PIC Light Chaser

This month I am continuing with the PIC projects that I started in August. To be able to build this circuit you must build the August circuit which allows you the ability to program PIC's.

The circuit this month is a simple 8 light chaser built around a PIC. This will demonstrate how easy it is to program a PIC and to utilize it in a circuit. The circuit works as follows. When power is supplied to the circuit the PIC resets and starts to process instructions that are programmed into it.
The program will turn on each LED in sequence with a small delay between each one. It will continue to do this until power is removed. The nice feature with this circuit is that you can program it to perform many complex lighting sequences. Normally you would have to rebuild a hardware based circuit to change the light sequence. With a PIC all you need to do is reprogram it and plug it back into the circuit. I am assuming that you will be using an IC socket. The circuit is show below and then I will discuss the program that is programmed into the PIC.

The program is listed below:

;File DEMO.ASM

;Assembly code for PIC16F84micro controller

;Blinks LED's on the outputs in a rotating pattern.

;With 75khz osc, each LED stays on half a second.

;CPU configuration

; (its a 16F84,RC Oscillator, watchdog timer off, power-up timer on)

processor 16f84

include <p16f84.inc>

_config _RC_OSC &_WDT_OFF &_PWRITE_ON

;Declare variables at 2 memory locations.

J equ H'1F';J=Address hex 1F

K equ H'1E' ;K=Address hex 1E

;Program

org 0 ;start at address 0

;Set port B as output and initialize it

movlw B'00000000' ;w : =00000000 binary

tris PORTB ;port B ctrl register := w

movlw B'00000001' ;w := 00000001 binary

movwf PORTB ;port B itself := w

;Rotate the bits of port B leftward

mloop: rlf PORTB,f

;Waste some time by executing nested loops.

movlw D'50' ;w := 50 decimal

movwf J ;J :=w

jloop: movwf K ;K :=w

kloop: decfsz J,f ;J = J -1, skip next if zero

goto kloop

decsz J,f ;J = J - 1, skip next if zero

goto jloop

;Do it all again

goto mloop

end

The program works as follows. The first few lines in the program are what is called comment lines. Comment lines assist us in documenting what each part of the programs function is. If a program is commented well, then it will be easier later own to understand why the program was written the way it was. Any line that begins with a semicolon is a comment line and will be ignored when the assembler is run. The assembler is another program that will convert these written instructions and convert them to binary data to be programmed into the PIC. The first true commands that the PIC will process is theprocessor, includeand_config.These instructions tells the assembler that it is using 16F84 instructions. The second instruction says to include a set of predefined constants in a file called P16F84.inc. Finally, the third instruction sets various configuration bits in the PIC to turn on the RC Oscillator, turn off the watch dog timer and turn on the automatic power up reset timer. That way the PIC will reboot every time power is applied. The twoequinstructions reserve memory space in the PIC's RAM for two variables, which is being called "J" and "K". The locations are in Hex 1E and 1F. Theses locations will be used to store counters to keep track of how many times a loop has been repeated. Theorginstruction tells the assembler that the program starts at location 0. in the program memory and that the actual program is next. The first real PIC instruction is a

movlwinstruction that clears the working register calledW. That number is then copied into theTRIScontrol register forPort B, setting pins 6 - 13 to outputs pins instead of input pins. Next the program puts binary00000001into theWregister and copies it toPort B. That lights the LED connected to pin 6. But before you have time to actually see the LED come on, the program executes anrlfinstruction that rotates the contents ofPort Bto the left , changing the data to00000010. That will light the LED on pin 7 instead. Repeating that instruction will give00000100, and then000010000and so on, making the LED's flash in a marching pattern. In between rotations, the program needs to wait about a half second so that action isn't to fast to see. That is the purpose of the delay loop. It stores the decimal number 50 in memory locations "J" and "K" usingdecfszinstruction to count down from 50 to 0. Thedecfszinstruction means " Decrement and skip next instruction if zero. The last instruction in the program ismloopwhich sends the program back to the beginning. Theendinstruction is not a CPU instruction. Instead it tells the assembler that the program is over. To compilethe program for the PIC we will be using a program that can be downloaded directly from the Microchip Web site. The program is called MPLAB. It is a developmental program for compiling and testing PIC programs. It will run under Microsoft Windows. The second piece of software that will be needed is the No Parts Programmer (NOPPP) that also works under Microsoft Windows. The use of the program is straight forward. You will load an object file that was created by MPLAB and program it into the PIC. After successfully programming the PIC and testing the circuit, try to modify the program to light the LEDS in a different pattern and then reprogram the PIC and test it.