After such an exhaustive, informative and descriptive answer you definitely need to open a bottle of beer and that's on me!

Thanks a million Stefan !

you have shown full steps on how to do the job with timers,and managing with only one phase ZCD pulses.

In our country, the power supply is very erratic and the mains freq. and Voltage are fluctuating with load conditions. In summers, we have power cuts/grid failures. Many people here have back up power supplies like Inverters and Generators. until recently most of the inverters owned by the people are Squarewave type or modified sine wave type. Pure sine wave inverters have come into market just 2 years back.

My design has to survive the freq & Voltage Fluctuations in these inverters.

so the idea of loading precalculated half period value into the TR is not a failsafe option.

i am working on 1Meg ZCD scheme to convert the square Wave on the i/o pin to continuous train of pulses for both phases of Zero crossings.

My idea is to use a Bridge infront of 1Meg resistor and use one more 1Meg resistor from i/o pin to GND, this results in positive falling pulses at the i/o pin at intervals of 10ms ( @ 50Hz freq)

i've simulated the above set up on Proteus, and will post the pics of the testing.

once again, thanks for sharing your thoughts and help.

Thanks Erik, can you explain me how to use a Zener to accomplish the job, and Zeners are dirt cheap here.

Regards,

Arun