Flash Memory Read Threshold Adjustment via Error Direction Statistics
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Solution Overview
Problem
NAND Flash memory devices experience increased read errors due to lobe overlap and retention effects, which shrink the effective working window and deteriorate programming accuracy over program/erase cycles, making it challenging to find optimal read thresholds for reliable data retrieval.
Innovation Solution
A method for dynamically adjusting read thresholds based on current read error direction statistics, using a combination of error evaluation and read threshold updates to minimize errors, including the application of a Kalman filter and maximal margin estimators to optimize read threshold settings without determining the contribution of each threshold to the error statistics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If read thresholds are fixed, then device complexity is low, but read reliability deteriorates due to lobe overlap and retention effects
Solution Approach 1:
The patent implements dynamic read threshold adjustment by continuously monitoring read error statistics and automatically modifying threshold values. The system transitions from static fixed thresholds to dynamic adaptive thresholds that respond to changing storage conditions, retention effects, and programming history, thereby maintaining high read reliability without requiring manual intervention.
Solution Approach 2:
The patent employs feedback mechanisms where read error direction statistics are collected and fed back into the threshold adjustment process. The system analyzes error patterns (e.g., MSB vs. LSB error dominance) and uses this feedback to automatically adjust read thresholds, creating a closed-loop system that continuously optimizes read reliability based on actual performance data.
2Measurement precision
If additional read operations are performed to optimize thresholds, then read accuracy improves, but productivity decreases due to increased read time
Solution Approach 1:
The patent performs preliminary threshold optimization by analyzing error direction statistics from previous reads and using this information to pre-adjust thresholds before the next read operation. This preliminary action eliminates the need for additional read operations during actual data retrieval, as the thresholds are already optimized based on historical performance data.
Solution Approach 2:
The system performs self-optimization by automatically adjusting its own read thresholds based on collected error statistics without requiring external intervention or additional read operations. The error evaluation circuit and threshold adjustment mechanism work autonomously to improve read accuracy while maintaining productivity, as the optimization process uses existing read data rather than requiring separate calibration reads.
Data Source
AI summary
There is provided a method for setting read thresholds to be used for reading multiple bits per cell flash memory cells, the method may include reading, by a read circuit, the flash memory cells using a set of current read thresholds to provide current read results; finding, by an error evaluation circuit, current read errors direction statistics associated with the current read results; determining multiple read threshold changes based upon the current read error direction statistics, without determining a contribution of each current read threshold to the current read error direction statistics; and altering multiple current read thresholds, by the multiple read threshold updates, to provide a set of next read thresholds.