ESP32 External RTC 32K

[mrrosen]

I have a board built by our team that is using an external clock source (not a crystal) for the external ESP32 RTC clock (32K). The source goes into XTAL32KP with XTAL32KN disconnected. However, whenever I set the CONFIG_ESP32_RTC_CLOCK_SOURCE_EXTERNAL_CRYSTAL to y via menuconfig in sdkconfig and build our project which uses the 2nd stage bootloader and am getting this message:

clk: still waiting for 32k oscillator to start up

Two questions:
1) Can the ESP32 use a single ended clock source for the 32K external input?
2) Is there more I have to do (if the answer to #1 is yes) to enable using this clock source in the IDF? (I have a version of the IDF from June 2017; I would provide a commit but the repo we are using has had its history erased in transfer )

[mrrosen]

Im still trying to get this to work, so I flashed the unit test application onto a few of our boards and scoped the signals. I tried running the rtc_clk unit tests and would get one of two possible outcomes from calibration:

Code: Select all

calibrate (0): 34.728 kHz
calibrate (1): 34.730 kHz
calibrate (2): 34.733 kHz
calibrate (3): 34.733 kHz
calibrate (4): 34.731 kHz
RTC_CAL_8MD256:
calibrate (0): 34.731 kHz
calibrate (1): 34.729 kHz
calibrate (2): 34.726 kHz
calibrate (3): 34.728 kHz
calibrate (4): 34.727 kHz
RTC_CAL_32K_XTAL:
calibrate (0): inf kHz
calibrate (1): inf kHz
calibrate (2): inf kHz
calibrate (3): inf kHz
calibrate (4): inf kHz
32K XTAL OSC has not started upswitching to RTC_SLOW_FREQ_8MD256: done
RTC_CAL_RTC_MUX:
calibrate (0): 34.725 kHz
calibrate (1): 34.726 kHz
calibrate (2): 34.729 kHz
calibrate (3): 34.729 kHz
calibrate (4): 34.725 kHz
RTC_CAL_8MD256:
calibrate (0): 34.728 kHz
calibrate (1): 34.727 kHz
calibrate (2): 34.729 kHz
calibrate (3): 34.731 kHz
calibrate (4): 34.732 kHz
RTC_CAL_32K_XTAL:
calibrate (0): inf kHz
calibrate (1): inf kHz
calibrate (2): inf kHz
calibrate (3): inf kHz
calibrate (4): inf kHz

Or:

Code: Select all

RTC_CAL_RTC_MUX:
calibrate (0): 34.728 kHz
calibrate (1): 34.727 kHz
calibrate (2): 34.726 kHz
calibrate (3): 34.727 kHz
calibrate (4): 34.726 kHz
RTC_CAL_8MD256:
calibrate (0): 34.728 kHz
calibrate (1): 34.730 kHz
calibrate (2): 34.730 kHz
calibrate (3): 34.728 kHz
calibrate (4): 34.731 kHz
RTC_CAL_32K_XTAL:
calibrate (0): 32.768 kHz
calibrate (1): 32.768 kHz
calibrate (2): 32.767 kHz
calibrate (3): 32.768 kHz
calibrate (4): 32.768 kHz
switching to RTC_SLOW_FREQ_32K_XTAL: done
RTC_CAL_RTC_MUX:
calibrate (0): 32.768 kHz
calibrate (1): 32.767 kHz
calibrate (2): 32.768 kHz
calibrate (3): 32.767 kHz
calibrate (4): 32.768 kHz
RTC_CAL_8MD256:
calibrate (0): 34.730 kHz
calibrate (1): 34.731 kHz
calibrate (2): 34.731 kHz
calibrate (3): 34.729 kHz
calibrate (4): 34.731 kHz
RTC_CAL_32K_XTAL:
calibrate (0): 32.767 kHz
calibrate (1): 32.768 kHz
calibrate (2): 32.768 kHz
calibrate (3): 32.768 kHz
calibrate (4): 32.768 kHz
switching to RTC_SLOW_FREQ_8MD256: done
RTC_CAL_RTC_MUX:
calibrate (0): 34.733 kHz
calibrate (1): 34.733 kHz
calibrate (2): 34.734 kHz
calibrate (3): 34.737 kHz
calibrate (4): 34.737 kHz
RTC_CAL_8MD256:
calibrate (0): 34.735 kHz
calibrate (1): 34.734 kHz
calibrate (2): 34.735 kHz
calibrate (3): 34.736 kHz
calibrate (4): 34.735 kHz
RTC_CAL_32K_XTAL:
calibrate (0): 32.767 kHz
calibrate (1): 32.768 kHz
calibrate (2): 32.767 kHz
calibrate (3): 32.768 kHz
calibrate (4): 32.768 kHz

In both cases, if I try to run the "Output 32k XTAL clock to GPIO25", I get a flat line while probing the XTAL_32KP line gets me the 32KHz clock as expected. Im maybe getting some noise one the 32K clock but I have to confirm with out hardware engineer if my setup is just a bit noisy. Other than that, even if it succeeds to calibrate, why am I getting a flat line on GPIO25?

[Shcreasey]

We also want to do this, any information regarding its viability would be appreciated.

If it is possible to use an externally generated clock to supply just XTAL_32k_P, can XTAL_32k_N be reused as a GPIO?

Thanks

[mrrosen]

We contacting ESP32 support directly and the response we got back told us to connect the XTAL32KN line to the oscillator, not the XTAL32KP line. We are going to try a rework to see if this fixes the strange issues we are seeing, so no definitive answers on this yet; but just for others looking for answers, this is what we have for now

[ESP_igrr]

Linking the related https://github.com/espressif/esp-idf/issues/1356 here.

[mrrosen]

As promised, a short update. We tried disconnecting our oscillator from the XTAL32KP and connected the XTAL32KN line to a function generator at 32.768KHz and it worked perfectly.

So, as with what ESP_iggr mentioned in the linked github issue; you must use the XTAL32KN line for the input of the oscillator if you arent using a crystal.