Change CPU frequency of nRF54L15

RE: Change CPU frequency of nRF54L15

Kenneth — Mon, 28 Apr 2025 15:49:48 GMT

Looks like it will cause a build error moving forward if not using 128MHz, ref:
https://github.com/nrfconnect/sdk-nrf/commit/9529161739fab464d5337edccf16ab4f54d9b310

There are no plans to support 64MHz.

They will add a note in the documentation that 128MHz should be used for the softdevice controller, so I guess overall it looks like 64MHz indirectly isn't supported by software.

Kenneth

RE: Change CPU frequency of nRF54L15

Emil Lenngren — Wed, 23 Apr 2025 16:05:42 GMT

[quote userid="46557" url="~/f/nordic-q-a/120810/change-cpu-frequency-of-nrf54l15/532570"]Bluetooth stack is only qualified with 128MHz so when using Bluetooth you have to run 128MHz.[/quote]

Where is this documented?

RE: Change CPU frequency of nRF54L15

Kenneth — Wed, 23 Apr 2025 11:17:37 GMT

I will ask internally to check if any of the designers can comment a bit on when CPU frequency can be safely changed or not.

Kenneth

RE: Change CPU frequency of nRF54L15

PaKa — Wed, 23 Apr 2025 11:10:18 GMT

Bluetooth stack is only qualified with 128MHz so when using Bluetooth you have to run 128MHz.

RE: Change CPU frequency of nRF54L15

Emil Lenngren — Wed, 23 Apr 2025 09:19:58 GMT

For CPU intense workloads, then yes, the 128 MHz frequency might be more optimal.

But when the MCU domain must be kept on due to some timing requirement, e.g. while busy-looping when waiting for a short SPI transfer to finish, or GRTC to wake up when requesting the current time, the 128 MHz option will of course consume more energy.

But in particular, due to these erratas https://docs.nordicsemi.com/bundle/errata_nRF54L15_Rev1/page/ERR/nRF54L15/Rev1/latest/anomaly_L15_39.html and https://docs.nordicsemi.com/bundle/errata_nRF54L15_Rev1/page/ERR/nRF54L15/Rev1/latest/anomaly_L15_20.html, the MCU domain will also need to stay on while using the radio. For example, during a radio reception like a BLE scan, according to my measurements, the system consumes around 11% more power with 128 MHz than if those errata would not exist and with 64 MHz, only around 8% more.

So yes, depending on the application, the optimal clock frequency is different for different scenarios.

And when I look in system_nrf54l.c, I see that the clock configuration can be changed using a define.

RE: Change CPU frequency of nRF54L15

Kenneth — Tue, 22 Apr 2025 18:57:04 GMT

Hi,

I am wondering if this is more of a theoretical problem rather than actual problem, because I would expect you want to run on 128MHz always. Considering the nRF54L15 don't have any Dynamic Voltage-Frequency Scaling I would expect the most power efficient would be to run on 128MHz and execute the task in half the time compared to 64MHz, or do you see scenarios where using 64MHz could be beneficial here.

I can find in \modules\hal\nordic\nrfx\mdk\system_nrf54l.c I it's configured to 128MHz in SystemInit(), I would expect this is the case for all images if not explicit changed.

Kenneth