Patent Publication Number: US-6704216-B1

Title: Dual match-line, twin-cell, binary-ternary CAM

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to static information storage and retrieval systems, and more particularly to associative memories, which are also referred to as content or tag memories. 
     2. Background Art 
     In a content addressable memory (CAM), an input word (the data-in word) consists of an ordered group of K bits which is compared bit-by-bit with a plurality of stored words in the CAM. If all of the bits of one of the stored words match those of the data-in word, the address of that stored word is output to the user. Sometimes more than one stored word matches the data-in word. In this case, usually only one address is output, based on a pre-specified priority criterion. 
     On the other hand, in many cases not all bits need to match when such comparisons are performed. Some bits in a stored word may then be masked out, and not compared with the corresponding bits in the data-in word. It is considered a legitimate match here if the stored word matches the data-in word for all but the masked bits. Technically, a masked bit always returns a match condition so that it does not affect the overall result. 
     A CAM wherein stored words must be compared bit-by-bit without exception is called a binary CAM, and its memory cells have two logic states: a “0” state and a “1” state. In contrast, a CAM that allows stored words to have masked bits is called a ternary CAM. The memory cell of a ternary CAM has three logic states: a “0” state, a “1” state, and a “DON&#39;T CARE” state that returns an unconditional match. 
     FIG. 1 shows a typical stored word in a binary CAM. It consists of a row of SRAM cells having differential outputs DSk and DSkB (DSk bar) which are compared with corresponding differential data-in lines Dlk and DlkB through a comparator. The set of connections for the comparator logic, also commonly termed a logic comparison function, is equivalent to (DSk*DlkB)+(DSkB*Dlk)=DSk(+)Dlk, which is effectively an EXCLUSIVE OR function. At the beginning of each compare cycle a match line (ML) is pulled high by connection to a voltage source V DD ) via a pre-charge gate (MP 0 ; typically a PMOS device), and all of the data lines Dlk and DlkB, where k=1 . . . K, are held low. The pre-charge gate is then turned off, leaving the match line floating. At the same time the Dlk data lines are driven by the input data. If DSk=Dlk then DSk (+)Dlk=0 and the cell k leaves the match line alone. This is a match condition. 
     On the other hand, if DSk≠Dlk then DSK(+)Dlk=1 and one of the series pairs of pass-gates (typically NMOS devices) will conduct and discharge the match line to ground. This is a non-match condition. Since it takes only one non-matched cell to discharge the match line to ground, a word is said to match with the data-in word if DSk=Dlk for all k=1 . . . K. That is if the match line stays high. Since all cells are counted in the compare here this is a binary CAM. The two states of the cell are: a “1” state with DSK=1 (high) and DSkB=0 (low); and a “0” state with DSK=0 and DSkB=1. 
     FIG. 2 shows a typical stored word in a ternary CAM. Since the memory cell of a ternary CAM needs three states, two SRAM cells sharing a comparator form a ternary cell. There are now three usable states: a “1” state, with DS 1 =1 and DS 2 =0; a “0” state with DS 1 =0 and DS 2 =1; and a “DON&#39;T CARE” state with DS 1 =DS 2 =0. The data-in lines Dl and DlB are connected in the same fashion as in FIG. 1, forming an EXCLUSIVE OR logic function. The pre-charged match line (ML) stays high if Dl 1 =1and the cell is in the “1” state or Dl 1 =0 and the cell is in the “0” state. When the cell is in the “DON&#39;T CARE” state, with both series pairs of pass-gates cut off and preventing the match line discharging to ground, a match condition is guaranteed and the cell has no effect on the overall result. Notice that the DSk and DSkB outputs of all odd numbered SRAM are the mirror images of the associated even numbered SRAM for easy layout. 
     As can be seen by comparing FIG.  1  and FIG. 2, the only difference between a binary CAM cell and a ternary CAM cell is that in a ternary cell a single comparator is shared by two SRAM cells. Most ternary CAM on the market today uses this configuration to save die area and to simplify component connection. Such CAM can still be used as a binary cell, but then the die area is not used efficiently, because two SRAM cells are being used to hold only one bit of data. 
     Another major concern regarding the efficient use of CAM is power consumption. As those skilled in the electronic arts well know, reducing power consumption is generally desirable in a circuit. However, in CAM it is an increasingly concern because CAM is often used in portable, battery operated devices and the consumption of power relates directly to how long such devices can be operated. Any reduction in CAM power consumption is therefore a highly desirable benefit. 
