Abstract:
Segmented CAM arrays are provided with dual-capture match line signal repeaters that support high speed pipelined search operations. A dual-capture match line signal repeater may extend between xR and xS segments of CAM cells within a respective row. This repeater provides high speed operation by quickly accessing the state (match or miss) of a match line segment when a corresponding segment of CAM cells connected to the match line segment undergo a respective stage of a pipelined search operation. If the match line segment is initially assessed as having a match signal thereon, then that match signal is passed to a next higher match line segment within the same row and a next stage search operation is commenced. However, if the match line segment is erroneously assessed as having a match signal thereon, when a miss condition was actually present in the corresponding segment of CAM cells, then the signal repeater will operate to capture a late miss signal and pass that late miss signal to the next higher match line segment to correct an error.

Description:
FIELD OF THE INVENTION 
     The present invention relates to integrated circuit memory devices and methods of operating same, and more particularly to content addressable memory devices and methods of operating same. 
     BACKGROUND OF THE INVENTION 
     In many memory devices, including random access memory (RAM) devices, data is typically accessed by supplying an address to an array of memory cells and then reading data from the memory cells that reside at the supplied address. However, in content addressable memory (CAM) devices, data is not accessed by initially supplying an address, but rather by initially applying data (e.g., search words) to the device and then performing a search operation to identify one or more entries within the CAM device that contain data equivalent to the applied data and thereby represent a “match” condition. In this manner, data is accessed according to its content rather than its address. Upon completion of the search operation, the identified location(s) containing the equivalent data is typically encoded to provide an address (e.g., CAM array block address+row address within a block) at which the matching entry is located. If multiple matching entries are identified in response to the search operation, then local priority encoding operations may be performed to identify a location of a best or highest priority matching entry. Such priority encoding operations frequently utilize the relative physical locations of multiple matching entries within the CAM device to identify a highest priority matching entry. An exemplary CAM device that utilizes a priority encoder to identify a highest priority matching entry is disclosed in commonly assigned U.S. Pat. No. 6,370,613 to Diede et al., entitled “Content Addressable Memory with Longest Match Detect,” the disclosure of which is hereby incorporated herein by reference. The &#39;613 patent also discloses the use of CAM sub-arrays to facilitate pipelined search operations. Additional CAM devices are described in U.S. Pat. Nos. 5,706,224, 5,852,569 and 5,964,857 to Srinivasan et al. and in U.S. Pat. Nos. 6,101,116, 6,256,216, 6,128,207 and 6,262,907 to Lien et al., the disclosures of which are hereby incorporated herein by reference. 
     CAM cells are frequently configured as binary CAM cells that store only data bits (as “1” or “0” logic values) or as ternary CAM cells that store data bits and mask bits. As will be understood by those skilled in the art, when a mask bit within a ternary CAM cell is inactive (e.g., set to a logic 1 value), the ternary CAM cell may operate as a conventional binary CAM cell storing an “unmasked” data bit. When the mask bit is active (e.g., set to a logic 0 value), the ternary CAM cell is treated as storing a “don&#39;t care” (X) value, which means that all compare operations performed on the actively masked ternary CAM cell will result in a cell match condition. Thus, if a logic 0 data bit is applied to a ternary CAM cell storing an active mask bit and a logic 1 data bit, the compare operation will indicate a cell match condition. A cell match condition will also be indicated if a logic 1 data bit is applied to a ternary CAM cell storing an active mask bit and a logic 0 data bit. Accordingly, if a data word of length N, where N is an integer, is applied to a ternary CAM array block having a plurality of entries therein of logical width N, then a compare operation will yield one or more match conditions whenever all the unmasked data bits of an entry in the ternary CAM array block are identical to the corresponding data bits of the applied search word. This means that if the applied search word equals {1011}, the following entries will result in a match condition in a CAM comprising ternary CAM cells: {1011}, {X011}, {1X11}, {10X1}, {101X}, {XX11}, {1XX1}, . . . , {1XXX}, {XXXX}. 
     Conventional techniques to reduce power consumption within CAM devices are disclosed in U.S. Pat. Nos. 6,191,969 and 6,191,970 to Pereira. In particular, the &#39;969 patent discloses a CAM array having CAM cells therein that include a discharge circuit connected between each cell and a fixed ground potential. Each of the discharge circuits includes a control terminal coupled to receive a control signal indicative of the logical state of a match line segment in a respective row. These discharge circuits may be turned off to prevent discharge of respective match line segments during a search operation. U.S. Pat. No. 6,243,280 to Wong et al. also discloses a technique to selectively precharge match line segments during a search operation. However, the match line precharge circuit described in the &#39;280 patent may suffer from relatively poor speed performance during a search operation. This poor speed performance may result whenever a wider timing margin is used in each stage of a search operation to account for worst case timing conditions. These worst case timing conditions can occur when only one CAM cell within a segment of CAM cells indicates a “miss” condition while all other CAM cells in the same segment indicate “match” conditions. Thus, in the &#39;280 patent, the timing margin associated with each stage of a search operation should be sufficient to account for the presence of a “worst case” miss signal before a decision can be made on whether to precharge a match line segment associated with a next segment of CAM cells. U.S. Pat. No. 6,430,074 to Srinivasan discloses a precharge circuit that uses selective look-ahead match line precharging techniques. The following patents also disclose subject matter relating to match line precharging: U.S. Pat. Nos. 6,101,115; 6,125,049; 6,147,891; 6,166,939; 6,240,001; 6,262,929 and 6,343,029. 
     U.S. Pat. No. 5,517,441 to Dietz et al. discloses the use of inverters and pull-down transistors to pass match line signals from one match line segment to another match line segment during a search operation. U.S. Pat. Nos. 5,446,685 and 5,598,115 to Holst also disclose the use of rail-to-rail (i.e., Vdd-to-Vss) pulsed ground signals during search operations. These pulsed ground signals may facilitate selective match line discharge operations. 
