Adaptive Read Threshold Voltage Tracking for SSDs
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Solution Overview
Problem
Existing adaptive tracking algorithms for solid state storage devices assume known gaps between default read threshold voltages, which is not always available, and fail to separately characterize each side of the voltage distribution about the mean, leading to suboptimal read threshold voltage adjustments and increased error rates over time.
Innovation Solution
The method involves characterizing statistical distributions on both sides of the mean for each memory level, estimating the gap between default read threshold voltages, and updating read threshold voltages based on these characteristics to improve robustness and accuracy, especially in asymmetric distributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If existing adaptive tracking algorithms are used to adjust read threshold voltages, then performance improvement is achieved, but reliability deteriorates due to increased error rates over time
Solution Approach 1:
The patent divides the voltage distribution characterization into two separate segments: one for the left side of the distribution and one for the right side. This segmentation allows independent tracking of asymmetry in the voltage distribution, enabling more accurate read threshold voltage adjustment and maintaining reliability over time while preserving performance improvements.
2Device complexity
If gap values between default read threshold voltages are assumed known, then device complexity is reduced, but measurement precision deteriorates due to inaccurate gap assumptions
Solution Approach 1:
The patent implements feedback by using collected statistics from voltage distribution measurements to iteratively estimate gap values. The algorithm collects statistics, estimates gaps based on these statistics, and uses the estimated gaps to refine subsequent measurements, thereby improving measurement precision without significantly increasing device complexity.
3Reliability
If separate characterization of each side of voltage distribution is implemented, then reliability is improved through better gap estimation, but device complexity increases
Solution Approach 1:
The patent applies local quality by characterizing the left and right sides of the voltage distribution with separate parameters (mean and standard deviation for each side). This localized characterization approach improves reliability by capturing asymmetry in the distribution while keeping the overall device complexity manageable through efficient statistical methods.
Data Source
AI summary
Methods and apparatus are provided for adaptive read threshold voltage tracking with separate characterization on each side of a voltage distribution about a distribution mean. A read threshold voltage for a memory is adjusted by determining statistical characteristics of two adjacent memory levels based at least in part on a type of statistical distribution of the memory levels and a distribution of data values read from cells using a plurality of read threshold voltages, wherein the statistical characteristics of the two adjacent memory levels are characterized independently on two sides about at least one mean of the statistical distribution; computing an adjusted read threshold voltage associated with the two adjacent memory levels by using the statistical characteristics of the two adjacent memory levels; and updating the read threshold voltage based on the adjusted read threshold voltage. The adjustment is optionally performed responsive to one or more read errors.