[time-nuts] Portable Time Standard

[Achim Gratz]

Joe Hobart writes:
> Your accuracy results are impressive.  I have questions:
>
>    What manufacture or brand DS3231 do you have?

There are two modules you can get easily from the usual places, prices
and delivery times vary wildly.  The first and larger variant used to be
called ZS-042, but now generally has "DS3231" in inverse print in that
spot on the PCB.  These usually have the "SN" variant of the chip, but
there are reports of random replacements with the "M" variant (which is
a bit different when it comes to temperature compensation and has some
errata to be aware of in the first revision of the chip).

You should almost surely not use this variant as its shipped: they
typically come with CR2032 coin cell that is not rechargeable, so you
need to disable the charging circuit that is there for use with LiR2032.
The charging circuit itself probably doesn't safely work with a LiR2032
either if you run with VCC at 5V (Arduino or something similar).  Then
there's a power indicator that is most often uselessly pulling current
and pullup resistors for the I²C lines that get in the way when your bus
already is terminated (most often directly by the µC these days) or you
want to cascade devices on the bus, which is easy since this module has
the bus lines looped through to an extra row of pads.

https://components101.com/modules/ds3231-rtc-module-pinout-circuit-datasheet
http://woodsgood.ca/projects/2014/10/21/the-right-rtc-battery/

The other variant can be directly plugged onto a rasPi:

https://www.raspberrypi.org/forums/viewtopic.php?t=161133

I have not used these, but you can find some comments that the cell on
these might need to be replaced more often than you'd like.


Regards
Achim.
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