UART only works while ST-Link is connected

[jm-vdai]

I'm using an ST-Link broken off from a NUCLEO board to program a custom board with an STM32G4A1 on it. The issue is that UART transmission only seems to work while the ST-Link is connected - probing the line with a scope shows no activity when the ST-link is disconnected from the board. This is especially confusing since the LED blinks in the while loop are still working, indicating that the program is actually running.

I've used this ST-Link with several custom boards at this point, and never had an issue like this. Any idea as to what might be causing this? The UART is in single wire half duplex mode, in case that matters.

[STea]

Hello @jm-vdai ,

can you give more details about the UART instance and the pins you are using ?

BR

 

[Andrew Neil]

Are you missing a ground connection?

Then, when the ST-Link is connected, it provides the ground ...

:thinking_face: