Abstract:
A semiconductor integrated circuit includes a plurality of output transistors each controlling the magnitude of an output voltage relative to the magnitude of a load current according to a control value indicated by an impedance control signal applied to a control terminal, a voltage monitor circuit outputing an output voltage monitor value indicating a voltage value of the output voltage, and a control circuit controling the magnitude of the control value according to the magnitude of an error value between a reference voltage indicating a target value of the output voltage and the output voltage monitor value, and controls based on the control value whether any of such transistors be brought to a conducting state. The control circuit increases a change step of the control value relative to the error value during a predetermined period according to prenotification signals for notifying a change of the load current in advance.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is Continuation of U.S. application Ser. No. 13/038,916, now U.S. Pat. No. 8,400,803, filed on Feb. 3, 2011, which in turn claims the benefit of Japanese Application No. 2010-45653, filed on Mar. 2, 2010, the disclosures of which Applications are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a content addressable memory device. 
     A content addressable memory array (CAM array) has a function to perform coincidence determination of stored data and supplied search data in addition to a read/write function of data, one entry which stores a search data word comprises plural CAM cells, and a word bit of a search candidate is stored in the CAM cells. In each entry, a match line to which the corresponding CAM cells are coupled in parallel is provided. When a search data word and a stored data word of an entry are in agreement, the corresponding match line is maintained in a state of “1”, and when in disagreement, the corresponding match line is driven into a state of “0.” 
     By identifying a voltage level of the match line, it is possible to determine whether data corresponding to the search data is stored in a table etc., for example. Such a content addressable memory is used for determining of a cache hit or cache miss in a communications router, a cache memory, etc., for example. Routing of an IP packet performed by a network router etc. is carried out by verifying that an IP address which is stored in a content addressable memory provided in the router is in agreement with an IP address which is inputted from the exterior. For example, on the basis of match line information indicative of a coincidence state of the content addressable memory provided in the router, a value which indicates the next destination address is written in the IP packet, and then the IP packet is transmitted from a corresponding port. 
     In searching operation of CAM, a match line which indicates a state of coincidence/non-coincidence between search data (for example, 80-bit width) and stored data (with an identical width to the search data) of the CAM array is pre-charged to a power supply voltage VDD at a pre-charge period. When the search data and the stored data are in disagreement (henceforth called as “Miss”) by searching, the match line is discharged by a transistor in a CAM cell to a ground voltage GND. When in agreement (henceforth called as “Hit”), on the other hand, discharging of the match line is not performed, but the level of the pre-charge voltage VDD is maintained. It is determined whether the stored data and the search data are in agreement or not, by the aid of the match line having such two states. 
     The following explains about current consumed at the time of search in a large-scale CAM array, for example, a 20M-bit (256K entries×80 bits) CAM array, which can perform a huge amount of data search concurrently. 
     Assuming that current consumed by one entry is I_ML when a search result is Miss (that is, charge and discharge current of one match line), current of I_ML×256K is consumed in the above-described large-scale CAM array. 
     At the time of searching, a search line which indicates a state of search data is charged to the VDD level when search data is “1” (HIGH), and discharged to a GND level when the searching finishes. Assuming that current consumed by a search line per one bit of search data and one entry is I_SL, current of I_SL×256K×80 is consumed in the above-described large-scale CAM array. 
     At the time of searching operation, a match amplifier circuit which determines a state of a match line of all the entries is also activated concurrently. Assuming that current consumed by a match amplifier circuit for one entry is I_ma, current of I_ma×256K is consumed in the above-described large-scale CAM array. 
     In this way, in the above-described large-scale CAM array, the current consumed by the match line, the search line, and the match amplifier circuit amounts to (I_ML×256K+I_SL×256K×80+I_ma×256K). This value of the consumption current is very large, and occupies the greater part of the entire consumption current. Moreover, the value of the consumption current increases in proportion to formation of a large-scale capacity and speeding up. 
     A configuration which reduces the consumption current I_ML in a match line in a CAM array is disclosed by Patent Document 1 (U.S. Pat. No. 6,191,969). That is, in configurations illustrated in FIG. 4 of Patent Document 1 and FIG. 3 of Patent Document 2 (U.S. Pat. No. 6,430,074), a match line of a CAM array is divided into plural blocks. When search result of the first stage block is of Miss, searching in the second stage block is controlled not to be performed, and the match lines of the second and latter stages are pre-charged (or discharged). Accordingly, the pre-charge current is reduced and power lowering is attained. 
     A configuration which reduces consumption current I_ML in a match line in a CAM array is disclosed also in Patent Document 2. That is, in FIG. 10A of Patent Document 2, by dividing match lines of a CAM array into three or more blocks, and controlling match line pre-charge of the third stage block according to search result (Hit, Miss) of the first stage block, formation of pipeline and speeding up of a clock are attained easily, and the pre-charge current is reduced and power lowering is attained. 
     A configuration which reduces consumption current I_SL in a search line in a CAM array is disclosed by Patent Document 3 (Japanese Patent Laid-open No. 2003-272386). That is, in Patent Document 3, a match line of a CAM array is divided into three or more, and pre-charge of the second stage is controlled by search result of the first stage. Accordingly, the pre-charge current is reduced. Furthermore, when all the entries are of non-coincidence in the first stage block, search lines of the third stage block and the latter stage blocks are rendered non-active. Accordingly, the consumption current of the search lines is reduced, and power lowering is attained. 
     A configuration which reduces consumption current I_ML in a match line in a CAM array is disclosed also in Patent Document 4 (Japanese Patent Laid-open No. 2009-158027). That is, in Patent Document 4, searching is performed for every memory cell array, and when search data is in agreement with any of data stored in the memory cell array, a search line driver unit is rendered non-active, and searching is not performed for the subsequent memory cell arrays. Accordingly, it become possible to reduce the number of match lines which need charging when returning to a standby state, and it becomes possible to attain reduction of consumption current. 
     Considering a case where 80 bits of search data are compared with 80 bits of stored data and non-coincidence (Miss) occurs, the number of bits in non-coincidence may be 1 bit-80 bits. When all of 80 bits are of non-coincidence, driving ability is great and access is quick, because match lines are drawn out to the GND side by search transistors of 80 bits. When only one bit is of non-coincidence, on the other hand, the match line is drawn out to the GND side by a search transistor of one bit, accordingly, the speed is slow. Accordingly, a worst access of searching is the non-coincidence of one bit (henceforth called one-bit miss). That is, an operating frequency in searching a CAM array is decided by the present access in most cases. 
     A configuration which attains speeding up at the time of a one-bit miss in a CAM array is disclosed by Patent Document 5 (U.S. Pat. No. 6,965,519). That is, in a configuration illustrated in FIG. 1 of Patent Document 5, a match line of a CAM array is divided into plurality, parasitic capacitance of the match line is reduced, and speeding up is attained by adopting a pipeline structure. 
     In Patent Document 6 (Japanese Patent Laid-open No. 2007-317342), by using a differential amplifier for an amplifier circuit of a match line, a very small amplitude signal of the match line is detected and speeding up of access at the time of a one-bit miss is attained. Moreover, a configuration which reduces consumption current I_ML in a match line in a CAM array is disclosed by Patent Document 6. That is, in a configuration illustrated in Patent Document 6, the pre-charge level of a match line is lowered to a middle voltage level lower than VDD/2, and a sense amplifier which can detect voltage of two states of the middle voltage level and a GND level is used. Accordingly, the pre-charge current of a match line is reduced. 
     Patent Document 7 (Japanese Patent Laid-open No. Hei 05 (1993)-127872) discloses a semiconductor integrated circuit comprising a sub sense line to which each output of bit comparator circuits of the corresponding group is summarized, and one main sense line for outputting a comparison result, to which each sub sense line is coupled in common via each drive element. 
     (Patent Document 1) U.S. Pat. No. 6,191,969 
     (Patent Document 2) U.S. Pat. No. 6,430,074 
     (Patent Document 3) Japanese Patent Laid-open No. 2003-272386 
     (Patent Document 4) Japanese Patent Laid-open No. 2009-158027 
     (Patent Document 5) U.S. Pat. No. 6,965,519 
     (Patent Document 6) Japanese Patent Laid-open No. 2007-317342 
     (Patent Document 7) Japanese Patent Laid-open No. Hei 05 (1993)-127872. 
     SUMMARY OF THE INVENTION 
     In response to a demand for speeding up beyond 300 Msps (search per second) in recent years, a match line is divided as in the previously existing technology, and pipeline structure is adopted to realize the speeding up. However, due to an increase in the number of pipeline stages accompanying the speeding up, the number of match amplifier circuits in operation has increased and consumption current has increased. 
     The breakdown of the consumption current in a CAM memory corresponding to a high-speed operation is given on an average by a ratio of (match amplifier circuit: search line: match line)=(1:2:4). It is assumed that current of a match amplifier circuit, current of a search line, and current of a match line is 1I, 2I, and 4I, respectively. In a method in which the number of pipeline stages is set to two stages in order to reduce pre-charge current of the match line, as in FIG. 4 of Patent Document 1 and FIG. 3 of Patent Document 2, the pre-charge current of the match line decreases from 4I to 2I, but the consumption current of the match amplifier circuit increases from 1I to 2I. Therefore, the consumption current as a whole decreases only from 7I to 6I, and the reduction effect is small. 
     In a method in which the number of pipeline stages is set to four stages in order to attain speeding up and to reduce pre-charge current of the match line as well, as in FIG. 10A of Patent Document 2, the pre-charge current of the match line decreases from 4 I to 2 I, but the consumption current of the match amplifier circuit increases from 1 I to 4I. Therefore, the consumption current as a whole rather increases from 7I to 8I, and there is no reduction effect. 
     In Patent Document 3, although pre-charge current of the match line decreases from 4I to 1I and charge and discharge current of the search line decreases from 2I to 1I, consumption current in the match amplifier circuit increases from 1I to 4I. Therefore, the consumption current as a whole decreases only from 7I to 6I, and the reduction effect is small. 
     With speeding up of a network, performance expected to CAM becomes high and it is predicted that searching at 500 Msps or more will be required in the near future. Therefore, in FIG. 3 of Patent Document 1 and in Patent Document 2, pre-charge (or discharge) operation (at the next clock) is becoming difficult. For example, in the case of 500 Msps, it is necessary to complete match line pre-charge or discharge of the next stage, after activating a match amplifier circuit and determining Miss/Hit within ½ cycle (1 ns). Accordingly, a design for realizing this situation is very difficult. 
     Patent Document 3 and Patent Document 4 both have failed to describe simultaneous pursuit of low power consumption and speeding up. 
     For that reason, the first purpose of the present invention is to provide a content addressable memory device which can manage to balance low power consumption and speeding up. 
     In a configuration illustrated in FIG. 1 of Patent Document 5, speeding up is attained by reducing parasitic capacitance of a match line of a CAM array by dividing the match line into plural parts, and by adopting pipeline structure. However, there is a problem that the number of circuits, such as a control circuit, a latch, and a flip-flop, increases due to adoption of the pipeline structure, and that a layout increases due to the increased number of circuits. 
     In a configuration illustrated in FIG. 1 of Patent Document 6, speeding up of access of a one-bit miss is attained by detecting a very small amplitude signal of a match line through the use of a differential amplifier for an amplifier circuit of a match line. However, the further speeding up by a future advanced process is considered to meet difficulty. The reason for the difficulty lies in the fact that, when miniaturization is advanced from now on, a line-to-line capacitance of a match line with an adjoining line will increase. Accordingly, parasitic capacitance of the match line does not decrease (or rather increases) and driving ability of a search transistor decreases by miniaturization. Therefore, access of a one-bit miss becomes slow. Since an amount of local variations of transistors becomes large by miniaturization and a sensitivity of an amplifier such as a differential amplifier for a match line deteriorates, it is likely that access deteriorates as a result. In order to avoid the situation, it is necessary to enlarge an amplifier circuit or to enlarge a search transistor in order to enlarge a signal amplitude of a match line at the time of a one-bit miss. Thereby the layout will increase greatly. 
     In the configuration of Patent Document 7, since match lines are formed hierarchically, a transistor for pre-charging a local match line and a circuit for controlling the transistor are needed. 
     For that reason, the second purpose of the present invention is to provide a content addressable memory device in which rapid access at the time of a one-bit miss is possible, without being accompanied by drastic increase in the number of circuits and the layout. 
     In Patent Document 6, reduction of consumption current of a match line is attained by lowering the voltage of a pre-charge level of the match line. However, because of the voltage lowered, a high sensitivity amplifier for amplifying a match line signal of a small amplitude and a reference voltage are needed. Therefore, amplifier control becomes more complicated than before, causing a problem that speeding up and realization of a high degree of integration are difficult. 
     For that reason, the third purpose of the present invention is to provide a content addressable memory device in which low power consumption is realizable, without being accompanied by drastic increase in the number of circuits and the layout. 
     A content addressable memory device according to one embodiment of the present invention comprises: a first memory array which comprises plural content addressable memory cells arranged in a matrix; plural first match lines each of which is coupled to each of plural content addressable memory cells belonging to a corresponding entry in the first memory array; plural first determination circuits each of which determines coincidence or non-coincidence of search data and data stored in a content addressable memory in an entry of the first memory array, according to a voltage of the first match line; a second memory array which comprises plural content addressable memory cells arranged in a matrix, plural second match lines each of which is coupled to each of plural content addressable memory cells belonging to a corresponding entry in the second memory array; plural second determination circuits each of which determines coincidence or non-coincidence of search data and data stored in a content addressable memory in an entry of the second memory array, according to a voltage of the second match line; and a control circuit which directs to start searching in the second memory array after two or more cycles after searching has been started in the first memory array, and directs to stop searching in the second memory array according to a voltage of the first match line after the searching in the first memory array. 
