Abstract:
A CAM cell comprises a pair of SRAM cells, each of which comprise a pair of cross coupled inverters for storing a data value and a pair of access devices for accessing a complementary pair of bit lines. The CAM cell ether comprises a pair of compare circuits, each for comparing said data value stored in one of said SRAM cells with a search data value provided on a corresponding search line. The CAM cell has an equivalent number of n-channel and p-channel devices. The CAM cell uses p-channel transistors as access transistors to the SRAM cells in order to improve the efficiency of the layout of the cell array. The implementation ensures a balanced number of p-channel and n-channel devices per cell while still providing excellent functional characteristics.

Description:
This application is a Continuation Application from U.S. application Ser. No. 09/894,900, filed Jun. 29, 2001, now U.S. Pat. No. 6,522,562 which claims priority from Canadian Application Serial No. 2,342,575, filed Apr. 3, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     Conventional content addressable memory (CAM) has been implemented primarily using static random access memory (SRAM) cells. SRAM-based CAMs have received widespread use due to the high access speed of SRAM memory cells and the static nature of the cells. Furthermore, SRAM cells can be manufactured using a pure-logic type fabrication process, which is commonly used for non-memory circuit blocks. 
     In addition to random access memory (RAM) functions of writing and storing data, the CAM also searches and compares the stored data to determine if the data matches search data applied to the memory. When the newly applied search data matches the data already stored in the memory, a match result is indicated, whereas if the search and stored data do not match, a mismatch result is indicated. CAMs are particularly useful for fully associative memories such as look-up tables and memory-management units. 
     Many current applications utilise ternary CAMs, which are capable of storing three logic states. For example, the three logic states are logic ‘0’, logic ‘1’ and “don&#39;t care”. Therefore, such CAM cells require two memory cells to store the logic states, as well as a comparison circuit for comparing stored data wit search data provided to the CAM. 
     In ternary form, each conventional SRAM-based CAM memory cell comprises a regular six-transistor (6T) SRAM cells. Therefore, SRAM-based CAM cells typically use 12 transistors to implement two 6T SRAM cells. That is, each SRAM cell requires 2 p-channel transistors and 2 n-channel transistors in a cross-coupled inverter relationship and a further 2 n-channel transistors as access devices from the bit lines. 
     Furthermore, four additional transistors are required for each ternary CAM memory cell for implementing an exclusive NOR function for comparing the search data with the stored data. For ternary CAM cells, a-channel devices are typically used in the comparison circuit. 
     Previous approaches in the art store data in a main memory cell and mask data in a mask memory cell. The comparison circuit is then either enabled or disabled by the mask memory cell contents. Examples of memory cells implementing such an approach are illustrated by U.S. Pat. No. 6,154,384, issued to Nataraj et al. and U.S. Pat. No. 6,108,227 issued to Voelkel. Although this approach is functional from a circuit point of view, difficulty arises when attempting to layout the elements of the CAM cells. The main problem is a non-optimised layout of the CAM cell, which takes up more silicon area than desired. 
     DRAM-based CAMs have also been proposed in the art. DRAM cells are typically physically smaller than SRAM cells. Therefore, DRAM-based CAMs have the advantage of being able to store much more data than SRAM-based CAMs for a given area due to the much smaller CAM cell size. However, because of the dynamic nature of the DRAM cell, which is used to implement a DRAM-based CAM cell, such cells require regular refresh operations in order to maintain the data. 
     U.S. Pat. No. 6,188,594 issued to Ong describes a CAM cell using only n-channel transistors. The CAM cell uses only n-channel transistors. The size of the cell is significantly reduced since the p-channel transistors are eliminated. The cell size is further reduced by using dynamic storage rather ta static storage in the CAM cell. The dynamic CAM cell as described has as few as six transistors, and a compact layout is facilitated. However, as previously mentioned, dynamic cells require regular refresh operations in order to maintain the data and such refresh circuitry takes up additional silicon area. 
     Therefore, there is a need for an SRAM-based CAM cell that achieves a more efficient spatial layout than the prior art, while maintaining the static characteristic of the SRAM-based CAM cell. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram of a ternary CAM half-cell according to an embodiment of the invention; 
         FIG. 2  is a circuit diagram of a full ternary SRAM-based CAM cell according to a first embodiment of the invention; 
         FIG. 3  is a circuit diagram of a full ternary SRAM-based CAM cell according to a second embodiment of the invention; 
         FIG. 4  is a plan view of a half-cell layout corresponding to circuit in  FIG. 1 ; and 
         FIG. 5  is a circuit diagram of a full ternary SRAM-based CAM cell according to the prior art. 
