Read Retry Threshold Voltage Selection for NAND Flash
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Solution Overview
Problem
NAND flash devices experience reduced reliability and increased latency due to voltage level shifts in cells over time, leading to inaccurate data reading and repeated read operations, especially under stressed conditions.
Innovation Solution
The implementation of a read retry table that dynamically updates read threshold voltages based on proximity to other voltages in the read voltage space, using a read retry neighbor table to ensure accurate data reading with lower latency by selecting new sets of read threshold voltages closest to the initial set after successful reads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If read retry operations are performed to handle voltage level shifts, then data reading reliability is improved, but read latency increases
Solution Approach 1:
The patent pre-calculates and stores multiple sets of read threshold voltages in a read retry table before actual read operations occur. When voltage shifts are detected, the system can immediately switch to pre-computed voltage sets without performing time-consuming calculations during the read operation, thus maintaining both reliability and low latency.
Solution Approach 2:
The patent implements dynamic selection of read threshold voltage sets based on detected voltage level shifts. The system adapts by choosing appropriate voltage sets from the read retry table according to the current state of the memory cells, allowing the read operation to respond dynamically to changing conditions without fixed, static thresholds.
2Device complexity
If fixed read threshold voltages are used, then device complexity is reduced, but measurement precision deteriorates due to voltage level shifts
Solution Approach 1:
The patent changes the read threshold voltage parameters based on detected voltage level shifts in memory cells. By storing multiple sets of threshold voltages and selecting appropriate sets based on cell state, the system maintains high measurement precision without requiring complex real-time adjustment mechanisms, thus balancing precision and complexity.
3Measurement precision
If multiple sets of read threshold voltages are stored in read retry table, then read accuracy is improved, but device complexity increases
Solution Approach 1:
The patent pre-computes and stores multiple sets of read threshold voltages in the read retry table during device initialization or idle periods. This preliminary action allows the device to maintain high read accuracy by having pre-prepared voltage sets available for immediate use, without requiring complex real-time computation hardware.
Solution Approach 2:
The patent creates multiple copies of read threshold voltage sets with different parameter values and stores them in the read retry table. Instead of implementing a single complex adaptive mechanism, the system uses simpler copied voltage sets that can be selected based on detected conditions, reducing overall device complexity while maintaining accuracy.
Data Source
AI summary
Embodiments describe a method for reading data from storage that includes selecting a block of memory to read, identifying a read retry table for reading the block, determining that the read retry table for the selected block of memory needs to be updated, and reading the block of memory using a new set of read threshold voltages from the read retry table. Responsive to a successful read operation using the new set of voltages, the method can also include replacing the initial set of read voltages in the first field with the new set of read voltages, and filling the plurality of subsequent fields in the read retry table with additional sets of read threshold voltages identified from a read retry neighbor table, where at least one of the additional sets of read voltages is closest in distance to the initial set of read voltages in read voltage space.