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IN PRACTICE

A PROJECT-BASED APPROACH

PIC in Practice

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PIC in Practice

A Project-Based Approach

D. W. Smith

iNr

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Contents

Introduction ix

1 Introduction to the PIC microcontroller 1

The aim of the book 1

Program memory 2

Microcontroller clock 3

The microcontroller system 3

Types of microcontroller 4

Microcontroller specification 5

Using the microcontroller 6

1 Microcontroller hardware 6

2 Programming the microcontroller 9

2 Programming the 16F84 microcontroller 11

Microcontroller inputs and output (I/O) 12

Timing with the microcontroller 12

Programming the microcontroller 12

Entering data 13

The header for the 16F84 14

Program example 16

Saving and assembling the code 19

PICSTART PLUS programmer 23

Programming flowchart 26

Problem: flashing two LEDs 26

Solution to problem, flashing two LEDs 27

3 Introductory projects 29

LED_Flasher2 29

SOS 30

Code for SOS circuit 30

Flashing 8 LEDs 33

Chasing 8 LEDs 35

Traffic lights 39

More than 8 outputs 45

4 Headers, porting code - which micro? 47

Factors affecting the choice of the microcontroller 47

Choosing the microcontroller 48

Headers 49

vi Contents

5 Using inputs 64

Switch flowchart 66

Program development 67

Scanning (using multiple inputs) 73

Switch scanning 73

Control application - a hot air blower 77

6 Understanding the headers 82

The 16F84 82

16F84 memory map 87

The 16F818 88

7 Keypad scanning 93

Programming example for the keypad 94

8 Program examples 110

Counting events 110

Look up table 115

7-Segment display 115

Numbers larger than 255 126

Long time intervals 133

One hour delay 136

9 The 16C54 microcontroller 139

Header for the 16C54 139

16C54 memory map 142

10 Alpha numeric displays 143

Display pin identification 144

Configuring the display 145

Writing to the display 146

Program example 146

Program operation 160

Display configuration 161

Writing to the display 162

Displaying a number 163

11 Analogue to digital conversion 166

Making an A/D reading 167

Configuring the A/D device 168

Analogue header for the 16F818 171
A/D conversion - example, a temperature sensitive

switch 1 74

Program code 176

Another example - a voltage indicator 178

Contents vii

12 Radio transmitters and receivers 186

Measuring the received pulse width 189

13 EEPROM data memory 199

Example using the EEPROM 200

14 Interrupts 207

Interrupt sources 208

Interrupt control register 208

Program using an interrupt 209

15 The 12 series 8 pin microcontroller 216

Pin diagram of the 12C508/509 216

Pin diagram of the 12F629 and 12F675 216

Features of these 12 series 217

The memory map of the 12C508 217

Oscillator calibration 218

I/O PORT, GPIO 219

Delays with the 12 series 220

Header for 12C508/9 220

Program application for 12C508 222

Program application using the 12F629/675 225

16 The 16F87X Microcontroller 229

16F87X family specification 229

The 16F872 microcontroller 230

16F87X memory map 232

The 16F872 header 233

16F872 application - a greenhouse control 236
Programming the 16F872 microcontroller

using PICSTART PLUS 242

Reconfiguring the 16F872 header 243

17 The 16F62X Microcontroller 245

16F62X oscillator modes 245

16F62X and 16F84 Pinouts 247

16F62X port configuration 247

16F62X memory map 248

The 16F62X headers 248

HEAD62RC.ASM 250

A 16F627 application - flashing an LED on and off 252

The 16F627 LED flasher code 253

Configuration settings for the 16F627 255

Other features of the 16F62X 255

viii Contents

18 Projects 257

Project 1 Electronic dice 257

Project 2 Reaction timer 266

Project 3 Burglar alarm 272

Fault finding 282

Development kits 285

19 Instruction set, files and registers 287

The PIC microcontroller instruction set 287

Registers 289

Instruction set summary 292

Appendix A Microcontroller data 299

Appendix B Electrical characteristics 301

Appendix C Decimal, binary and hexadecimal numbers 303

Appendix D Useful contacts 306

Index 307

Introduction

The microcontroller is an exciting new device in the field of electronics
control. It is a complete computer control system on a single chip,
microcontrollers include EPROM program memory, user RAM for storing
program data, timer circuits, an instruction set, special function registers,
power on reset, interrupts, low power consumption and a security bit for
software protection. Some microcontrollers like the 16F818/9 devices include
on board A to D converters.

The microcontroller is used as a single chip control unit for example in a
washing machine, the inputs to the controller would be from a door catch,
water level switch, temperature sensor. The outputs would then be fed to a
water inlet valve, heater, motor and pump. The controller would monitor the
inputs and decide which outputs to switch on i.e. close the door - water inlet
valve open - monitor water level, close valve when water level reached. Check
temperature, turn on heater, switch off heater when the correct temperature
is reached. Turn the motor slowly clockwise for 5 seconds, anticlockwise
for 5 seconds, repeat 20 times, etc. If you are not that maternal maybe you
prefer discos to washing - then you can build your own disco lights.

The microcontroller because of its versatility, ease of use and cost will change
the way electronic circuits are designed and will now enable projects to be
designed which previously were too complex. Additional components such as
versatile interface adapters (VIA), RAM, ROM, EPROM and address
decoders are no longer required.

One of the most difficult hurdles to overcome when using any new technology
is the first one - getting started! It was my aim when writing this book to
explain as simply as possible how to program and use the PIC microcon-
trollers. I hope I have succeeded.

Code examples in this book are available to download from:
http://books.elsevier.eom/uk//newnes/uk/subindex. asp?maintarget=companions/
defaultindividual.asp&isbn=0750648120

Dave Smith, B.Sc, M.Sc.

Senior Lecturer in Electronics

Manchester Metropolitan University

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Introduction to the PIC
microcontroller

A microcontroller is a computer control system on a single chip. It has
many electronic circuits built into it, which can decode written instructions and
convert them to electrical signals. The microcontroller will then step through
these instructions and execute them one by one. As an example of this a
microcontroller could be instructed to measure the temperature of a room and
turn on a heater if it goes cold.

Microcontrollers are now changing electronic designs. Instead of hard wiring
a number of logic gates together to perform some function we now use
instructions to wire the gates electronically. The list of these instructions given
to the microcontroller is called a program.

The aim of the book

The aim of the book is to teach you how to build control circuits using
devices such as switches, keypads, analogue sensors, LEDs, buzzers, 7 segment
displays, alpha-numeric displays, radio transmitters etc. This is done by intro-
ducing graded examples, starting off with only a few instructions and gradually
increasing the number of instructions as the complexity of the examples
increases.

Each chapter clearly identifies the new instructions added to your vocabulary.

The programs use building blocks of code that can be reused in many different
program applications.

Complete programs are provided so that an application can be seen working.
The reader is then encouraged to modify the code to alter the program in order
to enhance their understanding.

Throughout this book the programs are written in a language called assembly
language which uses a vocabulary of 35 words called an instruction set.
In order to write a program we need to understand what these words mean and
how we can combine them.

2 Introduction to the PIC microcontroller

The complete instruction set is shown in Chapter 19 Instruction Set, Files and
Registers.

All of the programs illustrated in the book are available from:
http://books.elsevier.com/uk//newnes/uk/subindex.asp?maintarget=
companions/defaultindividual.asp&isbn=0750648 1 20

You will of course need a programmer to program the instructions into the
chip. The assembler software, MPASM, which converts your text to the
machine code is available from Microchip on www.microchip.com this website
is a must for PIC programmers.

Program memory

Inside the microcontroller the program we write is stored in an area
called EPROM (Electrically Programmable Read Only Memory), this
memory is non-volatile and is remembered when the power is switched off.
The memory is electrically programmed by a piece of hardware called
a programmer.

The instructions we program into our microcontroller work by moving
and manipulating data in memory locations known as user files and registers.
This memory is called RAM, Random Access Memory. For example in
the room heater we would measure the room temperature by instructing the
microcontroller via its Analogue to Digital Control Register (ADCONO)
the measurement would then be compared with our data stored in one of
the user files. A STATUS Register would indicate if the temperature was
above or below the required value and a PORT Register would turn the
heater on or off accordingly. The memory map of the 16F84 chip is shown in
Chapter 6.

PIC Microcontrollers are 8 bit micros, which means that the memory locations,
the user files and registers are made up of 8 binary digits shown in Figure 1.1.

Bit is the Least Significant Bit (LSB) and Bit 7 is the Most Significant
Bit (MSB).

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bitO

MSB

-LSB

Figure 1.1 User file and register layout

Introduction to the PIC microcontroller 3

The use of these binary digits is explained in Appendix C.

When you make an analogue measurement, the digital number, which results,
will be stored in a register called ADRES. If you are counting the number
of times a light has been turned on and off, the result would be stored as an
8 bit binary number in a user file called, say, COUNT.

Microcontroller clock

In order to step through the instructions the microcontroller needs a clock
frequency to orchestrate the movement of the data around its electronic
circuits. This can be provided by 2 capacitors and a crystal or by an internal
oscillator circuit.

In the 16F84 microcontroller there are 4 oscillator options.

• An RC (Resistor/Capacitor) oscillator which provides a low cost solution.

• An LP oscillator, i.e. 32kHz crystal, which minimises power consumption.

• XT which uses a standard crystal configuration.

• HS is the high-speed oscillator option.

Common crystal frequencies would be 32kHz, 1MHz, 4MHz, 10MHz
and 20MHz.

Newer microcontrollers, such as the 16F818 and 12F629, have an oscillator
built on the chip so we do not need to add a crystal to them.

Inside the Microcontroller there is an area where the processing (the clever
work), such as mathematical and logical operations are performed, this is
known as the central processing unit or CPU. There is also a region where
event timing is performed and another for interfacing to the outside world
through ports.

The microcontroller system

The block diagram of the microcontroller system is shown in Figure 1.2.

CONTROL

OUTPUT

Figure 1.2 The basic microcontroller system

4 Introduction to the PIC microcontroller

• The input components would consist of digital devices such as, switches,
push buttons, pressure mats, float switches, keypads, radio receivers etc. and
analogue sensors such as light dependant resistors, thermistors, gas sensors,
pressure sensors, etc.

• The control unit is of course the microcontroller. The microcontroller will
monitor the inputs and as a result the program would turn outputs on and
off. The microcontroller stores the program in its memory, and executes the
instructions under the control of the clock circuit.

• The output devices would be made up from LEDs, buzzers, motors, alpha
numeric displays, radio transmitters, 7 segment displays, heaters, fans etc.

The most obvious choice then for the microcontroller is how many
digital inputs, analogue inputs and outputs does the system require.
This would then specify the minimum number of inputs and outputs (I/O)
that the microcontroller must have. If analogue inputs are used then the
microcontroller must have an Analogue to Digital (A/D) module inside.

The next consideration would be what size of program memory storage
is required. This should not be too much of a problem when starting out,
as most programs would be relatively small. All programs in this book fit into
a Ik program memory space.

The clock frequency determines the speed at which the instructions are
executed. This is important if any lengthy calculations are being undertaken.
The higher the clock frequency the quicker the micro will finish one task and
start another.

Other considerations are the number of interrupts and timer circuits required,
how much data EEPROM if any is needed. These more complex operations are
considered later in the text.

In this book the programs requiring analogue inputs have been implemented
on the 16F818 and 16F872 micros. Programs requiring only digital
inputs have used the 16F84 and 16F818. The 16F818 and 16F84 devices
have Ik of program memory and have been run using a 32.768kHz clock
frequency or the internal oscillator on the 16F818. There are over 100 PIC
microcontrollers, the problem of which one to use need not be considered until
you have understood a few applications.

Types of microcontroller

The list of PIC Microcontrollers is growing almost daily. They include devices
for all kinds of applications, for example the 18F8722 has 64k of EPROM
memory, 3938 bytes of RAM (User files), 1024 bytes of EEPROM, 16 10-bit

Introduction to the PIC microcontroller 5

A/D channels, a voltage reference, 72 inputs and outputs (I/O), 3-16 bit and
2-8 bit timers.

There are basically two types of microcontrollers, Flash devices and One
Time Programmable Devices (OTP).

The flash devices can be reprogrammed in the programmer whereas OTP
devices once programmed cannot be reprogrammed. All OTP devices however
do have a windowed variety, which enables them to be erased under ultra violet
light in about 15 minutes, so that they can be reprogrammed. The windowed
devices have a suffix JW to distinguish them from the others.

The OTP devices are specified for a particular oscillator configuration R-C,
LP, XT or HS. See Appendix A Microcontroller Data.

16C54 configurations are:

16C54JW Windowed device

16C54RC OTP, R-C oscillator

16C54LP OTP, LP oscillator, 32kHz

16C54XT OTP, XT oscillator, 4MHz

16C54HS OTP, HS oscillator, 20Mhz

In this book the two main devices investigated are the 16F84 and the 16F818
flash devices. The 16F84 at present is the main choice for beginners, but
should be replaced in popularity by the better and cheaper 16F818. They
have their program memory made using Flash technology. They can be
programmed, tested in a circuit and reprogrammed if required without the need
for an ultra violet eraser.

Microcontroller specification

You specify a device with its Product Identification Code.
This code specifies:

• The device number.

• If it is a Windowed, an OTP, or flash device. The windowed device is
specified by a JW suffix. OTP devices are specified by Oscillator Frequency,
and the Flash devices are specified with an F such as 16F84.

• The oscillation frequency, usually 04 for devices working up to 4MHz.,
10 up to 10MHz or 20 up to 20MHz. 20MHz devices are of course more
expensive than 4MHz devices.

• Temperature range, for general applications 0°C to +70°C is usually
specified.

6 Introduction to the PIC microcontroller

PART No. -XX X /XX

•Package L= PLCC
P = PDIP (standard plastic package)
SO = SOIC small outline IC
PQ = MQFP
JW = Windowed device (CERDIP)

•Temperature range - = 0°C to +70°C

I = -40°C to +85°C
E = -40°Cto+125°C

Frequency range 04 = 4MHz

04 = 10MHz
10 = 20MHz

Device i.e. 16C711

Figure 1.3 Product identification system

The Product Identification System for the PIC Micro is shown in Figure 1.3.

Using the microcontroller

In order to use the microcontroller in a circuit there are basically two areas
you need to understand:

1. How to connect the microcontroller to the hardware.

2. How to write and program the code into the microcontroller.

1 Microcontroller hardware

The hardware that the microcontroller needs to function is shown in
Figure 1.4. The crystal and capacitors connected to pins 15 and 16 of the
16F84 produce the clock pulses that are required to step the microcontroller
through the program and provide the timing pulses. (The crystal and capacitor
can be omitted if using an on board oscillator in e.g. 16F818). The 0.1 uF
capacitor is placed as close to the chip as possible between 5v and Ov. Its role is
to divert (filter) any electrical noise on the 5v power supply line to Ov, thus
bypassing the microcontroller. This capacitor must always be connected to
stop any noise affecting the normal running of the microcontroller.

Microcontroller power supply

The power supply for the microcontroller needs to be between 2v and 6v. This
can easily be provided from a 6v battery as shown in Figure 1.5.

Introduction to the PIC microcontroller 1

68 P 32kHz 16

16F84

V+

MCLR

I I
II 15

68p

Figure 1.4 The microcontroller circuit

Figure 1.5 Microcontroller power supply

The diode in the circuit drops 0.7v across it reducing the applied voltage to
5.3v. It provides protection for the microcontroller if the battery is acciden-
tally connected the wrong way round. In that case the diode would be reversed
biased and no current would flow.

7805, Voltage regulator circuit

Probably the most common power supply connection for the microcontroller
is a 3 terminal voltage regulator, I.C., the 7805. The connection for this is
shown in Figure 1.6.

The supply voltage, Yin, to the 7805 can be anything from 7v to 30v.

The output voltage will be a fixed 5v and can supply currents up to lamp.
So battery supplies such as 24v, 12v, 9v etc. can be accommodated.

8 Introduction to the PIC microcontroller

O-
Vin

O-

7805

-O

5v
-O

Figure 1.6 The voltage regulator circuit

Power dissipation in the 7805

Care must be taken when using a high value for Vin. For example if Vin = 24v
the output of the 7805 will be 5v, so the 7805 has 24-5 = 19v across it. If
it is supplying a current of 0.5amp to the circuit then the power dissipated
(volts x current) is 19 x 0.5 = 9.5watts. The regulator will get hot! and will
need a heat sink to dissipate this heat.

If a supply of 9v is connected to the regulator it will have 4v across it and
would dissipate 4 x 0.5 = 2watts.

In the circuits used in this book the microcontroller only requires a current
of 15uA so most of the current drawn will be from the outputs. If the output
current is not too large say < 100mA (0.1 A) then with a 9v supply the
power dissipated would be 4 x 0.1 = 0.4watts and the regulator will stay cool
without a heatsink.

Connecting switches to the microcontroller

The most common way of connecting a switch to a microcontroller is via
a pull-up resistor to 5v as shown in Figure 1.7.

10k

Ov
Figure 1.7 Connecting a switch to the microcontroller

When the switch is open, 5v, a logic 1 is connected to the micro.

When the switch is closed, Ov, a logic is connected to the micro.

Introduction to the PIC microcontroller 9

Some Microcontrollers such as the 16F84 and 16F818 have internal pull ups
connected to some of their I/O pins. PORTB in the above devices.

Figure 1.8 shows how the switch is connected using the internal pull up.

Ov
Figure 1.8 Connecting a switch using an internal pull up

Connecting outputs to the microcontroller

The microcontroller is capable of supplying approximately 20-25mA to an
output pin. So loads such as LEDs or small relays can be driven directly.
Larger loads require interfacing via a transistor, for dc or a triac, for ac.
Opto-coupled devices provide an isolated interface between the microcontroller
and the load.

The LED connection to the Micro is shown in Figure 1.9.

680R

Figure 1.9 Connecting an LED to the microcontroller

2 Programming the microcontroller

In order to have the microcontroller perform some controlling action you
need to communicate with it and tell it what those instructions are to be.
When we communicate with one another we use a spoken language, when
we communicate with a microcontroller we use a program language. The
program language for the PIC Microcontroller uses 35 words (instructions)

10 Introduction to the PIC microcontroller

in its vocabulary. A few more instructions are used in the bigger
microcontrollers.

In order to communicate with the microcontroller we need to know what
these 35 instructions are and how to use them. Not all 35 instructions are
used in this book. In fact you can write meaningful programs using only 5 or
6 instructions.

Programming the 16F84
microcontroller

Microcontrollers are now providing us with a new way of designing circuits.
Designs, which at one time required many Digital ICs and lengthy Boolean
Algebra calculations, can now be programmed simply into one Micro-
controller. For example a set of traffic lights would have required an oscillator
circuit, counting and decoding circuits plus an assortment of logic gate ICs.

In order to use this exciting new technology we must learn how to program
these Microcontrollers.

The Microcontroller I have chosen to start with is the 16F84-04/P, which
means it is a flash device that can be electrically erased and reprogrammed
without using an Ultra Violet Eraser. It can be used up to an oscillation
frequency of 4MHz and comes in a standard 18pin Plastic package.

It has 35 instructions in its vocabulary, but like all languages not all of the
instructions are used all of the time you can go a long way on just a few.
In order to teach you how to use these instructions I have started off with a
simple program to flash an LED on and off continually. This program
introduces you to 4 instructions in 5 lines of code.

You are then encouraged to write your own program to flash two LEDs on
and off alternately. The idea being, when you have understood my code you
can then modify it for your own program, thus understanding better. Once
you have written your first program you are then off and running. The book
then continues with further applications such as traffic lights and disco lights
to introduce more of the instructions increasing your microcontroller
vocabulary.

Instructions used in this chapter:

• BCF

• BSF

• CALL

• GOTO

12 Programming the 16F84 microcontroller

Microcontroller inputs and outputs (I/O)

The microcontroller is a very versatile chip and can be programmed to operate
in a number of different configurations. The 16F84 is a 13 I/O device, which
means it has 13 Inputs and Outputs. The I/O can be configured in any combi-
nation i.e. 1 input 12 outputs, 6 inputs 7 outputs, or 13 outputs depending
on your application. These I/O are connected to the outside world through
registers called Ports. The 16F84 has two ports, PORTA and PORTB. PORTA
is a 5-bit port it has 5 I/O lines and PORTB has 8 I/O.

Timing with the microcontroller

All microcontrollers have timer circuits onboard; some have 4 different timers.
The 16F84 has one timer register called TIMERO. These timers run at a speed
of Va of the clock speed. So if we use a 32,768Hz crystal the internal timer
will run at % of 32768Hz i.e. 8192Hz. If we want to turn an LED on for say
1 second we would need to count 8192 of these timing pulses. This is a lot
of pulses! Fortunately within the microcontroller there is a register called an
OPTION Register, that allows us to slow down these pulses by a factor of 2, 4,
8, 16, 32, 64, 128 or 256. The OPTION Register is discussed in the Instruction
Set, Files and Register section in Chapter 19. Setting the prescaler, as it is called
to divide by 256 in the OPTION register means that our timing pulses are now
8192/256 = 32Hz, i.e. 32 pulses a second. So to turn our LED on for 1 second
we need only to count 32 pulses in TIMERO, or 16 for 0.5 seconds, or 160 for
5 seconds etc.

Programming the microcontroller

In order to program the microcontroller we need to:

• Write the instructions in a program.

• Change the text into machine code that the microcontroller understands
using a piece of software called an assembler.

• Blow the data into the chip using a programmer.

Let's consider the first task, writing the program. This can be done on any text
editor, such as notepad. I prefer to use an editor supplied by the micro-
controller manufacturers, 'Microchip'. This software is called MPLAB and is
available free on www.microchip.com.

As you have seen above we need to configure the I/O and set the Prescaler
for the timing. If we do not set them the default conditions are that all PORT
bits are inputs. A micro with no outputs is not much use! The default for the
Prescaler is that the clock rate is divided by 2.

Programming the 16F84 microcontroller 13

The program also needs to know what device it is intended for and also what
the start address in the memory is.

If this is starting to sound confusing - do not worry, I have written a header
program, which sets the all the above conditions for you to use. These con-
ditions can be changed later when you understand more about what you are
doing.

The header for the 16F84 sets the 5 bits of PORTA as inputs and the 8 bits
of PORTB as outputs. It also sets the prescaler to divide by 256. We will use the
32,768Hz crystal so our timing is 32 pulses per sec. The program instructions
will run at % of the 32,768Hz clock, i.e. 8192 instructions per second. The
header also includes two timing subroutines for you to use they are DELAY 1 -
a 1 second delay and DELAYP5 - a half-second delay. A subroutine is a
section of code that can be called, when needed, to save writing it again.
For the moment do not worry about how the header or the delay subroutines
work. We will work through them, in Chapter 6, once we have programmed
a couple of applications.

Just one more point, the different ways of entering data.

Entering data

Consider the decimal number 37, this has a Hex value of 25 or a Binary value
of 0010 0101. The assembler will accept this as .37 in decimal (note the . is not
a decimal point) or as 25H in hex or B'00100101' in binary.

181 decimal would be entered as .181 in decimal, 0B5H in hex or B' 101 10101'
in binary. NB. If a hex number starts with a letter it must be prefixed with a
0, i.e. 0B5H not B5H.

NB. The default radix for the assembler MPASM is hex.

Appendix C. illustrates how to change between Decimal, Binary and
Hexadecimal numbers.

The PIC Microcontrollers are 8 bit micros. This means that the memory
locations, i.e. user files and registers contain 8 bits. So the smallest 8 bit number
is of course 0000 0000 which is equal to a decimal number (of course). The
largest 8 bit number is 1111 1111 which is equal to a decimal number of 255.
To use numbers bigger than 255 we have to combine memory locations. Two
memory locations combine to give 16 bits with numbers up to 65,536. Three
memory locations combine to give 24 bits allowing numbers up to 16,777,215

14 Programming the 16F84 microcontroller

and so on. These large numbers are introduced in Chapter 8, Numbers Larger
than 255.

The Header for the 16F84, HEADER84.ASM

The listing below shows the header for the 16F84 microcontroller. I suggest
you start all of your programs, for this chip, with this header, or a modified
version of it. A full explanation of this header file is given in Chapter 6.

; HEADER84.ASM for 16F84. This sets PORTA as an INPUT (NB lmeans

input).

; and PORTB as an OUTPUT (NB means output).

;The OPTION Register is set to /256 to give timing pulses of 1/32 of a second.

;1 second and 0.5 second delays are included in the subroutine section.

; EQUATES SECTION

TMRO

STATUS

PORTA

PORTB

TRISA

TRISB

EQU
EQU
EQU
EQU
EQU
EQU

OPTION_R EQU
ZEROBIT EQU
COUNT EQU

1 ;means TMRO is file 1.

3 ;means STATUS is file 3.

5 ;means PORTA is file 5.

6 ;means PORTB is file 6.

85H ;TRISA (the PORTA I/O selection) is file 85H

86H ;TRISB (the PORTB I/O selection) is file 86H

81H ;the OPTION register is file 81H

2 ;means ZEROBIT is bit 2.

OCH ;COUNT is file 0C, a register to count events.

LIST
ORG
GOTO

P=16F84

START

; we are using the 16F84.

;the start address in memory is

; goto start!

; Configuration Bits

__CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

SUBROUTINE SECTION.

;1 second delay.

Programming the 16F84 microcontroller 15

START TMRO.

READ TMRO INTO W.

TIME - 32

; Check TIME-W =
;Time is not = 32.
;Time is 32, return.

START TMRO.

READ TMRO INTO W.

TIME - 16

; Check TIME-W =

;Time is not = 16.
;Time is 16, return.

DELAY 1

CLRF

TMRO

LOOPA

MOVF

TMR0,W

SUBLW

.32

BTFSS

STATUS,
ZEROBIT

GOTO

LOOPA

RETLW

; 0.5 second

delay.

DELAYP5

CLRF

TMRO

LOOPB

MOVF

TMR0,W

SUBLW

.16

BTFSS

STATUS,
ZEROBIT

GOTO

LOOPB

RETLW

CONFIGURATION SECTION

START

BSF

STATUS,5

MOVLW

B'ooonnr

MOVWF

TRISA

MOVLW

B'00000000'

MOVWF

TRISB

MOVLW

B'ooooonr

MOVWF

OPTION_R

BCF

STATUS,5

CLRF

PORTA

CLRF

PORTB

;Turns to Bankl.

;5bits of PORTA are I/P

;PORTB is OUTPUT

;Prescaler is /256
;TIMER is 1/32 sees.

;Return to BankO.
;Clears PortA.
;Clears PortB.

;Program starts now.

END

;This must always come at the end of your code

NB. In the program any text on a line following the semicolon (;) is ignored by
the assembler software. Program comments can then be placed there.

The section is saved as HEADER84.ASM you can use it to start all of your
16F84 programs. HEADER84 is the name of our program and ASM is its
extension.

16 Programming the 16F84 microcontroller

Program example

The best way to begin to understand how to use a microcontroller is to start
with a simple example and then build on this.

Let us consider a program to flash an LED ON and OFF at 0.5 second
intervals. The LED is connected to PortB bit as shown in Figure 2.1.

68p

32kHz 16

68p

16F84

V+

MCLR

TOCKI

470R

LED1 Y>
Ov

T - 1 ^

Figure 2.1 Circuit diagram of the microcontroller flasher

Notice from Figure 2.1 how few components the microcontroller needs - 2 x
68pF capacitors, a 32.768kHz crystal for the oscillator and a 0.1 uF capacitor
for decoupling the power supply. Other oscillator and crystal configurations
are possible - see Microchip's data sheets for other combinations. I have
chosen the 32kHz crystal because it enables times of seconds to be produced
easily.

The program for this circuit can be written on any text editor, such as Notepad
or on Microchip's editor MPLAB.

Open HEADER84.ASM or start a new file and type the program in, saving as
HEADER84.ASM If using Notepad saveas type "All Files" to avoid Notepad
adding the extension .TXT

Once you have HEADER84.ASM saved on disk and loaded onto the screen
alter it by including your program as shown below:-

; HEADER84.ASM for 16F84. This sets PORTA as an INPUT (NB lmeans
input).

Programming the 16F84 microcontroller 17

; and PORTB as an OUTPUT (NB means output).

;The OPTION Register is set to /256 to give timing pulses of 1/32 of a second.

;lsecond and 0.5 second delays are included in the subroutine section.

«»'*■ «£« «f« %$* %S* +S* *£* *i* ^'* *1* ^'* ^'^ ^'* *»'* *1* ^'* *»'* *»'* ^'* *£* ^'* ^'* ^'* ^'* *£* ^'* ^'* ^'* ^'^ *1* ■»'* ^'* ^'^ ^'* *1* ^'*- ^'* ^'^ ^'* *1* ^'* *»'* ^'* ^'*- *£* ^'* ^'*- ^'* ^'* *1* *»'* ^'* *»'* ^'* *£*

• #T* *T* #T* *T* *T* *T* #T* *T* *T* *T* *T* #T* *T* *T* *T* #T* *T* *T* *T* *T* *T* *f* *T* *J* *T* *J* *T* *T* *T* *T* *T* *T* *T* *f* *T» *T* *T* #T* *T* *T* *T* *T* #T* *T* *T* *T* *T* *T* *T* *T* *T* *T* *T* *T* *T*

; EQUATES SECTION

TMRO

EQU

STATUS

EQU

PORTA

EQU

PORTB

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION,

EQU

81H

ZEROBIT

EQU

COUNT

EQU

OCH

LIST

= 16F84

ORG

GOTO

START

;means TMRO is file 1.

;means STATUS is file 3.

;means PORTA is file 5.

;means PORTB is file 6.

;TRISA (the PORTA I/O selection) is file 85H

;TRISB (the PORTB I/O selection) is file 86H

;the OPTION register is file 81H

;means ZEROBIT is bit 2.

; COUNT is file 0C, a register to count events.

;we are using the 16F84.

;the start address in memory is

;goto start!

; Configuration Bits

CONFIG H'3FF0'

;selects LP oscillator, WDT off, PUT on,
;Code Protection disabled.

SUBROUTINE SECTION.

; 1 second

delay.

DELAY 1

CLRF

TMRO

LOOPA

MOVF

TMR0,W

SUBLW

.32

BTFSS

STATUS,
ZEROBIT

GOTO

LOOPA

RETLW

;START TMRO.
;READ TMRO INTO W.
;TIME - 32

; Check TIME-W =
;Time is not = 32.
;Time is 32, return.

; 0.5 second delay.

18 Programming the 16F84 microcontroller

DELAYP5

CLRF

TMRO

LOOPB

MOVF

TMR0,W

SUBLW

.16

BTFSS

STATUS,
ZEROBIT

GOTO

LOOPB

RETLW

;START TMRO.
;READ TMRO INTO W.
;TIME - 16

; Check TIME-W =
;Time is not = 16.
;Time is 16, return.

CONFIGURATION SECTION.

START

BSF

STATUS,5

;Turns to Bankl.

MOVLW

B'ooonnr

;5bits of PORTA are I/P

MOVWF

TRISA

MOVLW

B'00000000'

MOVWF

TRISB

;PORTB is OUTPUT

MOVLW

B'ooooonr

;Prescaler is /256

MOVWF

OPTION_R

;TIMER is 1/32 sees.

BCF

STATUS,5

; Return to BankO.

CLRF

PORTA

;Clears PortA.

CLRF

PORTB

;Clears PortB.

;Program starts now.

BEGIN

END

BSF

PORTB,0

;Turn ON BO.

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,0

;Turn OFF BO.

CALL

DELAYP5

;Wait 0.5 seconds

GOTO

BEGIN

;Repeat

;YOU MUST END

How Does It Work?

The 5 lines of code starting at BEGIN are responsible for flashing the LED ON
and OFF. This is all the code we will require for now. The rest of the code, the
header is explained in Chapter 6 once you have seen the program working.

BEGIN is a label. A label is used as a location for the program to go to.
Linel the instruction BSF and its data PORTB, is shorthand for Bit Set
in File, which means Set the Bit in the File PORTB, where bitO is the
designated bit. This will cause PORTB, to be Set to a logic 1, in hardware
terms this means pin6 in Figure 2.1 is at 5v turning the LED on.

Programming the 16F84 microcontroller 19

NB. There must not be any spaces in a label, an instruction or its data. I keep
the program tidy by using the TAB key on the keyboard.

• Line2 CALL DELAYP5 causes the program to wait 0.5 seconds while the
subroutine DELAYP5 in the header is executed.

• Line3 BCF PORTB,0 is the opposite of Linel, this code is shorthand for
Bit Clear in File, which means Clear the Bit in the File PORTB, where bitO
is the designated bit. This will cause PORTB, to be Cleared to a logicO,
in hardware terms this means pin6 in Figure 2.1 is at Ov turning the LED off.

• Line4 CALL DELAYP5 is the same as Line2.

• Line5 GOTO BEGIN sends the program back to the label BEGIN to repeat
the process of flashing the LED on and off.

Any of the 8 outputs can be turned ON and OFF using the 2 instructions BSF
and BCF for example:

BSF PORTB,3 makes PORTB,3 (pin9) 5v.
BCF PORTB,7 makes PORTB,7 (pin 13) Ov.

Saving and assembling the code

The program is then saved as FLASHER. ASM. The next task is to assemble
this text into the HEX code that the microcontroller understands.

Open MPLAB the screen shown below in Figure 2.2 will open up.

Open the file FLASHER. ASM using the FILE menu as shown in Figure 2.3.

From the CONFIGURE Menu, Select Device then choose the micro 16F84 in
this example, as indicated in Figure 2.4.

Next choose CONFIGURE - Configuration Bits as shown in Figure 2.5 and
set as indicated.

Our configuration bits setting, select the LP Oscillator, turn the Watchdog
Timer Off, turn the Power Up Timer on and turn Code Protect off.

Notice the value of this configuration is 3FF0 in hex. This configuration setting
can be written into the header program so there is no need to here. The code is
_CONFIG H'3FF0'

The choice of configuration bit settings for the 16F84 are:

• the Oscillator, RC, LP, XT, HS. i.e. LP

• Watchdog Timer ON/OFF i.e. OFF

20 Programming the 16F84 microcontroller

* MPLABIDEv6.30

-ile Edit View Project Debugger Programmer Configure Window Help

Qg»Q 1 ^8 \S f

| c? G? H *

Figure 2.2 MPLAB initial screen

m MPLAB IDE v6, 30

Edit View Project Debugger Programmer
Ctrl+N

Configure Window Help

Mew

Open,..

Ctrl+O

\dttga *

H:\Micro\FLASHER.ASM

Save
Save As, ,
SaveAl

Ctrt+S

Ell®

Open Workspace,,,
Save Workspace
Save Workspace As. , .
Close Workspace

HEADER. ASH FOR 16F34. This sets PORTA -
;port. It also sets the 0PT>
;pulses of 1/32 of a second.
;1 second and 0.5 second deJ
; sub r out In* section

IKBO

EQU

PORTA

EQU

PORT©

EQU

OPTION P.

EQU

8IH

rRISA

EQU

esH

TPISB

EQU

3bH

STATUS

EQU

ZEROBIT

EQU

COUHT

EQU

OCH

***■*■**+**■*+♦♦*■»-■*■*•*■■**•*■*•*•***■*

LIST

P-16F84

ORG

GOTO

START

*♦***** + + ■*

Configuration Bits

Figure 2.3 Opening FLASHER.ASM

Programming the 16F84 microcontroller 21

Configure

Window Hel(

tfU

Select Device..,

Configuration Bits...
External Memory,,.
ID Memory..,

Settings*.,

Select Device

Device:

jiidLliU

Microchip Programmer Tool Support -

PICSTART Plus

MPLAB ICD 2

PRO MATE II

PICkrtI

MPLAB PM3

Microchip DehuggerTool Support
O MPLAB SIM MPLAB ICD 2

MP LAB SIM3Q

MPLAB ICE 2000

MPLAB ICE 4000

OPCM16XH0
OPCM16XH1

No Module

Cancel

Help

Figure 2.4 CONFIGURE - select device

1 Window Hdp

Select Device...

Configuration Bits... |

ID Memory...

■

Configuration Bits

- □ x!

1 Address

Value Category

Setting

Settings... 1

|20O7 3FFO Oscillator LP

Watchdog Tinier Off
Power Up Timer On
Code Protect Off

Figure 2.5 Configuration bits setting

22 Programming the 16F84 microcontroller

• Power Up Timer ON/OFF i.e. ON

• Code Protect ON/OFF i.e. OFF

Then we have to convert our text, FLASHER.ASM into a machine code file
FLASHER.HEX to do this choose PROJECT - Quickbuild Flasher.ASM as
shown in Figure 2.6.

Figure 2.6 Compiling FLASHER.ASM to FLASHER.HEX

If the program has compiled without any errors then MPLAB will return with
a message Build Succeeded as indicated in Figure 2.7. There may be some
warnings and messages but do not worry about them, the compiler has seen
something it wasn't expecting.

Incidently, I always have line numbers on my code to find my way around,
especially in larger programs. Line numbers can be turned on and off with the
path: EDIT - PROPERTIES.

Suppose that you have a syntax error in your code. The message Build Failed
will appear as shown in Figure 2.8. You then have to correct the errors.
MPLAB has indicated the error in the message box. If you 'double click' on the
error message then MPLAB will indicate, with an arrow, where the error is

Programming the 16F84 microcontroller 23

Output

Buid F^dnFtet

- n x 1

Naming \H J j H Ml ICkux* UbHtk Atin
¥arm ng [ 20 7 ] H : \M ICROvFLASHER . ASM
Warning! 207] H

\HICRO\FLASHER . ASK
\MICRO\FLASHER ASM
Varni ng [207] HAM ICRO\FXASHER ASM

«aramg[207]
Warning[207J
Warning! 20 7]
Warning [20 7 J

nHICROFLASHERASH
\MICROxFLASHER ASM
\MICROvFLASHER . ASM
\MICRQ\FLASHER.ASM

Warning! 20 7] H:\MICROxFLASHER.ASM

13
14
15
30

31
38
39

47
13

51
53

Message [302] H:\MI CRO vFLASHER , ASK
Message [302] H : NM ICROnFLASHER ASM
Message! 302] H:\HICROvFLASHER ASM
Warni ng [205] H Ml ICR0\FLASHER . ASM
Loaded H. \Hicro\FLASHER.COD
BUILD SUCCEEDED: Wed Apr 27 10 25 20 2005

bound label
Found label
Found label
Found label
Found label
Found label
Found label
Found label
Found label
Register in
Register in
Register in
Found direc

at ter column 1 .
after column 1.
after column 1
after column 1 .
af ter column 1 ,
after column 1
af ter column 1
after column 1 .
after column 1 .
operand not in
operand not in
operand not in
tive in column 1

(JKlbb} A

(STATUS) -

(ZEROBIT

(COUNT)

( DELAY 1)

(LOOPA)

(DELAYP5

(LOOPB)

(START)

bank

bank 0.

bank
(END)

Figure 2.7 Build Succeeded

in your code. Correct the errors and compile (Quickbuild) again to produce
an error free build.

