15 February 2019

Weird behavior of ZS-042 RTC modules (DS3231)

First of all: all these ZS-042 RTC modules were paid about 1 USD each and come from not-certified sources. 

One night I decided to measure the 32K output of some DS3231 ZS-042 RTC modules. I wanted to see what difference would make if I retouched the aging register. This experiment lead to few unexpected discoveries.

1) Maxim DS3231 datasheet mentions 32K output and 32.768 kHz interchangeably. Alright, it is a digital device, so 32K equals 32768 if K=1024 as used for bytes. But we are talking of Hz, not bytes, so I think should be K=1000. Proof? Search for "768" in the datasheet.

2) There are two quite different devices that share 6 characters of the part name - DS3231 - and are distinguished by their suffix: M or SN. The former "M" is 5 ppm with MEMS resonator, while the latter "SN" is 2 ppm with TCXO. In simple terms, "SN" is better than "M". But you can't choose when buying ZS-042 modules and you will receive either model

"M", "SN", "M":notice the deeper blue PCB of "SN"
3) All my ZS-042 modules with DS3231M output a frequency close to 32.700 kHz, while those with DS3231SN are close to 32.768 kHz. The "M" would be loosing 180 seconds/day, 1.5 h/month, but none of them does! Actually I have one DS3231M in my longest running Nixie clock and it is within 5 ppm specification.

Measuring "32K" output of DS3231M.
So, let's forget DS3231SN that are most likely second-hand/pulls but legit. How can a DS3231M that outputs 32700 Hz be so accurate as I observed? If it is genuine, Maxim has a way to calibrate the chip once, before it is sold. If it is a fake, then it is microcontroller with proper firmware and calibration to count the correct number of interrupts. I could sacrifice one and decap it, but then I lack the microscope to examine the silicon die.

Conclusion. Buy the DS3231SN if you need an interrupt rate of 32768 Hz. Otherwise for hobbyist use, they are both worth it.

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