Versatile feedback system for phase locked loop architecture

1. A phase locked loop (PLL) circuit comprising:

feedback division circuitry for receiving an output signal, the feedback division circuitry arranged to divide the output signal by a first division factor in a first mode of operation, and a second division factor in a second mode of operation;

a switchable feedback divider for dividing signal input to the feedback division circuitry by a mode-specific factor; and

a dynamically switchable divider circuit for dividing signal input to the feedback division circuitry responsive to a control signal.

2. A circuit as claimed in claim 1 , further comprising:

pre-division circuitry for receiving a reference signal, the pre-division circuitry arranged to divide the reference signal by a third division factor in the first mode of operation, and a fourth division factor in the second mode of operation.

3. A circuit as claimed in claim 2 , wherein the ratio of the first and third factors is substantially the same as the ratio of the second and fourth factors.

4. A circuit as claimed in claim 2 , further comprising lock detection circuitry for receiving the pre-divided reference signal and the feedback signal, configured to assert a control signal when the reference signal and the output signal are within a predetermined frequency of each other.

5. A circuit as claimed in claim 2 , further comprising timer circuitry configured to assert a control signal after a predetermined amount of time has elapsed since start up of the phase locked loop circuit.

6. A circuit as claimed in claim 2 , further comprising timer circuitry configured to assert a control signal after a predetermined amount of time has elapsed since reset of the phase locked loop circuit.

7. A circuit as claimed in claim 4 , wherein:

in the first mode of operation the control signal is not asserted; and

in the second mode of operation the control signal is asserted.

8. A circuit as claimed in claim 4 , wherein the feedback division circuitry further comprises:

a dynamically switchable divider circuit for dividing the signal input thereto by a first power of two responsive to the control signal.

9. A circuit as claimed in claim 4 , wherein the pre-division circuitry comprises:

a pre-divider for dividing the signal input thereto by a factor;

a second dynamically switchable divider circuit for dividing the signal input thereto by a second power of two responsive to the control signal.

10. A circuit as claimed in claim 9 , wherein the first and second powers of two are equal.

11. A circuit as claimed in claim 2 , wherein the pre-divided reference signal and the feedback signal are input to a phase frequency detector (PFD), which generates a PFD signal.

12. A circuit as claimed in claim 1 , wherein the output signal is generated by a controlled oscillator.

13. A circuit as claimed in claim 12 , wherein the controlled oscillator is a voltage controlled oscillator.

14. A circuit as claimed in claim 12 , wherein the controlled oscillator receives the PFD signal, and is responsive to the PFD signal.

15. A circuit as claimed in claim 14 , further comprising integration circuitry for modifying the PFD signal prior to it being received by the controlled oscillator.

16. A circuit as claimed in claim 1 , further comprising post division circuitry for dividing the output signal by a fifth factor to generate an output of the PLL.

17. A circuit as claimed in claim 4 , wherein the lock detection circuitry incorporates hysteresis.

18. A phase locked loop (PLL) circuit comprising:

feedback division circuitry for receiving an output signal, the feedback division circuitry arranged to divide the output signal by a first division factor in a first mode of operation, and a second division factor in a second mode of operation;

a switchable feedback divider for dividing by a first integer value in the first mode of operation and the second integer value in a second mode of operation; and

a dynamically switchable divider circuit for dividing signal input to the feedback division circuitry by a power of two in the first mode of operation.

19. A circuit as claimed in claim 18 wherein the first and second division factors of the pre-division circuitry are equal.

20. A circuit as claimed in claim 18 wherein the second integer is substantially equal to the product of the first integer and the power of two.

21. A circuit as claimed in claim 18 , wherein the first and second division factors of the pre-division circuitry are substantially equal.

22. A phase locked loop (PLL) circuit comprising:

feedback division circuitry for receiving an output signal, the feedback division circuitry arranged to divide the output signal by a first division factor in a first mode of operation, and a second division factor in a second mode of operation;

a switchable feedback divider for dividing signal input to the feedback division circuitry by a mode-specific factor;

a dynamically switchable divider circuit for dividing signal input to the feedback division circuitry responsive to a control signal; and

pre-division circuitry for receiving a reference signal, the pre-division circuitry arranged to divide the reference signal by a third division factor in the first mode of operation, and a fourth division factor in the second mode of operation, wherein the ratio of the first and third factors is substantially the same as the ratio of the second and fourth factors.

23. A circuit as claimed in claim 22 , further comprising lock detection circuitry for receiving the pre-divided reference signal and the feedback signal, configured to assert a control signal when the reference signal and the output signal are within a predetermined frequency of each other.

24. A circuit as claimed in claim 23 , wherein the lock detection circuitry incorporates hysteresis.