> The WDT is spec'ed a bit better than this, but not by much. Probably
> a borderline case; works mostly. OTOH, the internal oscillator many
> PICs have is plenty good enough

Yes, it is. My immediate thought was the original WDT's 18ms period,
which is between the two mains cycle times, 16.67ms (60Hz) and 20ms
(50Hz). Personally I've found WDT to be quite reliable in that regard
but, as you say, there are other options

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