> The problem is that there can be jitter on reaching the interrupt handler
> for the timer. This jitter is caused by 2-cycle instructions, other
> interrupt handlers, and interrupts being disabled at 'task' level.
>
> We can get around the jitter problem by always adding a constant to the
> timer, rather than trying to set it directly. What we are doing is
> effectively advancing time forward the rest of the 65536-5000 instructions
> to get the timer to interrupt again at the right point.
>
> ...
>
> fudge = ...number to account for instructions timer is turned off...
> magic_value = D'65536'-D'5000'+fudge
>
>     bcf        T1CON,TMR1ON    ; turn off timer
>     movlw   low(magic_value)
>     addwf   TMR1L,F
>     btfsc     STATUS,C
>     incf       TMR1H,F
>     movlw   high(magic_value)
>     addwf   TMR1H,F
>     bsf        T1CON,TMR1ON    ; turn it back on

You could do this an alternate way by leaving timer 1 alone to generate
regular interrupts.  You would then do the real interrupt task in the
interrupt service routine after waiting for timer1 to reach some small value
that is at least as large as what the longest latency could be.  Let's
assume the longest latency is well less than 256 instructions, which means
you can deal exclusively with TMR1L.  You could then use TMR1L to index into
a list of NOPs to de-jitter the interrupt.  Timer 1 would always be at a
guaranteed known value (and therefore this would be happening at a regular
time with no jitter) at the end of the NOPs.

Here's the general concept without some details:

         movwf   tmr1l, w    ;get timer 1 low byte
         addwf   pcl         ;skip ahead depending on how far along timer 1 is
         nop                 ;NOPs must be in same 256 word block as ADDWF PCL
         nop
         .
         .
         nop
         nop
;
;   Timer 1 is guaranteed to be exactly XXXX here.
;


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@cognivis.com, http://www.cognivis.com

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