>>
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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