     
    
    
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the invention, there is provided a ternary data content addressable memory (CAM) half-cell. The CAM half-cell comprises a static random access memory (SRAM) cell, which comprises a pair of cross-coupled inverters, for storing a data value and a pair of access devices for accessing a pair of complementary bit lines. The CAM half-cell further comprises a compare circuit for comparing the data value stored in the SRAM cell with a search data value provided on a search line. The CAM half-cell comprises an equivalent number of n-channel and p-channel devices. 
     In accordance with a further aspect of the invention, there is provided a CAM cell comprising a pair of SRAM cells, each of which comprise a pair of cross coupled inverters for storing a data value and a pair of access devices for accessing a complementary pair of bit lines. The CAM cell further comprises a pair of compare circuits, each for comparing said data value stored in one of said SRAM cells with a search data value provided on a corresponding search line. The CAM cell has an equivalent number of n-channel and p-channel devices. 
     In accordance with yet a further aspect of the invention, the CAM cell has only one p+ region to n+ region separation. 
     The ternary SRAM-based CAM cell uses p-channel transistors as access transistors to the SRAM cells in order to improve the efficiency of the layout of the cell array. The implementation ensures a balanced number of p-channel and n-channel devices per cell while still providing excellent functional characteristics. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 5 , an SRAM-based CAM cell that is standard in the art is illustrated generally by numeral  500 . The CAM cell comprises two 6T SRAM cells  502 . Each SRAM cell  502  comprises two p-channel transistors and two n-channel transistors in a cross-coupled inverter relationship  506 , and a firther two n-channel transistors  508  as access devices from a pair of bit lines  510 . The CAM cell further comprises a comparison circuit  512  with four additional n-channel transistors  508  for implementing an exclusive-NOR function for comparing search data with stored data. 
     The main problem with the implementation illustrated in  FIG. 5  is an imbalance between transistor types, which leads to a non-optimised layout of the CAM cell, Specifically, out of the total of 16 transistors, only four are p-channel devices. Moreover, all n-channel devices in a cell need to be positioned in a common p diffusion region. This region includes the n-channel access devices  508 , the n-channels of the cross-coupled inverters  506  and the n-channels of the comparison circuit  512 . The inevitable result is an unbalanced layout with regions containing the n-channels highly congested and wasted space around the two remaining p-channels used for the pull-up devices in the cross-coupled inverter transistors  504 . 
     It is a well-known design layout rule in the industry that n+ to p+ spacing is usually large relative to other design rules in a typical CMOS fabrication process. Also, the n+ to p+ spacing cannot contain transistors therein. Therefore, the aspect ratio of the cell should be made narrow. That is, the smaller dimension of a typical cell is in the direction of the line of the p-well separating n-channels and p-channels in the cell array. This minimises the area wasted in the p+ to n+ spacing. However, this is difficult to achieve given the imbalance between n-type and p-type devices in the conventional approach. 
     A reduction in ternary CAM cell area and optimization of a CAM cell layout is achieved by replacing n-channel access devices used for the SRAM cells with p-channel access devices and providing an active logic ‘0’ activated word line instead of an active logic ‘1’ activated word line. An SRAM cell with p-channel access devices is not normally used in conventional commodity or embedded SRAM applications due to the speed advantage of switching n-channel devices over p-channel devices. In a regular SRAM memory, the switching speed and other characteristics would suffer as a result. However, in a CAM cell, performance of the read/write is less critical than in a conventional SRAM cell since the primary task a CAM memory performs on a regular basis is a search and compare function. 
     Using p-channel access devices instead of n-channel access devices results in a full ternary CAM cell with a more balanced number of p-channel transistors and n-channel transistors. It is further preferable that the devices are balanced such that 8 n-channel devices and 8 p-channel devices are used in the layout. 
     Referring to  FIG. 1 , a CAM half-cell in accordance with an embodiment of the invention is illustrated generally by numeral  100 . The half-cell  100  comprises a complimentary bit line pair BL and {overscore (BL)}, a word line WL, a search line SL, a match line ML, cross-coupled inverter transistors P 1 , N 1 , P 2 , and N 2  and p-channel access devices P 3  and P 4 . 
