NAND Flash Threshold Voltage Control for Heterogeneous Block Reads
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Solution Overview
Problem
Existing memory systems face challenges in accurately maintaining threshold voltage settings for NAND flash memory due to variations in physical block quality and temperature changes, leading to reading errors that are not effectively mitigated by current error correction methods.
Innovation Solution
A memory system controller manages logical blocks with threshold voltage history and outlier tables, performs threshold voltage tracking for accurate reading, and employs temperature compensation and efficient compression methods to adapt to physical block variations and temperature changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If threshold voltage tracking is performed for each physical block to handle heterogeneous block quality, then reading accuracy improves, but system complexity and processing time increase
Solution Approach 1:
The patent segments the threshold voltage management by creating separate handling paths for normal blocks and outlier blocks. The outlier table specifically stores threshold voltages for problematic physical blocks, allowing targeted correction without complicating the entire system. This segmentation enables precise correction for heterogeneous blocks while maintaining simple default behavior for normal blocks.
Solution Approach 2:
The patent applies local quality by treating different physical blocks differently based on their characteristics. Normal blocks use the standard threshold voltage from the first table, while outlier blocks use corrected threshold voltages from the second table. This localized approach ensures high reading accuracy for problematic blocks without adding unnecessary complexity to the handling of normal blocks.
2Measurement precision
If threshold voltage tracking is performed for each physical block to handle heterogeneous block quality, then reading accuracy improves, but processing time increases
Solution Approach 1:
The patent performs preliminary action by pre-calculating and storing corrected threshold voltages for outlier physical blocks in the second table during manufacturing or initial characterization. When reading operations occur, the system simply retrieves the pre-computed values from the outlier table rather than performing complex real-time calculations, significantly reducing processing time while maintaining high accuracy.
Solution Approach 2:
The patent uses copying by creating a separate outlier table that copies and stores the corrected threshold voltage values for problematic blocks. This allows the system to quickly reference pre-determined correction values without re-performing the complex threshold voltage tracking process during actual read operations, thereby reducing processing time.
3Quantity of substance
If compression is applied to threshold voltage tables to reduce memory usage, then storage efficiency improves, but retrieval speed may decrease
Solution Approach 1:
The patent segments the threshold voltage storage into two separate tables: a first table for normal blocks and a second outlier table for problematic blocks. This segmentation allows the system to store only the necessary corrected values for outlier blocks separately, reducing overall memory usage while enabling fast retrieval through direct indexing into the appropriate table based on block characteristics.
Data Source
AI summary
According to one embodiment, a controller for a memory system including a nonvolatile memory with physical blocks sets logical blocks including the physical blocks, manages a first table having a first value indicating a threshold voltage to be used for reading data from each logical block, a second table for each physical block having a corrected threshold voltage that has been statistically determined to be an outlier in association with a second value indicating the corrected threshold voltage for the physical block. The controller performs a threshold voltage tracking for identifying a threshold voltage suitable for reading data from a physical block if a reading fails, and then retries the reading using the threshold voltage obtained by the threshold voltage tracking. The controller updates the first table if the physical block is not listed in the second table, but not otherwise.