Digital locking loop circuit and method of operation

1. A digital locking loop circuit comprising:

a digitally-controlled frequency generator;

a digital loop filter configured to output a digital control signal for inputting to the digitally controlled frequency generator; and

a multi-stage time-to-digital converter configured to detect phase error between an input reference clock signal and an output signal fed back from the digitally-controlled frequency generator, and to output the phase error as a control input to the digital loop filter, to cause the digital control signal output by the digital loop filter to adjust the digitally-controlled frequency generator to decrease the phase error; wherein the multi-stage time-to-digital converter includes:

a plurality of phase-error detection stages, each respective phase-error detection stage being configured to detect a respective phase error component at a respective resolution; and

combinatorial logic configured to combine the respective phase error components into a phase error signal.

2. The digital locking loop circuit of claim 1 wherein the plurality of phase-error detection stages comprises:

a first phase-error detection stage configured to detect phase error at a first resolution that is integer multiples of a time period of the output signal fed back from the digitally-controlled frequency generator;

a second phase-error detection stage configured to detect phase error at a second resolution that is a coarse fraction of the time period of the output signal fed back from the digitally-controlled frequency generator; and

a third phase-error detection stage configured to detect phase error at a third resolution that is a fine fraction of the time period of the output signal fed back from the digitally-controlled frequency generator, the fine fractions being smaller than the coarse fractions.

3. The digital locking loop circuit of claim 2 wherein:

the output signal fed back from the digitally-controlled frequency generator comprises a plurality of output phases;

the second phase-error detection stage operates on the plurality of output phases; and

the coarse fraction comprises a respective period of one the output phases.

4. The digital locking loop circuit of claim 3 wherein the first phase-error detection stage and the third phase-error detection stage both operate on a particular one of the plurality of output phases.

5. The digital locking loop circuit of claim 4 wherein:

the digitally-controlled frequency generator is a digitally-controlled oscillator configured to change its frequency in response to a digital control signal, whereby the digital locking loop circuit is a digital phase-locked loop circuit having an adjustable target frequency;

the digital phase-locked loop circuit further comprises a fractional phase interpolator on the particular one of the plurality of output phases, the fractional phase interpolator being configured to adjust the target frequency by a fractional amount; and

the second stage comprises conversion circuitry configured to compensate for a fractional phase on the particular one of the plurality of output phases.

6. The digital phase-locked loop circuit of claim 5 wherein the conversion circuitry is configured to determine a phase error attributable to the fractional phase interpolator and a delay attributable to the fractional phase interpolator.

7. The digital locking loop circuit of claim 3 wherein:

the coarse fraction bears a particular proportion to the time period; and

the fine fraction bears the particular proportion to the coarse fraction.

8. The digital locking loop circuit of claim 3 wherein the plurality of stages operate in parallel to provide different portions of the phase error signal.

9. The digital locking loop circuit of claim 1 wherein:

the digitally-controlled frequency generator is a digitally-controlled oscillator, whereby the digital locking loop circuit is a digital phase-locked loop circuit having an adjustable target frequency;

the digital phase-locked loop circuit further comprises a fractional phase interpolator configured to adjust the target frequency by a fractional amount, by shifting an edge of at least one cycle of the output signal fed back from the digitally-controlled oscillator; and

at least one stage of the plurality of stages comprises conversion circuitry configured to compensate for a fractional phase on the output signal fed back from the digitally-controlled oscillator.

10. The digital locking loop circuit of claim 1 wherein the plurality of stages operate in parallel to provide different portions of the phase error signal.

11. The digital locking loop circuit of claim 1 wherein the digitally-controlled frequency generator is a digitally-controlled oscillator configured to change its frequency in response to a digital control signal, whereby the digital locking loop circuit is a digital phase-locked loop circuit.

12. The digital locking loop circuit of claim 1 wherein the digitally-controlled frequency generator is a digital delay line including a plurality of controllable delay elements, whereby the digital locking loop circuit is a digital delay-locked loop circuit.

13. A method for controlling a digital locking loop circuit, the method comprising:

detecting phase error at a first resolution that is integer multiples of a time period of a feedback signal of the digital locking loop circuit;

detecting phase error at a second resolution that is a coarse fraction of the time period of the feedback signal of the digital locking loop circuit;

detecting phase error at a third resolution that is a fine fraction of the time period of the feedback signal of the digital locking loop circuit, the fine fractions being smaller than the coarse fractions; and

combining the phase error at the first resolution, the phase error at the second resolution, and the phase error at the third resolution, into a phase error signal that is a control input to the digital locking loop circuit.

14. The method of claim 13 wherein:

detecting phase error at the second resolution comprises operating on a plurality of output signal phases; and

the coarse fraction comprises a respective period of one the output signal phases.

15. The method of claim 14 wherein both the detecting phase error at the first resolution, and the detecting phase error at the third resolution, comprise operating on a particular one of the plurality of output phases.

16. The method of claim 15 further comprising:

interpolating a fractional phase of the particular one of the plurality of output phases fed back from the digitally-controlled oscillator by shifting an edge of at least one cycle of the particular one of the plurality of output phases, to adjust the target frequency by a fractional amount; wherein:

detecting phase error at the second resolution further comprises compensating for the fractional phase on the particular one of the plurality of output phases.

17. The method of claim 16 wherein the compensating for the fractional phase comprises determining a phase error attributable to interpolation of the fractional phase and a delay attributable to interpolation of the fractional phase.

18. The method of claim 13 wherein the detecting phase error at the first resolution, the detecting phase error at the second resolution, and the detecting phase error at the third resolution, are performed in parallel to provide different portions of the phase error signal.

19. The method of claim 13 further comprising:

interpolating a fractional phase on the feedback signal of the digital locking loop circuit by shifting an edge of at least one cycle of the feedback signal; wherein:

the generating a control input comprises compensating for the fractional phase on the feedback signal of the digital locking loop circuit.

20. The method of claim 19 wherein the compensating for the fractional phase comprises determining a phase error attributable to interpolation of the fractional phase and a delay attributable to interpolation of the fractional phase.