     According to one embodiment of the present invention, it is possible to realize a content addressable memory device which can manage to balance low power consumption and speeding up. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing illustrating roughly an entire configuration of a content addressable memory device according to Embodiment 1; 
         FIG. 2  is a drawing illustrating a configuration of a unit cell UC; 
         FIG. 3  is a drawing illustrating a configuration of an entry of a memory array A and a match amplifier A coupled to the entry; 
         FIG. 4  is a drawing illustrating a configuration of a block-B activation control circuit; 
         FIG. 5  is a drawing illustrating a configuration of an entry of a memory array B and a match amplifier B coupled to the entry; 
         FIG. 6  is a drawing for explaining search timing of a block A and a block B; 
         FIG. 7  is a drawing illustrating operating waveforms of the content addressable memory device according to Embodiment 1; 
         FIG. 8  is a drawing illustrating roughly an entire configuration of a conventional content addressable memory device; 
         FIG. 9  is a drawing illustrating a configuration of a match amplifier A of a match amplifier A zone illustrated in  FIG. 8 ; 
         FIG. 10  is a drawing for explaining search timing of a block A and a block B illustrated in  FIG. 8 ; 
         FIG. 11  is a drawing illustrating roughly operating waveforms of the content addressable memory device illustrated in  FIG. 8 ; 
         FIG. 12  is a drawing illustrating a configuration of a content addressable memory device according to Embodiment 2; 
         FIG. 13  is a drawing illustrating a configuration of an entry of a memory array A and a match amplifier A and a driver B which are coupled to the entry; 
         FIG. 14  is a drawing illustrating a configuration of an entry of a memory array B and a match amplifier B coupled to the entry; 
         FIG. 15  is a drawing illustrating operating waveforms of the content addressable memory device according to Embodiment 2; 
         FIG. 16  is a drawing enlarging a part of the operating waveforms illustrated in  FIG. 15 ; 
         FIG. 17  is a drawing illustrating a configuration of a content addressable memory device according to Embodiment 3; 
         FIG. 18  is a drawing illustrating a configuration of a match amplifier D of a block D; 
         FIG. 19  is a drawing for explaining search timing of blocks A-D according to Embodiment 3; 
         FIG. 20  is a drawing illustrating a configuration of a content addressable memory device according to Embodiment 4; 
         FIG. 21  is a drawing for explaining search timing of blocks A-D according to Embodiment 4; 
         FIG. 22  is a drawing illustrating a configuration of a content addressable memory device according to Embodiment 5; 
         FIG. 23  is a drawing for explaining search timing of blocks A-D according to Embodiment 5; 
         FIG. 24  is a drawing illustrating a configuration of a content addressable memory device according to Embodiment 6; 
         FIG. 25  is a drawing for explaining search timing of blocks A-D according to Embodiment 6; 
         FIG. 26  is a drawing illustrating roughly an entire configuration of a content addressable memory device according to Embodiment 7; 
         FIG. 27  is a drawing illustrating a configuration of one entry and a transfer circuit  14  and a match amplifier unit  10  corresponding to the entry, in the content addressable memory device according to Embodiment 7; 
         FIG. 28  is a drawing illustrating a configuration of one entry and a match amplifier corresponding to the entry, in a conventional content addressable memory device; 
         FIG. 29  is a drawing illustrating operating waveforms of the conventional content addressable memory device illustrated in  FIG. 28 ; 
         FIG. 30  is a drawing illustrating operating waveforms when an A side entry is of Miss and a B side entry is of Hit in Embodiment 7; 
         FIG. 31  is a drawing illustrating operating waveforms when the A side entry is of Hit and the B side entry is of Miss in Embodiment 7; 
         FIG. 32  is a drawing illustrating a configuration of one entry and a transfer circuit and a match amplifier unit corresponding to the entry, in a content addressable memory device according to Embodiment 8; 
         FIG. 33  is a drawing illustrating operating waveforms of the content addressable memory device according to Embodiment 8; 
         FIG. 34  is a drawing illustrating a configuration of one entry and a transfer circuit and a match amplifier unit corresponding to the entry, in a content addressable memory device according to Embodiment 9; 
         FIG. 35  is a drawing illustrating operating waveforms of the content addressable memory device according to Embodiment 9; 
         FIG. 36  is a drawing illustrating a configuration of one entry and a transfer circuit and a match amplifier unit corresponding to the entry, in a content addressable memory device according to Embodiment 10; 
         FIG. 37  is a drawing illustrating operating waveforms of the content addressable memory device according to Embodiment 10; 
         FIG. 38  is a drawing illustrating a configuration of one entry and a transfer circuit and a match amplifier unit corresponding to the entry, in a content addressable memory device according to Embodiment 11; 
         FIG. 39  is a drawing illustrating search timing of the content addressable memory device according to Embodiment 11; 
         FIG. 40  is a drawing illustrating operating waveforms of the content addressable memory device according to Embodiment 11; 
         FIG. 41  is a drawing illustrating a configuration of one entry and a transfer circuit and a match amplifier unit corresponding to the entry, in a content addressable memory device according to Embodiment 12; 
         FIG. 42  is a drawing illustrating a configuration of a content addressable memory device according to Embodiment 13; 
         FIG. 43  is a drawing illustrating an example of a switching control circuit; 
         FIG. 44  is a drawing illustrating operating waveforms of the content addressable memory device according to Embodiment 13; 
         FIG. 45  is a drawing illustrating operating waveforms of the conventional content addressable memory device; 
         FIG. 46  is a drawing illustrating current waveforms in Embodiment 13; 
         FIG. 47  is a drawing illustrating a configuration of a content addressable memory device according to Embodiment 14; 
         FIG. 48  is a drawing illustrating a configuration of a content addressable memory device according to Embodiment 15; 
         FIG. 49  is a drawing illustrating a configuration of a switching control circuit according to Embodiment 16; 
         FIG. 50  is a drawing illustrating a configuration of a content addressable memory device according to Embodiment 17; 
         FIG. 51  is a drawing illustrating a configuration of a content addressable memory device according to Embodiment 18; 
         FIG. 52  is a drawing illustrating a configuration of a content addressable memory device according to Embodiment 19; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, with reference to the accompanying drawings, the embodiments of the present invention are explained in detail. 
     (Embodiment 1) 
     (Configuration)  FIG. 1  illustrates roughly an entire configuration of a content addressable memory device according to Embodiment 1. 
     As illustrated in  FIG. 1 , the content addressable memory device according to Embodiment 1 comprises an array block, a clock control circuit  200 , a search data input circuit  990 , and a priority encoder  201 . 
     The clock control circuit  200  generates a search clock sclk&lt; 0 &gt;-sclk&lt; 5 &gt; based on a search command GSE and a clock camclk. 
     The search data input circuit  990  divides search data db&lt; 79 : 0 &gt; comprised of 80 bits into first data db&lt; 39 : 0 &gt; and second data db&lt; 79 : 40 &gt;, and supplies the first data to a search line driver zone  204  and the second data to a search line driver zone  209  after delaying it for a prescribed time (two cycles). 
     The search data input circuit  990  comprises a delay circuit  980  which is comprised of a latch circuit  211  and a latch circuit  970 . 
     The latch circuit  970  latches the search data db&lt; 79 : 40 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. The latch circuit  211  latches an output of the latch circuit  970 , according to the search clock sclk&lt; 2 &gt;. 
     The array block (80 bits×256 entries) is divided into a block A (the first half of 40 bits×256 entries) and a block B (the second half of 40 bits×256 entries). 
     The block A comprises a memory array A, a match amplifier A zone  206  adjoining the memory array A in the row direction, a search line driver zone  204  adjoining the memory array A in the column direction, a block-A control circuit  205  adjoining the match amplifier A zone  206  and the search line driver zone  204 , and a block-B activation control circuit  207  adjoining the match amplifier A zone  206  and the block-A control circuit  205  in the row direction. 
     The memory array A comprises a matrix of unit cells UC each of which serves as an content addressable memory cell (CAM cell). 
     The memory array A is divided into plural entries ERY in units of row. Each entry ERY is provided with a match line MLA to which a unit cell UC in the corresponding entry is coupled in parallel. A search line pair SLA (SL, /SL) which transmits the search data is provided in the column direction of the memory array A. A unit cell UC is provided corresponding to a cross point of the search line pair SLA and the match line MLA. 
     The match amplifier A zone  206  comprises a match amplifier A provided corresponding to each entry ERY. The match amplifier A is coupled to a match line MLA of the corresponding entry ERY, and determines coincidence/non-coincidence of search data and stored data of each entry in the memory array A. 
     The search line driver zone  204  comprises a search line driver for every search line SL. The search line driver A zone receives the 40-bit search data db&lt; 39 : 0 &gt; from the exterior. The search line driver supplies the VDD voltage or a ground voltage to the corresponding search line SL, according to search data db&lt;i&gt; from the exterior. 
     The block-A control circuit  205  controls components in the block A. The block-B activation control circuit  207  controls search activity of the block B based on a search result of the memory array A. 
     The block B comprises a memory array B, a match amplifier B zone  208  adjoining the memory array B in the row direction, a search line driver zone  209  adjoining the memory array B in the column direction, and a block-B control circuit  210  adjoining the match amplifier B zone  208  and the search line driver zone  209 . 
     The memory array B comprises a matrix of unit cells UC each of which serves as a content addressable memory cell (CAM cell). 
     The memory array B is divided into plural entries ERY in units of row. Each entry ERY is provided with a match line MLB to which a unit cell UC in the corresponding entry is coupled in parallel. A search line pair SLB (SL, /SL) which transmits the search data is provided in the column direction of the memory array B. A unit cell UC is provided corresponding to a cross point of the search line pair SLB and the match line MLB. 
     The match amplifier B zone  208  comprises a match amplifier B provided corresponding to each entry ERY. The match amplifier B is coupled to a match line MLB of the corresponding entry ERY, and determines coincidence/non-coincidence of search data and stored data of each entry in the memory array B. The match amplifier B outputs a search determination signal mao_n which indicates a search result in the entire of the memory array A and the memory array B. 
     The search line driver zone  209  comprises a search line driver for every search line SL. The search line driver B zone receives the 40-bit search data db&lt; 79 : 40 &gt; from the exterior via the delay circuit  980 . The search line driver supplies a VDD voltage or a ground voltage to the corresponding search line SL, according to search data db&lt;i&gt; from the exterior. 
     The block-B control circuit  210  controls components in the block B. The priority encoder  201  receives the search determination signal mao_n[ 0 ]-mao_n[ 255 ] from all the entries. The priority encoder  201  outputs an address of an entry with a high priority among the entries which have outputted a search determination signal of an “L” level. 
     (Unit cell)  FIG. 2  illustrates a configuration of a unit cell UC. 
       FIG. 2  illustrates an example of a configuration of a unit cell UC in the memory array A and the memory array B illustrated in  FIG. 1 . In  FIG. 2 , the unit cell UC comprises a SRAM cell SMC which stores one-bit data, N-channel MOS transistors (insulated-gate field effect transistors) TR 1  and TR 2  coupled in series between a match line ML and a ground node, and N-channel MOS transistors TR 3  and TR 4  coupled in series between the match line ML and the ground node. Gates of the MOS transistors TR 1  and TR 3  are coupled to the search lines SL and /SL, respectively. The MOS transistors TR 2  and TR 4  serve as search transistors, and the respective sources are coupled to the grand power supply. 
     Gates of the MOS transistors TR 2  and TR 4  are coupled to internal storage nodes /D and D of the SRAM cell SMC, respectively. Data bits which are complementary with each other are stored in the memory nodes D and /D. When the SRAM cell SMC stores data “1”, the internal storage node D is at a high level (H level), and the internal storage node /D is at a low level (L level). Therefore, in the present state, the MOS transistor TR 2  is in a conductive state and the MOS transistor TR 4  is in a non-conductive state. When the SRAM cell SMC stores data “0”, the states are reversed. 
     Search lines SL and /SL form the search line pair SLA or SLB illustrated in  FIG. 1 , and complementary data are transmitted at the time of searching. In the unit cell UC illustrated in  FIG. 2 , a word line and a bit-line pair for performing data writing and data reading of the SRAM cell SMC are provided. However, the word line and the bit-line pair are not illustrated in  FIG. 2 . 
     At the time of searching, it is assumed that the search data “1” is supplied, when the SRAM cell SMC stores “1” (the internal storage node D is at an H level). In the present case, the search line SL is at an H level and the complementary search line /SL is at an L level. Accordingly, the MOS transistors TR 2  and TR 3  are in a non-conductive state, and the match line ML maintains a pre-charge voltage level. On the other hand, when the internal storage node D of the SRAM cell SMC is at an H level and the search data of “0” is transferred to the search line SL, the search line SL is at an L level and the complementary search line /SL is at an H level. In the present case, the MOS transistors TR 3  and TR 4  are in a conductive state, and the match line ML is discharged from the pre-charge voltage level to the ground voltage level by the MOS transistor TR 4  which is serving as a search transistor. 
     Accordingly, when the unit cell UC illustrated in  FIG. 2  is used, it is possible to make binary determination of coincidence/non-coincidence of the search data and the stored data of an entry. Unit cells UC of the corresponding entry are coupled to the match line ML in parallel. When the unit cells UC of an entry ERY are all in a coincidence state, the match line ML maintains the pre-charge voltage level. On the other hand, when a unit cell for at least one bit in an entry is in a non-coincidence state, the match line ML is discharged via the present unit cell in the non-coincidence state, and the potential of the match line ML falls from the pre-charge voltage level. Accordingly, by amplifying the potential level of the match line ML with the match amplifier A or the match amplifier B, it becomes possible to make binary determination of coincidence/non-coincidence of the search data and the stored data of each entry. 
     (Entry of block A)  FIG. 3  illustrates a configuration of an entry of a memory array A and a match amplifier A coupled to the entry. 
     As illustrated in  FIG. 3 , the match amplifier A comprises a P-channel MOS transistor  214 . The P-channel MOS transistor  214  pre-charges the match line MLA to the VDD voltage, when an inverted pre-charge signal /MLPREA is activated to an L level. 
     The match amplifier A comprises a match amplifier circuit  212 , an inverter IV 1 , and a latch circuit  216 . 
     The match amplifier circuit  212  comprises a differential amplifier  12   a  to which a voltage of the match line A and a reference voltage VREF are inputted, and an N-channel MOS transistor  212   b  which couples the differential amplifier  12   a  to the grand power supply. The differential amplifier  12   a  amplifies a difference voltage of the voltage of the match line A and the reference voltage VREF, when a match amplifier activation signal MAEA is activated to an H level. 
