Bug in NCS v1.7.1 with 32kHz RC Oscillator and MCUboot enabled

Well, this one took me many hours of tracing to track down the root cause, so I hope it helps someone!

Note: I wasn't able to reproduce this directly with a sample included in the nRF Connect SDK. But it happens with my application, based on the "light_ctrl" mesh sample.

It occurs with the following 3-way combination (tested, only if all met);

  • CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y
  • CONFIG_BOOTLOADER_MCUBOOT=y
  • nRF Connect SDK v1.7.1 (didn't occur with v1.4.2 I last used)

I also had to enable CONFIG_BT_LL_SW_SPLIT=y  for zephyr controller to be able to fit in the flash of the nRF52833DK I'm using, but not sure if this is related (yes I intend to move to the soft device and know it is now recommended).

To fix I've changed to CONFIG_CLOCK_CONTROL_NRF_K32SRC_SYNTH=y

I've found these two issues which appear to have similar failure logs;

https://github.com/zephyrproject-rtos/zephyr/pull/25583

https://devzone.nordicsemi.com/f/nordic-q-a/76899/zephyr-build-smp_svr-for-thingy52-fails

Build error log below

Kevin