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I'm troubleshooting a real-time clock and suspect that under certain conditions the oscillator stops. How could I check if it is running, especially when the RTC chip is on battery power? It is Maxim Dallas DS1307 with external crystal.

The oscilloscope input capacitance would disturb the picture too much, I think. I thought about holding a coil (say, 100 turns diameter 1") close to the PCB traces and probing the coil with the oscilloscope.

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    \$\begingroup\$ 1) Use a 10:1 probe (yeah, DUH but I mention it anyway). 2) try probing on both contacts of the crystal oscillator contacts (I'm assuming that the crystal is separate), if the circuit is a Pierce oscillator, then one node is more sensitive than the other. 3) Try if probing though a 1 M ohm resistor works. \$\endgroup\$ Nov 11, 2021 at 14:30
  • \$\begingroup\$ In English we call it capacitance not capacity. I think in Germany, they call it the equivalent of capacity (Kapazität)? \$\endgroup\$
    – Andy aka
    Nov 11, 2021 at 14:30
  • \$\begingroup\$ Which real-time clock? Separate chip or MCU? Does it have buffered square wave output? \$\endgroup\$
    – Justme
    Nov 11, 2021 at 14:46
  • \$\begingroup\$ Some MCUs incorporate logic to detect osc faults (MSP430 for sure) \$\endgroup\$
    – user16324
    Nov 11, 2021 at 15:24

1 Answer 1

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enter image description here

Pin X1 is a high impedance input. If you probe there, probe capacitance will influence the oscillator.

However, pin X2 is the low impedance output of the internal inverter, so if you stick a X10 probe there, its capacitance will matter much less.

Then, there is a SQW/OUT pin that will output a square wave:

enter image description here

If you enable it in the I2C registers, then you can probe it and check its frequency without disturbing the oscillator at all.

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    \$\begingroup\$ Worked just fine with X10 probe at X2. Running on battery, so no square wave. \$\endgroup\$
    – Xpector
    Nov 11, 2021 at 17:14
  • \$\begingroup\$ So did you find what the problem was? Does it oscillate over the whole temperature and power supply range? \$\endgroup\$
    – bobflux
    Nov 11, 2021 at 18:02
  • \$\begingroup\$ I only had a healthy board on my desk so far. I still need to wait for the customer's faulty board and probe it without powering on. The oscillator should still be running on battery. Luckily, the failure happens under normal conditions. \$\endgroup\$
    – Xpector
    Nov 11, 2021 at 18:24

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