Content addressable memory cell and design methodology

Details Content addressable memory cell and design methodology Content addressable memory cell and design methodology

2000-09-09

2002-06-04

Nelms, David (Department: 2818)

Static information storage and retrieval

Associative memories

Ferroelectric cell

C365S189070, C365S094000

Reexamination Certificate

active

06400592

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Authorization Pursuant to 37 C.F.R. 1.71(e)
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
2. Field of the Invention
The field of the present invention relates generally to content addressable memories (CAMs), and more particularly to CAM cells for constructing high-speed, large-capacity CAM arrays with fill bit-wise and word-wise masking capability.
3. Related Art
A content addressable memory (CAM) is an associative memory device in which memory elements are identified (i.e. addressed) by their content, rather than by their physical location. A CAM generally includes a matrix of CAM cells arranged in rows and columns. Each CAM cell in this matrix usually includes a static memory element and comparison circuitry.
Typical uses for CAMs include pattern-matching memories for optical character recognition, and translation look-aside buffers for digital computers that employ virtual memory. In a traditional CAM, each CAM cell stores one bit of digital information in the static memory element. In some CAMs, each CAM cell employs “dual ended” logic, such that the cell makes available both a stored bit and its inverse. The static memory elements are connected with differential bit lines and a word line, thereby allowing read and write data operations. Typically, the comparison circuitry in each CAM cell is connected with separate differential compare bit lines, thus creating more flexibility in the timing of read, write and search operations.
An array of CAM cells, commonly arranged in a row, comprise a word of data. During a search operation, the CAM compares a search word (comparand) with a set of words stored within the CAM. One or more indicators associated with each word of data, commonly called match lines, produce a comparison result, which identifies the word(s) in the CAM that match the comparand. Thus a CAM usually requires more transistors and/or other circuit elements than standard Random Access Memory (RAM), but adds a “parallel processing” characteristic not found in a standard RAM.
One common use of a CAM is as a search tool for network address filtering. Network address filtering is a process for searching a router's station list for source and destination addresses and routing packets to the correct port(s). Modern telecommunication networks typically use digital packet or block type data formats as a means for dynamically routing data through a network. The advantages of such dynamically routed packetized data are well known. However, as data transfer rates continue to increase, the finite processing power available at network routers can cause communication bottlenecks, where data packets arrive faster than the router can determine where to route the incoming packets. Such bottlenecks can easily degrade the overall performance of a network.
A CAM can reduce processing bottlenecks by increasing the speed at which routing can be accomplished. Typical packet-based communication networks utilize digital “header” information at the beginning of each packet in the routing process. This header information comprises address fields defining source and destination information. Long addresses are necessary to accommodate the large number of potential addresses common in modern communication networks. These long addresses can substantially increase the time required to make routing decisions, thus requiring a fast searching technique during network address filtering. A CAM generally achieves this search performance by simultaneously comparing all entries stored in the CAM with a “comparand.”
FIG. 1
is a schematic block diagram illustrating a simple example of a conventional content addressable memory (CAM) utilizing dual-ended logic, with enlargements illustrating further details of a CAM cell and a CAM memory core cell. In
FIG. 1
, a CAM
100
is shown with only four rows and four columns, for a total of 16 CAM cells
110
, whereas in most practical applications the CAM would have many more CAM cells. Each row corresponds to a word, or part of a word, of data. Each column corresponds to a bit of data from each row or word. Each CAM cell
110
in a column is coupled with a local pair of bit lines
112
(b and bn) and a local pair of compare lines
114
(k and kn). Because the CAM
100
in this example uses dual ended logic, the local pair of bit lines
112
carry differential data representing a single bit of data, and the local pair of compare lines
114
carry differential data representing a single bit of comparison data.
Each CAM cell
110
in a row is connected to a word line
116
and a match line
118
. The word line
116
enables read and write access to the CAM cells
110
. The match line
118
provides an output for a comparison result. The multiple match lines
118
feed into an encoder
120
. Typically, the encoder
120
generates three output signals. A line designated HT is asserted high when one of the words stored in the CAM matches the comparand. A line designated AD, outputs a set of signals representing the address of the matched word. A line designated MLT is asserted high when multiple stored words match the comparand.
In some conventional designs, a priority encoder is used in place of the encoder
120
. The priority encoder functions in the same way except that in addition to asserting the MLT line high when multiple stored words match the comparand, it also outputs a set of signals on the AD line representing the address of the matched word in the lowest position in the CAM. Moreover, in some conventional designs a dynamic random access memory (DRAM) or a static random access memory (SRAM) are employed in place of the encoder
120
, thereby resulting in a CAM-DRAM or CAM-SRAM configuration, as is well known in the art.
CAM cell
110
a
in
FIG. 1
is an expanded view of a single CAM cell
110
, as may be found in a typical CAM. CAM cell
110
a
includes a CAM memory core cell
130
, a “comparator block” comprising two N-channel field effect transistors (FET)
132
and
134
, and a match line N-channel FET
136
. The CAM memory core cell
130
has two outputs designated C and CN, which are differential outputs representing a single bit of data stored in CAM memory core cell
130
. The first N-channel FET
132
connects to the positive compare line K and is gated by the output C. The second N-channel FET
134
connects to the negative compare line KN and is gated by the output CN. The output of the comparator block gates the match line N-channel FET
136
, thereby transmitting a comparison result for CAM cell
110
a
to the match line
118
during a compare operation.
Alternative conventional designs involve transmission of a comparison result to a match line in a wired-NOR configuration. In these designs, the first N-channel FET
112
would be gated by CN, and the second N-channel FET
134
would be gated by C. The match line N-channel FET
136
remains gated by the output of the comparator block, but instead of connecting a match line input with a match line output, it connects the match line
118
with a ground. In this fashion, the match line
118
is pulled low whenever a mismatch between the stored word and the comparand occurs. This configuration results in a fast comparison, but every time there is a mismatch, the match lines must be discharged and then charged up again. For large CAMs, this configuration results in excessive power consumption.
FIG. 1
further shows an expanded view of an example of a CAM memory core cell
130
a.
In a typical configuration,the CAM memory core cell
130
a
includes a static memory element
140
and two access N-channel FETs
142
and
144
. The static memory element
140
typically consists of two cross-coupled inverters.
While the CAM design represented by the C

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