Hi jose,

At 8Mhz, with a 1:1 prescaler on Timer1, and the count starting from 0, it will take over 32mS for the timer to overflow.

If you want it to take less time, you need to load a value in the TMR1H:L registers, so that it takes fewer "ticks" to make it overflow.

For 65uS, use 65406 ($FF7E).
That way it only takes 130 clock cycles to overflow.

@ 8Mhz, 1 clock cycle = 0.5uS [1/(8,000,000/4)]
and 130 * 0.5 = 65uS

But frankly, trying to get that kind of resolution using ON INTERRUPT is a gamble at best. ASM interrupts would be much better. Or, at least more predictable.

HTH,