A semiconductor device connected to a bus consisting of a plurality of signal lines, comprises a first pad connected with a discrete resistor corresponding to the impedance of the signal lines, a plurality of second pads respectively connected with the signal lines, a reference voltage generator for generating a reference voltage, a comparator for comparing the voltage on the first pad with the reference voltage to generate a control signal, a code generator for generating a code signal according to the control signal, a current source for supplying the first pad with variable current according to the code signal, and a data driver for driving data signals to the signal lines connected with the second pads according to the code signal. The code signal is used to match the impedance of the data driver with the impedance of the signal lines.