Serial error on nRF54L15DK when no terminal connected

RE: Serial error on nRF54L15DK when no terminal connected

ilnadi — Fri, 12 Dec 2025 16:34:05 GMT

Hi Syed,

Thank you for the reply. Since we will not have this situation once we have real hardware, we bypassed the problem

disabled uart30 from the USB bridge (using Board Configurator
connect the pins to a USB bridge that does not tri0state.

thank you, we can close this ticket

RE: Serial error on nRF54L15DK when no terminal connected

Syed Maysum Abbas Zaidi — Fri, 12 Dec 2025 15:02:08 GMT

Hi,

Thanks for the logs, actually when the COM port for uart30 is not opened on the PC, the on-board debugger keeps that UART interface tri-stated. This means the RX pin is not actively driven and can pick up noise. Even though you have bias-pull-up on the SoC RX pin in devicetree, the line between the SoC and the debugger can still be susceptible to noise when the debugger is not actively driving it. The async UART driver then interprets this noise as continuous RX activity, which shows up as “garbage” and buffer‑release events when no terminal is connected.

You can try to solve this by adding a stronger external pull-up resistor on the RX pin to keep the line stable. Once the RX line is actively driven instead of floating, the unwanted RX activity disappears.

Best Regards,
Syed Maysum

RE: Serial error on nRF54L15DK when no terminal connected

hmolesworth — Wed, 10 Dec 2025 21:08:30 GMT

Pull-ups are required for Rx to cover cases where the external transmitting device isn't (transmitting). Try adding this to pinctrl:

			/* Configure with pull-ups enabled for safety when Rx floating*/
			bias-pull-up;

Edit: Ah, just saw you already did this for UART30; how about UART20? Also best to add pull-ups to all uart pins as a precaution, including Tx as that reverts to GPIO when the UART is disabled

RE: Serial error on nRF54L15DK when no terminal connected

ilnadi — Wed, 10 Dec 2025 17:48:03 GMT

[quote userid="4142" url="~/f/nordic-q-a/126081/serial-error-on-nrf54l15dk-when-no-terminal-connected"]When we open a terminal to the COM port corresponding to uart20 but not to the COM port corresponding to uart20, we seem to receive a lot of garbage.[/quote]

Correcttion: When we open a terminal to the COM port corresponding to uart20 but not to the COM port corresponding to uart30, we seem to receive a lot of garbage.