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Controlling Search Currents in a Content Addressable Memory Array

IP.com Disclosure Number: IPCOM000081969D
Original Publication Date: 1974-Sep-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 47K

Publishing Venue

IBM

Related People

Kolchak, GM: AUTHOR

Abstract

The basic circuitry required to implement this technique is shown in Fig. 1. The content addressable memory (CAM) cell is enclosed by the dashed line. Q1, Q2, R1, R2, D1 and D2 form a cross-coupled memory element. D3 and D4 are "gating diodes" used during Read and Write. Q3 and Q4 are used to perform the search operation. A voltage clamp is required on each bit line. These are represented by Q5, D5, R5 and Q6, D6, R6. J1 and J0 are switchable current sources.

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Controlling Search Currents in a Content Addressable Memory Array

The basic circuitry required to implement this technique is shown in Fig. 1. The content addressable memory (CAM) cell is enclosed by the dashed line. Q1, Q2, R1, R2, D1 and D2 form a cross-coupled memory element. D3 and D4 are "gating diodes" used during Read and Write. Q3 and Q4 are used to perform the search operation. A voltage clamp is required on each bit line. These are represented by Q5, D5, R5 and Q6, D6, R6. J1 and J0 are switchable current sources.

A timing diagram is shown in Fig. 2. The required bit line potentials are obtained by switching current sources between the values II and I2, as shown in Fig. 3. I1 is selected to give a V standby such that neither the gating diodes, D3 and D4, nor the search transistors, Q3 and Q4, are conducting.

During search, the data stored in the cell is compared with the data encoded on the bit lines. If the search data agrees with the cell data, a match occurs and the match line remains at its standby value. If the search data is the complement of the stored data, a mismatch is said to occur and the match line is discharged by either Q3 or Q4. For a specified voltage transition on the match line, the search access time is a direct function of the current through the search transistors, Q3 or Q4. The I2 value of the bit line current sources is determined by:. I2 = Ic1 + n x Imm. Ic1 = Current through the voltage clamp to bias the bit line at V search. Imm = Current necessary to discharge the match line. n = Number of cells per bit line. If less than n cells mismatch the voltage clamp will provide the additional curr...