Method and apparatus for providing a clock generation circuit for digitally controlled frequency or spread spectrum clocking

• US 6,366,174 B1
• Filed: 02/21/2000
• Issued: 04/02/2002
• Est. Priority Date: 02/21/2000
• Status: Expired due to Term

• Current Assignee: Lexmark International Incorporated (Ninestar Corporation)
• Original Assignee: Lexmark International Incorporated (Ninestar Corporation)
• Inventors: Booth, James R., Hardin, Keith B., Berry, John B., Richey, John P.
• Primary Examiner(s): KINKEAD, ARNOLD M
• Application Number:
  US09/510,124
• Time in Patent Office: 771 Days
• Field of Search: 331/18, 331/78, 331/1.A, 327/106, 327/159, 375/376, 375/200
• US Class Current: 331/78
• CPC Class Codes: H03L 7/0898 the source or sink current values being variable H03L7/0896 takes precedence
• CPC Class Codes: H03L 7/1072 by changing characteristics of the charge pump, e.g. changing the gain
• CPC Class Codes: H03L 7/1806 the frequency divider comprising a phase accumulator generating the frequency divided signal
• CPC Class Codes: H04B 15/04 the interference being caused by substantially sinusoidal oscillations, e.g. in a receiver or in a tape-recorder
• CPC Class Codes: H04B 2215/064 Reduction of clock or synthesizer reference frequency harmonics
• CPC Class Codes: H04B 2215/067 by modulation dispersion

Abstract

An improved clock generation circuit is provided that operates with a single input clock frequency, and includes a Phase Locked Loop circuit (PLL) with a digital accumulator in the feedback loop, in which either the Most Significant Bit or the Carry Bit of the binary adder is used as the modulated feedback clock to the phase/frequency detector of the PLL. In one embodiment, a fixed add/phase amount is used to drive one of the inputs of the binary adder to generate a fixed output frequency. If it is desired to modulate the output frequency, then an Add Amount Modulator circuit can be provided that presents a varying numeric value to one of the inputs of the binary adder. The MSB or Carry Bit is communicated to an address look-up table, which then outputs an address to a memory circuit, which in turn presents a different add amount to the binary adder. If a periodic modulation is desired, the address look-up table will point to add amounts that create a particular periodic output frequency profile, which could include a Spread Spectrum profile. Certain optional circuits may be included, such as: a pre-multiply or pre-divide circuit to either increase or decrease the frequency of the input clock signal before it reaches the phase/frequency detector of the PLL; an output divider to provide a lower output clock frequency; a feedback divider to lower the VCO'"'"'s output frequency before it is directed into the digital accumulator; a synchronizing input (Sync Input); a synchronizing output (Sync Output); a Base Number register to keep the memory size of the look-up table to a reasonably small value; or a Start Number register to provide the add amount upon initialization.

Claims

1. A clock generating circuit, comprising:

(a) a frequency synthesizer circuit having;

(i) an error detector, (ii)an oscillator output circuit, and (iii) a feedback loop that includes an adder circuit;

(b) said frequency synthesizer circuit receiving;

(i) an input clock signal of a substantially constant frequency, which is in communication with said error detector, and (ii) an add/phase quantity which is in communication with said adder circuit, wherein said add/phase quantity is varied in a periodic manner, thereby causing an output frequency to change in a periodic manner;

(c) said adder circuit having a single bit output that is in communication with said error detector, said output acting as a modulated feedback clock signal, wherein said modulated feedback clock signal comprises a pulse signal that is generated at substantially equal time intervals per count sequence of said adder circuit and that is derived from one of (i) a Carry bit, or (ii) a most Significant Bit of said adder circuit;

(d) at least one latch circuit that temporarily holds an output valve of said adder circuit, and thereby creates said modified feedback clock signal; and

wherein said frequency synthesizer circuit generates a substantially accurate output frequency under control of said add/phase quantity, and wherein said add/phase quantity is generated by an Add Amount Modulator circuit, which comprises;

(i) an address look-up table, and (ii) a memory circuit that stores various add amount numeric quantities;

wherein said memory circuit provides a varying numeric quantity to said adder circuit over time.

2. The clock generating circuit as recited in claim 1, wherein said Add Amount Modulator circuit further comprises at least one of:

(i) a Base number register and a second adder circuit;

(ii) a Start number register and a multiplexer;

(iii) a Modulate ON input and a latching circuit;

or (iv) a data input to said memory circuit in which said memory circuit comprises RAM.