     The inverter IV 1  inverts an output of the match amplifier circuit  212 , and outputs it to the latch circuit  216 . According to the match amplifier activation signal MAEA, the latch circuit  216  latches an output of the inverter IV 1 , and outputs a block-A search determination signal maoa_n. The block-A search determination signal maoa_n turns to an L level, when all the unit cells UC of the corresponding entry of the memory array A are of Hit, and turns to an H level, when at least one unit cell UC of the corresponding entry of the memory array A is of Miss. 
     (Block B activation control circuit)  FIG. 4  illustrates a configuration of a block-B activation control circuit. 
     As illustrated in  FIG. 4 , the block-B activation control circuit  207  comprises a match OR circuit  299  and a latch circuit  220 . 
     The match OR circuit  299  comprises plural OR circuits OR 101 -OR 110  which are formed hierarchically. The match OR circuit  299  is activated by a search clock sclk&lt; 3 &gt;, and outputs an inverted logical product signal match_or of block-A search determination signals maoa_n&lt; 0 &gt;-maoa&lt; 255 &gt;. The match OR circuit  299  outputs the inverted logical product signal match_or at the timing of the search clock sclk&lt; 3 &gt;. 
     The latch circuit  220  latches the inverted logical product signal match_or, and outputs a search activation signal srch_act. When all the entries of the block A are of Miss, it is not necessary to search the block B. Therefore, the search activation signal srch_act is deactivated to an L level, and the block B does not perform searching. When at least one entry of the block A is of Hit, on the other hand, the search activation signal srch_act is activated to an H level, and the block B performs searching. 
     (Entry of block B)  FIG. 5  illustrates a configuration of an entry of a memory array B and a match amplifier B coupled to the entry. 
     As illustrated in  FIG. 5 , the match amplifier B comprises a P-channel MOS transistor  314 . The P-channel MOS transistor  314  pre-charges the match line MLB to the VDD voltage, when an inverted pre-charge signal /MLPREB is activated to an L level. 
     The match amplifier B comprises a match amplifier circuit  312 , an inverter IV 2 , and a latch circuit  316 . 
     The match amplifier circuit  312  comprises a differential amplifier  312   a  to which a voltage of the match line B and the reference voltage VREF are inputted, and an N-channel MOS transistor  312   b  which couples the differential amplifier  312   a  to the grand power supply. The differential amplifier  312   a  amplifies a difference voltage of the voltage of the match line B and the reference voltage VREF, when a match amplifier activation signal MAEB is activated to an H level. 
     The inverter IV 2  inverts an output of the match amplifier circuit  312 , and outputs it to the latch circuit  316 . According to the match amplifier activation signal MAEB, the latch circuit  316  latches an output of the inverter IV 2 , and outputs a block-B search determination signal maob_n. The block-B search determination signal maob_n turns to an L level, when all the unit cells UC of the corresponding entry of the memory array B are of Hit, and turns to an H level, when at least one unit cell UC of the corresponding entry of the memory array B is of Miss. 
     The inverted logical addition circuit  318  outputs an inverted logical addition of the block-A search determination signal maoa_n and the block-B search determination signal maob_n. 
     The inverted logical product circuit  320  outputs an inverted logical product of an output of the inverted logical addition circuit  318  and the search activation signal srch_act, as a search determination signal mao_n. 
     That is, the search determination signal mao_n turns to an H level indicating Miss, when the search activity signal srch_act is at an L level. The search determination signal mao_n turns to an L level indicating Hit, when the search activity signal srch_act is at an H level, the block-A search determination signal maoa_n is at an L level (that is, Hit), and the block-B search determination signal maob_n is at an L level (that is, Hit). 
     (Search timing)  FIG. 6  is a drawing for explaining search timing of the block A and the block B. 
     As illustrated in  FIG. 6 , the clock control circuit  200  generates the search clock sclk&lt; 0 &gt;-sclk&lt; 5 &gt; based on the clock camclk, when the search command GSE is activated to an H level. The block A performs searching at the timing of rising of the search clock sclk&lt; 0 &gt;. The block B performs searching at the timing of rising of the search clock sclk&lt; 4 &gt; which is delayed by two cycles from the search clock sclk&lt; 0 &gt;. That is, searching of two blocks is performed during three cycles. 
     (Operation)  FIG. 7  illustrates operating waveforms of the content addressable memory device according to Embodiment 1. 
     As illustrated in  FIG. 7 , first, the clock control circuit  200  changes the search clock sclk&lt; 0 &gt; to an H level, in response to the fact that the search command GSE has been activated at the timing of CLK# 1  of the clock camclk. The block-A control circuit  205  activates the pre-charge signal MLPREA to an H level, according to the fact that the search clock sclk&lt; 0 &gt; has changed to an H level. The P-channel MOS transistor  214  pre-charges the match line MLA of the memory array A, according to the fact that the pre-charge signal MLPREA has been activated to an H level. 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 1 &gt; to an H level at the timing of CLK# 1 B of the clock camclk. The search line driver of the block A activates the search line SLA of the block A, according to the fact that the search clock sclk&lt; 1 &gt; has changed to an H level. 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 1 &gt; to an L level and changes the search clock sclk&lt; 2 &gt; to an H level, at the timing of CLK# 2  of the clock camclk, the block-A control circuit  205  activates the match amplifier activation signal MAEA to an H level, according to the fact that the search clock sclk&lt; 1 &gt; has changed to an L level. The match amplifier circuit  212  amplifies a voltage of the match line MLA, according to the fact that the match amplifier activation signal MAEA has been activated to an H level. The latch circuit  216  latches an output of the match amplifier circuit  212 , and outputs a block-A search determination signal maoa_n[ 0 ] (in the case of the 0th entry). When the block-A search determination signal maoa_n[ 0 ] is at an L level, it is meant that all the unit cells UC of the 0th entry of the block A are of Hit. When the block-A search determination signal maoa_n[ 0 ] is at an H level, it is meant that at least one unit cell UC of the 0th entry of the block A is of Miss. The block-A control circuit  205  activates the match amplifier activation signal MAEA to an H level for a prescribed time and then deactivates it to an L level. The search line SLA is deactivated by the search line driver of the block A after the match amplifier activation signal MAEA is activated. After the search line SLA is deactivated, the block-A control circuit  205  activates the pre-charge signal MLPREA to an H level. The P-channel MOS transistor  214  pre-charges the match line MLA of the block A, according to the fact that the pre-charge signal MLPREA has been activated to an H level (accordingly, an inverted pre-charge signal /MLPREA has been activated to an L level). 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 3 &gt; to an H level at the timing of CLK# 2 B of the clock camclk. According to the fact that the search clock sclk&lt; 3 &gt; has changed to an H level, the match OR circuit  299  of the block-B activation control circuit  207  calculates an inverted logical product of the block-A search determination signals maoa_n&lt; 0 &gt;-maoa_n&lt; 255 &gt; which are outputted by the latch circuit  216  of all the entries of the block A, and outputs an inverted logical product signal match_or. When the inverted logical product signal match_or is at an H level, it is meant that all the unit cells UC are of Hit in at least one entry of the block A. When the inverted logical product signal match_or is at an L level, it is meant that at least one unit cell UC is of Miss in all the entries of the block A. 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 4 &gt; to an H level at the timing of CLK# 3  of the clock camclk. According to the fact that the search clock sclk&lt; 4 &gt; has changed to an H level, the latch circuit  220  of the block-B activation control circuit  207  latches the inverted logical product signal match_or, and outputs the search activation signal srch_act to the block B. The block-B control circuit  210  activates the pre-charge signal MLPREB to an H level, when the inverted logical product signal match_or outputted from the block A is activated to an H level. On the other hand, the block-B control circuit  210  does not activate the pre-charge signal MLPREB to an H level, when the inverted logical product signal match_or is deactivated to an L level. 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 5 &gt; to an H level at the timing of CLK# 3 B of the clock camclk. When the search activation signal srch_act outputted from the block A is activated to an H level according to the fact that the search clock sclk&lt; 5 &gt; has changed to an H level, the search line driver of the block B activates the search line SLB of the block B. On the other hand, the search line driver of the block B does not activate the search line SLB of the block B, when the search activation signal srch_act is deactivated to an L level. 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 5 &gt; to an L level at the timing of CLK# 4  of the clock camclk. According to the fact that the search clock sclk&lt; 5 &gt; has changed to an L level, the block-B control circuit  210  activates the match amplifier activation signal MAEB to an H level, when the search activation signal srch_act outputted from the block A is activated to an H level. The match amplifier circuit  312  amplifies a voltage of the match line MLB, according to the fact that the match amplifier activation signal MAEB has been activated to an H level. The latch circuit  316  latches an output of the match amplifier circuit  312 , and outputs the block-B search determination signal maob_n[ 0 ] (in the case of the 0th entry). When the block-B search determination signal maob_n[ 0 ] is at an L level, it is meant that all the unit cells UC of the 0th entry of the block B are of Hit. When the block-B search determination signal maob_n[ 0 ] is at an H level, it is meant that at least one unit cell UC of the 0th entry of the block B is of Miss. The inverted logical addition circuit  318  outputs an inverted logical addition signal of the block-A search determination signal maoa_n[ 0 ] and the block-B search determination signal maob_n[ 0 ]. The inverted logical addition signal is set to an H level only when both of the block-A search determination signal maoa_n[ 0 ] and the block-B search determination signal maob_n[ 0 ] are at an L level, that is, when all the unit cells of the 0th entry of the block A and the block B are of Hit. The inverted logical product circuit  320  outputs an inverted logical product of an inverted logical addition signal outputted by the inverted logical addition circuit  318  and the search activation signal srch_act, as the search determination signal mao_n. Only when all the CAM cells of the 0th entry of the block A and the block B are of Hit, and the search activation signal srch_act is at an H level (that is when all the CAM cells are of Hit in at least one entry of the block A), the search determination signal mao_n[ 0 ] becomes at an L level indicating Hit. 
     On the other hand, the block-B control circuit  210  does not activate the match amplifier activation signal MAEB to an H level, when the search activation signal srch_act outputted from the block A is deactivated to an L level. In this case, the search determination signal mao_n[n] (n=0-255) of all the entries is set to an H level indicating Miss by the inverted logical product circuit  320 . 
     The priority encoder  201  receives the search determination signals mao_n[ 0 ]-mao_n[ 255 ] from all the entries. The priority encoder  201  outputs an address of an entry of high priority among the entries which have outputted the search determination signal of an L level. 
     (Reference)  FIG. 8  illustrates roughly an entire configuration of a conventional content addressable memory device.  FIG. 9  illustrates a configuration of a match amplifier A of a match amplifier A zone illustrated in  FIG. 8 . 
       FIG. 10  is a drawing for explaining search timing of a block A and a block B illustrated in  FIG. 8 . 
     As illustrated in  FIG. 10 , a clock control circuit  1200  generates search clocks sclk&lt; 0 &gt;-sclk&lt; 3 &gt; based on the clock camclk, when a search command GSE is activated to an H level. The block A performs searching at the timing of rising of the search clock sclk&lt; 0 &gt;. The block B performs searching at the timing of rising of the search clock sclk&lt; 2 &gt; which is delayed by one cycle from the search clock sclk&lt; 0 &gt;. That is, searching of two blocks is performed during two cycles. 
       FIG. 11  illustrates roughly operating waveforms of the content addressable memory device illustrated in  FIG. 8 . As illustrated in  FIG. 11 , since the pre-charge signal MLPREB is activated to an H level based on the search clock sclk&lt; 2 &gt;, it is necessary to pre-charge or to discharge the match line MLB within a period Δt less than ½ cycle. When the frequency of the clock camclk becomes high, it will become difficult to complete pre-charging or discharging the match line MLB in the period Δt. 
     (Effects) The breakdown of the consumption current in a CAM memory is given on an average by a ratio of (match amplifier: search line: match line)=(1:2:4). Assuming that current of a match amplifier, current of a search line, and current of a match line is 1I, 2I, and 4I, respectively, in the present embodiment, the pre-charge current of the match line is reduced from 4I to 2I, there is no increase of the consumption current in the match amplifier, and the charge and discharge current of the search line is reduced from 2I to 1I. Therefore, the consumption current as a whole is reduced from 7I to 4I. 
     In the present embodiment, when the block A has an entry of which the search result is Hit, searching of the block B is performed. However, since search of almost all the entries results in Miss in the ordinary CAM (for example, only several searches are of Hit out of 256K entries in a 20M-byte CAM array), almost all the block A of the entire chip becomes of Miss, and almost all the block B becomes non-active. Therefore, in the entire chip, the consumption current becomes about 4/7, and a large reduction effect is obtained. 
     (Embodiment 2) 
     (Configuration)  FIG. 12  illustrates a configuration of a content addressable memory device according to Embodiment 2. 
     As illustrated in  FIG. 12 , the content addressable memory device according to Embodiment 2 is different from the content addressable memory device according to Embodiment 1 illustrated in  FIG. 1 , with respect to the parts of a match amplifier A plus driver zone  306  and a match amplifier B zone  308 . 
     The match amplifier A plus driver B zone  306  comprises a match amplifier A and a driver B which are provided corresponding to each entry ERY. The match amplifier A is coupled to a match line MLA of the corresponding entry ERY, and determines coincidence/non-coincidence of search data and stored data of each entry. The driver B pre-charges or discharges a match line MLB, according to a determination result in the match amplifier A. 
     The match amplifier B zone  308  comprises a match amplifier B provided corresponding to each entry ERY. The match amplifier B is coupled to the match line MLB of the corresponding entry ERY, and determines coincidence/non-coincidence of search data and stored data of each entry in the memory array B, and outputs a search determination signal mao_n which indicates a search result in the entire of the memory array A and the memory array B. 
     (Entry of block A)  FIG. 13  illustrates a configuration of an entry of a memory array A and a match amplifier A and a driver B which are coupled to the entry. 
     As illustrated in  FIG. 13 , the match amplifier A is the same as the match amplifier A of the match amplifier A illustrated in  FIG. 3 . The latch circuit  181  latches an output of the match amplifier A according to the search clock sclk&lt; 3 &gt;. 
