Method and apparatus for low power chip-to-chip communications with constrained ISI ratio
An efficient communications apparatus is described for a vector signaling code to transport data and optionally a clocking signal between integrated circuit devices. Methods of designing such apparatus and their associated codes based on a new metric herein called the “ISI Ratio” are described which permit higher communications speed, lower system power consumption, and reduced implementation complexity.
1. A method comprising:
receiving a set of symbols of a codeword of a vector signaling code in parallel over a plurality of wires of a multi-wire bus, each symbol received via a respective wire of the multi-wire bus;
generating a set of analog outputs, each analog output formed as a respective linear combination of a corresponding subset of symbols of the set of symbols of the codeword, the linear combination formed by a respective multi-input comparator (MIC) receiving the corresponding subset of symbols, the respective linear combination associated with a respective sub-channel code vector of a plurality of mutually orthogonal sub-channel code vectors of the vector signaling code, wherein a number of symbols in the corresponding subset of symbols corresponds to a number of non-zero elements in the respective sub-channel code vector, and wherein at least two analog outputs of the set of analog outputs are generated from linear combinations of subsets of symbols having a different number of symbols; and
generating a set of information bits from the set of analog outputs by slicing each analog output using a respective signal slicer circuit.
2. The method of claim 1 , wherein at least one analog output is sliced using a PAM-3 sheer.
3. The method of claim 1 , wherein at least one analog output is sliced using a PAM-4 sheer.
4. The method of claim 1 , wherein each sub-channel code vector is orthogonal to a common mode vector.
5. The method of claim 1 , wherein the set of analog outputs have equal vertical opening levels.
6. The method of claim 1 , wherein the vector signaling code has a quaternary alphabet.
7. The method of claim 6 , wherein the quaternary alphabet comprises a set of values [+/−1, +/−⅓].
8. The method of claim 1 , wherein sub-channel code vectors having three non-zero elements comprise a first non-zero element twice as large as each of a second non-zero element and a third non-zero element, and a position of the first non-zero element is aligned to where only one other sub-channel codeword vector has a non-zero element.
9. An apparatus comprising:
a plurality of multi-input comparators (MICs) configured to receiving a set of symbols of a codeword of a vector signaling code, each symbol received via a respective wire of a multi-wire bus, each MIC of the plurality of MICS configured to receive a respective subset of symbols of the set of symbols of the codeword and to generate a respective analog output of a set of analog outputs, the respective analog output formed as a respective linear combination of the respective subset of symbols, the respective linear combination associated with a respective sub-channel code vector of a plurality of mutually orthogonal sub-channel code vectors of the vector signaling code, wherein a number of symbols in the respective subset of symbols corresponds to a number of non-zero elements in the respective sub-channel code vector, wherein at least two analog outputs of the set of outputs are generated from linear combinations of subsets of symbols having a different number of symbols; and
a comparator configured to generate a set of information bits from the set of analog outputs by slicing each analog output of the set of analog outputs.
10. The apparatus of claim 9 , wherein the sheer circuit comprises a PAM-3 sheer configured to slice at least one analog output.
11. The apparatus of claim 9 , wherein the sheer circuit comprises a PAM-4 sheer configured to slice at least one analog output.
12. The apparatus of claim 9 , wherein each sub-channel code vector is orthogonal to a common mode vector.
13. The apparatus of claim 9 , wherein the set of analog outputs have equal output levels.
14. The apparatus of claim 9 , wherein the vector signaling code has a quaternary alphabet.
15. The apparatus of claim 14 , wherein the quaternary alphabet comprises a set of values [+/−1, +/−⅓].
16. The apparatus of claim 9 , wherein sub-channel code vectors having three non-zero elements comprise a first non-zero element twice as large as each of a second non-zero element and a third non-zero element, and a position of the first non-zero element is aligned to where only one other sub-channel codeword vector has a non-zero element.