In this chapter we talk about the magic of a microcontroller.
A single microcontroller can
replace many simple IC's and make a project so powerful it appears
to have "intelligence."
There are lots of microcontrollers on the market and lots of new
companies and manufacturers coming out with their own devices. It's so
complex that you almost don't know which path to travel.
The competition is so fierce that new products and designs are cheaper
and more powerful than the predecessors and this gets your head
Obviously you can't spread your talents too thinly and you can't learn
to program all the different devices. So, the simple answer is to start
with something that has been on the market for a while and has an
enormous backing - in the form of programs, projects and discussion
groups - that's
why we have chosen the PIC family.
But just before we discuss the PIC chip, here is little diversion.
Texas Instruments have introduced their own line of micros and a recent
email promoted their MSP430xxx series. These are a 16-bit
ultra-low-power devices and the suggested price started at 25 cents.
Thinking this was a very good entry price, I looked further to find the
actual part number and costing. Apart from not finding the part number,
the remainder of the website suggested the prices started from 49 cents,
not the 25 cents on the frontice page.
On further investigation I found very little in the way of documentation
or projects for any of the chips in this range and finally found a
supplier that sold the chip for 69 cents in quantities of 2,000 pieces!
Or single units for $2.30. This is a far-cry from the 25cent "bait."
I am still waiting for a reply to my email to "TIasia." asking for
clarification of the 25 cent device.
As I have said before, don't spread your resources too thinly.
Every time you entertain a new style of microcontroller you have to
learn a new set of instructions or advance to a higher-level language
where a compiler will create the code.
We have done all the background leg-work and come up with a PIC as the
That's why the microcontroller we will be discussing is the PIC12F629.
It is one of the simplest on the market, as we are starting at the
beginning of microcontroller design. This means everyone will be able to
pick up this amazing field of electronics.
There are thousands of pages on Talking Electronics website to help you
in all facets of electronics, including the designing of circuits
(called interfacing) to a microcontroller.
Some projects, in various magazines, don't tell you anything about the program in the micro and
projects give the chip a name such as "Hand Wand" and don't tell you
the type of chip in the circuit. We don't do that. We don't keep you in
the dark. We explain everything.
Everything we design has the full circuit diagram and all the code for
Our intention is to make you successful as an electronics engineer. The
author has been designing projects for over 30 years and sees customers
every day who say they have gained an enormous amount from reading
Talking Electronics projects and articles.
It's a lot of work but it is worth it.
To get into the microcontroller field is a lifetime career. You don't
know were it will lead, however I can guarantee you a position in the
You only have to concentrate on monitoring and the field is enormous.
It may be monitoring patients with terminal diseases, life-threatening
diseases, constant-monitoring-diseases or one of a thousand situations
that need attention.
Or you can take up hearing-loss, speech-loss, co-ordination-loss. And
then there are other areas such as security, shop-signs, automotive,
education, toys and lots more . . .
But before you can think about designing anything, you need to be able
to program a chip to carry out a task.
This is what this course is all about, It will give you the confidence
to tackle tasks you thought impossible. And produce designs beyond your
But before we can work in any of the fields we have mentioned, let's work on some simple projects to gain
Let's walk before we run.
IS DESIGNED AROUND A "COMPUTER"
Almost every new product is designed around a "computer" and to us, this means a "micro."
Devices such as talking key-chains, whistle detectors to locate your keys,
remote controls for garage doors, cars, DVDs, cable TV,
house alarms and toys are all designed around a single "computer" chip.
The reason is simple. Economics.
A "computer" chip is a device with many functions. It can
"remember" codes and inputs from the user and output to one or more
Rather than design a circuit using lots of transistors, chips and components, it
is much cheaper to use a single chip. It takes less board-space, costs less to
produce and creates a smaller product.
Here is a circuit for a
It is nothing more than a "door striker"
(a solenoid to release a door) that operates when a code is entered on
the 4x4 keypad. All the processing of the data from the keypad and code
remembering is done with a number of chips including a RAM. This can now
all be done in a micro and no other chips are required. This is how a
micro has changed electronics.
Micro's not only simplify designs, they reduce the size of the PC board
enormously. New designs are always going for compactness and the trend is to produce something
smaller than the previous model.
And it all revolves around a micro.
There are two choices. It can be custom-designed or "off the shelf."
Designing a chip will cost thousands of dollars. But in most cases you can buy one
"off the shelf" for only a few dollars and all you need to do is add the
These "universal" chips are called microcontrollers and are equivalent to buying
an exercise book and writing your own article.
