Semiconductor device and control method for power saving

1. A semiconductor device comprising:

a decoder decoding input signals having given combinations and generating a corresponding output signal;

a power control circuit reducing a supply of power to the decoder when detecting at least one of the given combinations of the, input signals applied to the decoder; and

a switch circuit composed of switching transistors that selectively connect a first terminal and plurality of second terminals and are controlled by the output signal of the decoder.

2. The semiconductor device as claimed in claim 1 , wherein the power control circuit has input terminals connected in parallel with the input terminals of the decoder, and the power control circuit reduces the supply of power when detecting an input applied to the input terminals of the power control circuit.

3. The semiconductor device as claimed in claim 1 , wherein reducing the supply of power by the power control circuit is implemented by disconnecting a power supply terminal of the decoder and the decoder from each other.

4. The semiconductor device as claimed in claim 3 , wherein the power control circuit comprises at least one switch that disconnects the power supply terminal and the decoder from each other.

5. The semiconductor device as claimed in claim 4 , wherein the power control circuit comprises a circuit that generates a signal for controlling said at least one switch.

6. The semiconductor device as claimed in claim 4 , wherein the power control circuit comprises a circuit that generates, based on the given combinations of the input signals applied to the input terminals of the decoder, a signal for controlling said at least one switch.

7. The semiconductor device as claimed in claim 4 , wherein the power control circuit comprises a circuit that generates, based on the given combinations of the input signals applied to the input terminals of the decoder, the common control signal from the input signals applied to the decoder.

8. The semiconductor device as claimed in claim 3 , wherein the power control circuit comprises multiple switches that disconnect the power supply terminal and the decoder, and the multiple switches include first and second switches controlled by a common control signal.

9. The semiconductor device as claimed in claim 8 , wherein the power control circuit comprises a circuit that generates the common control signal.

10. The semiconductor device as claimed in claim 1 , wherein the power control circuit is connected between the input terminals of the decoder and the decoder by bonding wires or metal interconnection lines.

11. The semiconductor device as claimed in claim 1 , wherein the decoder, the power control circuit and the switch circuit are formed on a single chip or separate chips.

12. The semiconductor device as claimed in claim 1 , wherein the decoder and the power supply circuit are formed on a chip, and the switch circuit is provided on another chip, which is connected to the chip by bonding wires.

13. The semiconductor device as claimed in claim 1 , wherein the decoder and the power control circuit are formed on a single chip or separate chips.

14. A method of controlling a semiconductor device having a decoder receiving input signals having given combinations and generating a corresponding output signal, and a switch circuit composed of switching transistors that selectively connect a first terminal and a plurality of second terminals are controlled by the output signal of the decoder, said method comprising the steps of:

detecting at least one of the given combinations of the input signals applied to the decoder; and

reducing a supply of power to the decoder when said at least one of the given combinations of the input signals is detected.