Clock generator and clock generating method capable of varying clock

Abstract

A clock generator including a frequency multiplier, a phase lock circuit and a frequency divider. The frequency multiplier generates a frequency multiplied clock by multiplying the frequency of an input clock. The phase lock circuit detects a phase difference between the input clock and a frequency divided clock, and generates, by delaying the frequency multiplied clock by an amount corresponding to the phase difference, a phase-locked clock with its phase locked with the input clock. The frequency divider detects in every fixed cycle a particular pulse of the phase-locked clock, and generates the frequency divided clock by dividing the phase-locked clock with reference to the particular pulse of the phase-locked clock. In particular, the frequency divider detects the particular pulse immediately previous to a falling edge of the input clock. This can reduce the phase difference between the input clock and the phase-locked clock, and hence to solve a problem of a conventional clock generator in that a delay time of a digital delay line in a phase lock circuit must be lengthened with a reduction in the multiplication number of the frequency multiplied clock, which requires a greater number of delay elements because of a large occupying area of the delay elements and a decoder, thereby increasing the circuit scale and cost of a chip to reduce the multiplication number of the frequency multiplied clock.