External device connection detection circuit

One kind of the external apparatus connection detecting circuit, comprising a first device arranged on a first detecting circuit and a second device arranged on a second detecting circuit, a detecting circuit is provided with a first section a detection pin, said second detecting circuit is provided with a second detecting pin of said first detection circuit further comprises a first comparator, the first comparator detecting the first the voltage on the pin with a first comparison voltage, the voltage on the first detection pin is changed according to whether the first detecting pin is connected with said second detecting pin is a first output terminal of said comparator is different from the first signal indicative of whether the device connected to the second device.