Sub-rate phase interpolator based clock data recovery architecture with phase skew correction
A sub-rate (such as half-rate I and Q) phase-interpolator based CDR architecture is configured to receive serial data signals and multiple sub-rate clock signals (such as generated by a VCO either integrated or external). The CDR includes multiple phase interpolators to generate, from respective sub-rate clock signals, respective PI (phase-interpolated) sub-rate clock signals. A CDR loop is configured to receive the input data and the PI sub-rate clock signals, and to generate multiple PI control signals, each to control a respective phase interpolator to align the PI sub-rate clock signals to the data edges. A skew-correction loop includes skew detection circuitry to generate a skew error signal from the PI sub-rate clock signals corresponding to a skew error between the PI sub-rate clock signals, and skew-correction offset circuitry to generate, from the skew error signal, a skew-correction offset signal to modify a selected PI control signal.
1. A circuit for use in a system including serial data communication, the circuit including clock and data recovery for received serial data, the circuit comprising:
a receiver interface to receive serial data signals with data symbols defined by data edges;
a clock interface to receive multiple sub-rate clock signals;
CDR (clock and data recovery) circuitry to receive the serial data signals and the sub-rate clock signals, including:
multiple phase interpolator circuitries to generate, from respective sub-rate clock signals, respective phase-interpolated (PI) sub-rate clock signals (collectively designated PI sub-rate clock signals);
a CDR loop to receive the input data and the PI sub-rate clock signals, and to generate multiple PI control signals, each to control a respective phase interpolator to align the PI sub-rate clock signals to the input data edges; and
a skew-correction loop including:
skew detection circuitry to generate a skew error signal from the PI sub-rate clock signals corresponding to a skew error between the PI sub-rate clock signals, and
skew-correction offset circuitry to generate, from the skew error signal, a skew-correction offset signal to modify a selected PI control signal.
2. The circuit of claim 1 , wherein the CDR circuitry includes in the CDR loop:
phase detector circuitry to generate an alignment error signal corresponding to a misalignment of a selected sub-rate clock signal with respect to the input data edges; and
phase interpolator control circuitry to generate the PI control signals based on the alignment error signal.
3. The circuit of claim 2 , wherein:
the phase detector circuitry is configured to generate a digital alignment error signal corresponding to the input data edge being early, late or aligned with the input data edge; and
the phase interpolator control circuitry comprises:
decoder circuitry to decode the digital alignment error signal to generate a decoded alignment error signal, and
PI selector circuitry to generate the PI control signals based on the decoded alignment error signal.
4. The circuit of claim 1 , wherein the PI control signals are digital PI control signals, and wherein:
the skew detection circuitry includes:
digitizer circuitry to generate a digitized skew error signal;
the skew-correction offset circuitry comprises:
skew-correction offset logic to generate, from the digitized skew error signal, a digital skew-correction offset signal; and
combiner circuitry to combine the digital skew-correction offset signal with at least one digital PI control signal.
5. The circuit of claim 4 , wherein the skew correction offset logic is configured:
to generate from the digitized skew error signal the digital skew-correction offset signal corresponding to an output of the skew detection circuitry changing polarity; and
to generate the digital skew-correction offset signal to cause a phase skew between the PI sub-rate clock signals to converge to a corresponding fraction of a UI (unit interval).
6. The circuit of claim 1 , further comprising:
sub-rate clock generation circuitry to generate the multiple input sub-rate clock signals for input to the multiple phase interpolator circuitries through the sub-rate clock input interface.
7. The circuit of claim 1 , wherein the sub-rate clock signals are half-rate in-phase (I) and quadrature (Q) clock signals, and wherein the CDR loop is operable to align the quadrature (Q) clock signal to the input data edges, and to sample the input data symbols with the in-phase (I) clock signal.
8. A system including serial data communication, the system comprising:
a circuit including receiver circuitry with a receiver interface to receive serial data signals with data symbols defined by data edges;
the circuit including one of clock circuitry to generate multiple sub-rate clock signals, or a clock interface to receive the multiple sub-rate clock signals which are generated external to the circuit;
the receiver circuitry including CDR (clock and data recovery) circuitry to receive the serial data signals and the sub-rate clock signals, including:
multiple phase interpolator circuitries to generate, from respective sub-rate clock signals, respective phase-interpolated (PI) sub-rate clock signals (collectively designated PI sub-rate clock signals);
a CDR loop to receive the input data and the PI sub-rate clock signals, and to generate multiple PI control signals, each to control a respective phase interpolator to align the PI sub-rate clock signals to the input data edges; and
a skew-correction loop including:
skew detection circuitry to generate a skew error signal from the PI sub-rate clock signals corresponding to a skew error between the PI sub-rate clock signals, and
skew-correction offset circuitry to generate, from the skew error signal, a skew-correction offset signal to modify a selected PI control signal.
