USB behavior when VDD below 3.3V

[EC.3]

Hello,

In a chip like STM32U585CIU6Q, where the VDDUSB is internally connected to the VDD, what is the behavior of the USB circuitry if the VDD is below 3.3V (e.g., at 2V)? Will the USB just stop working without affecting other non-USB functions, or it will stop working and cause problems to other non-USB functions?

Thanks

[Tesla DeLorean]

>>Will the USB just stop working..

That's not what happens, the USB Standard anticipates particular voltage levels on the D+/D- pins, failure to meet these doesn't mean it stops working, just that it is out-of-spec, and prone to malfunction, or be incompatible with other hardware that are not tolerant of marginal devices.

VDDUSB has generally been broken out, to permit those pins to be powered by an alternative supply, like from the USB 5V supply and regulators you have in the design when a cable is attached, and powered by a PC, HUB, etc. If it's not connected to anything, the pins don't need to be functional..