our USB Device (mass storage application) is working fine as long as 5V is attached to VDD5V during linux booting. I can then disconnect and reconnect the USB cable and I see my mass storage device again on the PC. But if 5V is NOT attached to VCC5V during booting I can't see my mass storage when connecting the PC. The PC even has not any action when I have the PC's device manager open. It seems like the USB interface of the iMX28 is "off".