Abstract:
The invention detects multiple matches between search and stored data in high-density content addressable memories. An input signal is derived from the matchlines, such that the input signal starts discharging form a predetermined precharge level towards a discharge level determined by the number of match conditions. A reference signal is generated such that it starts to discharge at the same time from the same precharge level towards a reference level which falls between the two discharge levels corresponding to single and double match condition. A latching differential amplifier is activated shortly thereafter to compare the input signal with the reference signal and thereby provide an indication whether a multiple single or no match occurs on the matchlines, after which the amplifier is deactivated. The disclosed circuit features a relatively fast detection with low current consumption.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to semiconductor memory devices and more specifically to detecting multiple matches between search and stored data in high-density Content Addressable Memory (CAM) systems.  
         BACKGROUND  
         [0002]    In many conventional memory systems, such as random access memory, binary digits (bits) are stored in memory cells, and are accessed by a processor that specifies a linear address that is associated with the given cell. This system provides rapid access to any portion of the memory system within certain limitations. To facilitate processor control, each operation that accesses memory must declare, as a part of the instruction, the address of the memory cell/cells required. As the amount of memory increases the number of bits per instruction needed to reference a memory cell also increases. This diminishes the efficiency of the system. In addition to this drawback, standard memory systems are not well designed for a content based search. Content based searches in standard memory require a software based algorithmic search under the control of the microprocessor. These searches are neither quick nor efficient in using processor resources.  
           [0003]    To overcome these inadequacies an associative memory system called Content Addressable Memory (CAM) has been developed. CAM allows cells to be referenced by their contents, so it first found use in lookup table implementations such as cache memory subsystems and is now rapidly finding use in networking systems. CAM&#39;s most valuable feature is its ability to perform a search and compare as a single operation. Specified user data and stored data can be compared and match and mismatch results can be returned by allowing the contents of a comparand register to be compared directly with all addresses held in a given memory system&#39;s address space simultaneously.  
           [0004]    In addition to the rows and columns of memory cells, that are common to most memory systems, CAM also has a matchline and a matchline detection circuit (DTC). The DTC is a sense amplifier that senses the changes in the logic state of a matchline for the cells in that row. The DTC detects a match or a mismatch during a CAM search and compare operation. Though in many early applications it was not a concern, presently the detection of multiple matches on a matchline is of great interest. Distinguishing between a match and mismatch condition is as simple as declaring a threshold voltage or current level between the match and mismatch levels, and determining on which side of the threshold the matchline level was. This cannot be done with the presence of multiple matches, as the effect that it has is simply to increase the decay rate of the voltage or current level.  
           [0005]    In a typical CAM system, various memory cells, arranged in a row, are connected by a matchline to a detection circuit (DTC). Running orthogonal to the matchlines are search lines which carry loaded search data to compare with stored data in the columns of CAM cells. Typically, matchlines are precharged to a matchline precharge voltage, for example, VDD. If a search result is a match, i.e. search data on the search line matches stored data in the CAM cell, then there is no change in the matchline voltage level, i.e. it remains at its precharge state, VDD. In the case of a mismatch, a direct current path is established between the respective matchline and VSS via the CAM cell and the voltage level on that matchline begins to decrease. This voltage drop is then detected by the respective DTC. Thus to detect a mismatch, each DTC, associated with each matchline, must detect whether a differing voltage level develops as a result of the search and compare operation. However, it is advantageous to determine whether a detected match has resulted from a single row or from multiple rows.  
           [0006]    In order to detect multiple matches, otherwise referred to as hits, the problem encountered is one of detecting different voltage levels which can develop on a line. U.S. Pat. No. 5,012,448 to Matsuoka et al describes a method for detecting a voltage level in a multilevel read only memory (ROM) cell. As shown in FIG. 1, the method and apparatus described therein use a pair of CMOS inverters that are operated as small signal amplifiers with an NMOS device used for current sensing. Both reference and active side sources are also fed into a current mirror. Though this design accomplishes the desired task of multilevel sensing, it does so at the cost of increased heat dissipation, and a limited output voltage swing. This increase in heat is a direct result of high power consumption by the discrete components and is a limiting factor in the attempt to shrink the component size so that it occupies less circuit area.  