One of the smallest and cheapest microcontrollers on the market is the
(data sheet 4.9MB .pdf)
It is an 8 pin chip with 5 pins that can be
connected to external devices such as globes, switches, relays, microphones,
photo devices, motors, etc
and 1 pin that is an INPUT only.
Sometimes you will need a transistor or other component between the chip and the
device to get them to "talk" to the chip, or get the chip
to drive the device. This
is called INTERFACING and will be covered in our discussion.
Even though the PIC12F629 is very small as far microcontrollers are
concerned, it can take the place of 10 or more individual chips, as can
be seen above.
All sorts of projects can be designed around the chip or you can get it to
perform "intelligent" commands such as playing a game or remember a sequence of
It may be small, but it's the place to start.
Every year, electronics is reducing is size, increasing in performance and
halving in cost.
New products are being released all the time using microcontrollers.
Devices such as auto-dialers for monitoring heart patients or diabetes suffers,
automatically dial a central station and download data to monitor the patient.
The computer decides if an alarm is to be raised.
The world is crying-out for devices to assist those unable to perform
But to be able to turn an idea into reality, you have to know the capability
of electronics and be able to carry out the design-work. You will lose-out
heavily if you pass your idea to another authority.
This is what this article is all about.
It shows some of the capabilities of a microcontroller and how to
get from the concept-stage to the finished product.
It's all a matter of doing things in small steps and
when these are combined together, you have something that will perform a task.
WHY PICK THE PIC12F629?
The first thing you will ask yourself is: Why pick the '12F629 for a project?
The answer is simple. It is an 8-pin chip with 6 pins that can be used as inputs
or outputs and these can be changed at any time during the running of the
program. Pin 4 (GP3) can only be used as an input pin. The only smaller PIC
micro is the PIC10F222. It is 20 to30 cents cheaper but does not have all the
features of a PIC12F629. See the article on
Each pin of the PIC12F629 will drive up to 25mA and this can be buffered with a transistor
to deliver higher currents.
The chip takes only a few micro amps when put into sleep mode and can
take the place of many other types of chips.
But best of all, the program you write and burn into the chip, can be protected
from prying eyes, so the project can be sold for material gain.
There are thousands of potential uses for this chip.
You only have to think of the medical field, security field, educational field
and use it as an interface device between buttons and a motor, lamp or speech
The best place to start is in your own household.
If you have an elderly person or know of someone that requires attention; study
the situation and develop something that solves the problem.
The same requirement will be needed for everyone else in the same predicament and if
the device you produce is cheap and reliable, you will have a ready-made market.
The main reason why we suggest a microcontroller is "ease
By writing a program and having all the operations (such as timing, delays, sequencing
etc) will be done via a program and the end-result can be modified to an individual's
requirement, so the chip can be up-graded without having to change any of the circuit
Not only that, many of the logic chips in "old-style" designs are becoming obsolete and difficult to
But best of all, you will have the quodos of knowing the project you have
designed uses the latest technology.
Don't worry about the code space of about 1,000 instructions. You will almost
NEVER fill the memory. The reason is simple. As you produce a larger and larger
program, you will refer to some of the previously written sub-routines and thus
you will be able to get a lot of new activity with a few instructions.
You can think of it this way. If you were to write a program on paper, using 25
lines per page, the 1,000 line program would require 40 pages. Some of the lines
of the program would consist of tables and if you allow 250 items of data, the
remaining space is 750 lines for your code.
THE PICAXE AND THE STAMP
You may have come across very popular microcontroller teaching systems called
"PICAXE" or "STAMP." These are both concepts that have been designed to teach
microcontroller chips using a higher level of language called BASIC (the
lowest level is called Machine Code).
The whole concept of these products is to get a project up and running, using a
microcontroller, in which all the sub-routines have been pre-programmed into the
chip and all you have to do is create a list of instructions to call the
necessary sub-routines to create a result. The simplest module in the PICAXE
range uses a PIC12F629 and the stamp uses a similar microcontroller.
This is a wonderful way to get a project "up-and-running" without having to learn how
to program the chip, but instead of having to learn 33 instructions for the
chip, you have to learn a language called "BASIC." This is totally different to anything we are doing.
We are using the 33 instructions that come with the chip and we are capable of
everything the chip is capable of doing.
Learning to program the other two "devices" is like getting someone to take your place at
University for 5 years and then give YOU the Engineering degree at the end of
BASIC language has nothing to do with PIC chips. The chip inside the product could be an ATMEL
chip and the outcome would be exactly the same.
If you want to produce a project and just see the end result, you can
use one of these concepts.
