60Hz timebase clock

Hello all, I am in the planning stages for a simple clock that uses numitron tubes for the display. That part is pretty much worked out. I decided against using a realtime clock chip as they do need an initial date setting and battery backup. My idea is to base it from the 60Hz power. For isolation and protection against spikes I plan to drive the LED side of an optic isolator using a small step down transformer. That should give nice clean 60Hz pulses on the output side. So far so good.

Now for the timekeeping part. How about preloading an 8 bit counter with 195 so it will overflow on the 60th pulse? Then the interrupt goes to the timekeeping code where each overflow is counted as a second. The time between these periods should be plenty for the necessary math and writing to the display.

Any comments appreciated.


Adam