           [0007]    U.S. Pat. No. 5,446,686 to Bosnyak and Santoro describes a method and an apparatus for detecting multiple address matches in a CAM. The described apparatus, as shown in FIGS. 2 a ,  2   b  and  2   c  uses a pair of ‘hit’ and ‘dhit’ lines that are connected to the power supply terminal (VDD) through a pair of pull-up PMOS transistors. A set of NMOS devices N 0 -N 3  on the ‘hit’ line operate in saturation since the voltages across the drain and source terminals are always above a threshold compared to the gate-to-source voltage (Vgs) of these devices. The reference transistor has a width to length ratio that is 1.5 times in dimension relative to any one of the NMOS devices N 0 -N 3  to provide a 1.5 times saturation current. The reference transistor Nref also operates in its saturation region. The Vgs of the Nref device is generated using a rather complex reference circuit to ensure appropriate compensation for temperature and voltage characteristics. Furthermore, the described apparatus uses a comparator to compare the current difference generated on the ‘hit’ and ‘dhit’ lines for a single and multiple match generation. The comparator is a two-stage area-intensive component that uses bipolar transistors and resistors for achieving the desired output.  
           [0008]    One limitation of this application is as follows. In order to operate the NMOS devices N 0 -N 3  and Nref in saturation, the pull-up PMOS devices connecting the ‘hit’ and ‘dhit’ lines to the power supply terminal have to have relatively low resistivity such that the voltage drop is not significant across them while the devices N 0 -N 3  and Nref turn on during sensing. This is required to ensure that the voltage across the drain-to-source channel does not change significantly. This is achieved at the cost of relatively high current consumption through the ‘hit’ and ‘dhit’ lines during the entire operation, which presents a limitation for multiple-match detection within high density CAMs.  
           [0009]    Furthermore, to operate Nref in saturation, a relatively complex compensating reference circuit is required to ensure compensation for temperature and voltage fluctuations in order to develop a constant reference current. The comparator itself is area consuming and consumes constant current. The requirement of bipolar devices presents a limitation for use in dynamic random access memories (DRAM), which are entirely CMOS based.  
           [0010]    In conclusion, the increased demand for large memory systems, and the desire to efficiently use these systems for more than simple sequential access requires a multilevel sense detector that in an ideal embodiment combines fast sensing with low power consumption.  
         SUMMARY OF THE INVENTION  
         [0011]    It is an object of this invention to provide a sense amplifier of a relatively high efficiency in detecting different voltage levels on a multiple-hit-line of a content addressable memory (CAM). It is another object of the invention to achieve multiple match detection using a relatively low power and comparatively low circuit area for detecting the differences between having multiple hits and non-multiple (zero or a single) hit.  
           [0012]    Therefore, in accordance with one aspect of the present invention, there is provided an apparatus, specifically a signal detection circuit, that comprises a differential amplifier having a sense node and a reference node, an activation means for switching the differential amplifier between an inactive phase and an active phase, an input means for providing the sense node with an input signal permitted to start changing from a predetermined precharge level towards a discharge level during said inactive phase, and a reference means for providing the reference node with a reference signal that starts changing from said precharge level towards a predetermined reference level during said inactive phase, whereby the differential amplifier provides a complementary output corresponding to the difference between the discharge and the reference levels when the difference between the input signal and the reference signal exceeds a detectable level during the active phase.  
           [0013]    In accordance with one embodiment of the present invention the differential amplifier comprises a latching circuit for providing the complementary output. Preferably the input and reference signals start changing from the precharge level at approximately the same time, whereas the active phase begins after a predetermined time delay from the time the input and reference signals start changing from the precharge level, said time delay being sufficient to permit building up a detectable difference between the input and reference signals.  
