nRF54l15 Making Use of Low Power Mode and an External Crystal Oscillator

[LeviRand]

Hi Nordic Team,

I have developed a couple applications for the nRF52833 using Zephyr and have now moved to the nRF54l15. For this new application, power consumption is a large risk and I have been looking into ways to reduce it. Combing through the product specification, I have found the deep sleep modes and RAM retention features but these are not ideal for my current application as our device needs to remain connected through Bluetooth while reducing power draw.

My main question is regarding how to make use of the chipset's low power mode and optional 32kHz crystal oscillator. Are there any drawbacks to using this low frequency mode? Some sources suggest that the low frequency task execution is independent of the high frequency tasks which further complicates my understanding of how this low frequency saves power overall.

From what I have found, other documentations don't specify whether entering this low power mode is set in the application level or handled by the Zephyr OS.  If it can be accessed by the application, what API should we use to change the clock frequency and are there any examples of how to accomplish this? 

With the low power mode, how do I select the external crystal oscillator using the Zephyr OS?

Respectfully,

[Einar Thorsrud]

Hi,

I am not sure what you refer to by low power mode, do you mean system ON sleep mode, which is the normal and typicall sleep mode on nRF5 devices? In this case, the nRF54 series will use the GRTC in much the same way as the older devices used the RTC, and this runs of the 32.768 kHz low power clock.

The 32.768 kHz clock can either come from an internal 32.768 kHz RC oscillator (LFRC) or an exteral 32.768 kHz clock. In for instance Bluetooth communication, using a 32.768 kHz crystall will reduce average power consumption because it gives a better sleep clock accuracy, which in turns means that the Rx window can be smaller (there is less poentiall clock drift that needs to be accounted for). You can see the effect of this in various roles and configurations using out Online Power Profiler.

If you have board files that include the LFXO (for instance files for a DK), you can disable it by for instance addnig this configuration to prj.conf:

# Clock config
CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y
CONFIG_CLOCK_CONTROL_NRF_K32SRC_XTAL=n

[LeviRand]

Hi Einar,

Thanks for responding to my post. Your info was very enlightening.

To continue the conversation, within the production specification for the nRF54l15, it describes two sub power modes: 

• Constant Latency
• Low-power

The documentation describes this Low-power mode as the default mode for System ON. Originally, I wasn't sure how much control over these modes the developer has. After reading the documentation over more, I don't believe I need to edit or control these sub modes.

 

Additionally, the info on the crystal oscillator's role was very helpful as well. Confirming I understand your response, if I wish to use my board's crystal oscillator for this improved Rx window and by extension reduced average power, I only need to add the two project configs you referenced? No other APIs or lfxo/hfxo devices in the project .overlay files need to be modified?

[Einar Thorsrud]

Hi,

You are right, low power mode is the default sleep mode. Constant latency has some uses, and in some cases some drivers or libraries may enable it for a short time, but disable it again. You only need to think about this specifically if you want constant latency for something specific (most likely not).

The snippet from my previous post enables the LFRC and disables the LFXO, and that is typically only something you want to do if you do not have a 32.768 kHz crystal. If you do, you often do not need to do anything as the default configuration of for isntance the DK board files is to use crystal. But if you instead set CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=n and CONFIG_CLOCK_CONTROL_NRF_K32SRC_XTAL=y, this will explicitly enable the LFXO. There are no other changes needed.

[LeviRand]

Great info and that answers my question. Thank you!