CHEAPIC

This tutorial attempts to explain how to get started programming PIC  
microcontrollers in the cheapest possible way.  To do this you will need 
the following:

1) IBM compatable PC.  An old 286 or 486 machine will do fine if it has 
   a COM port.  I assume you will be using DOS.

2) A text editor.  I use SLED, (Sam's little editor), written in the 
   '80's by Sam Willmont.  Only 17K in size, it is free.  get it at:
          ftp://ftp.funet.fi/pub/msdos/Simtel/editor
          download the file sled11.zip ,(31K)

3) An assembler.  MPASM from Microchip is free.  If you stick to just 
   the basics, it is pretty easy to use.  get it at:
   http://www.microchip.com/10/Tools/Picmicro/code/MPASM/asm2300/index.htm
   Once you unzip the file, throw away all files except mpasm.exe and 
   p16f84.inc.  
   The manual can be found at:
   http://www.microchip.com/10/Tools/PICmicro/code/MPASM/33014g/index.htm
   But it is in computerese.  Probably best to wait until you really need 
   it.

4) A programmer.  The one I will talk about appears at:
     http://websites.ntl.com/~matthew.rowe/micros/prog.htm 
   for other info from Matthew Rowe leave out the prog.htm, (keep the /)      
   I have pictures and construction notes for this programmer in the file
   com84bd.zip

   Another simple serial programmer I have built and works is at:
          http://www.jdm.homepage.dk/newpic.htm
           click on 'Ultra Lowcost for PIC16C84 Only'

   Both programmers get their power from the COM port.  Software I use with
   these programmers is called PIX. It can be found at:
          http://home5.swipnet.se/~w-53783
         download the latest version pix113b.zip

5) A PIC.  The PIC16F84 as an obvious choice.  Other OTP, (One Time 
   Programable), PICs require more expensive programmers, UV erasers etc.
   The data sheet for the 'F84' can be found at:
          http://www.microchip.com/10/Lit/PICmicro/16F8X/index.htm
            click on 30430c.pdf
   This is quite technical and better left as a reference also.
   
   One of the cheapest sources for the PIC16F84, (4mHz), and also the 78L05 
   used in the programmer is:
          http://www.phanderson.com/ordering.html

6) Various small parts.  Watch crystal, capacitors, LEDs, batteries etc.  
   I choose to run the PIC off three AA batteries.  This frees you from 
   the A.C. line, but means you have to watch current consumption.
   I have pictures and description of the demo board for these programs 
   in the file called cheapbd.zip

7)  Directions.  That is what this tutorial is all about.  Hopefully it 
    will save you the cost of a $30-$40 book.  Many short programs will 
    be presented.  You can assemble, run and then modify these to teach 
    yourself PIC programming. 

Text Editor:

You will be amazed at the capability of this little editor.  It will 
hold may separate text files at once with the capability to cut and 
paste between them.  Take some time to play with the editor and become 
familiar with its use.  Especially note operations using the keys F3, 
F4, F7, F8, alt-x, alt-c and alt-y.

Assembler:

You use the editor to write a program of instructions which you feed to 
the assembler, (MPASM).  Instructions occur one to a line and are made 
of short abbreviations that indicate what the instruction does.  One of 
these is NOP which stands for "no operation".  

To check out MPASM, try to assemble just one line with the instruction 
nop, ( capitals or lower case, doesn't matter). Create this one line 
file in the text editor.  Name this file 'nop.asm'.  You can run MPASM 
from the editor by using alt-y. When you type 'mpasm nop' you will 
probably get 2 errors.  A further explaination of errors is found in the 
error file 'nop.err'.

Hit the spacebar until you return to the program you were editing, ( 
nop.asm ), and load nop.err using F3.  One of the errors complains of no 
end statement, the other says you haven't identified the PIC you are 
using.  Hit F8 to return to nop.asm and add lines to make it read:

list p=16F84
nop
end

Save the file, (F4), and re-assemble, (alt-y, mpasm nop).  This time you 
will probably get no errors but 3 warnings.  Warnings can be ignored if 
you know what they mean and can ignore the consequences.  Errors always 
have to be fixed.  Using F3 you can load the new 'nop.err' file, (the 
old one is also still there).  The warnings state that each of the three 
lines started in column one.  Column one is reserved for 'labels' which 
identify the location in the program of the next instruction. 

Since we don't really need any labels, put a space or three at the 
beginning of each of the lines and re-compile.  This time you should get 
no errors or warnings.  If you use alt-y and type 'dir nop.*'  you will 
see there are 5 files with names beginning with 'nop'.  If you load the 
latest 'nop.err' you will find it blank, (get rid of the other 2 with 
F7).  

Some of the other files are quite interesting.  The file 'nop.lst' 
contains much useful information. Load it using F3 and look it over. 

You will find the program lines we just entered with a line number, 
in decimal, under 'LINE'.  For one line you will also find two 4 digit 
hexidecimal numbers.  These are the actual numbers and locations they 
will be entered in the PIC's program memory.  Each program memory 
location is 14 bits in size so it takes 4 hex digits to specify what goes 
in there, (under OBJECT CODE).  There are 1024 program memory locations
so it also takes 4 hex digits to specify the location, (under 'LOC').

You will also find the number of program memory words used, (1), and the 
number left, (1023).  

Programmer:

You follow this same process: edit source - assemble - check and correct 
errors in the source - re-assemble, until you get no errors on assembly.
When you finally get no errors you 'burn' the '.HEX' file, 'nop.hex' into
a PIC with the programmer.  You then take the PIC, place it in an actual 
circuit and start it up.  It may not do what you want, or nothing at 
all.  You then try to guess what is wrong, go back to the original 
source and do it all over again.  Fun huh?

The programs:

In the zip file with this text you should also find:

( bincnt.asm ) - Count in binary on 4 LEDs - registers; ports, the W 
register and TMR0 - use of T0IF in delays

( swcnt.asm ) - Count switch closures, display on LEDs - subroutines - 
debouncing switches - defining registers - instruction execution time

( eggtimer.asm ) - Countdown timer, minutes and 10's sec on LEDs - 
interrupt routines - saving without changing STATUS - addition and 
subtraction

( morsenbr.asm ) - Send numbers 0-9 out on piezo speaker - using tables

( xtal0.txt ) - discussion of using watch crystal and TMR0 with 16F84

I'm not finished.  I need your input.  send comments, questions etc to:

ockers@anl.gov