Patent ID: 7808306

Claim:
A power supply voltage control apparatus, comprising: a voltage control oscillator that generates a clock signal; a comparator that performs a phase comparison or a frequency comparison between a divided signal of a system clock signal and a divided signal of the clock signal generated by the voltage control oscillator; a power supply voltage generator that generates a power supply voltage to be supplied to the voltage control oscillator and at least one internal circuit, based on an output of the comparator, the power supply voltage generator comprising: a preset value storage that stores preset values that are set, by a predetermined value second than respective minimum operation power supply voltages corresponding to a plurality of system clock frequencies; a controller that, upon switching of a system clock frequency, reads out a preset value stored in the preset value storage, and outputs the read-out preset value as a power supply voltage preset value; and a digital-to-analog converter that generates the power supply voltage to be supplied to the voltage control oscillator and the at least one internal circuit, based on an output of the controller in addition to the output of the comparator, and wherein, after the system clock frequency is switched from a first system clock frequency to a second system clock frequency that is lower than the first system clock frequency, the controller reads out a preset value corresponding to the first system clock frequency from the preset value storage, and outputs the read-out preset value as the power supply voltage preset value.