     A conventional match line signal repeater is illustrated by  FIG. 1 . In particular,  FIG. 1  illustrates a segmented row of CAM cells  10  that utilizes serially connected inverters  11 – 14  to pass match line signals from lower match line segments to upper match line segments during a search operation. The segmented row  10  is illustrated as including three equal-length match line segments (x10 MLa, x10 MLb and x10 MLc) that are each electrically coupled to respective segments of CAM cells  12   a ,  12   b  and  12   c . Prior to commencement of a search operation, a plurality of active low precharge signals (PRECHARGE 1 – 3 ) are switched high-to-low in sequence during a precharge time interval. When this occurs, PMOS precharge transistors P 1 –P 3  turn on in sequence and precharge the three match line segments to logic 1 levels (e.g., Vdd). During this precharge time interval, pairs of differential comparand data lines (not shown), which are electrically connected to the segments of CAM cells  12   a ,  12   b  and  12   c , are globally masked (i.e., both the true and complementary data lines within each pair are pulled low). A search word is then applied to the data lines to commence a search operation. During the search operation, at least one match line segment is pulled low if one or more miss conditions exist in the illustrated row  10 . A worst case timing scenario may exist when only the leftmost CAM cell in the row  10  (i.e., CAM cell  0 ) indicates a miss and all other CAM cells (i.e., CAM cells  1 – 29 ) indicate a match (often referred to as a “hit”). When this occurs, a gradual pull-down of the first match line segment x10 MLa is accelerated from left-to-right across the match line segments x10 MLb and x10 MLc, by the inverters  11 – 14 . Thus, the inverters  11 – 14 , which may be designed to have relatively strong pull-down paths, can operate to increase the pull-down speed of the match line segments and thereby improve the worst case timing characteristics when search operations are performed. 
     Notwithstanding these conventional techniques to improve match line signal speed and reduce match line power consumption in segmented CAM arrays, there continues to be a need for additional techniques to further reduce power consumption and achieve high speed operation of CAM arrays having segmented match lines. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention include CAM devices that utilize advanced timing and power saving techniques to support high frequency search operations within large capacity CAM arrays. In some embodiments, segmented CAM arrays are provided with low power match line signal repeaters that support high speed pipelined search operations in an efficient manner. An exemplary match line signal repeater includes a dual-capture match line signal repeater that extends between xR and xS segments of CAM cells within a respective row, where R and S are positive integers. This repeater provides high speed operation by quickly accessing the state (match or miss) of a match line segment when a corresponding segment of CAM cells connected to the match line segment undergoes a respective stage of a pipelined search operation. If the match line segment is initially assessed as having a match signal thereon, then that match signal is passed to a next match line segment within the same row and a next stage search operation is commenced. However, if the match line segment is erroneously assessed as having a match signal thereon, when a miss condition was actually present in the corresponding segment of CAM cells, then the signal repeater will operate to capture a late miss signal and pass that late miss signal to the next higher match line segment, and thereby correct the error. 
     In particular, a dual-capture match line signal repeater may be configured to: (i) transfer a “early” match signal from a xR match line segment to a next higher xS match line segment during an early capture time interval; and then (ii) transfer the “late” miss signal, if present, from the xR match line segment to the xS match line segment during a late capture time interval that terminates after termination of the early capture time interval. In this manner, an early assessment of a match condition can be made in order to shorten the per-stage search cycle time. However, if the early assessment is erroneous because a worst case miss condition was actually present (resulting in a weak miss signal that is represented by a relatively gradual high-to-low transition of the match line), then the erroneous assessment is corrected and provided to the next segment of CAM cells while that next segment is undergoing the next stage of the search operation. However, because such an erroneous assessment is typically rare, the benefit of shorter search latency more than adequately compensates for the infrequent case when match line power is not conserved. 
     Additional embodiments of the present invention include methods of performing pipelined search operations within a segmented CAM array. These methods may include applying a first segment of a search word to first data lines that are electrically coupled to the first segment of CAM cells during a first stage of the pipelined search operation. Then, after a relatively short evaluation time period has elapsed, an early match signal, if present, is passed from a first match line segment associated with the first segment of CAM cells to a second match line segment associated with a second segment of CAM cells. This passing of the match signal may be performed while second data lines, which are electrically coupled to the second segment of CAM cells, are globally masked. Then, during a second stage of the pipelined search operation, a second segment of the search word is applied to the second data lines and a late miss signal is simultaneously passed from the first match line segment to the second match line segment, to thereby correct for the early passing of an erroneous match signal. 
     Still further embodiments of the present invention are described more fully hereinbelow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an electrical schematic of a row of CAM cells having a match line signal repeater therein, according to the prior art. 
         FIG. 2  is an electrical schematic of a row of CAM cells having a dual-capture match line signal repeater therein according to embodiments of the present invention. This row is divided into two halves, with  FIG. 2A  illustrating a left half (columns &lt;0:39&gt;) and  FIG. 2B  illustrating a right half (columns &lt;40:79&gt;). 
         FIG. 3  is a timing diagram that illustrates the timing of control signals applied to the dual-capture match line signal repeater of  FIG. 2 , according to additional embodiments of the present invention. 
         FIG. 4  is a block diagram of an XY ternary CAM (TCAM) cell that may be used in the embodiment of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention now will be described more fully herein with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout and signal lines and signals thereon may be referred to by the same reference characters. Signals may also be synchronized and/or undergo minor boolean operations (e.g., inversion) without being considered different signals. The suffix B (or prefix symbol “/”) to a signal name may also denote a complementary data or information signal or an active low control signal, for example. 
     Referring now to  FIG. 2 , a content addressable memory (CAM) array having a segmented row  100  of CAM cells therein will be described. The row  100 , which has been segmented into four equal width segments, is illustrated as having a logical width of 80 CAM cells, however, additional cells (not shown) may be added to one or more segments in order to provide column redundancy. Thus, in the illustrated embodiment, the segments each have a logical width of 20 CAM cells. These segments of CAM cells are described herein as a x20a segment, which spans columns 0–19, a x20b segment, which spans columns 20–39, a x20c segment, which spans columns 40–59, and a x20d segment, which spans columns 60–79. Other configurations of rows having a different number of segments and segments of unequal width are also possible. As illustrated by  FIG. 4 , the CAM cells may constitute ternary CAM (TCAM) cells, however, binary and quaternary CAM cells may also be used. In particular,  FIG. 4  illustrates an XY TCAM cell  20  that is connected to two pairs of bit lines ((BX, BXB) and (BY, BYB)) and one pair of data lines (D, DB). The bit lines provide write data (or read data) to (or from) the CAM array during write (or read) operations. In contrast, the data lines provide comparand data (i.e., search words) to the CAM array during search operations. The XY CAM cell  22  is also illustrated as being connected to a respective word line (WL), a respective match line (ML), a power supply line Vdd and a ground reference line Vss. 
