We have used opto-isolators to trigger an interrupt on the 8052 on zero-crossing. We have also used a 2N2222 circuit.

I put a little hysteresis in the software, though. After the interrupt, don't enable it again for at least half the expected period.

After start-up, I use a free-running timer to check several interrupt periods. I store the value and average. Then I check to see if any readings are more than nominally different than the others. I had to put this code in to make it work on both 50 Hz and 60 Hz...