IQ-skew adaptation for a symmetric eye in a SerDes receiver
Described embodiments provide for, in a receiver circuit, an adaptation process that adjusts the IQ-skew automatically to obtain proper eye centering in a data eye, thereby maximizing horizontal margin of the eye. The IQ-skew adaptation algorithm is realized with a ‘biased’ bang-bang phase detector (BBPD) oof a clock and data recovery circuit (CDR) that biases the weights applied to UP and DOWN outputs of the phase detector, rather than treating them equally. By weighting the BBPD UPs and DOWNs differently, the system locks to the left and right inner corners, and thereby is able to locate the center of the inner eye.
1. A method of generating a skew offset in a clock and data recovery (CDR) device, the method comprising:
generating a data eye from an input signal to a decision device, the input signal including a sequence of data symbols;
performing phase detection for each corner of a rectangle framing a center of the data eye by:
initializing a roaming latch to the corner of the data eye,
allowing the roaming latch to settle to a phase of the corner,
recording horizontal positions of the settled phase,
applying samples of the corner to a bank of L phase detectors, L being an integer greater than or equal to 1,
accumulating the outputs of the L phase detectors, each output representing a corresponding up or down value, to generate an accumulated up or an accumulated down value,
weighting the accumulated up or the accumulated down value, and
detecting a phase of the corner when the weighed accumulated up value and the weighted accumulate down value reach an equilibrium state;
determining a phase position of a center of the rectangle, wherein the center phase position represents a vertical and horizontal phase position of the center of the data eye;
detecting a difference between the center phase position and a current data sampling phase position, the detected difference between the center phase position and the current data sampling phase position being related to the skew offset; and
adjusting the current data sampling phase position based on the skew offset.
2. The method of claim 1 , wherein the weighting the accumulated up or the accumulated down value comprises comparing the accumulated up or the accumulated down value to a corresponding threshold.
3. The method of claim 2 , comprising setting each threshold so the roaming latch settles to a settled phase of the corner from a direction toward the center of the data eye.
4. The method of claim 1 , comprising repeating the method to adaptively set the current data sampling phase position at the center of the data eye.
5. The method of claim 1 , wherein the method is applied separately to even symbols and to odd data symbols.
6. The method of claim 1 , wherein the method is embodied in a Serializer/Deserializer (Ser Des) device.
7. Apparatus for generating a skew offset in a clock and data recovery (CDR) device, the apparatus comprising:
a decision device configured to generate a data eye from an input signal to the decision device, the input signal including a sequence of data symbols;
a phase detector configured to perform phase detection for each corner of a rectangle framing a center of the data eye, the phase detector comprising:
a bank of L phase detectors, L being an integer greater than or equal to 1, wherein the samples of the corner are applied to the bank of L phase detectors;
an accumulator configured to accumulate the outputs of the L phase detectors, each output representing a corresponding up or down value, and configured to generate an accumulated up or an accumulated down value; and
a combiner configured to apply weights to the accumulated up or the accumulated down value;
wherein the phase detector is configured to:
detect a phase of the corner when the weighted accumulated up value and the weighted accumulate down value reach an equilibrium state,
initialize a roaming latch to the corner of the data eye,
allow the roaming latch to settle to a phase of the corner, and
record horizontal positions of the settled phase; and
a controller configured to determine a phase position of a center of the rectangle, wherein the center phase position represents a vertical and horizontal phase position of the center of the data eye,
wherein the controller is configured to detect a difference between the center phase position and a current data sampling phase position, the detected difference between the center phase position and the current data sampling phase position being related to the skew offset; and
adjust the current data sampling phase position based on the skew offset.
8. The apparatus of claim 7 , wherein the combiner applies weights to the accumulated up or the accumulated down value with a comparator configured to compare the accumulated up or the accumulated down value to a corresponding threshold.
9. The apparatus of claim 8 , wherein the phase detector sets each threshold so the roaming latch settles to a settled phase of the corner from a direction toward the center of the data eye.
10. The apparatus of claim 7 , wherein the apparatus is configured to adaptively set the data sampling phase position at the center of the eye.
11. The method of claim 7 , wherein the apparatus is configured to separately adjust the current data sampling phase for even symbols and for odd data symbols.
12. The apparatus of claim 7 , wherein the apparatus is embodied in a Serializer/Deserializer (SerDes) device.
13. The apparatus of claim 7 , wherein the apparatus is embodied in an integrated circuit.
14. A non-transitory machine-readable storage medium, having encoded thereon program code, wherein, when the program code is executed by a machine, the machine implements a method for generating a skew offset in a clock and data recovery (CDR) device, comprising the steps of:
generating a data eye from an input signal to the decision device, the input signal including a sequence of data symbols;
performing phase detection for each corner of a rectangle framing a center of the data eye by:
initializing a roaming latch to the corner of the data eye,
allowing the roaming latch to settle to a phase of the corner,
recording horizontal positions of the settled phase,
applying samples of the corner to a bank of L phase detectors, L being an integer greater than or equal to 1,
accumulating the outputs of the L phase detectors, each output representing a corresponding up or down value, to generate an accumulated up or an accumulated down value,
weighting the accumulated up or the accumulated down value, and
detecting a phase of the corner when the weighted accumulated up value and the weighted accumulate down value reach an equilibrium state;
determining a phase position of a center of the rectangle, wherein the center phase position represents a vertical and horizontal phase position of the center of the data eye;
detecting a difference between the center phase position and a current data sampling phase position, the detected difference between the center phase position and the current data sampling phase position being related to the skew offset; and
adjusting the current data sampling phase position based on the skew offset.
15. The non-transitory machine-readable storage medium of claim 14 , wherein the weighting the accumulated up or the accumulated down value comprises comparing the accumulated up or the accumulated down value to a corresponding threshold.
16. The non-transitory machine-readable storage medium of claim 15 , comprising setting each threshold so the roaming latch settles to a settled phase of the corner from a direction toward the center of the data eye.
17. The non-transitory machine-readable storage medium of claim 14 , comprising repeating the method to adaptively set the current data sampling phase position at the center of the data eye.
18. The method of claim 1 , wherein the input signal is received on a communication channel as a serialized input.
19. The apparatus of claim 7 , wherein the input signal is received on a communication channel as a serialized input.
20. The non-transitory machine-readable storage medium of claim 14 , wherein the input signal is received on a communication channel as a serialized input.