     The driver B ( 182 ) comprises a logical addition circuit OR 182  and a P-channel MOS transistor TR 180 . The logical addition circuit OR 182  outputs a logical addition of a delayed block-A search determination signal maoa_n_lat outputted by the latch circuit  181 , and an inverted pre-charge signal /MLPREB. The P-channel MOS transistor TR 180  has a source coupled to the VDD voltage, a gate coupled to an output of the logical addition circuit OR 182 , and a drain coupled to the match line MLB. 
     When the inverted pre-charge signal /MLPREB is activated to an L level, and when the delayed block-A search determination signal maoa_n_lat is at an L level (indicating Hit), the match line MLB is pre-charged to the VDD voltage by the driver B ( 182 ). 
     The driver B ( 182 ) comprises a logical product circuit AND 182  and an N-channel MOS transistor TR 181 . The logical product circuit AND 182  outputs a logical product of a delayed block-A search determination signal maoa_n_lat outputted by the latch circuit  181 , and an inverted signal of the inverted pre-charge signal /MLPREB. The N-channel MOS transistor TR 181  has a source coupled to the ground power supply, a gate coupled to an output of the logical product circuit AND 182 , and a drain coupled to the match line MLB. 
     When the inverted pre-charge signal /MLPREB is activated to an L level, and when the delayed block-A search determination signal maoa_n_lat is at an H level (indicating Miss), the match line MLB is discharged to 0V by the driver B ( 182 ). 
     (Entry of block B)  FIG. 14  illustrates a configuration of an entry of a memory array B and a match amplifier B coupled to the entry. 
     As illustrated in  FIG. 14 , the match amplifier B comprises a match amplifier circuit  312 , an inverter IV 2 , a latch circuit  316 , and an inverted logical product circuit  320 . 
     The match amplifier circuit  312  comprises a differential amplifier  312   a  to which a voltage of the match line MLB and the reference voltage VREF are inputted, and an N-channel MOS transistor  312   b  for coupling the differential amplifier  312   a  to the ground power supply. The match amplifier circuit  312   a  amplifies a difference voltage of a voltage of the match line MLB and the reference voltage VREF, when the match amplifier activation signal MAEB is activated to an H level. 
     The inverter IV 2  inverts an output of the match amplifier circuit  312 , and outputs it to the latch circuit  316 . According to the match amplifier activation signal MAEB, the latch circuit  316  latches an output of the inverter IV 2 , and outputs the block-B search determination signal maob_n. The block-B search determination signal maob_n turns to an L level when all the unit cells UC of the corresponding entry of the memory array B are of Hit, and turns to an H level when at least one unit cell UC is of Miss. 
     The inverted logical product circuit  320  outputs an inverted logical product of an inverted signal of the block-B search determination signal maob_n and the search activation signal srch_act, as a search determination signal mao_n. The search determination signal mao_n turns to an L level (indicating Hit), only when the search activity signal srch_act is at an H level (at least one entry of the block A is of Hit) and the block-B search determination signal maob_n is at an L level (an entry of the block B is of Hit). 
     (Operation)  FIG. 15  illustrates operating waveforms of the content addressable memory device according to Embodiment 2. 
     As illustrated in  FIG. 15 , first, the clock control circuit  200  changes the search clock sclk&lt; 0 &gt; to an H level, in response to the fact that the search command GSE has been activated at the timing of CLK# 1  of the clock camclk. The block-A control circuit  205  activates the pre-charge signal MLPREA to an H level, according to the fact that the search clock sclk&lt; 0 &gt; has changed to an H level. The P-channel MOS transistor  214  pre-charges the match line MLA of the block A, according to the fact that the pre-charge signal MLPREA has been activated to an H level. 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 1 &gt; to an H level at the timing of CLK# 1 B of the clock camclk. The search line driver activates the search line SLA of the block A, according to the fact that the search clock sclk&lt; 1 &gt; has changed to an H level. 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 1 &gt; to an L level and changes the search clock sclk&lt; 2 &gt; to an H level, at the timing of CLK# 2  of the clock camclk. The block-A control circuit  205  activates the match amplifier activation signal MAEA to an H level, according to the fact that the search clock sclk&lt; 1 &gt; has changed to an L level. The match amplifier circuit  212  amplifies a voltage of the match line MLA, according to the fact that the match amplifier activation signal MAEA has been activated to an H level. The latch circuit  216  latches an output of the match amplifier circuit  212 , and outputs a block-A search determination signal maoa_n[ 0 ] (in the case of the 0th entry). When the block-A search determination signal maoa_n[ 0 ] is at an L level, it is meant that all the unit cells UC of the 0th entry of the block A are of Hit. When the block-A search determination signal maoa_n[ 0 ] is at an H level, it is meant that at least one unit cell UC of the 0th entry of the block A is of Miss. The block-A control circuit  205  activates the match amplifier activation signal MAEA to an H level for a prescribed time and then deactivates it to an L level. The search line SLA is deactivated by the search line driver of the block A after the match amplifier activation signal MAEA is activated. After the search line SLA is deactivated, the block-A control circuit  205  activates the pre-charge signal MLPREA to an H level. The P-channel MOS transistor  214  pre-charges the match line MLA of the block A, according to the fact that the pre-charge signal MLPREA has been activated to an H level (accordingly, an inverted pre-charge signal /MLPREA has been activated to an L level). 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 3 &gt; to an H level at the timing of CLK# 2 B of the clock camclk. According to the fact that the search clock sclk&lt; 3 &gt; has changed to an H level, the match OR circuit  299  of the block-B activation control circuit  207  calculates an inverted logical product of the block-A search determination signals maoa_n&lt; 0 &gt;-maoa_n&lt; 255 &gt; which are outputted by the latch circuit  216  of all the entries of the block A, and outputs an inverted logical product signal match_or. When the inverted logical product signal match_or is at an H level, it is meant that all the unit cells UC are of Hit in at least one entry of the block A. When the inverted logical product signal match_or is at an L level, it is meant that at least one unit cell UC is of Miss in all the entries of the block A. The latch circuit  181  latches the block-A search determination signal maoa_n[ 0 ] outputted by the match amplifier circuit  212 , according to the search clock sclk&lt; 3 &gt;, and outputs the delayed block-A search determination signal maoa_n_lat[ 0 ] (in the case of the 0th entry). 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 4 &gt; to an H level at the timing of CLK# 3  of the clock camclk. According to the fact that the search clock sclk has changed to an H level, the latch circuit  220  of the block-B activation control circuit  207  latches the inverted logical product signal match_or, and outputs it as the search activation signal srch_act to the block B. The block-B control circuit  210  activates the pre-charge signal MLPREB to an H level, according to the fact that the search clock sclk&lt; 4 &gt; has changed to an H level. The driver B discharges the match line MLB to 0V when the pre-charge signal MLPREB is activated to an H level and the delayed block-A search determination signal maoa_n_lat[ 0 ] is at an H level. That is, the match line MLB of each entry is discharged to 0V, when at least one unit cell UC of the same entry of the block A is of Miss. Accordingly, it is possible to notify the block B of the fact that the block A has been of Miss. The voltage of the match line MLB serves as a voltage indicative of Miss, even when the block B is of Hit. 
     On the other hand, the driver B pre-charges the match line MLB to VDD, when the pre-charge signal MLPREB is activated to an H level and the delayed block-A search determination signal maoa_n_lat[ 0 ] is at an L level. That is, when all the unit cells UC of the same entry of the block A are of Hit, the match line MLB is pre-charged to VDD. 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 5 &gt; to an H level at the timing of CLK# 3 B of the clock camclk. When the search activation signal srch_act outputted from the block A is activated to an H level according to the fact that the search clock sclk&lt; 5 &gt; has changed to an H level, the search line driver of the block B activates the search line SLB of the block B. On the other hand, the search line driver of the block B does not activate the search line SLB of the block B, when the search activation signal srch_act is deactivated to an L level. 
     Next, the clock control circuit  200  changes the search clock sclk&lt; 5 &gt; to an L level at the timing of CLK# 4  of the clock camclk. According to the fact that the search clock sclk&lt; 5 &gt; has changed to an L level, the block-B control circuit  210  activates the match amplifier activation signal MAEB to an H level, when the search activation signal srch_act outputted from the block A is activated to an H level. The match amplifier circuit  312  amplifies a voltage of the match line MLB, according to the fact that the match amplifier activation signal MAEB has been activated to an H level. The latch circuit  316  latches an output of the match amplifier circuit  312 , and outputs the block-B search determination signal maob_n[ 0 ] (in the case of the 0th entry). When the block-B search determination signal maob_n[ 0 ] is at an L level, it is meant that all the unit cells UC of the 0th entry of the block A and the block B are of Hit. When the block-B search determination signal maob_n[ 0 ] is at an H level, it is meant that at least one unit cell UC of the 0th entry of the block A and the block B is of Miss. 
     The inverted logical product circuit  320  outputs an inverted logical product of the search activation signal srch_act and the inverted signal of the block-B search determination signal maob_n[ 0 ] outputted by the latch circuit  316 , as the search determination signal mao_n. Only when all the unit cells UC of the 0th entry of the block A and the block B are of Hit and the search activation signal srch_act is at an H level (that is, all the unit cells UC in at least one entry of the block A are of Hit), the search determination signal mao_n[ 0 ] becomes at an L level indicating Hit. 
     On the other hand, the block-B control circuit  210  does not activate the match amplifier activation signal MAEB to an H level, when the search activation signal srch_act outputted from the block A is deactivated to an L level. In this case, the search determination signal mao_n[n] (n=0-255) of all the entries is set to an H level indicating Miss by the inverted logical product circuit  320 . 
     The priority encoder  201  receives the search determination signals mao_n[ 0 ]-mao_n[ 255 ] from all the entries. The priority encoder  201  outputs an address of an entry of high priority among the entries which have outputted the search determination signal of an L level. 
       FIG. 16  is a drawing enlarging a part of the operating waveforms illustrated in  FIG. 15 . Since the pre-charge signal MLPREB is activated to an H level based on the search clock sclk&lt; 4 &gt;, as illustrated in  FIG. 16 , it is sufficient for the match line MLB to be pre-charged or discharged in ½ cycle. Accordingly, it is possible to pre-charge or to discharge the match line MLB even when frequency of the clock camclk becomes high. 
     (Effects) As described above, the present embodiment adds a circuit which controls pre-charge of the match line of the memory array B using the search result for every entry of the memory array A. An entry of Hit of the memory array A pre-charges the match line of the same entry of the memory array B, and an entry of Miss of the memory array A does not pre-charge the match line of the same entry of the memory array B. 
     In the present embodiment, the memory array B is searched with a latency of two cycles after the memory array A is searched. Therefore, a problem that pre-charge control is becoming difficult due to speeding up in the previously existing technology is solved, and it becomes possible to make simultaneous pursuit of low power consumption and speeding up. 
     (Embodiment 3) 
     (Configuration)  FIG. 17  illustrates a configuration of a content addressable memory device according to Embodiment 3. 
     As illustrated in  FIG. 17 , in the content addressable memory device according to Embodiment 3, searching is performed in units of block of blocks A, B, C, and D which are formed by dividing the basic array block (80 bits) into four blocks in units of 20 bits. 
     The block A comprises a memory array A, a match amplifier A zone  439  adjoining the memory array A in the row direction, a search line driver A zone  444  adjoining the memory array A in the column direction, a block-A control circuit  445  adjoining the match amplifier A zone  439  and the search line driver A zone  444 , and a block-BCD activation control circuit  443  adjoining the match amplifier A zone  439  and the block-A control circuit  445  in the row direction. 
     The block B comprises a memory array B, a match amplifier B zone  440  adjoining the memory array B in the row direction, a search line driver B zone  446  adjoining the memory array B in the column direction, and a block-B control circuit  447  adjoining the match amplifier B zone  440  and the search line driver B zone  446 . 
     The block C comprises a memory array C, a match amplifier C zone  441  adjoining the memory array C in the row direction, a search line driver C zone  448  adjoining the memory array C in the column direction, and a block-C control circuit  449  adjoining the match amplifier C zone  441  and the search line driver C zone  448 . 
     The block D comprises a memory array D, a match amplifier D zone  442  adjoining the memory array D in the row direction, a search line driver D zone  450  adjoining the memory array D in the column direction, and a block-D control circuit  451  adjoining the match amplifier D zone  442  and the search line driver D zone  450 . 
     The block-BCD activation control circuit  443  has the same configuration as the block-B activation control circuit  207  according to Embodiment 1, and outputs a search activation signal srch_act and an inverted logical product signal match_or to the blocks B, C, and D. 
     As is the case with the block B according to Embodiment 1, the blocks B, C, and D operate when the search activation signal srch_act is activated to an H level, and do not operate when the search activation signal srch_act is deactivated to an L level. 
     A delay circuit  410  which delays search data of the block B comprises a latch circuit  416  and a latch circuit  418 . The latch circuit  418  latches search data db&lt; 39 : 20 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. The latch circuit  416  latches an output of the latch circuit  418 , according to the search clock sclk&lt; 2 &gt;. 
     A delay circuit  412  which delays search data of the block C comprises a latch circuit  424 , a latch circuit  422 , and a latch circuit  420 . The latch circuit  424  latches search data db&lt; 59 : 40 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. The latch circuit  422  latches an output of the latch circuit  424 , according to the search clock sclk&lt; 2 &gt;. The latch circuit  420  latches an output of the latch circuit  422 , according to the search clock sclk&lt; 4 &gt;. 
     A delay circuit  414  which delays search data of the block D comprises a latch circuit  432 , a latch circuit  430 , a latch circuit  428 , and a latch circuit  426 . 
     The latch circuit  432  latches search data db&lt; 79 : 60 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. The latch circuit  430  latches an output of the latch circuit  432 , according to the search clock sclk&lt; 2 &gt;. The latch circuit  428  latches an output of the latch circuit  430 , according to the search clock sclk&lt; 4 &gt;. The latch circuit  426  latches an output of the latch circuit  428 , according to the search clock sclk&lt; 6 &gt;. 