     The CAM cells in  FIG. 2  are grouped according to segment. The first segment of CAM cells  20   a , which spans columns 0–19 of the array, is electrically connected to a first x20a match line segment (shown as MLn — a, where “n” designates the row number). The second segment of CAM cells  20   b , which spans columns 20–39 of the array, is electrically connected to a second x20b match line segment (shown as MLn — b). The third segment of CAM cells  20   c , which spans columns 40–59 of the array, is electrically connected to a third x20c match line segment (shown as MLn — c). Finally, the fourth segment of CAM cells  20   d , which spans columns 60–79 of the array, is electrically connected to a fourth x20d match line segment (shown as MLn — d). 
     A dual-capture match line signal repeater is provided between each of the illustrated match line segments. In particular, a first dual-capture match line signal repeater  40   ab  is provided between the match line segments MLn — a and MLn — b, a second dual-capture match line signal repeater  40   bc  is provided between match line segments MLn — b and MLn — c, and a third dual-capture match line signal repeater  40   cd  is provided between match line segments MLn — c and MLn — d. 
     The first signal repeater  40   ab  is illustrated as including a first inverter  30   a  and a second inverter  32   b . The first inverter  30   a  may be defined internally by one PMOS pull-up transistor and one NMOS pull-down transistor. In contrast, the second inverter  32   b , which has a tri-state output, includes a pull-up path defined by two PMOS pull-up transistors and a pull-down path defined by two NMOS pull-down transistors. An input of the second inverter  32   b  is electrically connected to an output of the first inverter  30   a  by the first complementary match line segment MLBn — a. As illustrated, the uppermost PMOS pull-up transistor PEb within the second inverter  32   b  has a gate terminal that is responsive to a first evaluation control signal (shown as EV 1 ). The lowermost NMOS pull-down transistor NCb within the second inverter  32   b  has a gate terminal that is responsive to a first connect control signal (shown as CON 1 ). 
     The second signal repeater  40   bc  is illustrated as including a first inverter  30   b  and a second inverter  32   c . The first inverter  30   b  may be defined internally by one PMOS pull-up transistor and one NMOS pull-down transistor. The second inverter  32   c , which has a tri-state output, includes a pull-up path defined by two PMOS pull-up transistors and a pull-down path defined by two NMOS pull-down transistors. An input of the second inverter  32   c  is electrically connected to an output of the first inverter  30   b  by the second complementary match line segment MLBn — b. As illustrated, the uppermost PMOS pull-up transistor PEc within the second inverter  32   c  has a gate terminal that is responsive to a zeroth evaluation control signal (shown as EV 0 ). The lowermost NMOS pull-down transistor NCc within the second inverter  32   c  has a gate terminal that is responsive to a zeroth connect control signal (shown as CON 0 ). These evaluation and connect control signals may be generated by timing and control circuitry (not shown) that is synchronized to a clock signal (e.g., CLK 2 X), as illustrated by the timing diagram  50  of  FIG. 3 . 
     The third signal repeater  40   cd  is illustrated as including a first inverter  30   c  and a second inverter  32   d . The first inverter  30   c  may be defined internally by one PMOS pull-up transistor and one NMOS pull-down transistor. The second inverter  32   d , which has a tri-state output, includes a pull-up path defined by two PMOS pull-up transistors and a pull-down path defined by two NMOS pull-down transistors. As illustrated, the uppermost PMOS pull-up transistor PEd within the second inverter  32   d  has a gate terminal that is responsive to the first evaluation control signal (shown as EV 1 ). The lowermost NMOS pull-down transistor NCd within the second inverter  32   d  has a gate terminal that is responsive to the first connect control signal (shown as CON 1 ). 
     The first match line segment MLn — a is precharged to a logic 1 voltage (e.g., Vdd) just prior to commencement of a first stage of a pipelined search operation. This precharging operation is performed by PMOS pull-up transistor PUa, which is responsive to the zeroth evaluation control signal EV 0 . As described more fully hereinbelow with respect to the timing diagram  50  of  FIG. 3  and TABLES 1 and 2, a leading edge of the zeroth evaluation control signal, which is an active low signal, will operate to commence precharge of the first match line segment MLn — a. A pair of serially connected PMOS pull-up transistors P 1   a  and P 2   a  are also provided to support the first match line segment MLn — a at its precharged level, by offsetting leakage current losses that may occur in the first segment of CAM cells  20   a . As illustrated, a first current carrying terminal of the PMOS pull-up transistor P 1   a  (shown as a drain terminal) is electrically connected to the first match line segment MLn — a and a gate terminal of the PMOS pull-up transistor P 1   a  is electrically connected to an output of the first inverter  30   a . The gate terminal of the PMOS pull-up transistor P 2   a  is responsive to a bias signal (shown as PBIAS), which may be a signal having an adjustable or one time programmable (e.g., fuse programmable) dc voltage level (e.g., 0&lt;VPBIAS&lt;0.5(Vdd)). The order of the pull-up transistors P 1   a  and P 2   a  may be reversed. The operating characteristics of the PMOS pull-up transistors P 1   a  and P 2   a  are more fully described in commonly assigned U.S. application Ser. No. 10/323,236, filed Dec. 18, 2002, the disclosure of which is hereby incorporated herein by reference. 
     A second pair of serially connected PMOS pull-up transistors P 1   b  and P 2   b  are also provided to support any positive voltage on the second match line segment MLn — b, by offsetting leakage current losses that may occur in the second segment of CAM cells  20   b . Similarly, a third pair of serially connected PMOS pull-up transistors P 1   c  and P 2   c  are provided to support any positive voltage on the third match line segment MLn — c during search operations. Finally, a fourth pair of serially connected PMOS pull-up transistors P 1   d  and P 2   d  are provided to support any positive voltage on the fourth match line segment MLn — d during search operations. The fourth match line segment MLn — d terminates at an input of a final inverter  30   d , which passes a match/miss result to an input of a x80 capture latch  42 . As illustrated, the switching of the x80 capture latch  42  is synchronized with a trailing edge of the zeroth connect control signal CON 0 . The capture latch  42  generates a final active low match line signal MLBn, which may be provided to a priority encoder using conventional techniques. 
