A pad circuit includes a pad, a gate driving circuit, a voltage selection circuit, and an ESD detection/avoiding circuit. The gate driving circuit is used to discharge the ESD induced current. The ESD detection/avoiding circuit is used to isolate the ESD induced voltage. The voltage selection circuit selects a higher voltage from a power/ground terminal and the pad and outputs it to the gate driving circuit, so that the pad circuit can be used for the programming and 1/0 operations.