G400D based custom board boots up only if USB client is connected

Hi Everyone, I am debugging a HW custom PCB based on G400D. The SoM starts only if USB client is connected otherwise, the processor seems to ask for a reset (NRST pin is pulled down every 1-2 sec).

I have disconnected the 5V from USB client so it seems to be a problem with D+ and D-. Does anyone have any idea of what it could be? It seems to be a HW issue but don’t see clearly what is wrong. If D- and D+ are not connect the G400 does not start!!!

Just to inform you about investigation: sometimes the issues is not where it seems to be, I removed the voltage supervisor the board starts now without being obliged to connect the USB client cable… so it seems more a voltage level issue now… until next insight.

It may be a little more complicated the NRST line is not an input only but an IO as described by the section 14.4 of the user manual. Therefore some conflicts can occurs. Is there any tips GHI could give about such situation?