A timing controller includes a receiver, an internal clock generator, a first frequency converter, a first selector and a control signal generator. The receiver receives an image signal and a main clock signal having a first spread spectrum frequency from an external system, converts the main clock signal to a converted main clock signal and the image signal to a first converted image signal, and outputs the converted main clock signal as a first clock signal having a plurality of frequencies. The internal clock generator multiplies the frequencies of the first clock signal and generates a second clock signal having a frequency band within the multiplied frequencies of the first clock signal. The first frequency converter converts the second clock signal to a third clock signal having a second spread spectrum frequency. The first selector selects one of the second clock signal and the third clock signal in response to a first selection signal and outputs the selected one of the second clock signal and the third clock signal as a control clock signal. The control signal generator receives the control clock signal to generate a control signal synchronized with the control clock signal.