Method and circuitry for supplying clock to internal circuit

摘要

Clock supply circuitry comprises a phase-locked loop or PLL frequency multiplier for generating a frequency-multiplied clock signal having a frequency that is an integral multiple of the frequency of an input clock signal. The clock supply circuitry further includes a PLL output stability detecting circuit. When the clocksupply circuitry is made to return from a clock supply stopping state in which the PLL frequency multiplier is stopping the generation of the frequency-multiplied clock signal to a clock supply state in which the PLL frequency multiplier is generating and supplying the frequency-multiplied clock signal to an internal circuit, the PLL output stability detecting circuit determines whether the frequency-multiplied clock signal from the PLL frequency multiplier becomes stable. After the PLL output stability detecting circuit determines that the frequency-multiplied clock signal becomes stable, it supplies the frequency-multiplied clock signal from the PLL frequency multiplier as a system clock signal to the internal circuit.