NAND Flash Controller Wear Reduction via Adaptive Bias
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Solution Overview
Problem
As NAND flash memory scales, ensuring reliability and performance while extending endurance becomes increasingly challenging, particularly due to read disturb effects on non-selected cells, which lead to increased bit error rates and reduced lifespan.
Innovation Solution
A digital signal processor (DSP) building block within an advanced NAND flash controller dynamically adapts bias voltages during read operations, using lower bias voltages to minimize disturb effects on non-selected cells, and implements methods such as dummy reads and adaptive decoding processes to extend the number of reads before refresh is required, thereby reducing wear and increasing endurance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional read operations are performed with standard bias voltages, then read speed is maintained, but read disturb effects increase causing higher bit error rates and reduced memory lifespan
Solution Approach 1:
The patent implements dynamic bias voltage adjustment during read operations. The controller monitors read error rates and dynamically adapts the bias voltage level - using lower bias voltages when error rates are acceptable to reduce read disturb effects, and adjusting upward when needed to maintain read accuracy. This dynamic adaptation resolves the contradiction by making the bias voltage flexible rather than fixed, allowing the system to optimize between reliability and productivity in real-time based on actual memory block conditions.
Solution Approach 2:
The patent changes the physical parameter of bias voltage from a fixed conventional value to an adaptive variable parameter. By modifying the bias voltage parameter based on monitored read performance and error rates, the system can reduce read disturb effects (improving reliability/memory lifespan) while maintaining acceptable read speeds. The parameter change approach allows continuous optimization of the trade-off between read speed and memory endurance.
2Measurement precision
If higher bias voltages are used during reads, then read accuracy is improved, but wear on flash memory cells increases reducing endurance
Solution Approach 1:
The patent applies partial action by using bias voltages that are sufficient to achieve acceptable read accuracy but not excessively high. Instead of always using maximum bias voltages for guaranteed accuracy, the system uses the minimum necessary bias voltage level based on actual read performance monitoring. This partial action approach maintains adequate read accuracy while reducing the cumulative wear effect on flash memory cells, thereby extending memory endurance.
Solution Approach 2:
The patent implements a feedback mechanism where read error rates are monitored and used to adjust subsequent bias voltage levels. The system continuously measures read accuracy performance and feeds this information back to the bias voltage control logic. When error rates indicate sufficient accuracy, the feedback loop reduces bias voltage to minimize wear. This closed-loop feedback system resolves the contradiction by dynamically balancing read accuracy requirements against endurance preservation.
3Productivity
If read operations are performed frequently, then data access performance is improved, but read disturb effects accumulate increasing bit error rates
Solution Approach 1:
The patent implements dynamic adaptation of bias voltage in response to accumulated read operations. As read operations increase on a particular memory block, the system dynamically adjusts the bias voltage downward to reduce further read disturb accumulation. This dynamic response allows frequent data access (maintaining productivity) while adaptively reducing the bit error rate that would otherwise accumulate from repeated reads. The system monitors access patterns and adjusts bias voltage accordingly.
Solution Approach 2:
The patent uses feedback from monitored bit error rates to adjust bias voltage levels during subsequent read operations. When frequent reads cause error rate accumulation, the feedback mechanism detects this degradation and reduces bias voltage to mitigate further damage. This feedback loop enables the system to maintain high data access performance while actively managing and reducing bit error rates through adaptive bias voltage control based on actual memory block conditions.
Data Source
AI summary
A method for wear reduction of a flash memory module, the method may include reading data stored in a group of flash memory cells to provide a read data; wherein the reading comprise supplying a bias voltage that is lower than a write bias voltage; wherein the write bias voltage was supplied to the group of flash memory cells during a writing of the data to the group of flash memory cells; and decoding the read data, by applying a decoding process of a given complexity, to provide decoded data.