http://www.piclist.com/postbot.asp?id=3Dpiclist\2002\06\07\153942a

Thanks for the reply and yes I hadn't assigned the Prescaler in that =
example,BUT this is not the real problem.

I have written other code to select the prescaler for the TMR0 but is =
not that the PIC isn't counting/timing at the correct rate it's more =
drastic....=20

*** the TMR0 register is just RANDOMLY changing ****=20
I can see no sequence in the numbers at all. They appear to just change =
at will on every instruction.

When I run this in the simulator all is well and I get the TMR0 register =
changing as expected.=20
So why NOT on the real hardware???

I have now written it in assembler and still I must be missing something =
obvious...

; Use PIC16F84 and set radix system to decimal
        LIST P=3D16F877, R=3DDEC

;************************************************************************=
******
; Include header file
;************************************************************************=
******
#include "p16f877.inc"

#define TIMER_PRESET    180

;************************************************************************=
******
;                               equates
;************************************************************************=
******
; WORDS
status_temp             equ     0x0c
w_temp                  equ     0x0d


;************************************************************************=
******
;                            START of CODE
;************************************************************************=
******
        org     0x0                     ; reset vector
        goto    init                    ; jump over subroutines

        org     0x4                     ; Interrupts
        goto    isr                     ; Goto this service routine


;************************************************************************=
******
;                            initalisation
;************************************************************************=
******

init    clrf    PORTC                   ; set =3D 0
        bsf     STATUS,RP0              ; BANK1
        movlw   0x00
        movwf   TRISC

        movlw   b'11011111'             ;
        movwf   OPTION_REG              ; Set Timer ON and use prescaler =
of 256
        bcf     STATUS,RP0              ; Retrun to BANK0

        movlw   TIMER_PRESET
        movwf   TMR0                    ; Reset Timer0

        bsf     INTCON,T0IE             ; Turn Timer Interrupt On
        bsf     INTCON,INTE             ; Turn RB0 Change Interrupt On
        bsf     INTCON,GIE              ; Turn Global Interrupts On

;************************************************************************=
******
;                             main program
;************************************************************************=
******

main

background_loop
        goto    background_loop


 =
;************************************************************************=
******
;                       Interrupt Service Routines
;************************************************************************=
******
isr
; --------------------------- save context =
------------------------------------
context_save
        movwf   w_temp                  ; save off W and STATUS =
registers
        swapf   STATUS, w
        movwf   status_temp
        clrf    STATUS                  ; select Bank0

; ************************* TIMER TMR0 Interrupt =
******************************
tmr0_int
; ----------------------- Clear Interrupt Condition =
--------------------------
        bcf     INTCON,T0IF             ; Clear pending interrupt flag

; ------------- Reset TMR0 Timer State for next timed interrupt =
--------------
        movlw   TIMER_PRESET
        movwf   TMR0                    ; Reset Timer0
tmr0_int_end
; ******************** restore context saved on entry to ISR =
*******************
isr_end

context_restore
        swapf   status_temp, w          ; restore all registers
        movwf   STATUS
        swapf   w_temp, f
        swapf   w_temp, w

; -------------------- END of interrupt service routine =
-----------------------
        retfie                          ; Return from interrupt service =
routine
;************************************************************************=
******


;************************************************************************=
******
;************************************************************************=
******
        end
; ------------------------------ END OF FILE =
----------------------------------




---
Peter R Betts
http://www.piclist.com/member/PRB-TRL-AA7
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