Is there any specification for the allowed impedance to ground for the GND pin of a USB2.0 connection to an STM32 microcontroller?
I'm not using VBUS to power my PCB, so it's not really connected. If I don't connect the USB ground at all and only have DN and DP connected to the STM32F429VIT6 it doesn't work.
I have tried different resistances between the USB's ground and the MCU's ground (R105) and it's OK for 0 Ω, 1 kΩ, and 1.5 kΩ, but not 9.1 kΩ. I would like to have something better to go on than a value based on tests performed on a couple of host PCs. Is there an appropriate specification for this?
Ideally, I want VBUS and the USB's ground to be isolated. I know there are ICs that offer this functionality, but I already have a custom PCB with pads for a resistor here.
Note: I'm using a USB-C connector, but USB2.0 through it. CC1 and CC2 are connected to USB ground via 5.1 kΩ resistors.
Edit: The STM32 application note on USB includes the following: