CAM match line precharge

Details CAM match line precharge CAM match line precharge

2000-02-14

2001-05-29

Phan, Trong (Department: 2818)

Static information storage and retrieval

Associative memories

Ferroelectric cell

C365S203000

Reexamination Certificate

active

06240001

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to CAM match line precharge.
2. Related Art
A CAM (content addressable memory) is sometimes used in a computer system or device for storing and retrieving information. CAMs have the advantage the they can rapidly link associated data with known tags; it is thus possible to perform rapid lookup of the associated data once the tag is known. Known CAMs include comparison circuits for matching an input tag with each tag recorded in the CAM, so as to determine which if any of the row or lines in the CAM matches the tag.
In some known CAMs each row or line includes a set of memory cells, each of which includes a recorded match bit for comparison with the input tag. The comparison circuits operate by precharging a match line and coupling that match line to circuits for all the recorded match bits. If any recorded match bit is not the same as the input tag, the match line is discharged. Thus, most match lines are discharged, even though the CAM does not deliver the data associated with the unmatched tag.
One problem in the known art is that power consumption from precharging and discharging the match line is relatively high. This problem is exacerbated when the CAM is relatively large, that is, when the number of bits to be matched in the tag is relatively large. In addition to the amount of power consumed by precharge and discharge of the match line, the process of precharging and discharging the match line can be time consuming relative to the total time for the CAM to respond to the input tag. It would be advantageous to reduce both the relative amount of power consumption and the relative time required for operation of the CAM.
Accordingly, it would be desirable to provide a method and system for improved CAM match line precharge. This advantage is achieved in an embodiment of the invention in which the amount of power consumed by match line precharge and discharge is significantly reduced by use of a relatively lower discharge voltage. This has two further advantages: (1) discharging from a lower voltage is relatively faster, and (2) use of the lower voltage as an input to the sense amplifier speeds up comparisons.
SUMMARY OF THE INVENTION
The invention provides a method and system for improved CAM match line precharge, in which the amount of power consumed by match line precharge and discharge is significantly reduced. An independent match precharge voltage is introduced to the CAM, which is used to separately precharge the match line and is available for use by comparison circuits to attempt to match the input tag. In a preferred embodiment, the match precharge voltage is selected so as to reduce the relative power consumption for match line precharge and discharge by a factor of about five.
In a preferred embodiment, sense amplifiers in the comparison circuits are disposed so as to compare each discharged match line with the match precharge voltage itself, or to take advantage of the lower signal swing to speed up single-ended sensing schemes.


REFERENCES:
patent: 5598115 (1997-01-01), Holst
patent: 5617348 (1997-04-01), Maguire
patent: 5621677 (1997-04-01), Jones
patent: 6044005 (2000-03-01), Gibson et al.
patent: 6081441 (2000-06-01), Ikeda
patent: 6101115 (2000-08-01), Ross

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