The classic 70's LED clock used a 60Hz mains transformer and yes it was
easy enough to for the solid state electronics count pulses derived from
the low voltage secondary winding of the mains transformer. If your
appliances are from the 70's or 80's yes it is very likely the LED clock is
counting AC line frequency.

A vanishing small amount of consumer electronics in the past 10+ years uses
a mains transformer.. They will all be switching power supplies. This is
not just for cost savings, but to meet energy efficiency standards.

It is possible to get line frequency output from a switching power supply.

to have a line clock output but I haven't seen this as part of the power
supply in industrial equipment for decades, and never in consumer equipment
switching supplies.

Tim N3QE

On Fri, Feb 12, 2021 at 11:29 AM Thomas D. Erb tde@electrictime.com wrote:

On 2/12/21 8:23 AM, Thomas D. Erb 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.

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@luxfamily.com wrote:

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Mine peaked at +50s earlier today, it may have gone higher but didn't see.
Now down to +44s.
There may be a longer term offset, since this monitor was set arbitrarily
265.3 hours ago, and who knows what the official offset was at that point

Andy
www.g4jnt.com

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On Fri, 12 Feb 2021 at 18:55, Joseph B. Fitzgerald jfitzgerald@alum.wpi.edu
wrote:

at the time I grew up in Eastern Europe -- "communist time" -- they kept
he clocks using the line frequency as reference -- by counting the
periods during the day and week and for longer time for equal time
interval the "provided" equal number of line frequency periods, as
longer the time interval was as more precise was the time.  That way the
clocks were relative accurate. They could do it since everything was
"central governed".

On 2/12/2021 9:24 AM, Lux, Jim wrote:

It's still +47 seconds.    Perhaps they'll correct over the weekend.
Wind generation is high at the moment, contributing 13.64GW to the 34GW UK
total demand (it it 22:100 as I write).  Perhaps the high wind generation
level is the reason for the extended high frequency session.

Andy
www.g4jnt.com

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On Fri, 12 Feb 2021 at 21:38, Alex Pummer alex@pcscons.com wrote:

The joke in USA at that time was that the power companies adjusted their
frequency by calling the local telephone company (which was ATT/Bell
Telephone almost everywhere). In that era, you could call a local number
and get the time. The power companies used that time number, it was joked,
to adjust their frequency. The telephone companies, of course, just had an
electric clock that ran on the power line.

Jeremy

On Fri, Feb 12, 2021 at 1:38 PM Alex Pummer alex@pcscons.com wrote:

--
Jeremy Nichols
Sent from my iPad 6.

On Fri, 12 Feb 2021 18:23:54 +0000
Andy Talbot andy.g4jnt@gmail.com wrote:

Because you need accurate time or frequency.

E.g.: You have a USB connected device. The USB specs say
that the reference clock for the device must be accurate
to 0.2% (2000ppm) under all operation conditions (including
temperature). Yes, modern USB device implementations can get
away with a less accurate reference clock by locking the local
clock to the frame clock comming from USB. But that only works
for some classes of devices (i.e. has to run with 12MBit/s or less).
And it does not work for anything that can also be a USB host
as well (aka USB on the go).

Or: I was involved in the design of a logging device for shipment
tracking for insurance reason. Requirement from customer was to
achieve better than 10minutes over 2 years. That's 20ppm.
And we only got 10minutes after we told them that 1minute was
not physically possible given the size and power constraints.
And even that we only achieve when the parcel is constantly
in an air conditioned room, which, of course, is never the case.

Or: Any kind of radio/wireless application. Channel separation
requirements, even for low speed ISM band stuff are stringent
enough that you have to select your crystal carefully and can't
just take the cheapest one. Things that operate within the
2.4GHz band, like BT/BTLE, are even worse.

BTW: IoT devices are currently one of the major drivers behind
more accurate 32kHz crystals. Whether you have to wake up
for 10ms every hour or for 100ms makes a huge difference in
battery lifetime (in the order of factor 5). Similarly, cellphones
are a driving force behind (small) AT cut crystal accuracy..
or rather short-term drift. As less frequency drift means smaller
guard bands between different channels and within a channel. Which
directly translates into higher frequency utilization and thus
available bandwidth and money.

And we haven't even talked about anything that does precision
stuff, where having an accurate and stable clock source is often
paramount for having accurate measurment. Neither have we talked
about anything highspeed (i.e. beyond 50MHz) where timing margins
become low enough that being even 0.1% off would not do.

Keep in mind that the 1% RC oscillator is something relatively
new and they are 1% only at 25°C. Just 10 years ago, you
were lucky to get a device with an internal oscillator that would
be +/-10% at 25°C and 30% over temperature. Even a modern device
like the STM32F7xx family (IIRC 2-3 years old) is spec'ed at 4%
over temperature.

A standard AT cut crystal is 10-100ppm accuracy out of factory
at 25°C and with 100% accurate capacitive loading. After soldering,
you are probably off by another 10-30ppm. And, depending on the
actual cut angle, temperature variations add another 20-100ppm
on top of that. Yes, the "10ppm" value is misleading.

If you are talking about a 32kHz crystal, than its quadratic
temperaturure becomes a problem, E.g. at 0° you are already
off by an addtional -22ppm, at -10°C it's -43ppm. If we go
to the other extreme, it's -71ppm at 70°C and -106ppm at 80°C.
Those numbers, are of corse, if the temperature coefficient is
nominal. If you take the maximum tempco from the specs, the
numbers become -55ppm (-10°C), -28ppm (0°C), -91ppm (70°C)
and -136ppm (80°C). And we are still talking about quality
crystals here, with tightly controlled specs. A run of the
mill el-cheapo crystall will be quite a bit worse. Crystals
with >200ppm deviation over temperature are not uncommon.

Yes, this is a reason why Microcrystal crystals costs several
times of what you'd pay in China. And people are happy to pay that
premium as it shaves off a few dozen ppm from the end product and
crystals exhibit less aging, which in turn makes calibration
techniques work better. (My Swiss watches, after 20 years, are still
within 2ppm of nominal frequency over complete temperature range).

And, please do not forget that modern mains frequency control
is something quite recent as well. Especially outside (west) Europe.
Having mains frequency powered clocks being off several minutes
per month was the norm 50-70 years ago. This is, what drove
people to buy quartz crystal clocks back in the days.
Also, events have shown us, that gaining/losing a minute or two
within a month is still something you have to worry about
even with modern mains frequency control. Now think about places
where people don't have the Swiss, with their pedantic time keeping,
taking care of mains frequency.

		Attila Kinali

--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto