28.3.1.2 VUSB Power

For the internal transceiver of the USB peripheral to operate correctly and meet the USB specifications, the VUSB pin of the device requires a nominal 3.3V power supply, which can be provided by an external source or generated by an internal 5V (V_DD) to a 3.3V (V_USB) voltage regulator. See the USB Power Supply Configurations section for more information. For detailed electrical requirements of the V_USB power supply and characteristics of the internal regulator, refer to the Electrical Characteristics section. The SYSCFG peripheral controls the internal USB voltage regulator and checks the USB regulator status (ready or not ready). Refer to the SYSCFG - System Configuration section for additional information on these registers.