[time-nuts] Car Clock drift - the lowly 32kHz tuning fork crystal specs

Attila Kinali attila at kinali.ch
Sun Apr 9 09:50:13 EDT 2017

On Sun, 9 Apr 2017 07:45:23 -0400
Tim Shoppa <tshoppa at gmail.com> wrote:

> So my conclusion, is that all these car clocks must be temperature
> compensated. And they must've been doing this for several decades at this
> point.

Yes, definitely. Although in the 80s it was only the higher class cars.

> That shouldn't be too surprising - right next to the clock display on the
> dashboard is a digital thermometer. Maybe 30 or more years ago the
> temperature compensation was done by analog circuitry, but today I'm
> guessing there's a digital chip that takes the thermometer reading and
> numerically adjusts the divider word for the 32kHz oscillator to
> temperature compensate the clock digitally.

I'd rather guess that it is some RTC package with crystal, temp sensor
and battery in single package, and they use the internal temp sensor
of the RTC for the dashboard display.
> Is there a way to verify my guess at the TCXO method?

Beside opening up the dashboard and looking for the RTC or placing
the car in a climate chamber and measuring the temperature coefficient?
I don't think so.
> I'm guessing that all the better quartz wristwatches use a similar
> technology too. Maybe they have a different crystal cut that is closer to
> body temperature for the turnover point.

Most wristwatches do not have any temperature compensation. If worn, the
wristwatch is pretty close at the 25°C (the human body is a quite good
and temperature stable oven). The difference only starts to show when
the watch isn't worn for long periods of time.

			Attila Kinali
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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