Memory Controller Offset Calibration for Asymmetric Read Thresholds
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Solution Overview
Problem
Memory cells deteriorate over time due to repeated program/erase operations and high temperature exposure, leading to asymmetrical threshold voltage distributions and errors during read operations at valley voltage levels.
Innovation Solution
A memory controller employs an offset table to correct read voltage levels by determining an offset value based on cell count and valley voltage levels, using a read voltage calibration module to adjust the read operation accordingly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a valley search operation is performed to find the optimal read voltage level, then the read operation can be performed at a calibrated voltage level, but memory cell deterioration leads to asymmetrical threshold voltage distributions causing read errors
Solution Approach 1:
The patent performs a valley search operation beforehand to find the optimal read voltage level and stores this information in a lookup table. This preliminary calibration ensures that subsequent read operations use the correct voltage level, preventing read errors before they occur.
Solution Approach 2:
The patent dynamically adjusts the read voltage level based on the actual threshold voltage distribution of the memory cells. By detecting the valley point in the threshold voltage distribution and using this information to select appropriate read voltage levels from a lookup table, the system adapts to changes in memory cell characteristics over time.
2Productivity
If repeated program/erase operations are performed on memory cells, then data can be stored and retrieved multiple times, but memory cell deterioration occurs leading to threshold voltage distribution changes
Solution Approach 1:
The patent implements a feedback mechanism where the threshold voltage distribution is periodically measured and used to update the read voltage level selection. The valley search operation provides feedback about the current state of memory cells, allowing the system to adapt to deterioration from repeated program/erase cycles.
Solution Approach 2:
Before performing read operations, the system performs a valley search to determine the current threshold voltage distribution and selects appropriate read voltage levels in advance. This preliminary action ensures that read operations remain accurate even as memory cells deteriorate from repeated programming and erasing.
3Temperature
If high temperature exposure is applied to memory cells, then thermal stress testing can be performed, but memory cell deterioration accelerates causing read operation errors
Solution Approach 1:
The patent uses the valley search operation to detect changes in threshold voltage distribution caused by temperature variations. By measuring the actual distribution and using this feedback to select appropriate read voltage levels, the system compensates for thermal stress effects on memory cell performance.
Data Source
AI summary
Provided is a method of operating a memory controller for controlling a memory device including a plurality of memory cells, the method including transmitting a valley search command for threshold voltage distribution of the memory cells to the memory device, obtaining a cell count value and a valley voltage level from the memory device, obtaining a determined offset value by searching an offset table based on the cell count value and the valley voltage level, and determining a corrected read voltage level by correcting the valley voltage level based on obtained the offset value, wherein the memory device is controlled so that the memory device performs a read operation through the corrected read voltage level.