           [0014]    The signal detection circuit can be used for detecting a multiplicity of n match conditions on a plurality of m matchlines in a high density content addressable memory, where 2≦n&lt;m wherein the input signal is derived from the matchlines. To achieve this, the input means is coupled to a plurality of m matchlines, such that the discharge level is determined by the number of the match conditions, and the reference level falls between discharge level corresponding to n−1 match conditions and that corresponding to n match conditions. In a transistor circuit embodiment, the input means comprises a plurality of similarly sized m matchline devices connected in parallel to the sense node, wherein each matchline device is responsive to a corresponding one of the m matchlines and the reference means comprises a reference device and a plurality of m dummy devices connected in parallel to the reference node, wherein each dummy device corresponds to and has substantially similar electrical characteristics to one of the m matchline devices, and wherein the reference device has a size between that of first and second parallel combinations of n−1 and n matchline devices respectively. In practical CAM implementations n=2. The reference device is turned on by an enable signal to permit a change in the reference signal from the precharge level, at approximately the same time as the matchline devices receive their respective match data. The sense node and the reference node are separately coupled to a first power supply terminal through a pair of respective clamping devices having substantially similar electrical characteristics to one another and a sufficient size for precharging the sense node and the reference node during the inactive phase and for preventing the sense node and the reference node from reaching a voltage level lower than the threshold voltage of any one of the matchline devices. The clamping devices are preferred to have respective sizes suitable for limiting the current flow through the matchline devices, the dummy devices and the reference device substantially below saturation.  
           [0015]    Another embodiment of the present invention further comprises a logic circuit for turning off the pair of clamping circuits upon the differential amplifier switching to a latched state during the active phase. The activation means preferably comprises a circuit for enabling the differential amplifier only during the active phase.  
           [0016]    In accordance with another aspect of the present invention there is provided an apparatus, specifically a signal detection circuit for detecting a multiplicity of n match conditions occurring among a plurality of m matchlines within a CAM array where 2≦n&lt;m, comprising a differential amplifier having a sense node for receiving a time-varying input signal and a reference node for receiving a time-varying reference signal, activation means for switching the differential amplifier between an inactive phase and an active phase, precharge means for precharging the sense node and the input node to a predetermined precharge level, during the inactive phase, input means for deriving the input signal from the m matchlines, such that said input signal starts changing during the inactive phase from the precharge level to a discharge level determined by the number of the match conditions and reference means for generating the reference signal which starts changing during the inactive phase from the precharge level towards a reference level falling between the discharge levels corresponding to n and n−1 match conditions wherein the differential amplifier compares the input signal with the reference signal during the active phase and provides a corresponding complementary output that indicates whether the number of match conditions is below n or whether it is equal to or exceeds n.  
           [0017]    In accordance with yet another aspect of the present invention there is provided a method of detecting a multiplicity of n match conditions occurring among a plurality of m matchlines within a CAM array, where 2≦n&lt;m, comprising the steps of providing a differential amplifier, turning said differential amplifier into an inactive state, deriving an input signal from the m matchlines, such that said input signal starts changing from a predetermined precharge level towards a discharge level determined by the number of the match conditions, generating a reference signal that starts changing from the precharge level towards a reference level falling between the discharge level corresponding to n match conditions and that corresponding to n−1 match conditions, and turning the differential amplifier into an active state for comparing the input signal with the reference signal to obtain an indication whether the number of match conditions is below n or whether it is equal to or exceeds n.  
           [0018]    The invention avoids the otherwise conventional requirement for operating in saturation mode the devices driving the multiple hit lines and the reference line, thereby minimizing power consumption in the overall circuit.  
