The primary power input for the board will be +5V from the USB connector.  This will need to be regulated down to +3.3V for all the circuitry on the board.  In the case where the flash memory is populated on this board and it is connected to provide the image to the target FPGA, it should not be necessary to connect the USB cable and the board will draw power from the target board through the PMOD or UEXT connector.  To enable two different power sources, a diode is placed in-line with the USB power input and a Pch MOSFET is connected in-line with the PMOD/UEXT power pins.  The gate of the MOSFET is connected to a voltage divider from the USB power input so that the MOSFET is off when USB power is present, and enabled when only the PMOD/UEXT power is present.  The gate is also tied to PA28 on the MCU so that the controller can detect when USB power is present, and so that it can also override the signal to deliver power from USB to the PMOD or UEXT connectors for additional flexibility.