Patent ID: 8063514

Claim:
A power supply selection/detection circuit to select one external power supply from a plurality of external power supplies, comprising: for each of the plurality of external power supplies, a resistance element with one end connected to a respective one of the external power supplies and another end connected to a main power supply line, and a first voltage detector to receive a voltage of the respective external power supply and detect a voltage of the respective external power supply; and for at least one of the plurality of external power supplies, a second voltage detector to detect a voltage between the ends of the resistance element, and a switch connected between the respective external power supply and a ground to short-circuit or open-circuit between the respective external power supply and the ground according to an output of the second voltage detector, wherein the switch has one end connected to a further resistor and is open-circuited if a voltage from the at least one of the plurality of external power supplies exceeds a voltage on the main power supply line and short-circuited if a voltage from the at least one of the plurality of external power supplies is less than a voltage on the main power supply line.