NAND Flash LLR Estimation Using Threshold Slope Soft Decoding
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Solution Overview
Problem
NAND flash memory devices face errors due to stress conditions such as NAND noise and interference during programming and reading, leading to inaccuracies in programmed and read output, which existing technologies struggle to address effectively.
Innovation Solution
A method and system that estimate slope information of threshold voltage samples from a first read operation to generate soft information, which is then used to decode the result of a second read operation, improving endurance and resilience to retention and read-disturb stresses by adapting to dynamic stress conditions through online adaptation of soft information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional decoding methods are used for NAND flash memory, then device complexity remains low, but reliability deteriorates due to errors from stress conditions such as NAND noise and interference
Solution Approach 1:
The patent performs preliminary actions by conducting a first read operation to estimate slope information of threshold voltage samples before the actual decoding process. This preliminary estimation of soft information (slope values) allows the decoder to adapt to current stress conditions, improving reliability without significantly increasing complexity during the critical decoding phase.
Solution Approach 2:
The patent changes decoding parameters dynamically by adjusting soft information (LLR values) based on estimated slope information from threshold voltage samples. The decoder adapts its operation by modifying likelihood ratios according to measured slope values, enabling reliable decoding under varying stress conditions while maintaining manageable complexity through parameter adaptation rather than structural complexity.
2Measurement precision
If multiple read operations are performed to improve accuracy, then measurement precision improves, but loss of time increases due to additional read operations
Solution Approach 1:
The patent extracts only the essential information needed for decoding by performing a first read operation specifically to estimate slope information, then using this extracted soft information for decoding a second read operation. This selective extraction of slope values from threshold voltage samples provides measurement precision without requiring multiple full read operations, thus reducing time loss.
3Reliability
If soft information is generated and used for decoding, then reliability improves under stress conditions, but device complexity increases due to additional processing steps
Solution Approach 1:
The patent performs preliminary generation of soft information (slope estimation) from a first read operation before the actual decoding process. By pre-computing likelihood ratios based on slope information, the system improves reliability under stress conditions while keeping the decoding phase simpler, as the complex soft information generation is separated and optimized as a preliminary step.
Data Source
AI summary
A flash memory system may include a flash memory and a circuit for performing operations of the flash memory. The circuit may be configured to estimate slope information of a plurality of threshold voltage samples based on a first read operation on the flash memory with a first reference voltage. The circuit may be configured to generate soft information based on the estimated slope information. The circuit may be configured to decode a result of a second read operation on the flash memory based on the soft information.