>Micro uses RC clock & counter to generate a clock "tick"
>source at say 100Hz. If there is not at least one 50/60Hz
>pin input transition within two RC clock ticks then RTC
>clock source will be RC ticks, else RTC will use 50/60Hz
>ticks with divide by 50 or 60 to generate seconds. Seperate
>code determines if input is 50 or 60Hz... or perhaps this
>should be be a setup parameter as it could be too difiicult
>to distinguish 50Hz from 60Hz without an accurate timebase.

However I would also be tempted to have some sort of auto-calibration
routine that checked the average rate of your internal 100Hz against the
50/60Hz. By using an average over a 24hr period you should be able to deal
with RC clock variations as well. Should give your clock a more accurate
time if the power is off for a significant length of time.

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