SerDes data sampling gear shifter
A SerDes data sampling controller that includes a gear shifting data sampling clock that zeroes the data sampling skew at the center of the unit interval during the CDR phase lock stage, and then skews the data sample timing away from the center of the unit interval as the DFE coefficients adapt during the data transfer stage. This allows the controller to implement the best (unskewed) data sample timing during the CDR phase locking stage, and then skew the data sample timing after the DFE coefficients have adapted to provide the best (skewed) data sample timing for data bit sampling during the data transfer stage. The data sampling gear shifter may apply a variable skew value to the transition sampling or quadrature (Q) data sampling clock differentially varying the quadrature (Q) transition sampling clock from the inphase (I) data sampling clock.
1. A data sampling controller for a deserializer operative for transferring from a serial data stream into a parallel data stream, comprising:
an inphase data sampler configured to time inphase data samples of the serial data stream at data time intervals separated by a unit interval duration;
a transition data sampler configured to time transition samples of the serial data stream at transition time intervals nominally separated from the inphase data samples by one-half of the unit interval;
a decision feedback equalizer configured to apply feedback parameters to adaptively adjust the serial data stream causing asymmetry of a serial data eye defined by the serial data stream;
a gear shifter configured to apply a variable skew value causing a differential timing between the inphase data samples and the transition samples,
vary the skew value to cause the transition samples to track the asymmetry of the data eye, apply a first skew value during a phase locking stage while the data sampling controller synchronizes to the serial data stream, the first skew value causing a substantially quadrature relation between the data sampling clock and the transition sampling clock and apply a second skew value during a data transfer stage in response to adaptation of feedback control parameters, wherein the feedback control parameters are initialized to zero and the first skew value is set to zero during the phase locking stage and the feedback control parameters adapt to non-zero values and the second skew value is set to a non-zero value after the phase locking stage to track the asymmetry of the data eye causes by adaptation of feedback control parameters during the data transfer stage.
2. The data sampling controller of claim 1 , wherein the feedback control parameters are selected to compensate for inter-symbol interference in the serial data stream.
3. The data sampling controller of claim 2 , wherein the feedback control parameters comprise decision feedback equalizer parameters.
4. The data sampling controller of claim 3 , wherein the feedback control parameters further comprise a variable gain parameter.
5. The data sampling controller of claim 4 , wherein the feedback control parameters further comprise a linear equalizer parameter.
6. A method for deserializing data from a serial data stream into a parallel data stream, comprising the steps of:
timing inphase data samples of the serial data stream at data time intervals separated by a unit interval duration;
timing transition samples of the serial data stream at transition time intervals nominally separated from the inphase data samples by one-half of the unit interval;
applying feedback parameters to adaptively adjust the serial data stream causing asymmetry of a serial data eye defined by the serial data stream;
applying a variable skew value causing a differential timing between the inphase data samples and the transition samples;
varying the skew value to cause the transition samples to track the asymmetry of the data eye;
applying a first skew value causing a substantially quadrature relation between the data sampling clock and the transition sampling clock during a phase locking stage;
applying a second skew value during a data transfer stage after phase locking has been achieved;
initializing the feedback control parameters to zero;
setting the first skew value to zero during the phase locking stage;
adapting the feedback control parameters adapt to non-zero values; and
setting the second skew value to a non-zero value after the phase locking stage to track the asymmetry of the data eye caused by adaption of the feedback control parameters during the data transfer stage.
7. The method of claim 6 , further comprising the steps of adapting the feedback parameters to compensate for inter-symbol interference in the serial data stream.
8. The method of claim 6 , further comprising the steps of adapting the feedback parameters to adjust a gain parameter.
9. The method of claim 6 , further comprising the steps of adapting the feedback parameters to adjust a linear equalizer parameter.