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VBUS Sensing enable - USB FS STM32H7A3 - Nucleo Board

Sam A

Currently using Nucleo-H7A3ZI-Q to test the USB FS interface and using Cube FW 1.10.1 -

Using Nucleo end as USB Device mode with CDC class.

  1. The schematics of the nucleo board tells me both the VDD50USB and VDD33USB is internally self powered and tied together to 3.3VDC, I don't really require the USB Host to provide 5V.
  2. But in the Cube FW
    1. When the vbus sensing is enabled (hpcd_USB_OTG_HS.Init.vbus_sensing_enable = ENABLE), nucleo board already has the voltage divider for vbus sensing line that feeds to 5V tolerant pin PA9. Conditions below -
      1. If the USB Host is not providing 5V and
      2. If the vbus sensing is enabled then
      3. Result - USB doesn't work or doesn't get enumerated.
    2. When vbus sensing is disabled,
      1. Irrespective of USB Host providing 5V or not
      2. Result - USB works as expected and gets enumerated.
    3. When vbus sensing is enabled,
      1. Only when USB Host provides 5V
      2. Result - USB works as expected and gets enumerated.

I followed the AN4879 documentation to understand more on this topic of vbus sensing and it looks like vbus sensing is mandatory when self powered which in the case of nucleo board - yes its self powered but it seems like turning on the vbus sensing without the external 5V supply is not making the USB work at all. Does the term "self powered" mean setting nucelo as USB Host ? or does it mean self powered internally to turn on the USB transceiver (VDD50USB ON and VDD33USB ON) to bypass the internal USB regulator?

It almost looks like if vbus sensing is enabled, then it expects the external 5V from USB Host, otherwise the USB transceiver is shut off even though the VDD50USB and VDD33USB are powered internally and hard tied to 3.3 net as per schematics.

Is this the intended behavior of the Cube FW enabling this bit in this setup ?



Senior III

Did you get any a reply from ST staff regarding this issue?

There seems to be a lack of clear data on the USB front at the moment.