Derek ^ wrote:

The traditional approach assuming the outages were brief, was to use a
cheap oscillator (an astable mutivibrator) running from a back up
battery when the mains feed was down.

Yes, that's why I was careful to say "the biggest problem with using the
mains alone ..."

Cheap clock radios are the only things I've seen that have used crude RC
oscillators to cover mains outages, and their timekeeping is utter crap
- 5 min error in half an hour!

--
Andy