Patent ID: 7406144

Claim:
A clock generating circuit, comprising: a clock generator for receiving a reference clock signal and thereby generating an output clock signal; and a spread spectrum control circuit, coupled to the clock generator, for generating a modulated clock signal with a frequency variation according to the output clock signal and a modulation value, comprising: a modulation value generating circuit for outputting the modulation value; and a frequency control circuit, coupled to the clock generator and the modulation value generating circuit, for generating the modulated clock signal according to the output clock signal and the modulation value with which an average frequency of the modulated clock signal varies; wherein the modulation value varies with time in a predetermined manner so as to force the average frequency of the modulated clock signal to change up and down over time, the clock generator operates in a way being independent of the spread spectrum control circuit when the average frequency of the modulated clock signal changes up and down over time, and the output clock signal generated by the clock generator and the modulated signal generated by the spread spectrum control circuit are separate signals.