Jake,
thanks for explanation. Since I am C programmer on PIC for long time, I could read your program and understood.

The question is when actually the spark occurs. When GP1 is high or GP1 is low.

My test circuit fires spark when GP1's falling edge. So according to your program it means after dwell time (500usec).

>4. This interrupt service routine (the code you posted) turns on the GP1 pin, which tied to the SCR. It then
>waits the dwell time and turns the pin back off.

Jay.