     The search line driver A zone  444 , the search line driver B zone  446 , the search line driver C zone  448 , and the search line driver D zone  450  have the same function as the search line driver A zone and the search line driver B zone according to Embodiment 1. 
     The block-A control circuit  445 , the block-B control circuit  447 , the block-C control circuit  449 , and the block-D control circuit  451  have the same configuration as the block-A control circuit  205  and the block-B control circuit  210  according to Embodiment 1. 
     The match amplifier A, the match amplifier B, and the match amplifier C have the same configuration as the match amplifier A according to Embodiment 1. 
       FIG. 18  illustrates a configuration of the match amplifier D of the block D. As illustrated in  FIG. 18 , the match amplifier D according to Embodiment 3 is different from the match amplifier B according to Embodiment 1 with respect to the inverted logical addition circuit  918 . 
     The inverted logical addition circuit  918  outputs an inverted logical addition of a block-A search determination signal maoa_n[ 0 ], a block-B search determination signal maob_n[ 0 ], a block-C search determination signal maoc_n[ 0 ], and a block D search determination signal maod_n[ 0 ]. 
     (Search timing)  FIG. 19  is a drawing for explaining search timing of the blocks A-D according to Embodiment 3. 
     As illustrated in  FIG. 19 , the clock control circuit  434  generates the search clocks sclk&lt; 0 &gt;-sclk&lt; 9 &gt; based on the clock camclk, when the search command GSE is activated to an H level. The block A performs searching at the timing of rising of the search clock sclk&lt; 0 &gt;. The block B performs searching at the timing of rising of the search clock sclk&lt; 4 &gt; which is delayed by two cycles from the search clock sclk&lt; 0 &gt;. The block C performs searching at the timing of rising of the search clock sclk&lt; 6 &gt; which is delayed by one cycle from the search clock sclk&lt; 4 &gt;. The block D performs searching at the timing of rising of the search clock sclk&lt; 8 &gt; which is delayed by one cycle from the search clock sclk&lt; 6 &gt;. 
     (Effects) As described above, according to the present embodiment, it is possible to attain speeding up, by dividing the basic array block into four blocks, and performing searching in units of the divided blocks. According to the present embodiment, the search activity of the second-stage block, the third-stage block, and the fourth-stage block is controlled, depending on the search result of the first-stage block. Therefore, it is possible to reduce the consumption current to the maximum of about ¼. 
     (Embodiment 4) 
     (Configuration)  FIG. 20  illustrates a configuration of a content addressable memory device according to Embodiment 4. 
     As illustrated in  FIG. 20 , in the content addressable memory device according to Embodiment 4, searching is performed in units of block of blocks A, B, C, and D which are formed by dividing the basic array block (80 bits) into four blocks in units of 20 bits. 
     The block A comprises a memory array A, a match amplifier A zone adjoining the memory array A in the row direction, a search line driver A zone adjoining the memory array A in the column direction, and a block-A control circuit  545  adjoining the match amplifier A zone and the search line driver A zone. 
     The block B comprises a memory array B, a match amplifier B zone adjoining the memory array B in the row direction, a search line driver B zone adjoining the memory array B in the column direction, a block-B control circuit  547  adjoining the match amplifier B zone and the search line driver B zone, and a block-CD activation control circuit  543  adjoining the match amplifier B zone and the block-B control circuit  547  in the row direction. 
     The block C comprises a memory array C, a match amplifier C zone adjoining the memory array C in the row direction, a search line driver C zone adjoining the memory array C in the column direction, and a block-C control circuit  549  adjoining the match amplifier C zone and the search line driver C zone. 
     The block D comprises a memory array D, a match amplifier D zone adjoining the memory array D in the row direction, a search line driver D zone adjoining the memory array D in the column direction, and a block-D control circuit  551  adjoining the match amplifier D zone and the search line driver D zone. 
     The block-CD activation control circuit  543  has the same configuration as the block-B activation control circuit  207  according to Embodiment 1, and outputs the inverted logical product signal match_or and the search activation signal srch_act to the blocks C and D. 
     As is the case with the block B according to Embodiment 1, the block C and the block D operate when the search activation signal srch_act is activated to an H level, and do not operate when the search activation signal srch_act is deactivated to an L level. 
     A delay circuit  510  which delays the search data of the block B comprises a latch circuit  516 . The latch circuit  516  latches search data db&lt; 39 : 20 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. 
     A delay circuit  512  which delays search data of the block C comprises a latch circuit  522 , a latch circuit  520 , and a latch circuit  518 . The latch circuit  522  latches search data db&lt; 59 : 40 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. The latch circuit  520  latches an output of the latch circuit  522 , according to the search clock sclk&lt; 2 &gt;. The latch circuit  518  latches an output of the latch circuit  520 , according to the search clock sclk&lt; 4 &gt;. 
     A delay circuit  514  which delays search data of the block D comprises a latch circuit  530 , a latch circuit  528 , a latch circuit  526 , and a latch circuit  524 . The latch circuit  530  latches search data db&lt; 79 : 60 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. The latch circuit  528  latches an output of the latch circuit  530 , according to the search clock sclk&lt; 2 &gt;. The latch circuit  526  latches an output of the latch circuit  528 , according to the search clock sclk&lt; 4 &gt;. The latch circuit  524  latches an output of the latch circuit  526 , according to the search clock sclk&lt; 6 &gt;. 
     The match amplifier A, the match amplifier B, and the match amplifier C have the same configuration as the match amplifier A according to Embodiment 1. The match amplifier D has the same configuration as the match amplifier D according to Embodiment 3, illustrated in  FIG. 17 . 
     (Search timing)  FIG. 21  is a drawing for explaining search timing of the blocks A-D according to Embodiment 4. 
     As illustrated in  FIG. 21 , the clock control circuit  534  generates the search clocks sclk&lt; 0 &gt;-sclk&lt; 9 &gt; based on the clock camclk, when the search command GSE is activated to an H level. The block A performs searching at the timing of rising of the search clock sclk&lt; 0 &gt;. The block B performs searching at the timing of rising of the search clock sclk&lt; 2 &gt; which is delayed by one cycle from the search clock sclk&lt; 0 &gt;. The block C performs searching at the timing of rising of the search clock sclk&lt; 6 &gt; which is delayed by two cycles from the search clock sclk&lt; 2 &gt;. The block D performs searching at the timing of rising of the search clock sclk&lt; 8 &gt; which is delayed by one cycle from the search clock sclk&lt; 6 &gt;. 
     (Effects) As described above, according to the present embodiment, it is possible to attain speeding up, by dividing the basic array block into four blocks, and performing searching in units of the divided blocks. According to the present embodiment, the search activity of the third-stage block and the fourth-stage block is controlled, depending on the search results of the first-stage block and the second-stage block. Therefore, it is possible to reduce the consumption current to the maximum of about ½. 
     (Embodiment 5) 
     (Configuration)  FIG. 22  illustrates a configuration of a content addressable memory device according to Embodiment 5. 
     As illustrated in  FIG. 22 , in the content addressable memory device according to Embodiment 5, searching is performed in units of block of blocks A, B, C, and D which are formed by dividing the basic array block (80 bits) into four blocks in units of 20 bits. 
     The block A comprises a memory array A, a match amplifier A zone adjoining the memory array A in the row direction, a search line driver A zone adjoining the memory array A in the column direction, a block-A control circuit  650  adjoining the match amplifier A zone and the search line driver A zone, and a block-BCD activation control circuit  647  adjoining the match amplifier A zone and the block-A control circuit  650  in the row direction. 
     The block B comprises a memory array B, a match amplifier B zone adjoining the memory array B in the row direction, a search line driver B zone adjoining the memory array B in the column direction, a block-B control circuit  652  adjoining the match amplifier B zone and the search line driver B zone, and a block-CD activation control circuit  648  adjoining the match amplifier B zone and the block-B control circuit  652  in the row direction. 
     The block C comprises a memory array C, a match amplifier C zone adjoining the memory array C in the row direction, a search line driver C zone adjoining the memory array C in the column direction, and a block-C control circuit  654  adjoining the match amplifier C zone and the search line driver C zone. 
     The block D comprises a memory array D, a match amplifier D zone adjoining the memory array D in the row direction, a search line driver D zone adjoining the memory array D in the column direction, and a block-D control circuit  656  adjoining the match amplifier D zone and the search line driver D zone. 
     The block-BCD activation control circuit  647  has the same configuration as the block-B activation control circuit  207  according to Embodiment 1, and outputs the search activation signal srch_act0 and the inverted logical product signal match_or0 to the block B, as well as to a logical product circuit  657 . 
     The block-CD activation control circuit  648  has the same configuration as the block-B activation control circuit  207  according to Embodiment 1, and outputs the search activation signal srch_act1 and the inverted logical product signal match_or1 to the logical product circuit  657 . 
     The logical product circuit  657  outputs, to the blocks C and D, a search activation signal srch_act01 that is a logical product of the search activation signal srch_act0 and the search activation signal srch_act1, and an inverted logical product signal match_or01 that is a logical product of the inverted logical product signal match_or0 and the inverted logical product signal match_or1. 
     As is the case with the block B according to Embodiment 1, the block B operates when the search activation signal srch_act0 is activated to an H level, and does not operate when the search activation signal srch_act1 is deactivated to an L level. 
     As is the case with the block B according to Embodiment 1, the block C and the block D operate when the search activation signal srch_act01 is activated to an H level (that is when the search activation signal srch_act0 and the search activation signal srch_act1 are both activated to an H level), and do not operate when the search activation signal srch_act01 is deactivated to an L level. 
     A delay circuit  610  which delays search data of the block B comprises a latch circuit  618  and a latch circuit  616 . The latch circuit  618  latches search data db&lt; 39 : 20 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. The latch circuit  616  latches an output of the latch circuit  618 , according to the search clock sclk&lt; 2 &gt;. 
     A delay circuit  612  which delays search data of the block C comprises a latch circuit  626 , a latch circuit  624 , a latch circuit  622 , and a latch circuit  620 . The latch circuit  626  latches search data db&lt; 59 : 40 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. The latch circuit  624  latches an output of the latch circuit  626 , according to the search clock sclk&lt; 2 &gt;. The latch circuit  622  latches an output of the latch circuit  624 , according to the search clock sclk&lt; 4 &gt;. The latch circuit  620  latches an output of the latch circuit  622 , according to the search clock sclk&lt; 6 &gt;. 
     A delay circuit  614  which delays search data of the block D comprises a latch circuit  636 , a latch circuit  634 , a latch circuit  632 , a latch circuit  630 , and a latch circuit  628 . 
     The latch circuit  636  latches search data db&lt; 79 : 60 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. The latch circuit  634  latches an output of the latch circuit  636 , according to the search clock sclk&lt; 2 &gt;. The latch circuit  632  latches an output of the latch circuit  634 , according to the search clock sclk&lt; 4 &gt;. The latch circuit  630  latches an output of the latch circuit  632 , according to the search clock sclk&lt; 6 &gt;. The latch circuit  628  latches an output of the latch circuit  630 , according to the search clock sclk&lt; 8 &gt;. 
     The match amplifier A, the match amplifier B, and the match amplifier C have the same configuration as the match amplifier A according to Embodiment 1. The match amplifier D has the same configuration as the match amplifier D according to Embodiment 3, illustrated in  FIG. 17 . 
     (Search timing)  FIG. 23  is a drawing for explaining search timing of the blocks A-D according to Embodiment 5. 
     As illustrated in  FIG. 23 , the clock control circuit  638  generates the search clock sclk&lt; 0 &gt;-sclk&lt; 11 &gt; based on the clock camclk, when the search command GSE is activated to an H level. The block A performs searching at the timing of rising of the search clock sclk&lt; 0 &gt;. The block B performs searching at the timing of rising of the search clock sclk&lt; 4 &gt; which is delayed by two cycles from the search clock sclk&lt; 0 &gt;. The block C performs searching at the timing of rising of the search clock sclk&lt; 8 &gt; which is delayed by two cycles from the search clock sclk&lt; 4 &gt;. The block D performs searching at the timing of rising of the search clock sclk&lt; 10 &gt; which is delayed by one cycle from the search clock sclk&lt; 8 &gt;. 
     (Effects) As described above, according to the present embodiment, it is possible to attain speeding up, by dividing the basic array block into four blocks, and performing searching in units of the divided blocks. According to the present embodiment, the search activity of the second-stage block, the third-stage block, and the fourth-stage block is controlled, depending on the search result of the first-stage block, and the search activity of the third-stage block and the fourth-stage block is controlled, depending on the search results of the first-stage block and the second-stage block. Therefore, it is possible to reduce the consumption current to the maximum of about ¼. 
     (Embodiment 6) 
     (Configuration)  FIG. 24  illustrates a configuration of a content addressable memory device according to Embodiment 6. 
     As illustrated in  FIG. 24 , in the content addressable memory device according to Embodiment 6, searching is performed in units of block of blocks A, B, C, and D which are formed by dividing two basic array blocks (each basic array block: 80 bits) into four blocks in units of 40 bits. 
     The block A comprises a memory array A, a match amplifier A zone adjoining the memory array A in the row direction, a search line driver A zone adjoining the memory array A in the column direction, a block-A control circuit  719  adjoining the match amplifier A zone and the search line driver A zone, and a block-BD activation control circuit  716  adjoining the match amplifier A zone and the block-A control circuit  719  in the row direction. 
     The block B comprises a memory array B, a match amplifier B zone adjoining the memory array B in the row direction, a search line driver B zone adjoining the memory array B in the column direction, and a block-B control circuit  721  adjoining the match amplifier B zone and the search line driver B zone. 
     The block C comprises a memory array C, a match amplifier C zone adjoining the memory array C in the row direction, a search line driver C zone adjoining the memory array C in the column direction, a block-C control circuit  723  adjoining the match amplifier C zone and the search line driver C zone, and a block-BD activation control circuit  717  adjoining the match amplifier C zone and the block-C control circuit  723  in the row direction. 
     The block D comprises a memory array D, a match amplifier D zone adjoining the memory array D in the row direction, a search line driver D zone adjoining the memory array D in the column direction, and a block-D control circuit  725  adjoining the match amplifier D zone and the search line driver D zone. 
