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

[ThienNBK]

Hi everyone,

I’m working with the nRF54L05 MCU (SDK v2.9.0) for my application.

From the documentation, I read that in the system_off demo, when using the z_nrf_grtc_wakeup_prepare() function, the system defaults to the LFRC (internal low-frequency clock). However, I noticed that the timing is not accurate. I believe switching to the LFXO (external low-frequency crystal) would provide better accuracy.

My question is: how can I configure CLKSET to use LFXO instead of LFRC?

Many thanks in advance.

[Håkon Alseth]

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

 

However, I noticed that the timing is not accurate.

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

[ThienNBK]

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.

[Håkon Alseth]

Hi,

 

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

Håkon Alseth said:

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.

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