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.

;File DEMO.ASM

;Assembly code for PIC16F84 micro 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.

For more detail: PIC Light Chaser