If you want to produce something that may be converted to an income-producing
idea, these concepts will cost a lot more as you have to buy the
pre-programmed chip from special suppliers. Secondly, the chip is already so filled
with routines (called the BOOTSTRAP program) that it will only accept a few
instructions (about 30 - 80 BASIC instructions) that are placed in the EEPROM
section of the chip. One of the pins of the chip is used for programming, so the chip has one less in/out
If you are beginning programming, you can try them and see how exciting it is
to get something working. If you then want to produce a more-complex project,
and you run out of space in the chip, you can put the PICAXE chip in a
conventional programmer and erase the bootstrap program and use the chip in one of our projects. Nothing will be wasted!
Note: The BOOTSTRAP program can also be called the ROM Monitor program or
Monitor ROM as it is the operating system for the PICAXE system. Imagine if
every book you bought had a list of words in the front and how to create
sentences. What a waste of paper! That's how I feel about the MONitor system. It
reduces the capability of the chip enormously.
Our concept is to teach PROGRAMMING. When you do everything yourself, you can
change anything to suit the circumstances. When anything is done at
"arms-length" or by someone else, you have less control. Both our concept
and PICAXE or STAMP have
enormous potential and it's up to you to work out which path you want to take.
I prefer to be in charge of everything I do. That's why I show how to do
everything at the lowest level possible.
That's why our level is called hand-assembled "ASSEMBLY CODE."
The individual lines of code (instructions) create a program with an extension
".asm" and this program is actually made up of sub-routines. Each sub-routine is designed to
carry out a simple task such as recognizing the push of a button, turning on a LED
or creating a tone
in a speaker.
The place to start writing this code is on a template already laid-out with preliminary details.
For the chip we are featuring, the template is called
We have two templates:
will blink a LED to show that the program, chip and circuit, is working. This is a test program to prove
the writing, burning and circuit components are correct and functioning.
"blank12F629.asm" is a starting template.
To create a program for the chip, use blank12F629.asm,
Notepad.exe or Unzip
Notepad.zip in our Programming
folder on the left-side of the screen and load it with the blank12F629.asm
file by sliding it into Notepad. It will then
show in Notepad. Rename the file immediately to something such as "project1" and
save it. Work on "project1" file.
Don't get blink12F629.asm confused with blank12F629.asm
make programming easy, we have created a number of sub-routines and placed them
in a Library.
The library has also been formatted as a text file so you can copy and paste the
instructions directly into your program. Click
HERE for the library
as a text file.
DESIGNING A PROJECT
We have designed a lot of
projects using this PIC chip and the best place to start is with one of the
Some of them are surface-mount and some are programmed when the chip is
already fitted to the board. No matter which project you choose, you will learn
If doesn't matter if it is improving your soldering skills or simply identifying
components, every project will add to your knowledge.
Eventually you will want to design your own project and this is when your skills
will be tested.
The author has just spent 5 hours finishing a simple LED-light strip of over 300
LEDs for a set of doors in a bottle-shop.
The final design required 5 modifications and it's important to work of the
correct modification before going to the next.
Everything has to be done in the correct order and everything needs a lot of
The modifications were simple but in the finish it took 5 hours.
To be able to work on a project and make changes, you have to know what you are
doing. That's why we have produced so many project and presented them on the
website. Even though many of the projects do not use a microcontroller, they
offer other design features that will help you. That's why it's important to put
together as many projects as possible. They will all offer some feature that
will add to your knowledge.
If you think a new design will work first-time, you are fooling yourself. They
all require modifications and improvements.
So, here's the path: Pick a project and put it together.
Pick another and put it together
Try another project and build it.
Even if you set a goal of one project a week, it will take many months to build
up a store of "building blocks."
It takes time, money, patience and commitment to build up a store of knowledge.
But it will be worth it in the end.
Every society is promoting the need to have an understanding of technology and
the only way to get an electronics education is via self-study.
The range of material we have on the site will get you through to employment in
almost any field and it's all FREE! All you have to do is take
advantage of it and back up what you read with the practical side.
The following article discusses the PIC12F629 and its technical
features. You should also refer to the 120
page manual for more details on the workings of the chip.
by CHARLES MANNING
The Microchip PIC12F629 microcontroller is one of the smallest
chips in the range and is a good starting point for 'dedicated micro' applications.
It is low
cost and easy to program.
Electronic circuits are often used to monitor and control equipment.
This may be as simple as a sequencer for a model railway or sprinkler
system, or as complex as a real-time industrial controller. Where once
relays and logic gates were used, microcontrollers are now finding their
way into sophisticated and 'intelligent' control.
