I have a need to implement a USB device that is hardwired to a Linux MPU. For mechanical reasons, 3 wires would be ideal (D+, D-, and REF).
The device is self-powered, and while both the Linux MPU and the device are powered from same PSU, the device will boot much faster.
The device is hard-wired: no unplugging/plugging of cables when it's on. It is implemented via a mainstream STM32 with ST's libs.
>Do I need to add a 4th wire for 5V VBUS from host to device?
>What is the risk/danger if I don't?
>What is the role of VBUS sense per USB standard spec for self-powered devices? From what I can tell it is to avoid having live pull-ups on USB DP/DM, but that seems like a lame reason and there must be more.