Your theory seems correct so it's either hardware or software.  If
hardware, the most common problem would be using a series cut crystal
instead of a parallel cut.  To check the software use the Microchip
simulator.  Set a break point in the interrupt routine where you increment
your 1 ms counter, run to the break point, open the stopwatch and reset it,
run to next break point. you should be within a cycle of 1000, run it a few
times more to be sure of the behavior.

Even though I have an ICE2000, I test as much of the code as I can during
development with the simulator first.

Bob Bullock
Embedded Systems Consultant (currently available)

At 07:44 AM 11/26/2002 -0500, you wrote:
> >>
>I'm running PIC with 4MHz crystal oscillator and I've been trying to
>implement a clock using TMR2 interrupt. I initialized TMR2 with
>prescaler 4, postscaler 1:1 and period 250. So, I'm getting interrupts
>every millisecond. When a counter inside ISR reaches 1000 I increment
>seconds and zero the counter. Am I right so far?
><<
>
>Yep.  I've done this many times.  Your clock should be as accurate as your
>crystal.
>
> >>
>Anyway, clock lags
>behind about a second in 4 minutes.
><<
>
>There is something seriously wrong here.  Either your crystal isn't what you
>think it is (is it specified for parallel operation?), or you have a bug
>that missing some ticks.
>
>
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