32kHz RTC Lockup, Recommendations?

[vonnieda]

Hi all,

We have a custom board with a WROVER module, and we use an external 32kHz crystal for the RTC. In menuconfig I have "RTC clock source: External 32kHz crystal".

On about 5% of our boards, we're experiencing lockups of the ESP, which seem to be related to the RTC clock failing. I've confirmed this by changing the menuconfig setting on these boards to disable the external crystal, and the lockups stop in all cases.

We are using this crystal: https://www.digikey.com/product-detail/ ... ND/6578497

We originally started with 6pF load caps but with these the crystal would not even start. We then switched to 12pF which is where we are now, with about 5% of boards failing. In testing, we have found that 15pF appears to be stable, and > 30pF will not start.

So, my questions:
1. Are these lockups expected behavior for an external crystal that stops for some reason?
2. Are there any recommendations for which crystal and caps to use? The information in the ESP datasheet is very limited. For instance, see the attached image that shows information for an ATmega328PB. We'd expect something like this for the ESP.
3. Is there a way to dynamically disable the clock so I can avoid the lockups? If possible, I'd like to detect this condition and disable the clock on the next reboot. Or can this only be done via menuconfig.

Thanks,
Jason

[loboris]

Maybe you should try a 12pF or 12.5pF crystal, it is working for me.

[vonnieda]

Maybe you should try a 12pF or 12.5pF crystal, it is working for me.

Thanks for the response loboris. Can you share the exact part you are using? And if you are using load caps, can you tell me their value?

Thanks,
Jason

[WiFive]

https://github.com/espressif/esp-idf/co ... 6ed6836ff2

[vonnieda]

https://github.com/espressif/esp-idf/co ... 6ed6836ff2

Thanks WiFive - I'll give that a try. But as far as I can tell, this doesn't handle the clock stopping, does it? I believe the issue we have is that the clock eventually stops, since sometimes the lockups happen minutes after boot.

Thanks,
Jason