Unfortunately for the private individual this approach is often not
feasible since micro-controller development systems can be very
expensive. However this article will show you how to use this device and
explain most of the features of the chip.
A microcontroller is a 'computer in a chip', which has been designed
specially for control purposes. As
well as the central processing unit, most microcontrollers include
random access data memory (RAM), and read only memory (ROM), timers and
input/output lines; all in a single chip. Some also include more
complex features such as analog to digital converters, serial
communications channels and pulse width modulators. As is the case with
all electronic technologies, microcontrollers are becoming smaller and
many are now available in 'skinny DIP' or surface mount packages with
less than 20 pins.
A few years back, the use of miniature microcontrollers was only
feasible for large manufacturing companies producing tens of thousands
of each product. The design process was slow. Firstly, the design
engineers would use expensive design and modeling systems to lay out the
design of the device. The designs were sent off to the chip
manufacturer, who would take a few weeks (and a few thousand dollars) to
make a couple of sample parts for testing.
Hopefully the first design was right, but more likely some bug would be
found and this process would have to be repeated. Once the design was
tested, the manufacturer would then schedule a
production run of these parts.
This slow and costly process kept miniature microcontrollers out of
reach for a growing sector of the market, such as small engineering firms who
could not afford the large costs and long turnaround times. These firms
frequently have small budgets and their products are often cost
sensitive, have short production runs and need to be delivered quickly.
A few chip manufacturers realised this, and began producing miniature
microcontrollers with development systems that the smaller companies
could afford (a few hundred to a few thousand dollars).
This article is based on the PIC12F629
microcontroller, from Microchip Technologies, which has some features
which make it ideal for the amateur developer. Microchip's PIC series of
microcontrollers has created a lot of interest in the electronics
development community. Not a day goes by without exchanges about PICs on
international news networks like the Internet.
Unlike many micro-controllers, which have evolved from general purpose
microprocessors, the PIC series were purposefully designed from scratch
as small, cheap, feature-packed microcontrollers to suite both small and
large scale developers. PICs have a rather novel reduced instruction set
(RISC), which makes PIC code very compact and easy to learn. The
simplicity of a RISC core allows the PICs to combine low power usage and
high performance in small packages.
The main difference between the 12F629 and most other microcontrollers is
that the program is stored in onboard EEPROM, instead of EPROM. EEPROM
memory is erased electrically in a few milliseconds; EPROM typically
requires a few minutes exposure to ultra-violet light. Microchip
probably did this to allow new programs to be downloaded in the field.
EPROM-based reprogrammable devices are packaged in ceramic, with a
quartz window to admit ultra violet light; however the 12F629 comes in
the standard plastic packaging, which makes it cheap when compared with
other reprogrammable micro-controllers. As a bonus, EEPROM program
storage also does away with the need for an EPROM eraser.
The PIC12F629 is a CMOS device. This means it has a low power
consumption, typically 2mA or less at a clock speed of 4MHz and just a
few microamps in sleep mode. The 12F629 needs a 4.0 - 6.0V power supply
for XT, RC and LP oscillator configurations and a 4.5 - 5.5V supply for
the HS configuration.
The heart of any computing device is its processing core. The PIC has a
Harvard architecture, which uses separate memory access paths for
program and data. This improves performance, as program memory access
can be performed simultaneously with data memory access. Separate memory
access also allows the instruction word width to be different from the
data memory word width, a trick which the PIC designers have use to
great effect in implementing the RISC core.
Unlike most microcontrollers, the PIC has an eight-bit wide data word
and a 14-bit wide instruction word (earlier members of the family have
12-bit wide instruction words), allowing all PIC instructions to fit
into a single instruction word. This means that the program code is very
compact, the instructions execute faster and the design of the core is
simplified dramatically. As an example, Microchip quotes 2.24 times the
code density and 1.54 times the execution speed of a Motorola 68HC05 for
the same oscillator frequency (4MHz).
The 14-bit PIC core needs only 35 instructions and no 'strange'
addressing schemes, which simplifies programming. A PIC instruction
cycle takes four oscillator cycles. PIC instructions are decoded and
executed in one instruction cycle and in all case we talk about one
cycles for each instruction-execution. Fetching an instruction also takes
one instruction cycle.
The PIC has a pre-fetch pipeline, which allows an instruction to be
executed while the next instruction is being fetched. This allows most
instructions to execute in one instruction cycle; the exception being
where executing the instruction causes a change in the program counter.
The next instruction in the pipeline cannot be used and an extra machine
cycle is required to reload the pipeline. Examples of these include the
GOTO and RETURN instructions.