The error I have written into my code occurs in line 61, with the message,
'symbol not previously defined (PORT)'. I should have written PORTB the
compiler does not understand TORT'.

After successfully building the program, the HEX code is ready to be
programmed into the Microcontroller.

You can view your compilation using VIEW - PROGRAM MEMORY as
shown in Figure 2.9.

The FLASHER. HEX file is now ready to be programmed into the chip.

PICSTART PLUS programmer

If you do not have a programmer I would recommend Arizona Microchip's
own PICSTART PLUS. When Arizona bring out a new microcontroller as

24 Programming the 16F84 microcontroller

Bufci |rtid

Warning L
Varningf
Warning!
Warning [
W©rning[
Warning [
Warning [
Warning [
Message [
Message [

hFiet|

TuTTa
207] H
207] H
207] H
207] H
207] H
207] H
207] a
302] H
302] H
302] H

\HlCKUXfLAbHtK Abtt 1J hound label

xMICROnFLASHER ASM 14 Found label

\KICRO\FLASHER ASM 15 Found label

\HICROFLASHER ASM 30 Found label

\KICFO\FLASHER ASM 31 Found label

xKICROvFLASHER ASM 38 Found label

\KlCROxFLASHER ASM 39 Found label

\M1CR0\FLASH£R ASM 47 Found label

\MICRO\FLASHER ASM 49 Register in

\KICRO\FLASHER ASM 51 Register in

\KICRO\FLASHER ASM 53 Register in

alter column 1
after col turn 1 .
after column 1
after column 1
after column 1
after column 1
after column 1
after column 1.
operand not in
operand not in
operand not in

(STATUS) ^

(ZER0B1T

(COUNT)

(DELAY1)

(LOOPA)

(DEXAYP5

(LOOPB)

(START)

bank 0.

bank .

bank

Error F 1131 H HKRu •.FLAbHER AbM bl bvabol not Dreviouslv defined {PORT)

Warning[205] H XKICRO^FLASHER ASM 66 Found directive in column 1
Skipping link step. Hot all sources built successfully.
BUILD FAILED: Wed Apr 27 10:50:14 2005

END

H:\Micro\FIASHFR.ASM

STJUH

I- .[DI[X|

Untitle... .cPI n 5

Figure 2.8 Build failed

■ SF STATUS

£ ;T

tlOVLV

B'00011111"

;S bits o>

movwf

rami

KOVLW

KOVYJF

TI V£B

; PORTB IS

KOVLW

B' 00000111"

UDVYJF

OPTION P.

;PRISCAIBI

act

STATUS, 5

f Return v.e

CLEF

PORTA

; Clears PC

ri.p.F

PORTB

; Clears PC

;Prograa start* nou,

BEG If," BSV
CALL
1CF
CALL
GOTO

PORT,
DILAYP5

PORTB ,
DILAYP5

BECIR

,Tum OB BO.
;w*it 0,5 second*.

;Turn OFF BO.
,V*it 0.5 seconds.

; Repeat

;You MUST END.

1 Project Debugger Programmer

Configure Window

Help

• Project

• Output

Disassembly
Hardware Stack

ti fi?U

■

Program Memory

JijTj

| Line | Address

| Opcode

| Label

Disassembly A

He Registers

EEPROM
LCDPfed

Watch

Special Function Registers

0000
0001
0002
0003
0004
0005

I280D

GOTO START ! -^

4
5

0181
0601
3C20

1D03
2802

DELAY1
LOOPA

CLRF THRO
HOVF THRO,
SUBLM 0x20
BTFSS STATUS, 0x2
GOTO LOOPA

| <

[ Opcode Hex

Machine

Symbolic

Figure 2.9 Program memory

they do regularly, the driver software is updated and can be downloaded free
off the internet from MICROCHIP.COM.

Once installed on your PC it is opened from MPLAB i.e.

Switch on the PICSTART Plus Programmer.

Programming the 16F84 microcontroller 25

Select, Programmer - Select Programmer - PICSTART Plus, shown in
Figure 2.10.

Programme

Select Pic jrammei

Configure Window Hefci
None

Enable Programmer

Pw jram
Verfv

Blank Ched Al
Blani Ched OTP

Reset

Reset Program Statistics

Erase Flash Device

Download PICSTART OS

AboU

^;:r^TF _, :

Figure 2.10 Selecting the PICSTART plus programmer

Select Enable Programmer from the Programmer box, Figure 2.10.

The final stage is to program your code onto the chip. To do this click the
programming icon shown in Figure 2.11 or via the menu on Programmer -
Program.

Project Debugger Programmer Jools Configure Window H

DjjJ Dh D 3 Q K/ D>

Pass: Fail; Total:

Figure 2.11 Programming icon

After a short while the message success will appear on the screen.

You will be greeted with the success statement for a few seconds only, if
you miss it check the program statistics for Pass 1 Fail Total 1, which will be
continually updated.

The code has been successfully blown into your chip and is ready for use.

If this process fails - check the chip is inserted correctly in the socket, if it
is then try another chip.

So we are now able to use the microcontroller to switch an LED on and
off - Fantastic!

26 Programming the 16F84 microcontroller

But use your imagination. There are 35 instructions in your micro voca-
bulary. The PIC Microcontroller range at the moment includes devices with
64k bytes of EPROM-program memory, 3938 bytes of RAM-data memory,
1024 bytes of EEPROM, 72 Input and Output pins, 11 interrupts, 15 channel
A/D converter, 20MHz. clock, real time clock, 4 counter/timers, 55 word
instruction set. See Appendix A for a detailed list. If the 64k of EPROM or
3938 bytes of RAM is not enough your system can be expanded using extra
EPROM and RAM. In the end the only real limits will be your imagination.

Programming flowchart

Open MPLAB

Produce file FLASHER.ASM <

Quickbuild Flasher.ASM

Correct errors

Program Microcontroller

Problem: flashing two LEDs

There has been a lot to do and think about to get this first program into
the microcontroller and make it work in a circuit. But just so that you are
sure what you are doing - Write a program that will flash two LEDs on and
off alternately. Put LEDO on BO and LED1 on Bl. NB you can use the
file FLASHER.ASM it only needs two extra lines adding! Then save it as
FLASHER2.ASM

The circuit layout is shown in Figure 2.12.

Try not to look at the solution below before you have attempted it.

Programming the 16F84 microcontroller 27

68p 32kHz

-16
15

68p

16F84

B1
BO

V+
MCLR

680R

6 680R

=t0.1(i

Figure 2.12 Circuit to flash 2 LEDs

Solution to the problem, flashing two LEDs

The header is the same as in FLASHER. ASM. just include in the section,
program starts now, the following lines:

;Program starts now.

BEGIN

BSF

PORTB,0

;Turn ON BO.

BCF

PORTB,l

;Turn OFF Bl

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,0

;Turn OFF BO.

BSF

PORTB,l

;Turn ON Bl.

CALL

DELAYP5

;Wait 0.5 seconds

GOTO

BEGIN

;Repeat

END

28 Programming the 16F84 microcontroller

Did you manage to do this? If not have a look at my solution and see what the
lines are doing. Now try flashing 4 LEDs on and off, with 2 on and two off
alternately. You might like to have them on for 1 second and off for half a
second. Can you see how to use the 1 -second delay in place of the half-second
delay.

The different combinations of switching any 8 LEDs on PORTB should be
relatively easy once you have mastered these steps.

Perhaps the most difficult step in understanding any new technology is
getting started. The next chapter will introduce a few more projects similar to
Flasher. ASM to help you progress.

3
Introductory projects

New instructions used in this chapter:

• MOVLW

• MOVWF

• DECFSZ

Let's have a look at a few variations of flashing the LEDs on and off to
develop our programming skills.

LED_Flasher2

Suppose we want to switch the LED on for 2 seconds and off for 1 second.
Figure 2.1 shows the circuit diagram for this application. The code for this
would be:

;Program starts now.
BEGIN

BSF

PORTB,0

;Turn on BO

CALL

DELAY 1

;Wait 1 second

CALL

DELAY 1

;Wait 1 second

BCF

PORTB,0

;Turn off BO

CALL

DELAY 1

;Wait 1 second

GOTO

BEGIN

;Repeat

END

NB. This code would be added to HEADER84.ASM into the section called,
"Program starts now".

To do this open MPLAB, then FILE - OPEN HEADER84.ASM
Add the code and saveas LED_FLASHER2.ASM

The text would then be assembled by the MPLAB software and then blown
into the Microcontroller as explained in Chapter 2.

How does it work?

The comments alongside the code explain what the lines are doing. Because we
do not have a 2 second delay we wait for 1 second twice. You can of course
write a 2 second delay routine but we will be looking at this later.

30 Introductory projects

SOS

For our next example let us switch BO on and off just as we have been doing
but this time we will use delays of % second and 54 second. This is not
much different than we have done previously, but instead of turning an LED
on and off we will replace it by a buzzer. The program is not just going to
turn a buzzer on and off, but do it in a way that generates the signal, SOS.
Which is DOT,DOT,DOT DASH,DASH,DASH DOT,DOT, DOT. Where
the DOT is the buzzer on for % second and the DASH is the buzzer on for
Vi second with % second between the beeps.

The circuit diagram for the SOS circuit is shown in Figure 3.1.

68p 32kHz 16

68p

16F84

V+

MCLR
TOCKI

=r0.1(i

Figure 3.1 SOS circuit diagram

Code for SOS circuit

The complete code for the SOS circuit is shown below because an extra
subroutine, DELAYP25, has been added.

SOS. ASM for 16F84. This sets PORTA as an INPUT (NB lmeans input)

and PORTB as an OUTPUT (NB means output).

The OPTION Register is set to /256 to give timing pulses of 1/32 of a second.

1 second, 0.5 second and 0.25 second delays are included in the subroutine

section.

Introductory projects 31

;EQUATES SECTION

TMRO

EQU

STATUS

EQU

PORTA

EQU

PORTB

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION R

EQU

81H

ZEROBIT

EQU

COUNT

EQU

OCH

LIST

P=16F84

ORG

GOTO

START

;means TMRO is file 1.

;means STATUS is file 3.

;means PORTA is file 5.

;means PORTB is file 6.

;TRISA (the PORTA I/O selection) is file 85H

;TRISB (the PORTB I/O selection) is file 86H

;the OPTION register is file 81H

;means ZEROBIT is bit 2.

;COUNT is file OC, a register to count events.

;we are using the 16F84.

;the start address in memory is

;goto start!

Configuration Bits

CONFIG H'3FF0'

;selects LP oscillator, WDT off, PUT on,
;Code Protection disabled.

SUBROUTINE SECTION

;1 second delay.

DELAY 1

CLRF

TMRO

LOOPA

MOVF

TMR0,W

SUBLW

.32

BTFSS

STATUS,
ZEROBIT

GOTO

LOOPA

RETLW

;0.5 second delay.

DELAYP5

CLRF

TMRO

LOOPB

MOVF

TMR0,W

SUBLW

.16

BTFSS

STATUS,
ZEROBIT

GOTO

LOOPB

RETLW

START TMRO.

READ TMRO INTO W.

TIME - 32

;CheckTIME-W =

;Time is not = 32.
;Time is 32, return.

START TMRO.

READ TMRO INTO W.

TIME - 16

;CheckTIME-W =

;Time is not = 16.
;Time is 16, return.

32 Introductory projects

;0.25 second delay.

DELAYP25 CLRF

TMRO

;START TMRO.

LOOPC MOVF

TMR0,W

;READ TMRO INTO W.

SUBLW

;TIME - 8

BTFSS

STATUS,

ZEROBIT

;Check TIME-W =

GOTO

LOOPC

;Time is not = 8.

RETLW

;Time is 8, return.

CONFIGURATION SECTION

START BSF

STATUS,5

;Turns to Bankl.

MOVLW

B'oooinir

;5bits of PORTA are I/P

MOVWF

TRISA

MOVLW

B'OOOOOOOO'

MOVWF

TRISB

;PORTB is OUTPUT

MOVLW

B'ooooonr

;Prescaler is /256

MOVWF

OPTION_R

;TIMER is 1/32 sees.

BCF

STATUS,5

; Return to BankO.

CLRF

PORTA

;Clears PortA.

CLRF

PORTB

;Clears PortB.

;Program starts now.

BEGIN

BSF

PORTB,0

;Turn ON BO, DOT

CALL

DELAYP25

;Wait 0.25 seconds

BCF

PORTB,0

;Turn OFF BO.

CALL

DELAYP25

;Wait 0.25 seconds

BSF

PORTB,0

;Turn ON BO, DOT

CALL

DELAYP25

;Wait 0.25 seconds

BCF

PORTB,0

;Turn OFF BO.

CALL

DELAYP25

;Wait 0.25 seconds

BSF

PORTB,0

;Turn ON BO, DOT

CALL

DELAYP25

;Wait 0.25 seconds

BCF

PORTB,0

;Turn OFF B0.

CALL

DELAYP5

;Wait 0.5 seconds

BSF

PORTB,0

;Turn ON B0, DASH

CALL

DELAYP5

;Wait 0.5 seconds

Introductory projects 33

BCF

PORTB,0

;Turn OFF BO.

CALL

DELAYP25

;Wait 0.25 seconds

BSF

PORTB,0

;Turn ON BO, DASH

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,0

;Turn OFF BO.

CALL

DELAYP25

;Wait 0.25 seconds

BSF

PORTB,0

;Turn ON BO, DASH

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,0

;Turn OFF BO.

CALL

DELAYP5

;Wait 0.5 seconds

BSF

PORTB,0

;Turn ON B0, DOT

CALL

DELAYP25

;Wait 0.25 seconds

BCF

PORTB,0

;Turn OFF B0.

CALL

DELAYP25

;Wait 0.25 seconds

BSF

PORTB,0

;Turn ON B0, DOT

CALL

DELAYP25

;Wait 0.25 seconds

BCF

PORTB,0

;Turn OFF B0.

CALL

DELAYP25

;Wait 0.25 seconds

BSF

PORTB,0

;Turn ON B0, DOT

CALL

DELAYP25

;Wait 0.25 seconds

BCF

PORTB,0

;Turn OFF B0.

CALL

DELAYP5

;Wait 0.5 seconds

CALL

DELAY 1

CALL

DELAY 1

;Wait 2 seconds before returning.

GOTO

BEGIN

; Repeat

END

;YOU MUST END!!

How does it work?

I think the explanation of the code is clear from the comments. At the end of
the SOS the program has a delay of 2 seconds before repeating. This should be
a useful addition to any alarm project.

We will now move onto switching a number of outputs on and off. Consider
flashing all 8 outputs on PORTB on and off at Vi second intervals.

Flashing 8 LEDs

The circuit for this is shown in Figure 3.2.

This code is to be added to HEADER84.ASM as in LED FLASHER2.ASM

34 Introductory projects

Figure 3.2 Flashing 8 LEDs

;Program starts now.

BEGIN

BSF

CALL

BCF

PORTB,0

PORTB,l

PORTB,2

PORTB,3

PORTB,4

PORTB,5

PORTB,6

PORTB,7

DELAYP5

PORTB,0

PORTB,l

PORTB,2

PORTB,3

;Turn ON BO
;Turn ON Bl
;Turn ON B2
;Turn ON B3
;Turn ON B4
;Turn ON B5
;Turn ON B6
;Turn ON B7
;Wait 0.5 seconds
;Turn OFF BO
;Turn OFF Bl
;Turn OFF B2
;Turn OFF B3

Introductory projects 35

BCF

PORTB,4

;Turn OFF B4

BCF

PORTB,5

;Turn OFF B5

BCF

PORTB,6

;Turn OFF B6

BCF

PORTB,7

;Turn OFF B7

CALL

DELAYP5

;Wait 0.5 seconds

GOTO

BEGIN

END

Save the program as FLASH8.ASM, assemble and program the 16F84 as
indicated in Chapter 2.

There is an easier way than this of switching all outputs on a port, which we
look at later in this chapter with a set of disco lights.

Chasing 8 LEDs

Let's now consider the code to chase the 8 LEDs. The circuit of Figure 3.2 is
required for this. The code will switch BO on and off, then Bl, then B2 etc.

;Program starts now.

BEGIN

BSF

PORTB,0

;Turn ON BO

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,0

;Turn OFF BO

BSF

PORTB,l

;Turn ON Bl

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,l

;Turn OFF Bl

BSF

PORTB,2

;Turn ON B2

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,2

;Turn OFF B2

BSF

PORTB,3

;Turn ON B3

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,3

;Turn OFF B3

BSF

PORTB,4

;Turn ON B4

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,4

;Turn OFF B4

BSF

PORTB,5

;Turn ON B5

CALL

DELAYP5

;Wait 0.5 seconds

36 Introductory projects

BCF

PORTB,5

;Turn OFF B5

BSF

PORTB,6

;Turn ON B6

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,6

;Turn OFF B6

BSF

PORTB,7

;Turn ON B7

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTBJ

;Turn OFF B7

CALL

DELAYP5

;Wait 0.5 seconds

GOTO

BEGIN

END

This code once again is added to the bottom of HEADER84.ASM and is
saved as

CHASE8A.ASM

Now that we have chased the LEDs one way let's run them back the other
way and call the program CHASE8B.ASM. I think you know the routine
add the code to the bottom of HEADER84.ASM etc. So I will not mention
it again.

;CHASE8B.ASM

;Program starts now.

BEGIN

BSF

PORTB,0

;Turn ON BO

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,0

;Turn OFF BO

BSF

PORTB,l

;Turn ON Bl

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,l

;Turn OFF Bl

BSF

PORTB,2

;Turn ON B2

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,2

;Turn OFF B2

BSF

PORTB,3

;Turn ON B3

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,3

;Turn OFF B3

BSF

PORTB,4

;Turn ON B4

CALL

DELAYP5

;Wait 0.5 seconds

Introductory projects 37

BCF

PORTB,4

;Turn OFF B4

BSF

PORTB,5

;Turn ON B5

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,5

;Turn OFF B5

BSF

PORTB,6

;Turn ON B6

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,6

;Turn OFF B6

BSF

PORTB,7

;Turn ON B7

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,7

;Turn OFF B7

BSF

PORTB,6

;Turn ON B6

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,6

;Turn OFF B6

BSF

PORTB,5

;Turn ON B5

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,5

;Turn OFF B5

BSF

PORTB,4

;Turn ON B4

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,4

;Turn OFF B4

BSF

PORTB,3

;Turn ON B3

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,3

;Turn OFF B3

BSF

PORTB,2

;Turn ON B2

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,2

;Turn OFF B2

BSF

PORTB,l

;Turn ON Bl

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,l

;Turn OFF Bl

GOTO

BEGIN

END

Just one last flasher program. Let us switch each output on in turn leaving
them on as we go and then switch them off in turn. Try this for yourselves
before looking at the solution!

The program is saved as UPANDDOWN.ASM

38 Introductory projects

;Program starts now.

BEGIN

BSF

PORTB,0

;Turn ON BO

CALL

DELAYP5

;Wait 0.5 seconds

BSF

PORTB,l

;Turn ON Bl

CALL

DELAYP5

;Wait 0.5 seconds

BSF

PORTB,2

;Turn ON B2

CALL

DELAYP5

;Wait 0.5 seconds

BSF

PORTB,3

;Turn ON B3

CALL

DELAYP5

;Wait 0.5 seconds

BSF

PORTB,4

;Turn ON B4

CALL

DELAYP5

;Wait 0.5 seconds

BSF

PORTB,5

;Turn ON B5

CALL

DELAYP5

;Wait 0.5 seconds

BSF

PORTB,6

;Turn ON B6

CALL

DELAYP5

;Wait 0.5 seconds

BSF

PORTB,7

;Turn ON B7

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,7

;Turn OFF B6

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,6

;Turn OFF B6

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,5

;Turn OFF B5

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,4

;Turn OFF B4

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,3

;Turn OFF B3

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,2

;Turn OFF B2

CALL

DELAYP5

;Wait 0.5 seconds

BCF

PORTB,l

;Turn OFF Bl

CALL

DELAYP5

;Wait 0.5 seconds

Introductory projects 39

BCF

CALL

GOTO

PORTB,0

DELAYP5

BEGIN

;Turn OFF BO
;Wait 0.5 seconds

END

There are lots of other combinations for you to practice on. I'll leave you to
experiment further.

Consider another example of the delay routine:

Traffic lights

If you have ever tried to design a 'simple' set of traffic lights then you will
appreciate how much circuitry is required. An oscillator circuit, counters and
logic decode circuitry.

The microcontroller circuit is a much better solution even for this 'simple'
arrangement. The circuit is shown in Figure 3.3.

68p 32kHz

-16

68p

16F84

B5
B4
B3

V+
MCLR

6 x 680R

±0.1^1

Figure 3.3 Traffic lights circuit

40 Introductory projects

A truth table of the operation of the lights is probably a better aid to a solution
rather than a flowchart.

Traffic light truth table

Time

2sec

5sec

2sec

5 sec

2sec

REPEAT

Program listing for the traffic lights

TRAFFIC.ASM

Program starts now.

BEGIN

MOVLW
MOVWF
CALL

B'00100100' ;R1, R2 on.

PORTB

DELAY2 ;Wait 2 Seconds.

MOVLW
MOVWF
CALL

B'001 10100' ;R1, Al, R2 on.

PORTB

DELAY2 ;Wait 2 Seconds.

MOVLW
MOVWF
CALL

B'00001100' ;G1, R2 on.

PORTB

DELAY5 ;Wait 5 Seconds.

MOVLW
MOVWF
CALL

B'00010100' ;A1, R2 on.

PORTB

DELAY2 ;Wait 2 Seconds.

MOVLW
MOVWF

B'00100100' ;R1, R2 on.
PORTB

Introductory projects 41

CALL

DELAY2

;Wait 2 Seconds.

MOVLW

B'00100110'

;R1, R2, A2 on.

MOVWF

PORTB

CALL

DELAY2

;Wait 2 Seconds.

MOVLW

B'ooioooor

;R1, G2 on.

MOVWF

PORTB

CALL

DELAY5

;Wait 5 Seconds.

MOVLW

B'OOIOOOIO'

;R1, A2 on.

MOVWF

PORTB

CALL

DELAY2

;Wait 2 Seconds.

GOTO

BEGIN

END

How does it work

In a previous examples we turned LEDs on and off with the two commands
BSF and BCF, but a much better way has been used with the TRAFFIC.ASM
program.

The basic difference is the introduction of two more commands:

• MOVLW MOVe the Literal (a number) into the Working register.

• MOVWF MOVe the Working register to the File.

The data, in this example, binary numbers, are moved to W and then to the file
which is the output PORTB to switch the LEDs on and off. Unfortunately
the data cannot be placed in PORTB with only one instruction it has to go
via the W register.

So:

MOVLW B'OOIOOIOO'

MOVWF PORTB

clears B7,B6, sets B5, clears B4,B3, sets B2
and clears Bl, BO in the W register
moves the data from the W register to PORTB
to turn the relevant LEDs on and off.

All 8 outputs are turned on/off with these 2 instructions.

CALL DELAY2 and CALL DELAY5 waits 2 seconds and 5 seconds before

continuing with the next operation. DELAY2 and DELAY5 need adding to

the subroutine section as:

; 5 second delay.

DELAY5 CLRF TMRO ; START TMRO.

42 Introductory projects

LOOPC MOVF

TMR0,W ;

SUBLW

.160 ;

BTFSS

STATUS,ZEROBIT ;

GOTO

LOOPC ;

RETLW

;

; 2 second delay.

DELAY2 CLRF

TMRO ;

LOOPD MOVF

TMRO/W ;

SUBLW

.64 ;

BTFSS

STATUS,ZEROBIT ;

GOTO

LOOPD ;

RETLW

;READ TMRO INTO W.
;TIME - 160
;Check TIME - W =
;Time is not= 160.
;Time is 160, return.

START TMRO.
READ TMRO INTO W.
TIME - 64
Check TIME - W =
Time is not = 64.
;Time is 64, return.

The W register

The W or working register is the most important register in the micro. It is
in the W register were all the calculations and logical manipulations such as
addition, subtraction, and-ing, or-ing etc., are done.

The W register shunts data around like a telephone exchange re-routes tele-
phone calls. In order to move data from locationA to locationB, the data has
to be moved from locationA to W and then from W to location B

NB. If the three lines in the TRAFFIC. ASM program are repeated then any
pattern and any delay can be used to sequence the lights - you can make your
own disco lights!

Repetition (e.g. disco lights)

Instead of just repeating one sequence over and over, suppose we wish to repeat
several sequences before returning to the start as with a set of disco lights.

Consider the circuit shown in Figure 3.4. The 8 'Disco Lights' B0-B7 are to be
run as two sequences.

Sequence 1 Turn all lights on.
Wait.

Turn all lights off
Wait

Sequence 2 Turn B7-B4 ON, B3-B0 OFF
Wait
Turn B7-B4 OFF, B3-B0 ON

Wait

Introductory projects 43

8 x 680R

Figure 3.4 Disco lights

Suppose we wish Sequence 1 to run 5 times before going onto Sequence 2 to
run 10 times and then repeat. A section of program is repeated a number of
times with 4 lines of code shown below:

MOVLW .5 ;Move 5 into W

MOVWF COUNT ;Move W into user file COUNT

SEQ1

DECFSZ COUNT decrement file COUNT skip if zero.

GOTO SEQ1 ;COUNT not yet zero, repeat sequence

• The first two lines set up a file COUNT with 5. (Count is the first user file
and is found in memory location OCH.) 5 is first of all moved into W then
from there to file COUNT.

• SEQ1 is executed.

• The DECFSZ COUNT instruction, DECrement File and Skip if Zero,
decrements, takes 1 off, the file COUNT and skips GOTO SEQ1 if the count
is zero, if not zero then do SEQ1 again.

44 Introductory projects

This way SEQ1 is executed 5 times and COUNT goes from 5 to 4 to 3 to 2 to 1
to when we skip and follow onto SEQ2. SEQ2 is then done 10 times, say, and
the code would be:

MOVLW .10 ;Move 10 into W

MOVWF COUNT ;Move W into user file COUNT

SEQ2

DECFSZ COUNT decrement file COUNT skip if zero.

GOTO SEQ2 ;COUNT not yet zero, repeat sequence

Program code for the disco lights

;DISCO.ASM

;Program starts now.

BEGIN

SEQ1

SEQ2

MOVLW

MOVWF

COUNT

MOVLW

B'liiinir

MOVWF

PORTB

CALL

DELAYP5

MOVLW

B'00000000'

MOVWF

PORTB

CALL

DELAYP5

DECFSZ

COUNT

GOTO

SEQ1

MOVLW

.10

MOVWF

COUNT

MOVLW

B'l 11 10000'

MOVWF

PORTB

CALL

DELAYP5

MOVLW

B'oooonir

MOVWF

PORTB

CALL

DELAYP5

DECFSZ

COUNT

GOTO

SEQ2

GOTO

BEGIN

;Set COUNT = 5

;Turn B7-B0 ON
;Wait 0.5 seconds

;Turn B7-B0 OFF
;Wait 0.5 seconds
;COUNT-l, skip if 0.

;Set COUNT =10

;B7-B4 on, B3-B0 off

;Wait 0.5 seconds

;B7-B4 off, B3-B0 on
;Wait 0.5 seconds

;COUNT-l, skip if 0.

END

Introductory projects 45

Using the idea of repeating sequences like this any number of combinations
can be repeated. The times of course do not need to be of 0.5 seconds duration.
The flash rate can be speeded up or slowed down depending on the
combination.

Try programming a set of your own Disco Lights. This should keep you quiet
for hours (days!).

More than 8 outputs

Suppose we wish to have a set of disco lights in a 3 x 3 matrix as shown in
Figure 3.5. This configuration of course requires 9 outputs. We have 8 outputs
on PORTB so we need to make one of the PORTA bits an output also, say
PORTA bitO.

68p 32kHz

J 6
"15

68p

16FB4

B7
B6

B5
B4
B3

B2
B1
BO

V+
MCLR

11
10_

9 x 680R

=r0.1n

V\ V

\Z\ V

V\ V

Figure 3.5 9 Disco light set

46 Introductory projects

To change PORTA bitO from an input to an output change the lines in the
Configuration section from:

MOVLW

MOVWF

MOVLW

MOVWF

B'oooinir

TRISA

B'00011110'
TRISA

NB a 1 signifies an input a signifies an output.

So to set a '+' pattern in the lights we turn on B7, B4, Bl, B3 and B5, keeping
the others off. The code for this would be:

MOVLW
MOVWF
MOVLW
MOVWF

B'00000000'
PORTA
B'10111010'
PORTB

;A0 is clear

;B7, B5, B4, B3 and Bl are on

So to set an 'X' pattern in the lights we turn on B6, B4, B2, AO and BO, keeping
the others off. The code for this would be:

MOVLW
MOVWF
MOVLW
MOVWF

B'ooooooor

PORTA

B'01010101'
PORTB

;A0 is on

;B6, B4, B2 and BO are on

There are endless combinations you can make with 9 lights. In fact there are
512. That is 2 9 . This should give you something to go at!

Headers, porting
code - which micro?

Arizona Microchip the manufacturers of the PIC Microcontroller make
over 100 different types of microcontroller. How do we choose the correct one
for the job?

Factors affecting the choice of the microcontroller

When deciding on which Microcontroller to use for your application there are
a number of factors you will need to consider.

• How many inputs and outputs do you need. If you are using the program
FLASHER.ASM which only flashes 1 LED on and off then any PIC will
do this. If you are turning 8 outputs on and off then you will need
a microcontroller that has at least 8 I/O (of course). So an 8pin micro
i.e. 12F629 will not do because it only has 6 I/O.

• Do you need accurate timing? If so then you will need to add a crystal
to your micro to provide the clock. If timing is not that critical then
you can use a micro that has an on board oscillator such as the 16F818. You
can then omit the crystal and 2 capacitors. The timing accuracy is about
1%. This would do for FLASHER.ASM but not for a 24 hour clock.
1% is about 14 minutes a day.

• Are you making analogue measurements? If so you will need a micro with an
AtoD converter on it. The 16F818 has a 5 channel, 10 bit AtoD converter.
If you need more that 5 channels then you will need to use a micro with
more AtoD channels such as the 16F877 which has 8.

• What operating frequency do you require? The greater the frequency the
faster your code will execute. Most newer devices can operate up to 20MHz,
some even faster. Some older devices can only achieve 4MHz. The programs
in this book only require an operating speed of 4MHz.

• How many instructions are there in your program? The 16F818 has
space for Ik i.e. 1024 instructions. The 16F877 has 8k program memory
locations. All programs in this book require less than Ik of program
memory space.

• How many memory locations are required to store data? The 16F818 has
128 bytes of data memory, the 16F877 has 368.

48 Headers, porting code - which micro?

• Do you need to store data so that it will be saved if the power is removed or
lost? If so you need a micro with EEPROM data memory. The 16F818 has
128 bytes of EEPROM memory, the 16F877 has 256.

There may be other requirements that you need from your micro, which are
not considered in this book, such as:

• Number of timers

• Comparators

• Pulse width modulation

• In circuit debugging

• USB drivers.

Choosing the microcontroller

As I mentioned previously the FLASHER. ASM program which flashes 1 LED
on and off can be performed by any Micro. Well, that has narrowed the field
down! So which microcontroller do we use for that application? If you were
mass producing these flasher units the answer would probably be - use the
cheapest and smallest - the 12C508 is possibly the device then. But for small
scale production or one offs you will probably have (or develop) a favorite.
Probably the most common chip used by the beginner is the 16F84; this has
been around since about 1998. This micro has built up a very large fan base
which is why it is still widely used. People are using this chip because they are
used to using it! There is now another micro on the market which will do
everything that the 16F84 can do and more. This device is the 16F818.

The data sheets for the 16F84 and 16F818 are shown in Figures 4.1 and 4.2
respectively.

The main differences are that the 16F818 has 16 I/O, an on board oscillator
with 8 selectable frequencies, 128 bytes of data RAM, 128 bytes of EEPROM,
3 Timers one of them a 16 bit, 5 channel 10 bit AtoD converter. The 16F84 has
13 I/O, no on board oscillator, 68 bytes of data RAM, 64 bytes of EEPROM,
1 timer, no AtoD. The most surprising difference of all is that the 16F84 is
about 3 times the price of the 16F818!!

The programs in this book consist of 2 parts:

• A header section which tells the 'build' software which device we are using,
configures the device, i.e. defines which pins are inputs and outputs, sets the
timer rate and includes some timing delays if you require them in a
subroutine section.

Headers, porting code - which micro? 49

Devices included in this Data Sheet:

• PIC16F83

• PIC16F84

• PIC16CR83

• PIC16CR84

• Extended voltage range devices available
(PIC16LF8X, PIC16LCR8X)

High Performance RISC CPU Features:

• Only 35 single word instructions to learn

• All instructions single cycle except for program
branches which are two-cycle

• Operating speed: DC - 10 MHz clock input

DC - 400 ns instruction cycle

Pin Diagrams

Device

Program
Memory
(words)

Data
RAM
(bytes)

Date

EEPROM

(bytes)

MAX.
Freq
(MHz)

PIC16F83

512 Flash

PIC16F84

1 K Flash

PIC16CR83

512 ROM

PIC18CR84

1 KROM

• 14-bit wide instructions

• 8-bit data path

• 15 special function hardware registers

• Eight-level deep hardware stack

• Direct, indirect and relative addressing modes

• Four interrupt sources:

- External RB0/INT pin

- TMR0 timer overflow

- PORTE<7:4> interrupt on change

- Data EEPROM write complete

• 1000 erase/write cycles Flash program memory

• 10,000,000 erase/write cycles EEPROM data memory

• EEPROM Data Retention > 40 years

Peripheral Features:

• 13 I/O pins with individual direction control

• High current sink/source for direct LED drive

- 25 mA sink max. per pin

- 20 mA source max. per pin

• TMR0: 8-bit timer counter with 8-bit
programmable prescaler

Figure 4.1 The PIC 16F84 data sheet

PDIP, SOIC

RA2

RA3

RA4/ T0CKI

MCLR

Vss

RB0/INT

RB1

RB2

RB3

^ZJ~

2 3J

o o

O -n

31 2

RA1

RAO

OSC1/CLKIN

OSC2/CLKOUT

Vdd

RB7

RB6

RB5

RB4

Special Microcontroller Features:

• In-Circuit Serial Programming (ICSP™) - via two
pins (ROM devices support only Data EEPROM
programming)

• Power-on Reset (POR)

• Power-up Timer (PWRT)

• Oscillator Start-up Timer (OST)

• Watchdog Timer (WDT) with its own on-chip RC
oscillator for reliable operation

• Code-protection

• Power saving SLEEP mode

• Selectable oscillator options

CMOS Flash/EEPROM Technology:

• Low-power, high-speed technology

• Fully static design

• Wide operating voltage range:

- Commercial: 2.0V to 6.0V

- Industrial: 2.0Vto6.0V

• Low power consumption:

- < 2 mA typical @ 5V, 4 MHz

- 1 5 |iA typical @ 2V, 32 kHz

- < 1 u.A typical standby current @ 2V

• The second part of the program, entitled, 'Program starts now', is where you
write the code to perform your application.

The header program is unique to the particular microcontroller being used, but
the 'application code' entered after "Program starts now", is specific to the
application not the microcontroller. So any microcontroller that has i.e. the
required number of I/O or A/D can be used. As I mentioned before any
microcontroller can be used to execute the FLASHER. ASM code.

Headers

Just one point before we look at the headers. The 8 pin micros only have 6 I/O,
they do not have PORTA and PORTB pins, they have what is called a General

50 Headers, porting code - which micro?

Low-Power Features:

• Power Managed modes:

- Primary RUN: XT, RC oscillator,
87 uA, 1 MHz, 2V

- INTRC: 7 uA, 31.25 kHz, 2V

- SLEEP: 0.2 uA, 2V

• TimeM oscillator 1 .8 uA, 32 kHz, 2V

• Watchdog Timer: 0.7 uA, 2V

• Wide operating voltage range:

- Industrial: 2.0V to 5.5V

Oscillators:

• Three Crystal modes:

- LP, XT, HS: up to 20 MHz

• Two External RC modes

• One External Clock mode:

- ECIO:upto20 MHz

• Internal oscillator block:

- 8 user selectable frequencies: 31 kHz, 125 kHz,
250 kHz, 500 kHz, 1 MHz, 2 MHz, 4 MHz, 8 MHz

Peripheral Features:

• 16 1/0 pins with individual direction control

• High sink/source current: 25 mA

• TimerO: 8-bit timer/counter with 8-bit prescaler

• Timerl : 1 6-bit timer/counter with prescaler,
can be incremendet during Sleep via external
crystal/clock

• Timer2: 8-bit timer/counter with 8-bit period
register, prescaler and postscaler

• Capature, Compare, PWM (CCP) module:

- Capature is 16-bit, max. resolution is 12.5 ns

- Coampare is 16-bit, max. resolution is 200 ns

- PWM max. resolution is 10-bit

• 10-bit, 5-channel Analog-to-digital converter

• Synchronous Serial Port (SSP) with
SPI™ (Master/Slave) and l 2 C™ (Slave)

Pin Diagram

18-pin DIP, SOIC

RA2/AN2/Vref- -++-£

RA3/AN3/Vref+ -«-+-L

RA4/ AN4/T0 CKI **-+£

RA5/MCLR/Vpp — •»:

Vss— HI

RB0/INT «*■**

RB1/SDI/SDA«**-E

RB2/SDO/CCP1 «**"£

RB3/CCP1/PGM — »-L"

.1 ^ish-

2 „ 17 >

3 5 16 >

4 « 15 ]■

5 S 14 >

6 io 13 ]-

7 o 12 >

8 o. 11 >

9 10 >

•RA1/AN1
RA0/AN0
RA7/OSC1/CLKI
RA6/OSC2/CLKO
Vdd

•RB7/T10SI/PGD
RB6/T10SO/T1CKI/PGC
■RB5/SS
•RB4/SCK/SCL

Special Microcontroller Features:

• 100,000 erase/write cycles Enhanced FLASH
program memory typical

• 1 ,000,000 typical erase/write cycles EEPROM
data memory typical

• EEPROM Data Retection: > 40 years

• In-Circuit Serial Proramming™ (ICSP™)-
via two pins

• Processor read/write access to program memory

• Low Voltage Programming

• In-Circuit Debugging via two pins

Device

Program Memory

Data Memory

I/O Pins

10-bit
A/D (ch)

CCP

(PWM)

SSP

Timers

8/16-bit

FLASH
(bytes)

# Single Word
Instructions

SRAM
(bytes)

EEPROM
(bytes)

SPI

Slave
1 2 C

PIC16F818

1792

1024

126

128

2/1

PIC16F819

3584

2048

256

2/1

Figure 4.2 The PIC 16F818 and 16F819 data sheet

Purpose I/O or GPIO. So the instruction BSF PORTB,0 would have to be
changed to BSF GPIO,0.

