Set CLKSET on nRF54L05 to Use External Low-Frequency Crystal (LFXO)

RE: Set CLKSET on nRF54L05 to Use External Low-Frequency Crystal (LFXO)

Håkon Alseth — Fri, 22 Aug 2025 08:08:57 GMT

Hi,

You will have an added delay on the boot up sequence, as wakeup is from reset, as formerly mentioned:

[quote user="hkn"]Please note that it takes some time for the CPU to wake up, as it starts from a reset-state, to execute your wanted code.[/quote]

You cannot reliably use the current consumption period to measure precisely, as there will be added processing, and additional latency due to generic processing, such as these lines:

https://github.com/nrfconnect/sdk-zephyr/blob/v4.0.99-ncs1/drivers/timer/nrf_grtc_timer.c#L427-L431

Try set/clr a LED during your boot up instead, and also play around with the sleep interval, to see how this scales.

Please note that if you require full control of your timing, I would recommend using systemon idle instead, as you do not need a reset upon wakeup.

Kind regards,

Håkon

RE: Set CLKSET on nRF54L05 to Use External Low-Frequency Crystal (LFXO)

ThienNBK — Fri, 22 Aug 2025 03:05:42 GMT

Hi, thanks for your reply.

Regarding the timing issue: the expected time is 2 seconds, but when I measure it using the PPK II, the actual time I observe is about 2.006 seconds (see the attached image).

If the GRTC does not switch the clock source as you mentioned, how can I achieve more accurate timing? I’m developing a smartwatch application, so precise timekeeping is very important.

Many thanks.

RE: Set CLKSET on nRF54L05 to Use External Low-Frequency Crystal (LFXO)

Håkon Alseth — Thu, 21 Aug 2025 08:58:49 GMT

Hi,

The GRTC does not switch source in sleep/systemoff, as stated here in the docs:

https://docs.nordicsemi.com/bundle/ps_nrf54L15/page/grtc.html#ariaid-title2

[quote user=""]However, I noticed that the timing is not accurate. [/quote]

How much are you seeing?

Please note that it takes some time for the CPU to wake up, as it starts from a reset-state, to execute your wanted code.

Kind regards,

Håkon