Dynamic Read Calibration for QLC NAND Charge-Loss Shifts
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Solution Overview
Problem
Memory devices experience high read trigger rates due to temporal shifts in threshold voltage distributions, leading to increased latency and reduced quality of service (QoS) despite existing calibration techniques, especially in QLC NAND devices.
Innovation Solution
Dynamic calibration of read voltage levels based on data state metrics obtained from scans on memory devices, adjusting read voltage offsets for specific wordlines and pages to compensate for susceptibility to slow charge loss (SCL) and temporal shifts in threshold voltage distributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If static calibration techniques are used, then device complexity is reduced, but read trigger rates increase and quality of service deteriorates
Solution Approach 1:
The patent implements dynamic calibration by adjusting read voltage offsets based on real-time data state metrics obtained from scans. The calibration values are no longer static but are dynamically updated based on measured error rates and threshold voltage distribution shifts, allowing the system to adapt to temporal variations in memory device characteristics.
Solution Approach 2:
The system performs scans to obtain data state metrics, measures error rates and threshold voltage shifts, and uses this feedback information to update calibration values. This closed-loop feedback mechanism enables the system to detect performance degradation and automatically compensate for it by adjusting read voltage offsets.
2Productivity
If dynamic calibration is implemented, then read trigger rates are reduced, but measurement and detection complexity increases
Solution Approach 1:
The system performs scans and obtains data state metrics in advance of actual read operations. By pre-measuring error rates and threshold voltage distributions through scan operations, the calibration values are prepared beforehand, enabling quick adjustment without adding complexity to the critical read path.
Solution Approach 2:
The patent introduces scan operations as an intermediary process that bridges the gap between static calibration and actual read operations. These scan operations collect data state metrics that inform calibration adjustments, acting as a mediator that provides necessary measurement information without directly interfering with the read operation flow.
3Loss of time
If read voltage offsets are adjusted dynamically, then latency is reduced, but calibration process complexity increases
Solution Approach 1:
The system dynamically changes the read voltage parameter by adjusting offsets based on measured data state metrics. Instead of using fixed read voltages, the system modifies voltage levels according to actual threshold voltage distribution shifts and error rate measurements, optimizing read operations for current device conditions.
Solution Approach 2:
The calibration process is segmented into distinct components: scan operations for data collection, metric analysis for error rate and threshold voltage measurement, and calibration value generation for offset adjustment. This segmentation allows each component to be optimized independently and simplifies the overall calibration process management.
Data Source
AI summary
A system includes a memory device with multiple cells and a processing device to perform operations including: identifying a group of wordlines, each connected to a subset of cells, and assigning a specified charge loss classification value to that group. The operations can also include selecting a page level, selecting a first set of cells, determining, for the first set of cells, a value of a first data state metric, identifying a second set of cells charged to a specified charge state, and determining a value of a second data state metric. The operations can also include maintaining a skew counter of the second data state metric, identifying and updating a read reference voltage offset, as well as applying the updated read reference voltage offset in a read operation.