           [0019]    The differential amplifier detects a difference between voltages on the multiple-hit line and the reference line, as opposed to the prior art method of current detection. Thus the circuit consumes relatively negligible current without requiring additional stages for boosting signal levels to be detected, as in prior art solutions.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0020]    Exemplary embodiments of the invention will now be further described with references to the drawings in which same reference numerals designate similar parts throughout the figures thereof, and wherein:  
         [0021]    [0021]FIG. 1 shows a conventional sense amplifier for a multilevel sensing;  
         [0022]    [0022]FIG. 2 a ,  2   b , and  2   c  show a prior art apparatus for detecting multiple address matches in a CAM;  
         [0023]    [0023]FIG. 3 is a schematic diagram illustrating the structure of a multiple-hit-line to be sensed and a reference multiple-hit-line; as part of an embodiment of this invention;  
         [0024]    [0024]FIG. 4 is a schematic diagram illustrating a differential amplifier and associated circuits for use with the structure shown in FIG. 3, as a first embodiment of the present invention;  
         [0025]    [0025]FIG. 5 is a schematic diagram illustrating a differential application and associated circuits for use with the structure shown in FIG. 3, in a second embodiment of the present invention;  
         [0026]    [0026]FIG. 6 is a timing diagram of the multiple match detection operation performed by the first embodiment;  
         [0027]    [0027]FIG. 7 is a timing diagram of the multiple match detection operation performed by the second embodiment; and  
         [0028]    [0028]FIGS. 8 a ,  8   b  and  8   c  are timing diagrams to illustrate the three cases of multiple, single and no hit respectively in the second embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    [0029]FIG. 3 illustrates the structure of a multiple hit line MHL and a reference line MHLb as part of one embodiment of the present invention. The multiple hit line MHL is coupled to a number ( 128  in this embodiment) of matchlines ML 1 -ML 128 , in parallel, via NMOS matchline devices MD 1 -MD 128 , which all have similar dimensions and operating characteristics. Each of these matchline devices MD 1 -MD 128  is setup so that its source is connected to VSS, its drain is connected to MHL, and its gate is connected to the respective matchline thereby allowing the voltage level on MHL to be controlled by the multiple pull-down paths provided by the matchline devices MD 1 -MD 128 . The number of matchline devices used in an array of CAM cells is equal to the number of rows of CAM cells in that array, which in this embodiment is  128 . In turn, each matchline is connected to a row of CAM cells, which use the matchline to report the incidence of a hit (or a match) and a miss (or a mismatch).  
         [0030]    In the above structure, each of the matchlines ML 1 -ML 128  carries match or mismatch information about the cells in its row. In the event of a mismatch between search data and stored data in a CAM cell in a selected row, the corresponding matchline is pulled down to VSS (low), from a precharged state of VDD (high) by a corresponding matchline sense amplifier (not shown) whereas a match between search and stored data will result in that matchline remaining at the precharged VDD level. As long as at least one matchline is at the high level, MHL will be discharged towards VSS through at least one matchline device. Having more than one matchline reporting a hit (i.e. remaining at VDD) results in MHL discharging towards VSS faster. In the event that no matchline reports a hit, MHL remains at VDD because all the matchlines are low and none of the matchline devices turn on. The purpose of this embodiment is to dintinguish a multiple hit condition from a non-multiple hit condition where only one or no match occurs on MHL.  
         [0031]    The second line used by this embodiment is a reference line denoted by MHLb. This reference line has an equal number ( 128  in this embodiment) of NMOS dummy devices DD 1 -DD 128  as the number of matchline devices MD 1 -MD 128 , connected in a similar parallel manner, except that the gates are connected to the VSS line rather than the actual matchlines ML 1 -ML 128 , thereby each dummy device simulating a mismatch condition. These dummy devices DD 1 -DD 128  are chosen so as to have electrical characteristics similar to the matchline devices MD 1 -MD 128  for this purpose, and thus provide the same parasitic capacitance and resistive loading on MHLb as is present on MHL for noise reduction. As a result, any voltage or temperature variations will affect both MHL and MHLb lines equally, and there is no need for a voltage and temperature compensation using a reference generator or similar circuits for addressing this problem. In addition, there is an NMOS reference device RD on MHLb whose channel width falls between one and two times (in this embodiment 3/2) the channel width of any one of the matchline devices MD 1 -MD 128 . The gate of this reference device RD is connected to a matchline sense enable signal MLSEN which also activates the matchline sense amplifiers (not shown) each of which senses and latches relevant matchline data on its corresponding matchline. Match data is established on the matchlines ML 1 -ML 128  within a very short time after MLSEN is driven to VDD. Subsequently, in case of a match, one of the gates of the matchline devices MD 1 -MD 128  will be driven to VDD at approximately the same time the gate of the reference device RD is driven to VDD by MLSEN. The timing signal MLSEN is used to ensure that both MHL and MHLb start to change their voltages almost simultaneously. This allows for the current through MHL and MHLb to be controlled and limited, as opposed to the prior art solutions, wherein a constant current is continuously drawn.  
