Browse Prior Art Database

Dynamic Content Addressable Memory With Refresh Feature

IP.com Disclosure Number: IPCOM000041966D
Original Publication Date: 1984-Mar-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

Schuster, SE: AUTHOR

Abstract

A dense content addressable memory (CAM) is described herein which is dynamic and has only five devices. A method of refreshing the information stored in the cell is also described. In a CAM incoming information is compared to the information stored in the memory to determine if a match exists. CAMs have the potential for wide application. One of the limitations of present CAMs is the complexity and hence the area needed to implement the cell. A dynamic CAM cell having only five devices is shown in Fig. 1. The information to be stored in the cell and its complement are placed on the Yi and Yii lines, and the word line (Xi) of the row of selected cells is brought high. All the cells along the selected word line are loaded simultaneously.

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Dynamic Content Addressable Memory With Refresh Feature

A dense content addressable memory (CAM) is described herein which is dynamic and has only five devices. A method of refreshing the information stored in the cell is also described. In a CAM incoming information is compared to the information stored in the memory to determine if a match exists. CAMs have the potential for wide application. One of the limitations of present CAMs is the complexity and hence the area needed to implement the cell. A dynamic CAM cell having only five devices is shown in Fig. 1. The information to be stored in the cell and its complement are placed on the Yi and Yii lines, and the word line
(Xi) of the row of selected cells is brought high. All the cells along the selected word line are loaded simultaneously. As the selected word line rises, devices 10 and 12 will turn on transferring the information from Yi and Yii to storage nodes a and b. As the selected word line goes low, devices 10 and 12 turn off, and the information is dynamically stored on nodes a and b. After the information is written into the cell and the word line goes low, lines Yi and Yii are both brought low. Since either storage node a or storage node b is high, either device 13 or device 14 will be on and node c, the gate of device 15, will discharge. At this time, since device 15 for every cell will be off, the select line will be charged high either through a load device or a precharge device. Once the CAM is loaded, information can be compared to the memory contents to determine if the same information has been stored in the CAM. Assume storage node a is high and storage node b is low. If Yi is low and Yii is high, node c the gate of device 15 will remain low and the select line (Xii) will remain high, indicating a match. However, if Yi is high and Yii is low, node c will go high, turning on device 15, thereby discharging the select line (Xii) indicating a match has not occurred. Therefore, if one or more bits on the column lines (Yi and Yii) differ from the bits stored in a given row, the select line will go low. Only if all the bits are the same will the select line remain high - the match condition. It is possible to do compares on only a part of the input word by clamping the column lines Yi and Yii low on that portion of the word where one does not wish to make a comparison. It should be noted that unused rows can be deselected by storing a ...