Problem with USB_OTG again

[FredS]

        Dear helpers,

Thanks to valuable assistance from members of this forum I managed to get my project running on a STM Nucleo-H723ZG board, even with a USB-OTG connection to a VCP on my desktop PC. There is only one issue with this connection I can't solve. A Nucleo board has two USB-micro connectors, one for the ST-Link and one apparently for the USB-OTG. When I plug a USB-cable between this connector (CN13) and an USB-port on my desktop the Windows Device Manager shows a (new) port and I can stream my results to the Windows machine. So far, so good!

At first sight this looks fine, but: the Nucleo board appears to receive power via this USB connection, which I don't want to happen. It took me some time to become aware of this phenomenon, but I observed the MCU remained powered while I had switched my DC-supply off !  This also explains the changes in current output of my DC-supply: by minimal changes in voltage, the contribution of the two parallel sources shift from one source to the other.

However, the real reason I don't want the PC to supply power to my MCU is noise. My project is focussed on high precision measurements, for which I designed a galvanic isolation between the MCU and my front-end electronics. Although this isolation forms a barrier for unwanted signals, I want to avoid connection between the 5V from my desktop and the 5V supply for my setup (incl. the MCU).

After reading the information I understand (this excludes most of chapter 62 of RM0468, the Ref.Manual for H723, etc) I tried to create my private USB-connector, with the pins wired as follows:

• USB_OTG_HS_V+      => n.c.
• USB_OTG_HS_DP     => A12
• USB_OTG_HS_DM    => A11
• USB_OTG_HS_GND  => Gnd

To ensure I made no mistakes, I checked the electrical path from the pins on the Nucleo board up to the pins in the USB connector I plug in my desktop: all wires are fine. But still, no VCP appears in the Device Manager when I plug in the self created USB cable. I read several discussions on problems with OTG on this and other forums, without finding a clue for my case.

Can anyone help me out? I'm open for any solution where I can keep my fast USB to VCP connection, without linking the power of my desktop to the MCU.

Thank you in advance for your interest and time,

             Fred Schimmel

[TDK · Accepted solution]

The GND connection between the board and the PC will cause much more trouble than the 5V connection. You can use a USB isolator to isolate the data signals and ground.

If your code requires VBUS to be present in order for USB to be initialized, that can cause the issue. Since it works with VBUS connected, that would be the first thing I'd check. Debug your code, hit pause and examine the state when you think a connection should be there, but it isn't.