Flash Memory Read Accuracy via Binary Digit Distribution Analysis
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Solution Overview
Problem
Flash memory systems, particularly NAND-type flash memories, face challenges in accurately reading data due to disturbances in threshold voltage distribution caused by increased program/erase cycles and retention time, leading to incorrect data retrieval and uncorrectable errors.
Innovation Solution
A method and memory controller that utilize binary digit distribution characteristics of bit sequences read from memory cells to adaptively adjust control gate voltages, performing multiple read operations with different voltage settings to determine accurate readout information and reduce uncorrectable errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If original control gate voltage setting is used to read stored bits, then reading operation is simple and fast, but data accuracy deteriorates due to changed threshold voltage distribution
Solution Approach 1:
The patent applies preliminary action by performing a calibration process before the actual read operation. The memory controller first determines the threshold voltage distribution characteristics of the flash memory cells, then uses this information to adjust the control gate voltage settings. This preliminary calibration ensures that subsequent read operations use optimized voltage settings that account for P/E cycle effects, thereby maintaining high reading accuracy without requiring complex real-time adjustments during the actual read process.
Solution Approach 2:
The patent implements feedback by using the determined threshold voltage distribution characteristics to adjust the control gate voltage settings. The memory controller reads the threshold voltage distribution information, compares it with ideal distribution, and feeds back adjustments to the control gate voltage to compensate for shifts caused by P/E cycles. This closed-loop approach ensures accurate data retrieval despite changes in memory cell characteristics over time.
2Measurement precision
If multiple read operations with different voltage settings are performed, then data reading accuracy is improved, but reading time increases
Solution Approach 1:
The patent performs the threshold voltage distribution determination and control gate voltage optimization as a preliminary action during calibration phases or when data is first written. This pre-computed information is then stored and reused for subsequent read operations, allowing fast reads without repeating the complex multi-voltage measurements. The time-consuming accuracy improvements are achieved once, not repeatedly for every read operation.
Solution Approach 2:
The memory system uses its own threshold voltage distribution characteristics to automatically adjust its reading parameters. By determining the actual voltage distribution and using this information to optimize subsequent reads, the system serves itself, eliminating the need for external calibration equipment or repeated complex measurement sequences, thereby reducing reading time while maintaining accuracy.
3Reliability
If threshold voltage distribution changes are tracked and adjusted, then uncorrectable errors are reduced, but system complexity increases
Solution Approach 1:
The patent uses feedback by continuously monitoring the threshold voltage distribution and using this information to adjust control gate voltage settings. The memory controller determines the actual voltage distribution, compares it with expected distributions, and automatically adjusts reading parameters to compensate for shifts. This feedback mechanism reduces uncorrectable errors caused by P/E cycle effects without requiring complex error correction codes or redundant storage schemes.
Solution Approach 2:
The patent changes the operating parameters (control gate voltage settings) based on the determined threshold voltage distribution characteristics. Instead of adding complex hardware, the system dynamically adjusts voltage parameters to match the actual memory cell states. This parameter adaptation approach improves reliability by aligning read operations with actual memory conditions while keeping the overall system architecture relatively simple.
Data Source
AI summary
A method for reading data stored in a flash memory includes at least the following steps: controlling the flash memory to perform a plurality of read operations upon a plurality of memory cells included in the flash memory; obtaining a plurality of bit sequences read from the memory cells, respectively, wherein the read operations read bits of a predetermined bit order from the memory cells by utilizing different control gate voltage settings; and determining readout information of the memory cells according to binary digit distribution characteristics of the bit sequences.