Flash Memory Soft Information from Multi-Threshold Reads
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Solution Overview
Problem
Flash memory cells in NAND architecture face challenges in accurately reading charge levels due to variations in threshold voltage caused by charge leakage, overcharging, and interference from nearby cells, leading to increased bit error rates and the need for multiple read thresholds to improve reliability.
Innovation Solution
A method involving multiple read attempts using different sets of read thresholds to generate soft information, where each read threshold is used to calculate change-inducing thresholds, and combining these to produce coherent and compressed log-likelihood ratios for improved decoding accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple read thresholds are used to improve read reliability, then bit error rate decreases, but device complexity increases
Solution Approach 1:
The patent segments the read operation into multiple independent read attempts, each using a different read threshold. Instead of using all thresholds simultaneously in a complex manner, the system performs sequential read attempts with individual thresholds, then combines the results. This segmentation reduces the complexity of any single read operation while maintaining high reliability through multiple attempts.
Solution Approach 2:
The patent performs multiple read attempts with different read thresholds, using more read operations than the minimum single threshold would provide. This excessive action (multiple reads instead of one) ensures that even if some reads fail or produce errors, the combined results will achieve the desired reliability level.
2Reliability
If multiple read attempts are performed to reduce noise, then signal-to-noise ratio improves, but read time increases
Solution Approach 1:
The patent performs preliminary read attempts with different read thresholds before the final decoding stage. These preliminary reads gather information that helps in making more accurate decoding decisions later. By preparing this soft information in advance, the actual decoding process becomes more efficient and accurate.
Solution Approach 2:
The system uses the results from multiple read attempts as feedback to improve the decoding process. The soft information obtained from comparing read results against different thresholds provides feedback that guides the error correction decoder, allowing it to make more informed decisions and achieve better error correction with fewer iterations.
3Measurement precision
If soft information is generated through multiple reads, then decoding accuracy improves, but processing complexity increases
Solution Approach 1:
The patent introduces soft information as an intermediary between the raw read results and the final decoding process. Instead of directly decoding hard read results, the system first generates soft information that captures the reliability and confidence levels of each bit. This intermediary representation makes the subsequent decoding process more accurate while keeping the complexity manageable through structured information representation.
Data Source
AI summary
A system, a method and a non-transitory computer readable medium for generating soft information. The method may include performing a first set of read attempts of flash memory cells using a first set of read thresholds to provide first read results; calculating for each flash memory cell in response to the first read results, first cell information indicative of a first change-inducing read threshold; performing a second set of read attempts of the flash memory cells using a second set of read thresholds to provide second read results, calculating for each flash memory cell in response to the second read results, second cell information indicative of a second change-inducing read threshold; and generating, for each flash memory cell soft information in response to the first cell information and the second cell information of the flash memory cell.