     Operations performed within the segmented row  100  of CAM cells illustrated by  FIG. 2  will now be described more fully with reference to the timing diagram  50  of  FIG. 3  and the search operations illustrated by TABLES 1 and 2. In particular,  FIG. 3  illustrates the timing of five signals: CLK 2 X (a synchronizing clock signal having a period of T=2 ns), the zeroth evaluation control signal EV 0 , the first connect control signal CON 1 , the first evaluation control signal EV 1  and the zeroth connect control signal CON 0 . As described above, the evaluation control signals EV 0  and EV 1  are active low signals and the connect control signals CON 0  and CON 1  are active high signals. 
     When the zeroth evaluation control signal EV 0  switches high-to-low, the first match line segment MLn — a is precharged high to a logic 1 level and the PMOS pull-up transistor PEc within the second inverter  32   c  is turned on to enable pull-up of the third match line segment MLn — c (when the second complementary match line segment MLBn — b is maintained at a logic 0 level). When the first evaluation control signal EV 1  switches high-to-low, the PMOS pull-up transistor PEb within the second inverter  32   b  is turned on to enable pull-up of the second match line segment MLn — b (when the first complementary match line segment MLBn — a is maintained at a logic 0 level). Switching the first evaluation control signal EV 1  high-to-low also causes the PMOS pull-up transistor PEd within the second inverter  32   d  to turn on and enable pull-up of the fourth match line segment MLn — d (when the third complementary match line segment MLBn — c is maintained at a logic 0 level). 
     When the zeroth connect control signal CON 0  is switched low-to-high, the NMOS pull-down transistor NCc within the second inverter  32   c  is turned on to enable pull-down of the third match line segment MLn — c (when the second complementary match line segment MLBn — b switches to (or is held at) a logic 1 level). When the first connect control signal CON 1  is switched low-to-high, the NMOS pull-down transistor NCb within the second inverter  32   b  is turned on to enable pull-down of the second match line segment MLn — b (when the first complementary match line segment MLBn — a switches to (or is held at) a logic 1 level). Switching the first connect control signal CON 1  low-to-high also causes the NMOS pull-down transistor NCd within the second inverter  32   d  to turn on and thereby enable pull-down of the fourth match line segment MLn — d (when the third complementary match line segment MLBn — c switches to (or is held at) a logic 1 level). In addition, switching the zeroth connect control signal CON 0  high-to-low causes the x80 capture latch  42  to capture the signal at the output of the final inverter  30   d . This captured signal is reflected as the final match line signal MLBn. 
     Referring specifically now to the entries within TABLES 1 and 2 and the timing diagram  50  of  FIG. 3 , STAGE 1 of a pipelined search operation with respect to an odd search word (e.g., WORD1) and STAGE 3 of a pipelined search operation with respect to an even search word (e.g., WORD0) occur during the time interval from 0T to 1T, which spans 2 ns. During the time interval 0T to 0.5T, bits &lt;0:19&gt; of WORD1 are applied to the 20 pairs of differential data lines (D/DB&lt;0:19&gt;) associated with the first segment of CAM cells  20   a  and bits &lt;40:59&gt; of WORD0 are simultaneously applied to the 20 pairs of differential data lines (D/DB&lt;40:59&gt;) associated with the third segment of CAM cells  20   c . This application of data commences two side-by-side partial word search operations. Just prior to time 0T, the bit lines D/DB&lt;0:19&gt; and &lt;40:59&gt; are held low to represent a global mask condition. 
     When the first segment of WORD1 is applied to the data lines D/DB&lt;0:19&gt;, the first match line segment MLn — a will be pulled low (i.e., discharged) from a precharged high level if one or more miss conditions are present in the first segment of CAM cells  20   a . A “worst” case miss condition exists from a timing standpoint when only CAM cell&lt;0&gt; in the leftmost column of the CAM array detects a miss condition and all other CAM cells&lt;1:19&gt; detect a match condition (i.e., hit). In this case, CAM cell&lt;0&gt; will be solely responsible for pulling down the entire first match line segment MLn — a. Similar “worst” case miss conditions may also exist whenever only a single cell miss condition is present in one of the CAM cells&lt;1:19&gt;. When the third segment of WORD  0  is applied to the data lines D/DB&lt;40:59&gt;, the third match line segment MLn — c will be pulled low (or held low) if one or more miss conditions are present in the third segment of CAM cells  20   a.    
     Also during the time interval from 0T to 0.5T, the previously applied bits &lt;20:39&gt; of WORD0 remain on the differential data lines (D/DB&lt;20:39&gt;) associated with the second segment of CAM cells  20   b  and the previously applied bits &lt;60:79&gt; of WORD(−1) remain on the differential data lines (D/DB&lt;60:79&gt;) associated with the fourth segment of CAM cells  20   d . In addition, because the first evaluation control signal EV 1  is inactive at a logic 1 level and the first connect control signal CON 1  is inactive at a logic 0 level during the time interval from 0T to 0.5T, the tri-state output of second inverter  32   b  and the tri-state output of second inverter  32   d  will be disposed in high impedance states. This will isolate the first match line segment MLn — a from the second match line segment MLn — b (i.e., x20a ML and x20b ML are isolated from each other) and also isolate the third match line segment MLn — c from the fourth match line segment MLn — d (i.e., x20c ML and x20d ML are isolated from each other). 
     Moreover, because the zeroth connect control signal CON 0  switches low-to-high at time 0T, any miss signal generated on the second match line segment MLn — b (during a prior STAGE 2 of the search operation with respect to WORD0) will be captured as this miss signal passes through the second inverter  32   c . In particular, if a miss is present from the prior stage, then the second complementary match line segment MLBn — b will be high and the NMOS pull-down transistor NCc associated with the second inverter  32   c  will be turned on in response to the low-to-high switching of the zeroth connect control signal CON 0 . This will cause the output of the second inverter  32   c  to pull (or hold) the third match line segment MLn — c segment low. In many cases, the third match line segment MLn — c segment will not need to be pulled low if it already was low during an immediately prior stage of a search operation. Thus, switching CON 0  low-to-high enables the capture of a late miss signal from the second match line segment MLn — b during STAGE 3 of the search operation with respect to WORD  0 . Switching the zeroth connect control signal CON 0  low-to-high also operates to capture the output of the final inverter  30   d , which represents a x80 match condition with respect to a prior word (WORD(−1)) that has finished a fourth stage of its search. 