         [0032]    In this first embodiment, MHL and MHLb are both clamped to VDD using two PMOS voltage clamping devices, shown in FIG. 4 as M 1  and M 2 . For a single hit, MHL will be pulled down, after a brief settling time, by the matchline that indicates the hit to a voltage level slightly higher than MHLb. This is due to the fact that the matchline device that indicates the single hit will be on simultaneously with the clamping device M 1  thereby setting up a voltage divider. Since the width of the reference device RD is 3/2 times the width of any of the matchline devices MD 1 -MD 128 , MHLb will be driven lower than MHL. In the case of two or more hits (matches), MHL will reach a voltage level lower than that of MHLb, after the brief settling time, since the combined width of the matchline devices whose gates are at VDD is larger than that of the reference device RD. For the case of no matches in the array, the gates of all the matchline devices MD 1 -MD 128  are at VSS, thus MHL will stay at the precharged level while MHLb is driven to a lower voltage.  
         [0033]    [0033]FIG. 4 shows a differential amplifier DA and associated circuits used with the structure shown in FIG. 3 as a first embodiment of this invention. As indicated above, the PMOS clamping devices M 1  and M 2  are used for clamping MHL and MHLb to VDD. The gates of M 1  and M 2  are connected to VSS making both devices conduct as soon as the voltage level on MHL and MHLb drops below VDD-Vtp (the threshold voltage of a PMOS device). The source terminals of M 1  and M 2  are connected to VDD and the drain terminal of M 1  is connected to MHL while the drain of M 2  is connected MHLb. The sizes of both M 1  and M 2  are so chosen as to provide enough current to clamp MHL and MHLb to VDD but M 1  and M 2  should have such a size as not allow MHL and MHLb to reach a voltage level below Vtn, the threshold voltage of NMOS first and second sensing devices M 3  and M 4  used as part of the differential amplifier DA. With this configuration, each combination of a PMOS device (M 1  or M 2 ) with at least one NMOS device (MD 1 -MD 128  or RD, DD 1 -DD 128  respectively) provide a resistive voltage divider that develops the respective voltage levels on both MHL and MHLb to be detected by the differential amplifier DA. In order to achieve this voltage divider configuration the PMOS devices are relatively small thereby reducing the current flow therethrough. The ratio of transistor sizes (channel widths) of M 1  and any of the matchline devices MD 1 -MD 128  shown in FIG. 3, determines the voltage level to which MHL will get discharged when a match occurs. Similarly, the ratio of transistor sizes of M 2  and RD determines the voltage level MHLb will reach when RD turns on. These ratios are typically insensitive to temperature variations thus eliminating the need for specific measures for temperature compensation.  
         [0034]    A latching circuit of four latch transistor devices M 6 -M 9  together with a pair of sensing devices M 3  and M 4  form the latching differential amplifier DA for sensing the level difference developed between MHL and MHLb. An activation circuit that switches the differential amplifier DA between an inactive phase and an active phase is formed by three activation transistor devices M 5 , M 10 , and M 11 , the gates of which are connected together to an activation signal SHL. The inactive phase coincides with SHL being low and the MHL and MHLb being precharged to VDD, whereas the active phase coincides with SHL being high and MHL and MHLb being sensed. SHL is turned high after MLSEN is turned high by a specific time delay sufficient to allow the voltage difference between MHL and MHLb to reach a level detectable by the differential amplifier DA. This is to avoid DA responding to spurious voltages while MHL and MHLb are discharging towards their respective target levels. It is to be noted that even during the active phase, the differential amplifier DA being of a latching type draws current only during the switching of the latching circuit M 6 -M 9 , and that such switching occurs rather rapidly due to the positive feedback connections within the CMOS latching circuit M 6 -M 9 .  
