Spread spectrum clock generator

1. A method of clock frequency spreading, the method comprising:

adjusting an oscillation period of first and second delay control type oscillators at a predetermined control period interval according to a control signal;

comparing a phase of a clock signal obtained by frequency dividing an output clock signal of said first delay control type oscillator with a phase of a clock signal obtained by frequency dividing a reference clock signal;

generating a frequency control signal based on the comparing of the phase;

supplying the frequency control signal to said first delay control type oscillator as the control signal;

determining a maximum modulation value from a predetermined value, the frequency control signal, and a given modulation degree setting signal;

generating a modulation control signal within the maximum modulation value; and

receiving a value obtained by adding the modulation control signal to the frequency control signal as the control signal and outputting a frequency modulated clock signal.

2. The spread spectrum clock generation circuit according to claim 1 , wherein the value obtained by conversion of the minimum oscillation period of said delay control type oscillator by the control period interval of the oscillation period is set to be constant even when a delay value of an element is changed due to one of manufacturing variations and a condition used, and

wherein the frequency modulated clock signal with a degree of modulation maintained at a desired value is output from said one of said delay control type oscillator.

3. A spread spectrum clock generation circuit, comprising:

first and second delay control type oscillators, each for adjusting an oscillation period thereof at a predetermined control period interval, according to a control signal;

a phase comparator for comparing a phase of a clock signal obtained by frequency dividing an output clock signal of said first delay control type oscillator with a phase of a clock signal obtained by frequency dividing a reference clock signal;

an oscillation frequency control circuit, receiving a result of a phase comparison by said phase comparator, for generating a frequency control signal based on a received result of the phase comparison and supplying the frequency control signal to said first delay control type oscillator as the control signal;

a maximum modulation value determination circuit for determining a maximum modulation value from a predetermined value, the frequency control signal, and a given modulation degree setting signal; and

a modulation signal generation circuit for receiving the maximum modulation value from said maximum modulation value determination circuit and generating a modulation control signal within the maximum modulation value,

wherein said second delay control type oscillator receives a value obtained by adding the modulation control signal from said modulation signal generation circuit to the frequency control signal from said oscillation frequency control circuit as the control signal and outputs a frequency modulated clock signal.

4. The spread spectrum clock generation circuit according to claim 3 , wherein the predetermined value for said maximum modulation value determination circuit is set to a value obtained by converting a minimum oscillation period of one of the delay control type oscillators by the control period interval of the oscillation period thereof.

5. The spread spectrum clock generation circuit according to claim 3 , wherein a minimum value of the frequency control signal is set to a value obtained by converting a minimum oscillation period of one of the delay control type oscillators by the control period interval of the oscillation period thereof.

6. The spread spectrum clock generation circuit according to claim 3 , wherein a predetermined value (B) is set to a value obtained by dividing a minimum oscillation period of the second delay control type oscillator by the control period interval,

wherein said maximum modulation determination circuit outputs a value (B+C)×MS obtained by multiplying the modulation degree setting signal for setting a modulation degree (MS) by a value obtained by adding the predetermined value (B) to the frequency control signal (C), as the maximum modulation value (A),

wherein maximum period modulation of the output clock signal from said second delay control type oscillator is given by (B+C)×MS×D 0 (where D 0 indicates the control period interval),

wherein the frequency control signal (C) when a phase of the output clock signal of said first delay control type oscillator becomes a phase of the reference clock signal is given by T 0 +D 0 ×C, in which the minimum oscillation period is indicated by T 0 , and

wherein a modulation degree given by a formula (B+C)×MS×D 0 /(T 0 +D 0 ×C) is the modulation degree set value (MS) due to B=T 0 /D 0 .

7. The spread spectrum clock generation circuit according to claim 3 , wherein said phase comparator inputs a first frequency-divided clock signal obtained by frequency dividing the reference clock signal by a value M by a first frequency divider,

wherein a second frequency-divided clock signal obtained by frequency dividing an output of said first delay control type oscillator by N (in which the N is a predetermined positive integer) by a second frequency divider is supplied to said phase comparator, and

wherein the value M comprises a predetermined positive integer.