     According to a preferred aspect of the match line signal repeaters, if a high-to-low transition of the second match line segment MLn — b is relatively gradual in response to a respective STAGE 2 of a search operation, then the low-to-high transition of the second complementary match line signal MLBn — b may also be relatively gradual, but nonetheless recognized by the second inverter  32   c  when the NMOS pull-down transistor NCc turns on in response to the active zeroth connect control signal CON 0 . As described herein, a match signal represents a logic 1 signal on a match line and a miss signal represents a logic 0 signal on a match line. In contrast, a “late” miss signal can represent either a “strong” miss signal that is captured late (relative to a match signal) or a “weak” miss signal that is captured late. A “weak” miss signal represents a logic 0 signal that was developed slowly on a match line (i.e., the high-to-low transition of the match line is not sufficiently abrupt to classify the transition as a “strong” miss signal having a sharp falling edge). 
     At the commencement of the time interval from 0.5T to 1T, the zeroth connect control signal CON 0  switches high-to-low (at time 0.5T) to thereby turn off NMOS transistor NCc within the second inverter  32   c . The first evaluation control signal EV 1  also switches high-to-low to thereby turn on PMOS pull-up transistor PEb (within the second inverter  32   b ) and PMOS pull-up transistor PEd (within the second inverter  32   d ). This enables the “early” capture and passing of any logic 1 match signal from the first match line segment MLn — a to the second match line MLn — b, while data lines D/DB&lt;20:39&gt; are being globally masked (in preparation for STAGE 2 of the search operation with respect to WORD1). This also enables the “early” capture and passing of any logic 1 match signal from the third match line segment MLn — c to the fourth match line MLn — d, while data lines D/DB&lt;60:79&gt; are being globally masked (in preparation for STAGE 4 of the search operation with respect to WORD0). During this time interval from 0.5T to 1T, the second match line segment MLn — b will be isolated from the third match line segment MLn — c (because EV 0 =1 and CON 0 =0 and the output of the second inverter  32   c  is tri-stated), the first segment of WORD1 will remain on data lines D/DB&lt;0:19&gt; and the third segment of WORD0 will remain on data lines D/DB&lt;40:59&gt;. 
     STAGE 2 of a pipelined search operation with respect to search WORD1 and STAGE 4 of a pipelined search operation with respect to search WORD0 occur during the time interval from 1T to 2T. At the time point 1T, the first evaluation control signal EV 1  switches low-to-high and the first connect control signal CON 1  switches low-to-high. This enables a late miss signal, if any, to be passed from the first segment of CAM cells  20   a  to the second segment of CAM cells  20   b  (i.e., passed through second inverter  32   b ). This also enables a late miss signal, if any, to be passed from the third segment of CAM cells  20   c  to the fourth segment of CAM cells  20   d  (i.e., passed through second inverter  32   d ). Moreover, because the zeroth evaluation control signal EV 0  and the zeroth connect control signal CON 0  are held high and low, respectively, during the interval from 1T to 1.5T, the second match line segment MLn — b remains isolated from the third match line segment MLn — c. 
     During the STAGE 2 and STAGE 4 time intervals, bits &lt;20:39&gt; of WORD1 are applied to the 20 pairs of differential data lines (D/DB&lt;10:39&gt;) associated with the second segment of CAM cells  20   b  and bits &lt;60:79&gt; of WORD0 are simultaneously applied to the 20 pairs of differential data lines (D/DB&lt;60:79&gt;) associated with the fourth segment of CAM cells  20   d . This application of data commences two side-by-side partial word search operations. In addition, the first segment of WORD1 (i.e., bits &lt;0:19&gt;) is maintained on the first segment of data lines D/DB&lt;0:19&gt; and the third segment of WORD0 (i.e., bits &lt;40:59&gt;) is maintained on the third segment of data lines D/DB&lt;40:59&gt;. 
     When the second segment of WORD1 is applied to the data lines D/DB&lt;20:39&gt; during STAGE 2, the second match line segment MLn — b will be pulled high-to-low (i.e., discharged) if STAGE 1 resulted in a match condition and one or more miss conditions are present in the second segment of CAM cells  20   b . Alternatively, the second match line segment MLn — b will be pulled high-to-low if STAGE 1 resulted in an early capture of an erroneous match signal during the time interval 0.5T to 1T, followed by late capture of a “weak” miss signal during the time interval 1T to 1.5T. Finally, the second match line segment MLn — b will remain low during STAGE 2 if it was low at the beginning of STAGE 2 and STAGE 1 did not result in an early capture of a match signal during the time interval 0.5T to 1T. 
     Likewise, when the fourth segment of WORD0 is applied to the data lines D/DB&lt;60:79&gt; during STAGE 4, the fourth match line segment MLn — d will be pulled high-to-low (i.e., discharged) if STAGE 3 resulted in a match condition and one or more miss conditions are present in the fourth segment of CAM cells  20   d . Alternatively, the fourth match line segment MLn — d will be pulled high-to-low if STAGE 3 resulted in an early capture of an erroneous match signal during the time interval 0.5T to 1T, followed by late capture of a “weak” miss signal during the time interval 1T to 1.5T. Finally, the fourth match line segment MLn — d will remain low during STAGE 4 if it was low at the beginning of STAGE 4 and STAGE 3 did not result in an early capture of a match signal during the time interval 0.5T to 1T. 
     Next, at the commencement of the time interval from 1.5T to 2T, the first connect control signal CON 1  switches high-to-low (at time 1.5T) to thereby turn off NMOS transistor NCb within the second inverter  32   b  and NMOS transistor NCd within the second inverter  32   d . This operates to isolate the first match line MLn — a from the second match line MLn — b and also isolate the third match line from the fourth match line MLn — d. At time 1.5T, the zeroth evaluation control signal EV 0  switches high-to-low to thereby turn on PMOS pull-up transistor PUa (which precharges the first match line segment MLn — a and prepares it for STAGE 1 of a search operation) and turn on PMOS pull-up transistor PEc (within the second inverter  32   c ). This turn on of PMOS pull-up transistor PEc enables the “early” capture and passing of any logic 1 match signal from the second match line segment MLn — b to the third match line MLn — c, while data lines D/DB&lt;40:59&gt; are globally masked (in preparation for STAGE 3 of the search operation with respect to WORD1) and the second segment of WORD0 is maintained on the data lines D/DB&lt;20:39&gt;. The fourth segment of WORD0 is also maintained on data lines D/DB&lt;60:79&gt; during the time interval from 1.5T to 2T, to thereby support capture of a final match condition with respect to WORD  0  during the next time interval from 2T to 2.5T (See, e.g., TABLE 2). 
     STAGES 3 and 4 of the pipelined search operations with respect to WORD1 are next performed during the time intervals from 2T–3T and 3T–4T, respectively. These operations repeat the STAGE 3 operations and STAGE 4 operations described above with respect to WORD0. Moreover, during these final stage operations with respect to WORD1, STAGE 1 operations and STAGE 2 operations are performed with respect to a new word, WORD2. Accordingly, as illustrated best by the timing diagram of  FIG. 3  and TABLE 2, the odd words (WORD1, WORD3, . . . ) are searched in a first repeating STAGE1–STAGE4 sequence that spans 8 ns and the even words (WORD0, WORD2, . . . ) are searched in a second repeating STAGE1–STAGE4 sequence that is time shifted relative to the first repeating sequence by a time interval of 2T (4 ns). 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 0T to 0.5T 
                 APPLY SEARCH WORD1&lt;0:19&gt; 
               
