Patent ID: 7830174

Claim:
An input/output circuit operable in an input mode and an output mode, the input/output circuit comprising: an input/output terminal; a pull-up output transistor; a pull-down output transistor; a first logic circuit that operates in accordance with data and an enable signal, the first logic circuit including an output node connected to the gate of the pull-up output transistor and including a P-channel MOS transistor, wherein the first logic circuit activates the pull-up output transistor in the output mode and inactivates the pull-up output transistor in the input mode; a second logic circuit that operates in accordance with the data and the enable signal, wherein the second logic circuit operates the pull-down output transistor in a complementary manner with respect to the pull-up output transistor in the output mode, and the second logic circuit inactivates the pull-down output transistor in the input mode; a control circuit for maintaining the pull-up output transistor in the inactivated state regardless of voltage being applied to the input/output terminal in the input mode; a switch circuit for disconnecting the first logic circuit from a power supply when an input signal having voltage higher than the power supply voltage of the power supply is input to the input/output terminal in the input mode, the switch circuit including a P-channel MOS transistor; and a back gate control circuit for supplying back gates of the P-channel MOS transistors arranged in the first logic circuit and the switch circuit with back gate voltage having the same voltage as the input signal when the input signal is input to the input/output terminal in the input mode, wherein: the P-channel MOS transistor of the switch circuit is a power supply disconnection P-channel MOS transistor that is inactivated by the input signal when the input signal is input to the input/output terminal; and the switch circuit includes a transfer gate for connecting the input/output terminal and the gate of the power supply disconnection P-channel MOS transistor when the input signal is input to the input/output terminal.