I have a three digit LED display for a thermometer.  I had the multiplexing
working fine for the three digits when it was in my main code.  I simply
decremented a variable that was initialized with a value of 3.  For each
value, I turned on the respective digit, delayed for a while, and went to
the next one.  I wanted to make changes to the code that would affect the
time delay between displays so I thought I should put the digit
multiplexing code in a routine that would be called by a
TIMER0 rollover-initiated interrupt.  I tested the scheme with a called
routine that simply toggled an output, and the interrupt stuff all worked
well.

Once I got that far, I started working on the multiplexing math and logic
but have had no success.  This is my first time at using interrupts and I
suspect that I am doing something wrong that is messing up my code.  I show
three versions of my routine below.  The first worked fine and showed me
that I had the interrupt stuff right.  The next two show simple attempts at
asserting three outputs in succession, however without success.  Can
someone tell me the errors of my ways here?!?!?!?!  Plus, is there a
slicker easier way to do this?

Thanks.

Mark Peterson

DISPLAY
      movwf _w          ; Save
      swapf status,w          ; Save
      movwf _status           ; Save
      bcf   intcon,2          ; Clear interrupt bit

      movfw LOOP        ; LOOP to w
      movwf portb       ; Send to port

      LOOPDEC
      decfsz      LOOP        ; Equal to 0?
       goto       $+3         ; No, leave and do it again later.
      movlw d'3'        ; Yes, reinitialize LOOP variable.
      movwf LOOP        ; Yes, reinitialize LOOP variable.

      swapf _status,w   ; Restore.
      movwf status            ; Restore.
      swapf _w          ; Restore.
      swapf _w,w        ; Restore.
      retfie                  ; Return.

With the code above, bits 0 and 1 on Portb step through 11, 10, 01 with
each rollover of TIMER0.  With this success, I moved on to transform the
LOOP count into individual bit assertions.


DISPLAY
      movwf _w          ; Save
      swapf status,w          ; Save
      movwf _status           ; Save
      bcf   intcon,2          ; Clear interrupt bit

      movfw LOOP        ; LOOP to w
      call  DIGIT_T
      movwf portb

      LOOPDEC
      decfsz      LOOP        ; Equal to 0?
       goto       $+3         ; No, leave and do it again later.
      movlw d'3'        ; Yes, reinitialize LOOP variable.
      movwf LOOP        ; Yes, reinitialize LOOP variable.

      swapf _status,w   ; Restore.
      movwf status            ; Restore.
      swapf _w          ; Restore.
      swapf _w,w        ; Restore.
      retfie                  ; Return.

****Tabel located in main code******
DIGIT_T     ; 3 digit display table
      addwf pcl,f       ; Jump into lookup table
            ; _____321'
      retlw b'11111011' ; Segment 3
      retlw b'11111101' ; Segment 2
      retlw b'11111110' ; Segment 1

With the code above, portb bits 0, 1, & 2 are on all the time.  It's a
simple lookup table yet I can't see what I did wrong.


DISPLAY
      movwf _w          ; Save
      swapf status,w          ; Save
      movwf _status           ; Save
      bcf   intcon,2          ; Clear interrupt bit

      bsf   portb,0           ; Turn off 100 digit
      bsf   portb,1           ; Turn off 10 digit
      bsf   portb,2           ; Turn off 1 digit

      movfw LOOP        ; LOOP to w
      sublw d'3'        ; 3 - LOOP in w
      btfss status,2          ; LOOP = 3?
       goto LOOPDEC     ; No, leave.
      bcf   portb,2           ; Yes, turn on 100 digit
      goto  LOOPDEC     ; Yes, leave.

      movfw LOOP        ; LOOP to w
      sublw d'2'        ; 2 - LOOP in w
      btfss status,2          ; LOOP = 2?
       goto LOOPDEC     ; No, leave.
      bcf   portb,1           ; Yes, turn on 10 digit
      goto  LOOPDEC     ; Yes, leave.

      movfw LOOP        ; LOOP to w
      sublw d'1'        ; 1 - LOOP in w
      btfss status,2          ; LOOP = 1?
       goto LOOPDEC     ; No, leave.
      bcf   portb,0           ; Yes, turn on 1 digit
      goto  LOOPDEC     ; Yes, leave.

      LOOPDEC

      decfsz      LOOP        ; Equal to 0?
       goto       $+3         ; No, leave and do it again later.
      movlw d'3'        ; Yes, reinitialize LOOP variable.
      movwf LOOP        ; Yes, reinitialize LOOP variable.

      swapf _status,w   ; Restore.
      movwf status            ; Restore.
      swapf _w          ; Restore.
      swapf _w,w        ; Restore.
      retfie                  ; Return.

With the code above, portb,2 flashes at the rate that TIMER0 rolls over but
bits 0 and 1 are on steady.  Again, it's straight forawrd stuff that works
in regular main code but I can't get to work inside the interrupt-called
routine.


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