Methods and circuits for decision-feedback equalization with early high-order-symbol detection
A PAM-4 DFE receives an input signal distorted by inter-symbol interference (IS I) and expressing a series of symbols each representing one of four pulse amplitudes to convey two binary bits of data per symbol. High-order circuitry resolves the most-significant bit (MSB) of each two-bit symbol, whereas low-order circuitry 115 resolves the immediate least-significant bit (LSB). An immediate value of the MSB is used to select a set of IS I offsets used to resolve the LSB. Resolved values of the prior values of the MSB and LSB are then used to select the IS I offset for the immediate symbol.
1. A receiver comprising:
an input node configured to receive an input signal expressing a series of symbols, each of the symbols representing a symbol value having a high-order bit and a low-order bit, the series of symbols including an immediate symbol representing an immediate-symbol value and a prior symbol representing a prior-symbol value; and
low-order circuity coupled to the input node to receive the series of symbols, the low-order circuitry configured to select the low-order bit of the immediate-symbol value based on the high-order bit of the immediate-symbol value and the prior-symbol value.
2. The receiver of claim 1 , wherein the low-order circuitry selects the low-order bit of the immediate-symbol value based on the high-order bit of the prior-symbol value.
3. The receiver of claim 2 , wherein the low-order circuitry selects the low-order bit of the immediate-symbol value based on the low-order bit of the prior-symbol value.
4. A decision-feedback equalizer (DFE) comprising:
an input node configured to receive an input signal expressing a series of symbols, each of the symbols representing a symbol value having a high-order bit and a low-order bit, the series of symbols including an immediate symbol representing an immediate-symbol value and a prior symbol representing a prior-symbol value;
high-order circuitry coupled to the input node to receive the series of symbols, the high-order circuitry comprising:
high-order samplers configured to sample the immediate symbol with high-order offsets to acquire speculative high-order samples of the immediate symbol; and
a high-order multiplexer coupled to the high-order samplers to select the high-order bit of the immediate-symbol value from among the speculative high-order samples of the immediate symbol based on the high-order bit of the prior-symbol value; and
low-order circuity coupled to the input node to receive the series of symbols, the low-order circuitry configured to select the low-order bit of the immediate-symbol value based on the high-order bit of the immediate-symbol value and the prior-symbol value.
5. The DFE of claim 4 , the low-order circuitry configured to select the low-order bit of the immediate-symbol value based on the high-order bit of the immediate-symbol value, the high-order bit of the prior-symbol value, and the low-order bit of the prior-symbol value.
6. The DFE of claim 4 , the low-order circuit comprising:
a first low-order multiplexer configured to select between alternatively offset immediate-symbol levels responsive to the high-order bit of the immediate symbol.
7. The DFE of claim 6 , the low-order circuit further comprising:
low-order samplers configured to sample the selected alternatively offset immediate-symbol level with low-order offsets to acquire speculative low-order samples of the immediate symbol.
8. The DFE of claim 7 , the low-order circuit further comprising:
a second low-order multiplexer configured to select between the speculative low-order samples responsive to the prior-symbol value.
9. The DFE of claim 4 , wherein the high-order bit is a most-significant bit.
10. The DFE of claim 4 , wherein the low-order bit is a least-significant bit.
11. The DFE of claim 4 , wherein the low-order circuitry and the high-order circuitry are of a first DFE slice, the DFE further comprising an additional DFE slice coupled to the input node to receive the series of symbols.
12. The DFE of claim 11 , wherein the prior-symbol value is from the additional DFE slice.
13. A method for resolving an input signal expressing a series of symbols, each of the symbols expressed over a symbol time and representing a symbol value having a high-order bit and a low-order bit, the series of symbols including an immediate symbol representing an immediate-symbol value and a prior symbol representing a prior-symbol value, the method comprising:
resolving the high-order bit of the immediate symbol;
applying offsets to the immediate symbol to produce offset immediate symbols;
selecting one of the offset immediate symbols, a selected offset immediate symbol, responsive to the high-order bit of the immediate symbol;
applying second offsets to the selected offset immediate symbol to produce second offset immediate symbols; and
selecting one of the second offset immediate symbols responsive to at least one of the high-order bit of the prior-symbol value and the low-order bit of the prior-symbol value to resolve the low-order bit of the immediate symbol.
14. The method of claim 13 , wherein applying the offsets to the immediate symbol applies the offsets to at least one of the input signal and a reference against which the input signal is compared.
15. The method of claim 13 , wherein resolving the high-order bit of the immediate symbol comprises:
applying second offsets to the immediate symbol to produce second offset immediate symbols;
sampling the second offset immediate symbols; and
selecting one of the second offset immediate symbols as the high-order bit of the immediate symbol responsive to the high-order bit of the prior-symbol value.
16. The method of claim 13 , wherein the high-order bit is a most-significant bit.
17. The method of claim 13 , wherein the immediate symbol is adjacent the prior symbol in the series of symbols.
18. The method of claim 13 , wherein applying the offsets comprising injecting a current into a signal path of the series of symbols.
19. The method of claim 13 , wherein the input signal expresses the series of symbols at a symbol rate, the method further comprising sampling the selected one of the offset immediate symbols at a slice rate less than the symbol rate.