         [0035]    The circuit of FIG. 4 also includes an additional conventional output latch circuit LC driven by the LATCH signal through a switch SW which is turned on and off by a delay unit DU in response to the SHL signal. The delay unit provides two control signals SHL_DLY and SHL_DLYb which are time delayed versions of SHL. When SHL goes to a low voltage level the path to the latch circuit LC through the switch SW is disabled, while a high voltage level MSHL enables the latch circuit.  
         [0036]    [0036]FIG. 6 illustrates the relative timing sequence of the detection operation performed by the first embodiment combining the circuits of FIGS. 3 and 4. During the inactive phase when SHL is at VSS, first and second output nodes of the differential amplifier out and outb, respectively, are both precharged to VDD by M 10  and M 11 . SHL is driven to VDD to switch the differential amplifier DA into the active phase at the time when both MHL and MHLb have reached their respective voltage levels as determined by the match data on the matchlines ML 1 -ML 128  which ensure sufficient differential sensing margin. After this predetermined delay, SHL goes to VDD and M 5  turns on with its gate driven to VDD. In the case of a multiple hit, MHL settles to a lower voltage than MHLb, and the first sensing device M 3  being driven by MHL is turned on slightly while the second sensing device M 4  being driven by MHLb is turned on strongly. This provides a higher switching current through M 4  and M 7  which are connected together in series, and a lower current through M 3  and M 6  which are also connected together in series. The resulting differential current in the two paths rapidly switches, i.e. flips the latching circuit due to the positive feedback between its four latch transistors M 6 -M 9 . As a result, the first output node out is driven to VSS while the second output node outb stays high. Two pairs of output buffer transistor devices M 12 , M 13  and M 14 , M 15  provide first and second inverting buffers BUF 1  and BUF 2  respectively between the first and second output nodes out and outb, producing first and second respective output signals LATCH and LATCHb to be used in subsequent signal processing. In the case of a single match or no match, MHL is at a higher voltage level than MHLb, causing the first sensing transistor M 3  to turn on harder than the second sensing transistor M 4 . A differential current is generated in an opposite manner as the second output node outb is driven down to VSS while the first output node out stays high. The first output node out drives the first inverting buffer BUF 1  to provide the first output signal LATCH, which holds the information for further processing. It should be noted that the activation signal SHL remains at VDD only for the time duration it takes the sensed data to drive the latching circuit M 6 -M 9  (active phase). Since the first output node out provides the information on multiple hits in a relatively short time, current consumption through M 1  and M 2  is also for a relatively short time. This is clearly shown in FIG. 6, which illustrates the timing of a multiple-hit detection operation. As a result, the current flowing through the differential amplifier DA is only present during the switching of the latching circuit M 6 -M 9 .  
         [0037]    [0037]FIG. 5 shows a differential amplifier and associated circuits used with the structure shown in FIG. 3 as a second embodiment of the present invention wherein separate first and second PMOS precharge devices M 16  and M 17  respectively are provided in addition to the clamping devices M 1  and M 2 . The differential amplifier circuit consisting of M 3 , M 4  and M 6 -M 9  is the same as that shown in FIG. 4. Another difference between the first and second embodiments is that the pair of voltage clamping devices M 1  and M 2  have their respective gates controlled by the output of an inverter INV whose input gate is driven by the output of a NOR logic gate NLG. A first input of NLG is connected to the first output signal LATCH provided by the first inverting buffer BUF 1  formed by the first pair of buffer devices M 12  and M 13 , and a second input of NLG is connected to the second output signal LATCHb provided by the second inverting buffer BUF 2  formed by the second pair of buffer devices M 14  and M 15 . This connection of the gates of the voltage clamping devices M 1  and M 2  provides self-timing to turn off M 1  and M 2  as soon as the differential amplifier DA has latched the data and switched from its precharge state. This provides further reduction in power consumption since M 1  and M 2  are turned on only for a very limited time during the sensing interval.  
