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
