ESP32 Forum

Hi all,

I'm working on decreasing my deep sleep wakeup time. Currently the longest part of the process seems to be calibrating the 32kHz external crystal I'm using for RTC. With the recommended calibration cycles of 3000 it takes about 200ms.

I believe I understand all the menuconfig options for this. I know I can decrease it to lower startup time, but my question is: Why is calibration happening during deep sleep wakeup at all?

On cold boot it makes sense to calibrate the crystal, but when waking from deep sleep, presumably the crystal has been ticking along keeping time, so why recalibrate it when waking?

Here are the related sdkconfig options I'm using:
And logs from my test app:
Note, I have changed the log level of the "waiting for 32k" line to warning so I can disable most other logging and still see it.

Thanks,
Jason

The assumption is probably that most users prefer a more accurate clock over a faster wakeup, especially when using the internal oscillator.

You're better off skipping calibration in your case.

boarchuz wrote: ↑

Fri Aug 27, 2021 6:04 pm

The assumption is probably that most users prefer a more accurate clock over a faster wakeup, especially when using the internal oscillator.

You're better off skipping calibration in your case.

Thanks for your response, I don't think this applies. I'm not using the internal oscillator. I'm referring specifically to calibration happening during deep sleep wakeup. The question is: Why is calibration run during deep sleep wakeup when the calibration was already done during cold boot and the crystal has kept running while in deep sleep.

Thanks,
Jason