     The block-BD activation control circuit  716  has the same configuration as the block-B activation control circuit  207  according to Embodiment 1, and outputs a search activation signal srch_act0 and an inverted logical product signal match_or0 to the logical product circuit  726 . 
     The block-BD activation control circuit  717  has the same configuration as the block-B activation control circuit  207  according to Embodiment 1, and outputs the search activation signal srch_act1 and the inverted logical product signal match_or1 to the logical product circuit  726 . 
     The logical product circuit  726  outputs, to the blocks B and C, a search activation signal srch_act01 that is a logical product of the search activation signal srch_act0 and the search activation signal srch_act1, and an inverted logical product signal match_or01 that is a logical product of the inverted logical product signal match_or0 and the inverted logical product signal match_or1. 
     As is the case with the block B according to Embodiment 1, the block B and the block D operate when the search activation signal srch_act01 is activated to an H level (that is when the search activation signal srch_act0 and the search activation signal srch_act1 are both activated to an H level), and do not operate when the search activation signal srch_act01 is deactivated to an L level. 
     A delay circuit  701  which delays search data of the block B comprises a latch circuit  704  and a latch circuit  703 . The latch circuit  704  latches the search data db&lt; 79 : 40 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. The latch circuit  703  latches an output of the latch circuit  704 , according to the search clock sclk&lt; 2 &gt;. A delay circuit  702  which delays search data of the block D comprises a latch circuit  706  and a latch circuit  705 . The latch circuit  706  latches search data db&lt; 159 : 120 &gt; from the exterior, according to the search clock sclk&lt; 0 &gt;. The latch circuit  705  latches an output of the latch circuit  706 , according to the search clock sclk&lt; 2 &gt;. 
     The match amplifier A, the match amplifier B, and the match amplifier C have the same configuration as the match amplifier A according to Embodiment 1. The match amplifier D has the same configuration as the match amplifier D according to Embodiment 3. 
     (Search timing)  FIG. 25  is a drawing for explaining search timing of the blocks A-D according to Embodiment 6. 
     As illustrated in  FIG. 25 , the clock control circuit  707  generates the search clock sclk&lt; 0 &gt;-sclk&lt; 5 &gt; based on the clock camclk, when the search command GSE is activated to an H level. The block A and the block C perform searching at the timing of rising of the search clock sclk&lt; 0 &gt;. The block B and the block D perform searching at the timing of rising of the search clock sclk&lt; 4 &gt; which is delayed by two cycles from the search clock sclk&lt; 0 &gt;. 
     (Effects) As described above, according to the present embodiment, it is possible to attain speeding up, by dividing each of two basic array blocks into two blocks, and performing searching in units of the divided blocks. According to the present embodiment, the search activity of two second-stage blocks is controlled, depending on the search result (logical product) of two first-stage blocks. Therefore, it is possible to reduce the consumption current to the maximum of about ½. 
     (Embodiment 7) 
     (Configuration)  FIG. 26  illustrates roughly an entire configuration of a content addressable memory device according to Embodiment 7. 
     As illustrated in  FIG. 26 , the content addressable memory device according to Embodiment 7 comprises a array block  1 , a control circuit  8 , and a search data input circuit  4 . 
     The control circuit  8  controls the whole based on search data GSE and a clock camclk. At the same time, the control circuit  8  generates a pre-charge signal PRE, an inverted pre-charge signal /PRE, a search line activation signal SLd, an inverted search line activation signal SLd_n, and a match amplifier activation signal MAE. 
     The array block  1  (80 bits×256 entries) comprises a memory array A (the first half 40 bits×256 entries), a transfer controlling unit  661 , a memory array B (the second half 40 bits×256 entries), and a coincidence determination unit  2 . 
     The memory array A and the memory array B comprise a matrix of unit cells UC each of which serves as a content addressable memory cell (CAM cell). 
     The memory array A and the memory array B are divided into plural entries ERY in units of row. One entry ERY comprises unit cells UC of one row of the memory array A and the memory array B. One entry ERY is divided into an Aside entry including a unit cell UC belonging to the memory array A, and a B side entry including a unit cell UC belonging to the memory array B. 
     The A side entry is provided with a match line MLA to which plural unit cells UC belonging to the A side entry are coupled in parallel. Similarly, the B side entry is provided with a match line MLB to which plural unit cells UC belonging to the B side entry are coupled in parallel. 
     A search line pair SLP (SL, /SL) which transmits the search data is provided in the column direction of the memory array A and the memory array B. Unit cells UC are provided corresponding to cross points of the search line pair SLP and the match lines MLA and MLB. 
     The transfer controlling unit  661  comprises a transfer circuit  14  provided for every corresponding entry ERY. During the time of searching, the transfer circuit  14  separates the match line MLA provided in the A side entry ERY, from the match line MLB provided in the corresponding B side entry ERY, and transfers a voltage of the match line MLA to the match line MLB. Except for the time of searching, the transfer circuit  14  couples the match line MLA provided in the A side entry ERY to the match line MLB provided in the corresponding B side entry ERY. 
     The coincidence determination unit  2  comprises a match amplifier unit  10  provided corresponding to each B side entry ERY. The match amplifier unit  10  is coupled to the match line MLB of the corresponding B side entry ERY. 
       FIG. 27  illustrates a configuration of one entry and a transfer circuit  14  and a match amplifier unit  10  corresponding to the entry, in the content addressable memory device according to Embodiment 7. 
     As illustrated in  FIG. 27 , 40 unit cells UC 0 -UC 39  of the A side entry are coupled to the match line MLA. Similarly, 40 unit cells UC 40 -UC 79  of the B side entry are coupled to the match line MLB. 
     The transfer circuit  14  comprises a P-channel MOS transistor PM 1 , a P-channel MOS transistor PM 2 , a P-channel MOS transistor PM 3 , an N-channel MOS transistor NM 1 , an N-channel MOS transistor NM 2 , and an N-channel MOS transistor NM 3 . 
     Since the search line activation signal SLd is activated to an H level during a search period, the P-channel MOS transistor PM 1  is set to OFF, and the coupling between the match line A and the match line B is cutoff. Since the search line activation signal SLd is deactivated to an L level except for the search period, the P-channel MOS transistor PM 1  is set to ON, and the coupling between the match line A and the match line B is established. 
     The P-channel MOS transistor PM 3  and the N-channel MOS transistor NM 1  form an inverter. This inverter amplifies a voltage of the match line MLA. 
     The P-channel MOS transistor PM 2  is provided between a source of the P-channel MOS transistor PM 3  and the VDD power supply. The P-channel MOS transistor PM 2  receives an inverted search line activation signal /SLd at a gate thereof. 
     The N-channel MOS transistor NM 2  is provided between an output of the inverter and the ground power supply. The N-channel MOS transistor NM 2  receives an inverted search line activation signal /SLd at a gate thereof. 
     The N-channel MOS transistor NM 3  is provided between one end of the match line MLB and the ground power supply and serves as a discharging circuit. The N-channel MOS transistor NM 3  receives an output of the inverter at a gate thereof. 
     Since the inverted search line activation signal /SLd is deactivated to an L level during the search period, the P-channel MOS transistor PM 2  is set to ON, and the N-channel MOS transistor NM 2  is set to OFF. When the search result of the A side entry is of Miss and the voltage of the match line MLA is at an L level (0V) at this time, the gate voltage of the N-channel MOS transistor NM 3  turns to an H level (VDD) by the action of the inverter. Accordingly, the N-channel MOS transistor NM 3  is set to ON, and the match line MLB is discharged to an L level (0V). Since the N-channel MOS transistor NM 3  has larger driving ability than the search transistors TR 1  and TR 2  (or TR 3  and TR 4 ) comprised of two-staged NMOS transistors, it is possible to transfer the voltage of an L level of the match line MLA to the match line MLB at high speed. 
     When the search result of the A side entry is of Hit and the voltage of the match line MLA is at an H level, on the other hand, the gate voltage of the N-channel MOS transistor NM 3  turns to an L level by the action of the inverter. Accordingly, the N-channel MOS transistor NM 3  is set to OFF, and the match line MLB is not discharged to an L level (0V). 
     The match amplifier unit  10  comprises a VDD pre-charge circuit  18  and the match amplifier circuit  20 . 
     The VDD pre-charge circuit  18  comprises a P-channel MOS transistor PM 4  coupled to the match line MLB. 
     When the inverted pre-charge signal /PRE is activated to an L level except for the search period, the P-channel MOS transistor PM 4  pre-charges the match line MLB to an H level (VDD). Since the P-channel MOS transistor PM 1  is set to ON except for the search period, the P-channel MOS transistor PM 4  pre-charges the match line MLA to an H level (VDD) as well. 
     The match amplifier circuit  20  is coupled to the match line MLB and comprises two stages of inverters IV 1  and IV 2 . The inverters IV 1  and IV 2  are coupled to the VDD power supply and operate by the VDD voltage. When the match amplifier activation signal MAE is activated to an H level, the match amplifier circuit  20  amplifies the voltage of the match line MLB, and outputs the search determination signal MATCH_OUT. 
     When the A side entry is of Miss, the match line MLB is discharged to an L level by the transfer circuit  14 ; accordingly, the match amplifier circuit  20  outputs the search determination signal MATCH_OUT of an L level which indicates Miss. When the B side entry is of Miss, the match line MLB is discharged to an L level by the search transistors TR 1  and TR 2  (or TR 3  and TR 4 ) comprised of two-staged NMOS transistors; accordingly, the match amplifier circuit  20  outputs the search determination signal MATCH_OUT of an L level which indicates Miss. When the A side entry is of Hit and the B side entry is of Hit, the match line MLB is not discharged; accordingly, the match amplifier circuit  20  outputs the search determination signal MATCH_OUT of an H level which indicates Hit. 
     (Reference: a conventional configuration) By reference,  FIG. 28  illustrates a configuration of one entry and a match amplifier corresponding to the entry, in a conventional content addressable memory device. 
     In  FIG. 28 , a memory array is not divided, and a match line ML is coupled to unit cells UC for 80 bits; accordingly, parasitic capacitance of the match line ML becomes large. Consequently, a voltage change of the match line ML at the time of Miss becomes of a small amplitude. Therefore, a differential amplifier or a cross coupling amplifier, etc., which can amplify a signal of small amplitude, are employed as a match amplifier circuit  20 . This is because it is difficult that a circuit such as an inverter amplifies a signal of small amplitude at high speed. 
     On the other hand, in the present embodiment, the level of signal amplitude becomes two times larger because the load of the match line becomes half compared with the past. Therefore, it is possible to perform amplification at a high speed by an amplifier of a simple configuration, such as an inverter. 
     (Reference: conventional operation)  FIG. 29  illustrates operating waveforms of the conventional content addressable memory device illustrated in  FIG. 28 . 
     As illustrated in  FIG. 29 , before searching a memory array, the match line ML is pre-charged to VDD by a VDD pre-charge circuit. 
     When the search line activation signal SLd is activated, the search line SL is activated to an H level (VDD). When a corresponding entry is of Miss, the match line ML is discharged to an L level (0V) by a search transistor TR 2  or TR 4 . Here, Tcnv (in the order of several ns) is assumed to express a period in which the match line is discharged when one unit cell UC of an entry is of Miss. 
     After the voltage of the match line ML is discharged to 0V, the match amplifier activation signal MAE is activated to an H level. The match line ML is amplified by the match amplifier circuit  20 , and a search determination signal MATCH_OUT is outputted. 
     (Operation in the present embodiment)  FIG. 30  and  FIG. 31  illustrate operating waveforms of the content addressable memory device according to Embodiment 7. 
       FIG. 30  illustrates operating waveforms when the A side entry is of Miss and the B side entry is of Hit in Embodiment 7. 
     As illustrated in  FIG. 30 , before searching the memory array, the match lines MLA and MLB are pre-charged to VDD by the VDD pre-charge circuit  18 . 
     When the search line activation signal SLd is activated, the search line SL is activated to an H level (VDD). When the corresponding A side entry is of Miss, the match line MLA is discharged to an L level (0V) by the search transistor TR 2  or TR 4 . Since the parasitic capacitance is half compared with the past, the amplitude level of the match line MLA in the present embodiment becomes twice, and the voltage of the match line MLA is drawn out to an L level (0V) at a speed twice faster than the past. Therefore, when one unit cell UC of an entry is of Miss, the period in which the match line MLA is discharged is given by Tcnv/2. 
     Subsequently, the voltage of the match line MLA is amplified by an inverter comprised of the P-channel MOS transistor PM 3  and the N-channel MOS transistor NM 1 . It is assumed that the operating time of the present inverter is given by Tinv. 
     Furthermore, the voltage of the match line MLB is discharge to an L level (0V) by the N-channel MOS transistor NM 3 . When the driving ability of the N-channel MOS transistor NM 3  is increased four times greater than the driving ability of the search transistors TR 1  and TR 2  (or TR 3  and TR 4 ) which are comprised of two-staged NMOS transistors, the discharge time is given by Tcnv/8, because the parasitic capacitance of the match line MLB is half compared with the past. 
     After the voltage of the match line MLB is discharged to 0V, the match amplifier activation signal MAE is activated to an H level. The match line MLB is amplified by the match amplifier circuit  20 , and a search determination signal MATCH_OUT is outputted. 
     Accordingly, as illustrated in  FIG. 29 , in the past, the period after the search line SL is activated until the match amplifier circuit  20  starts amplification is given by Tcnv (in the order of several ns). In contrast with this, in the present embodiment, the period after the search line SL is activated until the match amplifier circuit  20  starts amplification is given by (⅝)×Tcnv+Tinv (Tinv is in the order of hundreds ps). That is, according to embodiments of the present invention, speeding up of about 1.5 times on an average can be attained compared with the past. 
       FIG. 31  illustrates operating waveforms when the A side entry is of Hit and the B side entry is of Miss in Embodiment 7. 
     Before searching the memory array, the match lines MLA and MLB are pre-charged to VDD by the VDD pre-charge circuit  18 . 