The following headers will be used in this book:

HEAD12C508.ASM

HEAD12F629.ASM

HEAD12F675.ASM

HEAD 16F627. ASM

HEADER84.ASM

HEAD16F818.ASM

HEAD 16F872. ASM

for the 12C508 and 12C509

for the 12F629

for the 12F675

for the 16F627 and 16F628

for the 16F84

for the 16F818 and 16F819

for the 16F872, 16F874 and 16F877

Headers, porting code - which micro? 51

;HEAD12C508.ASM FOR 12C508/9.
;Uses the internal 4MHz clock.

TMRO

OSCCAL

GPIO

STATUS

ZEROBIT

COUNT

TIME

EQU
EQU
EQU
EQU
EQU
EQU

1
5
6
3
2
07H

;TMR0 is FILE 1.

;GPIO is FILE 6.
;STATUS is FILE 3.
;ZEROBIT is Bit 2.
:USER RAM LOCATION.

EQU 08H ;TIME IS 39

LIST
ORG
GOTO

P=12C508

START

;We are using the 12C508.
;0 is the start address.
;goto start!

Configuration Bits
CONFIG H'OFEA'

;selects Internal RC oscillator, WDT off,
;Code Protection disabled.

SUBROUTINE SECTION.
; 1/100 SECOND DELAY

DELAY

CLRF

TMRO

LOOPA

MOVF

TMR0,W

SUBWF

TIME,W

BTFSS

STATUS,ZEROBIT

GOTO

LOOPA

RETLW

;P5 SECOND DELAY

DELAYP5

MOVLW

.50

MOVWF

COUNT

TIMEC

CALL

DELAY

DECFSZ

COUNT

GOTO

TIMEC

RETLW

;1 SECOND DELAY

DELAYP5

MOVLW

.100

MOVWF

COUNT

TIMED

CALL

DELAY

DECFSZ

COUNT

GOTO

TIMED

RETLW

;START TMRO
;READ TMRO IN W
;TIME - W
;CHECK TIME-W =

;RETURN AFTER TMRO = 39

52 Headers, porting code - which micro?

CONFIGURATION SECTION.

START MOVWF

OSCCAL

MOVLW

B'00001000'

;5 bits of GPIO are O,

TRIS

GPIO

MOVLW

B'ooooonr

OPTION

;PRESCALER is /256

CLRF

GPIO

;Clears GPIO

MOVLW

.39

MOVWF

TIME

**************************************

;Program starts now.

END

HEAD12F629.ASM FOR 12F629 using 4MHz internal RC

TMRO

EQU

TRISIO

EQU

85H

GPIO

EQU

STATUS

EQU

ZEROBIT

EQU

OPTION_R

EQU

81H

CMCON

EQU

19H

OSCCAL

EQU

90H

COUNT

EQU

20H

;TMR0is FILE 1.

;GPIO is FILE 6.
;STATUS is FILE 3.
;ZEROBIT is Bit 2.

;USER RAM LOCATION

LIST P = 12F629 ;We are using the 12F629.

ORG

GOTO START

;0 is the start address.
;goto start!

Configuration Bits
CONFIG H'3F84'

;selects Internal RC oscillator, WDT off,
;Code Protection disabled.

SUBROUTINE SECTION.

;1/100 SECOND DELAY
DELAY CLRF TMRO

LOOPA MOVF TMR0,W

;START TMRO
;READ TMRO IN W

Headers, porting code - which micro? 53

SUBLW

.39

BTFSS

STATUS,Z1

GOTO

LOOPA

RETLW

;P1 SECOND DELAY

DELAYP1

MOVLW

.10

MOVWF

COUNT

TIMEC

CALL

DELAY

DECFSZ

COUNT

GOTO

TIMEC

RETLW

;TIME - W

;CHECK TIME-W =

;RETURN AFTER TMR0 = 39

CONFIGURATION SECTION.

START

BSF STATUS,5 ;BANK1

MOVLW B'OOOOlOOr ;BITS 0,3 are I/P

MOVWF TRISIO

MOVLW B'00000111'

MOVWF OPTION_R ;PRESCALER is /256

CALL 3FFH

MOVWF OSCCAL Calibrates 4MHz oscillator

BCF

STATUS,5 ;BANK0

MOVLW 7H

MOVWF CMCON ;Turns off comparator

CLRF GPIO ;Clears GPIO

;Program starts now.

END

;HEAD12F675.ASM FOR 12F675 using 4MHz internal RC.

;TMR0is FILE 1.

GPIO is FILE 6.
STATUS is FILE 3.
ZEROBIT is Bit 2.

TMRO

EQU

TRISIO

EQU

85H

GPIO

EQU

STATUS

EQU

ZEROBIT

EQU

54 Headers, porting code - which micro?

EQU

ADSEL

EQU

9EH

ADCONO

EQU

1FH

ADRESH

EQU

1EH

OPTION_R

EQU

81H

CMCON

EQU

19H

OSCCAL

EQU

90H

COUNT

EQU

20H

USER RAM LOCATION

LIST
ORG
GOTO

P = 12F675 ;We are using the 12F675.
;0 is the start address.

START ;goto start!

Configuration Bits
CONFIG H'3F84'

;selects Internal RC oscillator, WDT off,
:Code Protection disabled.

SUBROUTINE SECTION.

;1/100 SECOND DELAY

DELAY

CLRF

TMRO

LOOPA

MOVF

TMR0,W

SUBLW

.39

BTFSS

STATUS,ZEROBIT

GOTO

LOOPA

RETLW

;P1 SECOND DELAY

DELAYP1

MOVLW

.10

MOVWF

COUNT

TIMEC

CALL

DELAY

DECFSZ

COUNT

GOTO

TIMEC

START TMRO
READ TMRO IN W
TIME - W
CHECK TIME-W =

:RETURN AFTER TMRO = 39

RETLW

CONFIGURATION SECTION.

START

BSF

STATUS,5

;BANK1

MOVLW

B'OOOlOOOl'

;A0 IS ANALOGUE,FOSC/8

MOVWF

ADSEL

Headers, porting code - which micro? 55

MOVLW

B'00001001'

MOVWF

TRISIO

MOVLW

B'00000111'

MOVWF

OPTION_R

CALL

3FFH

MOVWF

OSCCAL

BCF

STATUS,5

MOVLW

MOVWF

CMCON

CLRF

GPIO

BSF

ADCON0,0

;BITS 0,3 are I/P

;PRESCALER is /256

;Calibrates 4MHz oscillator

:BANK0

;Turns off comparator

;Clears GPIO

;Turns on A/D converter.

;Program starts now.
END

HEAD16F627.ASM for the 16F627/8, using the 37kHz internal RC
PortA bits to 7 are inputs
PortB bits to 7 are outputs
Prescaler/32

;EQUATES SECTION

TMRO

EQU

OPTION_R

EQU

PORTA

EQU

PORTB

EQU

TRISA

EQU

TRISB

EQU

STATUS

EQU

ZEROBIT

EQU

CARRY

EQU

EEADR

EQU

1BH

EEDATA

EQU

1AH

EECON1

EQU

1CH

EECON2

EQU

1DH

EQU

56 Headers, porting code - which micro?

WREN

EQU

PCON

EQU

OEH

COUNT

EQU

20H

LIST

P=16F627

;using the 627

ORG

GOTO

START

Configuration Bits

CONFIG H'3F10'

;selects Internal RC oscillator, WDT off,
;Code Protection disabled.

SUBROUTINE SECTION.

;0.1 SECOND DELAY
DELAYP1 CLRF

LOOPA

MOVF

SUBLW

BTFSS

GOTO

RETLW

TMRO ;Start TMRO

TMR0,W ;Read TMRO into W

.29 ;TIME - W

STATUS,ZEROBIT ;Check TIME-W =

LOOPA

:Return after TMRO = 29

;0.5 SECOND DELAY
DELAYP5 MOVLW

LOOPB

MOVWF

CALL

DECFSZ

GOTO

RETLW

COUNT

DELAYP1 ;0.1s delay

COUNT

LOOPB

:Return after 5 DELAYP1

;1 SECOND DELAY
DELAY 1 MOVLW

MOVWF
LOOPC CALL

DECFSZ

GOTO

RETLW

.10

COUNT

DELAYP1 ;0. Is delay

COUNT

LOOPC

;Return after 10 DELAYP1

Headers, porting code - which micro? 57

CONFIGURATION SECTION.

START

BSF

STATUS,5

;Bankl

MOVLW

B'llllllll'

MOVWF

TRISA

;PortA is input

MOVLW

B'OOOOOOOO'

MOVWF

TRISB

;PortB is output

MOVLW

B'00000100'

MOVWF

OPTION_R

;Option Register, TMRO/32

CLRF

PCON

;Select 37kHz oscillator.

BCF

STATUS,5

;BankO

CLRF

PORTA

CLRF

PORTB

MOVLW

MOVWF

1FH

;CMCON turns off compar

;Program starts now.
END

;HEADER84.ASM for the 16F84 using a 32kHz crystal

;EQUATES SECTION

TMRO

EQU

PORTA

EQU

PORTB

EQU

STATUS

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION_R

EQU

81H

ZEROBIT

EQU

COUNT

EQU

OCH

TMRO is FILE 1.

PORTA is FILE 5.

PORTB is FILE 6.

STATUS is FILE 3.

TRISA (the PORTA I/O selection)

TRISB (the PORTB I/O selection)

the OPTION register is file 81H

ZEROBIT is Bit 2.

USER RAM LOCATION.

LIST

P=16F84

;We are using the 16F84

ORG

;0 is the start address.

GOTO

START

;goto start!

58 Headers, porting code - which micro?

Configuration Bits

CONFIG H'3FF0'

;selects LP oscillator, WDT off, PUT on,
:Code Protection disabled.

SUBROUTINE SECTION.

;1 SECOND DELAY

DELAY 1

CLRF

TMRO

LOOPA

MOVF

TMR0,W

SUBLW

.32

BTFSS

STATUS,ZEROBIT

GOTO

LOOPA

RETLW

;0.5 SECOND DELAY

DELAYP5

CLRF

TMRO

LOOPB

MOVF

TMR0,W

SUBLW

.16

BTFSS

STATUS,ZEROBIT

GOTO

LOOPB

RETLW

START TMRO
READ TMRO IN W
TIME - W
CHECK TIME-W =

:RETURN AFTER TMRO = 32

START TMRO
READ TMRO IN W
TIME - W
CHECK TIME-W =

[RETURN AFTER TMRO = 16

CONFIGURATION SECTION.
START

BSF

STATUS,5

;Turn to BANK1

MOVLW

B'OOOlllir

;5 bits of PORTA are I/Ps

MOVWF

TRISA

MOVLW

B'00000000'

MOVWF

TRISB

;PORTB IS OUTPUT

MOVLW

B'00000111'

MOVWF

OPTION_R

;PRESCALER is /256

BCF

STATUS,5

;Return to BANKO

CLRF

PORTA

;Clears PORTA

CLRF
CLRF

PORTB
COUNT

;Clears PORTB

;Program starts now.

END

Headers, porting code - which micro? 59

HEAD818.ASM for 16F818. This sets PORTA as digital INPUT.
PORTB is an OUTPUT.
Internal oscillator of 31.25kHz chosen

The OPTION register is set to /256 giving timing pulses 32.768ms.
1 second and 0.5 second delays are included in the subroutine section.

;EQUATES SECTION

TMRO

EQU

STATUS

EQU

PORTA

EQU

PORTB

EQU

ZEROBIT

EQU

ADCON0

EQU

1FH

ADCON1

EQU

9FH

ADRES

EQU

1EH

CARRY

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION_R

EQU

81H

OSCCON

EQU

8FH

COUNT

EQU

20H

LIST

P =

:16F818

ORG

GOTO

START

means TMRO is file 1.

means STATUS is file 3.

means PORTA is file 5.

means PORTB is file 6.

means ZEROBIT is bit 2.

A/D Configuration reg.O

A/D Configuration reg.l

A/D Result register.

CARRY IS BIT 0.

PORTA Configuration Register

PORTB Configuration Register

Option Register

Oscillator control register.

COUNT a register to count events.

;we are using the 16F818.

;the start address in memory is

;goto start!

Configuration Bits

CONFIG H'3F10'

;sets INTRC-A6 is port I/O, WDT off, PUT

;on, MCLR tied to VDD A5 is I/O

;BOD off, LVP disabled, EE protect disabled,

;Flash Program Write disabled,

;Background Debugger Mode disabled,

;CCP function on B2,

:Code Protection disabled.

SUBROUTINE SECTION.

0.1 second delay, actually 0.099968s
DELAYP1 CLRF TMRO

LOOPB MOVF TMR0,W

;START TMRO.
;READ TMRO INTO W.

60 Headers, porting code - which micro?

SUBLW

;TIME-3

BTFSS

STATUS,ZEROBIT

;Check TIME-W =

GOTO

LOOPB

;Time is not = 3.

NOP

;add extra delay

NOP

RETLW

;Time is 3, return.

DELAYP5

MOVLW

MOVWF

COUNT

LOOPC

CALL

DELAYP1

DECFSZ

COUNT

GOTO

LOOPC

RETLW

;1 second delay.

DELAY 1

MOVLW

.10

MOVWF

COUNT

LOOPA

CALL

DELAYP1

DECFSZ

COUNT

GOTO

LOOPA

RETLW

Configuration Section

START

BSF

STATUS,5

;Turns to Bankl.

MOVLW
MOVWF

B'llllllir
TRISA

;8 bits of PORTA are I/P

MOVLW
MOVWF

B'00000110'
ADCON1

;PORTA IS DIGITAL

MOVLW
MOVWF

B'00000000'
TRISB

;PORTB is OUTPUT

MOVLW
MOVWF

B'00000000'
OSCCON

;oscillator 31.25kHz

MOVLW
MOVWF

B'OOOOOllT
OPTION R

;Prescaler is /256
;TIMER is 1/32 sees.

Headers, porting code - which micro? 61

BCF

STATUS,5

; Return to BankO

CLRF

PORTA

;Clears PortA.

CLRF

PORTB

;Clears PortB.

;Program starts now.

END

;HEAD872.ASM Header for 16F872 using 32kHz oscillator
;EQUATES SECTION

TMRO

EQU

OPTION_R

EQU

PORTA

EQU

PORTB

EQU

PORTC

EQU

TRISA

EQU

TRISB

EQU

TRISC

EQU

STATUS

EQU

ZEROBIT

EQU

CARRY

EQU

EEADR

EQU

ODH

EEDATA

EQU

OCH

EECON1

EQU

OCH

EECON2

EQU

ODH

EQU

WREN

EQU

ADCONO

EQU

1FH

ADCON1

EQU

1FH

ADRES

EQU

1EH

CHSO

EQU

GODONE

EQU

COUNT EQU 20H

LIST P=16F872

ORG

GOTO START

Configuration Bits

_CONFIG H'3F30' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

62 Headers, porting code - which micro?

SUBROUTINE SECTION.

;1 SECOND DELAY
DELAY 1 CLRF

LOOPA MOVF

TMR0,W

SUBLW

.32

BTFSS

STATUS,ZEROBIl

GOTO

LOOPA

RETLW

;0.5 SECOND DELAY

DELAYP5 CLRF

TMRO

LOOPB MOVF

TMR0,W

SUBLW

.16

BTFSS

STATUS,ZEROBIT

GOTO

LOOPB

RETLW

TMRO ;Start TMRO

Read TMRO into W

TIME - W

Check TIME-W =

;Return after TMRO = 32

Start TMRO

Read TMRO into W

TIME - W

Check TIME-W =

:Return after TMRO = 16

CONFIGURATION SECTION.

START

BSF

STATUS,5

;Bankl

MOVLW

B'llllllll'

MOVWF

TRISA

;PortA is input

MOVLW

B'00000000'

MOVWF

TRISB

;PortB is output

MOVLW

B'llllllll'

MOVWF

TRISC

;PortC is input

MOVLW

B'ooooonr

MOVWF

OPTION_R

;Option Register, TMRO/256

MOVLW

B'00000000'

MOVWF

ADCON1

;PortA bits 0,1,2,3,5 are analogue

BSF

STATUS,6

;BANK3

BCF

EECON1J

;Data memory on.

BCF

STATUS,5

BCF

STATUS,6

;BANK0 return

BSF

ADCON0,0

;turn on A/D

CLRF

PORTA

Headers, porting code - which micro? 63

CLRF PORTB
CLRF PORTC

;Program starts now.
END

These headers can be used for applications that use the corresponding
microcontrollers. E.g. Any one of them can be used with FLASHER. ASM.
Other applications may require functions that are not in all of the devices i.e.
AtoD.

The explanation of the operation of the headers will be dealt with later when
the individual micros are examined.

5
Using inputs

A control program usually requires more than turning outputs on and off.
They switch on and off because an event has happened. This event is then
connected to the input of the microcontroller to 'tell' it what to do next.
The input could de derived from a switch or it could come from a sensor
measuring temperature, light levels, soil moisture, air quality, fluid pressure,
engine speed etc.

Analogue inputs are dealt with later, in this chapter we will concern ourselves
with digital on/off inputs.

New instructions used in this chapter:

• BTFSC

• BTFSS

• CLRF

• MOVF

• SUBLW

• SUBWF

• RETLW

As an example let us design a circuit so that switch, SW1 will turn an LED
on and off.

The circuit diagram is shown in Figure 5.1.

This circuit is using the 16F84 microcontroller with a 32kHz crystal.

It can of course also be performed with any of the microcontrollers discussed
previously. Including the 16F818 using its internal oscillator, in which case
the crystal and 2 x 68pF capacitors are not required.

The program to control the hardware would use the following steps:

1. Wait for SW1 to close.

2. Turn on LED1.

3. Wait for SW1 to open.

Using inputs 65

|— SW1
Ov

68p 32kHz

n 15

68p

=T=0.1u.

Figure 5.1 Circuit diagram of the microcontroller switch

4. Turn off LED 1.

5. Repeat.

In the circuit diagram SW1 is connected to AO and LED1 to BO.

When the switch is closed AO goes low or clear. So we wait until AO is clear.
The code for this is:

BEGIN

BTFSC

GOTO

BSF

PORTA,0 (test bit in file PORTA skip if clear)

BEGIN

PORTB,0

• The command BTFSC is Bit Test in File Skip if Clear, and the instruction
BTFSC PORTA,0 means Test the Bit in the File PORTA, i.e. BitO, Skip
the next instruction if Clear. If AO is Clear Skip the next instruction
(GOTO BEGIN) if it isn't Clear then do not Skip and GOTO BEGIN to
check the switch again.

The program will check the switch thousands maybe millions of times a second,
depending on your clock.

• When the switch is pressed the program moves on and executes the
instruction BSF PORTB,0 to turn on the LED.

66 Using inputs

We then wait for the switch to open.

When the switch is open AO goes Hi or Set, we then wait until AO is Set i.e.

SWOFF

BTFSS

PORTA,0

GOTO

SWOFF

BCF

PORTB,0

GOTO

BEGIN

• The command BTFSS is Bit Test in File Skip if Set, and the instruction
BTFSS PORTA,0 means Test the Bit in the File PORTA, i.e. BitO, Skip
the next instruction if Set. If AO is Set Skip the next instruction (GOTO
SWOFF) if it isn't Set then do not Skip and GOTO SWOFF to check the
switch again.

• When the switch is set the program moves on and executes the instruction
BCF PORTB,0 to switch off the LED.

• The program then goes back to the label BEGIN, to repeat.

The program is now added to the header. (NB. Use the TAB to make your
listing easy to read.) It is then saved as SWITCH. ASM.

;SWITCH.ASM

;Program starts now.

BEGIN BTFSC

GOTO
BSF

SWOFF BTFSS

GOTO
BCF
GOTO

PORTA,0

BEGIN

PORTB,0

PORTA,0

SWOFF

PORTB,0

BEGIN

;Wait for SW1 to be pressed

;Turn on LED 1 .

;Wait for SW1 to be released.

;Switchoff LED1.
; Repeat sequence.

END

Switch flowchart

It will be obvious from the program listing of the solution to the switch
problem that listings are difficult to follow. A picture is worth a thousand
words has never been more apt than it is with a program listing. The picture of
the program is shown below in the flowchart for the solution to our initial
switch problem, Figure 5.2. Before a programming listing is attempted it is
very worthwhile drawing a flowchart to depict the program steps. Diamonds
are used to show a decision (i.e. a branch) and rectangles are used to show

Using inputs 67

Turn on LED1

Turn off LED1

Figure 5.2 Flowchart for the switch

a command. Each shape may take several lines of program to implement. But
the idea of the flowchart should be evident. Note that the flowchart describes
the problem - you can draw it without any knowledge of the instruction set.

Program development

From our basic switch circuit an obvious addition would be to include a
delay so that the LED would go off automatically after a set time.

Suppose we wish to switch the light on for 5 seconds, using AO as the switch
input. Figure 5.3 shows this Delay Flowchart.

The complete listing for this program for the 16F84 is shown below. I have
shown the complete code including the header because I have added a 5 second
delay in the subroutine section.

;DELAY.ASM

;EQUATES SECTION

TMRO
PORTA

EQU
EQU

;TMR0 is FILE 1.
;PORTA is FILE 5.

68 Using inputs

PORTB is FILE 6.

STATUS is FILE3.

TRISA (the PORTA I/O selection)

TRISB (the PORTB I/O selection)

the OPTION register is file 81H

ZEROBIT is Bit 2.

USER RAM LOCATION.

PORTB

EQU

STATUS

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION R

EQU

81H

ZEROBIT

EQU

COUNT

EQU

OCH

;We are using the 16F84.
;0 is the start address.
;goto start!

LIST P=16F84

ORG

GOTO START

Configuration Bits

_CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

•>

SUBROUTINE SECTION.

;5 second delay.

DELAY5

CLRF

TMRO ;

LOOPA

MOVF

TMR0,W ;

SUBLW

.160 ;

BTFSS

STATUS,ZEROBIT ;

GOTO

LOOPA ;

RETLW

;

;Start TMRO.
;Read TMRO into W.
;TIME - 160
;Check TIME-W =
;Time is not= 160.
;Time is 160, return.

CONFIGURATION SECTION.

START

BSF

STATUS,5

;Turn to BANK1

MOVLW

B'OOOlllir

;5 bits of PORTA are I/Ps

MOVWF

TRISA

MOVLW

B'00000000'

MOVWF

TRISB

;PORTB IS OUTPUT

MOVLW

B'00000111'

MOVWF

OPTION_R

;PRESCALER is /256

BCF

STATUS,5

;Return to BANKO

CLRF

PORTA

;Clears PORTA

CLRF

PORTB

;Clears PORTB

CLRF

COUNT

Using inputs 69

;Program starts now.
ON

BTFSC

PORTA,0

;Check button pressed.

GOTO

BSF

PORTB,0

;Turn on LED.

CALL

DELAY5

;CALL 5 second delay

BCF

PORTB,0

;Turn off LED.

GOTO

; Repeat

END

Set up PORTB as output.
Set PRESCALER to /256.

Figure 5.3 Delay flowchart

How does it work?

Turn ON LED.

Wait 5 seconds

Turn OFF LED.

We check to see if the switch has been pressed (clear). If not GOTO ON
and check again. If it has skip that line and Turn on the LED on BO.
The code is:

BTFSC

GOTO

BSF

PORTA,0 ;Check button pressed.

PORTB,0 ;Turn on LED.

Wait 5 seconds. The 5 second delay has been included for you in the
subroutine section. Code:

CALL

DELAY5

70 Using inputs

Turn the LED off and go back to the beginning. Code:

BCF
GOTO

PORTB,0
ON

;Turn off LED.

Try this next problem for yourselves, before looking at the solution.

Problem 1:

Using Port A bit as a start button and outputs on PortB

bits 0-3. Switch on Port B bits and 2 for % second, switch

off bits and 2.

Switch on Port B bits 1 and 3 for % second, switch off bits

1 and 3.

Repeat continuously.

The % second delay is provided for you.

The flowchart for the solution to problem 1 is shown in Figure 5.4

Program solution to problem 1 for the 16F84

;PROBLEMl.ASM

;EQUATES SECTION

TMRO

EQU

PORTA

EQU

PORTB

EQU

STATUS

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION_R

EQU

81H

ZEROBIT

EQU

COUNT

EQU

OCH

LIST

P=16F84

ORG

GOTO

START

TMRO is FILE 1.

PORTA is FILE 5.

PORTB is FILE 6.

STATUS is FILE 3.

TRISA (the PORTA I/O selection)

TRISB (the PORTB I/O selection)

the OPTION register is file 81H

ZEROBIT is Bit 2.

USER RAM LOCATION.

;we are using the 16F84.

;the start address in memory is

;goto start!

Using inputs 71

Configuration Bits

CONFIG H'3FF0'

;selects LP oscillator, WDT off, PUT on
:Code Protection disabled.

SUBROUTINE SECTION.

;0.25 second delay.

DELAY

CLRF

TMRO

;START TMRO.

LOOPA

MOVF

TMR0,W

;READ TMRO INTO W

SUBLW

;TIME - 8

BTFSS

STATUS,ZEROBIT

;Check TIME-W =

GOTO

LOOPA

;Time is not = 8.

RETLW

;Time is 8, return.

CONFIGURATION SECTION

START

BSF

MOVLW

MOVWF

MOVLW

MOVWF

MOVLW

MOVWF

BCF

CLRF

STATUS,5

B'OOOlllir

TRISA

B'00000000'

TRISB

B'ooooonr

OPTION_R
STATUS,5
PORTA
PORTB

;Turn to BANK1

;5 bits of PORTA are I/Ps.

;PORTB IS OUTPUT

;PRESCALER is /256
;Return to BANKO
;Clears PORTA
;Clears PORTB

;Program starts now.

BTFSC

PORTA,0

;Check button pressec

GOTO

REPEAT

MOVLW

B'oooooior

MOVWF

PORTB

;Turn on bits and 2

CALL

DELAY

; Va second delay

MOVLW

B'00001010'

MOVWF

PORTB

;Turn on bits 1 and 3

CALL

DELAY

; Va second delay

GOTO

REPEAT

; Repeat

END

72 Using inputs

Y <

Turn on BO, B2.

Wait 1/4 second.

Turn OFF BO, B2.
Turn ON B1, B3.

Wait 1/4 second.

Turn OFF B1, B3.

Figure 5.4 Flowchart for problem

How does it work?

• Wait for the switch on PORTA,0 to clear, with BTFSC PORTA,0 then
skip to

• MOVLW B'OOOOOIOT this sets up the data in the W register.

• MOVWF PORTB transfers the W register to PORTB and puts 5v on
BO and B2 only.

• CALL DELAY waits for l A second.

• MOVLW B'00001010' this sets up the data in the W register.

• MOVWF PORTB transfers the W register to PORTB and puts 5v on
Bl and B3 only.

• CALL DELAY waits for % second.

Using inputs 73

• GOTO REPEAT sends the program back to (my) label, REPEAT.
This will keep the lights flashing all the time without checking the switch
again.

Question. How do we make the program look at the switch, so that we can
control whether or not the program repeats?

Answer: Instead of GOTO REPEAT use GOTO BEGIN. The program will
then goto the label BEGIN instead of REPEAT and will wait for the switch
to be Clear before repeating.

Extra Work. Try and make the flashing routine more interesting by adding
more combinations.

Scanning (using multiple inputs)

Scanning (also called polling) is when the microcontroller looks at the condi-
tion of a number of inputs in turn and executes a section of program depending
on the state of those inputs.

Applications include:

• Burglar Alarms - when sensors are monitored and a siren sounds either
immediately or after a delay depending on which input is active.

• Keypad scanning - a key press could cause an LED to light, a
buzzer to sound or a missile to be launched. Just do not press the
wrong key!

Let's consider a simple example:

Switch scanning

Design a circuit so that if a switch is pressed a corresponding LED will light, i.e.

If SWO is Hi, (logic 1 or Set) then LEDO is on.

If SWO is Low, (logic or Clear) then LEDO is off.

If SW1 is Hi, (logic 1 or Set) then LED1 is on.

If SW1 is Low, (logic or Clear) then LED1 is off.

etc.

The circuit diagram for this is shown if Figure 5.5 and the corresponding
flowchart in Figure 5.6.

74 Using inputs

|— swo

Ov 5v

SW1

1K 1

SW2

Ov 5v

1K 2

SW3

68p 32kHz

i 1

.16

68p

BO
B1

B2
B3

16F84

V+
MCLR

4 x 680R

LEDO

LED1

LED2

LED3 V

iO.ln

Figure 5.5 Switch scanning circuit

The program for this switch scan is:

;SWSCAN.ASM using 16F84 and 32kHz crystal.
;EQUATES SECTION

TMRO

EQU

;TMR0is FILE 1.

PORTA

EQU

;PORTA is FILE 5

PORTB

EQU

;PORTB is FILE 6

Using inputs 75

STATUS

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION_R

EQU

81H

ZEROBIT

EQU

COUNT

EQU

OCH

;STATUS is FILE3.

;TRISA (the PORTA I/O selection)

;TRISB (the PORTB I/O selection)

;the OPTION register is file 81H

;ZEROBIT is Bit 2.

:USER RAM LOCATION.

LIST P=16F84

ORG

GOTO START

Configuration Bits

;We are using the 16F84.
;0 is the start address.
;goto start!

CONFIG H'3FF0'

;selects LP oscillator, WDT off, PUT on,
;Code Protection disabled.

CONFIGURATION SECTION.

START

BSF

STATUS,5

;Turn to BANK1

MOVLW

B'OOOllllT

;5 bits of PORTA are I/Ps

MOVWF

TRISA

MOVLW

B'OOOOOOOO'

MOVWF

TRISB

;PORTB IS OUTPUT

MOVLW

B'ooooonr

MOVWF

OPTION_R

;PRESCALER is /256

BCF

STATUS,5

;Return to BANKO

CLRF

PORTA

;Clears PORTA

CLRF

PORTB

;Clears PORTB

CLRF

COUNT

;Program starts now.

SWO

SW1

BTFSC

PORTA,0

;SwitchO pressed?

GOTO

TURNONO

;Yes

BCF

PORTB,0

:No, Switch off LEDO

BTFSC

PORTA, 1

;Switchl pressed?

GOTO

TURNON1

;Yes

BCF

PORTB, 1

:NO Switch off LED 1

76 Using inputs

SW2

BTFSC

PORTA,2

;Switch2 pressed?

GOTO

TURNON2

;Yes

BCF

PORTB,2

:NO Switch off LED2.

SW3

BTFSC

PORTA,3

;Switch3 pressed?

GOTO

TURNON3

;Yes

BCF

PORTB,3

:NO Switch off LED3.

GOTO

swo

;Rescan.

TURNONO

BSF

PORTB,0

;Turn on LEDO

GOTO

SW1

TURNON1

BSF

PORTB,l

;Turn on LED1

GOTO

SW2

TURNON2

BSF

PORTB,2

;Turn on LED2

GOTO

SW3

TURNON3

BSF

PORTB,3

;Turn on LED3

GOTO

SWO

END

How does it work?

• SWO is checked first with the instruction BTFSC PORTA,0. If the switch is
closed when the program is executing this line then we GOTO TURNONO.
That is the program jumps to the label TURNONO which turns on
LEDO and then jumps the program back to check SW1 at, of course, the
label, SW1.

• SW1 is then checked in the same manner and then SW2 and SW3.

Suppose we press the switch when the program is not looking at it. The
program lines are being executed at % of the clock frequency i.e. 32,768Hz
that is 8192 lines a second. The program will always catch you!

Try modifying the program so that the switches can flash 4 different routines
e.g. SWO flashes all lights on and off 5 times for 1 second.

Using inputs 11

Turn off LEDO

Turn off LED1

Turn off LED2

Turn off LED3

Figure 5.6 Flowchart for switch scan

Turn on LEDO

Turn on LED1

sSW2

SET?

Turn on LED2

Turn on LED3

Control application - a hot air blower

The preceding section outlined how to monitor inputs by looking at them
in turn. This application will 'read' all the bits on the port at once,
because we will be concerned with particular combinations of inputs
rather than individual ones.

78 Using inputs

The bits on the Input Port will be Os or Is and we can treat this binary pattern
like any other number in a file.

Consider a controller for a hot air radiator. When the water is warm the fan
will blow the warm air into the room. The heater and fan are controlled by
3 temperature sensors: (a) a room temperature sensor, (b) a boiler water
temperature sensor and (c) a safety overheating sensor. The truth table for
the system is shown in Table 5.1, where a 1 means hot and a means cold
for the sensors.

The block diagram for the system is shown in Figure 5.7.

Note A3, A4, A5, A6 and A7 are inputs and need to be connected to Ov. Do
not leave them floating - you would not know if they were or 1 ! Even though

Table 5.1 Truth table for the hot air system

INPUTS

OUTPUTS

A
6

A
5

Room
A2

Water
A1

OverH
AO

Heater
B1

Fan
BO

Over heat sensor —

Water temp sensor -

Room temp sensor

AO
A1
A2
A3
A4
A5
A6
A7

Fan
Heater

Figure 5.7 Block diagram for the hot air system

Using inputs 79

they are not being used they are still being read. NB. The inputs A5, A6 and
A7 do not exist on the 16F84.

There are 8 input conditions from our 3 sensors. So all 8 must be checked to
determine which condition is true.

Consider the first condition A2 = A1 = A0 = 0, i.e. PORTA reads 0000 0000.
How do we know that PORTA is 0000 0000? We do not have an instruction
which says "is PORTA equal to 0000 0000" or any other value for that matter.
So we need to look at our 35 instructions and come up with a way of finding
out what is the binary value of PORTA.

We check for this condition by subtracting 00000000 from it, if the answer is
zero then PORTA reads 00000000. I.e. 0000 0000 - 0000 0000 = (obviously).
But how do we subtract the two numbers and how do we know if the answer
is zero?

This is a very important piece of programming so read the next few lines
carefully.

• We first of all read PORTA into the W register with the instruction MOVF
PORTA, W that moves the data, (setting of the switches, Is or 0s), into W.

• We then subtract the number we looking for in this case 00000000 from W.

• We then need to know if the answer to this subtraction is zero. If it is
then the value on PORTA was 00000000. If the answer is not zero then the
value of the data on PORTA was not zero.

• So is the answer zero? Yes or No? The answer is held in a register called
the Status Register, in bit 2 of this register, called the zero bit. If the zero bit,
called a flag is 1, it is indicating that the statement is true the calculation
was zero. If the zero bit is that indicates the statement is false the answer
was not zero.

• We test the zero bit in the status register just like we tested the bit on
the switch connected to PORTA at the start of this chapter. We use the
command BTFSC and the instruction BTFSC STATUS,ZEROBIT.
If the zero bit is clear we skip the next instruction if it is set we have a
match and do not skip.

The code for this is:

MOVLW

B , 00000000 ,

;put 000000 in W

SUBWF

PORTA

subtract W from PORTA

BTFSC

STATUS,ZEROBIT

;PORTA = 00000000?

CALL

CONDA

;yes

80 Using inputs

CON DA is short for condition A where we require the heater on and the fan off.

• To check for A2 = Al =0 and A0= 1 we subtract 00000001. To check for
the next condition A2 = 0, Al = 1, A0 = we subtract 00000010, and so on
for the other 5 conditions.

MOVLW

B'ooooooor

;put 00000001 in W

SUBWF

PORTA

subtract W from PORTA

BTFSS

STATUS,ZEROBIT

;PORTA = 00000001?

CALL

CONDB

;yes

etc.

The opcode for this program CONTROL.ASM is:

;CONTROL.ASM

SUBROUTINE SECTION.

CONDA

BCF
BSF
RETLW

PORTB,0
PORTB,l

;turns fan off
;turns heater on

CONDB

BSF
BCF
RETLW

PORTB,0
PORTB,l

;turns fan on
;turns heater off

CONDC

BSF
BSF
RETLW

PORTB,0
PORTB,l

;turns fan on
;turns heater on

CONDD

BCF
BCF

PORTB,0
PORTB,l

;turns fan off
;turns heater off

RETLW

;Program starts now.

BEGIN

MOVLW

B'00000000'

;put 00000000 in W

SUBWF

PORTA

;PORTA - W

BTFSC

STATUS,ZEROBIT

;PORTA = 00000000?

CALL

CONDA

;yes

MOVLW

B'ooooooor

;put 00000001 in W

SUBWF

PORTA

;PORTA - W

BTFSC

STATUS,ZEROBIT

;PORTA = 00000001?

CALL

CONDB

;yes

Using inputs 81

MOVLW

B'00000010'

put 00000010 in W

SUBWF

PORTA

PORTA - W

BTFSC

STATUS,ZEROBIT

PORTA = 00000010?

CALL

CONDC

yes

MOVLW

B'00000011'

put 00000011 in W

SUBWF

PORTA

PORTA - W

BTFSC

STATUS,ZEROBIT

PORTA = 00000011?

CALL

CONDB

yes

MOVLW

B'OOOOOIOO'

put 00000100 in W

SUBWF

PORTA

PORTA - W

BTFSC

STATUS,ZEROBIT

PORTA = 00000100?

CALL

CONDD

yes

MOVLW

B'00000101'

put 00000101 in W

SUBWF

PORTA

PORTA - W

BTFSC

STATUS,ZEROBIT

PORTA = 00000101?

CALL

CONDB

yes

MOVLW

B'00000110'

put 00000110 in W

SUBWF

PORTA

PORTA - W

BTFSC

STATUS,ZEROBIT

PORTA = 000001 10?

CALL

CONDD

yes

MOVLW

B'ooooonr

put 000001 11 in W

SUBWF

PORTA

PORTA - W

BTFSC

STATUS,ZEROBIT

PORTA = 00000 111?

CALL

CONDB

yes

GOTO

BEGIN

END

Notice that the SUBROUTINE SECTION needs to be changed to include the
conditions, CONDA, CONDB, CONDC and CONDD. The DELAY

subroutines are not required in this example.

The program can be checked by using switches for the input sensors and
LEDs for the outputs.

There is more than one way of skinning a cat, another way of writing this
program is shown in Chapter 8, in the section on look up tables.

6
Understanding the headers

The 16F84

HEADER84.ASM The header for the 16F84.

Now that we have looked at a number of applications we are ready to under-
stand HEADER84.ASM introduced in Chapter 2.

• The header starts with a title that includes the name of the file, this is useful
when you are printing it out and details about what the program is doing.

HEADER84.ASM for 16F84. This sets PORTA as an INPUT (NB 1

means input) and PORTB as an OUTPUT
(NB means output). The OPTION
register is set to /256 to give timing pulses
of 1/32 of a second.
1 second and 0.5 second delays are
included in the subroutine section.

• The EQUATES section tells the software what numbers your words
represent. When you write your program you use mnemonics such as
PORTA, PORTB, TMRO, STATUS, ZEROBIT, COUNT, MYAGE. The
Assembler Program does not understand your words; it is looking for the
file number or the bit number. You have to tell it what these mean in
the Equates Section i.e. COUNT is File 0C, PortA is file 5, the STATUS
register is file 3, ZEROBIT is bit 2, etc. The memory map of the 16F84
in Table 6.1 shows the addresses of the registers and user files. The file
with address 0C is the first of the user files and I have called it COUNT,
it stores the number of times certain events have happened in my program.

I could have file 0D as COUNT2, file 0E as COUNT3, file OF as SECONDS
or WAIT etc.

