Problems with Uart21 and Xiao nrf54L15

RE: Problems with Uart21 and Xiao nrf54L15

Hieu — Tue, 14 Oct 2025 17:46:08 GMT

Hi Markus,

It is correct that you need Constant Latency mode to use P2 with a peripheral in the Peripheral Domain. This is mentioned in the datasheet here: Pin assignments. Sorry I missed this.

Keeping Constant Latency mode on all the time will increase power consumption though. The UART driver supports only requesting it as needed, and you can do so using CONFIG_NRF_SYS_EVENT.

The fact that k_yield() helps seem to be that the function changed timing just enough for things to work, but that shouldn't be a solution.

RE: Problems with Uart21 and Xiao nrf54L15

Hieu — Sat, 11 Oct 2025 20:54:05 GMT

I reproduced the issue with the nRF54L15 DK and the UART ASYNC API sample. However, in my case, adding k_yield() didn't help at all.

The fact that Constant Latency mode helps make sense, as the signal is passing power domains in the case of P2 pins. This is also a clue.

I will start some internal inquiry and update you when I learn something.

RE: Problems with Uart21 and Xiao nrf54L15

MarkusK — Thu, 09 Oct 2025 11:16:57 GMT

It is also working, when I include

#include <nrfx_power.h>

and change the PowerMode Constant Latency sub-power mode with the Funktion:

nrfx_power_constlat_mode_request();

But I have to say, that my understanding from the Power Driver is quiet limited.

RE: Problems with Uart21 and Xiao nrf54L15

MarkusK — Wed, 08 Oct 2025 13:23:42 GMT

Yes, this is right. It can be also some computing like a for-loop instead the k_yield(). But a k_msleep after uart_tx with the xiao nrf54l15 doesnt work properly with the uart21 on the default pins 2.07 and 2.08. Changing the pins from the uart21 to 1.04 and 1.05 everything works again. A try with direct polling on the other side on pins 2.07 and 2.08 did work.

RE: Problems with Uart21 and Xiao nrf54L15

Hieu — Wed, 08 Oct 2025 12:57:14 GMT

I think that is some good clues. While pins 2.07 and 2.08 can be assigned to UART21, they are on a different power domain, so some difference in behavior is less surprising. My apologies, this is one of the things I should have noticed from the compiled DTS file.

Do I understand right that:

With pins 2.07 and 2.08, you need to have k_yield() for UART to work.
With pins 1.04 and 1.05, your code work just fine without using k_yield()?

RE: Problems with Uart21 and Xiao nrf54L15

MarkusK — Wed, 08 Oct 2025 09:38:02 GMT

When I programm it as a thread the same problem occurs. I have still to place inside the thread:

while (tx_busy) {
            k_yield();
}

RE: Problems with Uart21 and Xiao nrf54L15

MarkusK — Wed, 08 Oct 2025 07:48:53 GMT

Its becomming even a little bit more strange. I used an overlay-file to test other free pins for the uart21 and it worked fine:

&pinctrl {
    uart21_default: uart21_default {
        group1 {
            /* TX = P1.04 */
            psels = <NRF_PSEL(UART_TX, 1, 4)>;
        };
        group2 {
            /* RX = P1.05 mit Pull-up */
            psels = <NRF_PSEL(UART_RX, 1, 5)>;
            bias-pull-up;
        };
    };
    
    uart21_sleep: uart21_sleep {
        group1 {
            psels = <NRF_PSEL(UART_TX, 1, 4)>,
                    <NRF_PSEL(UART_RX, 1, 5)>;
            low-power-enable;
        };
    };
};

When I set the default-pins for the Xiao-Board also in the overlay-file I dont get any output:

&pinctrl {
    uart21_default: uart21_default {
        group1 {
            psels = <NRF_PSEL(UART_TX, 2, 7)>;
        };
        group2 {
            psels = <NRF_PSEL(UART_RX, 2, 8)>;
            bias-pull-up;
        };
    };
    
    uart21_sleep: uart21_sleep {
        group1 {
            psels = <NRF_PSEL(UART_TX, 2, 7)>,
                    <NRF_PSEL(UART_RX, 2, 8)>;
            low-power-enable;
        };
    };
};

When I remove the pin-setting again from the overlay-file I get again the output which works only correct if I place the k_yield(). But at least I get an output. The zephyr.dts looks in both cases same for the pin settings.

I ordered also a nrf54l15-dk by digikey 3 weeks ago for testing, but unfortunaly there is a delivery delay.

RE: Problems with Uart21 and Xiao nrf54L15

Hieu — Tue, 07 Oct 2025 20:46:30 GMT

Hi MarkusK,

I can't figure out what could go wrong here...

What would happen if instead of running the loop in main(), you create a dedicated thread for it and run it inside? main() does run at a high priority.

What doesn't make sense to me is how k_yield() helps, because k_msleep() would have run regardless, and achieve a similar result.

Does the approach in the UART ASYNC API sample work for you? If yes, then that is the simplest way to get something working, though I am intrigued why the current setup doesn't work.

RE: Problems with Uart21 and Xiao nrf54L15

MarkusK — Fri, 03 Oct 2025 05:56:40 GMT

Hi Hieu,

i had to rename the .config to test.config to be able to upload the file.

devzone.nordicsemi.com/.../1185.zephyr.dts

devzone.nordicsemi.com/.../test_2E00_config

RE: Problems with Uart21 and Xiao nrf54L15

Hieu — Thu, 02 Oct 2025 13:14:32 GMT

Hi MarkusK,

Could you please share the following files?

Compiled Kconfig at <build dir>/<app name>/zephyr/.config
Compiled DeviceTree at <build dir>/<app name>/zephyr/zephyr.dts

Hieu