     When the search line activation signal SLd is activated, the search line SL is activated to an H level (VDD). When the corresponding B side entry is of Miss, the match line MLB is discharged to an L level (0V) by the search transistor TR 2  or TR 4 . Since the parasitic capacitance is half compared with the past, the amplitude level of the match line MLB in the present embodiment becomes twice, and the voltage of the match line MLB is drawn out to an L level (0V) at a speed twice faster than the past. Therefore, when one unit cell UC of an entry is of Miss, the period in which the match line MLB is discharged is given by Tcnv/2. 
     After a period of Tcnv/8+Tinv passes after that, the match amplifier activation signal MAE is activated to an H level at the same timing as the case where the A side entry is of Miss. The match line MLB is amplified by the match amplifier circuit  20 , and the search determination signal MATCH_OUT is outputted. 
     (Effects) As described above, according to the present embodiment, it is possible to attain speeding up of searching, by halving the load capacitance of the match line during the search period. 
     Since the amplitude level of the match line is small in the conventional device illustrated in  FIG. 28 , there is a problem that an amplifier circuit with a large layout area is needed in consideration of speeding up and local variation, and a control signal of the amplifier circuit becomes complicated. In contrast with this, according to embodiments of the present invention, the amplitude level of the match line becomes large and the voltage of the match line can be amplified by a simple amplifier circuit, such as an inverter; accordingly, it is possible to reduce the layout area. Since control of the amplifier circuit is also simple, a circuit which generates a signal for controlling the amplifier circuit can be reduced in size as well. Consequently, it is possible to reduce the layout and the circuit scale as the whole. 
     Signals for controlling the transfer circuit which is appended by the embodiments of the present invention are the search line activation signal SLd used from the past, and a signal /SLd which is the negative logic of SLd. Accordingly, it is not necessary to provide a special circuit for generating the signals for controlling the transfer circuit. 
     In the present embodiment, the VDD pre-charge circuit is coupled to the match line MLB. However, it is not limited to the case and the VDD pre-charge circuit may be coupled to the match line MLA. 
     (Embodiment 8) 
     (Configuration)  FIG. 32  illustrates a configuration of one entry and a transfer circuit and a match amplifier unit corresponding to the entry, in a content addressable memory device according to Embodiment 8. 
     The configuration illustrated in  FIG. 32  is different from the configuration of Embodiment 7 illustrated in  FIG. 27 , with respect to a match amplifier circuit  30 . 
     The match amplifier circuit  30  illustrated in  FIG. 32  comprises a cross-coupling-type differential amplifier DA 1  which is activated by a match amplifier activation signal MAE. The differential amplifier DA 1  is coupled to the VDD power supply and operates by the VDD voltage. At the time of activation, the differential amplifier DA 1  receives the reference voltage VREF and a voltage of the match line MLB, amplifies a difference of these voltages, and outputs a determination signal MATCH_OUT. The reference voltage VREF can be set as VDD-0.1V, for example (it is assumed that VDD=1.0V). 
     (Operation)  FIG. 33  illustrates operating waveforms of the content addressable memory device according to Embodiment 8. 
     According to Embodiment 7 illustrated in  FIG. 29 , when the match line MLB reaches 0V, the match amplifier activation signal MAE is activated and the match amplifier circuit  20  starts amplification of the match line MLB. 
     As compared with this, the match amplifier circuit  30  according to the present embodiment can amplify a signal even when the amplitude of the voltage of the match line MLB is small. Accordingly, as illustrated in  FIG. 33 , before the match line MLB reaches 0V, the match amplifier activation signal MAE is activated and the match amplifier circuit  30  starts the differential amplification of the match line MLB. 
     (Effects) In the present embodiment, timing at which the determination signal MATCH_OUT is outputted can be set earlier than in Embodiment 7. 
     (Embodiment 9) 
     (Configuration)  FIG. 34  illustrates a configuration of one entry and a transfer circuit and a match amplifier unit corresponding to the entry, in a content addressable memory device according to Embodiment 9. 
     The configuration illustrated in  FIG. 34  is different from the configuration according to Embodiment 8 illustrated in  FIG. 32  with respect to the method of division of a memory array. 
     In Embodiment 8, the number of columns of the memory array A and the memory array B is 40. That is, each of the A side entry and the B side entry comprises 40 unit cells UC. 
     As compared with this, in the present embodiment, the number of columns of the memory array A is 20, and the number of columns of the memory array B is 60. That is, the A side entry comprises 20 CAM cells CC, and the B side entry comprises 60 CAM cells CC. According to the configuration of the present embodiment, the number of unit cells UC coupled to the match line MLA becomes half and the length of the match line MLA also becomes half, compared with the configuration according to Embodiment 8. Accordingly, at the time of Miss, the voltage can be discharged to 0V at high speed. 
     (Operation)  FIG. 35  illustrates operating waveforms of the content addressable memory device according to Embodiment 9. 
     In the present embodiment, the number of unit cells UC coupled to the match line MLA becomes half, and the length of the match line MLA also becomes half compared with Embodiment 8. Accordingly, at the time of Miss, the voltage can be discharged to 0V at high speed. The period required for discharging is Tcnv/4. 
     In the present embodiment, the number of unit cells UC coupled to the match line MLB increases, and the length of the match line MLB also increases compared with Embodiment 8. However, since the differential amplifier DA 1  can also amplify a small amplitude signal, an increase in the period required for amplification can be disregarded mostly. 
     (Effects) In the present embodiment, timing at which the determination signal MATCH_OUT is outputted can be set yet earlier than in Embodiment 8. 
     (Embodiment 10) 
     (Configuration)  FIG. 36  illustrates a configuration of one entry and a transfer circuit and a match amplifier unit corresponding to the entry, in a content addressable memory device according to Embodiment 10. 
     The configuration illustrated in  FIG. 36  is different from the configuration according to Embodiment 7 illustrated in  FIG. 27  with respect to a transfer circuit  36 . 
     The transfer circuit  36  comprises an N-channel MOS transistor NMOS 1  in addition to the components of the transfer circuit  14  according to Embodiment 7. 
     The N-channel MOS transistor MNOS 1  has a drain coupled to the input of an inverter comprised of a P-channel MOS transistor PM 3  and an N-channel MOS transistor NM 1 , a source coupled to the ground, and a gate coupled to a node A which is an output of the inverter. 
     (Operation)  FIG. 37  illustrates operating waveforms of the content addressable memory device according to Embodiment 10. 
     As illustrated in  FIG. 37 , when the search line activation signal SLd is activated, the search line SL is activated to an H level (VDD). When the corresponding A side entry is of Miss, the match line MLA is discharged gradually by the search transistor TR 2  or TR 4 . When the voltage of the match line MLA reaches a threshold of the inverter (indicated as (1)), the voltage of the node A increases gradually by the action of the inverter (indicated as (2)). Subsequently, when the voltage of the node A reaches a threshold of the N-channel MOS transistor NMOS 1  (indicated as (3)), the voltage of the match line MLA is discharged rapidly by the N-channel MOS transistor NMOS 1 . 
     (Effects) In the present embodiment, timing at which the determination signal MATCH_OUT is outputted can be set earlier than in Embodiment 7. 
     (Embodiment 11) 
     (Configuration)  FIG. 38  illustrates a configuration of one entry and a transfer circuit and a match amplifier unit corresponding to the entry, in a content addressable memory device according to Embodiment 11. 
     Although  FIG. 38  depicts components disposed in two stages in the vertical direction, in fact all the components are extended horizontally. 
     In the present embodiment, the array block (80 bits×256 entries) is divided into a memory array A, a memory array B, a memory array C, and a memory array D, each in units of 20 bits. 
     As illustrated in  FIG. 38 , 20 unit cells UC 0 -UC 19  of an A side entry are coupled with a match line MLA. Similarly, 20 unit cells UC 20 -UC 39  of a B side entry are coupled with a match line MLB. Similarly, 20 unit cells UC 40 -UC 59  of a C side entry are coupled with a match line MLC. Similarly, 20 unit cells UC 60 -UC 79  of a D side entry are coupled with a match line MLD. 
     A transfer circuit  64  couples or separates the match line MLA and the match line MLB. In addition, when the search result of the A side entry is of Miss, and when the voltage of the match line MLA is at an L level (0V), the transfer circuit  64  discharges the match line MLB to an L level (0V). The transfer circuit  64  has the same configuration as the transfer circuit  14  according to Embodiment 7. However, the transfer circuit  64  is controlled by a search line activation signal SLab, and an inverted search line activation signal /SLab. 
     A transfer circuit  63  couples or separates the match line MLC and the match line MLD. In addition, when the search result of the C side entry is of Miss, and when the voltage of the match line MLC is at an L level (0V), the transfer circuit  63  discharges the match line MLD to an L level (0V). The transfer circuit  63  has the same configuration as the transfer circuit  14  according to Embodiment 7. However, the transfer circuit  63  is controlled by a search line activation signal SLcd, and an inverted search line activation signal /SLcd. 
     A latch circuit  72  latches an output of a match amplifier circuit  70 , and outputs a search determination signal MATCH_OUTab. A latch circuit  73  latches an output of a match amplifier circuit  69 , and outputs a search determination signal MATCH_OUT. 
     A VDD pre-charge circuit  68  is coupled to the match line MLB. The VDD pre-charge circuit  68  has the same configuration as the VDD pre-charge circuit  18  according to Embodiment 7. However, a P-channel MOS transistor PM 4  in the VDD pre-charge circuit  68  is activated by an inverted pre-charge signal /PREab. 
     A VDD pre-charge circuit  67  is coupled to the match line MLD. The VDD pre-charge circuit  67  comprises an inverted logical product circuit NAND 67 , a P-channel MOS transistor PM 4 , and an N-channel MOS transistor NM 67 . 
     The inverted logical product circuit NAND 67  outputs an inverted logical product of an inverted signal of an inverted pre-charge signal /PREcd, and a signal MATCH_OUTab which is an output of the latch circuit  72 . The P-channel MOS transistor PM 4  has a source coupled to the power supply VDD, a drain coupled to the match line MLD, and a gate coupled to an output of the inverted logical product circuit NAND 67 . 
     When the inverted pre-charge signal /PREcd is activated to an L level and the signal MATCH_OUTab is at an H level (that is when the match line LB is not discharged), the VDD pre-charge circuit  68  pre-charges the match line MLD by the VDD voltage. 
     The N-channel MOS transistor NM 67  serves as a discharging circuit, and has a source coupled to the ground, a gate coupled to a discharge signal Discd, and a drain coupled to the match line MLD. The N-channel MOS transistor NM 67  discharges the match line MLD to an L level (0V), when the discharge signal Discd is activated to an H level. 
     (Operation)  FIG. 39  illustrates search timing of the content addressable memory device according to Embodiment 11. 
     When searching is performed for the A side entry and the B side entry, the search line activation signal SLab is activated. After the searching of the A side entry and the B side entry, the search line activation signal SLcd is activated. Then, searching is performed for the C side entry and the D side entry. 
       FIG. 40  illustrates operating waveforms of the content addressable memory device according to Embodiment 11.  FIG. 40  illustrates the operating waveforms for the case where the A side entry is of Miss, the B side entry, the C side entry, and the D side entry are of Hit. 
     As illustrated in  FIG. 40 , before searching is performed for the memory array, the match lines MLA and MLB are pre-charged to VDD by the VDD pre-charge circuit  68 . 
     When the search line activation signal SLab is activated, the search lines SL of the A side entry and the B side entry are activated to an H level (VDD). When the corresponding A side entry is of Miss, the match line MLA is discharged to an L level (0V) by the search transistors TR 1  and TR 2  (or TR 3  and TR 4 ). Subsequently, the voltage of the match line MLA is amplified by the inverter comprised of the P-channel MOS transistor PM 3  and the N-channel MOS transistor NM 1 . Furthermore, the voltage of the match line MLB is discharge to an L level (0V) by the N-channel MOS transistor NM 3 . 
     After the voltage of the match line MLB is discharged to 0V, the match amplifier activation signal MAEab is activated to an H level. Subsequently, the voltage of the match line MLB is amplified by the match amplifier circuit  70 , and the search determination signal MATCH_OUTab of an L level is outputted from the latch circuit  72 . 
     Next, when the discharge signal Discd is activated to an H level, the match line MLD is discharged to the ground level by the NMOS transistor NM 67  which serves as a discharging circuit. When the inverted pre-charge signal /PREcd is activated to an L level, by the action of the inverted logical product circuit NAND 67  and the P-channel MOS transistor PM 4 , the match line MLD is pre-charged to the VDD voltage when the search determination signal MATCH_OUTab is at an H level, and the match line MLD is not pre-charged when the search determination signal MATCH_OUTab is at an L level. When the A side entry is of Miss as described above, the search determination signal MATCH_OUTab is at an L level; accordingly, the match line MLD is not pre-charged to the Vdd but maintains the ground level. 
     Subsequently, when the search line activation signal SLcd is activated, the search line SL of the C side entry and the D side entry is activated to an H level (VDD). The match line MLD maintains the ground level as described above, regardless of whether the C side entry and the D side entry are of Hit or Miss. 
     Subsequently, the match amplifier activation signal MAEcd is activated to an H level, the voltage of the match line MLD is amplified by the match amplifier circuit  69 , the search determination signal MATCH_OUTcd of an L level is outputted from the latch circuit  73 , and it is possible to determine that the present entry comprised of the match lines MLA, MLB, MLC, and MLD is of Miss. 
     (Effects) In the present embodiment, an array block is divided into four memory arrays A, B, C, and D, and it is possible to attain speeding up by searching in division units during the search period. Furthermore, pre-charge/discharge of the match line of blocks of the memory array C and the memory array D are controlled based on the search result of the memory array A and the memory array B. Accordingly, it is possible to attain speeding up, and at the same time, to realize reduction of consumption current to the maximum of about ½. 
     (Embodiment 12) 
     (Configuration)  FIG. 41  illustrates a configuration of one entry and a transfer circuit and a match amplifier unit corresponding to the entry, in a content addressable memory device according to Embodiment 12. 
     The configuration illustrated in  FIG. 41  is different from the configuration according to Embodiment 7 illustrated in  FIG. 27  with respect to a transfer circuit  40  and a ½ VDD pre-charge circuit  38 . 
     As illustrated in  FIG. 41 , the ½ VDD pre-charge circuit  38  comprises a P-channel MOS transistor PM 4  which has a source coupled to a power supply of VDD/2 in magnitude. 
