Hi Keith,

You asked about compensating for the first instructions within the ISR, but don't forget about the hardware call and any state savings on the stack that the compiler did for you before you arrived at the ISR. This amount is variable, depending on how the ISR was declared (are you using a different register set so it's not necessary to save everything?). You'll have to look at your generated asm code to see what compensations you should make for maximum accuracy.

Patrick's suggestion about T2 was the best though.

Beyond that, if you're doing long time intervals, remember that the xtal isn't extact...either initially or with time and temperature.

Dennis