Many embedded applications serve monitoring or control functions. This requires
- for example - the use of analog-to-digital converters, digital-to-analog converters,
pulse-width modulation, and suitable control algorithms (such as the ubiquitous
PID algorithm).
d) Operating systems
The simple operating system (OS) presented in this book is able to control a single
periodic task. We have not been able to consider OSs suitable for use with more
than one task.
In addition, a drawback with the simple OS presented in this book was that - in
response to a task longer than the tick interval - the system would 'lose ticks'
(Figure 11.1).
f System 'ticks' i><C Time
FIGURE 11.1 Losing ticks as a result of executing a 22 ms task using an OS with a 10 ms tick
interval
This type of OS is not suitable for use in high-reliability applications.
e) Multi-processor systems
All of the applications we have considered in this book have contained only one
embedded processor. However, in practice, an intruder alarm system (of the type
considered in Chapter 10) might well contain 6 processors, distributed in the vari-
ous sensors and the control panel, while a modern car will typically contain 50
embedded processors. Overall, multi-processor applications are becoming increas-
ingly common. Such processors may be linked together by means of a popular
serial standard such as RS-485 or the CAN bus.
:322 Chapter 11 p285-290 21/2/02 10:04 am Page 2
288 Embedded C
1 1 .4 Patterns for Time-Triggered Embedded Systems
If you are looking for a more advanced book on embedded systems after you finish
this book, then Patterns for Time-Triggered Embedded Systems (PTTES) may be
of interest. 32
PTTES is a large (1000-page) book that covers all of the topics highlighted in the
previous section. The overall approach is the same as this introductory book, but
PTTES covers this material at a more advanced level, and in more depth than was
appropriate here. PTTES also discusses hardware design for microcontroller-based
systems. In total, more than 70 patterns are described, complete with guidelines to
help you apply these techniques in your own projects: full source code for all of
the patterns is included on the PTTES CD.
The patterns described in PTTES include:
• Hardware patterns describing reset, oscillator and memory circuits. Numerous
complete hardware schematics for 8051-based designs are included.
• Several complete schedulers ('operating systems') for both single-processor and
multi-processor applications. These schedulers can support the scheduling of multi-
ple tasks, and are able to deal safely with tasks longer than the system tick interval.
• User-interface designs using switches, keypads, LED and liquid-crystal displays.
• Patterns for RS-232, RS-485, CAN, SPI and I 2 C serial communications.
• Patterns for analog-to-digital and digital-to-analog conversion using both on-
chip and external hardware.
• Patterns for pulse-width modulation (PWM).
• Patterns for control-system design (including implementations of industry-
standard 'PID' algorithms).
1 1 .5 Embedded Operating Systems
Like the present book, PTTES focuses on the use of the 8051 microcontroller. As
we discussed in Chapter 7 (Section 7.5), the co-operative, time-triggered operating
systems introduced in this book are used with a very wide range of embedded
systems.
If you would like to learn more about the design and implementation of operat-
ing systems for embedded environments, then a forthcoming book - Embedded
Operating Systems (EOS) - may be of interest. 33
32. Pont, M.J. (2001) Patterns for Time-Triggered Embedded Systems: Building reliable applications with
the 8051 family of microcontroller, ACM Press, New York. [ISBN 0-201-33138-1].
33. Pont, M.J. (in preparation) Embedded Operating Systems, Addison- Wesley, UK. Due for publica-
tion January, 2004.
:322 Chapter 11 p285-290 21/2/02 10:04 am Page 289
Where do we go from here? 289
EOS will describe the design and implementation of co-operative, time-
triggered, operating systems for a broad range of 8-, 16- and 32-bit processors.
11.6 Conclusions
In this closing chapter we have reviewed the material that has been presented
throughout this introductory book, and considered some further topics that you
will need to consider if you want to learn more about embedded systems. This
brings us to the end of 'Embedded C.
