Abstract:
A dynamic CAM cell has features that reduce the effect of noise within a CAM array. By shielding the matchline from the wordline, noise transmitted from the matchline to the wordline is reduced. By placing the searchline equally distant from a bitline and the bitline complement, the noise transmitted by the searchline is received equivalently by both the bitline and the bitline complement and therefore cancelled out.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to content addressable memory (CAM), and more specifically, to circuits and methods for reducing spurious noise in DRAM CAM cells.  
       BACKGROUND OF THE INVENTION  
       [0002]     An essential semiconductor device is semiconductor memory, such as a random access memory (RAM) device. A RAM allows a memory circuit to execute both read and write operations on its memory cells. Typical examples of RAM devices include dynamic random access memory (DRAM) and static random access memory (SRAM).  
         [0003]     Another form of memory is the content addressable memory (CAM) device. A CAM is a memory device that accelerates any application requiring fast searches of a database, list, or pattern, such as in database machines, image or voice recognition, or computer and communication networks. CAMs provide benefits over other memory search algorithms by simultaneously comparing the desired information (i.e., data in the comparand register) against the entire list of pre-stored entries. As a result of their unique searching algorithm, CAM devices are frequently employed in network equipment, particularly routers and switches, computer systems and other devices that require rapid content searching.  
         [0004]     In order to perform a memory search in the above-identified manner, CAMs are organized differently than other memory devices (e.g., DRAM and SRAM). For example, data is stored in a RAM in a particular location, called an address. During a memory access, the user supplies an address and reads into or gets back the data at the specified address.  
         [0005]     In a CAM, however, data is stored in locations in a somewhat random fashion. The locations can be selected by an address bus, or the data can be written into the first empty memory location. Every location has a pair of status bits that keep track of whether the location is storing valid information in it or is empty and available for writing.  
         [0006]     Once information is stored in a memory location, it is found by comparing every bit in memory with data in the comparand register. When the content stored in the CAM memory location does not match the data in the comparand register, the local match detection circuit returns a no match indication. When the content stored in the CAM memory location matches the data in the comparand register, the local match detection circuit returns a match indication, e.g., a match flag. If one or more local match detection circuits return a match indication, the CAM device returns a “match” indication. Otherwise, the CAM device returns a “no-match” indication. In addition, the CAM may return the identification of the address location in which the desired data is stored or one of such addresses if more than one address contained matching data. Thus, with a CAM, the user supplies the data and gets back the address if there is a match found in memory.  
         [0007]     Many current applications utilize 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 with search data provided to the CAM.  
         [0008]      FIG. 1  depicts a six transistor ( 6 T) dynamic ternary (DRAM) CAM cell  100  of the prior art. The cell  100  has an “x” bit and a “y” bit. For the x bit, data is written to and read out of the cell  100  via bitline BLx  110 , access transistor  160  and storage capacitor Cx  140 . For the y bit, data is written to and read out of the cell  100  via bitline BLy  112 , access transistor  162  and storage capacitor Cy  142 . The access transistors  160 ,  162  are controlled by a common wordline  132 . It should be understood that the storage capacitors can be discrete components or the parasitic capacitance of the line  132 . Alternately, other storage or memory devices may be used to store data in the cell  100 . Although not shown, other memory cells  100  in a column of a memory array are coupled either to bitline BLx  110  and bitline BLy  112  or bitline BLx*  114  and bitline BLy*  116 . Although CAM cell  100  is shown as a DRAM CAM cell, the CAM cell may also be implemented using other types of memory storage, e.g., the CAM cell may use SRAM memory cells.  
         [0009]     To store a logic “0” in the cell  100 , a “1” must be written into the x bit, and a “0” must be written into the y bit of the cell  100 . To store a logic “1” in the cell  100 , a “0” must be written into the x bit and a “1” must be written into the y bit of the cell  100 . If a “0” is stored in both the x and the y bits of the cell  100 , then the cell  100  will be masked for a search operation. If a “1” is stored in both the x and the y bits of the cell  100 , then the cell  100  will always indicate a mismatch for search operations.  
         [0010]     During a search operation, a search key/word is applied to search datalines SDx  120 , SDy  122 , each of which is coupled to the gate terminal of compare transistors  174 ,  176 , respectively. A first source/drain of the compare transistors  174 ,  176  is coupled to a common matchline  130 . A second source/drain of the transistors  174 ,  176  is coupled to a first source/drain of transistors  170 ,  172 , respectively. Each transistor pair  174 - 170  and  176 - 172  is referred to as a compare “stack.” The applied search key is compared to data stored in the cell  100  to see if there is a match. To search for a “0,” SDx is set to 0 and SDy is set to 1; to search for a “1,” SDx is set to 1, and SDy is set to 0.  