12F629 has 1024 instruction addresses for your program and this is the
space allowed for instructions and any table data. It also has 64
locations for SRAM and 22 for special function
It also has 128 bytes of EEPROM data, which lie outside the data space and are accessible via special purpose registers.
The EEPROM data can be read and modified at run-time and, unlike RAM
data, is retained when power is removed from the device.
The PIC devices based on the new 14-bit core support and in-circuit
serial programming mode. This is particularly relevant for the 12F629 because the program is kept in EEPROM.
A new program and EEPROM data can
be downloaded without having to disassemble the circuit or the
For example, I have designed a 12F629 based circuit which can operate
underwater. The 12F629 can be reprogrammed without having to open and
reseal the enclosure, and can even be reprogrammed underwater. The
serial programming mode means the programming circuit can be kept
The PIC core has an eight-deep hardware stack, which is used to store
return addresses for subroutine calls and the interrupt service routine.
The PIC12F629 can support eight different clock oscillator modes, which
are selected by programming the corresponding configuration fuses. The
LP, XT and HS modes cater for various frequency ranges of crystal or
ceramic resonator operation, as well as external clock input. Such an
oscillator is a must where accurate timing is required.
The RC mode requires only an external resistor and capacitor. The
frequency of oscillation is determined by the resistor and capacitor
values and, to a lesser extent, by the supply voltage and temperature.
This mode cuts down costs at the expense of reduced accuracy. Microchip
suggests that the resistor be kept in the range of 3k to 100k, to ensure oscillator stability and a reasonable degree of accuracy.
Similarly, a capacitor value of at least 20pF is suggested.
The PIC allows various power-on reset (POR) configurations. There are
three parts to the POR delay circuitry.
The power-up timer (PWRT) is a fixed internal RC timer of approximately
72 milliseconds. This timer is enabled by setting the PWRTE
After the PWRT has elapsed, the oscillator startup timer (OST) counts
1024 clock cycles for LP, XT and HS mode oscillators. This allows the
oscillator time to settle down to guarantee oscillator stability.
The MCLR pin allows for an external reset to be applied. It is usually
sufficient to hard wire this pin to Vdd and rely on the combination of
the PWRT and OST to provide a clean start up. A longer delay can be
applied by attaching external circuitry to the
A word of caution, though. The MCLR pin may be attached directly to Vdd,
but it must always be connected to ground via a resistor of at least 100
ohms. This resistor prevents a high current latch-up problem from
occurring. Thankfully this problem is not shared by other pins on the
The PIC devices support a power-down mode which shuts down the
oscillator, 'putting the device to sleep'. The PIC enters the power down
mode when the SLEEP instruction is executed. The primary purpose of this
mode is to conserve power, though it may also be used as a delay.
The normal power consumption, typically about 2mA, is very low, but this
drops to less than 10uA in SLEEP mode, which means that a small battery
can keep a PIC in SLEEP mode for a couple of years. This makes the PIC
suitable for number of circuits where very low power consumption is a major
Of course there isn't much point in having a PIC just sleeping. The
wake-up sequence runs the OST sequence for the oscillator to stabilise
(1024 oscillator cycles for LP, XT and HS mode oscillators, but
immediate for RC mode oscillators) and then resumes execution at the
instruction after the SLEEP instruction.
The PIC can be programmed to wake up on external interrupts and watch
dog timer expiry. Unfortunately, since the internal oscillator is turned
off, the timer/counter does not function and can't cause an interrupt.
This restricts time based wake-ups to the watchdog timer expiry which,
as mentioned below, is not entirely accurate. It does however still
allow the PIC to be asleep most of the time, waking periodically to
check for some condition, then going to sleep again.
Even the best designed circuits can sometimes freeze up or go a bit
berserk due to power spikes, the phase of the moon or other glitches.
When this happens with desktop
computers, it seems to be accepted as an annoying part of our high-tech
way of life. However if a microcontroller deeply embedded in an
appliance goes crazy, you end up with a washing machine stuck on 'spin'
or flooding the laundry. A watchdog timer provides a mechanism to help
increase the reliability of an embedded microcontroller.
The watchdog timer consists of a timer circuit which is built into the
PIC's reset circuitry. This timer must be periodically cleared by
executing a CLRWDT instruction. If the timer is allowed to expire, the
watchdog 'bites' and resets the PIC.
The timer period is approximately 18 milliseconds. The period is set by
an internal resistor and capacitor, so it is prone to minor change with
temperature, voltage and manufacturing variations. The period can be
lengthened up to approximately 2.3 seconds by using the prescaler available in the OPTION register.
