And I guess it makes sense that the overhead is in entering the interupt. Otherwise those cycles would be part of the "timer cycle" and not have to be adjusted for in the preload. I think at this point I have 2 options, either use pure ASM interupts and/or increase my timing to 8uS. Maybe I will look at how many instructions it takes to enter the interupt if using pure ASM. This will be the deciding factor I guess. Clearly there is some lag needed to enter the handler, but is it 24 cycles?