CAM Match Line Power Reduction via Pre-Compare Population Count Filtering
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Solution Overview
Problem
Content addressable memory (CAM) devices face significant power consumption issues due to the alternating charging and discharging of match lines during compare operations, which limits memory size and scalability, especially in ternary CAM devices where additional circuits increase power consumption and latency.
Innovation Solution
A pre-compare circuit that selectively enables match lines in the CAM array based on population counts of the search key and ternary words, determining whether a match is possible to avoid unnecessary pre-charging, thereby reducing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If match lines are pre-charged toward supply voltage prior to compare operation, then match detection accuracy is improved, but power consumption increases significantly
Solution Approach 1:
The patent applies preliminary action by performing a pre-compare operation using population counts before the actual compare operation. The pre-compare circuit calculates population counts of the search key and compares them with stored population counts to determine whether a match is possible, thereby avoiding unnecessary pre-charging of match lines and reducing power consumption while maintaining match detection accuracy when needed
Solution Approach 2:
The patent changes the parameter of match line charge state from always charged to selectively charged based on population count comparison results. By modifying the charging state parameter conditionally, the system achieves both power savings and accurate match detection when required
2Quantity of substance
If CAM array size and density are increased, then memory capacity is improved, but power consumption from match line charging increases
Solution Approach 1:
The pre-compare operation using population counts is performed in advance to filter out rows that cannot possibly match, reducing the number of match lines that need to be pre-charged for the actual compare operation. This allows larger CAM arrays to operate with lower power consumption by minimizing unnecessary charging operations across all rows
3Use of energy by moving object
If pre-compare operations are implemented, then power consumption is reduced, but device complexity increases
Solution Approach 1:
The patent introduces an intermediary pre-compare circuit that uses population count calculations as a filtering mechanism. This intermediary structure sits between the search key input and the main CAM array, performing preliminary filtering to determine which rows should undergo full comparison, thereby reducing overall power consumption with added computational steps
Solution Approach 2:
The patent replaces the traditional mechanical approach of charging all match lines with a computational approach using population count comparisons. By substituting the purely electrical charging mechanism with a logical comparison mechanism first, the system reduces unnecessary charging operations while managing complexity through algorithmic logic
Data Source
AI summary
A CAM device for comparing a search key with a plurality of ternary words stored in a CAM array includes one or more population counters, a pre-compare memory, and a pre-compare circuit. The present embodiments reduce the power consumption of CAM devices during compare operations between a search key and ternary words stored in a CAM array by selectively enabling the match lines in the CAM array in response to pre-compare operations between a set of population counts corresponding to the masked search key and a set of population counts corresponding to the ternary words stored in the CAM array.