         [0011]      FIG. 2  shows a CAM array  200  and associated circuits  250 ,  252 ,  254 ,  256 . The array  200  includes a plurality of CAM cells  100  organized as a plurality of rows and columns. Each row of CAM cells  100  is coupled to a respective wordline  132  and matchline  130 , where every CAM cell  100  in the same row is mutually coupled to the wordline  132  and matchline  130  corresponding to the row. Each column of CAM cells  100  is coupled to a respective search dataline SDx  120 , SDy  122 , and to bitlines BLx  110 , BLy  112 , BLx*  114 , BLy*  116 , where every CAM cell  100  in the same column is mutually coupled to the search dataline SDx  120 , SDy  122  and to either bitline BLx  110  and bitline BLy  112  or bitline BLx*  114  and bitline BLy*  116  corresponding to the column.  
         [0012]     Every wordline  132  is coupled to access/decode circuit  254  and to a respective wordline driver  284 . Every matchline  130  is coupled to access/decode circuit  256  and to a respective sense amplifier  286 . Every search dataline SDx  120 , SDy  122  is coupled to access/decode circuit  252  and to a respective search data driver  282 . Every bitline BLx  110 , BLy  112 , BLx*  114 , BLy*  116  is coupled to access/decode circuit  250 .  
         [0013]     It is known to orient the matchlines  130  substantially parallel to the wordlines  132  in the conventional CAM architecture (e.g.,  FIGS. 1 and 2 ). However, signals and other currents carried on the matchline  130  can create noise on an adjacent wordline  132 . Noise on a wordline  132  can affect a charge stored on an adjacent capacitor Cx  140  or Cy  142  ( FIG. 1 ), as the noise may cause the charge on the capacitor to leak. Accordingly, there is a desire and need to reduce noise on a CAM wordline that may be caused by the matchline.  
         [0014]     It is also known to orient the search datalines SDx  120 , SDy  122  substantially parallel to bitlines BLx  110 , BLy  112 , BLx*  114 , BLy*  116  in the conventional CAM architecture (e.g.,  FIGS. 1 and 2 ). However, signals and other currents carried on a search dataline SDx  120 , SDy  122  can create noise on an adjacent bitline BLx  110 , BLy  112 , BLx*  114 , BLy*  116 . In the operation of a CAM array, since a bitline is used to sense data and the complement of the bitline is used as a reference, if one of these two bitlines receives noise and the other does not, then data on the bitlines will be read or written incorrectly. Accordingly, there is a desire and need to reduce noise on a CAM bitlines that may be caused by the searchline.  
         [0015]     Storage of a signal in a DRAM memory cell is inherently more unstable than storing a similar charge in a SRAM memory cell. A DRAM cell is more sensitive to noise, while the SRAM has a strong resistance to noise. Consequently, spurious noise in a memory array is more likely to perturb a DRAM memory cell than a SRAM memory cell. Therefore, it is desirable to reduce the effect of spurious noise in the circuitry of the DRAM CAM array.  
       BRIEF SUMMARY OF THE INVENTION  
       [0016]     The present invention provides circuits that reduce the effect of spurious noise in the circuitry of a CAM array. In an exemplary embodiment, a grounded noise shield between a wordline and a matchline reduces the transmission of noise from the matchline to the wordline.  
         [0017]     In another exemplary embodiment, a searchline is positioned symmetrically over complimentary pairs of bitlines. As a result, noise generated by the searchline is received by both bitlines and the resulting signal errors cancel each other out. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     These and other advantages and features of the invention will be more clearly understood from the following detailed description of the invention which is provided in connection with the accompanying drawings, in which:  
         [0019]      FIG. 1  is a schematic circuit diagram of a CAM cell in the prior art;  
         [0020]      FIG. 2  is a block diagram of a CAM array including the CAM cell of  FIG. 1 ;  
         [0021]      FIG. 3  is a schematic circuit diagram of a CAM cell in accordance with an exemplary embodiment of the invention;  
         [0022]      FIG. 4 . is a mask layer diagram of the CAM cell of  FIG. 3  in accordance with an exemplary embodiment of the invention;  
         [0023]      FIG. 5  is a block diagram of a CAM array including the CAM cell of  FIG. 3  and associated circuits in accordance with an exemplary embodiment of the invention;  
         [0024]      FIG. 6  is a mask layer diagram showing a portion of a CAM array in accordance with another exemplary embodiment of the invention;  
         [0025]      FIG. 7  is a diagram of a semiconductor chip with an integrated circuit that includes a CAM array constructed in accordance with exemplary embodiment of the invention;  
         [0026]      FIG. 8  is a schematic diagram of a processor system employing the CAM array of  FIG. 7  as part of a CAM device, in accordance with another exemplary embodiment of the invention; and  
         [0027]      FIG. 9  is a schematic diagram of a router employing the CAM array of  FIG. 7  as part of a CAM device, in accordance with another exemplary embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to make and use the invention, and it is to be understood that structural, logical or other changes may be made to the specific embodiments disclosed without departing from the spirit and scope of the present invention.  
