Flash Memory Reference Voltage Optimization via Threshold Distribution Sensing
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Solution Overview
Problem
Existing memory systems face challenges in optimizing reference voltages for flash memory cells, which can lead to read errors due to changes in threshold voltage distributions caused by wear and tear or environmental changes, especially after repeated cycling operations.
Innovation Solution
A system and method that independently optimize each reference voltage based on the number of erase/program cycles, read errors, and time since last optimization, using a voltage generator and sensing modules to determine optimal voltage settings without relying on stored data or error correcting codes, allowing for Gray mapping and selection of specific bit pages for optimization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If reference voltages are optimized based on threshold voltage distribution changes, then data retrieval accuracy is improved, but device complexity increases due to additional sensing and control modules
Solution Approach 1:
The memory system performs self-diagnosis and self-optimization by using the sensing module to detect threshold voltage distributions and automatically adjusting reference voltages without external intervention. The control module enables the system to service itself by monitoring its own state and making necessary adjustments to maintain optimal performance.
Solution Approach 2:
The system implements a feedback mechanism where the sensing module continuously monitors threshold voltage distributions of memory cells and provides this information to the control module, which then adjusts reference voltages accordingly. This closed-loop feedback ensures that reference voltages remain optimized despite wear and environmental changes.
2Reliability
If optimization is performed after every cycle, then read error minimization is improved, but productivity decreases due to frequent optimization operations
Solution Approach 1:
The system dynamically adjusts the optimization frequency based on actual performance needs rather than following a fixed schedule. The control module determines when optimization is necessary by monitoring threshold voltage distribution changes, allowing the system to optimize only when needed rather than after every cycle, thus balancing reliability and productivity.
Solution Approach 2:
The system maintains continuous monitoring of threshold voltage distributions through the sensing module, ensuring that optimization opportunities are not missed while avoiding unnecessary optimization operations. This continuous awareness allows the system to perform useful optimization actions only when actual changes warrant them.
3Measurement precision
If reference voltage optimization uses stored data and ECC, then measurement precision is improved, but ease of operation worsens due to dependency on stored data availability
Solution Approach 1:
The invention extracts the optimization capability from dependency on stored data and ECC systems. By using the sensing module to directly measure threshold voltage distributions of actual memory cells, the system obtains the necessary information for optimization without requiring stored reference data or error correcting codes, thereby simplifying operation.
Solution Approach 2:
The sensing module serves multiple functions: it is used both for normal data reading operations and for threshold voltage distribution measurement during optimization. This multi-functionality eliminates the need for separate optimization data structures or ECC mechanisms, making the system easier to operate while maintaining high measurement precision.
Data Source
AI summary
A system including a divider module, a read module, a counting module, and a reference voltage setting module. The divider module is configured to select a voltage range in which to adjust a reference voltage used to read memory cells of nonvolatile memory, and to divide the voltage range into a plurality of bins, where each of the bins is defined by a pair of voltages. The read module is configured to perform a plurality of read operations by applying, to the memory cells, the voltages defining the bins. The counting module is configured to generate, in each of the read operations, counts of a number of memory cells having threshold voltages in each of the bins. The reference voltage setting module is configured to adjust, based on the counts, the reference voltage to a voltage selected from one or more voltages associated with the bins.