Clock generation circuit and clock generation method
A clock generation circuit and a clock generation method are provided, which are spread spectrum clock generation and accurate phase control of a reference clock signal and an output clock signal. An input divider unit 70 divides an input clock signal CLKR by 50 to output a divided input clock signal CLKS. A DLL circuit 80 operates to obtain delay control signals DCS 1 , DCS 2 . A modulation circuit 40 modulates, in response to the delay control signals DCS 1 , DCS 2 and a modulation signal MOD output from a modulation control circuit 50 , the divided input clock signal CLKS to output a modulation clock signal CLKN. A phase comparator 11 detects the phase difference between the modulation clock signal CLKN and a divided inner clock signal CLKM. A clock generation unit 20 generates an output clock signal CLKO having frequency corresponding to a phase difference signal from the phase comparator 11.
1. A clock generation circuit which receives a reference clock as an input and spreads a spectrum of an output clock according to a modulation signal, the clock generation circuit comprising:
a divider circuit for receiving either the reference clock or the output clock as a first clock, dividing the first clock, and outputting the first clock, which has been divided, as a divided first clock;
a first delay circuit for generating a plurality of delay divided first clocks having different phases, for the divided first clock;
a selector circuit for selectively outputting, in response to the modulation signal, one of the plurality of delay divided first clocks according to timing of an edge of the first clock present during time other than a period which is from an edge of the divided first clock to an edge of the delay divided first clock of which delay time is maximum; and
a phase comparator for making a comparison between a phase of the one of the reference clock and the output clock not received by the divider circuit as the first clock and a phase of the delay divided first clock selected by the selector circuit and outputting a signal according to a result of the comparison.
2. The clock generation circuit according to claim 1 ,
wherein the first delay circuit has a plurality of unit delay elements which are connected in series, and
wherein the unit delay elements output the delay divided first clocks, respectively.
3. The clock generation circuit according to claim 2 , comprising:
a second delay circuit which has elements having the same device configuration as of the first delay circuit and is provided for controlling bias signals for the plurality of unit delay elements such that the phase difference between a clock input to a first stage of the unit delay elements and a clock output from a last stage of the unit delay elements becomes equal to one cycle of clocks input to the first stage of the unit delay elements and for outputting said bias signals to the first delay circuit.
4. The clock generation circuit according to claim 3 , wherein the clock input to the first stage of the unit delay elements is the reference clock or the output clock.
5. The clock generation circuit according to claim 1 , wherein a cycle of the divided first clock is variably controlled according to the modulation signal.
6. The clock generation circuit according to claim 2 ,
wherein a cycle of the divided first clock is variably controlled according to the modulation signal, and
wherein the number of unit delay elements provided for the first delay circuit is smaller than a value obtained by dividing the cycle of the reference clock or the output clock by the delay time of the unit delay elements.
7. A clock generation method for receiving a reference clock as an input and spreading a spectrum of an output clock according to a modulation signal, the clock generation method comprising the steps of:
receiving either the reference clock or the output clock as a first clock, dividing the first clock, and outputting the first clock, which has been divided, as a divided first clock;
generating a plurality of delay divided first clocks having different phases, for the divided first clock;
selectively outputting, in response to the modulation signal, one of the plurality of delay divided first clocks according to timing of an edge of the first clock present during time other than a period which is from an edge of the divided first clock to an edge of the delay divided first clock of which delay time is maximum; and
making a comparison between a phase of the one of the reference clock and the output clock not outputted as the first clock and a the phase of the selected delay divided first clock to output a signal according to the a result of the comparison.
8. The clock generation method according to claim 7 , wherein the divided value of the reference clock or output clock is variably controlled according to the modulation signal.