     FIG. 3 depicts a logical approach, based on industry trends, to achieving a binary-ternary CAM. It should be noted, however, that the present inventors do not know of any products actually using this approach. FIG. 3 is included here as a useful comparison to the present invention. Yet another manner of achieving binary-ternary CAM, and one which takes a much different approach, is described in U.S. Pat. No. 6,362,992. 
     As can also be seen by comparing FIGS. 1-3, a common feature of CAM (and many other parallel comparison circuits, for that matter) is the use of a match line (ML). Only a few memory cells are shown in the examples, with all in a single row connected to only a single match line. In actual practice, there typically will be thousands of cells in each row and hundreds or thousands of such rows, each having a respective match line. Each such match line is pre-charged with a voltage and when a match condition occurs the match line discharges, nominally to ground. Conceptually similar circuits can be constructed, but are not common, where the match line is pre-grounded and a match condition causes charging to some voltage level which is detected. Unfortunately, these approaches to match detection have a number of problems. 
     One can consider just one row. In view of the large number of cells present, it should be clear that the match line for such a row will have an appreciable capacitance. The pre-charge sub-circuit (e.g., MP 0  in FIG. 1) has to handle this, and will typically take up a relatively large amount of die space or “real estate.” The row also necessarily operates at a particular clock rate, performing pre-charging, comparison, and match detection in each cycle. In this approach, the match line necessarily also runs at that clock frequency. However, as those skilled in the electronic arts also accept as a maxim, it is generally desirable to reduce sub-circuit frequency wherever possible, while maintaining overall system frequency. Lower operating frequency typically correlates with reduced power consumption, component life, etc. Additionally, switching high power and reactive loads at high frequency is undesirable because of possible collateral effects due to electro-magnetic radiation. 
     In summary, it is desirable to have a more flexible CAM, which can be efficiently applied as either binary or ternary CAM. It is also desirable to have CAM, in the form of binary CAM, ternary CAM, or configurable binary-ternary CAM, which consumes less power. And it is further desirable to have CAM which operates sub-circuits at low effective frequency, particularly including the match line sub-circuit. 
     DISCLOSURE OF INVENTION 
     Accordingly, it is an object of the present invention to provide a CAM which is configurable to operate in either binary mode or ternary mode, yet which is efficiently able to utilize all memory cells in either mode. 
     Another object of the invention is to provide a CAM which provides substantial power savings. 
     Briefly, one preferred embodiment of the present invention is a content addressable memory (CAM). The CAM includes an input sub-circuit to present input data to multiple cell sub-circuits. A further included match sub-circuit has a match-high line, a match-low line, and a pre-charge sub-circuit. The pre-charge sub-circuit is able to connect the match-high line to a voltage source, to charge it, and also to connect the match-low line to a ground, to discharge it. The cell sub-circuits each compare one bit of the input data with one bit of pre-stored storage data and determine whether pre-specified match criteria are met. If so, the cell sub-circuit connects the match-high line and match-low line, thereby signaling detection of a mismatch condition. 
     Briefly, another preferred embodiment of the present invention is a content addressable memory (CAM) for binary mode comparison of input bits with storage bits. The CAM includes an input sub-circuit to present the input bits to multiple cell sub-circuits. A further included match sub-circuit has a match-high line, a match-low line, a pre-charge sub-circuit, and multiple match-gates, at least equaling the cell sub-circuits in number. The pre-charge sub-circuit is able to controllably bring the match-high line to a high state and the match-low line to a low state. Each match-gate is able to operationally connect the match-high line with the match-low line in response to a match signal. The cell sub-circuits each store one of the storage bits and generate a respective match signal based on the states of the input bit and its storage bit, thereby permitting the CAM to compare the input bits with the storage bits to detect a mismatch condition. 
