Phase-locked loop slip detector

1. A phase-locked loop (PLL), comprising:

a phase-frequency detector (PFD) having a first PFD input, a second PFD input, and a PFD output, the PFD configured to generate a first signal on the PFD output, the first signal comprising pulses having pulse widths indicative of a phase difference between signals on the first PFD input and the second PFD input;

a low pass filter (LPF) having an LPF input and an LPF output, the LPF input coupled to the PFD output, wherein the LPF is configured to remove from the first signal the pulses having a pulse width less than a threshold phase difference;

a flip-flop having a clock input and a flip-flop output, the clock input coupled to the LPF output; and

a lock-slip control circuit coupled to the flip-flop output and to the first PFD input, the lock-slip control circuit configured to determine phase-lock and phase-slip based at least in part on a signal on the flip-flop output;

wherein the first PFD input is configured to receive a reference clock;

wherein the lock-slip control circuit is configured to assert a lock signal indicating that the PLL is in phase-lock responsive to no edges being detected on the flip-flop output for a predetermined number of cycles of the reference clock;

wherein the lock-slip control circuit is configured to de-assert the lock signal indicating that the PLL is not in phase-lock responsive to at least a first number of X edges being detected on the flip-flop output for a second number of cycles of the reference clock; and

wherein the lock-slip control circuit is configured to assert a quasi slip signal responsive to at least one edge being detected on the flip-flop output while the lock signal is asserted.

2. The PLL of claim 1 , wherein the flip-flop has a flip-flop input, and wherein the PLL includes an inverter coupled between the flip-flop output and the flip-flop input.

3. The PLL of claim 1 , wherein:

the first PFD input is configured to receive the reference clock; and

the lock-slip control circuit is configured to assert the lock signal indicating that the PLL is in phase-lock responsive to no edges being detected on the flip-flop output for the predetermined number of cycles of the reference clock.

4. The PLL of claim 1 , wherein:

the first PFD input is configured to receive the reference clock; and

the lock-slip control circuit is configured to de-assert the lock signal indicating that the PLL is not in phase-lock responsive to at least the first number of X edges being detected on the flip-flop output for the second number of cycles of the reference clock.

5. The PLL of claim 4 , wherein the lock-slip control circuit is configured to also de-assert the lock signal indicating that the PLL is not in phase-lock response to least a third number of X edges being detected on the flip-flop output for a fourth number of cycles of the reference clock, wherein the third number is larger than the first number, and the fourth number is larger than the second number.

6. The PLL of claim 5 , wherein the LPF has a frequency response such that a pulse on the first signal having a width less than a threshold, indicative of phase-lock, is filtered out of, and not present on, a LPF output signal on the LPF output.

7. A lock-slip detect circuit for a phase-locked loop (PLL), comprising:

a low pass filter (LPF) having an LPF input and an LPF output, the LPF input configured to receive a first signal comprises pulses having pulse widths indicative of a phase difference between clock signals, and the LPF configured to remove from the first signal the pulses having a pulse width less than a threshold phase difference;

a flip-flop having a clock input and a flip-flop output, the clock input coupled to the LPF output; and

a lock-slip control circuit coupled to the flip-flop output, the lock-slip control circuit configured to determine phase-lock and phase-slip based at least in part on a signal on the flip-flop output;

wherein a first of the clock signals is a reference clock;

wherein the lock-slip control circuit is configured to assert a lock signal indicating phase-lock between the clock signals responsive to no edges being detected on the flip-flop output for a predetermined number of cycles of the reference clock;

wherein the lock-slip control circuit is configured to de-assert the lock signal indicating phase slip between the clock signals responsive to at least a first number of X edges being detected on the flip-flop output for a second number of cycles of the reference clock; and

wherein the lock-slip control circuit is configured to assert a quasi slip signal responsive to at least one edge being detected on the flip-flop output while the lock signal is asserted.

8. The lock-slip detect circuit of claim 7 , wherein the flip-flop has a flip-flop input, and wherein the lock-slip detect circuit includes an inverter coupled between the flip-flop output and the flip-flop input.

9. The lock-slip detect circuit of claim 7 , wherein a first of the clock signals is the reference clock, and wherein the lock-slip control circuit is configured to assert the lock signal indicating phase-lock between the clock signals responsive to no edges being detected on the flip-flop output for the predetermined number of cycles of the reference clock.

10. The lock-slip detect circuit of claim 7 , wherein a first of the clock signals is the reference clock, and wherein the lock-slip control circuit is configured to de-assert the lock signal indicating phase slip between the clock signals responsive to at least the first number of X edges being detected on the flip-flop output for the second number of cycles of the reference clock.

11. The lock-slip detect circuit of claim 10 , wherein the lock-slip control circuit is configured to also de-assert the lock signal indicating phase slip response to least a third number of X edges being detected on the flip-flop output for a fourth number of cycles of the reference clock, wherein the third number is larger than the first number, and the fourth number is larger than the second number.

12. The lock-slip detect circuit of claim 7 , wherein the lock-slip control circuit comprises a counter.

13. A phase-locked loop (PLL), comprising:

a voltage-controlled oscillator (VCO) having a VCO output configured to generate an output clock;

a phase-frequency detector (PFD) having a first PFD input, a second PFD input, and a PFD output, the first PFD input configured to receive a reference clock, the second PFD input coupled to the VCO output, the PFD configured to generate a first signal on the PFD output, the first signal comprising pulses having pulse widths indicative of a phase difference between the reference clock and the output clock; and

a lock-slip detect circuit coupled to the PFD, the lock-slip detect circuit configured to determine phase-slip based at least in part on the first signal, wherein the lock-slip detect circuit includes:

a low pass filter (LPF) having an LPF output;

a flip-flop having a clock input, a flip-flop input, and a flip-flop output, the clock input coupled to the LPF output; and

a lock-slip control circuit coupled to the flip-flop output, the lock-slip control circuit configured to:

assert a lock signal indicating phase-lock between the output clock and the reference clock responsive to no edges being detected on the flip-flop output for a predetermined number of cycles of the reference clock; and

assert a quasi slip signal responsive to at least one edge being detected on the flip-flop output while the lock signal is asserted.

14. The PLL of claim 13 , wherein the lock-slip detect circuit comprises:

the LPF having an LPF input, the LPF input coupled to the PFD output; and

an inverter coupled between the flip-flop output and the flip-flop input.

15. The PLL of claim 14 , wherein the lock-slip detect circuit is configured to determine phase-slip based at least in part on a signal on the flip-flop output.

16. The PLL of claim 15 , wherein the lock-slip control circuit is configured to determine phase-slip responsive to an occurrence of at least a first number of pulses of the signal on the flip-flop output in a second number of cycles of the reference clock.