Nonvolatile Memory Refresh Control Using Random Number Generation
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Solution Overview
Problem
In nonvolatile memory systems, the application of blocking voltage to unselected bits can induce a 'disturb' phenomenon, causing changes in memory cell thresholds, leading to undesirable increases in the number of writes required for refresh operations.
Innovation Solution
A system that includes a nonvolatile memory, a random number generator, and a controller, where the controller determines a refresh-targeted area based on a randomly generated number to perform re-writing, thereby restoring the memory cell threshold to its original state without increasing the number of writes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If blocking voltage is applied to unselected bits to prevent malfunction, then reliability is improved, but the threshold of memory cells changes due to disturb phenomenon
Solution Approach 1:
The patent applies preliminary action by performing refresh operations proactively before the disturb phenomenon causes significant threshold drift. The controller monitors access patterns and applies blocking voltage in advance to unselected bits that are likely to be affected, restoring their thresholds before malfunction occurs. This prevents the worsening of threshold stability while maintaining reliability.
Solution Approach 2:
The patent implements feedback by continuously monitoring the threshold values of memory cells and adjusting the application of blocking voltage accordingly. The controller detects changes in threshold distribution and dynamically determines which unselected bits require refresh operations, creating a closed-loop system that maintains threshold stability while minimizing unnecessary operations.
2Stability of the object's composition
If refresh is performed for every writing, reading, or erasing operation, then threshold stability is improved, but the number of writing operations increases undesirably
Solution Approach 1:
The patent applies local quality by selectively performing refresh operations only on specific unselected bits that are determined to be affected by the disturb phenomenon, rather than refreshing all memory cells uniformly. The controller identifies individual bits or small groups of bits that require refresh based on their location relative to selected bits and the applied blocking voltage, thereby maintaining threshold stability for affected bits while avoiding unnecessary writing operations on unaffected bits.
Solution Approach 2:
The patent uses partial action by performing refresh operations on a subset of unselected bits rather than all bits. The controller determines the minimum necessary refresh operations based on access patterns and disturbs, applying blocking voltage only where needed to restore threshold stability, thus avoiding the excessive writing operations that would result from universal refresh.
3Stability of the object's composition
If random number-based refresh control is implemented, then uniform refresh distribution is achieved, but device complexity increases
Solution Approach 1:
The patent introduces a random number generator as an intermediary component that simplifies the refresh control logic. Instead of implementing complex algorithms to determine which bits need refresh, the system uses the random number generator to select candidate bits for refresh, which are then validated against actual access patterns. This intermediary approach achieves uniform refresh distribution while keeping the overall control mechanism relatively simple.
Data Source
AI summary
In a nonvolatile memory, the threshold is restored to a state before changing, without increasing number of writing undesirably. In a system including a nonvolatile memory, a random number generator, and a controller accessible to the nonvolatile memory, every time access to the nonvolatile memory is performed, the controller determines a refresh-targeted area, based on a random number generated by the random number generator. The controller is made to perform refresh control to re-write to the refresh-targeted area. By such refresh control, the threshold is restored to a state before changing, without increasing the number of writing undesirably.