         [0029]      FIG. 3  is a schematic diagram of a CAM cell  300  in accordance with an exemplary embodiment of the invention. The CAM cell  300  of  FIG. 3  differs from the CAM cell  100  of  FIG. 1  in that CAM cell  300  includes shieldline (SL)  390 . As seen in  FIG. 3 , shieldline  390  is disposed between and parallel to matchline  130  and wordline  132 . Shieldline  390  is desirably formed from copper) but may be constructed of any conductive material. Although not shown in  FIG. 3 , shieldline  390  is coupled to a ground potential (i.e., grounded). Because the grounded shieldline  390  is between the matchline  130  and wordline  132 , the shieldline  390  absorbs noise generated by the matchline  130 . For example, if matchline  130  operates at high frequencies, shieldline  390  isolates wordline  132  from the resulting high frequency emissions, which could otherwise couple to wordline  132  and produce noise and possibly signal errors. Consequently, the amount of noise received by the wordline  132  is reduced. The reduction of noise in the wordline  132  also reduces the effect that the noise on the wordline  132  has on the charge of the capacitors  140 ,  142  within the CAM cell  300 .  
         [0030]      FIG. 4 . shows a mask layer diagram of CAM cell  300  in accordance with an exemplary embodiment of the invention. The illustrated shieldline  390  is disposed between matchline  130  and wordline  132 . All three lines  390 ,  130 ,  132  are also disposed within the same metal layer to minimize the height required to build the CAM cell  300 .  
         [0031]      FIG. 5  is a block diagram of a CAM array  350  including the CAM cell  300  of  FIG. 3  and associated circuits  250 ,  252 ,  254 ,  256  constructed in accordance with an exemplary embodiment of the invention. In a preferred embodiment the shieldline  390  extends the entire width of the row of the CAM array  350 , corresponding to length of the wordline  132  in the row. Otherwise, the array  350  contains conventional CAM cells components  110 ,  112 ,  114 ,  116 ,  120 ,  122 ,  300  as described above.  
         [0032]      FIG. 6  is a mask layer diagram showing a portion of the CAM array  350  in accordance with another exemplary embodiment of the invention. As seen in  FIG. 6 , CAM array  350  includes at least one CAM cell  400 . The CAM array  350  differs from the CAM array  100  ( FIG. 1 ) in that the searchlines  420 ,  422  are each placed symmetrically over a complementary pair of bitlines  110 ,  114  and  112 ,  116 , respectively. By placing a searchline equidistant from a complementary pair of bitlines (e.g., searchline  420 , bitlines  110 ,  114 ), noise generated by the searchline  420  will be received equally by both bidines  110 ,  114 .  
         [0033]     Regarding each pair of complementary bitlines, when one bit line is used to sense data, the other is being used as a reference, and vice versa. When searchline  420 , for example, operates at high frequency, the resulting high frequency emissions could otherwise couple more strongly to one of the bitlines in the pair. This could produce noise on the more strongly coupled bitline and not on the other, and thus, data on the bitlines would be read or written incorrectly. If searchline  420  is positioned symmetrically and equidistant from bitlines  110  and  114 , however, the coupling of noise to both bitlines  110 ,  114  will be approximately equal. Therefore, there will be substantially the same noise in both lines  110 ,  114 , which will be offset and cancelled out. The cancellation of the noise reduces the effect of noise on the sense operation. In an exemplary embodiment, the searchline  420  is formed in a layer different from the metal layer that contains complementary bitlines  110 ,  114 ; the invention, however, is not limited to such an arrangement. For example, a metal layer containing the searchline  420  constitutes a first layer, and a second metal layer contains bitlines  110  and  114 , where the first layer is parallel to and above the second layer. Alternatively, the first layer is coplanar to the second layer.  
         [0034]     Although depicted separately in  FIGS. 3-6 , the concepts of the invention could be used together. That is, in another embodiment, a CAM cell has both a shieldline disposed between matchline and wordline, and searchlines placed symmetrically relative to complementary bitlines. This arrangement would have all of the benefits described above with regard to  FIGS. 3-6 .  
         [0035]      FIG. 7  depicts a CAM array  350 , as described in connection with  FIGS. 3-6  that is included on an integrated circuit formed on a semiconductor memory chip  1210  so that it may be incorporated into a router or other processor system (as described below).  
