But is stable for each board?

I remember that we discussed the same issue in the past. I recommended a circuit in

http://www.8052.com/forum/read.phtml?id=98954

It is:


I just have tested, whether there is a steep zero crossing with the LED and 1N6263. There is! No oscillation, or noise, just a steep zero crossing. So, signal is ok.

One disadvantage of using the built-in comparator of your micro is, that offset voltage drift isn't specified. This could indeed be the cause of mistriggering after some time!

Another cause of mistriggering could come from noisy supply voltage. So, you should filter the point where the parts of comparator are connected to supply voltage.

Prahlad, this circuit should work properly, like the original circuit from National does. But maybe an additional comparator is needed, instead of the internal one of your micro.

One last hint: Please don't omit the LED. Without it the transformer injects current into your supply voltage.

Kai