               
                 STAGES 1 &amp; 3 
                 DISCHARGE x20A ML if x20A MISS 
               
               
                 (EVAL) 
                 MAINTAIN SEARCH WORD0&lt;20:39&gt; ON 
               
               
                 (LATE 
                 D/DB&lt;20:39&gt; 
               
               
                 CAPTURE) 
                 APPLY SEARCH WORD0&lt;40:59&gt; 
               
               
                   
                 DISCHARGE x20C ML if x20C MISS and/or x20B 
               
               
                   
                 LATE MISS CAPTURE 
               
               
                   
                 MAINTAIN SEARCH WORD(−1)&lt;60:79&gt; ON 
               
               
                   
                 D/DB&lt;60:79&gt; 
               
               
                   
                 CAPTURE x80 MATCH/MISS (WORD(−1)) 
               
               
                   
                 ISOLATE x20A ML FROM x20B ML &amp; ISOLATE 
               
               
                   
                 x20C ML FROM x20D ML 
               
               
                 0.5T to 1T 
                 MAINTAIN SEARCH WORD1&lt;0:19&gt; ON D/DB&lt;0:19&gt; 
               
               
                 (EARLY 
                 PASS EARLY CAPTURE OF MATCH TO x20B ML 
               
               
                 CAPTURE) 
                 GLOBALLY MASK D/DB&lt;20:39&gt; 
               
               
                   
                 MAINTAIN SEARCH WORD0&lt;40:59&gt; ON 
               
               
                   
                 D/DB&lt;40:59&gt; 
               
               
                   
                 PASS EARLY CAPTURE OF MATCH TO x20D ML 
               
               
                   
                 GLOBALLY MASK D/DB&lt;60:79&gt; 
               
               
                   
                 ISOLATE x20B ML FROM x20C ML 
               
               
                 1T to 1.5T 
                 MAINTAIN SEARCH WORD1&lt;0:19&gt; ON D/DB&lt;0:19&gt; 
               
               
                 STAGES 2 &amp; 4 
                 APPLY SEARCH WORD1&lt;20:39&gt; 
               
               
                 (EVAL) 
                 DISCHARGE x20B ML if x20B MISS and/or x20A 
               
               
                 (LATE 
                 LATE MISS CAPTURE 
               
               
                 CAPTURE) 
                 MAINTAIN SEARCH WORD0&lt;40:59&gt; ON 
               
               
                   
                 D/DB&lt;40:59&gt; 
               
               
                   
                 APPLY SEARCH WORD0&lt;60:79&gt; 
               
               
                   
                 DISCHARGE x20D ML if x20D MISS and/or x20C 
               
               
                   
                 LATE MISS CAPTURE 
               
               
                   
                 ISOLATE x20B ML FROM x20C ML 
               
               
                 1.5T to 2T 
                 PRECHARGE x20A ML 
               
               
                 (EARLY 
                 GLOBALLY MASK D/DB&lt;0:19&gt; 
               
               
                 CAPTURE) 
                 MAINTAIN SEARCH WORD1&lt;20:39&gt; of 
               
               
                   
                 D/DB&lt;20:39&gt; 
               
               
                   
                 PASS EARLY CAPTURE OF MATCH TO x20C ML 
               
               
                   
                 GLOBALLY MASK D/DB&lt;40:59&gt; 
               
               
                   
                 MAINTAIN SEARCH WORD0&lt;60:79&gt; ON 
               
               
                   
                 D/DB&lt;60:79&gt; 
               
               
                   
                 ISOLATE x20A ML FROM x20B ML &amp; ISOLATE 
               
               
                   
                 x20C ML FROM x20D ML 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 0T to 0.5T 
                 APPLY SEARCH WORD1&lt;0:19&gt; 
                 APPLY SEARCH WORDO&lt;40:59&gt; 
               
               
                 STAGES 1 &amp; 3 
                 MAINTAIN SEARCH WORD(−1)&lt;60:79&gt; ON D/DB&lt;60:79&gt; 
                 MAINTAIN SEARCH WORD0&lt;20:39&gt; ON D/DB&lt;20:39&gt; 
               
               
                   
                 CAPTURE x80 MATCH/MISS (WORD(−1)) 
                 ISOLATE x20A ML FROM x20B ML &amp; ISOLATE x20C ML 
               
               
                   
                   
                 FROM x20D ML 
               
               
                   
                 DISCHARGE x20A ML if x20A MISS 
                 DISCHARGE x20C ML if x20C MISS and/or x20B LATE 
               
               
                   
                   
                 MISS CAPTURE 
               
               
                 0.5T to 1T 
                 PASS EARLY CAPTURE OF MATCH TO x20B ML 
                 PASS EARLY CAPTURE OF MATCH TO x20D ML 
               
               
                   
                 GLOBALLY MASK D/DB&lt;20:39&gt; &amp; ISOLATE x20B ML 
                 GLOBALLY MASK D/DB&lt;60:79&gt; 
               
               
                   
                 FROM x20C ML 
               
               
                 1T to 1.5T 
                 APPLY SEARCH WORD1&lt;20:39&gt; 
                 APPLY SEARCH WORD0&lt;60:79&gt; 
               
               
                 STAGES 2 &amp; 4 
                 MAINTAIN SEARCH WORD1&lt;0:19&gt; ON D/DB&lt;0:19&gt; 
                 MAINTAIN SEARCH WORD0&lt;40:59&gt; ON D/DB&lt;40:59&gt; 
               
               
                   
                 ISOLATE x20B ML FROM x20C ML 
               
               
                   
                 DISCHARGE x20B ML if x20B MISS and/or x20A LATE 
                 DISCHARGE x20D ML if x20D MISS and/or x20C LATE 
               
               
                   
                 MISS CAPTURE 
                 MISS CAPTURE 
               
               
                 1.5T to 2T 
                 PASS EARLY CAPTURE OF MATCH TO x20C ML 
                 PRECHARGE x20A ML 
               
               
                   
                 GLOBALLY MASK D/DB&lt;40:59&gt; &amp; ISOLATE x20C ML 
                 GLOBALLY MASK D/DB&lt;0:19&gt; &amp; ISOLATE x20A ML 
               
               
                   
                 FROM x20D ML 
                 FROM x20B ML 
               
               
                 2T to 2.5T 
                 APPLY SEARCH WORD1&lt;40:59&gt; 
                 APPLY SEARCH WORD2&lt;0:19&gt; 
               
               
                 STAGES 3 &amp; 1 
                 MAINTAIN SEARCH WORD1&lt;20:39&gt; ON D/DB&lt;20:39&gt; 
                 MAINTAIN SEARCH WORD0&lt;60:79&gt; ON D/DB&lt;60:79&gt; 
               
               
                   
