A display device includes an oscillator which generates a reference clock signal having a frequency corresponding to frequency information provided from an external device, a register which stores a signal parameter for the reference clock signal, and the signal parameter indicates the number of pulses of the reference clock signal included in one horizontal time, a data driver which applies data signals to data lines connected to pixels based on the one horizontal time, and a controller which changes the signal parameter based on a change in the frequency of the reference clock signal in a way such that the one horizontal time is maintained substantially constant.