Patent ID: 7865163

Claim:
A time constant automatic adjusting circuit comprising: a filter circuit capable of varying time constant, the filter circuit varying a phase of a clock signal to be input so as to output the clock signal; a phase comparison circuit comparing a phase of an output signal of the filter circuit with the phase of the clock signal, and generating output voltage according to a phase difference between the phase of the output signal of the filter circuit and the phase of the clock signal; at least three comparators comparing the output voltage with a plurality of different voltages; an up-down counter counting a number of output bits of either one of at least three different values, each values being set in accordance with an output result of the comparators; and a control circuit varying the time constant of the filter circuit in accordance with the number of output bits counted by the up-down counter; and a switch capable of connecting or disconnecting the clock signal to be input to the filter circuit and the clock signal to be output from the filter circuit to the phase comparison circuit.