Linear half-rate phase detector and clock and data recovery circuit
Method and apparatus for recovering a clock and data from a data signal. One method of the invention includes receiving the data signal having a first data rate, receiving the clock signal having a first clock frequency, alternating between a first level and a second level, wherein the first data rate is twice the first clock frequency. A first signal is generated by passing the data signal when the clock signal is at the first level, and storing the data signal when the clock signal is at the second level. A second signal is generated by passing the data signal when the clock signal is at the second level, and storing the data signal when the clock signal is at the first level. A third signal is generated by passing the first signal when the clock signal is at the second level, and storing the first signal when the clock signal is at the first level. A fourth signal is generated by passing the second signal when the clock signal is at the first level, and storing the second signal when the clock signal is at the second level. An error signal is generated by taking the exclusive-OR of the first signal and the second signal, and a reference signal is generated by taking the exclusive-OR of the third signal and the fourth signal.
1. A clock and data recovery apparatus comprising:
a voltage controlled oscillator configured to provide, at a clock output, a clock signal at substantially a first frequency;
a half-rate phase detector comprising a data input configured to receive a data input signal having a data rate and a data pattern, wherein the data rate is substantially twice the first frequency, the half-rate phase detector further comprising a clock input, coupled to the clock output of the voltage controlled oscillator, to receive the clock signal;
the half-rate phase detector further configured to provide a first signal and a second signal, and the first signal is dependent on the phase difference between the data input signal and the clock signal and also is dependent on the data pattern, and the second signal is dependent on the data pattern but is not dependent on the phase difference;
a charge pump coupled to receive the first signal and the second signal from the half-rate phase detector, wherein the charge pump combines the first signal and the second signal to generate an output signal that is dependent on the phase difference but is not dependent on the data pattern; and
a low-pass filter coupled to receive the output signal from the charge pump and configured to provide a signal to the voltage controlled oscillator.
2. The apparatus of claim 1 wherein the clock and data recovery apparatus is configured to correct data dependency associated with the first signal by scaling at least one signal to effectively subtract substantially one-half of the average value of the second signal from the first signal.
3. The apparatus of claim 1 wherein the half-rate phase detector is configured to adjust a magnitude of at least one signal to provide the first signal and the second signal.
4. The apparatus of claim 3 wherein the half-rate phase detector comprises XOR gates that are scaled differently to provide the first signal and the second signal.
5. The apparatus of claim 4 wherein an XOR gate that generates the first signal comprises at least one transistor that is either substantially twice as large as or substantially half as small as a corresponding at least one transistor in an XOR gate that generates the second signal.
6. The apparatus of claim 3 wherein the difference between the first signal and the second signal is not dependent on the data pattern but is dependent on the phase difference.
7. The apparatus of claim 6 wherein the charge pump is configured to generate the output signal as a differential voltage signal that is proportional to the difference between the first signal and the second signal.
8. The apparatus of claim 1 wherein the half-rate phase detector is configured to low pass filter the first signal and the second signal.
9. The apparatus of claim 8 wherein the half-rate phase detector comprises XOR gates configured to low pass filter the first signal and the second signal.
10. The apparatus of claim 9 wherein the XOR gates comprise PMOS transistors configured to low pass filter the first signal and the second signal.
11. The apparatus of claim 1 wherein the charge pump generates the output signal by subtracting the second signal from the first signal.
12. The apparatus of claim 11 wherein the clock signal has approximately a fifty percent duty cycle.
13. The apparatus of claim 11 wherein the voltage controlled oscillator comprises a ring oscillator.
14. The apparatus of claim 1 wherein the first signal comprises an error signal and the second signal comprises a reference signal.
15. The apparatus of claim 1 wherein the half-rate phase detector is configured to output demultiplexed data signals.
16. The apparatus of claim 1 wherein the half-rate phase detector is configured to:
generate an initial first signal and an initial second signal; and
adjust a magnitude of at least one of the initial first signal and the initial second signal to generate the first signal and the second signal, such that a relative magnitude of the first signal with respect to the second signal is substantially twice a relative magnitude of the initial first signal with respect to the initial second signal.
17. A clock and data recovery method comprising:
providing, by a voltage controlled oscillator, a clock signal at substantially a first frequency;
receiving, by a half-rate phase detector, a data input signal having a data rate and a data pattern, wherein the data rate is substantially twice the first frequency,
receiving, by the half-rate phase detector, the clock signal;
providing, by the half-rate phase detector, a first signal dependent on the phase difference between the data input signal and the clock signal and also dependent on the data pattern;
providing, by the half-rate phase detector, a second signal dependent on the data pattern but not dependent on the phase difference;
receiving, by a charge pump, the first signal and the second signal;
combining, by the charge pump, the first signal and the second signal to generate an output signal that is dependent on the phase difference but is not dependent on the data pattern; and
low-pass filtering the output signal to provide a signal to the voltage controlled oscillator.
18. The method of claim 17 comprising correcting data dependency associated with the first signal by scaling at least one signal to effectively subtract substantially one-half of the average value of the second signal from the first signal.
19. The method of claim 17 comprising adjusting a magnitude of at least one signal to provide the first signal and the second signal.
20. The method of claim 19 wherein the difference between the first signal and the second signal is not dependent on the data pattern but is dependent on the phase difference.
21. The method of claim 19 comprising generating the output signal as a differential voltage signal that is proportional to the difference between the first signal and the second signal.
22. The method of claim 17 wherein the half-rate phase detector low pass filters the first signal and the second signal.
23. The method of claim 17 wherein the charge pump generates the output signal by subtracting the second signal from the first signal.
24. The method of claim 17 wherein the first signal comprises an error signal and the second signal comprises a reference signal.
25. The method of claim 17 comprising providing, by the half-rate phase detector, demultiplexed data output signals.
26. The method of claim 17 comprising:
generating an initial first signal and an initial second signal; and
adjusting a magnitude of at least one of the initial first signal and the initial second signal to generate the first signal and the second signal, such that a relative magnitude of the first signal with respect to the second signal is substantially twice a relative magnitude of the initial first signal to the initial second signal.