DRAM Wordline Refresh Control for Variable Retention Times
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Solution Overview
Problem
DRAM cells require periodic refreshing to maintain charge, but existing methods do not account for varying retention times across cells, leading to inefficient energy consumption and command bandwidth usage.
Innovation Solution
Implementing different refresh intervals for wordlines based on minimum retention time, with shorter intervals for weak wordlines and longer intervals for strong wordlines, using a memory controller to issue fewer refresh commands and reduce energy consumption and bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If uniform refresh intervals are applied to all wordlines, then all cells are refreshed periodically to maintain charge, but energy consumption and command bandwidth are wasted on cells with longer retention times
Solution Approach 1:
The patent implements different refresh intervals for different wordlines based on their specific retention characteristics. Wordlines are classified into multiple groups (e.g., first group with shorter interval, second group with longer interval) according to their minimum retention times. This local differentiation allows the system to optimize energy consumption by avoiding excessive refreshing of cells with longer retention times while maintaining reliability for cells with shorter retention times.
Solution Approach 2:
The patent changes the refresh interval parameter dynamically based on wordline characteristics. Instead of using a single uniform refresh interval for all wordlines, the system assigns different refresh interval values to different wordline groups. This parameter variation enables the system to adapt to the diverse retention properties of cells across different wordlines, reducing overall energy consumption while maintaining data integrity.
2Productivity
If refresh commands are issued for all wordlines at uniform intervals, then all cells are refreshed, but command bandwidth is inefficiently utilized
Solution Approach 1:
The patent reduces the number of refresh commands by applying local quality differentiation. Instead of issuing refresh commands to all wordlines uniformly, the system issues commands only to specific wordline groups based on their retention characteristics. For example, during a long refresh period, only wordlines in the second group (with longer retention times) receive refresh commands, thereby reducing the total command bandwidth requirement while maintaining productivity.
Solution Approach 2:
The patent applies partial action by selectively refreshing only the necessary portion of wordlines at any given time. Rather than refreshing all wordlines at every refresh interval, the system performs partial refresh operations on specific wordline groups that require it based on their retention characteristics. This partial action approach reduces the quantity of refresh commands while maintaining overall system productivity.
3Loss of energy
If longer refresh intervals are used to reduce energy consumption, then energy efficiency improves, but cells with shorter retention times may lose charge
Solution Approach 1:
The patent resolves this contradiction by applying local quality principles to match refresh intervals with specific cell retention characteristics. Wordlines containing cells with shorter retention times are assigned to the first group with shorter refresh intervals, ensuring charge is maintained. Wordlines with cells having longer retention times are assigned to the second group with longer refresh intervals, reducing energy consumption. This localized matching ensures reliability is maintained where needed while optimizing energy efficiency elsewhere.
Solution Approach 2:
The patent segments the wordline population into multiple groups based on retention characteristics. By dividing wordlines into distinct segments (e.g., first group with shorter intervals, second group with longer intervals), the system can apply different refresh strategies to each segment. This segmentation allows the system to maintain short refresh intervals for vulnerable cells while using longer intervals for robust cells, thereby maintaining overall charge retention reliability while reducing total energy consumption.
Data Source
AI summary
DRAM cells need to be periodically refreshed to preserve the charge stored in them. The retention time is typically not the same for all DRAM cells but follows a distribution with multiple orders of magnitude difference between the retention time of cells with the highest charge loss and the cells with the lowest charge loss. Different refresh intervals are used for certain wordlines based on the required minimum retention time of the cells on those wordlines. The memory controller does not keep track of refresh addresses. After initialization of the DRAM devices, the memory controller issues a smaller number of refresh commands when compared to refreshing all wordlines at the same refresh interval.