                 ISOLATE x20A ML FROM x20B ML &amp; ISOLATE x20C ML 
                 CAPTURE x80 MATCH/MISS (SEARCH WORD0) 
               
               
                   
                 FROM x20D ML 
               
               
                   
                 DISCHARGE x20C ML if x20C MISS and/or x20B LATE 
                 DISCHARGE x20A ML if x20A MISS 
               
               
                   
                 MISS CAPTURE 
               
               
                 2.5T to 3T 
                 PASS EARLY CAPTURE OF MATCH TO x20D ML 
                 PASS EARLY CAPTURE OF MATCH TO x20B ML 
               
               
                   
                 GLOBALLY MASK D/DB&lt;60:79&gt; 
                 GLOBALLY MASK D/DB&lt;20:39&gt; &amp; ISOLATE x20B ML 
               
               
                   
                   
                 FROM x20C ML 
               
               
                 3T to 3.5T 
                 APPLY SEARCH WORD1&lt;60:79&gt; 
                 APPLY SEARCH WORD2&lt;20:39&gt; 
               
               
                 STAGES 4 &amp; 2 
                 MAINTAIN SEARCH WORD1&lt;40:59&gt; ON D/DB&lt;40:59&gt; 
                 MAINTAIN SEARCH WORD2&lt;0:19&gt; ON D/DB&lt;0:19&gt; 
               
               
                   
                 ISOLATE x20B ML FROM x20C ML 
               
               
                   
                 DISCHARGE x20D ML if x20D MISS and/or x20C LATE 
                 DISCHARGE x20B ML if x20B MISS and/or x20A LATE 
               
               
                   
                 MISS CAPTURE 
                 MISS CAPTURE 
               
               
                 3.5T to 4T 
                 PRECHARGE x20A ML 
                 PASS EARLY CAPTURE OF MATCH TO x20C ML 
               
               
                   
                 GLOBALLY MASK D/DB&lt;0:19&gt; &amp; ISOLATE x20A ML 
                 GLOBALLY MASK D/DB&lt;40:59&gt; &amp; ISOLATE x20C ML 
               
               
                   
                 FROM x20B ML 
                 FROM x20D ML 
               
               
                 4T to 4.5T 
                 APPLY SEARCH WORD3&lt;0:19&gt; 
                 APPLY SEARCH WORD2&lt;40:59&gt; 
               
               
                 STAGES 1 &amp; 3 
                 MAINTAIN SEARCH WORD1&lt;60:79&gt; ON D/DB&lt;60:79&gt; 
                 MAINTAIN SEARCH WORD2&lt;20:39&gt; ON D/DB&lt;20:39&gt; 
               
               
                   
                 CAPTURE x80 MATCH/MISS (SEARCH WORD1) 
                 ISOLATE x20A ML FROM x20B ML &amp; ISOLATE x20C ML 
               
               
                   
                   
                 FROM x20D ML 
               
               
                   
                 DISCHARGE x20A ML IF x20A MISS 
                 DISCHARGE x20C ML if x20C MISS and/or x20B LATE 
               
               
                   
                   
                 MISS CAPTURE 
               
               
                 4.5T to 5T 
                 PASS EARLY CAPTURE OF MATCH TO x20B ML 
                 PASS EARLY CAPTURE OF MATCH TO x20D ML 
               
               
                   
                 GLOBALLY MASK D/DB&lt;20:39&gt; &amp; ISOLATE x20B ML 
                 GLOBALLY MASK D/DB&lt;60:79&gt; 
               
               
                   
                 FROM x20C ML 
               
               
                 5T to 5.5T 
                 APPLY SEARCH WORD3&lt;20:39&gt; 
                 APPLY SEARCH WORD2&lt;60:79&gt; 
               
               
                 STAGES 2 &amp; 4 
                 MAINTAIN SEARCH WORD3&lt;0:19&gt; ON D/DB&lt;0:19&gt; 
                 MAINTAIN SEARCH WORD2&lt;40:59&gt; ON D/DB&lt;40:59&gt; 
               
               
                   
                 ISOLATE x20B ML FROM x20C ML 
               
               
                   
                 DISCHARGE x20B ML if x20B MISS and/or x20A LATE 
                 DISCHARGE x20D ML if x20D MISS and/or x20C LATE 
               
               
                   
                 MISS CAPTURE 
                 MISS CAPTURE 
               
               
                 5.5T to 6T 
                 PASS EARLY CAPTURE OF MATCH TO x20C ML 
                 PRECHARGE x20A ML 
               
               
                   
                 GLOBALLY MASK D/DB&lt;40:59&gt; &amp; ISOLATE x20C ML 
                 GLOBALLY MASK D/DB&lt;0:19&gt; &amp; ISOLATE x20A ML 
               
               
                   
                 FROM x20D ML 
                 FROM x20B ML 
               
               
                 6T to 6.5T 
                 APPLY SEARCH WORD3&lt;40:59&gt; 
                 APPLY SEARCH WORD4&lt;0:19&gt; 
               
               
                 STAGES 3 &amp; 1 
                 MAINTAIN SEARCH WORD3&lt;20:39&gt; ON D/DB&lt;20:39&gt; 
                 MAINTAIN SEARCH WORD2&lt;60:79&gt; ON D/DB&lt;60:79&gt; 
               
               
                   
                 ISOLATE x20A ML FROM x20B ML &amp; ISOLATE x20C ML 
                 CAPTURE x80 MATCH/MISS (SEARCH WORD2) 
               
               
                   
                 FROM x20D ML 
               
               
                   
                 DISCHARGE x20C ML if x20C MISS and/or x20B LATE 
                 DISCHARGE x20A ML if x20A MISS 
               
               
                   
                 MISS CAPTURE 
               
               
                 6.5T to 7T 
                 PASS EARLY CAPTURE OF MATCH TO x20D ML 
                 PASS EARLY CAPTURE OF MATCH TO x20B ML 
               
               
                   
                 GLOBALLY MASK D/DB&lt;60:79&gt; 
                 GLOBALLY MASK D/DB&lt;20:39&gt; &amp; ISOLATE x20B ML 
               
               
                   
                   
                 FROM x20C ML 
               
               
                 7T to 7.5T 
                 APPLY SEARCH WORD3&lt;60:79&gt; 
                 APPLY SEARCH WORD4&lt;20:39&gt; 
               
               
                 STAGES 4 &amp; 2 
                 MAINTAIN SEARCH WORD3&lt;40:59&gt; ON D/DB&lt;40:59&gt; 
                 MAINTAIN SEARCH WORD4&lt;0:19&gt; ON D/DB&lt;0:19&gt; 
               
               
                   
