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I'd like to know if it's possible to synchronize classical quartz clock by replacing inbuilt quartz crystal with DS3231's 32 kHz output. I need to synchronize my analog watch with RTC DS3231.

Am I supposed to simply remove quartz crystal and conenct DS3231 output? There are two wires coming from the watch IC, the crystal is connected between them. I think one is XTAL1 and the other one XTAL2, but which is which? I wonder is it even possible?

typical quartz watch circuit

I heard it's about connecting 32 kHz output to XTAL1, so "2?" and frequency corrector are not connected then? I'm clueless.

my watch. Which one is XTAL1?

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  • \$\begingroup\$ First you need to figure out which pin is the input and which is the output. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 23 '15 at 2:02
  • \$\begingroup\$ XTAL is a passive element that when placing in a circuit began to oscillate at a specific frequency. like RC oscillators. NO Absoultly Not. it's not a true concept, you can't do it. \$\endgroup\$ – HOPE Sep 23 '15 at 5:26
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Usually, yes - assuming you mean 32.768kHz when you say 32kHz, or the watch will run at the wrong rate.

First you need to know which IC pin is the input and which is the output, as Ignacio commented. A quick measurement with your oscilloscope will show a strong 32.768kHz signal at one pin on the crystal, and a weak signal at the other. The weaker signal is the input.

Now you need to connect the watch ground to the DS3231's ground, and capacitively couple the DS3231's 32.768kHz output into that input, removing the crystal of course. The correct value of capacitance is a guess, but 22-100pF is probably the range, and I would guess 33pf as a starting point. After the capacitance, you should see a signal approximately the same as the original. If it's much larger, reduce the capacitance and vice-versa. (You may have to remove the trimmer capacitance, or turn it to its minimum setting, i.e. maximise the input signal)

Oh and you still need the correct supply voltage of course - e.g. the original battery.

There should be a good clean copy of your 32.678kHz signal on the original (now unused) output pin.

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  • \$\begingroup\$ Thank you very much! I measured both pins and recognized input pin (that one where trimmer is connected = "2?"), now I am going to connect DS3231 32.678 kHz output through capacitor (33-100pF), but is there any pull-up resistor needed? 10k from DS3231 32.678 kHz to DS3231 Vcc? I also wonder if it is possible to supply my clock with 1.55 V and DS3231 with 3.7 V. Grounds connected together, of course. Thank you! \$\endgroup\$ – Herr Krum Sep 23 '15 at 16:01
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I have measured the 32kHz output signal of four DS3231 modules with a strange result. The board of the modules are labled ZS-042 and has a DS3231M chip mounted. Instead of the high accurate 32768 Hz signal I have measured 32641Hz, 32710Hz, 32730Hz and 32743Hz. When I enable the 1PPS signal at all four modules, the period is the same with little deviation of some ppm. The measured range of the 1PPS is between 1000.009ms and 999.994ms without waiting for long time stabilization.

All frequency measurements are done with a calibrated version of a TransistorTester. The calibration was done with a trimmer (10-35pF) at the crystal using a 1PPS signal from a GPS receiver as reference.

As a result of this measurement I would say, that the 32768 Hz of the DS3231M can not be used as precise reference.

Probably this is only a problem of the cheap DS3231M!

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  • \$\begingroup\$ Welcome to EE.SE. You should probably add in what instrumentation you used to take the measurements. \$\endgroup\$ – Transistor Jan 17 '18 at 20:35
  • \$\begingroup\$ I have used a calibrated TransistorTester module. The clock frequency has been calibrated with the PPS signal of a GPS receiver before the measurements. All four modules are measured in the same circuit with the same Tester with 16MHz clock. A second measurement with a 8MHz Tester has the same strange 32kHz results. \$\endgroup\$ – Karl-Heinz Kübbeler Jan 17 '18 at 20:56
  • \$\begingroup\$ That info would improve your answer considerably. I suggest that you add it in so that all the information is in your answer rather than scattered through the comments. \$\endgroup\$ – Transistor Jan 17 '18 at 20:58
  • \$\begingroup\$ The DS3231 has its frequency divider factory calibrated and temperature compensated, that's the whole magic of that chip. If you need a calibrated frequency output, you have to use the square wave output. There's a trade-off between frequency and precision of course. You could use an external PLL to get back to exactly (on the long run) 32768Hz from e.g. 1024Hz. \$\endgroup\$ – Janka Jan 17 '18 at 21:03
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The 32 kHz output of the DS3231M is not temperature compensated while it is for the non-M variant. The MEMS oscillator of the M model is highly sensitive to temperature and the frequency will drift like crazy on that unit. The temperature compensation takes as part of the timekeeping functionality, so it still meets spec. The nom M variant has temperature compensated time and frequency. See https://blog.heypete.com/2017/09/05/major-differences-between-the-ds3231-and-ds3231m-rtc-chips/

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