H743ZI2 USB OTG with external power source

[Smoun.1]

Hello,

I have a Nucleo H743ZI2 on which I am communicating with the micro AB USB port through USB OTG. Up until now I always had the main USB (STlink) plugged in to power the board. Eventually I want to power the nucleo board with an external 3.3 power source (another board in the system).

However whenever I do that (i.e. power the H7 with the external 3.3V source), the USB OTG won't enumerate and get detected by the PC.

The datasheet is not helping, actually, my understanding is that ST actually recommends the use of the an external power supply when using USB OTG. Quote:

1.It is recommended to power the Nucleo-144 board with an external power supply when using USB OTG or host function.

I am using the USB OTG as device only, SOF and VBus all disabled.

Does anyone know what I am missing here?

cheers

[Smoun.1]

I tried with an external 5V (to the 5V pin of course) and same result. No USB device detection whenever the STlink USB isn't connected.

[waclawek.jan]

And the processor works otherwise? Blinky blinks?

If yes, then debug in the old fashioned way: printf, pin wiggling...

JW

[Smoun.1]

Yes, everything worked as normal. I fixed the problem by supplying 3.3V to the STlink CN2 connector on pin 1.

i.e. the stlink MCU needs to be powered on to enable USB OTG. Not sure why though; it was a bit of a long shot but that worked.

[waclawek.jan]

Does the target have a crystal? I am not familiar with the H7 and the nucleo boards.

JW

[Smoun.1]

that's the strange part.

Yes it uses the crystal that's on the STlink. But I am using bypass.

So whenever using an external 3.3 my program would work, only the USB wouldn't be detected.

not sure why the H7 would be able to run the program without proper clocking. Fact it works shows it's not a clock issue, however that doesn't explain the USB wouldn't be detected whenever the STlink isn't powered on. Strange