3. The clock generating circuit as recited in claim 2, further comprising at least one of:

(i) a Sync Input signal that resets an address in said address look-up table to a predetermined address;

(ii) a Sync Output signal that synchronizes an external clock circuit to said frequency synthesizer circuit;

or (iii) a Clear Accumulator signal that sets a phase remainder amount in said adder circuit to a predetermined numeric value.

4. A method for controlling a spread spectrum clock generator circuit, said method comprising:

(a) providing an input clock signal to a frequency synthesizer circuit, and providing a controller, said frequency synthesizer circuit having an adder circuit in its feedback loop;

said frequency synthesizer circuit under control of said controller generating a plurality of output frequencies over time using a predetermined spread spectrum profile;

(b) generating a signal pulse for each count sequence of said adder circuit, and using said signal pulse as a modulated feedback clock that is communicated to an error detector of said frequency synthesizer circuit; and

(c) further controlling said frequency synthesizer circuit with an add/phase quantity, so as to generate a more accurate instantaneous output frequency.

5. The method as recited in claim 4, wherein said signal pulse is derived from one of:

(i) a Carry bit, or (ii) a Most Significant Bit of said adder circuit.

6. The method as recited in claim 5, wherein said adder circuit'"'"'s output signals are latched by flip-flops, thereby creating a digital accumulator circuit.

7. The method as recited in claim 4, wherein said signal pulse is generated at substantially equal time intervals per count sequence of said adder circuit.

8. The method as recited in claim 7, wherein said add/phase quantity is varied over time, thereby causing said output frequency to change value over time.

9. The method as recited in claim 4, wherein said error detector comprises one of:

a digital phase/frequency detector, an analog phase detector, a digital phase detector, an analog frequency detector, or a digital frequency detector.

10. A spread spectrum clock generating circuit, comprising:

(a) a frequency synthesizer circuit having;

(i) an error detector, (ii) an oscillator output circuit, and (iii) a feedback loop that includes an adder circuit;

(b) said frequency synthesizer circuit receiving;

(i) an input clock signal of a substantially constant frequency, which is in communication with said error detector, and (ii) an add/phase quantity which is in communication with said adder circuit;

(c) said adder circuit having an output that is in communication with said error detector, said output acting as a modulated feedback clock signal; and

(d) a controller that is configured;

(i) to control said frequency synthesizer circuit so that the oscillator output circuit generates a plurality of output frequencies over time using a predetermined spread spectrum profile, and (ii) to control said add/phase quantity so as to generate a more accurate instantaneous output frequency.

11. The clock generating circuit as recited in claim 10, further comprising at least one latch circuit that temporarily holds an output value of said adder circuit, and thereby creates said modulated feedback clock signal.

12. The clock generating circuit as recited in claim 11, wherein said modulated feedback clock signal comprises a pulse signal that is generated at substantially equal time intervals per count sequence of said adder circuit.

13. The clock generating circuit as recited in claim 12, wherein said modulated feedback clock signal comprises a pulse signal that is derived from one of:

(i) a Carry bit, or (ii) a Most Significant Bit of said adder circuit.

14. The clock generating circuit as recited in claim 10, wherein said add/phase quantity is generated by an N-bit register.

15. The clock generating circuit as recited in claim 10, wherein said add/phase quantity is generated by an Add Amount Modulator circuit, which comprises:

(i) an address look-up table, and (ii) a memory circuit that stores various add amount numeric quantities;

wherein said memory circuit provides a varying numeric quantity to said adder circuit over time.

16. The clock generating circuit as recited in claim 15, wherein said Add Amount Modulator circuit further comprises at least one of:

(i) a Base number register and a second adder circuit;

(ii) a Start number register and a multiplexer;

(iii) a Modulate ON input and a latching circuit;

or (iv) a data input to said memory circuit in which said memory circuit comprises RAM.

17. The clock generating circuit as recited in claim 16, further comprising at least one of:

(i) a Sync Input signal that resets an address in said address look-up table to a predetermined address;

(ii) a Sync Output signal that synchronizes an external clock circuit to said frequency synthesizer circuit;

or (iii) a Clear Accumulator signal that sets a phase remainder amount in said adder circuit to a predetermined numeric value.

18. The clock generating circuit as recited in claim 10, wherein said error detector comprises one of:

a digital phase/frequency detector, an analog phase detector, a digital phase detector, an analog frequency detector, or a digital frequency detector.