I am a fairly new user of PIC controllers.  I am using a PIC 16C74B to flash
a set of LEDs, located on PORTB, on and off at 5 second intervals.  I know
that there are several ways to accomplish this task; however, I have chosen
to user Timer2 with PostScaler=10, PR=250, and PreScaler=4 using a 4Mhz
clock yielding Internal clock rate of 1uS.  By my calculation this produces
an Interrupt every 10mS, which works fine.

Needing a delay of 5 secs forced the implementation of a inner and outer
loop, where the inner loop is executed 5 times and the outer loop is
executed a 100 times.  The problem is that when I simulate this in MPLAB and
use the StopWatch to view the number cycles, my program locks up around 50ms
-62ms.  If load this code into the PIC, the LEDS come on and stay on.  Is
there is glitch in my program, MPLAB, or I just missing it.

Thanks

Ronald

The code is shown Below:

                ORG     0x000                   ; processor reset vector
                clrf    PCLATH                  ; ensure page bits are
claared
                goto    main                    ; go to beginning of program


                ORG     0x004                   ; interrupt vector location
                movwf   w_temp                  ; save off current W
register contents
                movf    STATUS,w                ; move status register into
W register
                bcf     STATUS,RP0              ; ensure file register bank
set to 0
                movwf   status_temp             ; save off contents of
STATUS register
                movf    DelayValue,w            ; move status register into
W register
                movwf   DelayTemp               ; save off current
DelayValue contents






                btfsc   PIR1,TMR2IF             ; Handler for Timer2
                call    IntTimer2               ; Interrupt Handler for
Timer2

                bcf     STATUS,RP0              ; ensure file register bank
set to 0
                movf    status_temp,w           ; retrieve copy of STATUS
register
                movwf   STATUS                  ; restore pre-isr STATUS
register contents
                movf    DelayTemp,W             ; retrieve copy of delay
register
                movwf   DelayValue              ; restore pre-isr DelayValue
register
                swapf   w_temp,f
                swapf   w_temp,w                ; restore pre-isr W register
contents
                retfie                          ; return from interrupt
IntTimer2:
                bcf     PIR1,TMR2IF             ; Must Clear Int flag for
Timer2
                decf    Counter,F               ; Decrease Counter
                return



main:
                bcf     STATUS,RP0              ;
                movlw   0x04                    ;
                movwf   Counter                 ;
                movlw   0x63
                movwf   Outer

                call    InitTimer2              ;

                movlw   0xc0                    ;
                movwf   INTCON                  ;

                movlw   0x49                    ; Prescaler=16,
PostScaler=16
                movwf   T2CON                   ; Since OSC = 4Mhz timer =
1uS
                bsf     T2CON,TMR2ON            ; Start Timer

MainLoop:
                call    DecOuter
                call    LED
                goto    MainLoop

;***************************************************************************
******************
;*      Everything in this section represents either initialization code or
*
;*      helper functions.
*
;*
*
;*                      Ronald J.Cotton
*
;*                      02/02/2001
*
;***************************************************************************
******************


DecOuter:       decf    Outer,F
DecInner:       btfss   Counter,7               ;
                goto    DecInner
                movlw   0x05                    ;
                addwf   Counter,F               ; Put Counter back to 50
                btfss   Outer,7
                goto    DecOuter
                goto    ReInit
ReInit:
                movlw   0x64
                addwf   Outer,F
                return


LED:
                comf    PORTB,F                 ; Toggle PORTB
                return

InitTimer2:
                bcf     STATUS,RP0              ; Ensure Reg Bank 0
                clrf    T2CON                   ; Stop Timer2, Pre=1:1,
Post=1:1
                clrf    TMR2                    ; Clear Timer2 register
                clrf    PIR1                    ; Clear Interrupt Flag
register
                bsf     STATUS,RP0              ; Bank1
                clrf    PIE1                    ; Disable all Peripheral
Interrupts
                bsf     PIE1,TMR2IE             ; Enable only Timer2
                movlw   0xf9                    ; Timeout Value = 249
                movwf   PR2                     ;

                clrf    TRISB                   ; Make PortB output
                bcf     STATUS,RP0              ; Back to Bank0
                movlw   0xff                    ; All on
                movwf   PORTB                   ; Initialize LEDS to be on
                return


                END                             ; directive 'end of program'

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