nRF52840 Boot Time Optimization in NCS

RE: nRF52840 Boot Time Optimization in NCS

Vidar Berg — Thu, 09 Apr 2026 07:10:38 GMT

Thanks for confirming. I now remembered that the initialisation of the cryptocell RNG is quite time consuming. In addition to having CONFIG_SYSTEM_CLOCK_NO_WAIT enabled, please create a DT overlay to change the entropy source from the Cryptocell to the nRF RNG by changing the "zephyr,entropy" assignment in the "chosen" node:

/ {
	chosen {
		/*
		 * In some default configurations within the nRF Connect SDK,
		 * e.g. on nRF52840 and nRF9160, the chosen zephyr,entropy node
		 * is &cryptocell. This devicetree overlay ensures that default
		 * is overridden wherever it is set, as this application uses
		 * the RNG node for entropy exclusively.
		 */
		zephyr,entropy = &rng;
	};
};

RE: nRF52840 Boot Time Optimization in NCS

ilo — Thu, 09 Apr 2026 06:44:07 GMT

Hi,

I observed the same result after setting CONFIG_SYSTEM_CLOCK_NO_WAIT=y.

RE: nRF52840 Boot Time Optimization in NCS

Vidar Berg — Thu, 09 Apr 2026 05:38:49 GMT

Hello,

I suspect the delay is caused by the clock driver waiting on the 32 KHz LF clock to become calibrated. To confirm this, please try building with CONFIG_SYSTEM_CLOCK_NO_WAIT=y

Best regards,

Vidar