ALL-DIGITAL SPREAD SPECTRUM CLOCK GENERATING CIRCUIT WITH EMI REDUCTION EFFECT AND A METHOD FOR CONTROLLING THE SAME

摘要

An all-digital spread spectrum clock generating circuit with EMI reduction effect and a method for controlling the same utilize a digital spread-spectrum clock controlling unit to control a digital controlled oscillator, so that it can directly modulates an output clock frequency. Accordingly, the spectrum of the output clock frequency is spread, and the EMI effect due to the output clock signal is reduced. A spread-spectrum clock controller receives a reference clock counting signal and a dividing clock counting signal generated by a frequency detecting unit. After detecting and judging, the spread-spectrum clock controlling unit modulates and maintains a central frequency of the spread-spectrum clock periodically according to the two counting signals, thereby keeping a stability of the central frequency of the spread-spectrum clock signal and decreasing the complexity of the circuit design.