8. The spread spectrum clock generation circuit according to claim 3 , further comprising:

a first programmable counter for receiving said reference clock signal and outputting a first clock signal obtained by frequency dividing the reference clock by a first frequency division ratio;

a second programmable counter for receiving an output from said first delay control type oscillator and supplying a second clock signal obtained by frequency dividing the output from said first delay control type oscillator by N by a second frequency division ratio to a second input terminal of said phase comparator;

an adder for adding the frequency control signal from said oscillation frequency control circuit to the modulation control signal from said modulation signal generation circuit; and

a buffer for receiving an output of said second delay control type oscillator and outputting a modulated clock signal,

wherein said phase comparator receives an output of said first programmable counter at a first input terminal thereof,

wherein said maximum modulation value determination circuit receives the frequency control signal from said oscillation frequency control circuit and the modulation degree setting signal and determines a maximum modulation value, and

wherein said modulation signal generation circuit receives the maximum modulation value output from said maximum modulation value determination circuit and outputs a modulation control signal within the maximum modulation value.

9. The spread spectrum clock generation circuit according to claim 8 , wherein the predetermined value for said maximum modulation determination circuit is set to a value obtained by converting a minimum oscillation period of the second delay control type oscillator by a control period interval.

10. The spread spectrum clock generation circuit according to claim 8 , wherein a minimum value of the frequency control signal is set to a value obtained by converting a minimum oscillation period of the second delay control type oscillators by a control period interval.

11. The spread spectrum clock generation circuit according to claim 8 , further comprising:

a third programmable counter for receiving the output of said second delay control type oscillator and supplying to said buffer a signal obtained by frequency dividing the output of said second delay control type oscillator by a third frequency division ratio as an input.

12. A semiconductor integrated circuit device comprising the spread spectrum clock generation circuit according to claim 1 .

13. A method of clock frequency spreading, the method comprising:

adjusting an oscillation period of first and second delay control type oscillators at a predetermined control period interval according to a control signal;

comparing a phase of a clock signal obtained by frequency dividing an output clock signal of said first delay control type oscillator with a phase of a clock signal obtained by frequency dividing a reference clock signal;

generating a frequency control signal based on the comparing of the phase;

supplying the frequency control signal to said first delay control type oscillator as the control signal;

determining a maximum modulation value from a predetermined value, the frequency control signal, and a given modulation degree setting signal;

generating a modulation control signal within the maximum modulation value; and

receiving a value obtained by adding the modulation control signal to the frequency control signal as the control signal and outputting a frequency modulated clock signal.

14. The method according to claim 13 , further comprising converting a minimum oscillation period of one of the first and second delay control type oscillators by the control period interval of the oscillation period thereof.

15. The method according to claim 13 , further comprising setting a minimum value of the frequency control signal to a value obtained by converting a minimum oscillation period of one of the delay control type oscillators by the control period interval of the oscillation period thereof.

16. The method according to claim 13 , further comprising:

setting a predetermined value (B) to a value obtained by dividing a minimum oscillation period of the second delay control type oscillator by the control period interval;

outputting a value (B+C)×MS obtained by multiplying the modulation degree setting signal for setting a modulation degree (MS) by a value obtained by adding the predetermined value (B) to the frequency control signal (C), as the maximum modulation value (A);

outputting a maximum period modulation of the output clock signal determined by (B+C)×MS×D 0 (where D 0 indicates the control period interval);

determining a frequency control signal (C) when a phase of the output clock signal of said first delay control type oscillator becomes a phase of the reference clock signal is given by T 0 +D 0 ×C, in which the minimum oscillation period is indicated by T 0 ; and

determining a modulation degree given by a formula (B+C)×MS×D 0 /(T 0 +D 0 ×C) where a modulation degree set value (MS) due to B=T 0 /D 0 .

17. The method according to claim 13 , further comprising inputting a first frequency-divided clock signal obtained by frequency dividing the reference clock signal by M by a first frequency divider; and

dividing an output of said first delay control type oscillator by N by a second frequency divider to obtain a second frequency-divided clock signal,

wherein M comprises a predetermined positive integer, and

wherein N comprises a predetermined positive integer.

18. The method according to claim 13 , further comprising:

receiving said reference clock signal and outputting a first clock signal obtained by frequency dividing the reference clock by a first frequency division ratio;

receiving an output from said first delay control type oscillator and supplying a second clock signal obtained by frequency dividing the output from said first delay control type oscillator by N by a second frequency division ratio to a second input terminal of said phase comparator;

adding the frequency control signal to the modulation control signal;

receiving an output of said second delay control type oscillator and outputting a modulated clock signal in a buffer; and

receiving an output of said first programmable counter at a first input terminal thereof.

19. The method according to claim 13 , further comprising setting the predetermined value for said maximum modulation to a value obtained by converting a minimum oscillation period of the second delay control type oscillator by a control period interval.

20. The method according to claim 13 , further comprising setting a minimum value of the frequency control signal to a value obtained by converting a minimum oscillation period of the second delay control type oscillators by a control period interval.