Separate power supply

[Sigma Relief]

I am on my 2nd V-USB project for what essentially amounts to a battery powered high power stepper motor controller. The catch is the USB interface is optional and usually will not be present. I know this will cause some issues with software when I go to implement USB control, but for now, I'm working on board layout for the +5V rail. I don't know the finer points of the USB protocol, but my first thought is to simply leave the +5V from the PC unconnected. To my knowledge, the PC does not monitor the +5V rail for anything other than over current conditions, so it shouldn't care where the 3.3V logic is coming from. Please let me know If I am missing anything.

[PHermansson]

There is a setting in usbconfig.h (I think) about "self-powered" device.
I would try to use this, and connect the USB 5V to a I/O-pin. That can then be used in the code detect if USB is connected or not.