USB interface circuit and method for low power operation

Described examples include USB port controllers with a control circuit configured to switch from a normal first power mode to a second power mode for reduced power consumption in response a command from a port manager circuit, and to switch from the second power mode to the first power mode in response to detected activity on a communications connection, or a detected connection of a USB device to a USB port connector. After switching back to the first power mode in response to detected communications activity, the control circuit automatically switches operation of the USB port controller back to the second power mode unless a communications transaction addressed to the USB port controller is received within a non-zero certain time after switching from the second power mode to the first power mode.