A few months ago, I was informed that my project team would be needing me to
write some firmware for the PIC 16f877.  Since I won't be taking my
"Introduction to Microcontrollers" class until this summer, I began to teach
myself about assembler, the chip and MPLAB.  After running around in circles for
quite awhile, I found a copy of Myke's book and finally started making some real
progress. (THANKS MYKE!!!)

Earlier this week, I finally felt comfortable enough to start writing some code.
My team leader advised me to become very familiar with interrupts, so  I decided
to design an 8-bit binary counter, utilizing an interrupt routine.  The
algorithm is simple:

     All necessary registers are initialized
     The program waits in MAIN for TMR1 to overflow, setting TMR1IF
     The value in COUNTER is written to PORTC (which is connected to a bank of
LEDs)
     The COUNTER is incremented and the whole thing starts again.

I spent a few days stomping bugs using MPLAB SIM.  Once I got everything
working, I loaded my demo board and used MPLAB ICD to see it in action.  I
expected everything to work perfectly (silly me).  However, I have observed
three problems which only occur In ICD.

     1.  When I clear TRISC, bits <1:0> do not reset.

     2.  When I initialize TMR1H, TMR1L does not increment.  This is pretty
minor, but I        want to understand why this is happening.

     3.  When the TMR1IF sets, the program does not go to the interrupt vector.
It just stays in the     Main loop indefinitely.

These problems occur every time I load the program, and ONLY while stepping
through the code in ICD; everything is still fine if I step through in SIM.

Before I drive myself crazy trying to fix my code, I'm wondering if anyone has
seen anything like this before.  It occurs to me that I could have a bad chip,
but I can't ask around for a new one until Monday.

So what do you think?  I'd appreciate any pointers, because this behavior seems
really strange to me.

Thanks,
Wendy

;*************************************************************
;*               cntrdemo.asm
;*    A demonstration program for the PIC 16f877
;*

;*


;*                Wendy Olend
;*               24 April 2001
;*


;*************************************************************

   list p=16f877              ;include files
   include"p16f877.inc"

COUNTER   equ  0x2d ;register declaration

   org         0x00
   goto        Initialize

   org         0x04
Interrupt
   movf        COUNTER,w ;copy counter value and
   movwf  PORTC          ;send it to PORTC
   incf        COUNTER,f ;increment counter
   bcf         PIR1,0         ;reset TMR1IF
   movlw       0xf0      ;TEST ONLY
   movwf  TMR1L          ;TMR1 will count from 0000-FFFF
   movlw       0xff      ;in actual execution
   movwf  TMR1H
   retfie

Initialize
   bsf         STATUS,RP0     ;switch to bank1
   bsf         INTCON,7  ;EN GIE
   bsf         INTCON,6  ;EN peripherial interrupts
   bsf         PIE1,0         ;EN TMR1 interrupt
   clrf        TRISC          ;PORTC all outputs
   bcf         STATUS,RP0     ;switch to bank0
   clrf        PORTC          ;make sure PORTC is clear
   movlw       b'00001101'    ;TMR1 setup prescaler 1:1,osc EN,
   movwf  T1CON          ;internal clock, timer EN

   movlw       0xf0      ;TEST ONLY
   movwf  TMR1L          ;TMR1 will count from 0000-FFFF
   movlw       0xff      ;in actual execution
   movwf  TMR1H

   bcf         PIR1,0         ;clr TMR1 overflow flag
   movlw       0x01      ;initialize counter
   movwf  COUNTER
Main
  goto         Main      ;wait for TMR1 to overflow
                    ;then goto interrupt vector
  END

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