Never used interupts...but its time. Similar to another post of using TMR=
0,
where he wants to use a 4MHz clock (giving a 1MHz or 1uS state time) and =
time
for 1 second, I need to do a similar thing.  But I think the math is off,=

because if you use a /256 on the prescaler thats 256uS per tic for TMR0, =
and
you can only have a max of 256 tic for TMR0 before it rolls over thats
65,536us (a bit short of 1000000us for 1 second). Now the purpose of my
application is to generate a 1 second rate for several timers.  I could d=
o a
simple loop where I just delay and the decrement the count for the other
timers, but I need to have several timers running in parallel.

Since I can't generate a 1 second interupt (could change the clock but th=
en we
get into custom crystals as well) I figure that I set TMR0 to a value of =
159
(decimal) and the prescale to 256.  This would give me pretty close to a
quarter second for the interupt.  Thus when vectoring to the interupt, it=

would reload the value to TMR0, decrement a QTRCNT value, check for zero.=
  If
zero, that means 4 quarter seconds passed and thus one second....THEN....=
I can
decrement all my other timers...sounds simple? Not sure..how the interupt=
s
need to be set up and all that.  My init code has this:

; set up TIMER0 for interupt vector for every quarter second
        movlw   0x07
        movwf   OPTION_REG

        movlw   0xA0
        movwf   INTCON  ; set interupt for TMR0

        movlw   0x9E    ; intial set for overflow value
        movwf   TMR0    ;  to be reloaded on each interupt
        movlw   0x04    ; for 4 quarter seconds
        movwf   QTRCNT  =


Then...for the interupt....

        org     0x004   ; interupt vector
        movlw   0x9E    ; intial set for overflow value
        movwf   TMR0    ;  to be reloaded on each interupt
        decfsz  QTRCNT,1
        goto    NOTZ
        movlw   0x04
        movwf   QTRCNT  ; we reached the count so reload
        < decrement all the counters etc>
        bsf     INTCON, GIE             ; enable interupts again
        retfie
NOTZ:   =

;       bsf     PORTC,0
        bsf     INTCON, GIE             ; enable interupts again
        retfie

Comments?

Again, thanks for all the help on this list.  =


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