     Briefly, another preferred embodiment of the present invention is a content addressable memory (CAM) for ternary mode comparison of input bits with storage bits. The CAM includes an input sub-circuit to present the input bits to multiple composite cells. A further included match sub-circuit has a match circuit including a match-high line, a match-low line, a pre-charge sub-circuit, and multiple match-gates, at least equaling the cell sub-circuits in number. The pre-charge sub-circuit is able to controllably bring the match-high line to a high state and the match-low line to a low state. Each match-gate is able to operationally connect the match-high line with the match-low line in response to a match signal. The composite cells each store one of the storage bits and one of the mask bits as a ternary unit having three possible states: 1, 0, and X, wherein X represents masked. The composite cells then each generate a respective match signal based on the states of the input bit and the ternary unit, thereby permitting the CAM to compare one of the input bits with one of the storage bits and one of the mask bits to detect a mismatch condition. 
     And, briefly, another preferred embodiment of the present invention is a content addressable memory (CAM) for comparison of a data set in either binary mode or ternary mode. The CAM includes an input circuit suitable for presenting input bits from the data set to multiple composite cells. A further included match circuit has a match-high line, a match-low line, a pre-charge sub-circuit, and multiple match-gates, at least equaling the composite cells in number. The pre-charge sub-circuit is able to controllably bring the match-high line to a high state and the match-low line to a low state. Each match-gate is able to operationally connect the match-high line with the match-low line in response to a match signal. The composite cells selectively operate in either the binary mode or the ternary mode. In the binary mode, each composite cell stores two storage bits, receives two input bits from the input circuit, and generates two match signals respectively based on the states of an input bit and a storage bit, thereby permitting the CAM to compare the data set with the storage bits in binary manner to detect a mismatch condition. In the ternary mode, each composite cell stores one storage bit and one mask bit as a ternary unit having three possible states: 1, 0, and X, wherein X represents masked. The composite cells each receive one input bit from the input circuit and generate a respective match signal based on the states of the input bit and the ternary unit, thereby also permitting the CAM to compare the data set with the storage bits and the mask bits in ternary manner to detect a mismatch condition. 
     An advantage of the present invention is that it provides a CAM that is configurable to operate in either binary mode or ternary mode while efficiently utilizing all memory cells. 
     Another advantage of the invention is that it provides substantial power savings in input data handling, by floating signals that do not need to be changed, and thereby not requiring reconditioning (charging or discharging) of those signals for later use. 
     And another advantage of the invention is that it halves the operation rate or frequency of key sub-circuits which handle and consume substantial power. This directly avoids direct adverse frequency related effects in the CAM, such as electromagnetic interference. Concurrently, this provides substantial further power savings in match detection, by reduced operation of large devices and capacitances, while still providing match detection capability during every clock cycle. 
    
    
     These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The purposes and advantages of the present invention will be apparent from the following detailed description in conjunction with the appended figures of drawings in which: 
     FIG. 1 (background art) is a schematic diagram showing a typical word in a conventional binary CAM; 
     FIG. 2 (background art) is a schematic diagram showing a typical word in a conventional ternary CAM; 
     FIG. 3 is a schematic diagram showing a word in a dual-function CAM, a binary-ternary CAM; 
     FIG. 4 is schematic diagram showing a word in a binary-ternary CAM according to the present invention; 
     FIG. 5 is schematic diagram particularly showing one suitable match detection decoding circuit which may be used in the binary-ternary CAM; 
     FIG. 6 is a timing diagram depicting signal relationships at various points in the circuit shown in FIG. 5; 
     FIG. 7 is a block diagram showing the binary-ternary CAM applied in the context of a larger CAM having rows; and 
     FIG. 8 is a series of schematic diagrams showing examples of generally conventional binary and ternary cells improved by addition of a match detection scheme according to the present invention. 
    
    
     In the various figures of the drawings, like references are used to denote like or similar elements or steps. 
     DETAILED DESCRIPTION 
     Best Mode for Carrying out the Invention 
     A preferred embodiment of the present invention is a dual-match line, twin-cell binary-ternary CAM. As illustrated in the various drawings herein, and particularly in the view of FIG. 4, embodiments of the invention are depicted by the general reference character  10 . 
     In applications where both binary and ternary CAM is required on a single chip, it is important that both binary and ternary modes are supported in the most efficient manner. One method to implement such a “dual function” CAM is shown in FIG.  3 . The layout of the CAM array here is identical to that of a binary CAM, so that it functions exactly like a binary CAM when required. To use the array as a ternary CAM, one may simply ground the unused differential lines and feed the input word through the central data-in lines with multiplexers, as shown. The result is then a ternary cell, which functions exactly the same as the case in FIG.  2 . 
