Wide-range clock multiplier
A variable-frequency input clock signal and a reference clock signal are compared during a frequency-compare interval to generate a value that indicates a ratio of their frequencies. The frequency-ratio value is then applied to configure a wide-range frequency-locking oscillator for operation with a narrowed input frequency range. Because the narrowed input frequency range is targeted to the input clock frequency, the wide-range oscillator is able to rapidly lock to a frequency multiple of the input clock frequency. Because the frequency-compare interval is also brief, an extremely fast-locking, clock-multiplying operation may be effected over a relatively wide range of input clock frequencies.
1. A method of operation within an integrated circuit device, the method comprising:
determining a frequency ratio between an input clock signal and a first reference clock signal during a first interval; and
selecting, based at least in part on the frequency ratio, one of a plurality of frequency-multiplying oscillators to generate an output clock signal that is a frequency multiple of the input clock signal.
2. The method of claim 1 further comprising enabling one of the frequency-multiplying oscillators to generate the first reference clock signal.
3. The method of claim 2 wherein enabling the one of the frequency-multiplying oscillators to generate the first reference clock signal comprises establishing a free-running oscillation mode within the one of the frequency-multiplying oscillators.
4. The method of claim 1 wherein determining the frequency ratio between the input clock signal and the first reference clock signal comprises counting edges of the first reference clock signal over the first interval, wherein a start of the first interval corresponds to a first transition of the input clock signal and an end of the first interval corresponds to a second transition of the input clock signal.
5. The method of claim 4 wherein the first and second transitions of the input clock cycle define a cycle of the input clock signal.
6. The method of claim 1 wherein determining the frequency ratio between the input clock signal and the first reference clock signal comprises:
counting edges of the first reference clock signal over a first sub-interval that corresponds to a first cycle of the input clock signal to generate an edge-count value;
comparing the edge-count value to a threshold; and
counting edges of the first reference clock cycle over a second sub-interval if the edge-count indicates a number of edges of the first reference clock signal fewer than the threshold.
7. The method of claim 6 wherein the second sub-interval comprises a second cycle of the input clock signal.
8. The method of claim 6 further comprising incrementing a cycle-count value if the edge-count value indicates a number of edges of the first reference clock signal fewer than the threshold, the cycle-count value forming a field within a value indicative of the frequency ratio.
9. The method of claim 1 wherein the plurality of frequency-multiplying oscillators comprise respective frequency-locking ranges that are spectrally offset from one another, and wherein selecting one of the plurality of frequency-multiplying oscillators to generate the output clock signal comprises selecting the one of the plurality of frequency-multiplying oscillators having a frequency-locking range indicated by the frequency ratio to bound an oscillating frequency of the input clock signal.
10. The method of claim 1 further comprising determining a frequency ratio between the input clock signal and a second reference clock signal during the first interval, and wherein selecting one of the plurality of frequency-multiplying oscillators to generate an output clock signal that is a frequency multiple of the input clock signal comprises selecting the one of the plurality of frequency-multiplying oscillators to generate the output signal based on a combination of frequency-ratio values, including at least a first frequency-ratio value corresponding to the frequency ratio between the input clock signal and the first reference clock signal and a second frequency-ratio value corresponding to the frequency-ratio between the input clock signal and the second reference clock signal.
11. The method of claim 10 wherein selecting the one of the plurality of frequency-multiplying oscillators to generate the output signal based on the combination of frequency-ratio values comprises generating a sum of the frequency-ratio values, including at least a sum of the first and second frequency-ratio values.
12. An integrated circuit device comprising:
a frequency comparator to determine a frequency ratio between an input clock signal and a first reference clock signal during a first interval;
a plurality of frequency-multiplying oscillators; and
control logic to select, based at least in part on the frequency ratio, one of the plurality of frequency-multiplying oscillators to generate an output clock signal that is a frequency multiple of the input clock signal.
13. The integrated circuit device of claim 12 wherein the control logic comprises circuitry to enable one of the frequency-multiplying oscillators to generate the first reference clock signal.
14. The integrated circuit device of claim 13 wherein the circuitry to enable the one of the frequency-multiplying oscillators to generate the first reference clock signal comprises circuitry to establish a free-running oscillation mode within the one of the frequency-multiplying oscillators.
15. The integrated circuit device of claim 12 wherein the frequency comparator to determine the frequency ratio between the input clock signal and the first reference clock signal comprises a counter to count edges of the first reference clock signal over the first interval, wherein a start of the first interval corresponds to a first transition of the input clock signal and an end of the first interval corresponds to a second transition of the input clock signal.
16. The integrated circuit device of claim 15 wherein the first and second transitions of the input clock cycle define a cycle of the input clock signal.
17. The integrated circuit device of claim 12 wherein the frequency comparator to determine the frequency ratio between the input clock signal and the first reference clock signal comprises:
a counter to counting edges of the first reference clock signal over a first sub-interval that corresponds to a first cycle of the input clock signal to generate an edge-count value;
circuitry to compare the edge-count value to a threshold; and
circuitry to enable the counter to additionally count edges of the first reference clock cycle over a second sub-interval if the edge-count indicates a number of edges of the first reference clock signal fewer than the threshold.
18. The integrated circuit device of claim 17 wherein the second sub-interval comprises a second cycle of the input clock signal.
19. The integrated circuit device of claim 17 further comprising circuitry to increment a cycle-count value if the edge-count value indicates a number of edges of the first reference clock signal fewer than the threshold, the cycle-count value forming a field within a value indicative of the frequency ratio.
20. The integrated circuit device of claim 12 wherein the plurality of frequency-multiplying oscillators comprise respective frequency-locking ranges that are spectrally offset from one another, and wherein the control logic to select one of the plurality of frequency-multiplying oscillators to generate the output clock signal comprises circuitry to select the one of the plurality of frequency-multiplying oscillators having a frequency-locking range indicated by the frequency ratio to bound an oscillating frequency of the input clock signal.
21. The integrated circuit device of claim 12 wherein the frequency comparator comprises circuitry to determine a frequency ratio between the input clock signal and a second reference clock signal during the first interval, and wherein the control logic to select one of the plurality of frequency-multiplying oscillators to generate an output clock signal that is a frequency multiple of the input clock signal comprises circuitry to select the one of the plurality of frequency-multiplying oscillators to generate the output signal based on a combination of frequency-ratio values, including at least a first frequency-ratio value corresponding to the frequency ratio between the input clock signal and the first reference clock signal and a second frequency-ratio value corresponding to the frequency-ratio between the input clock signal and the second reference clock signal.
22. The integrated circuit device of claim 21 wherein the frequency comparator further comprises summing circuitry to generate the combination of frequency-ratio values.
23. An integrated circuit device comprising:
means for determining a frequency ratio between an input clock signal and a first reference clock signal during a first interval;
a plurality of frequency-multiplying oscillators; and
means for selecting, based at least in part on the frequency ratio, one of the plurality of frequency-multiplying oscillators to generate an output clock signal that is a frequency multiple of the input clock signal.