Patent ID: 7046047

Claim:
A clock switching circuit for switching and outputting a plurality of input clock signals having different frequencies, comprising: a first flip-flop group including at least one flip-flop for taking in an inverted signal of a select signal selecting a first input clock signal or a second input clock signal, said at least one flip-flop operating according to said first input clock signal; a second flip-flop group including at least one flip-flop for taking in said select signal, said at least one flip-flop operating according to said second input clock signal; a third flip-flop group including at least one flip-flop for taking in an output signal of said second flip-flop group, said at least one flip-flop operating according to said first input clock signal; a fourth flip-flop group including at least one flip-flop for taking in an output signal of said first flip-flop group, said at least one flip-flop operating according to said second input clock signal; and an output circuit for disabling output of one signal that is not selected by said select signal, out of said first input clock signal and said second input clock signal, wherein, in a case where each flip-flop of said first to fourth flip-flop groups operates on rising edge of an input signal from a clock input terminal, said output circuit disables and resumes output of said first input clock signal based on an output signal of a first gate circuit depending on whether the first input clock signal is selected, and disables and resumes output of said second input clock signal based on an output signal of a second gate circuit depending on whether the second input clock signal is selected, said first gate circuit performing logical OR operation on an output signal of said first flip-flop group and an inverted signal of an output signal of said third flip-flop group, said second gate circuit performing logical OR operation on an output signal of said second flip-flop group and an inverted signal of an output signal of said fourth flip-flop group.