Understanding the headers 83

;EQUATES SECTION

TMRO

EQU

PORTA

EQU

PORTB

EQU

STATUS

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION_R

EQU

81H

ZEROBIT

EQU

COUNT

EQU

OCH

TMRO is FILE 1.

PORTA is FILE 5.

PORTB is FILE 6.

STATUS is FILE 3.

TRISA (the PORTA I/O selection)

TRISB (the PORTB I/O selection)

the OPTION register is file 81H

ZEROBIT is Bit 2.

USER RAM LOCATION.

What chip are we using?

LIST

P=16F84

;we are using the 16F84.

ORG

;the start address in memory is

GOTO

START

;goto start!

LIST P=16F84 tells the assembler what chip to assemble the code for. ORG
means put the next line of code into program memory address 0, then follow
with next line in address 1 etc.

GOTO START makes the program bypass the subroutine section and GOTO
the label START which is where the device is configured before executing
the body of the program. The instruction GOTO START is placed in EPROM
address by ORG 0.

The line DELAY1 CLRF TMRO is then placed in program memory
address 1, etc.

• CONFIGURATION BITS

To avoid having to set the configuration bits when we come to program the
device they can be set in the code. You can change these bits if you require
in MPLAB, note the new number and substitute it in the code.
; Configuration Bits

CONFIG H'3FF0'

;selects LP oscillator, WDT off, PUT on,
;Code Protection disabled.

• SUBROUTINE SECTION.

The subroutine section consists of 2 subroutines DELAY 1 and DELAYP5.

A subroutine is a section of program, which is, used a number of times
instead of rewriting it and using up program memory. Just call it i.e. CALL

84 Understanding the headers

DELAY 1, at the end you RETURN to the program in the position you left
it. The stack is the register that remembers where you came from and returns
you back.

The DELAY 1 code is:

DELAY 1

CLRF

TMRO

;Start TMRO.

LOOPA

MOVF

TMR0,W

;Read TMRO into W

SUBLW

.32

;TIME - 32

BTFSS

STATUS,ZEROBIT

;Check TIME-W =

GOTO

LOOPA

;Time is not = 32.

RETLW

;Time is 32, return.

DELAY1 starts by clearing the register TMRO (timer 0), with CLRF TMRO,
i.e. CleaR the File TMRO. This sets the timer to zero and will be counting
TMRO pulses every 1/32 of a second.

LOOPA MOVF TMR0,W is move file TMRO into the working register, W.

We want to know when TMRO is 32, because then we will have had 32
TIMERO pulses, which is 1 second. This is done with a subtraction as in the
example earlier in this Chapter 5, in the section on the hot air blower.

The label LOOPA is there because we keep returning to it until TMRO reaches
the required value.

There is no instruction, which asks the micro is TMRO equal to 32. So we
have to use the instructions available. We subtract a number from W and ask
is the answer 0. If for example we subtract 135 from W and the answer is
then W contained 135 if the answer was not then W did not contain 135.
The status register contains a bit called a zerobit, it is bit2. Notice in the
EQUATES section I have put ZEROBIT EQU 2. So I can use ZEROBIT
in my code instead of 2 — I would soon forget what the 2 was supposed to
mean. The zerobit is set to a 1 when the result of a previous calculation is 0.
So a 1 means result was 0!!!! Think of this as a flag (because that's what it
is called), the flag is waving (a 1) to indicated the result is zero. We can
test this zerobit, i.e. look at it and see if it is a 1 or 0. We can skip the next
instruction if it is set, (a zero has occurred), by BTFSS STATUS,ZEROBIT
or skip if clear, (a zero has not occurred), by BTFSC STATUS,
ZEROBIT. Doesn't this read better than BTFSC 3,2 STATUS is Register3,
ZEROBIT is bit 2.

Understanding the headers 85

Lets look at this subroutine again.

DELAY 1

CLRF

TMRO

;START TMRO.

LOOPA

MOVF

TMR0,W

;READ TMRO INTO W

SUBLW

.32

;TIME - 32

BTFSS

STATUS,ZEROBIT

; Check TIME-W =

GOTO

LOOPA

;Time is not = 32.

RETLW

;Time is 32, return.

• We clear TMRO (CLRF TMRO).

• Then move TMRO into W (MOVF TMR0,W)

• SUBTRACT 32 from W which now holds TMRO value. (SUBLW .32)

• If W (hence TMRO) is 32 the zerobit is set, we skip the next instruction
and return from the subroutine with in W (RETLW 0)

• If W is not 32 then we do not skip and we GOTO LOOPA and put TMRO
in W and repeat until TMRO is 32.

DELAYP5 is a similar code but TMRO now is only allowed to count upto 16
i.e. a half-second (with 32 pulses a second). Note if you copy and paste, change
the name of the subroutine from DELAY 1 to DELAYP5, change the 32 to
16 and do not forget to change LOOPA to LOOPB. You cannot goto room 27
if there are two room 27s!

• CONFIGURATION SECTION:

START

BSF

STATUS,5

;Turn to BANK1

MOVLW

B'00011111'

;5 bits of PORTA are I/Ps

MOVWF

TRISA

MOVLW

B'00000000'

MOVWF

TRISB

;PORTB IS OUTPUT

MOVLW

B / 00000111 /

MOVWF

OPTION_R

;PRESCALER is /256

BCF

STATUS,5

;Return to BANKO

CLRF

PORTA

;Clears PORTA

CLRF

PORTB

;Clears PORTB

CLRF

COUNT

The instruction BSF STATUS, 5 sets bit 5 in the Status Register. As you can
see from the explanation of the Status Register bits in Chapter 19, bit 5 is
a page select bit which selects pagel giving us access to the Registers in the
page 1 (Bankl) column of the memory map in Table 6.1. The reason for pages
or banks is that we have an 8 bit micro. 8 bits can only address 256 files so

86 Understanding the headers

to identify a file we have it on a page, like a line in a book i.e. line 17 on page 40
instead of line 2475.

MOVLW B'0001 1111' ;5bits of PORTA are I/P

MOVWF TRISA

These 2 lines move 11111 into the data direction register to set the 5 bits of
PORTA as inputs. The 11111 is first moved to W (MOVLW B'OOOlllH')
and then into the data direction register with MOVWF TRISA. A 1 signifies
an input a an output.

MOVLW B'00000000' ;8bits of PORTB are O/P

MOVWF TRISB

These 2 lines move 00000000 into the data direction register to set the 8 bits
of PORTB as outputs. The 000000 is first moved to W and then into the
data direction register with MOVWF TRISB.

Port A and PortB can be configured differently if required. E.g. to make
the lower 4 bits of PortB outputs and the upper 4 bits inputs - alter the 2 lines
of the program with:

MOVLW B'l 11 10000'

MOVWF TRISB

The header also sets the internal clock to divide by 256 i.e. a 32.768kHz
clock gives a program execution of 32.768 kHz/4 = 8.192 kHz. If the prescaler is
set to divide by 256 this gives timing pulses of 32 a second.

The prescaler is configured with the 2 lines:

MOVLW B'00000 111' ;Prescaler is /256

MOVWF OPTION_R ;TIMER is 1/32 sec.

The OPTION register can be altered in the header to give faster timing pulses
if required, as described in the OPTION Register section in Chapter 19.

The line BCF STATUS,5 ;Return to BankO.

then returns to page on the memory map. The good news here is in the
programs in this book we only need to go into page 1 in the Configuration
Section. The body of the program, your section, resides in page 0.

Understanding the headers 87

We then finish the configuration section by clearing any outputs in PORTA
and PORTB with,

CLRF
CLRF

PORTA
PORTB

;Clears PortA.
;Clears PortB.

This will not affect any bits that are configured as inputs.

Just for good measure the COUNT file is also cleared with CLRF COUNT.

16F84 memory map

The Memory Map of the 16F84 is shown in Table 6.1.

This diagram shows the position of the Special Function Registers, i.e.
PORTA, PORTB, TMRO etc. in addresses 00 to OB and the location of
the User Files i.e. COUNT (the only one we have used up to now) occupying
locations 0C through to 4F.

These files are very important when writing our code. The Special Function
Registers enable us to tell the microcontroller to do things, i.e. set PORTB
up as an output port with TRISB, alter the rate of TMRO with the OPTION

Table 6.1 16F84 memory map

FILE
ADDRESS

FILE NAME

INDIRECT
ADDRESS

TMRO

OPTION

PCL

STATUS

FST

FSR

PORTA

TRISA

PORTB

TRISB

EEDATA

EECON1

EDADR

EECON2

PCLATH

INTCON

OC
4F

68
USER
FILES

BANK0

BANK1

88 Understanding the headers

register, find out if the result of a calculation is zero, +ve or — ve using the
STATUS register. TMRO of course tells us how much time has elapsed.

The other microcontroller which features frequently in this book, my
favourite, is the 16F818. We will look at its header and memory map now
and compare it to the 16F84 to see how they differ. After that you will be
able to distinguish between other micros.

The 16F818

HEAD818.ASM The header for the 16F818.

The code shown below is the header for the 16F818 that we first saw in
Chapter 4.

HEAD818.ASM for 16F818. This sets PORTA as digital INPUT.

PORTB is an OUTPUT.

Internal oscillator of 31.25kHz chosen

The OPTION register is set to /256 giving timing pulses of 32.768 ms.

1 second and 0.5 second delays are included in the subroutine section.

EQUATES SECTION

TMRO EQU

STATUS EQU

PORTA EQU

PORTB EQU

ZEROBIT EQU

ADCON0 EQU

ADCON1 EQU

ADRES EQU

CARRY EQU

TRISA EQU

TRISB EQU

OPTION_R EQU

OSCCON EQU

COUNT EQU

1 ;means TMRO is file 1.

3 ;means STATUS is file 3.

5 ;means PORTA is file 5.

6 ;means PORTB is file 6.

2 ;means ZEROBIT is bit 2.
1FH ;A/D Configuration reg.O
9FH ;A/D Configuration reg.l
1EH ;A/D Result register.

;CARRY IS BIT 0.

85H ;PORTA Configuration Register

86H ;PORTB Configuration Register OPTION_R

81H ;Option Register

8FH ;Oscillator control register.

;COUNT a register to count events.

20H

LIST
ORG
GOTO

P=16F818

START

;we are using the 16F818.

;the start address in memory is

:goto start!

Understanding the headers 89

Configuration Bits

CONFIG H'3F10'

;sets INTRC-A6 is port I/O, WDT off, PUT

;on, MCLR tied to VDD A5 is I/O

;BOD off, LVP disabled, EE protect disabled,

;Flash Program Write disabled,

;Background Debugger Mode disabled, CCP

;function on B2,

;Code Protection disabled.

SUBROUTINE SECTION.

;0.1 second delay, actually 0.099968s

DELAYP1

CLRF

TMRO

;START TMRO.

LOOPB

MOVF

TMR0,W

;READ TMRO INTO W

SUBLW

;TIME - 3

BTFSS

STATUS,ZEROBIT

; Check TIME-W =

GOTO

LOOPB

;Time is not = 3.

NOP

;add extra delay

NOP

RETLW

;Time is 3, return.

;0.5 second delay.

DELAYP5

MOVLW

MOVWF

COUNT

LOOPC

CALL

DELAYP1

DECFSZ

COUNT

GOTO

LOOPC

RETLW

;1 second delay.

DELAY 1

MOVLW

.10

MOVWF

COUNT

LOOPA

CALL

DELAYP1

DECFSZ

COUNT

GOTO

LOOPA

RETLW

CONFIGURATION SECTION.

START

BSF

STATUS,5

;Turns to Bankl

MOVLW
MOVWF

Bllllllir
TRISA

;8 bits of PORTA are I/P

90 Understanding the headers

MOVLW

B'OOOOOllO'

;PORTA IS DIGITAL

MOVWF

ADCON1

MOVLW

B'OOOOOOOO'

MOVWF

TRISB

;PORTB is OUTPUT

MOVLW

B'OOOOOOOO'

MOVWF

OSCCON

;oscillator 31.25kHz

MOVLW

B'ooooonr

;Prescaler is /256

MOVWF

OPTION_R

;TIMER is 1/32 sees.

BCF

STATUS,5

;Return to BankO.

CLRF

PORTA

;Clears PortA.

CLRF

PORTB

;Clears PortB.

;Program starts now.

END

We will now consider only the new additions to the previous HEADER84.
ASM for the 16F84.

PORTA is now an 8 I/O port, NB. PORTA,5 is input only.
ADCON0, ADCON1 and ADRES are Special Function Registers that will
enable us to instruct the microcontroller on how we want the A/D converter
to function. We will discuss these when we consider A/D conversion in
Chapter 11.

OSCCON allows us to set the value of the internal oscillator. We can
choose from 8MHz, 4MHz, 2MHz, 1MHz, 500kHz, 250kHz, 125kHz or
31.25 kHz. The use of OSCCON is described in the Register section in
Chapter 19.

CONFIGURATION BITS. There are more functions on the 16F818
than the 16F84 so there are more choices in the way it is configured.
Here we have selected the internal oscillator so we do not need the
crystal, that has freed up 2 I/O lines. The master clear, MCLR has been
switched internally to Vdd (5v) freeing up another I/O line, giving 16 I/O.
We have switched the brown out off this would reset the micro if the
supply voltage fell below a critical point avoiding erratic behaviour.
Low voltage programming has been switched off. EEPROM protection
and Program Write Protection has been disabled. Background Debugger
Mode has been disabled. The 16F818 is capable of working with
the Microchip In Circuit Debugger (ICD2). Capture and Compare

Understanding the headers 91

Pulse Width Module (CCP) not discussed in this book has been switched
onto B2.

• SUBROUTINE SECTION.

The 16F818 header described uses the internal 31.25kHz oscillator, which
does not lend itself so easily to times of seconds. I have had to write a
different code for the delays. A 31.25kHz clock gives timing pulses of
32|is which do not add up exactly to give a second. The delay loop similar
in its action to the 16F84 delay has had 2 NOP (no operation) instructions
added to make up the shortfall. The 0.1 second delay is therefore 0.099968s
which is as close as I could get it. If you really need accurate times you will
need to use a crystal for your timing. The internal oscillators are only about
1% accurate.

• CONFIGURATION SECTION.

Because the 16F818 has an A/D converter on board you need to tell it
which PORTA inputs are analogue and which are digital. Analogue inputs
are dealt with in Chapter 11 for now PORTA has been set to all digital
inputs with:

MOVLW B'00000110' ;PORTA IS DIGITAL

MOVWF ADCON1

The internal oscillator is set to 31.25kHz with:

MOVLW B'00000000'

MOVWF OSCCON ;oscillator 31.25kHz

This is a default condition and is therefore not required. I have included it
incase you are wondering how the frequency is set. You need to alter the
data in OSCCON to change the frequency, see Chapter 19.

Because the 16F818 has more functions than the 16F84 it follows that there
are more Special Function Registers to handle these extra functions. It also
has more user files.

These files are now arranged over 4 banks, BANKO, BANK1, BANK2 and
BANK3. The Banks are selected by the Bank Select bits (page select bits)
in the Status Register, RPO and RP1, bits 5 and 6, shown in Figure 6.1.

IRP RP1 RPO TO PD Z DC C

bit7 bitO

Figure 6.1 Status register bits

92 Understanding the headers

So 00 selects BankO
01 selects Bankl

10 selects Bank2

1 1 selects Bank3

For most applications in this book once we have configured the device we
will not need to change banks. The only time we do change is when we look
at applications involving the Data EEPROM.

The 16F818 memory Map is shown below in Figure 6.2.

File
Address
OOh
01 h
02h
03h
04h
05h
06h
07h
08h
09h
OAh
OBh
OCh
ODh
OEh
OFh
10h
1 1 h
12h
13h
14h
15h
16h
17h
18h
19h
1Ah
1Bh
1Ch
1Dh
1Eh
1Fh
20h

7Fh

File
Address
80h
81 h
82h
83h
84h
85h
86h
87h
88h
89h
8Ah
8Bh
8Ch
8Dh
8Eh
8Fh
90h
91h
92h
93h
94h
95h
96h
97h
98h
99h
9Ah
9Bh
9Ch
9Dh
9Eh
9Fh
AOh

BFh
COh

FFh

File
Address
100h
101h
102h
103h
104h
105h
106h
107h
108h
109h
10Ah
10Bh
10Ch
10Dh
10Eh
10Fh
110h

11Fh
120H

17Fh

File
Address
180h
181h
182h
183h
184h
185h
186h
187h
188h
189h
18Ah
18Bh
18Ch
18Dh
18Eh
18Fh
190h

19Fh
1A0h

1FFh

Indirect addr.(*)

Indirect addr.f)

Indirect addr.(*)

TMRO

OPTION

TMRC

OPTION

PCL

STATUS

FSR

PORTA

TRISA

PORTB

TRISB

PORTB

TRISB

PCLATH

INTCON

PIR1

PIE1

EEDATA

EECON1

PIR2

PIE2

EEADR

EECON2

TMR1L

PCON

EEDATH

Reserved* 1 *

TMR1H

OSCCON

EEADRH

Reserved* 1 *

T1CON

OSCTUNE

TMR2

T2CON

PR2

SSPBUF

SSPADD

SSPCON

SSPSTAT

CCPR1L

CCPR1H

CCP1CON

ADRESH

ADRESL

ADCON0

ADCON1

General
Purpose
Register

96 Bytes

General Purpose
Register

32 Bytes

accesses
20h-7Fh

accesses
40h-7Fh

BankO

Bank 1

Bank 2

Bank 3

Figure 6.2 16F818 memory map

We will now continue with some more applications and introduce some
more instructions and ideas. Each of these programs will be able to be executed
using a number of micros using the appropriate headers.

Keypad scanning

There are no new instructions used in this chapter

Keypads are an excellent way of entering data into the microcontroller. The
keys are usually numbered but they could be labeled as function keys for
example in a remote control handset in a TV to adjust the sound or colour etc.

As well as remote controls, keypads find applications in burglar alarms, door
entry systems, calculators, microwave ovens etc. So there are no shortage
of applications for this section.

Keypads are usually arranged in a matrix format to reduce the number of
I/O connections.

A 12 key keypad is arranged in a 3 x 4 format requiring 7 connections.
A 16 key keypad is arranged in a 4 x 4 format requiring 8 connections.

Consider the 12 key keypad. This is arranged in 3 columns and 4 rows as shown
in Table 7.1. There are 7 connections to the keypad - CI, C2, C3, Rl, R2, R3
and R4.

Table 7.1 12

Key keypad

Column"!, C1

Column2, C2

Column3, C3

Row1, R1

Row2, R2

Row3, R3

Row4, R4

This connection to the micro is shown in Figure 7.1.

The keypad works in the following way:

If for example key 6 is pressed then B2 will be joined to B4. For key 1 BO would
be joined to B3 etc. as shown in Figure 7.1.

The micro would set BO low and scan B3, B4, B5 and B6 for a low to see
if keys 1, 4, 7 or * had been pressed.

94 Keypad scanning

The micro would then set Bl low and scan B3, B4, B5 and B6 for a low to
see if keys 2, 5, 8 or had been pressed.

Finally B2 would be set low and B3, B4, B5 and B6 scanned for a low to see
if keys 3, 6, 9 or # had been pressed.

Figure 7.1 Keypad connection to the microcontroller

Programming example for the keypad

As a programming example when key 1 is pressed display a binary 1 on
PORTA, when key 2 is pressed display a binary 2 on PORTA etc.

Key displays 10. Key * displays 11. Key # displays 12.

This program could be used as a training aid for decimal to binary conversion.

The flowchart is shown in Figure 7.2.

Keypad scanning 95

Set PORTA as Output.

Set PORTB as MIXED I/O

CLEAR PORTA

PORTB = FF

^c^s

Is N

v Y

B3 = 0?

PORTA = 1

N yA
Is ^

v Y

54 = 0?

PORTA = 4

N X
Is X

\ Y

35 = 0?

PORTA = 7

N A

v Y

B6 = 0?

PORTA = 1 1

Figure 7.2 Keypad scanning flowchart

96 Keypad scanning

PORTB = FF
Clear B1

Figure 7.2 Continued

Keypad scanning 97

PORTB = FF
Clear B2

\ Y

PORT A = 12

Return to **

The program listing for the Keypad example for the 16F84 is shown below but
can be used with any 'suitable' microcontroller using the appropriate header.

N.B. PORTA has been configured as an output port and PORTB has been
configured with 3 outputs and 5 inputs, so the header will require modifying as
shown.

98 Keypad scanning

PORTB has internal pull up resistors so that the resistors connected to PORTB
in Figure 7.1 are not required.

;KEYPAD.ASM

;EQUATES SECTION

STATUS

EQU 3

PORTA

EQU 5

PORTB

EQU 6

TRISA

EQU 85H

TRISB

EQU 86H

OPTION_R

EQU 81H

LIST

= 16F84

ORG

GOTO

START

;means STATUS is file 3.
;means PORTA is file 5.
:means PORTB is file 6.

;we are using the 16F84.

;the start address in memory is

;goto start!

CONFIGURATION BITS

_Config H'3FF0' ;selects LP Oscillator, WDT off,

;Put on,
;code protection disabled.

CONFIGURATION SECTION

START BSF STATUS,5 ;Turns to Bankl.

;PORTA is OUTPUT

;PORTB is mixed I/O.
;Turn on pull ups.
; Return to BankO.
;Clears PortA.
;Clears PortB.

BSF

STATUS,5

MOVLW

B'OOOOOOOO'

MOVWF

TRISA

MOVLW

B'l 11 11000'

MOVWF

TRISB

BCF

OPTION_R,7

BCF

STATUS,5

CLRF

PORTA

CLRF

PORTB

;Program starts now.

COLUMN 1

BCF

PORTB,0

;Clear BO

BSF

PORTB, 1

;Set Bl

BSF

PORTB,2

;Set B2

Keypad scanning 99

CHECK1

BTFSC

PORTB,3

;Is B3 Clear?

GOTO

CHECK4

;No

MOVLW

;Yes, output 1.

MOVWF

PORTA

CHECK4

BTFSC

PORTB,4

;Is B4 Clear?

GOTO

CHECK7

;No

MOVLW

;Yes, output 4.

MOVWF

PORTA

CHECK7

BTFSC

PORTB,5

;Is B5 Clear?

GOTO

CHECK 11

;No

MOVLW

;Yes, output 7.

MOVWF

PORTA

CHECK 11

BTFSC

PORTB,6

;Is B6 Clear?

GOTO

COLUMN2

;No

MOVLW

.11

; Yes, output 1 1 .

MOVWF

PORTA

C0LUMN2

BSF

PORTB,0

;Set BO

BCF

PORTB,l

;Clear Bl

BSF

PORTB,2

;Set B2

CHECK2

BTFSC

PORTB,3

;Is B3 Clear?

GOTO

CHECK5

;No

MOVLW

;Yes, output 2.

MOVWF

PORTA

CHECK5

BTFSC

PORTB,4

;Is B4 Clear?

GOTO

CHECK8

;No

MOVLW

;Yes, output 5.

MOVWF

PORTA

CHECK8

BTFSC

PORTB,5

;Is B5 Clear?

GOTO

CHECK 10

;No

MOVLW

;Yes, output 8.

MOVWF

PORTA

CHECK 10

BTFSC

PORTB,6

;Is B6 Clear?

GOTO

COLUMN3

;No

MOVLW

.10

;Yes, output 10.

MOVWF

PORTA

COLUMN3

BSF

PORTB,0

;Set BO

BSF

PORTB,l

;Set Bl

BCF

PORTB,2

;Clear B2

CHECK3

BTFSC

PORTB,3

;Is B3 Clear?

GOTO

CHECK6

;No

MOVLW

;Yes, output 3.

MOVWF

PORTA

CHECK6

BTFSC

PORTB,4

;Is B4 Clear?

GOTO

CHECK9

;No

MOVLW

;Yes, output 6.

MOVWF

PORTA

100 Keypad scanning

CHECK9

CHECK12

BTFSC

PORTB,5

;Is B5 Clear?

GOTO

CHECK 12

;No

MOVLW

;Yes, output 9.

MOVWF

PORTA

BTFSC

PORTB,6

;Is B6 Clear?

GOTO

COLUMN 1

;No

MOVLW

.12

;Yes, output 12.

MOVWF

PORTA

GOTO

COLUMN 1

;Start scanning i

END

How does the program work?

Port configuration

The first thing to note about the keypad circuit is that the PORTA pins are
being used as outputs. On PORTB, pins BO, Bl and B2 are outputs and B3, B4,
B5 and B6 are inputs. So PORTB is a mixture of inputs and outputs. The
HEADER84.ASM program has to be modified to change to this new
configuration.

To change PORTA to an output port, the following two lines are used in the
Configuration Section:

MOVLW B'00000000' ;PORTA is OUTPUT
MOVWF TRISA

To configure PORTB as a mixed input and output port the following two lines
are used in the Configuration Section:

MOVLW B'll 111000'

MOVWF TRISB ;PORTB is mixed I/O. B0,B1,B2 are O/P.

Scanning routine

The scanning routine looks at each individual key in turn to see if one is being
pressed. Because it can do this so quickly it will notice we have pressed a key
even if we press it quickly.

The scanning routine first of all looks at the keys in columnl i.e. 1, 4, 7 and *.
It does this by setting B0 low, Bl and B2 high. If a 1 is pressed the B3 will
be low, if a 1 is not pressed then B3 will be high. Because pressing a 1 connects
B0 and B3.

Similarly if 4 is pressed B4 will be low if not B4 will be high.

Keypad scann ing 1 1

If 7 is pressed B5 will be low if not B5 will be high.
If * is pressed B6 will be low if not B6 will be high.

In other words when we set BO low if any of the keys in column 1 are pressed
then the corresponding input to the microcontroller will go low and the
program will output the binary number equivalent of the key that has been
pressed.

If none of the keys in column 1 are pressed then we move onto column2.

The code for scanning column 1 is as follows:

These 3 lines set up PORTB with B0 = 0, Bl = 1 and B2= 1.

COLUMN1

BCF

PORTB,0

;Clear BO

BSF

PORTB, 1

;Set Bl

BSF

PORTB,2

;Set B2

These next 4 lines test input B3 to see if it clear if it is then a 1 is placed on
PORTA, then the program continues. If B3 is set then we proceed to check
to see if key 4 has been pressed, with CHECK4.

CHECK1

BTFSC

PORTB,3

;Is B3 Clear?

GOTO

CHECK4

;No

MOVLW

;Yes, output 1

MOVWF

PORTA

;to PORTA

These next 4 lines test input B4 to see if it clear if it is then a 4 is placed
on PORTA, then the program continues. If B4 is set then we proceed to check
to see if key 7 has been pressed, with CHECK7.

CHECK4

BTFSC

PORTB,4

;Is B4 Clear?

GOTO

CHECK7

;No

MOVLW

;Yes, output 4.

MOVWF

PORTA

These next 4 lines test input B5 to see if it clear if it is then a 7 is placed on
PORTA, then the program continues. If B5 is set then we proceed to Check to
see if key * has been pressed, with CHECK11.

CHECK7

BTFSC

PORTB,5

;Is B5 Clear?

GOTO

CHECK 11

;No

MOVLW

;Yes, output 7.

MOVWF

PORTA

102 Keypad scanning

These next 4 lines test input B6 to see if it clear if it is then an 1 1 is placed on
PORTA, then the program continues. If B5 is set then we proceed to check the
keys in column2, with COLUMN2.

CHECK11 BTFSC PORTB,6

GOTO COLUMN2

MOVLW .11

MOVWF PORTA

Is B6 Clear?

Yes, output 11.

These 3 lines set up PORTB with BO = 1, Bl = and B2 = 1

COLUMN2 BSF

PORTB,0

;Set BO

BCF

PORTB, 1

;Clear Bl

BSF

PORTB,2

;Set B2

We then check to see if key2 has been pressed by testing to see if B3 is clear,
if it is then a 2 is placed on PORTA and the program continues. If B3 is set
then we proceed with CHECK5. This code is:

CHECK2

BTFSC
GOTO

PORTB,3
CHECK5

;Is B3 Clear?

;No

MOVLW
MOVWF

.2
PORTA

;Yes, output 2

The program continues in the same manner checking 5, 8 and 10 (0). Then
moving onto column3 to check for 3, 6, 9 and 12 (#). After completing the scan
the program then goes back to continue the scan again.

It takes about 45 lines of code to complete a scan of the keypad.
With a 32,768Hz crystal the lines of code are executed at % of this speed i.e.
8192 lines per second. So the scan time is 45/8192= 5.5ms. This is why
no matter how quickly you press the key the microcontroller will be able to
detect it.

Security code

Probably one of the most useful applications of a keypad is to enter a code to
turn something on and off such as a burglar alarm or door entry system.

In the following program KEYS3.ASM the sub-routine SCAN, scans the
keypad, waits for a key to be pressed, waits 0.1 seconds for the bouncing
to stop, waits for the key to be released, waits 0.1 seconds for the bouncing

Keypad scann ing 103

to stop and then returns with the key number in W which can then be
transferred into a file.

This is then used as a security code to turn on an LED (PORTA, 0) when
3 digits (137) have been pressed and turn the LED off again when the same
3 digits are pressed. You can of course use any 3 digits.

;KEYS3.ASM
;EQUATES SECTION

ZEROBIT

EQU

TMRO

EQU

STATUS

EQU

PORTA

EQU

PORTB

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION_R

EQU

81H

NUM1

EQU

OCH

NUM2

EQU

ODH

NUM3

EQU

OEH

;means STATUS is file 3.
;means PORTA is file 5.
;means PORTB is file 6.

LIST P= 16F84 ;we are using the 16F84.

ORG
GOTO

START

;the start address in memory is
;goto start!

;SUB-ROUTINE SECTION

SCAN

NOP

COLUMN 1

BCF

PORTB,0

;Clear BO

BSF

PORTB, 1

;Set Bl

BSF

PORTB,2

;Set B2

CHECK 1

BTFSC

PORTB,3

;Is B3 Clear?

GOTO

CHECK4

;No

CALL

DELAYP1

CHECK1A

BTFSS

PORTB,3

GOTO

CHECK1A

CALL

DELAYP1

RETLW

CHECK4

BTFSC

PORTB,4

;Is B4 Clear?

GOTO

CHECK7

;No

CALL

DELAYP1

104 Keypad scanning

CHECK4A

BTFSS

PORTB,4

GOTO

CHECK4A

CALL

DELAYP1

RETLW

CHECK7

BTFSC

PORTB,5

;Is B5 Clear?

GOTO

CHECK 11

;No

CALL

DELAYP1

CHECK7A

BTFSS

PORTB,5

GOTO

CHECK7A

CALL

DELAYP1

RETLW

CHECK 11

BTFSC

PORTB,6

;Is B6 Clear?

GOTO

COLUMN2

;No

CALL

DELAYP1

CHECK11A

BTFSS

PORTB,6

GOTO

CHECK11A

CALL

DELAYP1

RETLW

.11

COLUMN2

BSF

PORTB,0

;Set BO

BCF

PORTB,l

;Clear Bl

BSF

PORTB,2

;Set B2

CHECK2

BTFSC

PORTB,3

;Is B3 Clear?

GOTO

CHECK5

;No

CALL

DELAYP1

CHECK2A

BTFSS

PORTB,3

GOTO

CHECK2A

CALL

DELAYP1

RETLW

;Yes, output 2.

CHECK5

BTFSC

PORTB,4

;Is B4 Clear?

GOTO

CHECK8

;No

CALL

DELAYP1

CHECK5A

BTFSS

PORTB,4

GOTO

CHECK5A

CALL

DELAYP1

RETLW

;Yes, output 5.

CHECK8

BTFSC

PORTB,5

;Is B5 Clear?

GOTO

CHECKO

;No

CALL

DELAYP1

CHECK8A

BTFSS

PORTB,5

GOTO

CHECK8A

CALL

DELAYP1

RETLW

;Yes, output 8.

Keypad scann ing 105

CHECKO

BTFSC

PORTB,6

;Is B6 Clear?

GOTO

COLUMN3

;No

CALL

DELAYP1

CHECKOA

BTFSS

PORTB,6

GOTO

CHECKOA

CALL

DELAYP1

RETLW

;Yes, output 10.

COLUMN3

BSF

PORTB,0

;Set BO

BSF

PORTB,l

;Set Bl

BCF

PORTB,2

;Clear B2

CHECK3

BTFSC

PORTB,3

;Is B3 Clear?

GOTO

CHECK6

;No

CALL

DELAYP1

CHECK3A

BTFSS

PORTB,3

GOTO

CHECK3A

CALL

DELAYP1

RETLW

;Yes, output 3.

CHECK6

BTFSC

PORTB,4

;Is B4 Clear?

GOTO

CHECK9

;No

CALL

DELAYP1

CHECK6A

BTFSS

PORTB,4

GOTO

CHECK6A

CALL

DELAYP1

RETLW

;Yes, output 6.

CHECK9

BTFSC

PORTB,5

;Is B5 Clear?

GOTO

CHECK 12

;No

CALL

DELAYP1

CHECK9A

BTFSS

PORTB,5

GOTO

CHECK9A

CALL

DELAYP1

RETLW

;Yes, output 9.

CHECK 12

BTFSC

PORTB,6

;Is B6 Clear?

GOTO

COLUMN 1

;No

CALL

DELAYP1

CHECK 12A

BTFSS

PORTB,6

GOTO

CHECK12A

CALL

DELAYP1

RETLW

.12

;Yes, output 12.

;3/32 second

delay.

DELAYP1

CLRF

TMRO

;Start TMRO.

LOOPD

MOVF

TMR0,W

;Read TMRO into W.

SUBLW

;TIME-3

106 Keypad scanning

BTFSS STATUS,ZEROBIT

GOTO LOOPD

RETLW

CONFIGURATION SECTION

;Check TIME-W =
;Time is not= 3.
;Time is 3, return.

START

BSF

STATUS,5

;Turns to Bankl.

MOVLW

B'OOOOOOOO'

;PORTA is OUTPUT

MOVWF

TRISA

MOVLW

B'11111000'

MOVWF

TRISB

;PORTB is mixed I/O.

MOVLW

B'00000111'

MOVWF

OPTION_R

BCF

STATUS,5

;Return to BankO.

CLRF

PORTA

;Clears PortA.

CLRF

PORTB

;Clears PortB.

;Program starts now.

;Enter 3 digit code here

MOVLW

;First digit

MOVWF

NUM1

MOVLW

;Second digit

MOVWF

NUM2

MOVLW

;Third digit

MOVWF

NUM3

BEGIN

CALL

SCAN

;Get 1st number

SUBWF

NUM1,W

BTFSS

STATUS,ZEROBIT

;IS NUMBER = 1?

GOTO

BEGIN

;No

CALL

SCAN

;Get 2nd number

SUBWF

NUM2,W

BTFSS

STATUS,ZEROBIT

;IS NUMBER = 3?

GOTO

BEGIN

;No

CALL

SCAN

;Get 3rd number.

SUBWF

NUM3,W

BTFSS

STATUS,ZEROBIT

;IS NUMBER = 7?

GOTO

BEGIN

;No

BSF

PORTA,0

;Turn on LED, 137 entered

TURN_OFF

CALL

SCAN

;Get 1st number again

SUBWF

NUM1,W

BTFSS

STATUS,ZEROBIT

;IS NUMBER = 1?

GOTO

TURN_OFF

;No

CALL

SCAN

;Get 2nd number

Keypad scanning 107

SUBWF

NUM2,W

BTFSS

STATUS,ZEROBIT

;IS NUMBER = 3?

GOTO

TURN_OFF

;No

CALL

SCAN

;Get 3rd number.

SUBWF

NUM3,W

BTFSS

STATUS,ZEROBIT

;IS NUMBER = 7?

GOTO

TURN_OFF

;No

BCF

PORTA,0

;Turn off LED.

GOTO

BEGIN

END

How does the program work?

The ports are configured as in the previous code KEYPAD. ASM.

The KEYS3.ASM program looks for the first key press and then it
compares the number pressed with the required number stored in a user
file called NUM1. It then looks for the second key to be pressed. But because
the microcontroller is so quick, the first number could be stored and the
program looks for the second number, but our finger is still pressing the
first number.

Anti-bounce routine

Also when a mechanical key is pressed or released it does not make or break
cleanly, it bounces around. If the micro is allowed too, it is fast enough to see
these bounces as key presses so we must slow it down.

• We look first of all for the switch to be pressed.

• Then wait 0.1 seconds for the switch to stop bouncing.

• We then wait for the switch to be released.

• We then wait 0.1 seconds for the bouncing to stop before continuing.

The switch has then been pressed and released indicating one action.
The 0.1 second delay is written in the Header as DELAYP1.

Scan routine

The scan routine used in KEYS3.ASM is written into the subroutine.

When called it waits for a key to be pressed and then returns with the number
just pressed in W. It can be copied and used as a subroutine in any program
using a keypad.

• The scan routine checks for key presses as in the previous example
KEYPAD.ASM, Columnl checks for the numbers 1, 4, 7 and 11 being
pressed in turn.

108 Keypad scanning

If the 1 is not pressed then the routine goes on to check for a 4.

If the 1 is pressed then the routine waits 0.1 second for the bouncing to stop.

The program then waits for the key to be released.

Waits again 0.1 seconds for the bouncing to stop,

and then returns with a value of 1 in W.

Code for CHECK 1:

CHECK1

BTFSC

PORTB,3

GOTO

CHECK4

CALL

DELAYP1

CHECK1A

BTFSS

PORTB,3

GOTO

CHECK1A

CALL

DELAYP1

RETLW

;Is B3 Clear? Pressed?

;No

;Antibounce delay, B3 clear

;Is B3 Set? Released?

;No

;Antibounce delay, B3 Set

; Return with 1 in W.

If numbers 4, 7 or 1 1 are pressed the routine will return with the corresponding
value in W.

If no numbers in column 1 are pressed then the scan routine continues on to
column2 and column3. If no keys are pressed then the routine loops back to the
start of the scan routine to continue checking.

Storing the code

The code i.e. 137 is stored in the files NUM1, NUM2, NUM3 with the
following code:

MOVLW

;First digit

MOVWF

NUM1

MOVLW

;Second digit

MOVWF

NUM2

MOVLW

;Third digit

MOVWF

NUM3

Checking for the correct code

• We first of all CALL SCAN to collect the first digit, which returns with the
number pressed in W.

• We then subtract the value of W from the first digit of our code stored in
NUM1 with:

SUBWF NUM1/W.

This means SUBtract W from the File NUM1. The (,W) stores the result of
the subtraction in W. Without (,W) the result would have been stored
in NUM1 and the value changed!

Keypad scanning 109

• We then check to see if NUM1 and W are equal, i.e. a correct match. In this
case the zerobit in the status register would be set. Indicating the result
NUM1— W = zero. This is done with:

BTFSS STATUS,ZEROBIT

We skip and carry on if it is set, i.e. a match. If it isn't we return to BEGIN
to scan again.

• With a correct first press we then carry on checking for a second and
if correct a third press to match the correct code.

• When the correct code is pressed we turn on our LED with:

BSF PORTA,0

• We then run through a similar sequence and wait for the code to turn off
the LED.

