Relaxed 433 encoding to reduce coupling and power noise on PAM-4 data buses
Methods of operating a serial data bus divide series of data bits into sequences of one or more bits and encode the sequences as N-level symbols, which are then transmitted at multiple discrete voltage levels. These methods may be utilized to communicate over serial data lines to improve bandwidth and reduce crosstalk and other sources of noise.
1. A method of operating a serial data bus, the method comprising:
configuring a first set of multiple data lanes of the serial data bus for PAM-4 communication, wherein PAM-4 encodes two bits of data on a data lane of the bus as one of four different voltage levels (symbols) to facilitate communication;
configuring the data lanes not in the first set for PAM-433 communication, wherein PAM-433 utilizes PAM-4 (four-level encoding) encoding for some symbols, and three-level encoding for other symbols on a bus, wherein encoding a symbol is the physical configuration of a line driver circuit of the serial data bus to drive the voltage on the data lane to a particular value;
the data lanes for the PAM-4 communication being separated from one another by at least one of the data lanes for the PAM-433 communication.
2. The method of claim 1 , further comprising, for each data lane of the data lanes for the PAM-433 communication:
dividing a series of data bits for communication on the data lane into a plurality of sequences of five bits each;
encoding a first two bits of each of the sequences of five bits on the data lane as a four-level symbol; and
encoding a next three bits of each of the sequences of five bits on the data lane as two three-level symbols.
3. The method of claim 2 , wherein the two three-level symbols are selected to eliminate a possibility of maximum voltage deltas between four-level symbols on the serial data bus.
4. The method of claim 2 , further comprising:
selectively encoding a next three bits of each of the sequences of five bits on the serial data bus on condition that the first two bits of each of the sequences of five bits encodes as a four-level symbol at a highest symbol voltage level or a lowest symbol voltage level utilized by the serial data bus.
5. The method of claim 2 , wherein the two three-level symbols comprise a first three-level symbol and a second three-level symbol, a voltage level of the first three-level symbol being either (a) at most two voltage steps below a voltage level of the four-level symbol, or (b) at most two voltage steps above the voltage level of the four-level symbol.
6. The method of claim 2 , wherein the two three-level symbols comprise a first three-level symbol and a second three-level symbol, and further comprising:
the sequences of five bits comprising a first sequence of five bits for communication on the serial data bus, and a second sequence of five bits for communication after the first sequence of five bits on the serial data bus; and
a voltage level of the second three-level symbol of the first sequence of five bits being either (a) at most two voltage steps below a voltage level of the four-level symbol of the second sequence of five bits, or (b) at most two voltage steps above the voltage level of the four-level symbol of the second sequence of five bits.
7. The method of claim 1 , further comprising, for each data lane of the data lanes for PAM-433 communication:
dividing a series of data bits for communication on the data lane into sequences of seven bits;
encoding a first four bits of each of the sequences of seven bits onto the data lane as two four-level symbols; and
encoding a next three bits of each of the sequences of seven bits onto the data lane as two three-level symbols.
8. The method of claim 7 , wherein the data lanes for PAM-433 communication comprise a first data lane and a second data lane, and transmission of the two four-level symbols is staggered in time between the first data lane and the second data lane.
9. The method of claim 8 , wherein staggering the transmission of the two four-level symbols between the first data lane and the second data lane comprises transmission of two four-level symbols for the first data lane during a different clock interval than transmission of two four-level symbols for the second data lane.
10. The method of claim 7 , further comprising:
selectively encoding the next three bits of each of the sequences of seven bits on the data lane on condition that a second two bits of each of the sequences of seven bits encodes as a four-level symbol at a highest symbol voltage level or a lowest symbol voltage level utilized by the data lane.
11. The method of claim 7 , wherein the two three-level symbols comprise a first three-level symbol and a second three-level symbol, a voltage level of the first three-level symbol being either (a) at most two voltage steps below a voltage level of a second one of the two four-level symbols, or (b) at most two voltage steps above the voltage level of the second one of the two four-level symbols.
12. The method of claim 7 wherein the two three-level symbols comprise a first three-level symbol and a second three-level symbol, and further comprising:
the sequences of seven bits comprising a first sequence of seven bits for communication on the data lane, and a second sequence of seven bits for communication on the data lane after the communication of the first sequence of seven bits on the data lane; and
a voltage level of the second three-level symbol of the first sequence of seven bits being either (a) at most two voltage steps below a voltage level of a second one of the two four-level symbols of the first sequence of seven bits, or (b) at most two voltage steps above the second one of the two four-level symbols of the first sequence of seven bits.
13. The method of claim 1 , further comprising:
configuring one data lane of the serial data bus for PAM-4 communications of exclusively CRC values.
14. The method of claim 1 , further comprising:
configuring one data lane of the serial data bus for communication of CRC values utilizing symbols consisting of two voltage levels.
15. The method of claim 1 , further comprising:
terminating a data transfer request on each of the data lanes for PAM-433 communication with a symbol consisting of two voltage levels.
16. The method of claim 1 , the serial data bus consisting of nine data lanes.
17. The method of claim 16 , wherein three of the nine data lanes are configured for PAM-4 communication and are each separated from one another by two data lanes configured for PAM-433 communication.
18. The method of claim 1 , the serial data bus consisting of ten data lanes.
19. The method of claim 18 , wherein four of the ten data lanes are configured for PAM-4 communication and are each separated from one another by two data lanes configured for PAM-433 communication.
20. The method of claim 19 , wherein one of the four data lanes configured for PAM-4 communication is a dedicated CRC data lane.