A scanning circuit includes n input ports K1˜Kn arranged in n rows L1˜Ln; and m−2 output ports W2˜(m−1) arranged in m columns P1˜Pm. The n rows and the m columns define a switch matrix including n*m switches. Ends of the switches in the same row are connected to one of the n input ports K1˜Kn, respectively. The ends of the switches in the column P1 are connected to ground. The ends of the switches in the same column of the columns P2˜P(m−1) are connected to a power supply VCC via resistors R2˜R(m−1) and the m−2 output ports W2˜W(m−1), respectively. The ends of the switches in the columns Pm are connected to ground via a resistor Rs and the power supply VCC via the resistors R2˜R(m−1) and diodes D2˜D(m−1), respectively.