ST-LINK V3 VCP "backpowers" board

[svcguy]

Hello,

I'm developing a board with the BlueNRG-M2 module. I'm also using an ST-LINK V3 with uVision to debug/program. I'm also using the VCP made available by the ST-LINK V3 for some good old fashioned printf telemetry. The VCP is connected through the STDC14 connector to the ST-LINK V3. When I use the VCP (specifically once I start PuTTY), I get a voltage on the VDD rail of the board of around 2.4V without the boards normal power source active. I wired up some flywires to a 2x7 connector and found that there is 3.3V potential between VCP_TX and VCP_RX and ground when the debugger is connected to USB. The voltage is present when using both a V3 MINI and a V3 SET (no isolation board). Schematic is attached. TX and RX are connected to the module, the STDC14 and the pin header and nothing else. I'm also lead to believe by the BlueNRG-M2 datasheet that there isn't anything going on inside the module either.

Questions:

Is this normal?

To combat this could I put some blocking diodes on the TX and RX lines and still maintain UART/VCP functionality?

Thanks,

Andy

[TDK]

Seems normal to me. UART lines are idle high.

Schottky diodes or similar could be used to limit the current injection on these lines. They will affect the signal integrity, but it is unlikely to be an issue at typical UART speeds. Or just avoid connecting a debugger to an unpowered board.

[svcguy]

The effect on current consumption is my main concern. The B-STLINK-ISOL should also take care of the issue as well? (provided I can find one)

[TDK]

A level shifter could potentially prevent this, but I'm unsure how that one in particular is implemented.