Dynamic Read Pass Voltage Control for Flash Memory
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Solution Overview
Problem
Non-volatile storage devices, such as EEPROM and flash memory, experience issues with read disturb and data retention due to changes in threshold voltage over time, especially in fresh memory that has not undergone many program/erase cycles, leading to program noise and data retention problems.
Innovation Solution
A non-volatile storage system dynamically adjusts the read pass voltage based on the number of program/erase cycles, using a higher voltage for cycled memory to address data retention issues and a lower voltage for fresh memory to mitigate read disturb, optimizing voltages through simulation, testing, and monitoring to ensure accurate data retrieval.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high read pass voltage is applied to unselected word lines, then data retention in cycled memory is improved, but read disturb in fresh memory worsens
Solution Approach 1:
The patent applies dynamics by making the read pass voltage adjustable rather than fixed. The system dynamically selects between a first read pass voltage (higher) for cycled memory and a second read pass voltage (lower) for fresh memory based on the memory's cycle history. This resolves the contradiction by adapting the voltage level to the specific condition of the memory, preventing read disturb in fresh memory while ensuring data retention in cycled memory.
Solution Approach 2:
The patent changes the voltage parameter based on memory condition. By monitoring whether memory cells have undergone many program/erase cycles, the system switches between two distinct voltage levels for reading operations. This parameter change approach allows optimization for both fresh and cycled memory states, resolving the trade-off between read disturb and data retention.
2Reliability
If operating parameters are optimized for cycled memory, then data retention improves, but performance of fresh memory deteriorates
Solution Approach 1:
The system dynamically adjusts operating parameters based on memory condition. By detecting whether memory is in a fresh state or has undergone many program/erase cycles, the controller selects appropriate read pass voltages and timing parameters. This dynamic adaptation ensures optimal performance for fresh memory while maintaining data retention for cycled memory.
Solution Approach 2:
The patent implements parameter changes by switching between different read pass voltage levels and potentially other operating parameters based on the memory's program/erase cycle history. This allows the system to optimize for data retention when memory is cycled while maintaining high performance when memory is fresh, resolving the contradiction between reliability and productivity.
3Device complexity
If a fixed read pass voltage is used, then device complexity is reduced, but reliability under varying conditions deteriorates
Solution Approach 1:
The patent introduces dynamic voltage selection based on memory condition without significantly increasing device complexity. The controller monitors program/erase cycle counts and automatically selects the appropriate read pass voltage level. This dynamic approach maintains reliability across varying memory conditions while keeping the implementation relatively simple through automated detection and selection.
Solution Approach 2:
The system changes the read pass voltage parameter based on detected memory condition (fresh vs. cycled). By implementing this parameter change through automated monitoring and control logic, the patent improves reliability for data retrieval accuracy while managing device complexity through systematic voltage selection rather than complex hardware modifications.
Data Source
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AI summary
A first read pass voltage is determined and optimized for cycled memory. One or more starting read pass voltages are determined for one or more dies. The system dynamically calculates a current read pass voltage based on the number of program/erase erase cycles, the first read pass voltage and the respective starting read pass voltage. Data is read from one or more non-volatile storage elements using the calculated current read pass voltage.