Hi,
For some appliances however, I question if a clock display is really
needed. A toaster certainly wouldn't need anything more than an RC. A
couple of percent error on how long I warm up lunch probably won't matter!
However, some appliances and consumer grade equipment do utilize RC
oscillators. We have a remote controlled fan we've had in the house for
about a decade. The remote quit working, and being the cheapskate I am,
decided to look into it.
Long story short, the remote and receiver in the fan both used micro
controllers that have internal RC's for clocks. After a decade one or
the other drifted far enough out of spec that it didn't work. I luckily
stumbled across the fact that heating the micro on the fan side allowed
the remote to work again.
The End fix was to RTV a resistor to the top of the micro and wire it
across 5V unregulated power. The little bit of heat corrected the
frequency enough that the remote works better than it ever did.
Unfortunately, I don't have any ADEV plots or similar of the oscillator.
It would have been neat to measure the frequency, and change in
frequency over temperature. But there's no clock output pin. I couldn't
come up with an easy way to pick that RC up outside of the micro.
The irony of the whole exercise, is that the fan effectively needed to
have it's frequency source ovenized to keep me cool on warm days!
Dan
On 2/13/2021 5:22 AM, [email protected] wrote:
Message: 2 Date: Sat, 13 Feb 2021 01:27:41 +0100 From: Attila Kinali
<[email protected]> To: Discussion of precise time and frequency
measurement <[email protected]> Subject: [time-nuts] The need for
quartz crystals and mains frequency (was: Mains Frequency) Message-ID:
<[email protected]> Content-Type:
text/plain; charset=UTF-8 On Fri, 12 Feb 2021 18:23:54 +0000 Andy Talbot
<[email protected]> wrote:
Why should the microcontroller have a crystal at all?
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.
Many have factory trimmed RC oscillators, typical 1% accuracy, because
accurate timing for other than timekeeping is rarely needed.
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 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.
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
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