Static information storage and retrieval – Read/write circuit – Precharge
Reexamination Certificate
2000-02-01
2001-07-17
Le, Vu A. (Department: 2824)
Static information storage and retrieval
Read/write circuit
Precharge
C365S049130
Reexamination Certificate
active
06262929
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a pre-charging circuit and method for a word match line (also called a match line) of a content addressable memory (CAM) of a semiconductor circuit, and furthermore a CAM word circuit including the pre-charging circuit.
BACKGROUND OF THE INVENTION
The CAM is a memory device that can search for stored data that match the reference data and read out information related to the matching data, such as an address indicating where the matching data is stored. With advancement of semiconductor technologies, there is a demand for CAMs with higher speed and less power dissipation.
A bit, indicating whether or not a word (which is a storing unit consisting of a plurality of bits specified by an address) is valid for, is often attached to each word of the CAM. This bit is called a “valid bit” and normally stored in a memory cell called a “valid cell,” provided in the CAM.
Normally, the valid bit attached to a word is controlled so that it may be set to a “true” value when valid data is written to the word. In other words, the “true” value (e.g., a bit “1”) of the valid bit is stored in the valid cell. In addition, resetting to a “false” value is performed either by resetting the valid bits of all the words at once or by resetting the valid bit one by one for each word. In other words, the valid bit in the valid cell is set to a “false” value (e.g., a bit “0”).
FIG. 1
is a diagram showing a conventional word configuration of a CAM having a valid cell. In the CAM of
FIG. 1
, when a searching operation is performed, both the signal from a word match line
1
indicating the match or mismatch of a word and the output of the valid bit held in the valid cell
2
are input to a NAND gate
3
and the logical AND operation between them is performed.
FIG. 2
is a diagram showing another conventional word configuration of a CAM having a valid cell. The CAM of
FIG. 2
has the configuration where a valid cell
2
is connected to a word match line in parallel with other CAM cells within a word and connect to inverter pair
4
. In this case, the “true” value of the valid bit is equated to a match in other bits within the word, while the “false” value of the valid bit is equated to a mismatch in other bits within the word.
The conventional methods of
FIGS. 1 and 2
can attain the functional object of the operation of searching for a word matching input data from among valid words. These methods, however, have the disadvantage that power is wastefully dissipated, because the searching operation is done also for invalid words and the match lines are charged and discharged there. There are match lines by the number of addresses. The parasitic capacitance of the match line is large when the width of the word is big, for instance, when it is as big as 64 bits. Therefore, if there are a greater number of invalid words, power dissipation will become greater and wasteful heat will be generated.
Thus, there is a need ways to attain a reduction in the average power dissipation for the CAM searching operation without having impact on CAM performance. There is also a need for a circuit and a method which control pre-charging of a word match line of a CAM by employing a valid bit.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a pre-charging circuit for a word match line of a content addressable memory (CAM) having valid cells, comprising means for controlling pre-charging of the word match line in accordance with the valid bit stored in the valid cell.
According to the present invention, there is provided a CAM word circuit with a valid cell, comprising: a word match line; a plurality of CAM cells each connected in parallel to the word match line; and a pre-charging circuit connected to the word match line, the pre-charging circuit controlling pre-charging of the word match line in accordance with a valid bit stored in the corresponding valid cell.
According to the present invention, there is provided a method of pre-charging a word match line of a CAM having valid cells, comprising the steps of: storing a valid bit in the valid cell; and pre-charging the word match line in accordance with the valid bit stored in the valid cell.
These and other aspects of the invention are described in further detail below.
REFERENCES:
patent: 5617348 (1997-04-01), Maguire
Miyatake Hisatada
Mori Yohtaro
Tanaka Masahiro
Capella Steven
International Business Machines - Corporation
Le Vu A.
Nguyen Vanthu
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