The watchdog timer is enabled by programming the WDTE fuse in the
configuration fuses. As mentioned above, the timer may also be used to
wake the PIC from power-down mode.
The idea of an interrupt can be easily explained by an analogy. Consider
that you are sitting at your desk performing some task. You have a
phone, but it has a tiny indicator lamp instead of a ringer. This means
that you need to periodically stop what you are doing and check whether
the lamp is lit. If you don't do this often enough, you may miss a call;
do it too often and you will never get any of your work done. This is
analogous to polling.
Trying to write a program like this is very difficult, since the code
must be peppered with regular checks for external events. This situation
gets even more difficult if timing is critical.
Now replace the telephone's indicator lamp with the more familiar
ringer. You can now get on with your work. When the phone rings, you can
stop what you are doing, take the call, then get back to your original
task. This is analogous to interrupt handling.
When the microcontroller hardware detects an interrupt condition, it
saves the foreground task's current execution context on a stack, then
starts executing the interrupt handler code — starting at a special
location called the interrupt vector. This is location 04 in program
memory. When the interrupt handler
completes its task, it executes a 'return from interrupt' instruction
which restores the suspended execution context and the foreground task
continues as if nothing had happened. Since the hardware checks for the
interrupt events, all the software writer needs to do is specify a
single interrupt servicing routine and enable the appropriate
Interrupts allow the foreground processing code to be considered
separately from the background 'event handling' code. Interrupts really
help to reduce the coupling between the fore ground task and the
making the code more robust and easier to understand. A bonus is that
this code becomes more modular, which means that it is easier to build
reusable software components.
Here is the Instruction-set for the
PIC12F629. The letter "f" indicates a file (also called a register) from 20h to 5Fh.
The letter "d" is the destination for the result of the operation and when d=0,
the result will be placed in the W (working) register. If d=1 the result will be
placed in the file itself.
The letter "C" = Carry flag in the Status register. "DC" = Digit Carry in the
Status register. "Z" = Zero flag in the Status register.
These instructions are described in more detail in the
.pdf file for PIC12F629 and
on our Instruction Set page.
The PIC Instruction Set is very simple and can be
mastered in a short time. The table below shows the PIC12F629
Instruction Set in contracted form. For more details on each
ADDLW 00 to FF
Add literal to W
C, DC, Z
Add W to f
ANDLW 00 to
AND literal with W
AND W with f
Test bit, skip if clear
Test bit, skip if set
Clear watchdog timer
Decrement f, skip if zero
Increment f, skip if zero
OR literal with W
OR W with f
Move f to destination
00 to FF
Move literal to W
Move W to f
Load OPTION register
Return from interrupt
00 to FF
Return with literal in W
Rotate f left
Rotate f right
Enter sleep mode
00 to FF
Subtract W from k
C, DC, Z
Subtact W from f
C, DC, Z
Load tristate register
00 to FF
XOR literal with W
XOR W with f
The PIC has one working register, the W register. It has many purposes
and one of its simplest uses is a "carrier" file or register.
If you want to load one of the files (20h to 5F) with a value, the value
is initially loaded into W. The value is called a literal and can
be from 01 to 0FFh. These values are hex (hexadecimal) values. For the
table of hex values from 00 to FF see:
(Decimal to Binary to HEX).
The value is then moved from W to the required file
(this is a total of two instructions). W is also often used as a dummy
register where the results of an operation are to be discarded.
PIC's files are called data memory or 'data registers' or 'data
It is useful to identify the various operand types to understand how
the instructions work. The first type are the byte-oriented operations
on file registers. This is where all the 8 bits in a file are operated
on. A typical instruction is:
These typically have two operands, referred to as f and d, where f is
the file number (20h in this case) and d is the destination of the
result (1 in this case).
The f operand is a seven-bit address which selects which file
register is being manipulated. The d operand is a single bit which
selects the destination of the result; 0 selects the W register and 1
selects the file register specified in f.
Note that the two clear instructions, CLRF and CLRW, really fall into
this category although they do not have two operands. CLRF, the clear
file instruction, requires only the f operand and the destination is
implicitly the file register; CLRW, the clear W instruction, is
effectively a CLRF instruction with the destination set to W.
The second type is the bit-oriented file register operations. These
operations have two operands, f and b, a seven-bit file register
address and three-bit bit address respectively. These allow any bit in
the file registers to be set, cleared or tested.
The third type of operand requires a single literal byte operand. The
mnemonics for all of these operations end in LW, indicating a literal
value operation on the W register.
