RE: 'AT Host' and 'Modem Shell' via P28, P29, 5V and GND

Syed Maysum Abbas Zaidi — Wed, 24 Dec 2025 16:30:06 GMT

Hi,

Yes it looks correct, SW6 (IFMCU DISCONNECT) is the right and intended solution for your use case.

When SW6 is in the default (left) position, the nRF9160 UART is routed through the on-board interface MCU (which also provides the USB virtual COM port). In this mode, applications such as AT Host and Modem Shell effectively depend on the USB CDC interface being present, which is why UART activity on P28/P29 only starts once a USB serial connection is opened.

By switching SW6 to right position, the interface MCU is physically disconnected as mentioned here. This allows the nRF9160 GPIOs to be used directly by external circuitry without interference or "blocking" from the USB-related logic. This is the intended configuration when using the nRF9160 as a standalone modem.

You are correct that you will need to move SW6 back to the left position to flash the device via USB (Firmware development mode) and It is very unlikely you have damaged the board as DK should be able to support this mode of operation. Thanks.

Best Regards,
Syed Maysum

RE: 'AT Host' and 'Modem Shell' via P28, P29, 5V and GND

kpedersen — Mon, 22 Dec 2025 21:46:59 GMT

MCU DISCONNECT (SW6) seems to be the answer

This little switch that not only controls the power from VIN (which previously was indeed disabled to prevent over-power when USB is also plugged) but also seems to cause that blocking until a USB connection was established.

Perhaps someone can confirm this as the right approach? I am also assuming that I will need to switch it back to be able to flash it again.

Hopefully my fiddling hasn't damaged the board! ;)