:322 Chapter 11 p285-290 21/2/02 10:04 am Page 290
:322 Index p291-304 21/2/02 10:05 am Page 291
Index
8051 microcontrollers
address space 28-9
basic features xi-xii, 3
BDATA area 26-7, 50
circuit diagrams 4
customizing 7
Extended 5-6, 18
external interface 18-20
flash memory 13, 55
interrupts 30-2, 93, 118, 147, 150, 167-9
I/O pins 29
memory architecture 26-8
memory organization 25-6
memory types 23-5
operating modes 33-4
oscillator frequency 21-3, 38-9, 91-2, 117,
125, 158
ports 47-50
power consumption 32-4, 159
power supply 20
pull-up resistors 67-9
reset requirements 20
serial interface 32, 217, 222-4
signed data and 93
Small 4-5, 18
special function registers 47-9, 58-9,
117-18, 119
Standard 3, 18
switch bounce 69-70,174-5
THx and TLx registers 119-20
timers 29-30, 116-20, 129, 161-6, 222-4
types 1 7
UARTchip 222-4
8052 microcontrollers 17
ADCs see analog-to-digital converters
addresses
bit and byte addresses 26-7
space 28-9
analog-to-digital converters (ADCs) 130
Application-Specific ICs (ASICs) 7
ARM microcontrollers 166
ASCII 219
asynchronous data transmission 219
Atmel AT89C52 microcontroller 55
Atmel AT89C55WD microcontroller 22
Atmel AT89S53 microcontroller 160
audio museum guides 144-5
autopilot systems 113-15,189-90
baud rates 219, 222-4
BDATA area (memory) 26-7, 50
BIOS (basic input/output system) 9
bitwise operators 62-7, 93
breadboards 13
C programming language
basic features 9
bit variables 50
bitwise operators 62-7, 93
delay functions 50-5, 116, 120-4, 125-8,
130-4, 151
header files 86, 88-96
interrupt service routines 147-50, 157
libraries 85-8
object-oriented programming 82-8
port pins, controlling 46-50
port pins, reading 58-69, 94-6, 257
sEOS operating system and 152-66
structured programming 81-2
:322 Index p291-304 21/2/02 10:05 am Page 292
292
NDEX
super loops 10-12, 45-6, 60-1, 143, 146
switch input, reading 70-5, 115-16, 134-6, 258
timeout loops 129-42
typedef statements 92-3
C++ programming language
overheads 83-4
strengths and weaknesses 84
CAN see Controller Area Network computer bus
code
restrictions on use xiv-xv
see also embedded code
CODE area (memory) 26-8, 31
compilers see Keil compiler
Controller Area Network (CAN) computer bus 5-7
hardware issues 286
monitoring and control 287
multi-processor systems 287
multi-state systems 189-215
operating systems and 9-12
periodic functions 145-7, 157
requirements 145-7
field-programmable gate arrays (FPGAs) 7
flash memory 13, 55
flash ROM 25
FPGAs see field-programmable gate arrays
Graham, I. 82
Dallas High Speed Microcontroller 22
Dallas Ultra High Speed Microcontroller 22
DATA area (memory) 26-8, 31
digital-to-analog converters 144, 145
Dolphin Integration 7
DRAM see dynamic RAM
dynamic RAM (DRAM) 24
EEPROM see electrically-erasable programmable
read-only memory
electrically-erasable programmable read-only
memory (EEPROM) 25
electronics, knowledge of xiii
embedded code
choice of programming language 7-8
compiling 12, 14
'Hello World' program 36, 43-55, 96-102
real-time constraints 113-15
structure 81-2
super loops 10-12, 45-6, 60-1, 143, 146
testing 12-15
user interface 57-8
Embedded Operating Systems 288-9
embedded systems
definition xi, 1
examples xii-xiii, 2
hardware simulators xiii
Keil 8051 simulator 14, 37-42, 52-5
hexadecimal notation 25-6
Infineon C167 microcontroller 166
Infineon C501 microcontroller 33
Infineon C515C microcontroller 224
Infineon C517A microcontroller 224
interrupt service routines (ISRs) 147-50, 157,
168-9
interrupts 30-2, 93, 118, 147, 150, 167-9, 173-4
I/O pins 29
ISRs see interrupt service routines
Jalote, P. 83
Keil 8051 simulator 14
configuration 37-9
debugging 39-40
running 40-2, 52-4
Keil compiler xiii, 36, 39
mask read-only memory 24
memory
BDATA area 26-7, 50
DATA and CODE areas 26-8, 31
:322 Index p291-304 21/2/02 10:05 am Page 293
INDEX 293
organization 25-6
serial communication and 224-5
types 23-5
Microchip PIC microcontrollers 166
microcontrollers
choice of 2-3
flash memory 13
see also 8051 microcontrollers;
8052 microcontrollers
microprocessors see 8051 microcontrollers;
microcontrollers
