UART0 TX pin sometimes pulled low after suspending uart0 and putting MCU into System OFF

RE: UART0 TX pin sometimes pulled low after suspending uart0 and putting MCU into System OFF

Mike Austin (LPI) — Sun, 07 Sep 2025 23:16:20 GMT

Hi Ressa,

I managed to solve my problem. It was pretty simple in the end.

Stupidly, I had a printk() statement that I called just before putting the MCU into System OFF. I understand now that the firmware will return from that printk() statement the moment the final character has been sent for TX-ing, so the MCU may be in the middle of transmitting that last character still, when the sys_poweroff() call is made.

Once the sys_poweroff() call is made, the current state of all GPIO is "frozen" during the System OFF period, so if the UART had TX low at that point, that is the state that will latched for the TX pin during System OFF.

And because its effectively a race condition, the likelihood that TX will be either low or high when the MCU enters System OFF will be somewhat random, which is essentially what I was seeing.

I have removed that printk() statement, and now reconfigure the TX pin as a GPIO, set it high, then call sys_poweroff().

That ensures the TX pin behaviour is as I would want it to be

Regards,

Mike

RE: UART0 TX pin sometimes pulled low after suspending uart0 and putting MCU into System OFF

Ressa — Mon, 01 Sep 2025 14:43:02 GMT

Hi Mike,

Could you provide the schematic of the design or at least UART pins connections, and how they are connected to the LEDs ? Also could you provide the source code here? I would like to see if it is reproducible here. Do you know exactly how many devices having this issue out of how many samples? And also the amount of current it is drawing vs normal situation ?

RE: UART0 TX pin sometimes pulled low after suspending uart0 and putting MCU into System OFF

Mike Austin (LPI) — Fri, 29 Aug 2025 14:50:26 GMT

Hi Ressa,

Not sure that answers my question.

That ticket seems to be how to configure the UART so that the sleep configuration doesn't get applied. Specifically, Vidar says: "if you don't want the 'uart1_sleep' group to be applied on suspend"

Why does applying the UART sleep group cause my TX pin to get set low?

I would have though putting the UART device into suspend and then calling sys_poweroff() was all that was needed?

Is this random GPIO behavior some sort of errata?

Regards,

Mike

RE: UART0 TX pin sometimes pulled low after suspending uart0 and putting MCU into System OFF

Ressa — Fri, 29 Aug 2025 12:45:56 GMT

Hi,

Have you looked into this ticket? It sounds related to your observation:
(+) pm_device_action_run for UART causes GPIOs to switch states - Nordic Q&A - Nordic DevZone - Nordic DevZone

Best regards,
Ressa