                 ISOLATE x20B ML FROM x20C ML 
               
               
                   
                 DISCHARGE x20D ML if x20D MISS and/or x20C LATE 
                 DISCHARGE x20B ML if x20B MISS and/or x20A LATE 
               
               
                   
                 MISS CAPTURE 
                 MISS CAPTURE 
               
               
                 7.5T to 8T 
                 PRECHARGE x20A ML 
                 PASS EARLY CAPTURE OF MATCH TO x20C ML 
               
               
                   
                 GLOBALLY MASK D/DB&lt;0:19&gt; &amp; ISOLATE x20A ML 
                 GLOBALLY MASK D/DB&lt;40:59&gt; &amp; ISOLATE x20C ML 
               
               
                   
                 FROM x20B ML 
                 FROM x20D ML 
               
               
                 8T to 8.5T 
                 APPLY SEARCH WORD5&lt;0:19&gt; 
                 APPLY SEARCH WORD4&lt;40:59&gt; 
               
               
                 STAGES 1 &amp; 3 
                 MAINTAIN SEARCH WORD3&lt;60:79&gt; ON D/DB&lt;60:79&gt; 
                 MAINTAIN SEARCH WORD4&lt;20:39&gt; ON D/DB&lt;20:39&gt; 
               
               
                   
                 CAPTURE x80 MATCH/MISS (SEARCH WORD3) 
                 ISOLATE x20A ML FROM x20B ML &amp; ISOLATE x20C ML 
               
               
                   
                   
                 FROM x20D ML 
               
               
                   
                 DISCHARGE x20A ML IF x20A MISS 
                 DISCHARGE x20C ML if x20C MISS and/or x20B LATE 
               
               
                   
                   
                 MISS CAPTURE 
               
               
                   
               
             
          
         
       
     
     The timing diagram of  FIG. 3  also demonstrates alternative embodiments that modify the timing of the connect control signals CON 0  and CON 1 . For example, the dotted lines A and B in  FIG. 3  represent an earlier rising edge and a later falling edge of the first connect control signal CON 1 , respectively. Similarly, the dotted lines C and D in  FIG. 3  represent an earlier rising edge and a later falling edge of the zeroth connect control signal CON 0 , respectively. In the event the connect control signal CON 0  (CON 1 ) has a rising edge as shown by the dotted line C (A), then the dual-capture match line signal repeater will operate to capture early miss signals and early match signals during the same time intervals. Nonetheless, late capture of late miss signals will still be provided. Moving the rising edges of the connect control signals CON 0  and CON 1  forward in time as shown by the dotted lines C and A, respectively, may be less preferred because it typically results in greater power consumption, particularly if the second inverters  32   b ,  32   c  and  32   d  are switched slowly in response to “weak” miss signals (gradual high-to-low transition of a respective match line segment). Slow switching of an inverter results in higher current consumption because it causes a direct resistive path to form between the power supply lines (Vdd and Vss) when the PMOS and NMOS transistors are simultaneously conductive. 
     As illustrated by TABLE 3, each CAM array includes a bit line and data line driver circuit (not shown) that drives the data lines (D/DB&lt;0:79&gt;) with segments of the search words during the pipelined search operations. These data line driving operations are interleaved with global masking operations that may be implemented using a dedicated mask cell sub-array containing global mask cells. An exemplary dedicated mask cell sub-array is more fully described in commonly assigned U.S. application Ser. No. 10/386,400, filed Mar. 11, 2003, the disclosure of which is hereby incorporated herein by reference. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 D/DB 
                 D/DB 
                 D/DB 
                 D/DB 
               
               
                 STAGES 
                 TIME 
                 &lt;0:19&gt; 
                 &lt;20:39&gt; 
                 &lt;40:59&gt; 
                 &lt;60:79&gt; 
               
               
                   
               
             
             
               
                 1 AND 3 
                 0T to 0.5T 
                 WORD 1 
                 WORD 0 
                 WORD 0 
                 WORD −1 
               
               
                   
                 0.5T to 1T 
                 WORD 1 
                 MASK 
                 WORD 0 
                 MASK 
               
               
                 2 AND 4 
                 1T to 1.5T 
                 WORD 1 
                 WORD 1 
                 WORD 0 
                 WORD 0 
               
               
                   
                 1.5T to 2T 
                 MASK 
                 WORD 1 
                 MASK 
                 WORD 0 
               
               
                 3 AND 1 
                 2T to 2.5T 
                 WORD 2 
                 WORD 1 
                 WORD 1 
                 WORD 0 
               
               
                   
                 2.5T to 3T 
                 WORD 2 
                 MASK 
                 WORD 1 
                 MASK 
               
               
                 4 AND 2 
                 3T to 3.5T 
                 WORD 2 
                 WORD 2 
                 WORD 1 
                 WORD 1 
               
               
                   
                 3.5T to 4T 
                 MASK 
                 WORD 2 
                 MASK 
                 WORD 1 
               
               
                 1 AND 3 
                 4T to 4.5T 
                 WORD 3 
                 WORD 2 
                 WORD 2 
                 WORD 1 
               
               
                   
                 4.5T to 5T 
                 WORD 3 
                 MASK 
                 WORD 2 
                 MASK 
               
               
                 2 AND 4 
                 5T to 5.5T 
                 WORD 3 
                 WORD 3 
                 WORD 2 
                 WORD 2 
               
               
                   
                 5.5T to 6T 
                 MASK 
                 WORD 3 
                 MASK 
                 WORD 2 
               
               
                 3 AND 1 
                 6T to 6.5T 
                 WORD 4 
                 WORD 3 
                 WORD 3 
                 WORD 2 
               
               
                   
                 6.5T to 7T 
                 WORD 4 
                 MASK 
                 WORD 3 
                 MASK 
               
               
                 4 AND 2 
                 7T to 7.5T 
                 WORD 4 
                 WORD 4 
                 WORD 3 
                 WORD 3 
               
               
                   
                 7.5T to 8T 
                 MASK 
                 WORD 4 
                 MASK 
                 WORD 3 
               
               
                 1 AND 3 
                 8T to 8.5T 
                 WORD 5 
                 WORD 4 
                 WORD 4 
                 WORD 3 
               
               
                   
                 8.5T to 9T 
                 WORD 5 
                 MASK 
                 WORD 4 
                 MASK 
               
               
                 2 AND 4 
                 9T to 9.5T 
                 WORD 5 
                 WORD 5 
                 WORD 4 
                 WORD 4 
               
               
                   
                 9.5T to 10T 
                 MASK 
                 WORD 5 
                 MASK 
                 WORD 4 
               
               
                   
               
             
          
         
       
     
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.