nRF52840 Boot Time Optimization in NCS

[ilo]

Hi everyone,

I'm experiencing an unexpectedly long boot time on my nRF52840 device using NCS v3.2.0. Could you help me verify if this boot time is normal, or suggest any possible causes?

Setup:

• Platform: nRF52840
• SDK: NCS v3.2.0
• Code Base: peripheral_hids_keyboard (with custom modifications)
• Board Configuration: Custom board, currently without bootloader

What I did:
To measure boot time, I added timing signal logs at different stages of the boot process. Here are my results:

EARLY
                ~4.8ms
PRE_KERNEL_1
                434ms <-- take long time on here
PRE_KERNEL_2
                76us
POST_KERNEL
                459us
main thread

The code I used for the measurement is as follows:

int prek1_marker(const struct device *dev) {
    gpio_pin_configure(GPIO1_DEV, 2, GPIO_OUTPUT);
    gpio_pin_set(GPIO1_DEV, 2, 1);
    return 0;
}
SYS_INIT(prek1_marker, PRE_KERNEL_1, 0);

int prek2_marker(const struct device *dev) {
    gpio_pin_set(GPIO1_DEV, 2, 0);
    return 0;
}
SYS_INIT(prek2_marker, PRE_KERNEL_2, 0);

int postk_marker(const struct device *dev) {
    gpio_pin_set(GPIO1_DEV, 2, 1);
    return 0;
}
SYS_INIT(postk_marker, POST_KERNEL, 0);

int main(void)
{
    gpio_pin_set(GPIO1_DEV, 2, 0);
    // ...
}

If anyone has experience with boot timing on nRF52840 or suggestions on what could cause the delay at PRE_KERNEL_1, your input would be greatly appreciated. Thank you!

[Vidar Berg]

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

[ilo]

Hi,

I observed the same result after setting CONFIG_SYSTEM_CLOCK_NO_WAIT=y.

[Vidar Berg]

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;
	};
};