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DS3231M - Improve accuracy with Aging register?

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Peabody:
It seems that all the usual sources continue to list DS3231 RTC modules as having the characteristics of the DS3231SN, but what they ship is the DS3231M, which is inferior in several ways.  It uses twice as much power when running on the coin cell, but the main thing is it reportedly doesn't keep time very well.

The DS3231SN uses an actual 32768Hz crystal, and a temperature compensation method that switches load capacitors in and out to keep the frequency constant.  But the DS3231M uses a MEMS resonator, presumably running at a much higher frequency, and apparently only changes the divider count when the temperature changes.

Both chips also have an Aging register, which lets you add your own correction, up or down, to what the temperature algorithm produces.  I'd like to know whether the Aging register could be used to make the M as accurate as the SN, or whether the M is inherently worse.  I assume temperature would affect the MEMS oscillator much more than a crystal oscillator, so it may be that there's just a limit to what can be achieved with the M.

Unfortunately, the 32K output of the M is divided down directly from the raw MEMS oscillator, and is unaffected by the temperature adjustments used to actually keep time, so it can't be used to measure the effect of Aging register changes.  And the square wave output is limited to 1Hz in the M, so measuring the effect of Aging register adjustments could be a long-term project.

Anyway, I wondered if anyone here might have experience using the Aging register on the M, and could advise me whether it's of any practical use.

tooki:

--- Quote from: Peabody on March 28, 2023, 05:52:29 pm ---It seems that all the usual sources continue to list DS3231 RTC modules as having the characteristics of the DS3231SN, but what they ship is the DS3231M, which is inferior in several ways.  It uses twice as much power when running on the coin cell, but the main thing is it reportedly doesn't keep time very well.

--- End quote ---
While the increase from 2ppm to 5ppm is huge as a percentage of each other, you have to keep it in perspective: both are still extremely good. 2ppm and 5ppm work out to 63 and 158 seconds per year, respectively. A standard quartz watch crystal is 20ppm, or 52 seconds per month!

So while the M is a lot worse than the SN, it’s merely a drop from “spectacular” to “excellent”. It most certainly doesn’t deserve the description “doesn’t keep time very well”.

Peabody:
I understand, but what I see various places is that the M often does a lot worse than its spec (a few minutes a month).  Not so the SN.  Both chips claim to be factory trimmed, so I don't know why the M in particular would be off so much.

tooki:
I wonder if the Chinese have finally started counterfeiting it. If so, that might explain it.

iMo:
DS is now ADI so go into their EZone - I can see there some "M" related questions already..
https://ez.analog.com/clock_and_timing/f/q-a/566628/ds3231m-oscillator-failure-stopping

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