     It is thus apparent that such utilization of a twin-cell to perform binary/ternary dual functions can be accomplished as long as differential signals are available and the two SRAM cells used can be encoded into three states. The present invention uses such a concept, and at the same time saves dynamic power as is now described. 
     Consider the circuit shown in FIG.  4 . The dual-function, twin-cell binary-ternary CAM (CAM  10 ) in this circuit consists of two identical sub-circuits: sub-circuit  12  and sub-circuit  14 , each of which stores a single bit of information in a SRAM cell  16 . Circuitry for storing the bits of information in the SRAM cells  16  has been omitted here for clarity in presenting the salient points of the invention, but such circuitry may be entirely conventional in any case. 
     Collectively a pair of the sub-circuits  12 ,  14  form a composite cell  18 . Internal details of one possible SRAM cell  16  are shown in insert A. 
     In binary mode, each SRAM cell  16  provides two states and the sub-circuits  12 ,  14  behave as two independent binary CAM cells. By combining the sub-circuits  12 ,  14  and properly encoding the four states provided by the two SRAM cells  16 , however, it is possible to store and detect the three states required by the ternary mode. 
     Let us consider the binary mode first. Since the sub-circuits  12 ,  14  operate independently, it suffices to investigate only the sub-circuit  12 . A cross-coupled pair inside the SRAM cell  16  defines two states: a “1” state with a X node  20  high (DS 1 =1) and a Y node  22  low (DS 1 B=0), and a “0” state with the X node  20  low (DS 1 =0) and the Y node  22  high (DS 1 B=1). Note that these nodes  20 ,  22  (DS 1 , DS 1 B here in binary mode) respectively control a first pass gate  24  and a second pass gate  26 . When the SRAM cell  16  is in the “1” state, the X node  20  is high and a third node  28  follows an input line  30  (Dl 1 B), and when the SRAM cell  16  is in the “0” state, the Y node  22  is high and the third node  28  follows an input line  32  (Dl 1 ). Together, the X node  20  and the input line  32  (DS 1  and Dl 1 ) form an EXCLUSIVE OR logic function. The third node  28 , in turn, controls a first match gate  34 . Once this first match gate  34  is turned on, the potential of a match-high line  36  (MHL) and a match-low line  38  (MLL) equalize. 
     Each compare cycle consists of three phases: a data input phase, a compare phase, and a sampling phase. During the first phase, the data input phase, the CAM  10  is initialized to one of two possible states and data is input to it through the data-in sub-circuit  44 . The two possible states are a charged-high state and a charged-low state. The CAM  10  is in the charged-high state when a first pre-charge device  40  is turned on and a second pre-charge device  42  is turned off. The match-high line  36  then goes high and the match-low line  38  floats. The CAM  10  is in the charged-low state when the first pre-charge device  40  is turned off and the second pre-charge device  42  is turned on. The match-high line  36  then floats and the match-low line  38  goes low. 
     Under control of the data-in sub-circuit  44  a data bit is driven on the input lines  30 ,  32  differentially (as Dl 1 B and Dl 1 ). When the SRAM cell  16  is in the “1” state (DS 1 =1,DS 1 B=0) the first pass gate  24  is on and the third node  28  follows the input line  30  (Dl 1 B). If the input line  32  is high (Dl 1 =1), the input line  30  (Dl 1 B) is low and the first match gate  34  is off. This is a match condition. Alternately, if the input line  32  is low (Dl 1 =0) the input line  30  (Dl 1 B=1) is high, so the third node  28  will go high and the first match gate  34  will turn on. This is a mismatch condition. 
     Similarly if the SRAM cell  16  is in the “0” state (DS 1 =0, DS 1 B=1), the second pass gate  26  is on and the third node  28  follows the input line  32  (Dl 1 ). The first match gate  34  is off and a match condition now occurs if the input line  32  is low (Dl 1 =0). Whereas, the first match gate  34  is on and a mismatch condition occurs if the input line  32  is high (Dl 1 =1). 
