RTC accuracy , 32.768kHz Crystal

nir hasidim

Hi all,

We develop an ultra low power application.
Accurate time stamp is very important for us. We can not set the RTC in field.

We do calibrate the RTC before the installation.

Most of the 32.768kHz crystals have -20ppm drift. Some of them gets to +-10ppm (about 1-2 seconds drift per day).

We can not tolerate more than few seconds clock drift in a year.

Is there any way we can trust the RTC time stamp after few years without setting it??

What is the best crystal for the kind of application (low drift, low power, low tolerance, best temperature stability)?
The best method to calibrate it before going out to the field?

Thanks,
Nir

over 12 years ago

0 Ilmars over 12 years ago

Guru 46710 points

You need atomic clock. Search for Symmetricom SA.32m. Other solution would be to include GPS receiver which is used to periodically (once per day?) adjust time reference.

0 Jens-Michael Gross over 12 years ago in reply to Ilmars

Guru 227245 points

If the device is to operate without 1000km radius around Braunschweig, Germany, then a DCF77 receiver might be a low-cost low-power solution. Actually you only need a trimmed inductor, a diode, a transistor and some software to get the current time out of thin air. :)

Ilmars said:
You need atomic clock.

Funny, when I read the original quesiton, I immediately had the same idea.

0 Kelly Petlig over 12 years ago

Prodigy 10 points

One of the most accurate small clock-sources I've found is a DS3232 from Maxim.

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RTC accuracy , 32.768kHz Crystal