Well I just read Darrels thread, & website on using instant interrupts. This should make what I want to do much easier. I can keep a running average of the frequency of the supply (which can dip using generators) then use the two opto method as in the paper to more reliably detect the zero cross point. Using a few timers to then sort out the firing of the thyristors.

Thanks to Darrel this may not be beyond me at all.

Just need to brush up on using hardware peripherals, rather than using PBP for ADC, timing etc.

Thank's for your ideas guys

Regards
Bob...