9. The circuit of claim 8 , wherein the CDR circuitry includes in the CDR loop:
phase detector circuitry to generate an alignment error signal corresponding to a misalignment of a sub-rate clock signal with respect to the input data edges; and
phase interpolator control circuitry to generate the PI control signals based on the alignment error signal.
10. The circuit of claim 9 , wherein:
the phase detector circuitry is configured to generate a digital alignment error signal corresponding to the input data edge being early, late or aligned with the input data edge; and
the phase interpolator control circuitry comprises:
decoder circuitry to decode the digital alignment error signal to generate a decoded alignment error signal, and
PI selector circuitry to generate the PI control signals based on the decoded alignment error signal.
11. The circuit of claim 8 , wherein the PI control signals are digital signals, and wherein:
the skew detection circuitry includes:
digitizer circuitry to generate a digitized skew error signal;
the skew-correction offset circuitry comprises:
skew-correction offset logic to generate, from the digitized skew error signal, a digital skew-correction offset signal; and
combiner circuitry to combine the digital skew-correction offset signal with at least one digital PI control signal.
12. The circuit of claim 11 , wherein the skew correction offset logic is configured:
to generate from the digitized skew error signal the digital skew-correction offset signal corresponding to an output of the skew detection circuitry changing polarity; and
to generate the digital skew-correction offset signal to cause a phase skew between the PI sub-rate clock signals to converge to a corresponding fraction of a UI (unit interval).
13. The circuit of claim 8 , wherein the sub-rate clock signals are half-rate in-phase (I) and quadrature (Q) clock signals, and wherein the CDR loop is operable to align the quadrature (Q) clock signal to the input data edges, and to sample the input data symbols with the in-phase (I) clock signal.
14. A method of receiving serial data communication with clock and data recovery, the method comprising:
receiving serial data signals with data symbols defined by data edges;
generating multiple sub-rate clock signals;
generating, with multiple phase interpolator circuitries, from respective sub-rate clock signals, respective phase-interpolated (PI) sub-rate clock signals (collectively designated PI sub-rate clock signals);
in a CDR loop, receiving the input data and the PI sub-rate clock signals, and to generate multiple PI control signals, each to control a respective phase interpolator to align the PI sub-rate clock signals to the input data edges; and
in a skew-correction loop:
generating a skew error signal from the PI sub-rate clock signals corresponding to a skew error between the PI sub-rate clock signals, and
generating, from the skew error signal, a skew-correction offset signal to modify a selected PI control signal.
15. The method of claim 14 , wherein the CDR loop comprises:
generating an alignment error signal corresponding to a misalignment of a sub-rate clock signal with respect to the input data edges; and
generating the PI control signals based on the alignment error signal.
16. The method of claim 15 :
wherein generating the alignment error signal comprises generating a digital alignment error signal corresponding to the input data edge being early, late or aligned with the input data edge; and
wherein generating the PI control signals comprises:
decoding the digital alignment error signal to generate a decoded alignment error signal, and
generating the PI control signals based on the decoded alignment error signal.
17. The method of claim 14 , wherein the PI control signals are digital signals, and wherein the skew-correction loop comprises:
generating a digitized skew error signal;
generating, from the digitized skew error signal, a digital skew-correction offset signal; and
combining the digital skew-correction offset signal with at least one digital PI control signal.
18. The method of claim 17 , wherein the skew-correction loop further comprises:
generating from the digitized skew error signal the digital skew-correction offset signal corresponding to an output of the skew detection circuitry changing polarity; and
generating the digital skew-correction offset signal to cause a phase skew between the PI sub-rate clock signals to converge to a corresponding fraction of a UI (unit interval).
19. The method of claim 14 , wherein the sub-rate clock signals are half-rate in-phase (I) and quadrature (Q) clock signals, and wherein the CDR loop is operable to align the quadrature (Q) clock signal to the input data edges, and to sample the input data symbols with the in-phase (I) clock signal.