     P 2  is coupled between a positive supply voltage  102  and a first node  104 . N 2  is coupled between the first node  104  and a ground supply voltage  106 . Both P 2  and N 2  are gated by a second node  108 . P 1  is coupled between a positive supply voltage  102  and the second node  108 . N 1  is coupled between the second node  108  and a ground supply voltage  106 . Both P 1  and N 1  are gated by the first node  104 . 
     The first node  104  is coupled to bit line BL via access transistor P 3 . P 3  is gated by the word line WL. The second node  108  is couple to bit line {overscore (BL)} via access transistor P 4 . P 4  is also gated by the word line WL. The p-channel access devices P 3  and P 4  selectively connect the cross-coupled inverters to complementary bit lines BL and {overscore (BL)} which carry read/write data. 
     The match line ML is coupled to ground via serially coupled transistors N 3  and N 4 . N 4  is gated by the search line SL and N 3  is gated by the second node  108 . As can be seen from  FIG. 1 , there are four p-channel transistors and four n-channel transistors comprising the half-cell as opposed to two p-channel transistors and six n-channel transistors as discussed regarding the prior art approach. 
     Referring to  FIG. 2  a full ternary CAM cell in accordance with an embodiment of the present invention is illustrated generally by numeral  200 . The full ternary CAM cell comprises 8 p-channel transistors and 8 n-channel transistors. The transistors of the first SRAM cell component of the full ternary CAM cell are numbered similarly to the corresponding transistors in  FIG. 1  for convenience. For the second SRAM cell component of the CAM cell, the cross-coupled inverter transistors are labelled P 12 , N 12 , P 11  and N 11 , the access transistors are labelled P 13  and P 14 , and the transistors serially coupled between the match line ML and ground are labelled N 14  and N 13  respectively. It will be noted that for a full ternary CAM cell there are two complementary bit line pairs, BL 1 , {overscore (BL 1 )} and BL 2 , {overscore (BL 2 )} and two search lines SL 1  and SL 2 . 
     The general operation of the full ternary CAM cell  200  illustrated in  FIG. 2  is now described. To perform a write operation, data to be stored in the CAM cell is loaded onto bit line pairs BL 1 , {overscore (BL 1 )}, and BL 2 , {overscore (BL 2 )}. The word line WL is asserted active logic ‘0’ tuning on p-channel access transistors P 3 , P 4 , P 13  and P 14 . The data carried on the complementary bit line pairs is thereby written into the two SRAM cells and the word line is de-asserted. 
     For a read operation, the complementary bit line pairs are precharged to VDD/2. The word line is asserted active logic ‘0’ and the data from the SRAM cells is read onto the bit line pairs. The data then is transferred to data buses (not shown). 
     For a search and compare operation, the match line is precharged to logic ‘1’ and data is placed on the search lines SL 1  and SL 2 . Typically, search data and stored data are provided in such a manner that in the case of a mismatch a change occurs in the match line state. It is preferable to change the match line state for a mismatch rather than a match because a mismatch is a more infrequent occurrence. Therefore, a change in match line state will occur infrequently, reducing power dissipated by discharging match lines. The match line ML is precharged to a logic ‘1’ and a mismatch discharges the match line to ground, whereas in the case of a match no change occurs in the state of the match line. Alternatively, in another match line sensing approach, the match line is precharged to logic ‘0’ and detection of a match is made by pulling up with a device that is weaker than the two series devices holding the match line at logic ‘0’. 
     If the CAM cell  200  stores a logic ‘1’ in the left SRAM cell and a logic ‘0’ in the right SRAM cell, SL 1  has logic ‘1’, and SL 2  has logic ‘0’, a mismatch will result as follows. The output of the left SRAM cell provides a logic ‘1’ to transistor N 3 , turning it on. The search line SL 1  provides a logic ‘1’ to transistor N 4 , turning it on. Since N 3  and N 4  are both turned on, they provide a path to discharge the match line ML to ground and thus indicate a mismatch. 
