54l15. UART communication issue with low temperature.

RE: 54l15. UART communication issue with low temperature.

Vidar Berg — Thu, 28 May 2026 09:37:51 GMT

Hello,

Just to be clear, the current consumption will remain high as long as the UART receiver is enabled, regardless of which API you use or whether it is using HFINT or HFXO.

There is an open PR in Zephyr to handle the clock requests internally in the driver:

https://github.com/zephyrproject-rtos/zephyr/pull/105415

RE: 54l15. UART communication issue with low temperature.

twistwind2 — Thu, 28 May 2026 09:04:21 GMT

hello uart will be enable all the time, but we use asynchronus api. CONFIG_UART_ASYNC_API is activated. So, it should be ok for us. Thanks for your help. Do you know if hf_clock_request() will be no more necessary in future release?

RE: 54l15. UART communication issue with low temperature.

Vidar Berg — Thu, 21 May 2026 09:10:27 GMT

Hi,

As long as you have the UART receiver is enabled it is not possible to achieve a floor current in the 2-3 uA range as it requires the HF clock to be running. Is the plan to keep the UART enabled all the time? In that case, I recommend you consider the LP UART driver: https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/drivers/uart_nrf_sw_lpuart.html

RE: 54l15. UART communication issue with low temperature.

twistwind2 — Wed, 20 May 2026 14:48:36 GMT

Hi Vidar, i integrated your solution into your sample : it is working. it solves the issue. Thanks for your response. I have got 2 questions: Is this solution permit to have efficient low power? (into future our product needs to run with ~= 10 µA max). Into our dts, i selected clock like this. here:

&clock {
    status = "okay";
    hfclk {
        source = "external";
    };
    lfclk {
        source = "xtal";
    };
};

This solution should be enougth to work with low temperature without onoff clock request, rigth?

I don't understand why XO is not running with echo bot sample or our project (No ble for moment)

RE: 54l15. UART communication issue with low temperature.

Vidar Berg — Tue, 19 May 2026 13:01:58 GMT

Hello,

Please request the HFXO through the "onoff" API as shown in the hf_clock_request() function here: https://github.com/nrfconnect/sdk-nrf/blob/f35cddd506a9a779305feb0f52fcbf99b03bee23/tests/drivers/grtc/grtc_clk_output/src/main.c#L49 at the beginning of main() if you are not already doing that. This API is globally reference counted, which means the HFXO will be kept running even if the bluetooth stack or another SDK module releases its request for this clock source.

Best regards,

Vidar