     At the end of the first phase, the input data should be stable and all of the match gates  34  for a given row of SRAM cells  16  should be turned either on or off. The changing of the polarity of pre-charge signal marks the beginning of the second phase, the compare phase. During this phase, the first pre-charge device  40  and second pre-charge device  42  change from on to off, or from off to on, depending on their previous states. Since in a CAM array the SRAM cells  16  of an entire row are connected to the match-high line  36  and the match-low line  38 , it takes just one single non-matching cell unit, like sub-circuit  12 , to equalize the match-high line  36  to the match-low line  38 . On the other hand, the match-high line  36  stays high only if all of the bits in the SRAM cells  16  match the bits appearing in the data-in sub-circuit  44 . In other words, if any match gate  34  is turned on in a given row, i.e., a mismatch condition; the match-high line  36  and match-low line  38  will be equalized, and both signals will stay either high or low, depending on the pre-charge condition. If no match gate  34  in a row is turned on, i.e., a match condition; the match-high line  36  will stay high and the match-low line  38  will stay low. 
     After both of the match lines  36 ,  38  settle down, the third phase begins, the sample phase. The rising edge of a clock signal (FIG. 5) marks the beginning of this phase. During this phase, a sense amplifier (FIG. 5) samples both of the match lines  36 ,  38 . If they are the same a mismatch condition exists, and if the match-high line  36  is high and the match-low line  38  is low a match condition exists. 
     In ternary mode, each compare cycle can also be viewed as having three phases: a data input phase, a compare phase, and a sampling phase. During the first phase, the data input phase, the CAM  10  is initialized to one of two possible states and data is input to the CAM through the data-in sub-circuit  44 . A data bit drives the input line  32  and an input line  48  differentially (Dl Band Dl 1 ) and the input line  30  and an input line  50  are driven to ground. 
     The SRAM cells  16  in the sub-circuits  12 ,  14  (CK 1  and CK 2 ) are combined such that the composite cell  18  forms three states: a “1” state where (DS 1 , DS 2 )=(1, 0); a “0” state where (DS 1 , DS 2 )=(0, 1); and a “DON&#39;T CARE” state where (DS 1 , DS 2 )=(0, 0). The state (DS 1 , DS 2 )=(1, 1) is not allowed. In these expressions a “1” means a high voltage and “0” means a low voltage, as is customary in this art. 
     If the composite cell  18  is in the “1” state, the second pass gate  26  is on and a third pass gate  54  is off. If Dl=1, the input line  32  (Dl 1 B) will be low and the input line  48  (Dl 1 ) will be high. Both the third node  28  and a fifth node  56  are low, and both of the first match gate  34  and a second match gate  52  are off. This is a match condition. On the other hand, if Dl=0, the input line  32  (Dl 1 B) will be high and the input line  48  (Dl 1 ) will be low. The third node  28  will then follow the input line  32  (Dl 1 B), turning on the first match gate  34 . This is therefore a mismatch condition. Similar analysis will show the validity for the case of the “0” state. For the “DON&#39;T CARE” state, since both the second pass gate  26  and the third pass gate  54  are off, the match gates  34 ,  52  are off. A match condition is therefore guaranteed. It follows that by appropriate multiplexing and by encoding the states of the composite cell  18 , the two binary SRAM cells  16  behave like a single ternary cell. 
     At the end of the first phase, the input data should be stable and all of the match gates  34 ,  52  for a given row should be either on or off. The changing of the polarity of pre-charge signal marks the beginning of the second phase, the compare phase. During this phase, much the same as in the binary mode, the pre-charge devices  40 ,  42  change from on to off, or from off to on, depending on their previous states. Since the SRAM cells  16  of a whole row in a CAM array are connected to the match lines  36 ,  38 , it takes only one single non-matching cell unit, like the sub-circuit  12 , to equalize the match lines  36 ,  38 . On the other hand, the match-high line  36  stays high only if all bits in the SRAM cells  16  match the bits appearing in the data-in sub-circuit  44 . In other words, if any match gate  34 ,  52  is turned on in a given row, i.e., a mismatch condition exists; the match lines  36 ,  38  will be equalized and both will stay either high or low. If no match gate  34 ,  52  in a row is turned on, i.e., a match condition exists; the match-high line  36  will stay high and the match-low line  38  will stay low. 
     After both of the match lines  36 ,  38  settle down, the third phase begins, the sample phase. This is also much the same as in the binary mode; the rising edge of the clock (FIG. 5) marks the beginning of this phase. During this phase, the sense amplifier (FIG. 5) samples both of the match lines  36 ,  38 . If they are the same a mismatch condition exists, and if the match-high line  36  is high and the match-low line  38  is low a match condition exists. 
