A timing controller adapted to a flat display includes a voltage detecting circuit, a clock generator, a first multiplexer and a second multiplexer. The voltage detecting circuit detects a variation of an operating voltage and thus outputs a reset signal. The clock generator outputs a start signal and a first clock signal. The first multiplexer is controlled by the reset signal and coupled to the start signal and a constant voltage. The second multiplexer is controlled by the reset signal and coupled to the first clock signal and a second clock signal. A frequency of the second clock signal is obviously higher than a frequency of the first clock signal.