Interface circuit

1. An interface circuit, comprising:

a connector terminal configured to receive an input signal in which one of signal levels is substantially direct current;

an oscillating circuit configured to generate and output an input level determination signal having a frequency higher than a frequency of the input signal;

an output control circuit having a first enable terminal connected to a first power supply node and a second enable terminal receiving the input signal via the connector terminal and an input terminal connected to the oscillating circuit to control whether to output the input level determination signal or not, based on a signal level of the input signal;

an AC coupling element coupled to the output control circuit to transmit the input level determination signal output from the output control circuit; and

a receiving circuit coupled to the AC coupling element to receive the input level determination signal, the receiving circuit comprises a clamp circuit that limits an upper limit value of the input level determination signal and includes a protective diode connected between an input node of the receiving circuit and a power supply terminal,

wherein the oscillating circuit continues to output the input level determination signal when the output control circuit stops outputting the input level determination signal.

2. The interface circuit according to claim 1 , further comprising:

another connector terminal configured to receive another input signal in which one of signal levels is substantially direct current; and

another output control circuit coupled to the another connector terminal and the oscillating circuit to control whether to output the input level determination signal or not, based on a signal level of the another input signal,

wherein the input terminal of the output control circuit via which the output control circuit receives the input level determination signal and another input terminal of the another output control circuit via which the another output control circuit receives the input determination signal are connected electrically in parallel regarding to an output terminal of the oscillating circuit via which the oscillating circuit outputs the input level determination signal.

3. The interface circuit according to claim 1 , wherein the receiving circuit comprises a pulse detector that outputs a connection detecting signal based on presence of the input level determination signal.

4. The interface circuit according to claim 1 , wherein the AC coupling element transmits an AC component of the input level determination signal to the receiving circuit through a capacitor.

5. The interface circuit according to claim 1 , wherein the AC coupling element transmits an AC component of the input level determination signal to the receiving circuit through a transformer.

6. The interface circuit according to claim 1 , wherein the input signal is a binary signal having a first logic level and a second logic level different from the first logic level, and a duration of the first logic level and a duration of the second logic level are not fixed.

7. The interface circuit according to claim 1 , wherein the input signal indicates whether or not other device is connected to the interface circuit through a cable.

8. The interface circuit according to claim 1 , wherein the input signal is an HPD signal defined in an HDMI standard or a DVI standard.

9. A interface circuit, comprising:

a connector terminal configured to receive an input signal in which one of signal levels is substantially direct current;

an oscillating circuit configured to generate and output an input level determination signal having a frequency higher than a frequency of the input signal via an output terminal;

an output control circuit having an input terminal, and coupled to the connector terminal and the oscillating circuit to receive the input level determination signal via the input terminal and to control whether or not to output the input level determination signal, in accordance with a signal level of the input signal, to an AC coupling element which is to be coupled to the output control circuit and configured to transmit the input level determination signal a receiving circuit coupled to the AC coupling element to receive the input level determination signal to a receiving circuit comprising a clamp circuit that limits an upper limit value of the input level determination signal and includes a protective diode connected between an input node of the receiving circuit and a power supply terminal;

another connector terminal configured to receive another input signal in which one of signal levels is substantially direct current; and

another output control circuit having another input terminal, and coupled to the another connector terminal and the oscillating circuit to receive the input level determination signal via the another input terminal and control whether or not to output the input level determination signal, in accordance with a signal level of the another input signal, to another AC coupling element which is to be coupled to the another output control circuit and configured to transmit the input level determination signal,

wherein the input terminal of the output control circuit and the another input terminal of the another output control circuit are connected electrically in parallel regarding to the output terminal of the oscillating circuit.

10. The interface circuit according to claim 9 , further comprising:

an AC coupling element coupled to the output control circuit to transmit the input level determination circuit output from the output control circuit.