Notice that if you enter an incorrect digit you return to BEGIN or
TURN_OFF. If you forget what key you have pressed then press an incorrect
one and start again.

You could of course modify this program by adding a fourth digit to the
program then turn on the LED. In which case you use another user file
called NUM4. You could of course use a different code for switching off
the output.

You can also beep a buzzer for half a second to give yourself an audible
feedback that you had pressed a button.

As an extra security measure you could wait for a couple of seconds if an
incorrect key had been pressed, or wait for 2 minutes if three wrong numbers
had been entered.

The keypad routine opens up many different circuit applications.

The SCAN routine can be copied and then pasted into any program using
the keypad. Then when you CALL SCAN the program will return with the
number pressed in W for you to do with it as you wish.

8
Program examples

New instructions used in this chapter:

• INCF

• INCFSZ

• DECF

• ADDWF

Counting events

Counting of course is a useful feature for any control circuit. We may wish to
count the number of times a door has opened or closed, or count a number of
pulses from a rotating disc. If we count cars into a car park we would
increment a file count every time a car entered, using the instruction INCF
COUNT. If we needed to know how many cars were in the car park we would
have course have to reduce the count by one every time a car left. We would do
this by DECF COUNT. To clear the user file COUNT to start we would
CLRF COUNT. In this way the file count would store the number of cars
in the car park. If you prefer COUNT could be called CARS. It is a user file
call it what you like.

Let's look at an application.

Design a circuit that will count 10 presses of a switch, then turn an LED on and
reset when the next ten presses are started. The hardware is that of Figure 5.1
with A0 as the switch input and BO as the output to the LED.

There are two ways to count, UP and DOWN. We usually count up and know
automatically when we have reached 10. A computer however knows when it
reaches a count of 10 by subtracting the count from 10. If the answer is zero,
then bingo. A simpler way however is to start at 10 and count down to zero -
after 10 events we will have reached zero without doing a subtraction. Zero
for the microcontroller is a really useful number.

Program examples 111

Set PORTB as Output.

Set COUNT to 10.

Decrement COUNT

Turn on LED

Figure 8.1 Initial counting flowchart

The initial flowchart for this problem is shown in Figure 8.1.

To ensure that the LED is OFF after the switch is pressed for the eleventh time
put in TURN OFF LED after the switch is pressed, as shown in Figure 8.2.

N.B. The switch will bounce and the micro is fast enough to count these
bounces, thinking that the switch has been pressed several times. A 0.1 second
delay is inserted after each switch operation to allow time for the bounces to
stop.

The final flowchart is shown in Figure 8.2.

112 Program examples

Set PORTB as Output.

Set COUNT to 10.

Wait 0.1 seconds

Turn off LED.
R 1

Wait 0.1 seconds.

Decrement COUNT

Turn on LED

Figure 8.2 Final counting flowchart

The program for the counting circuit

;COUNT84.ASM using the 16F84 with a 32kHz. crystal
;EQUATES SECTION

TMRO
STATUS

EQU
EQU

;means TMRO is file 1.
means STATUS is file 3,

Program examples 113

PORTA EQU

PORTB EQU

TRISA EQU

TRISB

EQU

OPTION_R EQU
ZEROBIT EQU
COUNT EQU

5
6
85H

86H

81H

2
OCH

means PORTA is file 5.
means PORTB is file 6.
TRISA (the PORTA I/O selection) is
file 85H

TRISB (the PORTB I/O selection) is
file 86H

the OPTION register is file 81H
means ZEROBIT is bit 2.
COUNT is file OC, a register to count
;events.

LIST P= 16F84 ;we are using the 16F84.

ORG ;the start address in memory is

GOTO START ;goto start!

;Configuration Bits

_CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

SUBROUTINE SECTION.

;3/32 second delay.
DELAY CLRF
LOOPA MOVF
SUBLW
BTFSS
GOTO
RETLW

TMRO

TMR0,W

STATUS, ZEROBIT

LOOPA

START TMRO.

READ TMRO INTO W.

TIME - 3

Check TIME-W =

Time is not = 3.

Time is 3, return.

CONFIGURATION SECTION

START BSF

STATUS,5

MOVLW
MOVWF

; Turns to Bankl.

B'0001 1 111' ;5bits of PORTA are I/P
TRISA

MOVLW
MOVWF

B'00000000'
TRISB

;PORTB is OUTPUT

114 Program examples

MOVLW

B'ooooonr

;Prescaler is /256

MOVWF

OPTION_R

;TIMER is 1/32 sees.

BCF

STATUS,5

;Return to BankO.

CLRF

PORTA

;Clears PortA.

CLRF

PORTB

;Clears PortB.

;Program starts now.

BEGIN MOVLW

.10

PRESS

RELEASE

MOVLW

.10

MOVWF

COUNT

BTFSC

PORTA,0

GOTO

PRESS

CALL

DELAY

BCF

PORTB,0

BTFSS

PORTA,0

GOTO

RELEASE

CALL

DELAY

DECFSZ

COUNT

GOTO

PRESS

BSF

PORTB,0

GOTO

BEGIN

;Put 10 into COUNT.
;Check switch is pressed

;Wait for 3/32 seconds.
;TURN OFF LED.
;Check switch is released.

WAIT for 3/32 seconds.
Dec COUNT skip if 0.
Wait for another press.
Turn on LED.
Restart

END

How does it work?

• The file COUNT is first loaded with the count i.e. 10 with:

MOVLW
MOVWF

.10
COUNT

;Put 10 into COUNT.

We then wait for the switch to be pressed, by PORTA, going low:

PRESS BTFSC PORTA,0 ;Check switch is pressed

GOTO PRESS

Anti-bounce:

CALL DELAY

Turn off the LED on B0:

BCF PORTB,0

Wait for switch to be released

;Wait for 3/32 seconds.

RELEASE

BTFSS
GOTO

PORTA,0
RELEASE

;Check switch is released.

Anti-bounce:
CALL DELAY

;Wait for 3/32 seconds.

Program examples 115

Decrement the file COUNT, if zero turn on LED and return to begin.
If not zero continue pressing the switch.

DECFSZ

COUNT

;Dec COUNT skip if 0.

GOTO

PRESS

;Wait for another press.

BSF

PORTB,0

;Turn on LED.

GOTO

BEGIN

;Restart

This may appear to be a lot of programming to count presses of a switch,
but once saved as a subroutine it can be reused in any other programs.

Look up table

A look up table is used to change data from one form to another i.e.
pounds to kilograms, °C to °F, inches to centimeters etc. The explanation of
the operation of a look up table is best understood by way of an example.

7-Segment display

Design a circuit that will count and display on a 7-segment display, the number
of times a button is pressed, up to 10. The circuit diagram for this is shown in
Figure 8.3.

I— SW1

68p 32kHz 16

I=l

68p

16F84

BO
B1
B2
B3
B4
B5
B6

V+
MCLR

7 x 680R

±. 0.1 LI

Figure 8.3 Circuit diagram of the 7-segment display driver

116 Program examples

The flowchart for the 7-Segment Display Driver is shown in Figure 8.4.

Figure 8.4 Initial flowchart for the 7-segment driver

This is a basic solution that has a few omissions:

• The switch bounces when pressed.

• Clear the count at the start.

• The micro counts in binary, we require a 7-segment decimal display.
So we need to convert the binary count to drive the relevant
segments on the display.

• When the switch is released it bounces.

The amended flowchart is shown in Figure 8.5.

Program examples 117

Set PORTB as output.
Clear PORTB.
Clear COUNT.

Wait 0.1 seconds

Increment count

Convert binary count
to 7 segment format.

Display Count

Wait 0.1 seconds.

Figure 8.5 Amended flowchart for 7-segment display

118 Program examples

Set PORTB as output.
Clear PORTB.
Clear COUNT.

— ►*<—

Wait 0.1 seconds

Increment count

Convert binary count
to 7 segment format.

Display Count

Wait 0.1 seconds.

Clear Count

Figure 8.6 Final flowchart for 7-segment display

The flowchart is missing just one thing! What happens when the count reaches
10? The counter needs resetting (it would count up to 255 before resetting). The
final flowchart is shown in Figure 8.6.

Now about this look up table:

Table 8.1 shows the configuration of PORTB to drive the 7-segment display.
(Refer also to Figure 8.3).

Program examples 119

Table 8.1 Binary code to drive 7-segment display

NUMBER

PORTB

The look up table for this is:

CONVERT ADDWF

RETLW

B'oinonr

RETLW

B'oiooooor

RETLW

B'ooinoir

RETLW

B'onoioir

RETLW

B'oioonor

RETLW

B'01101110'

RETLW

B'01111100'

RETLW

B'oiooooir

RETLW

B'oimiir

RETLW

B'oioonir

How does the look up table work?

Suppose we need to display a 0.

We move into W and CALL the look up table, here it is called CONVERT.

The first line says ADD W to the Program Count, since W = then go to the
next line of the program which will return with the 7-segment value 0.

Suppose we need to display a 6.

Move 6 into W and CALL CONVERT. The first line says ADD W to the
Program Count, since W contains 6 then go to the next line of the program and
move down 6 more lines and return with the code for 6, etc.

Just one more thing: To check that a count has reached 10, subtract 10 from
the count if the answer is 0, bingo!

120 Program examples

The program listing for the complete program is:

;DISPLAY.ASM

;EQUATES SECTION

EQU

2 ;

TMRO

EQU

1 ;

STATUS

EQU

3 ;

PORTA

EQU

5 ;

PORTB

EQU

6 ;

TRISA

EQU

85H ;

TRISB

EQU

86H ;

OPTION R

EQU

81H ;

ZEROBIT

EQU

2 ;

COUNT

EQU

OCH ;

;means PC is file 2.

;means TMRO is file 1.

;means STATUS is file 3.

;means PORTA is file 5.

;means PORTB is file 6.

;TRISA (the PORTA I/O selection) is file 85H

;TRISB (the PORTB I/O selection) is file 86H

;the OPTION register is file 81H

;means ZEROBIT is bit 2.

;COUNT is file 0C, a register to count events.

LIST P= 16F84 ;we are using the 16F84.

ORG ;the start address in memory is

GOTO START ;goto start!

;Configuration Bits

__CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

SUBROUTINE SECTION.

;3/32 second delay.

DELAY

CLRF

TMRO

;START TMRO.

LOOPA

MOVF

TMR0,W

;READ TMRO INTO W

SUBLW

;TIME - 3

BTFSS

STATUS, ZEROBIT

;CheckTIME-W =

GOTO

LOOPA

;Time is not = 3.

RETLW

;Time is 3, return.

CONVERT

ADDWF

RETLW

B'onionr

RETLW

B'oiooooor

RETLW

B'ooinoir

RETLW

B'01101011'

RETLW

B'oioonor

RETLW

B'01101110'

Program examples 121

RETLW

B'01111100'

RETLW

B'oiooooir

RETLW

B'oiiinir

RETLW

B'oioonir

CONFIGURATION SECTION

START BSF

STATUS,5

:Turns to Bankl.

MOVLW
MOVWF

B'ooomir

TRISA

;5bits of PORTA are I/P

MOVLW
MOVWF

BCF

CLRF

B'OOOOOOOO'
TRISB

B'00000111'
OPTION_R

STATUS,5

PORTA

PORTB

PORTB is OUTPUT

Prescaler is /256
TIMER is 1/32 sees.

Return to BankO.
Clears PortA.
;Clears PortB.

;Program starts now.

CLRF

COUNT ;

PRESS BTFSC

PORTA,0 ;

GOTO

PRESS ;

CALL

DELAY ;

INCF

COUNT ;

MOVF

COUNT,W ;

SUBLW

.10 ;

BTFSC

STATUS,ZEROBIT ;

CLRF

COUNT ;

MOVF

COUNT,W ;

CALL

CONVERT ;

MOVWF

PORTB ;

RELEASE BTFSS

PORTA,0 ;

GOTO

RELEASE ;

CALL

DELAY ;

GOTO

PRESS ;

Set COUNT to 0.
Test for switch press.
Not pressed.
Antibounce wait 0.1 sec.
Add 1 to COUNT.
Move COUNT to W.
COUNT- 10, W is altered.
Is COUNT - 10 = 0?
Count = 10 Make Count =
Put Count in W again.
Count is not 10, carry on.
Output number to display.

Is switch released?
Not released.
Antibounce wait 0.1 sec.
Look for another press.

END

122 Program examples

How does the program work?

• The file count is cleared (to zero) and we wait for the switch to be pressed.
PRESS

CLRF

COUNT

;Set COUNT to 0.

BTFSC

PORTA,0

;Test for switch press.

GOTO

PRESS

;Not pressed.

Wait for 0.1 seconds, Anti-bounce.
CALL DELAY

Add 1 to COUNT and check to see if it 10:

INCF COUNT ;Add 1 to COUNT.

MOVF COUNT,W ;Move COUNT to W.

SUBLW .10 ;COUNT-10, W is altered.

BTFSC STATUS,ZEROBIT ;Is COUNT - 10 = 0?

If COUNT is 10, Clear it to and output the count as 0. If the COUNT is
not 10 then output the count.

Count = 10 Make Count =
Put Count in W again.
Count is not 10, carry on.
Output number to display.

CLRF

COUNT

MOVF

COUNT,W

CALL

CONVERT

MOVWF

PORTB

Wait for the switch to be released and

the presses.

PLEASE BTFSS

PORTA,0

GOTO

RELEASE

CALL

DELAY

GOTO

PRESS

;Is switch released?
;Not released.
;Antibounce wait 0.1 sec.
;Look for another press.

Test your understanding

• Modify the program to Count up to 6 and reset.

• Modify the program to Count up to F in HEX and reset.

A look up table to change °C to °F is shown below, called DEGREE

DEGREE

ADDWF

RETLW

.32

RETLW

.34

RETLW

.36

RETLW

.37

;ADD W to Program Count.
;0°C =32°F
;1°C = 34°F
;2°C = 36°F
;3°C = 37°F

Program examples 123

RETLW

.39

4°C =

39°F

RETLW

.41

5°C =

41°F

RETLW

.43

6°C =

43°F

RETLW

.45

7°C =

45°F

RETLW

.46

8°C =

46°F

RETLW

.48

9°C =

48°F

RETLW

.50

10°C

= 50°F

RETLW

.52

11°C

= 52°F

RETLW

.54

12°C

= 54°F

RETLW

.55

13°C

= 55°F

RETLW

.57

14°C

= 57°F

RETLW

.59

15°C

= 59°F

RETLW

.61

16°C

= 61°F

RETLW

.63

17°C

= 63°F

RETLW

.64

18°C

= 64°F

RETLW

.66

19°C

= 66°F

RETLW

.68

20°C

= 68°F

RETLW

.70

21°C

= 70°F

RETLW

.72

22°C

= 72°F

RETLW

.73

23°C

= 73°F

RETLW

.75

24°C

= 75°F

RETLW

.77

25°C

= 77°F

RETLW

.79

26°C

= 79°F

RETLW

.81

27°C

= 81°F

RETLW

.82

28°C

= 82°F

RETLW

.84

29°C

= 84°F

RETLW

.86

30°C

= 86°F

Another application of the use of the look up table is a solution for a previous
example i.e. the "Control Application - A Hot Air Blower." Introduced in
Chapter 5.

In this example when PORTA was read the data was treated as a binary
number, but we could just as easily treat the data as decimal number.

i.e. A2 Al A0 = 000 or
= 001 or 1
= 010 or 2
= 011 or 3
= 100 or 4
= 101 or 5
= 110 or 6
= 1 1 1 or 7

124 Program examples

The look up table for this would be:

CONVERT

ADDWF

RETLW

B'00000010'
B'00000001'

B'ooooooir

B'00000001'
B'00000000'
B'00000001'
B'00000000'
B'00000010'

;0 on PORTA turns on Bl
;1 on PORTA turns on BO
;2 on PORTA turns on B1,B0
;3 on PORTA turns on BO
;4 on PORTA turns off B1,B0
;5 on PORTA turns on BO
;6 on PORTA turns off B1,B0
;7 on PORTA turns on Bl

The complete program listing for the program DISPLAY2 would be:
;DISPLAY2.ASM

;EQUATES SECTION

EQU

TMRO

EQU

STATUS

EQU

PORTA

EQU

PORTB

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION R

EQU

81H

ZEROBIT

EQU

COUNT

EQU

OCH

;Program Counter is file 2.

;means TMRO is file 1.

;means STATUS is file 3.

;means PORTA is file 5.

;means PORTB is file 6.

;TRISA (the PORTA I/O selection) is

;file 85H

;TRISB (the PORTB I/O selection) is

;file 86H

;the OPTION register is file 81H

;means ZEROBIT is bit 2.

;COUNT is file OC, a register to count

:events.

LIST P= 16F84 ;we are using the 16F84.

ORG ;the start address in memory is

GOTO START ;goto start!

Configuration Bits

_CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

:Code Protection disabled.

Program examples 125

SUBROUTINE SECTION.

CONVERT

ADDWF

RETLW

B'00000010'
B'00000001'

B'ooooooir

B'00000001'
B'00000000'
B'00000001'
B'00000000'
B'00000010'

on PORTA

1 on PORTA

2 on PORTA

3 on PORTA

4 on PORTA

5 on PORTA

6 on PORTA

7 on PORTA

turns on Bl
turns on BO
turns on B1,B0
turns on BO
turns off Bl, BO
turns on BO
turns off Bl, BO
turns on Bl

CONFIGURATION SECTION

START BSF

MOVLW
MOVWF

STATUS,5

B'00011111'
TRISA

: Turns to Bankl.

;5bits of PORTA are I/P

MOVLW
MOVWF

BCF

CLRF

B'00000000'
TRISB

STATUS,5

PORTA

PORTB

;PORTB is OUTPUT

B'00000 111' ;Prescaler is /256
OPTION_R ;TIMER is 1/32 sees.

;Return to BankO.
;Clears PortA.
iClears PortB.

;Program starts now.

BEGIN

MOVF

PORTA,W

,Read PORTA into W

CALL

CONVERT

,Obtain O/Ps from I/Ps

MOVWF

PORTB

,switch on O/Ps

GOTO

BEGIN

,repeat

END

How does the program work?

• The program first of all reads the value of PORTA into the working
register, W:

MOVF

PORTA,W

126 Program examples

• The CONVERT routine is called which returns with the correct setting of
the outputs in W. i.e. If the value of PORTA was 3 then the look up table
would return with 00000001 in W to turn on B0 and turn off Bl:

CALL CONVERT ;Obtain O/Ps from I/Ps.

MOVWF PORTB ;switch on O/Ps

• The program then returns to check the setting of PORTA again.

Numbers larger than 255

The PIC Microcontrollers are 8 bit devices, this means that they can easily
count up to 255 using one memory location. But to count higher then more
than one memory location has to be used for the count.

Consider counting a switch press up to 1000 and then turn on an LED to show
this count has been achieved. The circuit for this is shown in Figure 8.7.

SW1

68p 32 kHz 1 6

68p

16F84

V+
MCLR

470 R
LED1 iv^

T 0.1(1

Figure 8.7 Circuit for 1000 count

To count up to 1000 in decimal i.e. 03E8 in hex, files COUNTB and COUNTA
will store the count (a count of 65535 is then possible).

COUNTB will count up to 03H then when COUNTA has reached E8H, LED1
will light indicating the count of 1000 has been reached.

The flowchart for this 1000 count is shown in Figure 8.8.

Program examples 127

Set PORTB as Output.
Set Prescaler to / 256.

Clear PORTB.
Clear COUNTA.
Clear COUNTB.

Wait 0.1 seconds

Wait 0.1 seconds.

Increment COUNTA

INCREMENT COUNTB

Figure 8.8 Count of 1000 flowchart

128 Program examples

TURN onLEDI

Figure 8.8 Continued

Flowchart explanation

• The program is waiting for SW1 to be pressed. When it is, there is a delay
of 0.1 seconds to allow the switch bounce to stop.

• The program then looks for the switch to be released and waits 0.1 seconds
for the bounce to stop.

Program examples 129

• 1 is then added to COUNTA and a check is made to see if the count
has overflowed i.e. reached 256. (255 is the maximum it will hold, when it
reaches 256 it will reset to zero just like a two digit counter would reset to
zero going from 99 to 100.)

• If COUNTA has overflowed then we increment COUNTB.

• A check is made to see if COUNTB has reached 03H, if not we return to
keep counting.

• If COUNTB has reached 03H then we count presses until COUNTA reaches
E8H. The count in decimal is then 1000 and the LED is lit.

Any count can be attained by altering the values COUNTB and COUNTA
are allowed to count up to i.e. to count up to 5000 in decimal which is 1388H.
Ask if COUNTB = 13H then count until COUNTA has reached 88H.

The program listing

;CNT 1000. ASM

;EQUATES SECTION

TMR0

EQU

STATUS

EQU

PORTA

EQU

PORTB

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION R

EQU

81H

ZEROBIT

EQU

COUNTA

EQU

0CH

COUNTB

EQU

0DH

means TMR0 is file 1.

means STATUS is file 3.

means PORTA is file 5.

means PORTB is file 6.

TRISA (the PORTA

I/O selection) is file 85H

TRISB (the PORTB I/O selection) is file 86H

the OPTION register is file 81H

means ZEROBIT is bit 2.

USER RAM LOCATION.

LIST P= 16F84 ;we are using the 16F84.

ORG ;the start address in memory is

GOTO START ;goto start!

;Configuration Bits

_CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

130 Program examples

SUBROUTINE SECTION.

;3/32 second delay.

DELAY CLRF

TMRO

LOOPA MOVF

TMRO/W

SUBLW

BTFSS

STATUS,ZEROBIT

GOTO

LOOPA

RETLW

START TMRO.

READ TMRO INTO W.

TIME - 3

Check TIME-W =

Time is not = 3.

Time is 3, return.

CONFIGURATION SECTION

START BSF

STATUS,5 ;Turns to Bankl

MOVLW B'0001 1 111' ;5bits of PORTA are I/P

MOVWF TRISA

MOVLW
MOVWF

B'00000000'
TRISB

:PORTB is OUTPUT

MOVLW B'00000 111' ;Prescaler is /256

MOVWF OPTION R :TIMER is 1/32 sees.

BCF

CLRF

STATUS,5 ;Return to BankO.
PORTA ;Clears PortA.

PORTB :Clears PortB.

;Program starts now.

CLRF

COUNTA

CLRF

COUNTB

PRESS BTFSC

PORTA,0

GOTO

PRESS

CALL

DELAY

RELEASE BTFSS

PORTA,0

GOTO

RELEASE

CALL

DELAY

;Check switch pressed

;Wait for 3/32 seconds.
;Check switch is released.

;Wait for 3/32 seconds.

Program examples 131

INCFSZ

COUNTA

;Inc. COUNT skip if 0.

GOTO

PRESS

INCF

COUNTB

MOVLW

03H

;Put 03H in W.

SUBWF

COUNTB,W

;COUNTB - W (i.e. 03)

BTFSS

STATUS,ZEROBIT

;IS COUNTB = 03H

GOTO

PRESS

;No

PRESS 1 BTFSC

PORTA,0

;Check switch pressed.

GOTO

PRESS1

CALL

DELAY

;Wait for 3/32 seconds.

RELEASE1 BTFSS

PORTA,0

;Check switch released.

GOTO

RELEASE 1

CALL

DELAY

;Wait for 3/32 seconds.

INCF

COUNTA

MOVLW

0E8H

;Put E8 in W.

SUBWF

COUNTA

;COUNTA - E8.

BTFSS

STATUS,ZEROBIT

;COUNTA = E8?

GOTO

PRESS 1

;No.

BSF

PORTB,0

;Yes, turn on LED1.

STOP GOTO

STOP

;stop here

END

How does the program work?

• The two files used for counting are cleared.

CLRF
CLRF

COUNTA
COUNTB

As we have done previously we wait for the switch to be pressed and released
and to stop bouncing:

;Check switch pressed

;Wait for 3/32 seconds.
:Check switch is released.

PRESS

BTFSC

PORTA,0

GOTO

PRESS

CALL

DELAY

RELEASE

BTFSS

PORTA,0

GOTO

RELEASE

CALL

DELAY

;Wait for 3/32 seconds.

132 Program examples

We addl to file COUNTA and check to see if it zero. If it isn't then continue
monitoring presses. (The file would be zero when we add 1 to the 8 bit
number 1111 1111, it overflows to 0000 0000):

INCFSZ
GOTO

COUNTA
PRESS

;Inc. COUNT skip if 0.

If the file COUNTA has overflowed then we add 1 to the file COUNTB, just
like you would do with two columns of numbers. We then need to know if
COUNTB has reached 03H. If COUNTB is not 03H then we return to
PRESS and continue monitoring the presses.

INCF

COUNTB

MOVLW

03H

;Put 03H in W.

SUBWF

COUNTB,W

;COUNTB - W (i.e. 03)

BTFSS

STATUS,ZEROBIT

;IS COUNTB = 03H?

GOTO

PRESS

;No

Once COUNTB has reached 03H we need only wait until COUNTA reaches
0E8H and we would have counted up to 03E8H i.e. 5000 in decimal. Then
we turn on the LED.

PRESS 1 BTFSC

PORTA,0

Check switch pressed.

GOTO

PRESS1

CALL

DELAY

Wait for 3/32 seconds

RELEASE1 BTFSS

PORTA,0

Check switch released

GOTO

RELEASE 1

CALL

DELAY

Wait for 3/32 seconds

INCF

COUNTA

MOVLW

0E8H

Put E8 in W.

SUBWF

COUNTA

COUNTA E8.

BTFSS

STATUS,ZEROBIT

COUNTA = E8?

GOTO

PRESS1

No.

BSF

PORTB,0

Yes, turn on LED1.

STOP GOTO

STOP

stop here

This listing can be used as a subroutine in your program to count up to
any number to 65535 (or more if you use a COUNTC file). Just alter
COUNTB and COUNTA values to whatever values you wish, in the two
places marked * in the program.

Program examples 133

Question. How would you count up to 20,000?

Answer. (Have you tried it first!!).

20,000 = 4E20H so COUNTB would count up to 4EH and COUNTA
would then count to 20H.

Question. How would you count to 100,000?

Answer. 100,000 = 0186A0H, you would use a third file COUNTC to count to
01H, COUNTB would count to 86H and COUNTA would count to A0H.

Programming can be made a lot simpler by keeping a library of subroutines.
Here is another

Long time intervals

Probably the more frequent use of a large count is to count TMR0 pulses to
generate long time intervals. We have previously seen in the section on delay
that we can slow the internal timer clock down to 1/32 seconds. Counting
a maximum of 255 of these gives a time of 255 x 1/32 = 8 seconds. Suppose
we want to turn on an LED for 5 minutes when a switch is pressed.

5 minutes = 300 seconds = 300 x 32 (1/32 seconds) i.e. a TMR0 count of 9600.
This is 2580 in hex. The circuit is the same as Figure 8.7 for the 1000-count
circuit, and the flowchart is shown in Figure 8.9.

Explanation of the flowchart

1. Wait until the switch is pressed, the LED is then turned on.

2. TMR0 is cleared to start the timing interval.

3. TMR0 is moved into W (read) to catch the first count.

4. Then wait for TMR0 to return to zero, (the count will be 256) i.e. 100
in hex.

5. COUNTA is then incremented and steps 3 and 4 repeated until COUNTA
reaches 25H.

6. Wait until TMR0 has reached 80H.

7. The count has reached 2580H i.e. 9600 in decimal. 5 minutes has elapsed
and the LED is turned off.

134 Program examples

Set PORTB as output

Set Prescalerto /256

Clear PORTB

Clear COUNTA

Turn onLED
Clear TMRO

Move TMRO into W.

Increment COUNTA

Move TMRO into W.

Turn off LED

Figure 8.9 Flowchart for the 5 minute delay

Program examples 135

Program listing for 5 minute delay

;LONGDLY.ASM

;EQUATES SECTION

TMRO EQU 1

STATUS EQU 3

PORTA EQU 5

PORTB EQU 6

TRISA EQU 85H

TRISB EQU 86H

OPTION_R EQU 81H

ZEROBIT EQU 2

COUNTA EQU OCH

;means TMRO is file 1.

;means STATUS is file 3.

;means PORTA is file 5.

;means PORTB is file 6.

;TRISA (the PORTA I/O selection) is file 85H

;TRISB (the PORTB I/O selection) is file 86H

;the OPTION register is file 81H

;means ZEROBIT is bit 2.

;COUNT is file OC, a register to count events.

LIST P= 16F84 ;we are using the 16F84.

ORG ;the start address in memory is

GOTO START ;goto start!

;Configuration Bits

_CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

iCONFIGURATION SECTION

START BSF

MOVLW
MOVWF

BCF

CLRF

STATUS,5 ;Turns to Bankl.

B'000 1 1 1 1 1 ' ;5bits of PORTA are I/P
TRISA

B'OOOOOOOO'
TRISB

B'ooooonr

OPTION_R

STATUS,5

PORTA

PORTB

PORTB is OUTPUT

Prescaler is /256
TIMER is 1/32 sees.

Return to BankO.
Clears PortA.
;Clears PortB.

;Program starts now.

PRESS

CLRF
BTFSC

COUNTA
PORTA,0

;Check switch pressed.

136 Program examples

GOTO

PRESS

;No

BSF

PORTB,0

;Yes, turn on LED

CLRF

TMRO

;Start TMRO.

WAITO

MOVF

TMRO/W

;Move TMRO into W

BTFSC

STATUS,ZEROBIT

;IsTMR0 = 0.

GOTO

WAITO

;Yes

WAIT1

MOVF

TMR0,W

;No, move TMRO into W.

BTFSS

STATUS,ZEROBIT

GOTO

WAIT1

;Wait for TMRO to overflow

INCF

COUNTA

increment COUNTA

MOVLW

25H

SUBWF

COUNTA,W

;COUNTA - 25H

BTFSS

STATUS,ZEROBIT

;Is COUNTA = 25H

GOTO

WAITO

;COUNTA < 25H

WAIT2

MOVF

TMRO/W

;COUNTA = 25H

MOVLW

80H

SUBWF

TMRO/W

;TMR0 - 80H

BTFSS

STATUS,ZEROBIT

;IsTMR0 = 80H

GOTO

WAIT2

;TMR0 < 80H

BCF

PORTB,0

;TMR0 = 80H, turn off LED

END

The explanation of this program operation is similar to that of the count to
1000, done earlier in this chapter.

This listing can be used as a subroutine and times upto 65535 x 1/32 seconds
i.e. 34 minutes can be obtained.

Problem: Change the listing to produce a 30 minute delay.
Hint. 1800sec in hex is 0708H.

One hour delay

Another and probably a simpler way of obtaining a delay of say 1 hour, is

• write a delay of 5 seconds,

• CALL it 6 times, this gives a delay of 30 seconds,

• put this in a loop to repeat 120 times, i.e. 120 x 30 seconds = 1 hour.

This code for the 1 hour subroutine will look like:-

ONEHOUR

MOVLW

.120

;put 120 in W

MOVWF

COUNT

;load COUNT with 120

LOOP

CALL

DELAY5

;Wait 5 seconds

CALL

DELAY5

;Wait 5 seconds

Program examples 137

CALL

DELAY5 ;

CALL

DELAY5 ;

CALL

DELAY5 ;

CALL

DELAY5 ;

DECFSZ

COUNT ;

GOTO

LOOP ;

RETLW

;

Wait 5 seconds
Wait 5 seconds
Wait 5 seconds
Wait 5 seconds
Subtract 1 from COUNT
Count is not zero.
;RETURN to program.

The program for the one-hour delay

ONEHOUR.ASM for 16F84.

This sets PORTA as an INPUT (NB 1

means input) and PORTB as an OUTPUT

(NB means output). The OPTION

of 1/32 of a second.

lhour and 5 second delays are

included in the subroutine section.

TMRO

EQU

STATUS

EQU

PORTA

EQU

PORTB

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION R

EQU

81H

ZEROBIT

EQU

COUNT

EQU

OCH

EQUATES SECTION

;means TMRO is file 1.

;means STATUS is file 3.

;means PORTA is file 5.

;means PORTB is file 6.

;TRISA (the PORTA I/O selection) is file 85H

;TRISB (the PORTB I/O selection) is file 86H

;the OPTION register is file 81H

;means ZEROBIT is bit 2.

;COUNT is file OC, a register to count events.

LIST P= 16F84 ;we are using the 16F84.

ORG ;the start address in memory is

GOTO START ;goto start!

;Configuration Bits

_CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

SUBROUTINE SECTION.

;1 hour delay.
ONEHOUR

MOVLW

.120

;put 120 in W

138 Program examples

load COUNT with 120
Wait 5 seconds
Wait 5 seconds
Wait 5 seconds
Wait 5 seconds
Wait 5 seconds
Wait 5 seconds
Subtract 1 from COUNT
Count is not zero.
RETURN to program.

START TMRO.
READ TMRO INTO W.
TIME - 160
Check TIME-W =
Time is not= 160.
Time is 160, return.

CONFIGURATION SECTION

MOVWF

COUNT

LOOP CALL

DELAY5

CALL

DELAY5

CALL

DELAY5

CALL

DELAY5

CALL

DELAY5

CALL

DELAY5

DECFSZ

COUNT

GOTO

LOOP

RETLW

;5 second delay.

DELAY5 CLRF

TMRO

LOOPB MOVF

TMR0,W

SUBLW

.160

BTFSS

STATUS,ZEROBIT

GOTO

LOOPB

RETLW

START BSF

STATUS,5 ;Turns to Bankl.

MOVLW

B'ooonnr

;5bits of PORTA are I/P

MOVWF

TRISA

MOVLW

B'00000000'

MOVWF

TRISB

;PORTB is OUTPUT

MOVLW

B'ooooonr

;Prescaler is /256

MOVWF

OPTION_R

;TIMER is 1/32 sees.

BCF

STATUS,5

;Return to BankO.

CLRF

PORTA

;Clears PortA.

CLRF

PORTB

iClears PortB.

;Program starts now.

BSF PORTB,0 ;Turn on B0

CALL ONEHOUR ;Wait 1 Hour.

BCF PORTB,0 ;Turn off B0.

STOP GOTO STOP :STOP!

END

9
The 16C54 microcontroller

The 16C54 is an example of a one time programmable (OTP) device.

The 16C54 device was brought out before the 16F84.

The main difference between them is that the 16C54 is not electrically erasable,
it has to be erased by UV light for about 15 minutes.

The 16C54 JW version is UV erasable.

The 16C54LP is a one time (only) programmable (OTP), 32 kHz version.

You would use a 16C54 JW for development and then program a OTP device
for your final circuit. The OTP device has to be selected for the correct
oscillator i.e. LP for 32kHz crystal, XT for 4MHz, HS for 20MHz and R-C for
an R-C network.

The header for use with the 16C54 is shown below.

Header for the 16C54

HEADER54.ASM for 16C54. This sets PORTA as an INPUT (NB 1

EQUATES SECTION

TMRO

EQU

STATUS

EQU

PORTA

EQU

PORTB

EQU

ZEROBIT

EQU

COUNT

EQU

;means TMRO is file 1.
;means STATUS is file 3.
;means PORTA is file 5.
;means PORTB is file 6.
;means ZEROBIT is bit 2.
;means COUNT is file 7,

140 The 16C54 microcontroller

TIME

EQU 8

;a register to count events
;file8 where the time is stored.

LIST

P=16C54

; we are using the 16C54.

ORG

01FFH

;the start address in memory is IFF at the
;end.

GOTO

START

; goto start!

ORG

SUBROUTINE SECTION.

; 1 second delay.
DELAY 1 CLRF

LOOPA

MOVLW

MOVWF

MOVF

SUBWF

BTFSS

GOTO

RETLW

TMRO

.32

TIME

TMR0,W

TIME,W

STATUS,ZEROBIT

LOOPA

;START TMRO.

;Time = 32/32 sees.
;Read TMRO into W.
;TIME - 32, result in W.
;Check TIME-W =
;Time is not = 32.
;Time is 32, return.

; 0.5 second delay.

DELAYP5
LOOPB

CLRF

MOVLW

MOVWF

MOVF

SUBWF

BTFSS

GOTO

TMRO

.16

TIME

TMR0,W

TIME,W

STATUS,ZEROBIT

LOOPB

RETLW

; START TMRO.

Time = 16/32 sees.
READ TMRO INTO W.
TIME - 16
Check TIME-W =
Time is not =16.
Time is 16, return.

CONFIGURATION SECTION

START

MOVLW

TRIS

MOVLW

TRIS

MOVLW

B'oooonir

PORTA

B'00000000'
PORTB

B'ooooonr

;4 bits of PORTA are I/P

;PORTB is OUTPUT

;Prescaler is /256

The 16C54 microcontroller 141

OPTION

CLRF PORTA

CLRF PORTB

TIMER is 1/32 sees.
Clears PortA.
Clears PortB.

;Program starts now.

This header can now be used to write programs for the 16C54 Microcontroller.

There are a number of differences between the 16F84 and the 16C54 that the
header has taken care of, but be aware of the differences when writing your
program.

• The 16C54 does not use Banks so there is no need to change from one to the
other.

• There are only 7 Registers on the 16C54 (see 16C54 Memory Map
Table 9.1). So the user files start at number 7. i.e. COUNT EQU 7, TIME
EQU 8.

• The 16C54 does not have the instruction SUBLW. So in the DELAY
subroutine the delay is moved into a file called TIME. (NB. TIME
EQUATES TO 8) Then the delay in the file is subtracted from W, giving the
same result as for the 16F84.

• Why bother using the 16C54? The reprogrammable 16C54 i.e. 16C54JW is
more expensive than the 16F84. But the one time programmable (OTP)
16C54 i.e. 16C54/04P is cheaper. So when your design is final you can blow
the program into the cheaper 16C54/04P. Why bother with the expensive
16C54JW and not the 16F84 for program development? I don't know! Only
convenience - not having to change the program.

• The 16C54JW has to be erased under an ultra violet lamp for about 15
minutes - this is a bind if you are impatient, you may need a couple.

• Pin 3 is only a TOCKI pin it does not double as A4 like the 16F84 and must
be pulled high if the TOCKI is not being used.

142 The 16C54 microcontroller

16C54 memory map

Table 9.1 16C54 memory map

FILE ADDRESS

FILENAME

INDIRECT ADDRESS

TMRO

STATUS

FSR

PORTA

PORTB

USER FILE

10
Alpha numeric displays

Using an Alpha Numeric Display in a project can bring it alive. Instead
of showing a number on a 7 segment display the Alpha Numeric Display could
indicate The Temperature is 27°C\ Instructions can also be given on screen.

This section details the use of a 16 character by 2 line display, which incor-
porates an HITACHI HD44780 Liquid Crystal Display Controller Driver
Chip. The HD44780 is an industry standard also used in displays other than
Hitachi (fortunately). The chip is also used as a driver for other display
configurations i.e. 16x1, 20x2, 20x4, 40x2 etc. It has an on board
character generator ROM which can display 240 character patterns.

The circuit diagram connecting the Alpha Numeric Display to the 16F84
is shown in Figure 10.1. This configuration is for the HD44780 driver and
can be used with any of the displays using this chip.

