Fast locking phase-locked loop

• US 8,643,414 B1
• Filed: 02/12/2013
• Issued: 02/04/2014
• Est. Priority Date: 02/13/2012
• Status: Active Grant

• Current Assignee: Rambus Inc.
• Original Assignee: Rambus Inc.
• Inventors: Navid, Reza
• Primary Examiner(s): Houston, Adam
• Application Number:
  US13/765,352
• Time in Patent Office: 357 Days
• Field of Search: 327/147, 327/156
• US Class Current: 327/156
• CPC Class Codes: H03L 2207/08 Modifications of the phase-locked loop for ensuring constant frequency when the power supply fails or is interrupted, e.g. for saving power
• CPC Class Codes: H03L 7/10 for assuring initial synchronisation or for broadening the capture range

Abstract

A phase-locked loop is placed in a low-power mode. The input to the variable-frequency oscillator is stored before the low-power mode is entered. Then, when the phase-locked loop is awakened, the previous input to variable-frequency oscillator is held at the input to the variable-frequency oscillator. While the input to variable-frequency oscillator is being held, the phase of the feedback signal is calibrated to the reference signal. Once the phase difference between the feedback signal and the reference signal is minimized, the normal feedback operation of the phase-locked loop is enabled.

Claims

1. A phase-locked loop, comprising:

a variable-frequency oscillator to produce a first signal;

a phase detector to receive a reference signal and a feedback signal;

a phase-alignment circuit to receive the first signal and an output of the phase detector, the phase-alignment circuit to produce a phase adjusted signal based on the first signal, the phase adjusted signal to be used as the feedback signal;

a loop filter to receive the output of the phase detector; and

,

wherein, after a reactivation of the phase-locked loop but before the loop filter controls the variable-frequency oscillator to adjust a frequency of the first signal, a phase adjustment of the feedback signal is to be made by the phase-alignment circuit to calibrate a phase differential between the feedback signal and the reference signal.

2. The phase-locked loop of claim 1, wherein said variable-frequency oscillator is a digitally controlled oscillator (DCO).

3. The phase-locked loop of claim 1, wherein, based on the output of the phase detector, the phase-alignment circuit is to adjust the phase adjusted signal to minimize a phase error between the reference signal and the feedback signal.

4. The phase-locked loop of claim 1, wherein the phase-alignment circuit comprises a phase-mixer.

5. The phase-locked loop of claim 4, wherein the phase-alignment circuit comprises glitch-free multiplexers to adjust the phase adjusted signal to within ¼

of a cycle.

6. The phase-locked loop of claim 5, wherein the phase adjusted signal is to be further adjusted by the phase mixer to phase align the feedback signal over a full cycle of the reference signal.

7. The phase-locked loop of claim 2, wherein a DCO input value determines a frequency of the first signal and a first DCO input value is to be used before deactivation and after the reactivation of the phase-locked loop but before the loop filter adjusts the DCO input value thereby to cause the DCO to adjust a frequency of the first signal.

8. The phase-locked loop of claim 1, further comprising:

a phase-mixer to receive the first signal and to output a second signal, the second signal to have an approximately constant delay between the reference signal and the second signal regardless of different delays between the reference signal and the first signal.

10. The phase-locked loop of claim 7, wherein the variable oscillator is a digitally controlled oscillator (DCO).

11. The phase-locked loop of claim 7, wherein, based on the output of the phase detector, the phase-alignment circuit is to adjust the phase adjusted signal to minimize a phase error between the reference signal and the feedback signal.

12. The phase-locked loop of claim 7, wherein the phase-alignment circuit comprises a phase-mixer.

13. The phase-locked loop of claim 12, wherein the phase-alignment circuit comprises glitch-free multiplexers to adjust the phase adjusted signal to within ¼

of a cycle.

14. The phase-locked loop of claim 13, wherein the phase adjusted signal is to be further adjusted by the phase mixer to phase align the feedback signal over a full cycle of the reference signal.

15. The phase-locked loop of claim 10, wherein a DCO input value determines a frequency of the first signal and a first DCO input value is to be used before deactivation and after the reactivation of the phase-locked loop but before the loop filter adjusts the DCO input value thereby causing the DCO to adjust a frequency of the first signal.

9. A phase-locked loop, comprising:

a variable oscillator to generate a first signal;

a phase detector to receive a reference signal and a feedback signal;

a phase-alignment circuit to receive the first signal and an output of the phase detector, the phase-alignment circuit to generate a phase adjusted signal based on the first signal,

a loop filter to receive the output of the phase detector; and

,

wherein, after a reactivation of the phase-locked loop but before the loop filter controls the variable oscillator to adjust a frequency of the first signal, a phase adjustment of the phase adjusted signal is to be made by the phase-alignment circuit to calibrate a phase differential between the phase adjusted signal and the reference signal.

16. The phase-locked loop of claim 9, further comprising:

a phase-mixer to receive the first signal and to output a second signal, the second signal to have an approximately constant delay between the reference signal and the second signal thereby compensating for variations in the phase differential between the phase adjusted signal and the reference signal.

17. A method of locking a phase-locked loop, comprising:

holding a first input of a variable-frequency oscillator, the first input determining a frequency of an output signal of the variable-frequency oscillator, the variable-frequency oscillator being part of a feedback loop of the phase-locked loop;

while the first input is being held, producing a phase adjusted signal based on the output signal of the variable-frequency oscillator, the phase adjusted signal controlled to minimize a phase difference between a reference signal and the phase adjusted signal, as measured by a phase detector; and

,

after the phase difference between a reference signal and the phase adjusted signal is minimized, allowing the feedback loop of the phase-locked loop to adjust this first input to match the frequency of the output signal of the variable-frequency oscillator to a frequency of the reference signal.

18. The method of claim 17, wherein the first input comprises a digital value.

19. The method of claim 17, wherein the first input is held at a value corresponding to prior lock condition of the phase-locked loop.

20. The method of claim 17, wherein producing the phase adjusted signal based on the output signal of the variable-frequency oscillator comprises controlling a phase-mixer to minimize the phase difference between the reference signal and the phase adjusted signal, as measured by the phase detector.

21. The method of claim 17, further comprising:

holding, after the phase difference between a reference signal and the phase adjusted signal is minimized, a second input that determines a phase of the phase adjusted signal.