WARNING: Untested code ahead!

What we really need is a way to ensure that Timer 1 interrupts every 5000
instructions or 1 millisecond.

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.

Here is my suggestion (this code would be in the interrupt handler)

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

Also, sorry this is in ASM, not C, but the same thing should work in C.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)




----- Original Message -----
From: Brent Brown <brent.brown@CLEAR.NET.NZ>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, September 28, 2000 6:32 AM
Subject: Re: [PIC]: interrupt accuracy


> Hi Bryce,
>
> Welcome to the PIC list!
>
> Like Jinx said, I think you just need to reload the counter directly.
> Do it as soon as possible after the interrupt to minimize timing
> errors. TMR1 increments at 1/4 Fosc, with 20MHz XTAL that's
> 5MHz so you should be able to get 200ns resolution. Don't know
> how much to allow for interrupt latency. Try some code like this:-
>
> interrupt void int_routine(void){
>   if(TMR1IF){
>     TMR1H = 0xEC;
>     TMR1L = 0x77;
>     TMR1IF = 0;
>     count++;
>   }
> }
>
> Brent Brown
> Electronic Design Solutions
> 16 English Street
> Hamilton, New Zealand
> Ph/fax: +64 7 849 0069
> Mobile: 025 334 069
> eMail:  brent.brown@clear.net.nz
>
> --
> http://www.piclist.com hint: To leave the PICList
> mailto:piclist-unsubscribe-request@mitvma.mit.edu
>
>

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