> 110V input throught two 10K resistors in series an H11AA814A
> optocoupler. collector of optocoupler pulled to 5V via a 22k resistor.
> collector fed into GP2 a 12F629. the PIC is running on interal clock

I use something similar, works fine

http://home.clear.net.nz/pages/joecolquitt/txless.html

In your actual application you'll need to quantify when the PIC
reacts to the zero-cross pulse (ie when the z-c pulse passes
through the transition voltages to cause a "1" or "0"). This would
be most easily done with a dual trace scope. z-c pulse on one
channel, PIC reaction pulse on the other

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