Hi,
some comments:
obviously, if the highest time you can reach with one rollover is 65536
usec, then it is NOT possible to reach a quarter second in one pass. Do
the following:
Preload TMR with 0x0C, which is, in turn 256 - 244. Then overflow will
occur in every 62464 usec. If you count it, the every 16th event has
nearby 1 seconds between them (999424 usec).
2nd remark: the BSF INTCON,GIE in the ISR is completely superfluous,
because of RETFIE exactly does it.
3rd remark: you must re-set INTCON.T0IF otherwise ISR will immediately
re-entered.

Context saving is necessary, I do not want to repeat comments already
done.
Regards,
Imre


On Wed, 8 Nov 2000, Hardware Engineering wrote:

> Never used interupts...but its time. Similar to another post of using TMR0,
> 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 do 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 then 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 interupts
> 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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