System and method for low power searching in content addressable memories using sampling search words to save power in compare lines

Abstract

An invention is provided for lowLow power searching in a CAM usinguses sample words to save power in the compare lines. The inventionA method includes comparing a sample section of stored data to a corresponding sample section of search data on a plurality of rows in the CAM. If a sample section of the stored data on any row of the plurality of rows is equivalent to the corresponding sample section of the search data, a remaining section of search data is allowed to propagate to the local compare lines coupled to the remaining section of the stored data of each row. However, if the sample section of the stored data is different from the corresponding sample section of the search data, the local compare lines coupled to the remaining section of the stored data on each row are latched.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application is a continuation-in-part of U.S. patent application Ser. No. 09 / 943,653, filed Aug. 30, 2001, now U.S. Pat. No. 6,577,519 and entitled “System and Method for Low Power Search in Content Addressable Memories Using Sample Search Words,” which is incorporated herein by reference; this application is related to: 1) U.S. patent application Ser. No. 09 / 944,251, filed Aug. 30, 2001, and entitled “System and Method for Low Power Search in Content Addressable Memories Using Non-Precharged Compare lines,” and 2) U.S. patent application Ser. No. 09 / 944,256, filed Aug. 30, 2001, and entitled “System and Method for Low Power Search in Content Addressable Memories Using Non-Precharged Match lines,” each of which is incorporated herein be reference.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]This invention relates generally to memory circuits, and more specifically to low power search techniques in content addressab...

AI Technical Summary

Benefits of technology

[0019]Broadly speaking, embodiments of the present invention address these needs by utilize sample words to reduce power usage in compare lines. In one embodiment, a method for low power searching in a CAM is disclosed. The method includes comparing a sample section of stored data to a corresponding sample section of search data on a plurality of rows in the CAM. If a sample section of the stored data on any row of the plurality of rows is equivalent to the corresponding sample section of the search data, a remaining section of search data is allowed to propagate to the local compare lines coupled to the remaining section of the stored data of each row. However, if the sample section of the stored data on every row of the plurality of rows is different from the corresponding sample section of the search data, the local compare lines coupled to the remaining section of the stored data on each row are latched.

[0020]In an additional embodiment, a match line is disclosed for a CAM. In this embodiment, the match line is one of a plurality of match lines forming a group of match lines. Each match line includes a sample match line coupled to a first set of CAM cells, and a sub-match line coupled to a second set of CAM cells. Each CAM cell in the second set of CAM cells is coupled to local compare lines that are in electrical communication with global compare lines via a plurality of local compare line latches. Coupled to the local compare line latches is a compare line propagation control circuit. In operation, the compare line propagation control circuit latches the local compare lines if a sample section of search data corresponding to the first set

Problems solved by technology

The long length of the global match lines results in each global match line having a large capacitance.

As a result, every transition on a global match line requires a large amount of power to overcome the large capacitance.

Since the output signals from the bit pattern entries propagate to the global match lines, every transition in the output signal results in a large power drain on the CAM.

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