CIM eDRAM Replica Arrays for Leakage-Aware Refresh Timing
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Solution Overview
Problem
Existing CIM architectures using eDRAM face challenges in maintaining accurate MAC operation results due to data leakage, leading to potential errors in refresh timing based on memory cell retention time, which is not suitable for analog MAC operations.
Innovation Solution
An apparatus and method that includes a replica array with replica MAC arrays and a refresh determination module to compare voltage levels and adaptively adjust refresh timing, ensuring accurate MAC operation results by detecting potential errors and generating refresh commands.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If refresh timing is adjusted based on data retention time (DRT) of memory cells, then power consumption is reduced by performing refreshes as slowly as possible, but MAC operation accuracy deteriorates due to charge leakage in eDRAM cells
Solution Approach 1:
The patent creates a replica array that copies the structure and functionality of the main CIM cell array. The replica array includes replica MAC units that perform the same analog MAC operations as the main array, allowing independent monitoring of charge retention without affecting the primary computational function. This copying approach enables separate optimization of refresh timing based on actual charge leakage characteristics observed in the replica.
Solution Approach 2:
The replica array acts as an intermediary monitoring system that observes charge retention characteristics in real-time and provides feedback signals to control the refresh operation of the main CIM array. The replica MAC units serve as sensors that detect when charge leakage begins to affect computational accuracy, triggering refresh operations only when necessary rather than periodically.
2Reliability
If refresh operations are performed periodically based on DRT, then data storage reliability is maintained, but computational accuracy deteriorates due to charge changes in eDRAM before value flipping
Solution Approach 1:
The system implements a feedback mechanism where the replica MAC units continuously monitor the charge state of eDRAM cells and compare it against reference values. When the charge level deviates beyond a threshold that would affect MAC operation accuracy, the replica generates a feedback signal to trigger a refresh operation in the main CIM array, ensuring computational accuracy without unnecessary refreshes.
Solution Approach 2:
The replica array performs preliminary monitoring of charge retention characteristics before actual MAC operations are affected. By detecting charge leakage trends in advance through the replica units, the system can trigger refresh operations proactively to prevent accuracy degradation in the main computational array.
3Quantity of substance
If eDRAM is used instead of SRAM to increase CIM capacity, then memory density is improved, but data retention deteriorates due to floating node leakage
Solution Approach 1:
The system transitions from static periodic refresh timing to dynamic adaptive refresh timing. The refresh interval is continuously adjusted based on real-time charge retention measurements from the replica array, allowing the system to optimize between eDRAM's high density and its leakage characteristics by refreshing only when and where charge degradation is detected.
Solution Approach 2:
The patent changes the refresh timing parameter from a fixed periodic interval based on conservative DRT estimates to a dynamic parameter adjusted according to actual charge retention measurements. The replica array enables measurement of the actual charge decay rate, allowing the refresh timing parameter to be optimized for each operational context and eDRAM cell state.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Adaptive refresh timing prevents errors in MAC operations by accurately determining when to refresh eDRAM cells, maintaining accurate computational results in CIM systems.
Implementation Method 1
eDRAM with a smaller cell size than SRAM is increasingly being used in CIM. In eDRAM, data is stored in a storage node (SN), which is a floating node. However, there is a problem that the data stored in the storage node (SN) may change in value due to leakage.
Implementation Method 2
a refresh determination module that compares voltage levels of signals output from each of the plurality of replica MAC arrays with MAC operation results according to weights stored in replica cells of the plurality of replica MAC arrays and activates a refresh enable signal for refreshing memory cells of the CIM cell array
Data Source
AI summary
An apparatus for adaptively adjusting refresh timing of a CIM based on eDRAM includes a replica array including a plurality of replica MAC arrays, each having replica cells arranged in a direction and number according to the direction and number in which memory cells performing MAC operations together in a CIM cell array are arranged and a refresh determination module that compares voltage levels of signals output from each of the plurality of replica MAC arrays with MAC operation results according to weights stored in replica cells of the plurality of replica MAC arrays and activates a refresh enable signal for refreshing memory cells of the CIM cell array.