Hi everybody,

I am working on a routine with 12F675. Since the frequency of the ac line is quite predictable I am sensing the zero cross then starting the sample at almost the peak of the AC waveform. As you know that the average dc voltage is 0.707 times the ac RMS, so it is quite easy to sense it as long as it is sine wave. I am then shifting it out via an opto-coupler. By the way I am using a transformer less power supply (read non-isolated) for the 8 pin PIC.
Microchip has got an appnote on the same (PS).

Regards

Sougata