Adaptive Reference Voltage Shift for MLC Flash Read Accuracy
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Solution Overview
Problem
Conventional MLC flash memory experiences read errors due to cycle and retention issues, as the threshold voltage distribution shifts over time, making it necessary to adapt reference voltages for accurate data retrieval.
Innovation Solution
The method involves determining a new set of reference voltages by finding a threshold voltage in the initial distribution that matches the shifted distribution's cell count, allowing for a calculated shift to correct the initial reference voltages, thereby ensuring accurate data reading from MLC flash memory.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If constant reference voltages are used for reading data from MLC flash memory, then device complexity is reduced and ease of operation is improved, but read errors occur due to threshold voltage distribution shifts after program/erase cycles and retention time
Solution Approach 1:
The patent applies preliminary action by pre-calculating and storing the relationship between program/erase cycle counts, retention time, and threshold voltage distribution shifts in a lookup table during manufacturing. This allows the system to directly query the appropriate reference voltage offset without performing complex real-time calculations, thus improving read accuracy while minimizing additional computational complexity.
Solution Approach 2:
The patent replaces the mechanical/electrical approach of physically adjusting reference voltages with a computational approach using lookup tables and offset calculations. Instead of hardware-based voltage adjustment mechanisms, the system uses software-based offset values stored in memory, which are added to base reference voltages to compensate for drift, thereby reducing device complexity while maintaining reliability.
2Reliability
If reference voltages are adapted to compensate for threshold voltage distribution shifts, then read accuracy is improved, but additional processing time and computational complexity are introduced
Solution Approach 1:
The patent performs the complex analysis of threshold voltage distribution shifts and reference voltage optimization during the manufacturing process, storing the results in lookup tables. During actual read operations, the system only needs to query the lookup table and apply a pre-computed offset, reducing the time penalty to minimal arithmetic operations rather than full recalibration.
Solution Approach 2:
The patent applies partial action by implementing reference voltage adaptation only when necessary - specifically, by detecting changes in program/erase cycle count or retention time thresholds that indicate significant distribution shifts. This selective adaptation approach avoids continuous recalibration overhead while maintaining read accuracy when drift occurs.
3Measurement precision
If the threshold voltage distribution is monitored and used to adjust reference voltages, then measurement precision is improved, but device complexity and difficulty of detecting and measuring increase
Solution Approach 1:
The patent extracts the complex task of threshold voltage distribution analysis and reference voltage optimization from the operational memory system and relocates it to the manufacturing process. During manufacturing, comprehensive measurements and optimizations are performed, and the results are stored as lookup tables. This extraction eliminates the need for complex real-time measurement and analysis capabilities in the operational device.
Solution Approach 2:
The patent creates a simplified copy of the threshold voltage distribution characteristics by storing pre-computed reference voltage offsets in lookup tables during manufacturing. Instead of requiring the operational system to perform complex distribution analysis, it uses these copied characteristics to directly determine the appropriate voltage adjustments, significantly reducing measurement and analysis difficulty.
Data Source
AI summary
A method and system for adaptively finding reference voltages for reading data from a multi-level cell (MLC) flash memory is disclosed. According to one embodiment, a first total number of cells of the flash memory above a first threshold voltage in a shifted threshold voltage distribution is provided. Search to find a second threshold voltage such that a second total number of the cells above the second threshold voltage is approximate to the first total number. An initial reference voltage or voltages of the initial threshold voltage distribution are shifted with an amount approximate to a voltage difference between the second threshold voltage and the first threshold voltage, thereby resulting in a new reference voltage or voltages for reading the data from the MLC flash memory.