     Since either of the match lines is floating during the data input phase, the data-in lines do not have to be connected to the ground. In the case an input data bit does not change from one compare cycle to the next no dynamic power is consumed by the data-in lines during the data input phase. Since there usually are many data-in lines that do not change, this aspect of the present inventive CAM  10  provides a substantial power savings. 
     FIG. 5 is a schematic diagram particularly showing one suitable decoding circuit  60  for match detection, which may be used in the CAM  10 , such as that depicted in FIG.  4 . For simplicity only one CAM cell is shown, together with the decoding circuit  60 . FIG. 6 is a timing diagram depicting signal relationships at various points in the circuit shown in FIG.  5 . 
     As shown in FIG. 6 a clock signal  62  is provided and a pre-charge signal  64  in the decoding circuit  60  is basically the clock signal  62  divided-by-two. Note that the pre-charge signal  64  in this embodiment follows the falling edge of the clock signal  62  and switches either low-to-high or high-to-low once every clock cycle. At the rising edge of the clock signal  62 , data-in drives a Dl line  66  (and its differentially related DlB line  68  in FIG. 5; e.g., signals on input lines  30 ,  32 ,  48  in FIG.  4 ), and stays valid throughout the whole clock cycle. The levels of the match-high line  36  (MHL) and the match-low line  38  (MLL) are then sensed during the low time of each cycle of the clock signal  62  and latched by the rising edge of the next clock cycle. 
     With reference also to FIG. 4, a cell node  70  (e.g., the third node  28  or the fifth node  56 ) exhibits the EXCLUSIVE OR function of a DS node  72  (e.g., the X node  20  (DS in FIG. 5) and the Dl line  66  (e.g., input lines  32 ,  48 ). For simplicity it is assumed that the DS node  72  is low (DS=0) all of the time and that the Dl line  66  switches. As shown in FIG. 6, whenever the Dl line  66  and the DS node  72  are different (Dl≠DS), the cell node  70  goes high and brings the match-high line  36  (MHL) and the match-low line  38  (MLL) together, so that (MHL, MLL)=(0, 0) or (1, 1) at the end of each cycle of the clock signal  62 , depending on the level of the pre-charge signal  64  (1 or 0). This is the non-match condition. 
     On the other hand, if Dl line  66  and the DS node  72  are the same (Dl=DS), the cell node  70  is low and the match-high line  36  is cut off from the match-low line  38 . Since the pre-charge signal  64  always switches once during each clock cycle, the match-high line  36  is charged high and the match-low line  38  is discharged low during the clock cycle and (MHL, MLL)=(1, 0) at the rising edge of the next clock cycle. This is therefore the match condition. 
     Since the pre-charge signal  64  switches at half the rate of the clock signal  62 , the dynamic power consumed by large devices, like the pre-charge devices  40 ,  42 , and large capacitances like the match-high line  36  and the match-low line  38  are all cut by half, as compared with the conventional method in which pre-charge and detection occur during every clock cycle. As power consumption in CAM is closely related to the pre-charge rate, this aspect of the inventive CAM  10  accordingly provides yet more substantial power savings. 
     As those skilled in the electronic arts are well aware, reducing the rate or frequency of operation of circuits, all or in part, provides a veritable cornucopia of benefits. For instance, lower speed components may be used and signal interference is reduced between components and the circuit environment, generally. Additionally, when both power savings and frequency reduction can be concurrently achieved, the benefit is often synergistically increased. 
     Next let us consider a sensing sub-circuit  80  consisting of devices  82 ,  84 ,  86 , and  88 . Notice that the match-low line  38  follows the Dl line  66  or the DlB line  68  through two gates, the Dl pass gate  90  and the match gate  92 , or the DlB pass gate  94  and the match gate  92 . Its high level is therefore V DD −2*V TN  where V TN  is the threshold voltage of an NMOS device (in this embodiment), and is typically rather low. For device  86  a natural NMOS device having a threshold voltage that is nominally positive, but around “0,” is preferably used. With device  84  and device  86  an AND function controlled by the match-high line  36  is formed at an AND node  96 . To avoid a worst case in which the threshold voltage of the device  86  goes slightly negative, and hence never cuts off, the device  82  is preferably a PMOS type and is used to supply power during sensing, while the device  88  post-conditions (discharges) the AND node  96  after sensing. Care must be taken that the switching threshold of an inverter  98  lies within the switching window of the AND node  96  under all circuit conditions. As shown in FIG. 6, a resulting latched voltage  100  (VO) goes high when a match occurs. 
