a high voltage detecting signal such as 5v is sent to the microcontroller 33 therefore the microcontroller 33 can determine whether it is the earphone microphone or the usb apparatus which is connected to the audio port 10 according to the detecting signal available from the detecting unit 31 the microcontroller 33 is also connected to the switch 35 one end of the switch 35 is connected to the first contact 11 and the second contact 12 another end of the switch 35 is selectably and exclusively connected either to the usb circuit 50 or to the audio circuit 70 the microcontroller 33 connects either the usb circuit 50 or the audio circuit 70 to the audio port 10 according to the detecting signal when the high voltage detecting signal is received from the detecting unit 31 the microcontroller 33 connects the usb circuit 50 to the audio port 10 when the low voltage detecting signal is received from the detecting signal the microcontroller 33 connects the audio circuit 70 to the audio port 10 the power management unit 37 is connected to the microcontroller 33 and the audio port 10 the power management unit 37 includes a transistor m a sampling resistor r 2 and a hot swap controller 371 in this embodiment the transistor m is a metal oxide semiconductor field effect transistor mosfet a source of the transistor m is connected to the third contact 13 and also grounded by a capacitor c a drain of the transistor m is connected to the power supply 5v by the sampling resistor r 2 a gate of the transistor m is connected to the hot swap controller 371 the hot swap controller 371 includes two sampling contacts samp 1 samp 2.