[time-nuts] Mains Frequency

[Andy Talbot]

Why should the microcontroller have a crystal at all?
Many have factory trimmed RC oscillators, typical 1% accuracy, because
accurate timing for other than timekeeping is rarely needed.
A minute per month is 10ppm, typical of a bog standard crystal, and given
the choice of that or mains timing for a clock, I'd use the latter any day.

Re the power supply, they may use direct rectification from mains
nowadays.   Capacitive dropper and bridge with a few smoothing and surge
suppression components.
My 20+ year old  microwave doesn't though - that has a real small
transformer in it that audibly hums .

Andy
www.g4jnt.com



On Fri, 12 Feb 2021 at 18:06, Lux, Jim <jim at luxfamily.com> wrote:

>
> Maybe.. you've got to condition the AC from the secondary side of the
> transformer and use a pin to bring it in on, which requires at least 2
> or 3 passive components, and you already have a crystal for the
> microcontroller (thinking here of oven timers and the like, which have a
> numeric display).  These applications are super price sensitive, and
> those 2 or 3 components cost money, in components, in board space, and
> in assembly costs. Pennies to be sure, but...
>
> And the fact that my appliances drift on the order of a minute in a
> month, differently. So maybe some count cycles and some have a rock.
>
>
>
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