68p

32kHz 16

68p

16F84

A0
Al
A2
B0
Bl
B2
B3
B4
B5
B6
B7

V+
MCLR

\1_
_1_8_
]_
6_
7

n_
n_

_y_

R/W

DISPLAY

Vss Vo

0.1 |J

Figure 10.1 The 16F84 driving the alpha numeric display

144 Alpha numeric displays

Display pin identification

This display configuration shows 11 outputs from the Microcontroller,
3 control lines and 8 data lines connecting to the display.

R/W is the read/write control line, RS is the register select and E is the
chip enable.

The R/W line tells the display to expect data to be written to it or to have
data read from it. The data that is written to it is the address of the character,
the code for the character or the type of command we require it to perform
such as turn the cursor off.

The R/S line selects either a command to perform (R/S = 0) i.e. clear display,
turn cursor on or off, or selects a data transfer (R/S =1).

The E line enables, (E = 1) and disables, (E = 0) the display.

There is much more to this display than we are able to look at here. If you
wish to know more about them you will need to consult the manufacturers
data book.

If we use 1 1 lines to drive the display that would only leave 2 lines for the
rest of our control with the 16F84. We could of course use a micro with
22 or 33 I/O. The display can however be driven with 4 data lines instead
of 8, 4 bits of data are then sent twice. This complicates the program a little -
but since I have done that work in the header it requires no more effort
on your part.

Also the R/W line is used to write commands to the micro and read the
busy line which indicates when the relatively slow display has processed the
data. If we allow the micro enough time to complete its task then we do not
have to read the busy line we can just write to the display. The R/W line can
then be connected to Ov in a permanent write mode and we do not require
a read/write line from the micro.

We will therefore only require 4 data lines and 2 control lines to drive the
display leaving 7 lines available for I/O on the 16F84.

This 6 line control for the display is shown in Figure 10.2.

Alpha numeric displays 145

68p

32kHz 16

68p

16F84

BO
Bl
B2
B3

V+
MCLR

_13

DISPLAY

D4
D5
D6
D7

Vss Vo R/W

1 3

0.1(1

Figure 10.2 Driving the alpha numeric display with 6 control lines

Configuring the display

Before writing to the display you first of all have to configure it. That means
tell it if you are:

(a) using a 4 bit or 8 bit Microcontroller,

(b) using a 1 or 2 line display,

(d) turning the display on or off,

(e) turning the cursor on or off,

(f) incrementing the cursor or not. The cursor position increments after a
character has been written to the display.

In the program shown below the display has been set up in the Configuration
Section with Function Set at 32H to use a 4 bit Microcontroller with a 2 line
display and Font size of 5 x 7 dots. The Display is turned on and Cursor
turned off with OCH and the Cursor set to increment with 06H. This
information was obtained from the display data sheet.

146 Alpha numeric displays

Writing to the display

• To write to the display you first of all set the address of the cursor
(where you want the character to appear). The Cursor address locations
are shown in Figure 10.3 Linel address starts at 80H. Line2 address starts
at COH.

Figure 10.3 Cursor address location

• Then tell the display what the character code is, e.g. A has the code 41H,
B has the code 42H, C is 43H, is 30H, 1 is 31 H, 2 is 32H etc.

To print an A on the screen - first enable the display, send 2 to PORTA,
send the code 41 H to PORTB and CLOCK this data.

These instructions have been written in the Subroutine Section so all you
have to do is CALL A.

To write HELLO on the display the program would be:

CALL H
CALL E
CALL L
CALL L
CALL O

Program example

The program below is the listing to spell out MICROCONTROLLERS AT
THE MMU.

Then CONTACT DAVE SMITH. Together with the time delays.

;ANHEAD84.ASM Header for the alpha numeric display using 6 I/O

TMRO

EQU

1 ;

STATUS

EQU

3 ;

PORTA

EQU

5 ;

PORTB

EQU

6 ;

TRISA

EQU

85H ;

TRISB

EQU

86H ;

OPTION R

EQU

81H ;

;means TMRO is file 1.

;means STATUS is file 3.

;means PORTA is file 5.

;means PORTB is file 6.

;TRISA (the PORTA I/O selection) is file 85H

;TRISB (the PORTB I/O selection) is file 86H

;the OPTION register is file 81H

Alpha numeric displays 147

ZEROBIT EQU 2

COUNT EQU OCH

;means ZEROBIT is bit 2.

;COUNT is file OC, a register to count events.

LIST

P=16F84

ORG

GOTO

START

;we are using the 16F84.

;the start address in memory is

;goto start!

; Configuration Bits

_CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

; SUBROUTINE SECTION.

;3 SECOND DELAY

DELAY3

CLRF

TMRO

LOOPA

MOVF

TMR0,W

SUBLW

.96

BTFSS

STATUS,ZEROBIT

GOTO

LOOPA

RETLW

;P1 SECOND DELAY

DELAYP1

CLRF

TMRO

LOOPC

MOVF

TMR0,W

SUBLW

BTFSS

STATUS,ZEROBIT

GOTO

LOOPC

RETLW

CLOCK

BSF
NOP

PORTA,2

BCF

PORTA,2

NOP

RETLW

MOVLW

2 ;e

MOVWF

PORTA

MOVLW

Start TMRO

Read TMRO into W

TIME - W

Check TIME-W =

:return after TMRO = 96

Start TMRO

Read TMRO into W

TIME - W

Check TIME-W =

;return after TMRO = 3

;enables the display

148 Alpha numeric displays

MOVWF

PORTB

CALL

CLOCK

MOVLW

;41 is code for A

MOVWF

PORTB

CALL

CLOCK

;clock character onto display.

RETLW

MOVLW

;enables the display

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

;42 is code for B

MOVWF

PORTB

CALL

CLOCK

;clock character onto display.

RETLW

MOVLW

;enables the display

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

;clock character onto display.

RETLW

MOVLW

;enables the display

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

;clock character onto display.

RETLW

MOVLW

;enables the display

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

Alpha numeric displays 149

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

150 Alpha numeric displays

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

OAH

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

OBH

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

OCH

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

ODH

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

Alpha numeric displays 151

MOVWF

PORTB

CALL

CLOCK

MOVLW

OEH

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

OFH

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

;clock character onto display.

;enables the display

;clock character onto display.

152 Alpha numeric displays

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

;clock character onto display.

Alpha numeric displays 153

NUMO

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

OAH

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

;clock character onto display.

;enables the display

154 Alpha numeric displays

NUM1

NUM2

NUM3

NUM4

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

Alpha numeric displays 155

NUM5

NUM6

NUM7

NUM8

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

;enables the display

;clock character onto display.

156 Alpha numeric displays

NUM9 MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

GAP MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

DOT MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

OEH

MOVWF

PORTB

CALL

CLOCK

RETLW

CLRDISP CLRF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

CALL

DELAYP1

RETLW

;enables the display

;clock character onto display.

CONFIGURATION SECTION.

START

BSF

STATUS,5

;Turns to Bankl

MOVLW

B'00000000'

;PORTA is O/P

MOVWF

TRISA

Alpha numeric displays 157

MOVLW

B'OOOOOOOO'

MOVWF

TRISB

;PORTB is OUTPUT

MOVLW

B'ooooonr

;Prescaler is /256

MOVWF

OPTION_R

;TIMER is 1/32 sees.

BCF

STATUS,5

;Return to BankO.

CLRF

PORTA

;Clears PortA.

CLRF

PORTB

;Clears PortB.

;Display Configuration

MOVLW

03H

JUNCTION SET

MOVWF

PORTB

;8bit data (default)

CALL

CLOCK

CALL

DELAYP1

;wait for display

MOVLW

02H

JUNCTION SET

MOVWF

PORTB

;change to 4bit

CALL

CLOCK

;clock in data

CALL

DELAYP1

;wait for display

MOVLW

02H

;FUNCTION SET

MOVWF

PORTB

;must repeat command

CALL

CLOCK

;clock in data

CALL

DELAYP1

;wait for display

MOVLW

08H

;4 bit micro

MOVWF

PORTB

;using 2 line display.

CALL

CLOCK

;clock in data

CALL

DELAYP1

MOVLW

;Display on, cursor off

MOVWF

PORTB

;0CH

CALL

CLOCK

MOVLW

OCH

MOVWF

PORTB

CALL

CLOCK

CALL

DELAYP1

MOVLW

;Increment cursor, 06H

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

158 Alpha numeric displays

;Program starts now.

BEGIN

CALL

CLRDISP

CLRF

PORTA

MOVLW

Cursor at top lej

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

CALL

display M

CALL

DELAYP1

wait 0.1 seconds

CALL

display I

CALL

DELAYP1

wait 0.1 seconds

CALL

Etc.

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

Alpha numeric displays 159

CLRF

PORTA

MOVLW

OCH

;Cursor on second line, C3

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

CALL

DELAYP1

CALL

DELAYP1

CALL

GAP

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

GAP

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAY3

;wait 3 seconds

CALL

CLRDISP

MOVLW

;Cursor at top left, 80H

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

160 Alpha numeric displays

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CLRF

PORTA

MOVLW

OCH

;Cursor on 2nd line

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

GAP

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAYP1

CALL

DELAY3

;wait 3 seconds

GOTO

BEGIN

END

Program operation

• PORTA and PORTB are configured as outputs in the CONFIGURATION
SECTION.

Alpha numeric displays 161

Display configuration

• In the Display Configuration Section, the Register Select (R/S) line, A Ion
the microcontroller, is set low by CLRF PORTA in the Configuration
Section.

• R/S = ensures that the data to the display will change the registers. Later
R/S = 1 writes the characters to the display.

• The display is expecting its data to arrive via 8 lines, but to save I/O lines
we will use 4 and write them twice. The code to do this and also tell the
driver chip the display is a two line display is:

MOVLW

03H

;FUNCTION SET

MOVWF

PORTB

;8bit data (default)

CALL

CLOCK

CALL

DELAYP1

;wait for display

MOVLW

02H

;FUNCTION SET

MOVWF

PORTB

;change to 4bit

CALL

CLOCK

;clock in data

CALL

DELAYP1

;wait for display

MOVLW

02H

;FUNCTION SET

MOVWF

PORTB

;must repeat command

CALL

CLOCK

;clock in data

CALL

DELAYP1

;wait for display

MOVLW

08H

;4 bit micro

MOVWF

PORTB

;using 2 line display.

CALL

CLOCK

iclock in data

The data is set up on PORTB using BO, 1,2 and 3. As in

MOVLW
MOVWF

03H
PORTB

;FUNCTION SET

This data is then clocked into the display by pulsing the Enable line, (E, A2
on the micro) high and then low with:

CLOCK

BSF

PORTA,2

NOP

BCF

PORTA,2

NOP

RETLW

162 Alpha numeric displays

CALL DELAYP1 , waits for 0.1 seconds to give the display time to activate
before continuing.

When the display has been configured to: Turn on, switch the cursor off, and
increment the cursor after every character write. We are then ready to write to
the display.

Writing to the display

• The display is cleared if required with:

CALL CLRDISP

• The address of the character is first written to the display, say, the 80H
position (top left hand corner).

CLRF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

;Cursor at top left, 80H

Notice the 8 is sent first followed by the 0.

To write to the position mid-way along the top line the address would be
88H. So the 80H in the code above would be replaced by 88H.

• In order to write the letter 'M' in the display at the position defined.
We CALL M and use the code 4DH, NB. Send the 4 first followed by
the D. The Register Select Line, R/S, Al on the micro, is set to 1 for the
character write option. The code is:

MOVLW

;enables the display

MOVWF

PORTA

;sets A 1 = 1

MOVLW

;send data 4

MOVWF

PORTB

CALL

CLOCK

MOVLW

ODH

;send data D

MOVWF

PORTB

CALL

CLOCK

;clock character W M

RETLW

Alpha numeric displays 163

In this way any one of the 240 characters available can be shown on the
display.

The program continues by printing out the rest of the message. A delay of
0.1 seconds is maintained after printing each character to give the effect
of the message being typed out.

All the Capital Letters and numbers to 9 have been included in the header
so you can easily enter your own message.

The complete character set for the display showing all 240 characters is
illustrated in Figure 10.4.

Displaying a number

Suppose we wish to display a number thrown by a dice, for example a 4. We
could use the instruction CALL NUM4, but we would not have known
previously that the number was going to be a 4. The throw of the dice
would be stored in a user file called, say, THROW and THROW would then
have 4 in it.

Now the code for is 30H

The code for 1 is 31H

The code for 2 is 32H

Etc.

If we wanted to display the number 4 the code is:

NUM4

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

CALL

CLOCK

MOVLW

MOVWF

PORTB

CALL

CLOCK

RETLW

;enables the display
;34H is the code for 4

;clock character onto display.

If the 4 is in the file THROW, we can display this with the code:

MOVLW

MOVWF

PORTA

MOVLW

MOVWF

PORTB

;enables the display

164 Alpha numeric displays

CALL CLOCK

MOVF THROW,W ;number comes from the file

MOVWF PORTB

CALL CLOCK ;clock character onto display.

RETLW

Notice how the value of the number now has come from the file.

This code would then display any number in the file THROW.

If you measured a temperature as 27°C, you would probably store the 2 in
a file TEMPTENS (tens of degrees) and the 7 in a file TEMPUNIT (units
of degrees).

You can then modify the code above to display:

THE TEMPERATURE

IS 27°C.

The T would be located at address C5H on the display. The temperature
would then be written at locations C8H and C9H. There would be no need
to rewrite the message just rewrite the temperature as it changed, after first
moving the cursor to address C8H.

Alpha numeric displays 165

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Figure 10.4 Alpha numeric display character set

Analogue to digital
conversion

Up to now we have considered inputs as being digital in operation i.e. the input
is either a or 1. But suppose we wish to make temperature measurements,
but not just hot or cold (1 or 0). We may for example require to:

(a) Sound a buzzer if the temperature drops below freezing.

(b) Turn a heater on if the temperature is below 18°C.

(d) Turn on an alarm if the temperature goes above 30°C.

We could of course have separate digital inputs, coming from comparator
circuits for each setting. But a better solution is to use 1 input connected to an
analogue to digital converter and measure the temperature with that.

Figure 11.1 shows a basic circuit for measuring temperature. It consists of
a fixed resistor in series with a thermistor (a temperature sensitive resistor).

The resistance of the thermistor changes with temperature causing a change
in the voltage at point X in Figure 11.1.

Thermister

22k

Figure 11.1 Temperature measuring circuit

Analogue to digital conversion 167

As the temperature rises the voltage at X rises.

As the temperature decreases the voltage at X reduces.

We need to know the relationship between the temperature of the thermistor
and the voltage at X. A simple way of doing this would be to place the
thermistor in a cup of boiling water (100°C) and measure the voltage at X.
As the water cools corresponding readings of temperature and voltage can be
taken. If needed a graph of these temperature and voltage readings could
be plotted.

Making an A/D reading

In the initial example let us suppose:

• 0°C gave a voltage reading of 0.6v

• 18°C gave a reading of 1.4v

• 25°C gave a reading of 2.4v

• 30°C gave a reading of 3.6v

The microcontroller would read these voltages and convert them to an 8-bit
number where Ov is and 5v is 255. I.e. a reading of 51 per volt or a resolution
of l/51v, i.e. 1 bit is 19.6mv.

So 0°C = 0.6v = reading of 3 1 (0.6 x 5 1 = 30.6)
18°C=1.4v = 71 (1.4x51=71.4)

25°C = 2.4v= 122 (2.4 x 51 = 122.4)
30°C = 3.6v= 184 (3.6 x 51 = 183.6)

If we want to know when the temperature is above 30°C the microcon-
troller looks to see if the A/D reading is above 184. If it is, switch on the
alarm, if not keep the alarm off. In a similar way any other temperature
can be investigated - not just the ones listed. With our 8 bits we have
255 different temperatures we can choose from. The PIC 16C773 and PIC
16C774 have 12-bit A/D converters and can have 4096 different temperature
points.

Analogue to Digital conversion was introduced to the PIC Microcontrollers
with the family called 16C7X devices: 16C71, 16C73 and 16C74. Table 11.1
shows some of the specifications of these devices.

Table 11.1 16C7X Device specifications

Device

I/O

A/D Channels

Program
Memory

Data
Memory

Current
Source/Sink

16C71

25mA

16C73

192

25mA

16C74

192

25mA

168 Analogue to digital conversion

This family of devices has now been superceded by the 16F87X devices
shown in Table 11.2.

Table 11.2

16F87X Devices

Device

I/O

A/D Channels

Program
Memory

Data
Memory

Current
Source/Sink

16F870

128

25mA

16F871

128

25mA

16F872

128

25mA

16F873

192

25mA

16F874

192

25mA

16F876

368

25mA

16F877

368

25mA

The device I shall consider in this section is the 16F818. The Device Family
Specifications are shown in Table 11.3.

Table 11.3 16F818/9 Device specifications

Device

I/O

A/D Channels

Program
Memory

Data
Memory

Current
Source/Sink

16F818

128

25mA

16F819

256

25mA

The 16F818 device needs extra registers that the 16F84 does not have,
to handle the A/D processing.

The 16F818 has 5 Analogue Inputs ANO, AN1, AN2, AN3 and AN4.

Configuring the A/D device

In order to make an analogue measurement we have to configure the device.
HEAD818.ASM has to have the CONFIGURATION SECTION changed
to make some of the PORTA inputs Analogue inputs. PORTB has been set
as an output port.

To configure the 16F818 for A-D measurements three registers need to be
set up.

ADCONO
ADCON1
ADRES

Analogue to digital conversion 169

ADCONO

The first of the A/D registers, ADCONO is A to D Control Register 0.
ADCONO is used to:

• Switch the A/D converter on with ADON, bitO. This bit turns the A/D
on when set and off when clear. The A/D once it is turned on can be left
on all of the time but it does draw a current of 90uA, compared to the rest
of the microcontroller which draws a current of 15uA.

• Instruct the microcontroller to execute a conversion by setting the
GO/DONE bit, bit2. When the GO/DONE bit is set the micro does an
A/D conversion. When the conversion is complete the hardware clears the
GO/DONE bit. This bit can be read to determine when the result is ready.

• Set the particular channel (input) to make the measurement from. This is
done with two Channel Select bits, CHSO, CHS1 and CHS2, bits 3, 4 and 5.

The Register ADCONO is shown in Figure 11.2.

Bit7

BitO

CHS2

CHS1

CHSO

GO/DONE

ADON

1 =A/D ir

_1=A/Don.
0=A/D off.

i progress.

0=A/D finished.
Analogue channel

select.

000=channel 0,AN0
001=channel 1,AN1
010=channel2,AN2
011=channel3, AN3
100=channel4,AN4

Figure 11.2 ADCONO Register

ADCON1

In ADCON1, A to D Conversion Register 1, only bits 0, 1, 2 and 3 are used.

They are the Port Configuration bits, PCFGO, PCFG1, PCFG2, and PCFG3
that determine which of the pins on PORTA will be analogue inputs and
which will be digital.

170 Analogue to digital conversion

The ADCON1 register is illustrated in Figure 11.3 and the corresponding
Analogue and Digital inputs are shown in Table 11.4.

Bit7

BitO

PCFG3

PCFG2

PCFG1

PCFGO

A/D Port
configuration bits.

Figure 11.3 ADCON1 Register

Table 11.4 ADCON1 Port configuration

PCFG

AN4

AN3

AN2

AN1

AN0

Vref+

Vref-

0000

Vdd

Vss

0001

Vref+

AN3

Vss

0010

Vdd

Vss

0011

Vref+

AN3

Vss

0100

Vdd

Vss

0101

Vref+

AN3

Vss

011X

Vdd

Vss

1000

Vref+

Vref-

AN3

AN2

1001

Vdd

Vss

1010

Vref+

AN3

Vss

1011

Vref+

Vref-

AN3

AN2

1100

Vref+

Vref-

AN3

AN2

1101

Vref+

Vref-

AN3

AN2

1110

Vdd

Vss

1111

Vref+

Vref-

AN3

AN2

As mentioned previously the microcontroller will convert an analogue
voltage between and 5v to a digital number between and 255. But suppose
our analogue readings of say, temperature, go from 0.6v representing a
temperature of 0°C to 3.6v representing a temperature of 30°C. It would
make sense to have our analogue range go from 0.6v to 3.6v. We can set
this by using two reference voltages. One at the low setting of 0.6v called
Vref—, connected to AN2. The other setting of 3.6v for Vref+, connected
to AN3. The two right hand columns in Table 1 1.4 show that PCFG Set at 1000
will set the A/D configuration using AN3 and AN2 as the reference voltages.
In this book I have not used any reference voltages but have used 5v, Vdd and
Ov. Vss as the references.

Analogue to digital conversion 111

ADRES

• The third register is ADRES, the A to D RESult register. This is the file
where the result of the A/D conversion is stored. If several measurements
require storing then the number in ADRES needs to be transferred to a
user file before it is overwritten with the next measurement. The 16F818
micro is a 10 bit A/D. The top 8 bits are stored in ADRESH and
the lower 2 bits in ADRESL. In this book I am only using 8 bits and have
called the file ADRES.

Analogue header for the 16F818

;HEAD818A.ASM for 16F818.

This sets PORTA as analogue/digital

INPUTs.

PORTB is an OUTPUT.

Internal oscillator of 31.25kHz chosen

The OPTION register is set to /256 giving

timing pulses 32.768ms.

1 second and 0.5 second delays are

included in the subroutine section.

EQUATES SECTION

TMRO

EQU

1 ;

STATUS

EQU

3 ;

PORTA

EQU

5 ;

PORTB

EQU

6 ;

ZEROBIT

EQU

2 ;

ADCON0

EQU

1FH ;

ADCON1

EQU

9FH ;

ADRES

EQU

1EH ;

CARRY

EQU

;

TRISA

EQU

85H ;

TRISB

EQU

86H ;

OPTION_R

EQU

81H ;

OSCCON

EQU

8FH ;

COUNT

EQU

20H ;

;means TMRO is file 1.

;means STATUS is file 3.

;means PORTA is file 5.

;means PORTB is file 6.

;means ZEROBIT is bit 2.

;A/D Configuration reg.O

;A/D Configuration reg.l

;A/D Result register.

;CARRY IS BIT 0.

;PORTA Configuration Register

;PORTB Configuration Register

;Option Register

;Oscillator control register.

;COUNT a register to count events.

LIST

P=16F818

;we are using the 16F818.

ORG

;the start address in memory is

GOTO

START

;goto start!

172 Analogue to digital conversion

Configuration Bits

CONFIG H'3F10'

sets INTRC-A6 is port I/O, WDT off, PUT

on, MCLR tied to VDD A5 is I/O

BOD off, LVP disabled, EE protect disabled,

Flash Program Write disabled,

Background Debugger Mode disabled, CCP

function on B2,

Code Protection disabled.

SUBROUTINE SECTION.

;0.1 second delay, actually 0.099968s

DELAYP1

CLRF

TMRO ;

LOOPB

MOVF

TMRO/W ;

SUBLW

.3 ;

BTFSS

STATUS,ZEROBIT ;

GOTO

LOOPB ;

NOP

RETLW

;

;START TMRO.
;READ TMRO INTO W.
;TIME-3
;CheckTIME-W =

;Time is not = 3.
;add extra delay

;Time is 3, return.

;0.5 second delay.

DELAYP5 MOVLW .5

MOVWF COUNT

LOOPC

CALL
DECFSZ
GOTO
RETLW

DELAYP1

COUNT

LOOPC

;1 second delay.

DELAY 1 MOVLW .10

MOVWF COUNT

LOOPA

CALL
DECFSZ
GOTO
RETLW

DELAYP1

COUNT

LOOPA

Analogue to digital conversion 173

CONFIGURATION SECTION.

START

BSF

STATUS,5

MOVLW

Biiiinir

MOVWF

TRISA

MOVLW

B'00000100'

MOVWF

ADCON1

MOVLW

B'00000000'

MOVWF

TRISB

MOVLW

B'OOOOOOOO'

MOVWF

OSCCON

MOVLW

B'ooooonr

MOVWF

OPTION_R

BCF

STATUS,5

BSF

ADCON0,0

CLRF

PORTA

CLRF

PORTB

;Turns to Bankl.

:8 bits of PORTA are I/P

;PORTB is OUTPUT

;oscillator 31.25kHz

;Prescaler is /256
;TIMER is 1/32 sees.

;Return to BankO.

;Clears PortA.
;Clears PortB.

;Program starts now.

END

Head818A.ASM explained

HEAD818A.ASM is similar in operation to HEAD818.ASM outlined in
Chapter 6, with the following extras:

The Carry Bit in the status register, that indicates if a calculation is +ve
or — ve, it is bit and has been equated to 0.

174 Analogue to digital conversion

In the Configuration Section AO, Al and A3 are set as Analogue inputs,
A2, A4, A5, A6 and A7 are set up as digital inputs with:

MOVLW B'OOOOOIOO'
MOVWF ADCON1

The A/D converter is switched on with:

BSF

ADCON0,0

A/D Conversion - example, a temperature sensitive
switch

To introduce the working of the A/D converter we will consider a simple
example, i.e. Turn an LED on when the Temperature is above 25°C and turn
the LED off when it is below 25°C.

The diagram for this Temperature Switch Circuit is shown in Figure 11.4.

Thermister

6 680R

22k

T 0.1*1

LED Vv

Figure 11.4 Temperature switch circuit

Analogue to digital conversion 175

Taking the A/D reading

The A/D converter has been switched on in the header and it automatically
looks at Channel unless told otherwise. In order to make the measurement
the GO/DONE bit, bit2 is set and we wait until it is cleared with:

BSF ADCON0,2 ;Take measurement, set GO/DONE

WAIT BTFSC ADCON0,2 ;Wait until GO/DONE is clear

GOTO WAIT

The measurement will then be in the A/D Result register, ADRES.

Determining if the temperature is above or below 25° C

Suppose the voltage on the analogue input, Channel 0, A0 is 2.4v when the
temperature is 25°C. The required A/D reading for 2.4v is 2.4x51 = 122.
We therefore need to know when the A/D reading is above and below 122,
i.e. above and below 25°C.

Previously we have seen how to tell if a value is equal to another by subtracting
and looking at the zerobit in the status register (Chapter 5).

There is another bit, bit in the status register called the Carry Bit, which
indicates if the result of a subtraction is +ve or — ve. If the Carry Bit is set
the result was +ve, if the bit is clear the result was — ve. So we can tell if the
number is above or below a defined value.

The code for this is:

;Move Analogue result into W

;Do 122 - ADRES, i.e. 122-W

;Check the carry bit. Clear if AD RES > 122 i.e. -ve

; Routine to turn off LED

;Routine to turn on LED

The analogue measurement is moved from ADRES into W where we can
subtract it from 122. NB. The subtraction always does, Value — W.

The carry bit tells us if the A/D result is above or below 122.

N.B. If the result of the subtraction is zero the carry is also 1. It must be 1 or 0.
Being +ve or zero does not matter in this example.

We have then found out if the result is equal to or above 122, or if it is less
than 122.

MOVF

ADRES,W ;

SUBLW

.122 ;

BTFSC

Status, Carry ;

GOTO

TURNOFF ;

GOTO

TURNON ;

176 Analogue to digital conversion

When the measurement is made we then goto one of two subroutines,
TURNON or TURNOFF. These subroutines are not very grand but they
could easily be more complicated, even hundreds of lines long.

Program code

The full code for this Temperature Sensitive Switch Program is shown below
as TEMPSENS.ASM

TEMPSENS.ASM.

This sets PORTA as analogue/digital INPUTs.

PORTB is an OUTPUT.

Internal oscillator of 31.25kHz chosen

The OPTION register is set to /256 giving timing

pulses 32.768ms.

1 second and 0.5 second delays are included in the

subroutine section.

;EQUATES SECTION

TMRO

EQU

STATUS

EQU

PORTA

EQU

PORTB

EQU

ZEROBIT

EQU

ADCON0

EQU

1FH

ADCON1

EQU

9FH

ADRES

EQU

1EH

CARRY

EQU

TRISA

EQU

85H

TRISB

EQU

86H

OPTION_R

EQU

81H

OSCCON

EQU

8FH

COUNT

EQU

20H

means TMRO is file 1.

means STATUS is file 3.

means PORTA is file 5.

means PORTB is file 6.

means ZEROBIT is bit 2.

A/D Configuration reg.O

A/D Configuration reg.l

A/D Result register.

CARRY IS BIT 0.

PORTA Configuration Register

PORTB Configuration Register

Option Register

Oscillator control register.

COUNT a register to count events

LIST

P=16F818

;we are using the 16F818.

ORG

;the start address in memory is

GOTO

START

;goto start!

Analogue to digital conversion 111

; Configuration Bits

CONFIG H'3F10'

;sets INTRC-A6 is port I/O, WDT off, PUT

;on, MCLR tied to VDD A5 is I/O

;BOD off, LVP disabled, EE protect disabled,

;Flash Program Write disabled,

;Background Debugger Mode disabled, CCP

;function on B2,

;Code Protection disabled.

SUBROUTINE SECTION.

;Turn on LED on BO
;Return to monitor

;Turn off LED on BO
;Return to monitor

CONFIGURATION SECTION.
START

TURNON

BSF

PORTB,0

GOTO

BEGIN

TURNOFF

BCF

PORTB,0

GOTO

BEGIN

BSF

STATUS,5

;Turns to Bankl.

MOVLW

B'llllllll'

;8 bits of PORTA are I/P

MOVWF

TRISA

MOVLW

B'00000100'

;A0,A1 and A3 are analoj

MOVWF

ADCON1

MOVLW

B'00000000'

MOVWF

TRISB

;PORTB is OUTPUT

MOVLW

B'00000000'

MOVWF

OSCCON

;oscillator 31.25kHz

MOVLW

B'ooooonr

;Prescaler is /256

MOVWF

OPTION_R

;TIMER is 1/32 sees.

BCF

STATUS,5

;Return to BankO.

BSF

ADCON0,0

;Turn ON A/D

CLRF

PORTA

;Clears PortA.

CLRF

PORTB

;Clears PortB.

178 Analogue to digital conversion

;Program starts now.

ADCON0,2 ;Take measurement, set GO/DONE

BEGIN BSF

WAIT

BTFSC ADCON0,2 ;Wait until GO/DONE is clear

GOTO WAIT

MOVF ADRES,W ;Move Analogue result into W

SUBLW .122 ;Do 122-ADRES, i.e. 122-W

BTFSC STATUS,

CARRY ; Clear if ADRES > 1 22

GOTO TURNOFF ;Routine to turn off LED

GOTO TURNON iRoutine to turn on LED

END

Another example - a voltage indicator

Previously we have looked at a single input level. But with our 8 bit micro we
could look at 255 different input levels.

Suppose we wish to use the LEDs connected to PORTB to indicate the voltage
on the Analogue Input ANO. So that as the voltage increases then the number
of LEDs lit also increases.

In HEAD818.ASM we have configured the micro so that the voltage reference
is Vdd i.e. the 5v supply. This was done with the instructions:

MOVLW
MOVWF

B'OOOOOIOO'
ADCON1

This means that 5v will give a digital reading of 255 in our 8 bit register
ADRES. The resolution of this register is 5v/255= 19.6mV.

Suppose our LED ladder was to increment in 0.5v steps as indicated below:

Vin = 0-0.5v
Vin = 0.5-1. Ov
Vin=1.0-1.5v
Vin=1.5-2.0v

All LEDs off,
BO on,
Bl on,
B2 on,

0.5v = 0.5/5x255 = 25.5 = 26
1.0v= 1/5x255 = 51

1.5v= 1.5/5x255 = 76.5 = 77
2.0v = 2/5x255=102

Analogue to digital conversion 179

Vin = 2.0-2.5v

B3 on,

Vin = 2.5-3.0v

B4 on,

Vin = 3.0-3.5v

B5 on,

Vin = 3.5-4.0v

B6 on,

Vin = 4.0-5.0v

B7 on.

2.5v = 2.5/5 x 255 = 127.5 = 128
3.0v = 3/5x255=153
3.5v = 3.5/5 x 255 = 178.5 = 179
4.0v = 4/5x255 = 204

The circuit diagram for this voltage indicator is shown in Figure 11.5 and the
Flowchart is shown in Figure 11.6.

Figure 11.5 Circuit for the voltage indicator

180 Analogue to digital conversion

Figure 11.6 Flowchart for the voltage indicator

Analogue to digital conversion 181

Turn on LED4.

Turn on LED5.

Turn on LED6.

Turn on LED7.

Figure 11.6 Continued

182 Analogue to digital conversion

Voltage indicator, program solution

HEAD818A.ASM is altered to produce the program VOLTIND.ASM for the
Voltage Indicator Circuit.

VOLTIND.ASM

This sets PORTA as analogue/digital

INPUTS. PORTB is an OUTPUT.

Internal oscillator of 31.25kHz chosen

The OPTION register is set to /256 giving timing

pulses 32.768ms.

1 second and 0.5 second delays are included in the

subroutine section.

; EQUATES SECTION

TMRO

EQU

means TMRO is file 1.

STATUS

EQU

means STATUS is file 3.

PORTA

EQU

means PORTA is file 5.

PORTB

EQU

means PORTB is file 6.

ZEROBIT

EQU

means ZEROBIT is bit 2.

ADCON0

EQU

1FH

A/D Configuration reg.O

ADCON1

EQU

9FH

A/D Configuration reg.l

ADRES

EQU

1EH

A/D Result register.

CARRY

EQU

CARRY IS BIT 0.

TRISA

EQU

85H

PORTA Configuration Register

TRISB

EQU

86H

PORTB Configuration Register

OPTION_R

EQU

81H

Option Register

OSCCON

EQU

8FH

Oscillator control register.

COUNT

EQU

20H

COUNT a register to count events

********

*********

t****************************

LIST

= 16F818

;we are using the 16F818.

ORG

;the start address in memory is

GOTO

START

;goto start!

; Configuration Bits

CONFIG H'3F10'

;sets INTRC-A6 is port I/O, WDT off, PUT on,
;MCLR tied to VDD A5 is I/O
;BOD off, LVP disabled, EE protect disabled,
;Flash Program Write disabled,

Analogue to digital conversion 183

;Background Debugger Mode disabled, CCP

;function on B2,

;Code Protection disabled.

CONFIGURATION SECTION.

START

BSF

MOVLW

MOVWF

MOVLW

MOVWF

MOVLW

MOVWF

BCF

MOVLW

MOVWF

CLRF

STATUS,5

B'OOOllllT

TRISA

B'OOOOOOIO'

ADCON1

B'OOOOOOOO'

TRISB

STATUS,5

B'OOOOOOOr

ADCONO

PORTA

PORTB

;Turns to Bankl.

;5bits of PORTA are I/P

;A0, Al are analogue
;A2, A3 are digital I/P.

;PORTB is OUTPUT

;Return to BankO.
;Turns on A/D converter,
;and selects channel ANO
;Clears PortA.
;Clears PortB.

;Program starts now.

BEGIN BSF

ADCON0,2 ;

WAIT BTFSC

ADCON0,2 ;

GOTO

WAIT

MOVF

ADRES,W ;

CLRF

PORTB ;

SUBLW

.26 ;

BTFSC

STATUS,CARRY ;

GOTO

BEGIN ;

MOVF

ADRES,W ;

BSF

PORTB,0 ;

SUBLW

.51 ;

BTFSC

STATUS,CARRY ;

GOTO

BEGIN ;

MOVF

ADRES,W ;

BSF

PORTB, 1 ;

SUBLW

.77 ;

BTFSC

STATUS,CARRY ;

GOTO

BEGIN ;

Take Measurement.
Wait until reading done.

Move A/D Result into W
Clear PortB.

26-,W. W is altered
Is W> or <26
Wis <26 (0.5v)

Move A/D Result into W
Turn on BO.
51-,W. W is altered
Is W> or <51

Wis <51 (l.Ov)

Move A/D Result into W
Turn on Bl.
77-,W. W is altered
Is W> or <77
Wis <77 (1.5v)

184 Analogue to digital conversion

MOVF

ADRES,W

;Move A/D Result into W

BSF

PORTB,2

;Turn on B2.

SUBLW

.102

;102-,W. W is altered

BTFSC

STATUS,CARRY

;Is W> or < 102

GOTO

BEGIN

;Wis <102 (2.0v)

MOVF

ADRES,W

;Move A/D Result into W

BSF

PORTB,3

;Turn on B3.

SUBLW

.128

;128-,W. W is altered

BTFSC

STATUS,CARRY

;Is W> or <128

GOTO

BEGIN

;Wis <128 (2.5v)

MOVF

ADRES,W

;Move A/D Result into W

BSF

PORTB,4

;Turn on B4.

SUBLW

.153

;153-,W. W is altered

BTFSC

STATUS,CARRY

;Is W> or <153

GOTO

BEGIN

;Wis <153 (3.0v)

MOVF

ADRES,W

;Move A/D Result into W

BSF

PORTB,5

;Turn on B5.

SUBLW

.179

;179-,W. Wis altered

BTFSC

STATUS,CARRY

;Is W> or <179

GOTO

BEGIN

;Wis <179 (3.5v)

MOVF

ADRES,W

;Move A/D Result into W

BSF

PORTB,6

;Turn on B6.

SUBLW

.204

;204-,W. W is altered

BTFSC

STATUS,CARRY

;Is W> or <204

GOTO

BEGIN

;Wis <204(4.0v)

BSF

PORTB,7

;Turn on B7.

GOTO

BEGIN

END

Operation of the voltage indicator program

The code to make the analogue measurement is the same as in the Temperature
Switch Circuit. Once the measurement has been taken the program checks
to see if the digital value of the input is >26 if it is BO LED is switched on.
The program then checks to see if the measurement is > 51, if so then Bl LED
is lit. If the reading is > 77 then B2 LED is lit etc. When the value is less
than the one being checked then the program branches back to the beginning,
makes another measurement and the cycle repeats.

Analogue to digital conversion 185

NB. After the A/D reading the LEDs are cleared before being turned on,
in case the voltage has dropped.

To check if a reading (or any number) is > say 26.

Put the number into W.

Take W from 26 i.e. 26-W by SUBLW .26

If the result is +ve, the number is < 26 and the carry bit is set in the Status
Register. If the number is > 26 the result is — ve and the carry bit is clear.

Problem

To check your understanding of the previous section, try this.

Turn a red LED on only when the input voltage is above 3v and turn a
yellow LED on only when the input voltage is below lv and turn a green LED
on only when the voltage is between lv and 3 v.

Hint

Check for voltage > 3v if true GOTO RED

If not check for voltage < lv if true GOTO YELLOW

If false then GOTO GREEN.

Radio transmitters and

receivers

Radio circuits used to frighten me but now with the introduction of low cost
modules the radio novice like myself can transmit data easily.

This section details the use of the 418 MHz Radio Transmitter and Receiver
Modules (RT1-418 and RR3-418). They do not need a license to operate
and there are many varieties available. The transmitters only have 3
connections, 2 power supply and one data input, the transmitting aerial
is incorporated on the unit. The receiver has 4 connections, 2 power supply,
1 aerial input and 1 data output. The receiving aerial only needs to be
a piece of wire about 25cm long.

