One of my nRF52840-DK outputs modem commands via UART on bootup

[mlac]

Hi guys,

I'm having a very weird issue. I flashed an application of an empty main() function to my nRF52840-DK boards. When I power up one of these boards and quickly connect to its UART console, the following appears:

T^SQPORT?
AT
AT
AT
AT
AT
AT

There's about a 3-second delay between the individual lines, which look like AT commands of a modem.

Only the following nRF52840-DK is affected, which features an nRF5340 interface MCU:

My other nRF52840-DK, which utilizes an ATSAM3U2C interface MCU, is not affected by this issue:

Any idea what's going on with the affected board and how to fix it?

[Kazi Afroza Sultana]

Hello,

Could you please try to erase and recover the DK (which shows problem on UART) using nrfjprog command?

Also you can try to change the USB cable for this DK and try again the same process? When DK is powered by USB port J2, UART is there by default. Changing the cable may show some difference? 

Thanks.

BR
Kazi

[mlac]

Hi Kazi,

I've used the following commands:

nrfjprog --family nRF52 --eraseall
nrfjprog --recover

Afterward, I reflashed the firmware and the issue persists.

[Kazi Afroza Sultana]

Hello,

Instead of empty main could you please try this sample https://github.com/nrfconnect/sdk-zephyr/tree/main/samples/basic/minimal. 

Any change after changing cable? 

[mlac]

The problem persists with an alternative USB cable.

The problem doesn't persist with the minimal sample, but this sample is irrelevant because it doesn't expose a console. However, I can reproduce the issue when adding the CONFIG_SHELL=y Kconfig option to the minimal sample. I think this Kconfig option shouldn't make the issue surface.

[Kazi Afroza Sultana]

Hello,

Can you add CONFIG_UART_CONSOLE=y in the prj.conf file?