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I am using MPLAB 6.50.

I could be doing something stupid, but I am having
trouble simulating a simple isr that is supposed to
happen every 208 cycles. When using the stopwatch, I
was getting 209. I simplified the problem in a test
project, and the symptoms are the same, TMR0 is losing
track of time.

Adding a value to TMR0 inhibits counting for 2 cycles
right? So I assume that adding 2 to TMR0 doesnt change
the net outcome?

I wrote a sample program to interupt naturally on TMR0
and the stopwatch says it interupts every 256 cycles
-- good.
Now if I add 2 to TMR0 during the interupt, it happens
every 257 cycles!

When I step through the code and watch TMR0, it doesnt
increment until the 4th instruction after the add,
whereas it should be the 3rd instruction.

Has anyone else noticed this problem, or am I missing
something? I have attached the test program if anyone
cares to give it a try.

Thanks,
- Ben



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Content-Type: text/plain; name="test.asm"
Content-Description: test.asm
Content-Disposition: inline; filename="test.asm"

        LIST    P=PIC12F629
        RADIX   dec

        INCLUDE "p12f629.inc"

        __CONFIG _INTRC_OSC_NOCLKOUT & _WDT_OFF

        CBLOCK 0x20   ; RAM starts at address 20h
W_TEMP
STATUS_TEMP
        ENDC


        org 0
        goto init
        nop
        nop
        nop

;------------------- INTERUPT ---------------------------
;as per data sheet
        MOVWF W_TEMP ;copy W to temp register,could be in either bank
        SWAPF STATUS,W ;swap status to be saved into W
        BCF STATUS,RP0 ;change to bank 0 regardless of current bank
        MOVWF STATUS_TEMP ;save status to bank 0 register
;------------------- ISR ---------------------------
isr:
        btfsc TMR0, 0   ;get rid of the jitters
        goto $+1

        bcf INTCON, T0IF

        movlw 2
        addwf TMR0, f
        movfw TMR0
        movfw TMR0
        movfw TMR0
        movfw TMR0      ;tmr0 incs during this instruction
        movfw TMR0      ;tmr0 doesnt inc untill the pc is at this line
        movfw TMR0
        movfw TMR0

;------------------- END INTERUPT ---------------------------

        SWAPF STATUS_TEMP,W;swap STATUS_TEMP register into W,
                                                ;sets bank to original state
        MOVWF STATUS ;move W into STATUS register
        SWAPF W_TEMP,F ;swap W_TEMP
        SWAPF W_TEMP,W ;swap W_TEMP into W
        retfie

;------------------- INIT ---------------------------
init:
        bcf STATUS, RP0 ;switch to bank 0

        clrf TMR0

        ;switch to bank 1 for option register
        bsf STATUS, RP0

        bsf OPTION_REG, PSA ;set prescaler to WDT (1:1 for tmr0)
        bcf OPTION_REG, T0CS ;timer 0 is a timer

        ;switch back to bank 0
        bcf STATUS, RP0

        clrf TMR0       ;start counting 2 instr from now
        bsf INTCON, T0IE        ;enable timer 0 interupts
        bsf INTCON, GIE         ;enable interupts

mainloop:

        nop             ;jitter simulator
        nop
        nop

        goto mainloop


        end

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