     If the CAM cell stores a logic ‘0’ in the left SRAM cell and a logic ‘1’ in the right SRAM cell, a match condition will result as follows. The output of the left SRAM cell provides a logic ‘0’ to the gate of transistor N 3 , leaving it turned off. The search line SL 1  provides a logic ‘1’ to the gate of transistor N 4 , turning it on. However, since N 3  and N 4  are serially connected, a path to ground does not exist for discharging the match line ML to ground. Similarly, the right SRAM cell provides a logic ‘1’ to transistor N 13 , turning it on. The search line SL 2  provides a logic ‘0’ to transistor N 14 , leaving it turned off. Therefore, similarly to the left SRAM cell, transistors N 13  and N 14  do not provide a path to discharge the match line ML to ground. As a result, the match line remains precharged to logic ‘1’ indicating a match condition. 
     If the CAM cell stores a logic ‘0’ in both the right and left SRAM cells a “don&#39;t care” state exists. The output from each SRAM cell produces a logic ‘0’. The logic ‘0’ is provided to the gate of transistors N 3  and N 13 , ensuring that a match condition is detected regardless of the data provided on the search lines SL 1 , SL 2 , and the match line remains unchanged. 
     This description of the basic operation only covers one possible match line detection scheme. However other approaches, including those common in the art as well as proprietary approaches, may be implemented without departing from the scope of the invention. 
     Referring to  FIG. 3 , an alternate embodiment of the invention is illustrated generally by numeral  300 . In the present embodiment, access devices of the SRAM cells N 23 , N 24 , N 33 , N 34  are n-channel devices and the transistors of the comparison circuit P 23 , P 24 , P 33 , P 34  are p-channel devices. The operation is similar to the operation of the embodiment illustrated in  FIG. 2  with the appropriate voltages reversed for devices of different polarities, as will be apparent to a person skilled in the art. For example, the word line WL is asserted active logic ‘1’. Further, the match line ML is logic ‘0’ and a mismatch charges the match line ML to logic ‘1’. 
     Referring to  FIG. 4 , a layout of a ternary CAM half-cell in accordance with the present embodiment is illustrated generally by numeral  400 . The layout  400  corresponds to the circuit  100  illustrated in FIG.  1 . For convenience, the transistor labels from  FIG. 1 , that is P 1 , P 2 , P 3 , P 4 , N 1 , N 2 , N 3 , and N 4 , are provided for indicating corresponding areas in the layout  400 . In the layout  400 , broken lines enclose regions representing active semiconductor areas  405  (for example, diffusion or ion-implanted areas). These areas include p-type active regions  405   a  and n-type active regions  405   b . Thick, solid, continuous lines enclose a polysilicon layer  410  while thin solid continuous lines enclose a metal  1  layer  420 . The metal  1  layer  420  provides a metal interconnect between a plurality of metal contacts  404 . The metal contacts  404  are represented by squares with an × symbol therein. Of special note is the metal  1  layer  420  connection for the cross coupled inverters formed by P 2 , N 2 , and P 1 , N 1 . Other higher metal layers (there are typically several metal layers) are not illustrated for simplicity. These include the search lines SL, complementary bit lines BL and {overscore (BL)}, which are in a metal  3  M 3  layer. These and other layers will be apparent to a person skilled in the art. 
     As can be seen in  FIG. 4  The p-channel devices P 1 , P 2 , P 3 , and p 4  are grouped at the top of the figure, using a single n-well, while the n-channel devices N 1 , N 2 , N 3 , and N 4  are grouped at the bottom, using a single p-well. This grouping results in a well-balanced use of cell area. Further, the compare circuitry N 3  and N 4  is separated spatially from the access devices P 3  and P 4 , which yields a well-packed efficient layout with a desirably narrow aspect ratio. As such, only one p+ region to n+ region separation is necessary for the entire cell unlike prior art approaches which required at least two p+ region to n+ region separations. Further advantages of the layout described above include having the connections to the search transistors (N 3 , N 4 ) at the opposite end of the connections to the access transistors (P 3 , P 4 ). This separation eases congestion in the upper layers of metal. Furthermore, the cell is close to the minimum width set by transistor geometries, local interconnect (or metal  1 ), and upper metals simultaneously. 
     A minimal width and improved aspect ratio mean smaller area and reduced match line length, which is important to increasing speed and reducing power consumption. Analysis reports demonstrate that prior art approaches using a 0.13 um pure logic process utilise a cell size that is approximately 40% larger than a cell implemented using a layout in accordance with the present invention. 
     Although the invention has been described with reference to specific embodiments, various modifications will become apparent to a person skilled in the art with departing from the spirit of the invention.