UART reception on nRF54L10 not working reliably without RTT Viewer or debugger attached.

RE: UART reception on nRF54L10 not working reliably without RTT Viewer or debugger attached.

Vidar Berg — Thu, 26 Feb 2026 06:24:57 GMT

Excellent, thanks for confirming what the issue was! This is documented in the "Pin Assigments" chapter of the data sheet: https://docs.nordicsemi.com/bundle/ps_nrf54L15/page/chapters/pin.html. This also includes the Cross power-domain use section linked to earlier.

I would also recommend you have a look at this devacademy course at https://academy.nordicsemi.com/courses/nrf54l-series-express-course/lessons/lesson-2-power-domains-event-system-and-gpio/ which explains why we have this architecture.

RE: UART reception on nRF54L10 not working reliably without RTT Viewer or debugger attached.

Paul Vogel0 — Wed, 25 Feb 2026 20:20:21 GMT

Thanks!!!! this was the problem. I would have sworn I saw some place where the docs said that uart21 and P2.07 & P2.08 were in the same power domain. Can you tell me where that information is at?

As far as I'm concerned now, this ticket can be closed.

RE: UART reception on nRF54L10 not working reliably without RTT Viewer or debugger attached.

Vidar Berg — Wed, 25 Feb 2026 16:31:54 GMT

UARTE21 and P2 are actually in different domains (the numbering can be a bit confusing) and this matches the behaviour you're observing. The solution is to request the constant latency power mode while the UARTE is enabled as described here: https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/app_dev/device_guides/nrf54l/pinmap.html#managing_cross_power-domain_pin_mapping

RE: UART reception on nRF54L10 not working reliably without RTT Viewer or debugger attached.

Paul Vogel0 — Wed, 25 Feb 2026 16:12:50 GMT

That sounds like a plausible cause, but I think the pin / uart selection for this are in the same power domain. We are using the QFAA package. The output from the GNSS arrives on P2.07 and is sent to GNSS from P2.08. uarte21 is being used for tor interface

Is there any chance the TRACEDATA functions could be interfering?

RE: UART reception on nRF54L10 not working reliably without RTT Viewer or debugger attached.

Vidar Berg — Wed, 25 Feb 2026 15:47:41 GMT

Hello,

Thanks for creating a new ticket. Please check if your uart instance and the GPIO port assigned to it reside in different power domains or not. Using a peripheral with a port from another power domain is referred to as Cross power-domain use in the DS and is only supported for selected pins and not in the lowest power mode. The reason I am asking is that when you connect the RTT Viewer, the chip enters debug interface mode and in this mode, all power domains remain powered on during sleep. Which in this case is equivalent to using the constant latency sub power mode mentioned in the linked section in the datasheet.

Best regards,

Vidar