A Novel Ternary Content Addressable Memory Cell

Ternary content addressable memories (TCAM) are used for parallel searching. The parallel searching, results high speed but consumes more power. For higher search speed applications, NOR type matchline TCAMs are useful. The NOR type matchline TCAM needs high power; therefore, the power reduction is the major objective of many reported designs. Here, a novel TCAM cell is proposed. The proposed Ternary CAM cell power consumption is 32% lesser than the NOR type matchline TCAM cell. Simulations are performed using cadence 45-nm technology.


Introduction
Now a day, internet is becoming the part of day to day life. Speed of the internet is mainly depends on its searching algorithms. But searching speed of search algorithms using software is low. Searching using hardware elements like memories plays a vital role in internet speed. Earlier Random Access memories (RAM) are used for this purpose.But RAM takes address as an input and gives content as an output.Usually internet users search for a content by giving content as an input. In this regard, RAM has its own limitations. Content addressable memories are the best choice for this kind. Content addressable memories, takes content as an input and gives address as an output. CAM helps in finding the location of the content and RAM is used to retrieve the data from that location. Data search using CAM can be done as shown in Fig.1. Here the objective is to retrieve the data "01101" from the memory location.RAM gives the data "01101" if address is specified. This address is obtained from CAM.CAM takes "01101" as an input and gives the address location "01" as an output. Now RAM takes the address location "01" as an input and gives the desired content "01101" as an output. So for high speed applications, Content addressable memories are very useful.
Basically Content addressable memories are of two types, namely binary content addressable memory (BiCAM) and ternary content addressable memory (TCAM). TCAM has a special feature that it can support wildcard entry where as BiCAM does not support.So TCAM is preferable than BiCAM.The most important application of TCAM is packet forwarding.TCAMs are also used in hardwares for routing applications such as network router, cache memories [3]- [6]. TCAM designer's objective is to increase the search speed and to reduce the power consumption. Conventional TCAM is shown in Fig.2 [2] . In Fig. 1,"C" represents TCAM cell and ML 0 , ML 1 , ML 2 , ML 3 matchlines corresponding to matchlines 0,1,2,3 respectively. SL 1 , SL 2 ,SL 3 and their complements represents search lines. TCAM is a memory element in which information is stored in rows and search happens simultaneously. Content addressable memory, takes content as an input and gives address as an output. If stored data matches with the search data then it gives address in which the data stored. A match or mismatch information is accessed by the sense amplifier. As shown in Fig.2, every TCAM cell in a dataline is connected to a common matchline (ML). Initially all matchlines are charged to high voltage. ML value maintains at high voltage if there is a match. Otherwise the respective matchline discharges. To start a new search, all matchlines should be at high voltage means ML needs to be charged again. Thus, the frequent charging and discharging happens in ternary content addressable memories. Frequent charging and discharging leads to more power dissipation.In the proposed cell, there is no need of frequent charging and discharging of matchline [1]. So there will be low power dissipation. The remainder of this short-lived is organized as follows: Section 2 describes the Ternary Content addressable memory cell operation. Proposed TCAM cells are described in Section 3. In Section 4, the performance comparison results obtained from simulations of TCAM cells have been presented and Section 5 concludes this short-lived.

TCAM Cell
Content addressable memories are of two types. Those are binary CAM and Ternary CAM. 0 and 1 can be stored in binary CAM cell where as in TCAM cell 0,1, and don't care(X) can also be stored. TCAM gives wildcard entry because of its storage capability of don't care (X). TCAM is the most widely used CAM compared to binary CAM. TCAM cells are of NAND & NOR type. In NAND type TCAM cells are connected in series where as in NOR type cells are connected in parallel. NAND type TCAM cell offers low power dissipation but low speed where as NOR type TCAM cells offer high speed but at a cost high power dissipation. So in general NOR type cells are preferred compared to NAND type in high speed applications. NOR type TCAM cell and its encoding is shown in Fig. 3& Table  1 respectively. To store three states two bits are essential so TCAM cell requires two SRAM cells. In TCAM cell, a logic "0" is stored by setting A = 0 and B = 1, Logic "1" by setting A = 1 & B = 0 and don't care by setting A = 1 & B = 1. Here A= 0 and B = 0 is unused state. In TCAM cell, search a logic "0" happens by setting SL = 0 andSL ̅̅̅ = 1, Logic "1" by setting SL = 1 andSL ̅̅̅ = 0 and don't care by setting SL = 1 and SL ̅̅̅ = 1 .  NOR type TCAM matchline structure is shown in Fig.4. Here TCAM cell is connected to supply voltage through pMOStransistor. Traditionally, CAM operation is divided into three parts. Namely data store, matchlineprecharge& data search. During precharge phase pMOS transistor is in ON state so ML is connected to supply voltage. In evalution phase, pMOS transistor needs to be OFFsothat"pre" value needs to be high.Initially matchline is charged to high voltage by setting "pre" value low. During evalution if there is a mismatch, matchline discharges. So for every new search, matchline is needed to charge to supply voltage.This type of charging and discharging causes highpower dissipation.

Proposed TCAM Cell
Proposed TCAM cell power consumption is lesser than the existing design. Proposed TCAM cell is shown in Fig. 5

Results
Proposed Ternary CAM and conventional Ternary CAM cells were implemented using cadence 45nm technology at a supply voltage of 1V. Simulations are carried out for 300ns by storing logic "0" in the cells. Evalution is carried out by providing logic "0", logic "1" and don't care(X).Waveforms of conventional TCAM and proposed TCAM cells are shown in figures 6 and 7 respectively. Precharge and evalution phases are indicated for conventional TCAM as shown in figure 6. Evalution phase is indicated for proposed TCAM as shown in figure 7. The area and speed of the proposed cell is same as conventional. From the waveforms it has been observed that, average power consumption of conventional TCAM cell is 8.66μW and for proposed TCAM cell is 5.84μW.

Conclusion
Conventional NOR type TCAM cell is simulated and observed that it consumes more power. The proposed TCAM cell is also simulated using cadence and observed that it consumes 32% less power than the conventional. Speed of the proposed cell is same as conventional cell. Proposed TCAM cell uses stored values for matchlinewhere asmatchline of conventional TCAM cell is connected to supply voltage through a transistor.