The last operand type is the 11-bit program address used by the GOTO and
Unconditional branching is supported by the GOTO and CALL instructions.
Conditional execution is supported by four instructions, DECFSZ, INCFSZ,
BTFSC and BTFSS. All these instructions skip
the execution of the next instruction if a certain test condition is
met. For example:
Here the DECFSZ instruction is the decrement test, decrementing
the "test" register and putting the result in the file itself.
The letter ,f indicates the result is placed in the file. The
instruction is skipped if the result of this decrementing was not zero.
In other words, the micro will normally go to the instruction below the DECFSZ
unless the result of the file is zero. This allows loops to be created
(such as for delays):
loop2 DECFSZ test,f
and when the result is zero, the micro advances down the program to
Do not use
if you want to decrement a file to zero (such as in a loop) as the
instruction will only put the result of the decrement into w and the
file will remain with its original value!
The PIC supports some special
instructions which manipulate special PIC features. These are the CLRWDT,
SLEEP, OPTION and TRISIO instructions.
Part of the reason why the PIC RISC core can be kept so simple is the
rich set of Special Function Registers (SFRs). The PIC12F629 supports 16 SFRs, which are used to access special features. All of the SFRs
are mapped into the PIC data space, and are accessed as if they were
normal RAM data locations.
IND0 and FSR: This pair allow data memory to be indirectly
accessed. The address of the data memory to be accessed is placed in the
FSR. FSR is file 04. The specified data memory can then be accessed as if it were in the
IND0 register. The following code fragment may explain this better.
; put value '10h' into FSR
IND0,W ;accessing INDO actually
accesses data at address '10'
Indirect memory access is often useful for iterative
(pronounced it'-ter-a-tive is an adjective
meaning repetitious) operations such as
reading or writing to buffers.
RTCC: The RTCC SFR gives access to the RTCC register mentioned
above. This register may be read or modified.
PCL and PCLATH: These registers allow access to the low
and high bytes of the current program counter (PC). The program counter
holds the ad dress of the instruction which is currently being
executed. Accessing the PC is not done commonly, except as a way to
perform table look-ups or implement state machines.
Note that modifying the program counter invalidates the instruction
pipeline. This means that an extra instruction cycle is required to load
the next instruction.
STATUS: The status register holds the current arithmetic status,
reset status and page preselector bits.
C is the carry/borrow bit. This bit is set if a carry out or
borrow out occurred in the last arithmetic instruction (ADDWF, ADDLW,
SUBWF, SUBLW). This bit is also modified by the rotate instructions (RLF
DC is the digit carry/borrow bit. Its semantics (the way it
works) are identical to
C for arithmetic operations, except that it reflects a carry out or
borrow out from bit 4 of the W register. This bit is not affected by
Z is the zero bit. This bit is set if the result of an
arithmetic, logical or MOVF instruction is zero.
PD is the power-down bit. This is set to 1 during power-up or by
a CLRWDT command. It is set to 0 by a SLEEP command.
TO is the time out bit. This is set to 1 during power up and by
the CRLWDT and SLEEP commands. This bit is set to zero by a watchdog
The PD and TO bits may be used in conjunction to determine the cause of
the last reset.
The PIC architecture supports 1024 bytes of data space (that is: 1024
lines of code or 1024 "instructions and table values etc").
RP1 and RP0 are the register page select bits for direct
addressing of data space.
IRP should be maintained at 0.
The PIC12F629 supports the entire 1024 locations of the data space. This
means a GOTO instruction can direct the micro to any part of the
OPTION: The option register controls some of the special
functions available on the PIC devices. The OPTION instruction is
used to load this register from the W register. By default all the bits
are 1. The OPTION register is called OPTION_REG
PS2, PS1 and PS0 are the prescaler bits. These control the
prescaler division rate. PSA is the prescaler assignment bit. This
assigns the prescaler to the TIMER (timer0) if PSA=0. If PSA=1 it
assigns the prescaler to the watchdog timer (WDT).
RTE controls the RTCC signal edge. The RTCC will increment on a low to
high transition if this bit is zero, otherwise the RTCC will increment
on a high to low transition. This is only useful if the RTS bit is set.
RTS selects the RTCC signal source. The RTCC will increment on internal
clock transitions if this bit is zero, other wise the RTCC will
increment on transitions of the RTCC pin.
INTEDG selects interrupt edge sensitivity of the INT pin (GPIO2). The
interrupt will occur on falling edges (high to low transitions) if this
bit is 1, other wise the interrupts will occur on rising edges (low
to high transitions).