     Using a natural device for device  86  is a recommendation. However, say, in a case where such is not available, more conventional methods can also be considered, including using PMOS devices for pass-gates  90 ,  94 . 
     In summary, as has been described above particularly with respect to FIG. 4, the present CAM  10  can be configured, as desired, to function as a binary or ternary type CAM. And as has also been described, particularly with respect to FIG. 5-6, the CAM  10  provides substantial power savings in two particular regards. It should be appreciated, however, that the necessarily limited examples which can be presented in a discussion such as this cannot depict all possible details of all possible embodiments of the invention. For example, the examples used have employed SRAM type memory cells, but those skilled in the art will appreciate that the present invention has applicability with many other types of memory. Similarly, while a dual-function binary-ternary CAM has been presented as the inventor&#39;s presently preferred embodiment, there is no reason why the approaches to power saving and sub-circuit frequency reduction taught herein cannot also be applied to single function CAM of either binary or ternary types. Indeed, since match detection is widely used with circuitry other than CAM, there is no reason why the approaches to match detection taught herein cannot also be used with such other circuitry. 
     FIG. 7 is a block diagram showing the SRAM cells  16  of the CAM  10  extended in to the context of a larger CAM  102  having a plurality of rows  104 . Accordingly, it can be seen that the present invention can be applied in large CAM based memory schemes, as is increasingly common. 
     Finally, FIG. 8 is a series of schematic diagrams showing examples of generally conventional binary and ternary cells improved by addition of a match detection scheme according to the present invention. Internal details of binary CAM cells are shown in items (a) and (b), and internal details of ternary CAM cells are shown in items (c), (d) and (e). 
     Accordingly, while various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the invention should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 
     INDUSTRIAL APPLICABILITY 
     The present CAM  10  is well suited for use in a wide variety of applications. Comparing FIG. 1 (background art) and FIG. 4 reveals that when used as binary CAM, the CAM  10  employs a different gate arrangement than is conventional. By employing three gates in association with each memory cell, power savings is achieved in two particular manners. Firstly, the lines (MHL, MLL) used for match detection are floated, so that the input data lines do not have to be grounded. Secondly, the lines (MHL, MLL) used for match detection are operated at half of the overall clock frequency. Comparing FIG. 2 (background art) and FIG. 4 similarly reveals that when used as ternary CAM, the CAM  10  also employs a non-conventional gate arrangement. But again, this provides the dual types of power savings discussed above. Comparing FIG.  3  and FIG. 4 reveals yet a further distinction. As discussed elsewhere, FIG. 3 depicts a logical approach to implementing a binary-ternary CAM. However, rather than use four gates per memory cell as in FIG. 3, the invention may employ only three gates per cell and be used as a binary-ternary CAM which additionally provides the noted power savings. Accordingly, the inventive CAM  10  may be implemented as a binary CAM, a ternary CAM, or a selectively configurable binary-ternary CAM and it may provide substantial benefits in all of these roles. 
     In straight forward manner the principals of the CAM  10  may be extended and used in larger schemes, such as the CAM  102  of FIG. 7 in which a number of cells are arranged into multiple rows, each having a common match sensing sub-circuit. Arrangement in columns or into sub arrays is likewise possible. The CAM  10  may also be implemented in a wide range of conventional memory and logic types, using essentially conventional manufacturing processes. This, in turn, facilitates immediate application of the inventive CAM  10  in discrete integrated circuits and modules, or as a sub-part of larger assemblies such as Systems on a Chip (SoC). 
     The range of ultimate uses for CAM is also wide, and growing very rapidly, so a detailed discussion is not possible. But one particularly promising area of use for the present CAM  10  that deserves mention is in portable devices. Such devices often require sparing use of power and the inventive CAM  10  clearly excels over conventional CAM in this regard. 
     For the above, and other reasons, it is expected that the CAM  10  of the present invention will have widespread industrial applicability. Therefore, it is expected that the commercial utility of the present invention will be extensive and long lasting.