         [0038]    Yet another difference from the circuit of the first embodiment shown in FIG. 4 is that instead of using the clamping devices M 1  and M 2  to precharge MHL and MHLb, the precharge devices M 16  and M 17  are used to precharge the MHL and MHLb lines using the PREb-MHL signal. Therefore, in this second embodiment as shown in FIG. 5, the voltage clamping devices M 1  and M 2  are used only to provide, the logic levels on MHL and MHLb and not for precharging as used in the first embodiment. If M 1  and M 2  were used for precharging as in the first embodiment, the transient current required for precharging MHL and MHLb through any PMOS transistor would be larger than the current needed to develop the voltage levels on these two lines, since in the second embodiment, M 1  and M 2  can be minimum sized transistors. Therefore, if M 1  and M 2  were used to perform the precharge, their required larger device sizes would result in a larger DC current power dissipation when developing the voltage levels on MHL and MHLb during the active phase. In order to address this larger power dissipation, the second embodiment of FIG. 5 uses the precharge transistors M 16  and M 17  to perform the precharge more efficiently.  
         [0039]    [0039]FIG. 7 illustrates the detection operation performed by the second embodiment combining the circuits of FIGS. 3 and 5 in the case of a single hit. As shown in ths figure, as soon as a difference between MHL and MHLb is detected by the differential amplifier DA, both lines begin to be discharged towards VSS by virtue of the feedback circuit from the two amplifier outputs out and outb vial the logic gate NLG and the inverter INV.  
         [0040]    [0040]FIGS. 8 a ,  8   b  and  8   c  illustrate the three possible outcomes of the search and compare operation on the circuitry described above for the first and second embodiments in FIG. 6. FIG. 8 a  illustrates the case of a multiple hit where the search and compare operation has resulted in more than one hit (or match) between search data and stored data in a particular row of the CAM. Note that after a brief period of time, MHL&#39;s voltage level decreases substantially below that of MHLb, as described earlier, due to the combined pull-down action of multiple matchline devices from ML 1 -ML 128  (shown in FIG. 3) being turned on simultaneously. These matchline devices corresponding to the multiple hits have a combined width which is greater than the width of the reference device RD and therefore pull MHL to a voltage level lower than that of MHLb. Detection of the voltage difference between MHL and MHLb occurs once the two lines are sufficiently far apart in voltage to be sensed by the sensing devices M 3  and M 4 . Shortly thereafter, the second output node outb obtains a corresponding signal from the differential amplifier as described above, MHL and MHLb are both discharged to VSS, and then MHL and MHLb are precharged once again in preparation for the next sensing operation during the next active phase. Similarly, in FIG. 8 b , the case of a single hit is illustrated. Note that the outcome is opposite to that shown in FIG. 8 a , with MHLb being pulled to a voltage level lower than that of MHL since the single matchline transistor that experiences a hit on its associated matchline has a width which is less than that of the reference transistor RD. The rest of the sensing operation proceeds as described with reference to FIG. 8 a . Finally, in FIG. 8 c , the case of no hits is illustrated. Here both MHL and MHLb remain at their precharged state since none of the matchline devices of FIG. 3 are enabled. As can be seen from these three illustrative timing charts, the present invention can rapidly distinguish between the three cases of multiple, single or no hits in a CAM using relatively little power, thereby permitting the fabrication of CAM arrays with large cell densities.  
         [0041]    Of course, numerous variations and adaptations may be made to the particular embodiments of the invention described above, without departing from the spirit and scope of the invention, which is defined in the claims. Whereas the embodiments described above are particularly relevant to high density CAM arrays requiring detection of multiple hits, the invention is also applicable to the detection of data lines having a time varying voltage level with no pre-set reference.