GPPU enables the weak pull-ups on
GPIO pins when this bit is set to zero,
otherwise the pull-ups are disabled. The pull-ups are disabled by
INTCON: This register controls which interrupts are enabled and
identifies which interrupts have occurred. The interrupt mechanism is
very flexible, to allow it to be tuned to react only to specific events.
GPIF is the GPIO interrupt flag. This is set when any of the GPIO input pins change. This flag must be reset in software.
INTF is the interrupt flag. This is set when the GP2/INT interrupt
occurs. This flag must be reset in software.
T0IF is the Timer0 overflow interrupt flag. This is set when Timer0 overflows. This flag must be reset in software.
GIE is the global interrupt enable flag. If this flag is zero then no
interrupts are enabled, otherwise the interrupts specified.
GPIO and TRISIO: These control access to
the 12F629's six I/O pins. All of these pins can be used as
inputs and all can be used as output except GP3 (pin4) which is input
only. The TRISIO register controls whether the pins in GPIO are input or output. The TRISIO
register can be set just like any other register, providing you access
it in Bank 1. All output have a push-pull output
drivers. GPIO2 has a Schmitt trigger input which gives a
better noise immunity than the standard TTL level inputs of the other
All inputs have 'weak' pull-ups (typically 100uA - 47k).
The weak pull-up is only active while the pin is in input mode and only
if enabled by the RBPU bit in the OPTION register.
A word of caution. The actual write to an I/O pin only
happens at the end of an instruction cycle, but the input state is
latched at the beginning of a read operation. If a read instruction
follows a write instruction, it is suggested that at least one
instruction be inserted between the write and read, to allow the pin
voltage to stabilise for the read operation.
EEDATA, EEADR, EECON1 and EECON2: These four registers control
access to the 128 bytes of EEPROM data on the 12F629. EEDATA and EEADR
work in a similar fashion to the IND0 and FSR pair. The address of the
byte to be accessed is written into the EEADR register; the data at that
address can then be accessed via the EEDATA register. This is relatively
straightforward for reading the data, but writing the EEDATA is a bit
An EEPROM write cycle is a lengthy affair, taking up to 10 milliseconds.
The WR bit in the EECON1 register is cleared by the EEPROM programming
logic when the write cycle completes. The programming logic may also be
configured to generate an interrupt on completion of the write cycle.
The EECON1 register is used to access the EEPROM control logic. It has
the following five bits.
RD is the read control bit. Setting RD to 1 initiates an EEPROM read.
This bit is cleared after a read by hardware and can not be cleared in
software. A read takes one clock cycle.
WR is the write control bit. The EEPROM write cycle is initiated by per
forming a fixed sequence of accessing EECON2 then setting WR to 1. WR is
cleared by hardware when the write cycle has completed and, like the RD
bit, cannot be cleared in software. As mentioned above, the write
operation takes about 10 milliseconds.
WREN is the EEPROM write enable bit. All writes to EEPROM are inhibited
when this bit is zero, otherwise they are allowed.
WRERR is the write error flag. This bit is set if a reset occurs and
prevents the write operation from completing.
EEIF is the EEPROM write completion interrupt flag. This is set when a
write cycle completes. This will generate an interrupt if enabled by the
EEIE bit in the INTCON register.
EECON2 is not a physical register and is only used for performing part
of the write sequence.
The configuration word controls the PIC's hardware features and is not a
register or part of the program instruction space. It is not accessible
at runtime, but can be accessed by the programmer. The following
functions are supported.
FOSC1 and FOSC0 are used to select the oscillator type. By default, the
RC oscillator is configured.
WDTE enables the watchdog timer feature. This is the only way to control
the watchdog timer. In particular, the watchdog timer cannot be disabled
by software. The watchdog timer is enabled by default.
PWRTE is the power-up timer enable fuse. This controls the operation of
the power-up reset circuitry. This is enabled by default.
CP is the code protection fuse.
If code protection is enabled, reading the PIC will not produce any
meaningful output and the program cannot be over written. This prevents
unauthorised duplication of the program.
This fuse also serves two extra purposes in the 12F629. Firstly, it also
prevents the EEPROM data from being accessed from the programmer.
Secondly, it provides a mechanism for erasing the EEPROM data and
The configuration word is created by placing "CONFIG" in the first or
second column and putting a double underbar in front: __CONFIG
In the next column is placed a number of features that are required to
be implemented by the micro, such as code-protection on or off, watchdog
timer on or off etc. Here is a typical __config with 4 features (more are normally required)
__CONFIG _CP_OFF & _WDT_OFF & _PWRTE_ON & _INTRC_OSC_NOCLKOUT