Methods and circuits for decision-feedback equalization using compensated decision regions

1. A decision-feedback equalizer (DFE) comprising:

an input node to receive an input signal expressing a series of symbols, each of the symbols representing one of a set of possible symbol values;

a first open-loop stage coupled to the input node to receive the series of symbols, the first open-loop stage to compare each symbol in the series of symbols with a set of reference-voltage levels to identify a voltage region;

a second open-loop stage coupled to the first open-loop stage to derive from the voltage region a set of tentative decisions representing a subset of the set of possible symbol values; and

a feedback stage coupled to the second open-loop stage to select one of the set of tentative decisions as a final decision responsive to a prior final decision;

wherein the first open-loop stage selects the set of reference-voltage levels from a gamut of reference-voltage levels responsive to a level of each symbol in the series of symbols;

wherein the comparing of each symbol relative to the set of reference-voltage levels successively compares the symbol to each reference-voltage level in the set of reference-voltage levels; and

wherein the first open-loop stage performs a binary search for the voltage region by the successive comparison of the symbol to each reference-voltage level in the set of reference-voltage levels.

2. The DFE of claim 1 , wherein the first open-loop stage, the second open-loop stage, and the feedback stage are of a first DFE slice, the DFE further comprising additional DFE slices each coupled to the input node to receive the series of symbols and produce a respective series of final decisions.

3. The DFE of claim 2 , wherein the feedback stage of each DFE slice receives the prior final decision from another of the DFE slices.

4. The DFE of claim 2 , further comprising a reference-level generator to generate a gamut of reference-voltage levels, including the set of reference-voltage levels, and apply the gamut of reference-voltage levels to the first open-loop stage of each of the DFE slices.

5. The DFE of claim 1 , further comprising a look-up table, coupled to the second open-loop stage, to store a mapping between the voltage regions and subsets of the set of symbol values.

6. The DFE of claim 1 , the second open-loop stage including a substage that further selects the set of tentative decisions from a prior set of tentative decisions.

7. A decision-feedback equalizer (DFE) comprising:

an input node to receive an input signal expressing a series of symbols, each of the symbols representing one of a set of possible symbol values;

a reference-level generator to provide a gamut of reference-voltage levels; and

an open-loop stage coupled to the input node to receive the series of symbols, the open-loop stage to compare a present level of each symbol in the series of symbols to a subset of the gamut of reference-voltage levels to identify a voltage region for the symbol;

wherein the open-loop stage selects the subset of reference-voltage levels responsive to the present level.

8. The DFE of claim 7 , wherein comparing the present level of each symbol with the subset of reference-voltage levels successively compares the present level to each reference-voltage level in the subset of the reference-voltage levels.

9. The DFE of claim 8 , wherein the open-loop stage performs a binary search for the voltage region by the successive comparing.

10. The DFE of claim 9 , wherein the open-loop stage is of a first DFE slice, the DFE further comprising additional DFE slices each coupled to the input node to receive the series of symbols and sample the received series of symbols at respective sample timings.

11. The DFE of claim 10 , wherein each of the DFE slices provides a final decision to another of the DFE slices.

12. The DFE of claim 7 , further comprising:

a second open-loop stage coupled to the first-mentioned open-loop stage to derive from the identified voltage region a set of tentative decisions representing a subset of the possible symbol values.

13. The DFE of claim 12 , further comprising:

a feedback stage coupled to the second open-loop stage to select one of the tentative decisions as a final decision responsive to a prior final decision.

14. The DFE of claim 12 , wherein the second open-loop stage further derives the set of tentative decisions from a prior set of tentative decisions.

15. A method of interpreting an input signal expressing a series of symbols, each of the symbols representing one of a set of possible symbol values, the method comprising:

performing, for each symbol, a binary search for a voltage region corresponding to the symbol, the binary search comprising comparing each symbol in the series of symbols to a set of reference-voltage levels selected from a gamut of reference-voltage levels, and wherein the comparing of each symbol performs the binary search for the voltage region by sampling the symbol relative to each reference-voltage level in the set of reference-voltage levels;

deriving, from the voltage region, a set of tentative decisions representing a subset of the possible symbol values; and

selecting one of the tentative decisions as a final decision responsive to a prior final decision.

16. The method of claim 15 , wherein the set of reference-voltage levels is a subset of the gamut of reference-voltage levels, the method further comprising selecting the subset of the gamut of reference-voltage levels responsive to the comparing.

17. The method of claim 15 , wherein deriving the set of tentative decisions considers a prior set of tentative decisions.