Motorola HC08 microcontrollers 166
Motorola MPC500 microcontrollers 166
multi-state (input/time) systems
design and implementation 204-5
washing machine controller program 205-15
multi-state (time) systems
animatronic dinosaur program 198-204
implementation 192
traffic light sequencing program 192-8
multi-tasking systems 157
object-oriented programming 82-8
operating systems xii
embedded 9-12, 147-88
see also sEOS operating system
oscillator frequency 21-3, 38-9, 91-2, 117,
125, 158
classification 82
programs
animatronic dinosaur program 198-204
data acquisition program 237-52
elapsed time display program 225-37
goat-counting program 75-80, 103-11, 129-30
'Hello World' program 35-6, 43-55, 96-102
intruder alarm system program 255-83
milk pasteurization program 174-87
remote-control robot program 252-3
traffic light sequencing program 192-8
washing machine controller program 205-15
PROM see programmable read-only memory
RAM see random access memory
random access memory (RAM) 23
read-only memory (ROM) 23-5
registers
special function registers 47-9, 58-9,
117-18, 119
THx and TLx registers 119-20
ROM see read-only memory
RS-232 communication protocol 32, 217-19
asynchronous data transmission 219
baud rates 219, 222-4
data transmission 220-1
flow control 220
UARTchip 222-4
Patterns for Time-Triggered Embedded Systems
xiv, 288
personal digital assistants (PDAs) 2
Philips 8Xc552 microcontroller 130, 137
ports 47-50
controlling 46-50
reading 58-69, 94-6, 257
processors see 8051 microcontrollers;
microcontrollers
programmable read-only memory (PROM) 24
programming languages
choice of 7-8
sEOS operating system 152-6, 258
animatronic dinosaur program 198-204
co-operative and pre-emptive scheduling 169-72
execution time 1 72-3
interrupts, disabling 173-4
locking mechanisms 170-1
milk pasteurization program 174-87
porting 161, 166
power saving 159-60
reliability 172
tasking 157
tick intervals 1 5 7-8
:322 Index p291-304 21/2/02 10:05 am Page 2
294
NDEX
time-triggered and event-triggered systems
166-9
timers, using 161-6
traffic light sequencing program 193-8
washing machine controller program 205-15
serial communication 32
asynchronous data transmission 219
baud rates 219, 222-4
data transmission 220-1
flow control 220
memory requirements 224-5
programs 225-53
standards 217-19
UARTchip 222-4
SFRs see special function registers
software see C programming language; code;
embedded code; programs
Sommerville, I. 84
special function registers (SFRs) 47-9, 58-9
TCONSFR 117-18
TMODSFR 119
SRAM see static RAM
static RAM (SRAM) 24
Stroustrup, B. 84
super loops 10-12, 45-6, 60-1, 143, 146
switch input, reading 70-5, 115-16, 134-6, 258
tasks 157
timeout loops 129-42
timers 29-30, 116-20, 129, 161-6, 222-4
Triscend E5 processors 7
typedef statements 92-3
UARTchip 222-4
UV erasable programmable read-only memory
(UVEPROM) 24-5
Winbond W77E58 microcontroller 22
Xilinx Foundation 7
:322 Index p291-304 21/2/02 10:05 am Page 295
:322 Index p291-304 21/2/02 10:05 am Page 296
:322 Index p291-304 21/2/02 10:05 am Page 297
;322 Index p291-304 21/2/02 10:05 am Page 29
:322 Index p291-304 21/2/02 10:05 am Page 299
:322 Index p291-304 21/2/02 10:05 am Page 300
:322 Index p291-304 21/2/02 10:05 am Page 301
:322 Index p291-304 21/2/02 10:05 am Page 302
:322 Index p291-304 21/2/02 10:05 am Page 303
Licensing Agreement
This book comes with a CD software package. By opening this package, you are
agreeing to be bound by the following:
The software contained on this CD is, in many cases, copyrighted, and all rights
are reserved by the individual licensing agreements associated with each piece of
software contained on the CD. THIS SOFTWARE IS PROVIDED FREE OF CHARGE,
AS IS, AND WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR
IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Neither the
book publisher nor its dealers and its distributors assumes any liability for any
alleged or actual damages arising for the use of this software.
303
:322 Index p291-304 21/2/02 10:05 am Page 3
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