NAND Read Voltage Estimation Using Corrected Threshold Histograms
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Solution Overview
Problem
In NAND type flash memory systems, the change in threshold voltage due to data retention and read disturbance leads to increased bit error rates, deteriorating the reliability of the memory system, and existing techniques for estimating read voltage are time-consuming and require extensive measurement.
Innovation Solution
A memory system that includes a nonvolatile memory and a memory controller, which generates a histogram of threshold voltage ranges and uses an estimation function to accurately determine the read voltage by correcting parameters from multiple error mitigation encoding schemes, thereby improving the accuracy of read voltage estimation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If read voltage is estimated by performing measurement using approximately seven reference read voltages, then the reliability of the memory system is improved, but the processing time becomes long
Solution Approach 1:
The patent pre-calculates and stores voltage distribution information (histograms) and read disturbance characteristics during manufacturing or initial setup. When reading data, the system retrieves this pre-computed information and applies it to estimate the current threshold voltage distribution, avoiding the need to perform time-consuming measurements with multiple reference read voltages at read time. This preliminary preparation resolves the contradiction by providing reliable voltage estimation data in advance.
Solution Approach 2:
The patent creates a simplified model or copy of the complex voltage distribution behavior by storing histogram data and disturbance characteristics that represent the full seven-voltage measurement results. This copied representation allows the system to estimate read voltage quickly using pre-computed statistical data rather than performing complete measurements, thereby reducing processing time while maintaining reliability.
2Measurement precision
If threshold voltage measurement is performed using actual NAND memory, then the accuracy of read voltage estimation is improved, but the device complexity increases
Solution Approach 1:
The patent extracts only the essential voltage distribution information (histograms) and read disturbance characteristics from the complex actual NAND memory measurements. By separating and storing only these critical parameters rather than the full measurement dataset, the system achieves accurate read voltage estimation without requiring the complete measurement apparatus or complex real-time measurement processes, thus reducing device complexity while maintaining precision.
Solution Approach 2:
The patent transforms the complex physical measurement problem into a parameter-based computational problem. Instead of performing complex physical measurements during operation, the system uses pre-measured parameters (histogram data, disturbance characteristics) to calculate threshold voltage distribution. This parameter transformation simplifies the device requirements while preserving measurement accuracy.
3Reliability
If the number of reference read voltages is increased to improve estimation accuracy, then the reliability is improved, but the productivity decreases
Solution Approach 1:
The patent performs the computationally intensive task of analyzing multiple reference read voltage measurements during a preliminary setup phase, storing the results as histogram data and disturbance characteristics. During actual read operations, the system uses this pre-processed information for rapid voltage estimation, achieving both high accuracy (equivalent to using multiple reference voltages) and high productivity (fast read operations) by separating the heavy computation from the time-critical read path.
Data Source
AI summary
A memory system includes a nonvolatile memory and a memory controller. The memory controller determines a read voltage for first data encoded by a first encoding scheme, by generating a first histogram indicating the number of memory cells for each threshold voltage, and estimating the read voltage using: (a) the first histogram that is corrected based on a first parameter of the first encoding scheme, and a second parameter of a second encoding scheme, and an estimation function for estimating a read voltage for second data encoded by the second encoding scheme, or (b) the uncorrected first histogram, and the estimation function that is corrected based on the first and second parameters, or (c) the first histogram after partial correction based on the first and second parameters, and the estimation function after partial correction based on the first and second parameters.