The basic circuit diagram of the radio system is shown in Figure 12.1.

The microcontroller generates the data and then passes the data pulses to
the transmitter. The receiver receives the data pulses and a microcontroller
decodes the information and processes it.

A microcontroller-radio system could measure the temperature outside and
transmit this temperature to be displayed on a unit inside.

10k

A0 r,

16F84

470R

Figure 12.1 Radio data transmission system

Radio transmitters and receivers 187

How does it work?

The transmitter

Data is generated by the microcontroller say by pressing a switch or from
a temperature sensor via the 16F818 doing an A/D conversion. Suppose this
data is 27H, this would then be stored in a user file, called, say, NUMA.

So file NUMA would appear as shown in Figure 12.2.

NUMA,7 NUMA,6 NUMA,5 NUMA,4 NUMA,3 NUMA,2 NUMA,1

NUMA,0

10 11

Figure 12.2 File NUMA containing 27H

The data then needs to be passed from the micro to the data input of the
transmitter. The transmitter output will then be turned on and off by the
data pulses. The length of time the transmitter is on will indicate if the data
was a 1, a or the transmission start pulse.

I have decided to use a start bit that is 7.5ms wide, a 5ms pulse to represent
a logic 1 and a 2.5ms pulse to represent a logic 0. All pulses are separated by
a space of 2.5ms. The pulse train for NUMA is then as shown in Figure 12.3.

Start

Figure 12.3 NUMA pulse train

In order to generate this train the software turns the output on for the 7.5ms
start pulse, off for 2.5ms, on for 5ms for the first 1, off for 2.5ms, on for
5ms for the next logic 1, off for 2.5ms, on for 5ms for the next logic 1, off
for 2.5ms, on for 2.5ms for the logic 0, etc.

To generate the data each bit in the file NUMA is tested in turn. If the bit is
then the output is turned on for 2.5ms, if the bit is 1 then the output
is turned on for 5ms. The code for this data would be:

BSF

PORTB,0

;Transmit start pulse

CALL

DELAY3

;7.5ms Start pulse

BCF

PORTB,0

;Transmit space

CALL

DELAY 1

:Delay 2.5ms

188 Radio transmitters and receivers

TESTAO

BTFSC

NUMA,0

;Test NUMA,0

GOTO

SETAO

;If NUMAO = 1

GOTO

CLRAO

;If NUMAO =

SETAO

BSF

PORTB,0

;Transmit 1

CALL

DELAY2

;Delay 5ms

GOTO

TESTA 1

CLRAO

BSF

PORTB,0

;Transmit

CALL

DELAY 1

;Delay 2.5ms

GOTO

TESTA 1

TEASTA1

BCF

PORTB,0

;Transmit space

CALL

DELAY 1

BTFSC

NUMA,1

;Test NUMA,1

GOTO

SETA1

;If NUMAO = 1

GOTO

CLRA1

;If NUMAO =

SETA1

BSF

PORTB,0

CALL

DELAY2

GOTO

TESTA2

CLRA1

BSF

PORTB,0

CALL

DELAY 1

GOTO

TESTA2

This bit testing is repeated until all 8 bits are transmitted.

The receiver

The receiver works the opposite way round. The data is received and stored
in a file NUMA. Several data bytes could be transmitted depending on
how many switches are used. Or the data may be continually varying from
a temperature sensor. In this example we are only looking for one byte
i.e. the number 27H which was transmitted. The data is passed from the
receiver to the input AO of the microcontroller.

We wait to receive the 7.5ms start bit. When this is detected we then measure
the next 8 pulses.

If a pulse is 5ms wide then a one has been transmitted and we SET the
relative bit in the file NUMA. If the pulse is only 2.5ms long then we leave
the bit CLEAR.

Radio transmitters and receivers 189

Measuring the received pulse width

Measuring the width of a pulse is a little more difficult than setting a pulse
width. Consider the pulse in Figure 12.4.

Start

Finish

CLRF

TMRO

Figure 12.4 Measuring the width of a pulse

READ

TMRO

The input is continually tested until it goes high and then the timer, TMRO,
is cleared to start timing. The input is continually tested until it goes low and
then the value of TMRO is read. This is done by:

MOVF

TMR0,W which puts the value of TMRO into W.

We can then check to see if the pulse is 5ms long i.e. a logic 1, if not then
a shorter pulse means a logic was transmitted. If the pulse is greater than
3.5ms then it must be a logic 1, at 5ms. If the pulse is less than 3.5ms then
it must be a logicO. TMRO will hold a value of 3 after a time of 3.5ms, so we
check to see if the width of the pulse is greater or less than 3.

The code for this is:

TESTAOH

BTFSS

PORTA,0

;wait for Hi transmission

GOTO

TESTAOH

CLRF

TMRO

; start timing

TESTAOL

BTFSC

PORTA,0

;wait for Lo transmission

GOTO

TESTAOL

MOVF

TMR0,W

;read value of TMRO

SUBLW

;3-W or 3-TMRO

BTFSC

STATUS,

CARRY

;Is TMRO > 3 i.e. a logicl

BSF

NUMA,0

;Yes.

190 Radio transmitters and receivers

This measuring of the pulse width continues until all 8 pulses are read and
the relevant bits stored in the file NUMA. A TMRO value >6 indicates the
pulse was a Start pulse.

We then check to see if the number stored in the file NUMA is 27H. This
is done as we have done before by subtracting 27H from it, if the answer is
zero, i.e. 27—27 = 0, then the number transmitted was 27H and we turn on the
LED. It seems such a waste to go to all this trouble to turn an LED on. I hope
you can be a little more imaginative — this is only an example.

The complete codes for the transmitter and receiver are shown below as
TX.ASM and RX.ASM.

The OPTION register has been set to produce timing pulses of 1ms.

Transmitter program code

TX.ASM

;tx.asm transmits code from a switch.

TMRO

EQU

STATUS

EQU

PORTA

EQU

PORTB

EQU

TRISA

EQU

85H

TRISB EQU

OPTION_R EQU

ZEROBIT EQU

COUNT EQU

NUMA EQU

86H

81H

2
OCH

ODH

;means TMRO is file 1.

;means STATUS is file 3.

;means PORTA is file 5.

;means PORTB is file 6.

;TRISA (the PORTA I/O selection)

;is file 85H

;TRISB (the PORTB I/O selection)

;is file 86H

;the OPTION register is file 81H

;means ZEROBIT is bit 2.

;COUNT is file 0C, a register to

xount events.

LIST P= 16F84 ; we are using the 16F84.

ORG ;the start address in memory is

GOTO START ; goto start!

Configuration Bits

__CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

; Code Protection disabled.

Radio transmitters and receivers 191

SUBROUTINE SECTION.

;2.5ms SECOND DELAY

DELAY 1

CLRF

TMRO

LOOPA

MOVF

TMR0,W

SUBLW

BTFSS

STATUS,ZEROBIT

GOTO

LOOPA
RETLW

;Start TMRO

;Read TMRO into W

;TIME-W

;Check TIME-W =

:Return after TMRO = 32

;5ms SECOND DELAY

DELAY2

CLRF

TMRO

LOOPB

MOVF

TMR0,W
SUBLW .3

BTFSS

STATUS,ZEROBIT

GOTO

LOOPB

RETLW

;Start TMRO

;Read TMRO into W

;TIME-W

;Check TIME-W =

:Return after TMRO = 2

;7.5ms SECOND DELAY

DELAY3

CLRF

TMRO

LOOPC

MOVF

TMR0,W
SUBLW .6

BTFSS

STATUS,ZEROBIT

GOTO

LOOPC

RETLW

;Start TMRO

;Read TMRO into W

;TIME-W

;CHECK TIME-W =

:Return after TMRO = 3

CONFIGURATION SECTION

START

BSF

STATUS,5

MOVLW

B'ooonnr

MOVWF

TRISA

MOVLW

B'00000000'

MOVWF

TRISB

MOVLW

B'00000010'

MOVWF

OPTION_R

BCF

STATUS,5

CLRF

PORTA

CLRF

PORTB

;Turns to Bankl.

;5bits of PORTA are I/P

PORTB is OUTPUT

Prescaler is /256
PRESCALER is /8,1ms

;Clears PortA.
;Clears PortB.

192 Radio transmitters and receivers

;Program starts now.

BEGIN

BTFSC

PORTA,0

;wait for switch press

GOTO

BEGIN

MOVLW

27H

;Put 27H into W

MOVWF

NUMA

;PUT 27H into NUMA

BCF

PORTB,0

CALL

DELAY 1

BSF

PORTB,0

;Transmit START

CALL

DELAY3

;wait 7.5ms

TESTAO

BCF

PORTB,0

;Transmit space

CALL

DELAY 1

;wait 2.5ms

BTFSC

NUMA,0

;Test NUMA,0

GOTO

SETAO

;If NUMAO = 1

GOTO

CLRAO

;If NUMAO =

SETAO

BSF

PORTB,0

;Transmit 1

CALL

DELAY2

;wait 5ms

GOTO

TESTA 1

CLRAO

BSF

PORTB,0

;Transmit

CALL

DELAY 1

;wait 2.5ms

TESTA 1

BCF

PORTB,0

CALL

DELAY 1

BTFSC

NUMA,1

GOTO

SETA1

GOTO

CLRA1

SETA1

BSF

PORTB,0

CALL

DELAY2

GOTO

TESTA2

CLRA1

BSF

PORTB,0

CALL

DELAY 1

TESTA2

BCF

PORTB,0

CALL

DELAY 1

BTFSC

NUMA,2

GOTO

SETA2

GOTO

CLRA2

Radio transmitters and receivers 193

SETA2

BSF

PORTB,0

CALL

DELAY2

GOTO

TESTA3

CLRA2

BSF

PORTB,0

CALL

DELAY 1

TESTA3

BCF

PORTB,0

CALL

DELAY 1

BTFSC

NUMA,3

GOTO

SETA3

GOTO

CLRA3

SETA3

BSF

PORTB,0

CALL

DELAY2

GOTO

TESTA4

CLRA3

BSF

PORTB,0

CALL

DELAY 1

TESTA4

BCF

PORTB,0

CALL

DELAY 1

BTFSC

NUMA,4

GOTO

SETA4

GOTO

CLRA4

SETA4

BSF

PORTB,0

CALL

DELAY2

GOTO

TESTA5

CLRA4

BSF

PORTB,0

CALL

DELAY 1

TESTA5

BCF

PORTB,0

CALL

DELAY 1

BTFSC

NUMA,5

GOTO

SETA5

GOTO

CLRA5

SETA5

BSF

PORTB,0

CALL

DELAY2

GOTO

TESTA6

194 Radio transmitters and receivers

CLRA5

BSF

PORTB,0

CALL

DELAY 1

TESTA6

BCF

PORTB,0

CALL

DELAY 1

BTFSC

NUMA,6

GOTO

SETA6

GOTO

CLRA6

SETA6

BSF

PORTB,0

CALL

DELAY2

GOTO

TESTA7

CLRA6

BSF

PORTB,0

CALL

DELAY 1

TESTA7

BCF

PORTB,0

CALL

DELAY 1

BTFSC

NUMA,7

GOTO

SETA7

GOTO

CLRA7

SETA7

BSF

PORTB,0

CALL

DELAY2

CLRF

PORTB

GOTO

BEGIN

CLRA7

BSF

PORTB,0

CALL

DELAY 1

CLRF

PORTB

GOTO

BEGIN

END

Receiver program code:

;RX.ASM

TMRO EQU 1 ;means TMRO is file 1.

STATUS EQU 3 ;means STATUS is file 3.

PORTA EQU 5 ;means PORTA is file 5.

PORTB EQU 6 ;means PORTB is file 6.

TRISA EQU 85H ;TRISA (the PORTA I/O selection) is file 85H

TRISB EQU 86H ;TRISB (the PORTB I/O selection) is file 86H

OPTION_R EQU 81H ;the OPTION register is file 8 1H

Radio transmitters and receivers 195

ZEROBIT EQU 2 ;means ZEROBIT is bit 2.

CARRY EQU

COUNT EQU OCH ;COUNT is file OC, a register to count events.

NUMA EQU ODH

LIST

P=16F84

;we are using the 16F84.

ORG

;the start address in memory is

GOTO

START

;goto start!

;Configuration Bits

_CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

CONFIGURATION SECTION.

START BSF

STATUS,5 ;Turns to Bankl.

MOVLW

B'oooinir

MOVWF

TRISA

MOVLW

B'OOOOOOOO'

MOVWF

TRISB

MOVLW

B'OOOOOOIO'

MOVWF

OPTION_R

BCF

STATUS,5

CLRF

PORTA

CLRF

PORTB

BCF

STATUS,5

CLRF

PORTA

CLRF

PORTB

;5bits of PORTA are I/P

;PORTB is OUTPUT

;Return to BankO.
;Clears PortA.
;Clears PortB.
; Return to BANKO
;Clears PORTA
;Clears PORTB

;Program starts now.

BEGIN

CLRF

NUMA

WAITHI BTFSS PORTA,0

;Wait for HI Transmission

196 Radio transmitters and receivers

GOTO
CLRF

TESTST BTFSC
GOTO
MOVF
SUBLW
BTFSC
GOTO

TESTAOH BTFSS
GOTO
CLRF

TESTAOL BTFSC
GOTO
NOP
MOVF
SUBLW
BTFSS
BSF

TESTA1H BTFSS
GOTO
CLRF

TESTA1L BTFSC
GOTO
NOP
MOVF
SUBLW
BTFSS
BSF

TESTA2H BTFSS
GOTO
CLRF

TESTA2L BTFSC
GOTO
NOP
MOVF
SUBLW
BTFSS
BSF

TESTA3H BTFSS
GOTO
CLRF

WAITHI

TMRO

PORTA,0

TESTST

TMRO/W

STATUS,CARRY

WAITHI

PORTA,0

TESTAOH

TMRO

PORTA,0

TESTAOL

TMRO/W

.3
STATUS,CARRY

NUMA,0

PORTA,0
TESTA 1H
TMRO
PORTA,0
TESTA 1L

TMRO/W

.3
STATUS,CARRY

NUMA,1

PORTA,0

TESTA2H

TMRO

PORTA,0

TESTA2L

TMRO/W

.3
STATUS,CARRY

NUMA,2

PORTA,0

TESTA3H

TMRO

;Wait for LOW Transmission
;Test for START PULSE

;5-W or 5-TMRO
;SKIPIFTIME>5
;NOT START BIT

;wait for Hi transmission

;start timing

;wait for Lo transmission

;read value of TMRO
;3-W or 3-TMRO
;Is TMRO > 3 i.e. a logicl
;Yes, 1 was transmitted.

;Wait for pulse

;Wait for LO.

;1 was transmitted
;Wait for pulse

;Wait for Lo.

;1 was transmitted
;Wait for pulse

Radio transmitters and receivers 197

TESTA3L BTFSC
GOTO
NOP
MOVF
SUBLW
BTFSS
BSF

TESTA4H BTFSS
GOTO
CLRF

TESTA4L BTFSC
GOTO
NOP
MOVF
SUBLW
BTFSS
BSF

TESTA5H BTFSS
GOTO
CLRF

TESTA5L BTFSC
GOTO
NOP
MOVF
SUBLW
BTFSS
BSF

TESTA6H BTFSS
GOTO
CLRF

TESTA6L BTFSC
GOTO
NOP
MOVF
SUBLW
BTFSS
BSF

TESTA7H BTFSS
GOTO
CLRF

PORTA,0
TESTA3L

TMR0,W

.3
STATUS,CARRY

NUMA,3

PORTA,0

TESTA4H

TMRO

PORTA,0

TESTA4L

TMR0,W

.3
STATUS,CARRY

NUMA,4

PORTA,0

TESTA5H

TMRO

PORTA,0

TESTA5L

TMR0,W

.3
STATUS,CARRY

NUMA,5

PORTA,0

TESTA6H

TMRO

PORTA,0

TESTA6L

TMR0,W

.3
STATUS,CARRY

NUMA,6

PORTA,0

TESTA7H

TMRO

;Wait for Lo

;1 was transmitted

;Wait for pulse

;Wait for Lo

;1 was transmitted

;Wait for pulse

;Wait for Lo

;1 was transmitted
;Wait for pulse

;Wait for Lo

;1 was transmitted

;Wait for pulse

198 Radio transmitters and receivers

TESTA7L BTFSC
GOTO
NOP
MOVF
SUBLW
BTFSS
BSF

MOVLW

SUBWF

BTFSS

GOTO

BSF

GOTO

PORTA,0
TESTA7L

TMRO/W

.3
STATUS,CARRY

NUMA,7

;Wait for Lo

;1 was transmitted

27H

NUMA/W ;NUMA-27

STATUS,ZEROBIT

BEGIN ;If NUMA is not 27

PORTB,0 ;Turn on LED.

BEGIN

END

Using the transmit and receive subroutines

The transmit and receive subroutines may seem a little complex, but all you
need to do in your code is call them.

• To transmit

Put the data you wish to transmit in the file NUMA then CALL
TRANSMIT. The data in the file NUMA is transmitted.

• To receive

CALL RECEIVE, the received data will be present in the file NUMA
for you to use.

These programs have illustrated how to switch an LED on (this could be
a remote control for a car burglar alarm). You may of course want to add
more lines of code to be able to turn the LED off. This could be done in
the receiver section by waiting for say 2 seconds and on the next transmission
turn the LED off, providing of course the code was again 27H. Other codes
could of course be added for other switches or keypad buttons, the possibilities
are endless.

The transmitter and receiver micros could be hard wired together first to
test the software without the radio link. The radio transmitter and receiver
can then replace the wire to give a wireless transmission.

13
EEPROM data memory

One of the special features of the 16F84, the 16F818 and some other micros
is the EEPROM Data Memory. This is a section of Memory not in the
usual program memory space. It is a block of data like the user files, but
unlike the user files the data in the EEPROM Data Memory is saved when
the microcontroller is switched off, i.e. it is non-volatile. Suppose we were
counting cars in and out of a car park and we lost the power to our circuit.
If we stored the count in EEPROM then we could load our count file with
this data and continue without loss of data, when the power returns.

To access the data, i.e. read and write to the EEPROM memory loca-
tions, we must of course instruct the microcontroller. There are 64 bytes of
EEPROM memory on the 16F84, 128 on the 16F818 and 256 on the 16F819.
So we must tell the micro which address we require and if we are reading
or writing to it.

When reading we identify the address from to 3Fh (for the 16F84) using the
address register EEADR. The data is then available in register EEDATA.
When writing to the EEPROM data memory we specify the data in the register
EEDATA and the location in the register EEADR.

Two other files are used to enable the process, they are EECON1 and
EECON2, two EEPROM control registers.

The Register EECON1 is shown below in Figure 13.1.

Bit 0, RD is set to a 1 to perform a read. It is cleared by the micro when

the read is finished.
Bit 1, WR is set to a 1 to perform a write. It is cleared by the micro when

the write is finished.
Bit 2, WREN, WRite ENable a 1 allows the write cycle, a prohibits it.

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bitO

EEPGD

EEIF

WRERR

WREN

Figure 13.1 The EECON1 register

200 EEPROM data memory

Bit 3, WRERR reads a 1 if a write is not completed, reads a if the write
is completed successfully.

Bit 4, EEIF interrupt flag for the EEDATA it is a 1 if the write operation
is completed, it reads if it is not completed or not started (for the
16F84). This bit has another purpose for the 16F818. We have not
used this bit in this book.

Bit 7, EEPGD, Program/Data EEPROM Select Bit. (Not used on 16F84.)
This bit allows either the program memory or the data memory to
be selected. selects Data, 1 selects program memory.

Example using the EEPROM

As usual, I think the best way of understanding how this memory works
is to look at a simple example.

Suppose we wish to count events, people going into a building, cars going
into a carpark etc. So if we loose the power to the circuit the data is still
retained. The circuit for this is shown in Figure 13.2.

Switch 1 is used to simulate the counting process and the 8 LEDs on
PORTB display the count in binary. (This is a good chance to practice
counting in binary.) The switch of course needs de-bouncing.

Remember the idea of this circuit, we are counting events and displaying
the count on PORTB. But if we loose power - when the power is re-applied
we want to continue the count as if nothing had happened.

So when we switch on we must move the previous EEPROM Data into the
COUNT file.

The flowchart is shown below in Figure 13.3.

Just a couple of points before we look at the program:

1. It is a good idea to make sure the EEPROM DATA MEMORY is reset
at the very beginning. This can be done by writing OOh to EEPROM
DATA address OOh when we blow the program into the chip - this is done
with the following lines of code.

ORG 21 OOH

DE OOH

2100H is the address of the first EEPROM data memory file i.e. OOh.

EEPROM data memory 201

8 x 680R

Figure 13.2 Switch press counting circuit

DE is Define EEPROM data memory, so we are initializing it with OOh,
and of course 2101H is EEPROM address 1 etc.

Data can also be written into the EEPROM using MPLAB, with VIEW,
EEPROM and writing the data in the EEPROM box as shown in
Figure 13.4.
2. Reading and Writing to EEPROM data is not as straightforward as with
user files, you probably suspected that! There is a block of code you need
to use -just add it to your program as required.

When reading EEPROM data at address to the file COUNT then
CALL READ. The subroutine written in the header.

When writing the file COUNT to EEPROM data address 0, CALL
WRITE.

202 EEPROM data memory

Move EEPROM DATA to COUNT

Move Count to EEPROM Data.

Figure 13.3 The switch press count flowchart

1 Project

Debugger

Programmer Toots Configure Window

Help

Project
Output

# a? B * m

Toolbars

►

1 Disassembly Listing

2 Hardware Stack

3 Program Memory

4 File Registers

I 5 EEPROM

7 Watch

3 Special Function Registers

■

EEPROM

Address

00 01

OS 06 07

OJL

OF |

0000

| FF

FF FF FF FF FF

ooio

FF FF

FF FF FF FF FF

0020

FF FF

FF FF FF FF FF

0030

FF FF

FF FF FF FF FF

0040

FF FF

FF FF FF FF FF

OOSO

FF FF

FF FF FF FF FF

0060

FF FF

FF FF FF FF FF

0070

FF FF

FF FF FF FF FF

Figure 13.4 Writing EEPROM data

EEPROM data memory 203

EEPROM program code

The complete program EEDATAWR.ASM is shown below:

EEDATAWR.ASM

This program will count and display switch

presses.

The count is saved when the power is removed

and continues when the

power is re-applied.

TMRO

EQU

1 ;

PORTA

EQU

5 ;

PORTB

EQU

6 ;

TRISA

EQU

85H ;

TRISB

EQU

86H ;

OPTION_R

EQU

81H ;

STATUS

EQU

3 ;

ZEROBIT

EQU

2 ;

COUNT

EQU

OCH ;

EEADR

EQU

9 ;

EEDATA

EQU

8 ;

EECON1

EQU

8 ;

EECON2

EQU

9 ;

EQU

;

EQU

1 ;

WREN

EQU

2 ;

;TMR0is FILE 1.

;PORTA is FILE 5.

;PORTB is FILE 6.

;TRISA (the PORTA I/O selection)

;TRISB (the PORTB I/O selection)

;the OPTION register is file 81H

;STATUS is FILE 3.

;ZEROBIT is Bit 2.

;USER RAM LOCATION.

;EEPROM address register

;EEPROM data register

;EEPROM control register 1

;EEPROM control register2

;read bit in EECON1

; Write bit in EECON1

; Write enable bit in EECON1

LIST

P=16F84

ORG

2100H

00H

ORG

GOTO

START

;We are using the 16F84.
;ADDRESS EEADR
;put 00H in EEADR

;0 is the start address.
;goto start!

;Configuration Bits

_CONFIG H'3FF0' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

204 EEPROM data memory

SUBROUTINE SECTION.

;0.1 SECOND DELAY

DELAYP1

CLRF

TMRO

,Start TMRO

LOOPA

MOVF

TMRO/W

,Read TMRO into W

SUBLW

,TIME - W

BTFSS

STATUS,ZEROBIT

,CHECK TIME-W =

GOTO

LOOPA

RETLW

, Return after TMRO =

;Put EEDATA into COUNT
READ MOVLW

;read EEDATA from EEADR
into W

MOVWF

EEADR

BSF

STATUS,5

;BANK1

BSF

EECONl,RD

BCF

STATUS,5

;BANK0

MOVF

EEDATA,W

MOVWF

COUNT

RETLW

;WRITE COUNT INTO EEDATA

WRITE

BSF

STATUS,5

;BANK1

BSF

EECONl,WREN

;set WRITE ENABLE

BCF

STATUS,5

;BANK0

MOVF

COUNT,W

;move COUNT to EEDATA

MOVWF

EEDATA

MOVLW

;set EEADR to receive

EEDATA

MOVWF

EEADR

BSF

STATUS,5

;BANK1

MOVLW

55H

;55 and AA initiates write cycle

MOVWF

EECON2

MOVLW

OAAH

MOVWF

EECON2

BSF

EECON1/WR

;WRITE data to EEADR

WRDONE

BTFSC

EECONl,WR

GOTO

WRDONE

;wait for write cycle to complete

EEPROM data memory 205

BCF EEC0N1,WREN

BCF STATUS,5 ;BANK0

RETLW

CONFIGURATION SECTION.

START

BSF

STATUS,5

MOVLW

B'OOOllllT

MOVWF

TRISA

MOVLW

MOVWF

TRISB

MOVLW

B'ooooonr

MOVWF

OPTION_R

BCF

STATUS,5

CLRF

PORTA

CLRF

PORTB

CLRF

COUNT

;Turn to BANK1

;5 bits of PORTA are I/Ps.

;PORTB IS OUTPUT

;PRESCALER is /256
;Return to BANKO
;Clears PORTA
;Clears PORTB

;Program starts now.

PRESS

RELEASE

CALL READ ;read EEPROM data into COUNT

MOVF COUNT,W

MOVWF PORTB ;Display previous COUNT (if any)

BTFSC PORTA,0 ;wait for switch press

GOTO PRESS

CALL DELAYP1 ;antibounce

BTFSS PORTA,0 ;wait for switch release

GOTO RELEASE

CALL DELAYP1 ;antibounce

INCF COUNT ;add 1 to COUNT

MOVF COUNT,W ;put COUNT into W

MOVWF PORTB ;move W (COUNT) to PORTB to

display
CALL WRITE ;write COUNT to EEPROM

address
GOTO PRESS ;return and wait for press

END

206 EEPROM data memory

Microchip are continually expanding their range of microcontrollers and
a new series of flash micros have been introduced, namely the 16F87X series
which include 8k of program memory, 368 bytes of user RAM, 256 bytes of
EEPROM data memory and an 8 channel 10 bit A/D converter. So now
analogue measurements can be stored and saved in EEPROM Data!

Interrupts

New instructions used in this chapter:

• RETFIE

We all know what interrupts are and we don't like being interrupted.
We are busy doing something and the phone rings or someone arrives at
the door.

If we are expecting someone, we could look out of the window every now
and again to see if they had arrived or we could carry on with what we are
doing until the doorbell rings. These are two ways of receiving an interrupt.
The first when we keep checking in software terms is called polling, the second
when the bell rings is equivalent to the hardware interrupt.

We have looked at polling when we used the keypad to see if any keys
had been pressed. We will now look at the interrupt generated by the hardware.

Before moving onto an example of an interrupt consider the action of the
door in a washing machine. The washing cycle does not start until the door
is closed, but after that the door does not take any part in the program.
But what if a child opens the door, water could spill out or worse!! We
need to switch off the outputs if the door is opened. To keep looking at
the door at frequent intervals in the program (software polling) would be
very tedious indeed, so we use a hardware interrupt. We carry on with the
program and ignore the door. But if the door is opened the interrupt
switches off the outputs - spin motor etc. If the door had been opened
accidentally then closing the door would return back to the program for
the cycle to continue.

This suggests that when an interrupt occurs we need to remember what the
contents of the files were. i.e. the STATUS register, W register, TMRO and
PORT settings so that when we return from the interrupt the settings are
restored. If we did not remember the settings, we could not continue where we
left off, because the interrupt switches off all the outputs and the W register
would also be altered, at the very least.

208 Interrupts

Interrupt sources

The 16F84 has 4 interrupt sources.

• Change of rising or falling edge of PORTB,0.

• TMRO overflowing from FFh to OOh.

• PORTB bits 4-7 changing.

• DATA EEPROM write complete.

The 16F818/9 has 9 interrupt sources, and of course need extra bits in
the interrupt registers to handle them. The additional interrups used in the
16F818/9 are

• A/D conversion complete

• Synchronous Serial Port Interrupt

• TMR1 overflowing

• TMR2 overflowing

• Capture Compare Pulse Width Modulator Interrupt.

These interrupts can be enabled or disabled as required by their own
interrupt enable/disable bits. These bits can be found in the interrupt control
register INTCON for the 16F84 and also on the Peripheral Interrupt Enable
Register 1, PIE1 on the 16F818/9.

In this section we will be looking at the interrupt caused by a rising or falling
edge on PORTB,0.

Interrupt control register

The Interrupt Control Register INTCON, file OBh is shown in Figure 14.1.

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bitO

GIE

EEIE

TOIE

INTE

RBIE

TOIF

INTF

RBIF

Figure 14.1 The interrupt control register, INTCON of 16F84

Bit 6 in this register is designated as the Peripheral Interrupt Enable Bit,

PEIE for the 16F818/9.

Before any of the individual enable bits can be switched ON, the Global

Interrupt Enable (GIE) bit 7 must be set, i.e. a 1 enables all unmasked

interrupts and a disables all interrupts.

Bit 6 EEIE (16F84) is an EEPROM data write complete interrupt enable

bit, a 1 enables this interrupt and a disables it.
Bit 6 PEIE (16F818/9) is the bit that permits enabling of the extra,

peripheral bits.

Interrupts 209

Bit 5 TOIE is the TMRO overflow interrupt enable bit, a 1 enables this

interrupt and a disables it.
Bit 4 INTE is the RBO/INT Interrupt Enable bit, a 1 enables this interrupt

and a disables it.
Bit 3 RBIE is the RB Port change (B4-B7) Interrupt enable bit, a 1 enables

it and a disables it.
Bit 2 TOIF is the flag, which indicates TMRO has overflowed to generate

the interrupt. 1 indicates TMRO has overflowed, indicates it hasn't.

This bit must be cleared in software.
Bit 1 INTF is the RBO/INT Interrupt flag bit which indicates a change on

PORTB,0. A 1 indicates a change has occurred, a indicates it hasn't.
Bit RBIF is the RB PORT Change Interrupt flag bit. A 1 indicates

that one of the inputs PORTB,4-7 has changed state. This bit must be

cleared in software. A indicates that none of the PORTB,4-7 bits

have changed.

Program using an interrupt

As an example of how an interrupt works consider the following example:

Suppose we have 4 lights flashing consecutively for 5 seconds each. A switch
connected to BO acts as an interrupt so that when BO is at a logic an interrupt
routine is called. This interrupt routine flashes all 4 lights ON and OFF
twice at 1 second intervals and then returns back to the program providing
the switch on BO is at a logic 1.

I have used the 16F818 for this application.

The circuit diagram for this application is shown in Figure 14.2.

One thing to note from the circuit the 16F818 chip has internal pull-up
resistors on PORTB so BO does not need a pull up resistor on the switch.

The interrupt we are using is a change on BO, we are therefore concerned
with the following bits in the INTCON register, i.e. INTE bit4 the enable
bit and INTF bitl the flag showing BO has changed, and of course GIE
bit7 the Global Interrupt Enable Bit.

Program operation

When BO generates an interrupt the program branches to the interrupt service
routine. Where? Program memory location 4 tells the Microcontroller where
to go to find the interrupt service routine.

210 Interrupts

Figure 14.2 Interrupt demonstration circuit

Program memory location 4 is then programmed using the org statement as:

ORG
GOTO

4
ISR

;write next instruction in program memory location 4
;jump to the Interrupt Service Routine.

The interrupt service routine

The Interrupt Service Routine, ISR, is written like a subroutine and is
shown below:

;Interrupt Service Routine

MOVWF

SWAPF

MOVWF

MOVF

MOVWF

MOVF

MOVWF

MOVLW
MOVWF
CALL

W_TEMP

STATUS,W

STATUS_T

TMR0,W

TMR0_T

PORTB,W

PORTB_T

OFFH

PORTB

DELAPY1

;Save W
;Save STATUS
;Save TMRO
;Save PORTB

;turn on all outputs.
;1 second delay

Interrupts 211

SW HI

MOVLW

MOVWF

CALL

MOVLW

MOVWF

CALL

MOVLW

MOVWF

CALL

BTFSS

GOTO

PORTB

DELAPY1

OFFH

PORTB

DELAPY1

PORTB

DELAPY1

PORTB,0

SW HI

;turn off all outputs
;1 second delay

;turn on all outputs.
;1 second delay

;turn off all outputs
;1 second delay

;wait for switch to be HI.

SWAPF

MOVWF

MOVF

MOVWF

MOVF

MOVWF

MOVF

BCF
RETFIE

STATUS_T,W

STATUS

TMR0_T,W

TMRO

PORTB_T,W

PORTB

W TEMP,W

;Restore STATUS

;Restore TMRO

;Restore PORTB
:Restore W

INTCON, INTF ;Reset Interrupt Flag

;Return from the interrupt

Operation of the interrupt service routine

The interrupt service routine operates in the following way.

• When an interrupt is made the Global Interrupt Enable is cleared
automatically (disabled) to switch off all further interrupts. We would
not wish to be interrupted while we are being interrupted.

• The registers W, STATUS, TMRO and PORTB are saved in temporary
locations W_TEMP, STATUS_T, TMR0_T and PORTB_T.

• The interrupt routine is executed, the lights flash on and off twice. This
is a separate sequence than before to show the interrupt has interrupted
the normal flow of the program. NB. The program has not been looking
at the switch that generated the interrupt.

• We then wait until the switch returns HI.

• The temporary files W_TEMP, STATUS_T, TMR0_T and PORTB_T are
restored back into W, STATUS, TMRO and PORTB.

• The PORTB, interrupt flag INTCON,INTF is cleared ready to indicate
further interrupts.

• We return from the interrupt, and the Global Interrupt Enable bit is
automatically set to enable further interrupts.

212 Interrupts

Program of the interrupt demonstration

The complete code for this program is shown below as INTFLASH.ASM.

INTFLASH.ASM Flashing lights being interrupted by a switch on BO.
Using 16F818
EQUATES SECTION

TMRO

STATUS

PORTA

PORTB

TRISA

TRISB

INTCON

ZEROBIT

CARRY

GIE

INTE

INTF

OPTION_R

ADCONO

ADCON1

ADRES

OSCCON

COUNT

TMR0_T

W_TEMP

STATUS_T

PORTB_T

COUNTA

EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU

EQU
EQU
EQU
EQU

85H

86H

OBH

81H

1FH

9FH

1EH

8FH

20H

21H
22H
23H

24H

means TMRO is file 1.

means STATUS is file 3.

means PORTA is file 5.

means PORTB is file 6.

TRISA (the PORTA I/O selection) is file 85H

TRISB (the PORTB I/O selection) is file 86H

Interrupt Control Register

means ZEROBIT is bit 2.

CARRY IS BIT 0.

Global Interrupt bit

BO interrupt enable bit.

BO interrupt flag

A/D Configuration reg.O
A/D Configuration reg.l
A/D Result register.
Oscillator control register.
COUNT a register to count events,
a register to count events
TMRO temporary file
W temporary file
STATUS temporary file
;PORTB temporary file

EQU 25H

LIST

P=16F818

ORG

GOTO

ORG

GOTO

;we are using the 16F818.
;the start address in memory is

START ;goto start!

4 ;write to memory location 4

ISR ;location4 jumps to ISR

;Configuration Bits
CONFIG H'3F10'

;sets INTRC-A6 is port I/O, WDT off, PUT

Interrupts 213

on, MCLR tied to VDD A5 is I/O

BOD off, LVP disabled, EE protect disabled,

Flash Program Write disabled,

Background Debugger Mode disabled, CCP

function on B2,

Code Protection disabled.

SUBROUTINE SECTION

;0.1 second delay, actually 0.099968s

DELAYP1 CLRF

TMRO

LOOPB

MOVF

TMR0,W

SUBLW

BTFSS

STATUS,ZEROBIT

GOTO

LOOPB

NOP

RETLW

;5 second

delay.

DELAY5

MOVLW

.50

MOVWF

COUNTA

LOOPC

CALL

DELAYP1

DECFSZ

COUNTA

GOTO

LOOPC

RETLW

;1 second

delay.

DELAY 1

MOVLW

.10

MOVWF

COUNT

LOOPA

CALL

DELAYP1

DECFSZ

COUNT

GOTO

LOOPA

RETLW

START TMRO.

READ TMRO INTO W.

TIME-3

Check TIME-W =

Time is not = 3.
add extra delay

;Time is 3, return.

;Interrupt Service Routine.

ISR

MOVWF

SWAPF

MOVWF

MOVF

MOVWF

MOVF

MOVWF

W_TEMP

STATUS,W

STATUSJT

TMR0,W

TMR0_T

PORTB,W

PORTB T

;Save W
;Save STATUS
;Save TMRO
;Save PORTB

214 Interrupts

MOVLW

OFFH

MOVWF

PORTB

turn on all outputs.

CALL

DELAY 1

1 second delay

MOVLW

MOVWF

PORTB

turn off all outputs

CALL

DELAY 1

1 second delay

MOVLW

OFFH

MOVWF

PORTB

turn on all outputs.

CALL

DELAY 1

1 second delay

MOVLW

MOVWF

PORTB

turn off all outputs

CALL

DELAY 1

1 second delay

SW_HI BTFSS

PORTB,0

GOTO

SW_HI

wait for switch to be HI.

SWAPF

STATUS_T,W

MOVWF

STATUS

Restore STATUS

MOVF

TMR0_T,W

MOVWF

TMRO

Restore TMRO

MOVF

PORTB_T,W

MOVWF

PORTB

Restore PORTB

MOVF

W_TEMP,W

Restore W

BCF

INTCONJNTF

Reset Interrupt Flag

RETFIE

Return from the interrupt

CONFIGURATION SECTION

START

BSF

STATUS,5

MOVLW

B'liiinir

MOVWF

TRISA

MOVLW

B'00000110'

MOVWF

ADCON1

MOVLW

B'ooooooor

MOVWF

TRISB

MOVLW

B'00000000'

MOVWF

OSCCON

MOVLW

B'ooooonr

MOVWF

OPTION R

;Turns to Bankl.

;8 bits of PORTA are I/P

:PORTA IS DIGITAL

;PORTB,0 is I/P

;oscillator 31.25kHz

;Prescaler is /256
:TIMER is 1/32 sees.

Interrupts 215

BCF

STATUS,5

Return to BankO.

CLRF

PORTA

Clears PortA.

CLRF

PORTB

Clears PortB.

BSF

INTCON,GIE

Enable Global Interrupt

BSF

INTCONJNTE

Enable BO interrupt

;Program starts now.

