[time-nuts] Speaking of precise 32.768 kHz based clocks: DS3231

Poul-Henning Kamp phk at phk.freebsd.dk
Sat Apr 16 17:06:27 UTC 2011


Peripherally relevant to the recent discussion about accurate wristwatches
I want to point out an interesting chip:  Dallas/Maxim DS3231

This is a CMOS clock with built in 32.768 kHz X-tal and a couple of
neat tricks:

Every 64 seconds, it measures the temperature, and adjust the capacitive
loading of the X-tal to match.

They claim:
(http://www.maxim-ic.com/datasheet/index.mvp/id/4627)

# Accuracy ±2ppm from 0°C to +40°C
# Accuracy ±3.5ppm from -40°C to +85°C 

And so far I have no reason to doubt them.

That is not half bad, for a chip which can run for a week on a 1F
super-capacitor...

But, the _really_ neat thing is that it has an "Xtal aging" register,
where the capacitive loading can be tuned, obviously necessary in
the absense of an external trimmer.  A quick test indicates that
this happens in steps of roughly 0.05PPM

Highly recommended...

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.



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