         [0036]      FIG. 8  illustrates an exemplary processing system  1300  that employs a CAM array  350  as described in relation to  FIGS. 3-7 . The processing system  1300  includes one or more processors  301  coupled to a local bus  304 . A memory controller  302  and a primary bus bridge  303  are also coupled the local bus  304 . The processing system  1300  may include multiple memory controllers  302  and/or multiple primary bus bridges  303 . The memory controller  302  and the primary bus bridge  303  may be integrated as a single device  306 .  
         [0037]     The memory controller  302  is also coupled to one or more memory buses  307 . Each memory bus accepts memory components  308 . Any one of memory components  308  may contain a semiconductor chip  1210  as described in relation to  FIG. 7  or a CAM array  350  described in connection with  FIGS. 3-6 .  
         [0038]     The memory components  308  may be a memory card or a memory module. The memory components  308  may include one or more additional devices  309 . For example, in a SIMM or DIMM, the additional device  309  might be a configuration memory, such as a serial presence detect (SPD) memory. The memory controller  302  may also be coupled to a cache memory  305 . The cache memory  305  may be the only cache memory in the processing system. Alternatively, other devices, for example, processors  301  may also include cache memories, which may form a cache hierarchy with cache memory  305 . If the processing system  1300  include peripherals or controllers which are bus masters or which support direct memory access (DMA), the memory controller  302  may implement a cache coherency protocol. If the memory controller  302  is coupled to a plurality of memory buses  307 , each memory bus  307  may be operated in parallel, or different address ranges may be mapped to different memory buses  307 .  
         [0039]     The primary bus bridge  303  is coupled to at least one peripheral bus  310 . Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus  310 . These devices may include a storage controller  311 , a miscellaneous I/O device  314 , a secondary bus bridge  315 , a multimedia processor  318 , and a legacy device interface  320 . The primary bus bridge  303  may also be coupled to one or more special purpose high-speed ports  322 . In a personal computer, for example, the special purpose port might be the Accelerated Graphics Port (AGP), used to couple a high performance video card to the processing system  1300 .  
         [0040]     The storage controller  311  couples one or more storage devices  313 , via a storage bus  312 , to the peripheral bus  310 . For example, the storage controller  311  may be a SCSI controller and storage devices  313  may be SCSI discs. The I/O device  314  may be any sort of peripheral. For example, the I/O device  314  may be a local area network interface, such as an Ethernet card. The secondary bus bridge may be used to interface additional devices via another bus to the processing system. For example, the secondary bus bridge may be a universal serial port (USB) controller used to couple USB devices  317  via to the processing system  1300 . The multimedia processor  318  may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to additional devices such as speakers  319 . The legacy device interface  320  is used to couple legacy devices, for example, older styled keyboards and mice, to the processing system  1300 .  
         [0041]     The processing system  1300  illustrated in  FIG. 8  is only an exemplary processing system with which the invention may be used. While  FIG. 8  illustrates a processing architecture especially suitable for a general-purpose computer, such as a personal computer or a workstation, it should be recognized that well known modifications can be made to configure the processing system  1300  to become more suitable for use in a variety of applications. For example, many electronic devices that require processing may be implemented using a simpler architecture that relies on a CPU  301  coupled to memory components  308  and/or memory devices  309 . The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices.  
         [0042]      FIG. 9  is a simplified block diagram of a router  1310  as may be used in a communications network, such as, e.g., part of the Internet backbone. The router  1310  contains a plurality of input lines  1312  and a plurality of output lines  1314 . When data is transmitted from one location to another, it is sent in a form known as a packet. Oftentimes, prior to the packet reaching its final destination, that packet is first received by a router, or some other device. The router  1310  then decodes that part of the data identifying the ultimate destination and decides which output line  1314  and what forwarding instructions are required for the packet.  
         [0043]     Generally, CAMs are very useful in router applications because historical routing information for packets received from a particular source and going to a particular destination is stored in the CAM of the router. As a result, when a packet is received by the router  1310 , the router already has the forwarding information stored within its CAM. Therefore, only that portion of the packet that identifies the sender and recipient need be decoded in order to perform a search of the CAM to identify which output line  1314  and instructions are required to pass the packet onto a next node of its journey.  
         [0044]     Still referring to  FIG. 9 , router  1310  contains the added benefit of employing a semiconductor memory chip  1210  containing a CAM array  350  ( FIG. 7 ) of the invention. Thus, the router  1310  benefits from a CAM array  350  having reduced noise and overall better operation than conventional CAM arrays (e.g.,  FIG. 1 ).  
         [0045]     Although the systems described above with respect to  FIGS. 7-9  are discussed in reference to incorporating an exemplary embodiment of the invention, the systems are not so limited and can incorporate any of the embodiments of the invention described above.  
         [0046]     While preferred embodiments of the invention have been described in the illustrations above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.