PAM-4 Bus-Invert Coding for Simultaneous Switching Noise
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Solution Overview
Problem
Parallel single-ended buses suffer from high self-induced noise due to simultaneous switching, which causes power supply voltage modulation, reducing timing margins and voltage margins by coupling noise onto signals and reference voltages.
Innovation Solution
The technique reduces the maximum di/dt by utilizing interactions between most significant bits (MSBs) and least significant bits (LSBs) in a PAM-4 single-ended parallel bus, rather than independently encoding them, and introduces a data bus inverse (DBI) bit to encode the polarity of the bus, thereby reducing the worst-case di/dt by 67% and average current consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If independent encoding of MSBs and LSBs is used in PAM-4 signaling, then encoding simplicity is maintained, but maximum di/dt remains high causing excessive self-induced noise
Solution Approach 1:
The patent merges the encoding of MSBs and LSBs by considering their combined weight rather than encoding them independently. The encoder calculates the total weight as the sum of MSB weight and LSB weight, then applies bus-invert encoding based on this combined weight, reducing peak di/dt by up to 67% compared to independent encoding
Solution Approach 2:
The patent changes the encoding parameter from independent bit encoding to combined weight encoding. By monitoring the total weight (sum of MSB and LSB weights) and applying inversion when the combined weight exceeds a threshold, the system reduces simultaneous switching noise while maintaining encoding efficiency
2Object-generated harmful factors
If conventional low-weight bus-invert coding is applied, then maximum di/dt is reduced by 50%, but an extra bit (lane) must be added to the group of lanes
Solution Approach 1:
The patent segments the PAM-4 signal into MSB and LSB components, each with their own weight characteristics. By treating these segments differently and combining their weights strategically, the system achieves superior di/dt reduction (67%) without adding extra lanes, unlike conventional approaches that treat all bits uniformly
Solution Approach 2:
The patent introduces a new dimension to the encoding problem by considering the temporal and hierarchical relationship between MSBs and LSBs in PAM-4 signaling. Instead of treating all bits at the same level, the system applies different weighting and inversion strategies to MSBs and LSBs, achieving better noise reduction without increasing lane count
3Use of energy by moving object
If PAM-4 signaling is used instead of PAM-2, then signal amplitude is reduced by 3× improving power efficiency, but simultaneous switching noise becomes more critical due to higher switching frequency
Solution Approach 1:
The patent applies the proven bus-invert encoding technique from PAM-2 systems to PAM-4 signaling by creating a adapted version that accounts for the multi-level nature of PAM-4. The encoder copies the weight-monitoring and inversion logic from conventional systems but modifies it to handle combined MSB-LSB weights in PAM-4, achieving 67% di/dt reduction while maintaining PAM-4's power efficiency
Solution Approach 2:
The patent changes the encoding parameters specifically for PAM-4 by introducing combined weight calculation that considers both MSB and LSB contributions. The inversion threshold and weight summation are adjusted to match PAM-4's four-level structure, enabling effective noise reduction while preserving the lower signal amplitude advantage of PAM-4
Data Source
AI summary
A system comprising a PAM-4 transmitter coupled data lanes includes a least significant bit section and a most significant bit section for the symbols generated on each lane. A controller to determine a state of the PAM-4 transmitter and selectively inverts a polarity of the symbol bits on the lanes based on the state.