BEGIN

MOVLW

MOVWF

CALL

MOVLW

MOVWF

CALL

MOVLW

MOVWF

CALL

MOVLW

MOVWF

CALL

GOTO

B'00000010'

PORTB

DELAY5

B'00000100'

PORTB

DELAY5

B'00001000'

PORTB

DELAY5

B'OOOIOOOO'

PORTB

DELAY5

BEGIN

;Turn on Bl

;wait 5 seconds
;Turn on B2

;wait 5 seconds
;Turn on B3

;wait 5 seconds
;Turn on B4

;wait 5 seconds

END

The 4 lights are flashing on and off slowly enough (5 second intervals)
so that you can interrupt part way through taking BO low via the switch,
(make sure BO is hi when starting). The interrupt service routine then flashes
all the lights on and off twice at 1 second intervals.

When returning from the interrupt with BO hi again, the program resumes
from where it left off, i.e. if the 2nd LED had been on for 3 seconds it
would come back on for the remaining 2 seconds and the sequence would
continue.

The 12 series 8 pin
microcontroller

Arizona Microchip have a range of microcontrollers with 8 pins. They include
types with Data EEPROM and A/D converters. In this section we will cover
the 12C508 and 12C509, which are one time programmable devices and the
flash 12F629 and 12F675 (electronically) reprogrammable devices.

The device memory specifications are shown in Table 15.1.

Table 15.1 12C508/509, 12F629 and 12F675 memory specifications

Device

EEPROM

User Files

Registers

12C508

512 x 12

12C509

1024 x 12

12F629

1024 x 14

12F675

1024 x 14

Pin diagram of the 12C508/509

Vdd

GP5/OSC1/CLKIN

GP4/OSC2

GP4/MCLR/VPP

c
c
c

32 32

o o

ro ro

O O

ui oi

o o

<o CO

. Vss

.> GPO

-► GP1

-► GP2/T0CKI

Figure 15.1 Pin diagram of the 12C508/9

Pin diagram of the 12F629 and 12F675

Vdd
GP5/T1Ckl/OSC1/CLKIN

GP4/AN3/T1 G/OSC2/CLOUT

■n

GP3/MCLR/VPP

Figure 15.2 Pin diagram of the 12F629 and 12F675

Vss

GP0/AN0/CIN+/ICSPDAT
GP1/AN1/CIN-/Vref/ICSPCLK
GP2/AN2/T0CKI/INT/COUT

The 12 series 8 pin microcontroller 217

Features of these 12 series

One of the special features of this Micro is that it has 8 pins, but 6 of them can
be used as I/O pins, the remaining 2 pins being used for the power supply.
There is no need to add a crystal and capacitors, because a 4MHz oscillator
is built on board! If you wish to use a clock other than the 4MHz provided,
then you can connect an oscillator circuit to pins 2 and 3 (as in the 16F84).
That leaves you with of course only 4 I/O.

Being an 8 pin device means of course it is smaller than an 18 pin device
and cheaper. The on board oscillator means that the crystal and timing
capacitors are not required, reducing the component count, size and cost even
further. So if your application requires no more than 6 I/O these are devices
to use. They have useful applications in burglar alarm circuits and the radio
transmitter circuits we have looked at previously.

The memory maps of the 12C508 and 12F629/675

The memory map of the 12C508 is shown in Figure 15.3, showing the 7 registers
and 25 user files. Figure 15.4 shows the 12F629/675 map.

The 12C509 has 16 extra user files mapped in Bankl.

There is no longer a PORTA or PORTB because we only have 6 I/O, they
are in a port called GPIO (General Purpose Input Output), File 6.

Address

File

01h

TMRO

02h

PCL

03h

STATUS

04h

FSR

05h

OSCCAL

06h

GPIO

07h

General

Purpose

Registers

(User files)

1Fh

Figure 15.3 12C508 Memory map

218 The 12 series 8 pin microcontroller

Address

OOH

INDADRESS

01H

TMRO

02H

PCL

03H

STATUS

04H

FSR

05H

GPIO

06H

07H

08H

09H

OAH

PCLATH

OBH

INTCON

OCH

PIR1

ODH

OEH

TMR1L

OFH

TMR1H

10H

T1CON

11H

12H

13H

14H

15H

16H

17H

18H

19H

CMCON

1AH

1BH

1CH

1DH

1EH

ADRESH

1FH

ADRESL

20H
5FH

General
Purpose
Register

64 bytes

BANKO

BANK1

Figure 15.4 12F629/675 Memory map

Address

80H

INDADRR

81 H

OPTION REG

82H

PCL

83H

STATUS

84H

FSR

85H

TRISIO

86H

87H

88H

89H

8AH

PCLATH

8BH

INTCON

8CH

PIE1

8DH

8EH

PCON

8FH

90H

OSCCAL

91H

92H

93H

94H

95H

WPU

96H

IOCB

97H

98H

99H

VRCON

9AH

EEDATA

9BH

EEADR

9CH

EECON1

9DH

EECON2

9EH

ADRESL

9FH

ANSEL

Oscillator calibration

Apart from the small size of this device an appealing feature is that the
oscillator is on board. The file OSCCAL is an oscillator calibration file used
to trim the 4MHz oscillator.

The 4MHz oscillator takes its timing from an on board R-C network, which is
not very precise. So these chips have a value that can be put into OSCCAL

The 12 series 8 pin microcontroller 219

to trim it. This value is stored in the last memory address i.e. OlFFh for the
12C508 and 03FFh for the 12C509 and 12F629/675.

• Trimming the 12C508/9

The code, which is loaded by the manufacturer in the last memory location
for the 12C508/9, is MOVLW XX where XX is the trimming value. The last
memory location is the reset vector i.e. when switched on the micro goes to this
location first, it loads the calibration value into W and the program counter
overflows to OOOh and continues executing the code. To use the calibration
value, in the Configuration Section write the instruction MOVWF OSCCAL,
which then moves the manufacturers calibration value into the timing circuit.

There is one point to remember - if you are using a windowed device then the
calibration value will be erased when the memory is erased. So make a note
of the MOVLW XX code by looking in MPLAB with: VIEW-PROGRAM
MEMORY and program it back in by ORG 01FFH MOVLW XX.

• Trimming the 12F629/675

A calibration instruction is programmed into the last location of program
memory, i.e. 3FFH. The instruction is RETLW XX, where XX is the calibra-
tion value. This value is placed in the OSCCAL register to set the calibration
value of the internal oscillator. This is done in the 12F629 header as

CALL 3FFH ;call instruction at location 3FFH

MOVWF OSCCAL ;move calibration value to OSCCAL

The trimming can be ignored if required - but it only requires 1 or 2 lines
of code, so why not use it.

I/O PORT, GPIO

The GPIO, General Purpose Input/Output, is an 8 bit I/O register, it has 6 I/O

lines available so bits GPIO to 5 are used, bits 6 and 7 are not.

N.B. GPIO bit3 is an input only pin so there is a maximum of 5 outputs.

• For the 12C508 GPIO pins 0,1 and 3 can be configured with weak pull ups
by writing to OPTION,6 (bit 6 in the OPTION register).

• For the 12F629/675 all GPIO pins except GPI03 can be configured with
weak pull ups. This is done by setting the relevant bits in the Weak Pull Up
Register, WPU. When in

Bank 1 MOVLW B'OO 1 1 1 1 1 '

MOVWF WPU

Will turn on all the weak pull ups.

220 The 12 series 8 pin microcontroller

WPU5 WPU4 WPU2 WPU1 WPUO

bit 7 bit

Figure 15.5 Weak pull up register

Delays with the 12 series

We have previously used a 32kHz. Crystal with the 16F84 device, but now
we are going to use the internal 4MHz clock.

A 4MHz clock means that the basic timing is % of this i.e. 1MHz. If we
set the OPTION register to divide by 256 this gives a timing frequency of
3906Hz. In the headers for the 12C508/9, 12F629 and 12F675 I have (as with
the 16F84) included a one second and a 0.5 second delay. In order to achieve
a one second delay from a frequency of 3906Hz I first of all produced a delay of
1/100 second by counting 39 timing pulses i.e. 3906Hz/39= 100.15= 100Hz
approx., called DELAY. A one second delay, subroutine DELAY 1 then counts
100 of these DELAY times (i.e. 100 x 1/100 second), and of course a delay of
0.5 seconds would count 50.

Just before we look at the headers - we do not have an instruction SUBLW
on the 12C508. I have therefore set up a file called TIME that I have written
39 into. I then move TMR0 into W and subtract the file TIME (39d) from
it to see if TMR0 = 39 i.e. 1/100 of a second has elapsed.

WARNING: The 12C508 and 509 micros only have a two level deep stack.
Which means when you do e.g. a one second delay, CALL DELAY 1 this
then calls another subroutine, i.e. CALL DELAY. You have used your two
levels and cannot do any further calls without returning from one at least
one of those subroutines. If you did make a third CALL the program would
not be able to find its way back!

Header for 12C508/9

;HEAD12C508.ASM FOR 12C508/9.

TMR0

EQU

TMROis FILE 1.

OSCCAL

EQU

Oscillator calibration

GPIO

EQU

GPIO is FILE 6.

STATUS

EQU

STATUS is FILE 3.

ZEROBIT

EQU

ZEROBIT is Bit 2.

COUNT

EQU

07H

USER RAM LOCATION

TIME

EQU

08H

TIME IS 39

The 12 series 8 pin microcontroller 221

LIST P=12C508 ;We are using the 12C508.

ORG ;0 is the start address.

GOTO START ;goto start!

Configuration Bits

_CONFIG H'OFEA' ;selects internal RC oscillator, WDT off,

;code protection disabled

SUBROUTINE SECTION.

; 1/100 SECOND DELAY

DELAY CLRF

TMRO

;Start TMRO

LOOPA MOVF

TMR0,W

;Read TMRO into W

SUBWF

TIME,W

;TIME-W

BTFSS

STATUS,ZEROBIT

;Check TIME-W=0

GOTO

LOOPA

RETLW

;Return after TMRO

;1 SECOND DELAY

DELAY 1 MOVLW

.100

MOVWF

COUNT

TIMEA CALL

DELAY

DECFSZ

COUNT

GOTO

TIMEA

RETLW

;l/2 SECOND DELAY

DELAYP5 MOVLW

.50

MOVWF

COUNT

TIMEB CALL

DELAY

DECFSZ

COUNT

GOTO

TIMEB

RETLW

= 39

; CONFIGURATION SECTION.

START

MOVWF
MOVLW
TRIS
MOVLW

OSCCAL

B'00001000'
GPIO

B'ooooonr

Calibrate oscillator.

5 bits of GPIO are O/Ps.

Bit3 is Input

222 The 12 series 8 pin microcontroller

OPTION

;PRESCALER is /256

CLRF

GPIO

;Clear GPIO

MOVLW

.39

MOVWF

TIME

;TIME = 39

;Program starts now.

Program application for 12C508

There are 5 I/O on the 12C508 i.e. GPIO bits 0,1,2,4 and 5. Bit3 is an input
only. For our application we will chase 5 LEDs on our outputs backwards and
forwards at 0.5 second intervals.

The Circuit diagram is shown in Figure 15.6.

GPIO0
GPI01

GPI02
GPI04

GPI05

Vdd

5 x 680R

-I Y

=r0.1n

Figure 15.6 LED chasing circuit for the 12C508

The 12 series 8 pin microcontroller 223

Notice that the only other component required is the power supply decoupling
capacitor, O.luF, no oscillator circuit is required.

The program for the LED Chasing Project, LED_CH12.ASM is shown below.

;LED_CH12.ASM Program to chase 5 LEDs with the 12C508

TMRO

EQU

;TMR0 is FILE 1.

OSCCAL

EQU

GPIO

EQU

;GPIO is FILE 6.

STATUS

EQU

;STATUS is FILE 3.

ZEROBIT

EQU

;ZEROBIT is Bit 2.

COUNT

EQU

07H

;USER RAM LOCATION

TIME

EQU

08H

;TIME IS 39

LIST P=12C508 ;We are using the 12C508.

ORG
GOTO

START

;0 is the start address.
;goto start!

Configuration Bits

CONFIG H'OFEA'

;selects Internal RC oscillator, WDT off,
;Code Protection disabled.

SUBROUTINE SECTION.

DELAY

CLRF

TMRO

LOOPA

MOVF

TMR0,W

SUBWF

TIME/W

BTFSS

STATUS,ZEROBIT

GOTO

LOOPA

RETLW

;1 SECOND DELAY

DELAY 1

MOVLW

.100

MOVWF

COUNT

TIMEA

CALL

DELAY

DECFSZ

COUNT

GOTO

TIMEA

RETLW

;l/2 SECOND DELAY

DELAYP5

MOVLW

.50

MOVWF

COUNT

Start TMRO

Read TMRO into W

TIME - W

Check TIME-W=0

;Return after TMRO = 39

224 The 12 series 8 pin microcontroller

TIMEB

CALL

DELAY

DECFSZ

COUNT

GOTO

TIMEB

RETLW

CONFIGURATION SECTION.

START

MOVWF OSCCAL Calibrate oscillator.

MOVLW B'00001000' ;5 bits of GPIO are O/Ps.

TRIS GPIO ;Bit3 is Input

MOVLW B'00000111'

OPTION ;PRESCALER is /256

CLRF GPIO ;Clear GPIO

MOVLW .39

MOVWF TIME :TIME = 39

;Program starts now.

BEGIN

MOVLW

MOVWF

CALL

MOVLW

MOVWF

CALL

MOVLW

MOVWF

CALL

MOVLW

MOVWF

CALL

MOVLW

MOVWF

CALL

MOVLW

MOVWF

CALL

MOVLW

MOVWF

CALL

MOVLW

B'ooooooor

GPIO
DELAYP5

B'00000010'

GPIO

DELAYP5

B'00000100'

GPIO

DELAYP5

B'00010000'

GPIO

DELAYP5

B'00100000'

GPIO

DELAYP5

B'OOOIOOOO'

GPIO

DELAYP5

B'00000100'

GPIO

DELAYP5

B'00000010'

;turn on LEDO

;turn on LED1

;turn on LED2

;turn on LED3

;turn on LED4

;turn on LED3

;turn on LED2

;turn on LED1

The 12 series 8 pin microcontroller 225

MOVWF

GPIO

CALL

DELAYP5

GOTO

BEGIN

END

The program is similar in content to the 16F84 programs used previously, but
with the following exceptions:

• A file TIME, file 8, has been set up which has had 39 loaded into it, in the
Configuration Section. This is used to determine when TMRO has reached
a count of 39, time of 0.01 seconds, which is then used in the timing
subroutines.

• In the Configuration Section the first instruction the program encounters
is MOVWF OSCCAL. This moves the calibration value which has just
been read by MOVLW XX, from location 1FFH, the first instruction, into
the calibration file OSCCAL.

• GPIO is used in the program instead of the usual PORTA and PORTB.

Program application using the 12F629/675

To perform the LED chasing action of the previous example in Figure 15.6
using the 12F675 the following code would be required.

;LED_CH675.ASM FOR 12F675 using 4MHz internal RC.

TMRO EQU

TRISIO EQU

GPIO EQU

STATUS EQU

ZEROBIT EQU

GO EQU

ADSEL EQU

ADCON0 EQU

ADRESH EQU

OPTION_R EQU

CMCON EQU

OSCCAL EQU

COUNT EQU

85H

9EH

1FH

1EH

81H

19H

90H

20H

;TMR0is FILE 1.

GPIO is FILE 6.
STATUS is FILE 3
ZEROBIT is Bit 2.

;USER RAM LOCATION.

LIST
ORG
GOTO

P=12F675

START

;We are using the 12F675.
;0 is the start address.
;goto start!

226 The 12 series 8 pin microcontroller

Configuration Bits

CONFIG H'3F84'

;selects Internal RC oscillator, WDT off,
;Code Protection disabled.

SUBROUTINE SECTION.

; 1/1 00 SECOND DELAY

DELAY CLRF TMRO

;START TMRO

LOOPA MOVF TMR0,W

;READ TMRO IN W

SUBLW .39

;TIME-W

BTFSS STATUS,ZEROBIT

;CHECK TIME-W=0

GOTO LOOPA

RETLW

;RETURN AFTER T

;P1 SECOND DELAY

DELAYP1 MOVLW

.10

MOVWF

COUNT

TIMEC CALL

DELAY

DECFSZ

COUNT

GOTO

TIMEC

RETLW

;P5 SECOND DELAY

DELAYP5 MOVLW

.50

MOVWF

COUNT

TIMED CALL

DELAY

DECFSZ

COUNT

GOTO

TIMED

RETLW

= 39

CONFIGURATION SECTION.

START BSF STATUS,5 ;BANK1

MOVLW B'000 10000' ;A11 I/O are digital (12F675 only)

MOVWF ADSEL

MOVLW B'00001000' ;Bit3 is IP

MOVWF TRISIO

MOVLW B'OOOOOllT

MOVWF OPTION R :PRESCALER is /256

The 12 series 8 pin microcontroller 227

CALL 3FFH

MOVWF OSCCAL Calibrates 4MHz oscillator

BCF

STATUS,5 ;BANK0

MOVLW

MOVWF

CMCON

;Turns off comparator

CLRF

GPIO

;Clears GPIO

BSF

ADCON0,0

;Turns on A/D converter

;Program starts now.

BEGIN MOVLW

MOVWF
CALL
MOVLW
MOVWF
CALL
MOVLW
MOVWF
CALL
MOVLW
MOVWF
CALL
MOVLW
MOVWF
CALL
MOVLW
MOVWF
CALL
MOVLW
MOVWF
CALL
MOVLW
MOVWF
CALL
GOTO

END

B'0000000 1 ' ;turn on LEDO

GPIO

DELAYP5

B'000000 1 0' ;turn on LED 1

GPIO

DELAYP5

B'00000100' ;turn on LED2

GPIO

DELAYP5

B'00010000' ;turn on LED3

GPIO

DELAYP5

B'OO 100000' ;turn on LED4

GPIO

DELAYP5

B'OOOIOOOO' ;turn on LED3

GPIO

DELAYP5

B'00000100' ;turn on LED2

GPIO

DELAYP5

B'000000 1 0' ;turn on LED 1

GPIO

DELAYP5

BEGIN

The differences in the code between the 12C508 and 12F675 are:

MOVLW B'00010000' ;A11 I/O are digital (12F675 only)
MOVWF ADSEL

228 The 12 series 8 pin microcontroller

These two lines are used to inform the 12F675 that the inputs are all
digital. Change the data to make the inputs analogue - refer to manufacturers
data. These two lines are not required for the 12F629 which does not have
any A/D.

• CALL 3FFH

MOVWF OSCCAL Calibrates 4MHz oscillator

These lines are used to calibrate the internal 4MHz oscillator.

• MOVLW 7H

MOVWF CMCON ;Turns off comparator

The 12F629/675 have analogue comparators, which we have not looked at.
They need to be turned off to use the I/O pins. The default is that the
comparators are on!

There are numerous other 12 series microcontrollers but once you have
understood how to move from the 12C508/9 to the 12F629/675 you will be
able to migrate to the rest.

16
The 16F87X microcontroller

The 16F87X range includes the devices, 16F870, 16F871, 16F872, 16F873,
16F874, 16F876 and 16F877. They are basically the same device but differ in
the amounts of I/O, analogue inputs, program memory, data memory (RAM)
and EEPROM data memory that they have.

The 16F87X have more I/O, program memory, data memory, EEPROM data
memory and analogue inputs than the 16F818.

16F87X family specification

Device

Program
Memory

EEPROM

Data Memory

(bytes)

RAMBytes

Pins

I/O

10 bit A/D
Channels

16F870

128

16F871

128

16F872

128

16F873

128

192

16F874

128

192

16F876

256

368

16F877

256

368

16F87X memory map

The 16F87X devices have more functions than we have seen previously.
These functions of course need registers in order to make the various selections.

The memory map of the 16F87X showing these registers is shown in
Figure 16.3.

The 16F87X devices have a number of extra registers that are not required
in the applications we have looked at. For an explanation of these registers
please see Microchip's website @ www.microchip.com, where you can
download the data sheet as a pdf (portable document file), which can be read
using Adobe Acrobat Reader.

230 The 16F87X microcontroller

MCLR/Vpp/THV Z

•

□ B7/PGD

A0/AN0 Z

□ B6/PGC

A1/AN1

□ B5

A2/AN2/Vref-

□ B4

A3/AN3/Vref+ IZ

□ B3/PGM

A4/T0CKI

Z B2

A5/AN4/SS

Z B1

Vss

Z BO/INT

0SC1/CLKIN

Z Vdd

0SC2/CLK0UT

| — | Vss

C0/T1OSO/T1CLKI

□ C7/RX/DT

C1/T10SI/CCP2

□ C6/TX/CK

C2/CCP1

□ C5/SD0

C3/SCK/SCL

□ C4/SD1/SDA

Figure 16.1 The 16F870/2/3/6 pinout

The 16F872 microcontroller

In order to demonstrate the operation of the 16F87X series we will consider
the 16F872 device. This is a 28pin device with 22 I/O available on 3 ports.
PortA has 6 I/O, PortB has 8 I/O and PORTC has 8 I/O. Of the 6 I/O available
on PortA 5 of them can be analogue inputs. The header for the 16F872,
HEAD872.ASM, configures the device with 5 analogue inputs on PortA,
8 digital inputs on PortC and 8 outputs on PortB. The port configuration for
the device is shown in Figure 16.4.

The 16F872 has been configured in HEAD872.ASM, using a 32 kHz crystal,
to allow all the programs used previously to be copied over with as little
alteration as possible.

The 16 F87X microcontroller 231

Devices included in this Data Sheet:

• PIC16F873 • PIC16F876

• PIC16F874 • PIC16F877
Microcontroller Core Features:

• High performance RISC CPU

• Only 35 single word instructions to learn

• All single cycle instructions except for program
branches which are two cycle

• Operating speed: DC - 20 MHz clock input

DC - 200 ns instruction cycle

• Up to 8K x 1 4 words of FLASH Program Memory,
Up to 368 x 8 bytes of Data Memory (RAM)

Up to 256 x 8 bytes of EEPROM Data Memory
Pinout compatible to the PIC16C73B/74B/76/77
Interrupt capability (up to 14 sources)
Eight level deep hardware stack
Direct, indirect and relative addressing modes
Power-on Reset (POR)
Power-up Timer (PWRT) and
Oscillatior Start-up Times (OST)
Watchdog Timer (WDT) with its own on-chip RC
oscillator for reliable operation
Programmable code-protection
Power saving SLEEP mode
Selectable oscillator options
Low power, high speed CMOS FLASH/EEPROM
technology
Fully static design

In-Circuit Serial Programming™ (ICSP) via two
pins

Single 5V In-Circuit Serial Programming capability
In-Circuit Debugging via two pins
Processor read/write access to program memory
Wide operating voltage range: 2.0V to 5.5V
High Sink/Source Current: 25 mA
Commercial and Industrial and Extended temperature
ranges
Low-power consumption:

- < 2 mA typical @ 3V, 4 MHz

- 20 uA typical @3V, 32 kHz

- < 1 U.A typical standby current

Pin Diagram

PDIP

MCLR/Vpp

RA0/AN0

RA1/AN1

RA2/AN2A/REF-

RA3/AN3/Vref +

RA4/T0CKI

RA5/AN4/SS

RE0/RD/AN5

RE1/WR/AN6

RE2/CS7AN7

Vdd

Vss

OSC1/CLKIN

OSC2/CLKOUT

RC0/T1OSO/T1CKI

RC1/T10SI/CCP2

RC2/CCP1

RC3/SCK/SCL

RD0/PSP0

RD1/PSP1

RB7/PGD

RB6/PGC

RB5

RB4

RB3/PGM

RB2

RB1

RB0/INT

Vdd

Vss

RD7/PSP7

RD6/PSP6

RD5/PSP5

RD4/PSP4

RC7/RX/DT

RC6/TX/CK

RC5/SDO

RC4/SDI/SDA

RD3/PSP3

RD2/PSP2

Peripheral Features:

• TimerO: 8-bit timer/counter with 8-bit prescaler

• Timerl : 1 6-bit timer/counter with prescaler,
can be incremented during SLEEP via external
crystal/clock

• Timer2: 8-bit timer/counter with 8-bit period
register, rescaler and postscaler

• Two Capture, Compare, PWM modules

- Capture is 16-bit, max. resolution is 12.5 ns

- Compare is 16-bit, max. resolution is 200 ns

- PWM max. resolution is 10-bit

• 10-bit multi-channel Analog-to-Digital converter

• Synchronous Serial Port (SSP) with SPI™ (Master
mode) and l 2 C™ (Master/Slave)

• Universal Synchronous Asychronous Receiver
Transmitter (USART/SCI) with 9-bit address detection

• Parallel Slave Port (PSP) 8-bits wide, with
external RD, WR and CS~controls (40/44-pin only)

• Brown-out detection circuitry for
Brown-out Reset (BOR)

Figure 16.2 The 16F87X data sheet

232 The 16F87X microcontroller

Address

File Name
BankO

File Name
Bankl

File Name
Bank2

File Name
Bank3

OOh

Ind.Add

01 h

TMRO

Option

TMRO

Option

02h

PCL

03h

Status

04h

FSR

05h

PORTA

TRISA

06h

PORTB

TRISB

PORTB

TRISB

07h

PORTC

TRISC

08h

PORTD

TRISD

09h

PORTE

TRISE

OAh

PCLATH

OBh

INTCON

OCh

PIR1

PIE1

EEDATA

EECON1

ODh

PIR2

PIE2

EEADR

EECON2

OEh

TMR1L

PCON

EEDATH

OFh

TMR1H

EEADRH

10h

T1CON

General
Purpose
Register
96 bytes

1 1 h

TMR2

SSPCON2

12h

T2CON

PR2

13h

SSPBUF

SSPADD

14h

SSPCON

SSPSTAT

15h

CCPR1L

16h

CCPR1H

17h

CCP1CON

18h

RCSTA

TXSTA

19h

TXREG

SPBRG

1Ah

RCREG

1Bh

CCPR2L

1Ch

CCPR2H

1Dh

CCP2CON

1Eh

ADRESH

ADRESL

1Fh

ADCONO

ADCON1

General
Purpose
Register
96 bytes

General
Purpose
Register
80 bytes

6Fh

7FH

Figure 16.3 The 16F87X memory map

The 16F87X microcontroller 233

5 Analogue Inputs

8 Digital Inputs

1 Digital Input

ANO(AO)
AN1(A1)
AN2(A2)
AN3(A3)
AN4(A5)

CO
C1
C2
C3
C4
C5
C6
C7

BO
B1
B2
B3
B4
B5
B6
B7

8 Outputs

Figure 16.4 Port configuration of the 16F872

The 16F872 header

HEAD872.ASM
;EQUATES SECTION

TMRO

EQU

OPTION R

EQU

PORTA

EQU

PORTB

EQU

PORTC

EQU

TRISA

EQU

TRISB

EQU

TRISC

EQU

STATUS

EQU

ZEROBIT

EQU

CARRY

EQU

EEADR

EQU

ODH

EEDATA

EQU

OCH

EECON1

EQU

OCH

EECON2

EQU

ODH

EQU

WREN

EQU

ADCONO

EQU

1FH

ADCON1

EQU

1FH

ADRES

EQU

1EH

234 The 16F87X microcontroller

CHSO EQU

GODONE EQU
COUNT EQU

3
2
20H

LIST
ORG
GOTO

P=16F872

START

; SUBROUTINE SECTION.

;1 SECOND DELAY
DELAY 1 CLRF

LOOPA

MOVF

SUBLW

BTFSS

GOTO

RETLW

TMRO ;Start TMRO

TMRO/W ;Read TMRO into W

.32 ;TIME-W

STATUS,ZEROBIT ;Check TIME-W =

LOOPA

;Return after TMRO = 32

;0.5 SECOND DELAY
DELAYP5 CLRF TMRO

LOOPB MOVF

SUBLW

BTFSS

GOTO

RETLW

;Start TMRO
TMR0,W ;Read TMRO into W

.16 ;TIME-W

STATUS,ZEROBIT ;Check TIME-W =
LOOPB
;Return after TMRO = 16

CONFIGURATION SECTION.

START BSF STATUS,5 ;Bankl

MOVLW B'llllllir
MOVWF TRISA ;PortA is input

MOVLW B'00000000'

MOVWF TRISB ;PortB is output

MOVLW B'llllllir

MOVWF TRISC ;PortC is input

MOVLW B'OOOOOllT

MOVWF OPTION_R ;Option Register, TMRO / 256

The 16F87X microcontroller 235

MOVLW

B'OOOOOOOO'

MOVWF

ADCON1

;PortA bits 0, 1, 2, 3, 5 are analogue

BSF

STATUS,6

;BANK3

BCF

EECON1J

;Data memory on.

BCF

STATUS,5

BCF

STATUS,6

;BANK0 return

BSF

ADCON0,0

;turn on A/D.

CLRF

PORTA

CLRF

PORTB

CLRF

PORTC

;Program starts now.

Explanation of HEAD872.ASM

Equates Section

• We have a third port, PORTC file 7 and its corresponding TRIS file,
TRISC file 7 on Bankl. The TRIS file sets the I/O direction of the port bits.

• The EEPROM data file addresses have been included. EEADR is file ODh
in Bank2, EEDATA is file OCh in Bank2, EECON is file OCh in Bank3
and EECON2 is file ODh in Bank3.

• The EEPROM data bits have been added. RD the read bit is bit 0, WR the
write bit is bit 1, WREN the write enable bit is bit 2.

• The Analogue files ADRES, ADCON1 and ADCON2 have been included
as have the associated bits CHSO channel select bit 3 and the GODONE
bit, bit 2.

List Section

• This of course indicates the microcontroller being used, the 16F872 and
that the first memory location is 0. In address is the instruction GOTO
START that instructs the micro to bypass the subroutine section and goto
the configuration section at the label START.

Subroutine Section

• This includes the 2 delays DELAY 1 and DELAYP5 as before.
Configuration Section

• As before we need to switch to Bankl to address the TRIS files to configure
the I/O. PORTA is set as an input port with the two instructions

236 The 16F87X microcontroller

MOVLW B'OOOOOllT
MOVWF TRISA

PORTB and PORTC are configured in a similar manner using TRISB and

TRISC.

The Option register is configured with the instructions

MOVLW B'OOOOOllT
MOVWF OPTION_R

The A/D register is configured with the instructions

MOVLW B'00000000'
MOVWF ADCON1

Setting PORTA bits 0, 1, 2, 3 and 5 as analogue inputs.

We turn to Bank3 by setting Bank select bit, STATUS,6 (bit 5 is still set)

so that we can address EECON1, the EEPROM data control register. BSF

EECON1 then enables access to the EEPROM program memory when

required.

We then turn back to BankO by clearing bits 5 and 6 of the Status register

and clear the files PortA, PortB and PortC.

16F872 Application - a greenhouse control

In order to demonstrate the operation of the 16F872 and to develop our
programming skills a little further consider the following application.

• A greenhouse has its temperature monitored so that a heater is turned
on when the temperature drops below 15°C and turns the heater off when
the temperature is above 17°C.

• A probe in the soil monitors the soil moisture so that a water valve will
open for 5 seconds to irrigate the soil if it dries out. The valve is closed and
will not be active for a further 5 seconds to give the water time to drain into
the soil.

• A float switch monitors the level of the water and sounds an alarm if
the water drops below a minimum level.

The circuit diagram for the greenhouse control is shown in Figure 16.5 and
the flowchart is drawn in Figure 16.6.

Greenhouse program

In order to program the analogue/digital settings consider the NTC
Thermister. As the temperature increases the resistance of the thermister
will decrease and so the voltage presented to ANO will increase.

The 16F87X microcontroller 237

Thermistor

Soil Moisture Probe

Float Switch \

Ov
Figure 16.5 Greenhouse control circuit

Let us assume the voltage is 2.9v at 15°C and 3.2v at 17°C they correspond
to digital readings of 2.9 x 51 = 147.9 i.e. 148 and 3.2 x 51 = 163.2 i.e. 163.
(N.B. 5v = 255, so lv=51 we are using an 8 bit A/D.)

Our program then needs to check when ANO goes above 163 and below 148.

As the soil dries out its resistance will increase. Let us assume in our
application dry soil will give a reading of 2.6v, (on AN1), i.e. 2.6 x 51 = 132.6
i.e. 133. So any reading above 133 is considered dry.

The float switch is a digital input showing 1 if the water level is above
the minimum required and a if it is below the minimum.

Greenhouse code

The code for the greenhouse uses HEAD872.ASM with the program
instuctions added and saved as GREENHO.ASM.

238 The 16F87X microcontroller

Turn off heater

Turn on heater

Turn on water valve
Wait 5 seconds
Turn off water valve
Wait 5 seconds

Figure 16.6 Greenhouse control flowchart

The 16F87X microcontroller 239

;GREENHO.ASM
;EQUATES SECTION

TMRO

EQU

OPTION_R

EQU

PORTA

EQU

PORTB

EQU

PORTC

EQU

TRISA

EQU

TRISB

EQU

TRISC

EQU

STATUS

EQU

ZEROBIT

EQU

CARRY

EQU

EEADR

EQU

ODH

EEDATA

EQU

OCH

EECON1

EQU

OCH

EECON2

EQU

ODH

EQU

WREN

EQU

ADCONO

EQU

1FH

ADCON1

EQU

1FH

AD RES

EQU

1EH

CHSO

EQU

GODONE

EQU

COUNT

EQU

20H

*****************************

LIST

P=16F872

ORG

GOTO START

Configuration Bits

_CONFIG H'3F30' ;selects LP oscillator, WDT off, PUT on,

;Code Protection disabled.

SUBROUTINE SECTION.

;1 SECOND DELAY

DELAY 1 CLRF TMRO

LOOPA MOVF TMR0,W

SUBLW .32

BTFSS STATUS,ZEROBIT

Start TMRO

Read TMRO into W

TIME-W

Check TIME-W =

240 The 16F87X microcontroller

GOTO
RETLW

;0.5 SECOND DELAY
DELAYP5 CLRF
LOOPB MOVF
SUBLW
BTFSS
GOTO
RETLW

;5 SECOND DELAY
DELAY5 CLRF

LOOPC

HEAT ON

MOVF

SUBLW

BTFSS

GOTO

RETLW

BSF
GOTO

HEAT_OFF BCF
GOTO

WATER_ON BSF

CALL
BCF
CALL
GOTO

LOOPA

TMRO

TMR0,W

.16

STATUS,ZEROBIT

LOOPB

TMRO

TMR0,W

.160

STATUS,ZEROBIT

LOOPC

PORTB,l
DELAY5
PORTB,l
DELAY5
WATER

;Return after TMRO = 32 >

Start TMRO

Read TMRO into W

TIME-W

Check TIME-W =

; Return after TMRO = 16

Start TMRO

Read TMRO into W

TIME-W

Check TIME-W =

; Return after TMR0= 160

PORTB,0 ;Turn heater on

SOIL ;Check soil moisture

PORTB,0 ;Turn heater off

SOIL ;Check soil moisture

;Turn water on

;Turn water off

:Check water level

ALARM_ON BSF

GOTO

ALARM_OFF BCF
GOTO

PORTB,2 ;Turn alarm on

BEGIN ;Repeat the process

PORTB,2 ;Turn alarm off

BEGIN ;Repeat the process

; CONFIGURATION SECTION.

START

BSF

STATUS,5

;Bankl

MOVLW

B'liiinir

MOVWF

TRISA

;PortA is input

MOVLW

B'00000000'

MOVWF

TRISB

;PortB is output

The 16F87X microcontroller 241

MOVLW

B'liiinir

MOVWF

TRISC

;PortC is input

MOVLW

B'ooooonr

MOVWF

OPTION_R

;Option Register, TMRO/256

MOVLW

B'OOOOOOOO'

MOVWF

ADCON1

;PortA bits 0, 1, 2, 3, 5 are
;analogue

BSF

STATUS,6

;BANK3

BCF

EECONl,7

;Data memory on.

BCF

STATUS,5

BCF

STATUS,6

;BANK0 return

BSF

ADCON0,0

;turn on A/D.

CLRF

PORTA

CLRF

PORTB

CLRF

PORTC

;Program starts now.

;Check the temperature on ANO

BEGIN

BCF

ADCON0,CHS0

;C to select ANO

BSF

ADCON0,GODONE

WAIT1

BTFSC

ADCON0,GODONE

GOTO

WAIT1

MOVF

ADRES/W

SUBLW

.163

;163- W

BTFSS

STATUS,CARRY

;Cif W > 163 i.e. hot
;(above 17°C)

GOTO

HEAT OFF

MOVF ADRES/W
SUBLW .148

;148-W

BTFSC STATUS,CARRY

GOTO

HEAT ON

;S if W < 148 i.e. cold
;(below 15°C)

;Check the soil moisture on AN1

SOIL BSF ADCON0,CHS0

BSF ADCON0,GODONE

WAIT2 BTFSC ADCON0,GODONE

GOTO WAIT2

;S to select AN1

242 The 16F87X microcontroller

MOVF

ADRES,W

SUBLW

.133

;133-W

BTFSS

STATUS,CARRY

;C if W > 133 i.e. dry

GOTO

WATER_ON

;Check water is above

minimum

WATER BTFSC

PORTQO

;C if below minimum

GOTO

ALARM_OFF

GOTO

ALARM ON

END

Explanation of code

In the previous analogue circuits in Chapter 11 we only used 1 analogue
input on ANO. We now have two analogue inputs on ANO and AN1. When
making an analogue measurement we must specify which analogue channel
we wish to measure. The default is ANO when moving to AN1 we select AN1
by setting channel select bitO i.e. BSF ADCON0,CHS0.

When moving back to ANO clear the channel select bit. The 8 channels, ANO to
AN7 are seclected using bits, CHS2, CHS1, CHSO.

• The temperature is read on ANO with and then checked to see if it is
greater than 17°C, by subtracting the A/D reading from 163 (the reading
equating to 17°C). The carry bit in the status register indicates if the result
is +ve or — ve being set or clear. We then go to turn off the heater if the
temperature is above 17°C or check if the temperature is below 15°C.
In which case we turn on the heater.

• The soil moisture is checked next. AN1 is selected and the reading compared
this time to 133 indicating dry soil. The program either goes to turn on
the water valve if the soil is dry or continues to check the water level if the
soil is wet.

• If the water level is below minimum then the alarm sounds, if above
minimum the alarm is turned off. The program then repeats the checking
of the inputs and reacts to them accordingly.

Programming the 16F872 microcontroller
using PICSTART PLUS

Once the pogram GREENHO.ASM has been saved it is then assembled using
MPASMWIN. The next step as previously is to program GREENHO.HEX
into the micro using PICSTART PLUS.

This process has been outlined in Chapter 2, but there are a few more selections
to attend to in the 'Device Specification' Section.

The 16F87X microcontroller 243

• Select the device 16F872, if this device is not available you will require a later
version of MPLAB, obtainable from www.microchip.com.

• Set the fuses.

Configuration bits

The configuration bit settings when programming the 16F872 for the
Greenhouse program are shown in Figure 16.7.

Configuration Bits

Address

Value

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