     The match line MLB is pre-charged to the magnitude of VDD/2, by the action of the ½ VDD pre-charge circuit  38 . 
     The size of the P-channel MOS transistor PM 3  and the N-channel MOS transistor NM 1  of the transfer circuit  40  is set up so that the inverter which comprises these transistors may output an L level, when the voltage of the magnitude of VDD/2 is inputted. 
     The transfer circuit  40  comprises a transfer gate which is comprised of a P-channel MOS transistor PM 40  and an N-channel MOS transistor  41 . The transfer gate couples or separates the match line MLA and the match line B under the control of the search line activation signal SLd and the inverted search line activation signal/SLd. 
     (Effects) Since the pre-charge level of the match lines MLA and MLB is reduced in the present embodiment, it is possible to attain power lowering. Although the amplitude level of the match line MLB becomes small, it is possible to amplify a small amplitude voltage of the match line MLB at high speed, by using the differential amplifier DA 1 . 
     In the present embodiment, by controlling the size of the P-channel MOS transistor PM 3  and the N-channel MOS transistor NM 1  which forms the inverter, the inverter which comprises these transistors is set up so as to output an L level, when the voltage of the magnitude of VDD/2 is inputted. However, the configuration is not limited to the above. As an alternative, the power supply voltage of the inverter may be set as VDD/2. 
     (Embodiment 13) 
     (Configuration)  FIG. 42  illustrates a configuration of a content addressable memory device according to Embodiment 13. 
     As illustrated in  FIG. 42 , the search lines SL[ 0 ]-SL[m], and /SL[ 0 ]-/SL[m] are collectively called a search line SL. The match lines ML[ 0 ]-ML[n] are collectively called a match line ML. Floating lines VS[ 0 ]-VS[n] are collectively called a floating line VS. 
     The CAM cell array comprises a matrix of unit cells UC each of which serves as a content addressable memory cell (CAM cell). 
     The memory array is divided into plural entries ERY in units of row. Each entry ERY is provided with a match line ML and a floating line VS to which unit cells UC in the entry are coupled in parallel. A search line SL which transmits search data is provided in the column direction of the memory array. A unit cell UC is provided corresponding to a cross point of the search line SL and the match line ML. 
     A common node VMA comprises a first wring section  810  and a second wring section  812 ; the first wring section  810  is wired in the vertical direction with respect to the match line ML and coupled to one end of the match line ML, and the second wring section  812  is wired in the vertical direction with respect to the search line SL and coupled to one end of the search line SL. 
     A SL driver  110  for driving each search line SL is provided to each search line SL. 
     The SL driver  110  supplies the VDD voltage (activation) or the ground voltage (deactivation) to the corresponding search line SL according to search data from the exterior. 
     The SL driver  110  comprises an inverted logical product circuit NAND 101 , a P-channel MOS transistor TR 104  coupled to the power supply VDD, an N-channel MOS transistor TR 105 , and an N-channel MOS transistor TR 106  which receives an inverted switch activation signal SL 2  ML_n. 
     The inverted logical product circuit NAND 101  outputs an inverted logical product of a search line activation signal SLE and search data GSL, to an inverter comprised of the P-channel MOS transistor TR 104 , the N-channel MOS transistor TR 105 , and the N-channel MOS transistor TR 106 . 
     The N-channel MOS transistor TR 106  is provided between the N-channel MOS transistor TR 105  and the ground power supply. The N-channel MOS transistor TR 106  turns to OFF when the inverted switch activation signal SL 2  ML_n is activated to an L level, and turns to ON when the inverted switch activation signal SL 2  ML_n is deactivated to an H level. 
     The SL driver  110  drives the search line SL into a high impedance state (Hi-Z state), when the inverted switch activation signal SL 2  ML_n is at an L level and the search line activation signal SLE is at an L level. 
     The SL driver  110  supplies a voltage of an H level to the search line SL to activate it, when the inverted switch activation signal SL 2  ML_n is at an H level, the search line activation signal SLE is at an H level, and the search data GSL is at an H level. 
     The SL driver  110  supplies a voltage of an L level to the search line SL to deactivate it, when the inverted switch activation signal SL 2  ML_n is at an H level, the search line activation signal SLE is at an H level, and the search data GSL is at an L level. 
     A SL switch  112  is provided to each search line SL for coupling each search line SL to the common node VMA. 
     The SL switch  112  comprises an inverted logical product circuit NAND 102  which receives a switch activation signal SL 2 ML and the search data GSL, and a P-channel MOS transistor TR 212  which receives an output of the inverted logical product circuit NAND 102 . The P-channel MOS transistor TR 212  is set to ON when the switch activation signal SL 2 ML and the search data GSL are activated to an H level, and couples the activated search line SL to the common node VMA. The reason why only the activated search line SL is coupled to the common node VMA is because charge distribution efficiency would worsen if a non-activated search line (of which the voltage is the ground level) is coupled. 
     A P-channel MOS transistor TR 101  is provided to each match line ML for pre-charging each match line ML. 
     The P-channel MOS transistor TR 101  has a source coupled to the power supply voltage VDD, a drain coupled to the match line ML, and a gate to which an inverted pre-charge signal /MLPRE is inputted. The P-channel MOS transistor TR 101  pre-charges the match line ML to the power supply voltage VDD, when the inverted pre-charge signal /MLPRE is activated to an L level. 
     A match amplifier  106  is provided to each match line ML for amplifying voltage of each match line ML. 
     The match amplifier  106  is coupled to the match line ML of the corresponding entry ERY, and determines coincidence/non-coincidence of search data and stored data of each entry. 
     The match amplifier  106  comprises an inverter INV 102  coupled to the match line ML, an inverter INV 103  which receives an output of the inverter INV 102 , and a latch circuit  114  which receives an output of the inverter INV 103 , and outputs a search determination signal ML_OUT. 
     A ML switch  118  is provided to each match line ML for coupling each match line ML to the common node VMA. 
     The ML switch  118  comprises an inverter INV 101  which receives the inverted switch activation signal SL 2  ML_n, and an N-channel MOS transistor TR 112  which receives an output of the inverter INV 101 . 
     The N-channel MOS transistor TR 112  is provided between one end of the match line ML and the common node VMA, and receives an output of inverter INV 101  at the gate thereof. 
     When the inverted switch activation signal SL 2  ML_n is activated to an L level, the N-channel MOS transistor TR 112  is set to ON and couples the match line ML to the common node VMA. 
     An N-channel MOS transistor TR 103  is provided to each floating line VS. 
     The N-channel MOS transistor TR 103  has a drain coupled to the corresponding floating line VS, a source coupled to the ground, and a gate which receives the inverted switch activation signal SL 2  ML_n. 
     The N-channel MOS transistor TR 103  couples the source of the search transistors TR 1  and TR 2  (or TR 3  and TR 4 ) to the ground level so that the match line ML can be discharged, when the inverted switch activation signal SL 2  ML_n is deactivated to an H level, that is, when the search line SL and the match line ML are not coupled with each other via the common node VMA (in a search period etc.). 
     The N-channel MOS transistor TR 103  floats the source of the search transistors TR 1  and TR 2  (or TR 3  and TR 4 ), when the inverted switch activation signal SL 2  ML_n is activated to an L level, that is, when the match line ML and the search line SL are coupled with each other via the common node VMA. 
     The reason is as follows. Since the voltage of the search line SL becomes a middle potential Vma by coupling the search line SL and the match line ML, a penetration current path from the match line ML to the ground is created and a penetration path to the ground is created also to the search line SL coupled to the match line ML. In order to prevent the present situation, the N-channel MOS transistor TR 103  is provided to float the source of the search transistors TR 1  and TR 2  (or TR 3  and TR 4 ). When the search result is of Miss while the search line SL is activated, the worst case of a period to discharge the match line ML to the ground level is given by discharging of the match line ML to the ground level by a one-bit search transistor. By providing the transistor TR 103  which has a sufficiently larger driving ability than the one-bit search transistor, the period to discharge the match line ML to the ground level in the case of the search result of Miss becomes equivalent to the past, accordingly, it is possible to realize the discharging without degrading the characteristics of the past search transistor. 
     The control circuit  815  outputs the pre-charge signal MLPRE, the inverted pre-charge signal /MLPRE, and the search line activation signal SLE, based on the clock CAMCLK and the search data GSL. The control circuit  815  comprises a switching control circuit as illustrated in  FIG. 43 . 
     The switching control circuit  161  comprises a logical addition circuit OR 101  and an inverter INV 161 . The logical addition circuit OR 101  generates a logical addition of the clock CAMCLK (search line activation signal SLE) and the pre-charge signal MLPRE, and outputs the logical addition as the inverted switch activation signal SL 2  ML_n. The inverter INV 161  reverses the inverted switch activation signal SL 2  ML_n, and outputs it as the switch activation signal SL 2 ML. 
     After the clock CAMCLK (search line activation signal SLE) is set to an L, until the pre-charge signal MLPRE is activated to an H level, the switching control circuit  161  activates the inverted switch activation signal SL 2  ML_n to an L level, and after some delay (since it passes through the inverter), the switching control circuit  161  activates the switch activation signal SL 2 ML to an H level. This is logic which activates the switch activation signal SL 2 ML, in a period other than the search period and the pre-charge period, and the search line SL and the match line ML are coupled in the period. 
     (Operating waveform)  FIG. 44  illustrates operating waveforms of the content addressable memory device according to Embodiment 13. 
     As illustrated in  FIG. 44 , the SL driver  100  activates the search line SL to VDD, when the search line activation signal SLE is at an H level and the search data of an H level is supplied. When the search line SL is activated and search result of any one of unit cells UC coupled to the match line ML is of Miss, the match line ML is discharged and changes toward an L level (the ground level). 
     Subsequently, when the inverted switch activation signal SL 2  ML_n is activated to an L level for a prescribed period (a period of about hundreds of ps), the match line ML and the common node VMA are coupled by the ML switch  118 . The SL driver  110  drives the search line SL to a Hi-Z state, when the inverted switch activation signal SL 2  ML_n is at an L level. 
     After the search line SL is driven to the Hi-Z state, the switch activation signal SL 2 ML is activated to an H level for a prescribed period (a period of about hundreds of ps), the search line SL activated to an H level (VDD level) by the SL switch  112  (that is, a search line to which the search data GSL of an H level has been supplied by the search line driver  110 ) and the common node VMA are coupled. 
     Accordingly, when the inverted switch activation signal SL 2  ML_n is at an L level and the switch activation signal SL 2 ML is at an H level, the activated search line SL and all the match lines ML are coupled via the common node and all of these lines become at the same potential Vma. 
     When the pre-charge signal MLPRE is activated to an H level after the search line SL and all the match lines ML become at the same potential, the P-channel MOS transistor TR 101  pre-charges the match line ML from the common potential Vma to VDD. The search line driver  110  deactivates the search line SL from the common potential Vma to an L level (the ground level). 
     (Reference: Operation of a conventional content addressable memory device) Operating waveforms of a conventional content addressable memory device are illustrated in  FIG. 45  for comparison. 
     As illustrated in  FIG. 45 , the SL driver activates the search line SL to VDD, when the search line activation signal SLE is at an H level and search data of an H level is supplied. When the search line SL is activated and search result of any one of unit cells UC coupled to the match line ML is of Miss, the match line ML is discharged and changes toward an L level (the ground level). 
     Subsequently, when the search line activation signal SLE changes to an L level, the search line driver deactivates the search line SL from VDD to an L level (the ground level). 
     Subsequently, when the pre-charge signal MLPRE is activated to an H level, the P-channel MOS transistor pre-charges the match line ML from the ground level to VDD. 
     In  FIG. 45 , since the match line ML is not coupled to the search line SL, the match line ML is charged to an H level (VDD), without passing through the middle common voltage Vma. Therefore, the consumption current of the past content addressable memory device is larger than that of the content addressable memory device according to the present embodiment. 
     (Current reduction effect) A current reduction effect of the present embodiment is explained. 
     A case where all the match lines ML are of Miss, that is, a case where consumption current becomes the maximum, is compared. It is assumed that parasitic capacitance of the match line ML is CML, and that parasitic capacitance of the search line SL is CSL. 
     In the past, the consumption current is given by CML×VDD. As compared with this, in the present embodiment, by coupling the search line SL charged to VDD level (accordingly, having a charge (CSL×VDD)) to the match line ML of the ground level, the voltage Vma of the match line ML becomes CSL/(CSL+CML)×VDD. Subsequently, the consumption current Ic necessary to charge the match line ML to VDD becomes CML×{1−CSL/(CSL+CML)}×VDD. 
     When the parasitic capacitance CSL of the match line ML and the parasitic capacitance CML of the search line SL are equal, the consumption current is given as follows: Ic=(CML/2)×VDD. This is a half of that in the past. 
     When the parasitic capacitance CML of the match line ML is ½ of the parasitic capacitance CSL of the search line SL, the consumption current is given as follows: Ic=(CML/3)×VDD. This is one third of that in the past. 
     (Reduction effect of current variation (dI/dt)) A reduction effect of current variation dI/dt is considered as follows. 
     When the parasitic capacitance CML of the match line ML and the parasitic capacitance CSL of the search line SL are equal, the match line ML is pre-charged to 0V or to VDD in the past. As compared with this, in the present embodiment, the match line ML is pre-charged from (VDD/2) to VDD. That is, dI/dt of the match line ML in the present embodiment is relaxed to ½ of that in the past. 
     In the past, the search line SL is discharged from VDD to 0V. As compared with this, in the present embodiment, the search line SL is discharged from (VDD/2) to 0V. That is, dI/dt of the search line SL in the present embodiment is relaxed to ½ of that in the past. 
       FIG. 46  illustrates current waveforms in Embodiment 13. As illustrated in  FIG. 46 , the VDD current is relaxed 33% and the VSS current is relaxed 67%. 
     (Effects) As described above, according to the present embodiment, the voltage of the activated search line SL is used for pre-charging of the match line ML. Accordingly, it is possible to reduce the consumption current and the current variation. 
     It is sufficient that one ML switch is provided for one match line ML, and that one SL switch is provided for one search line SL. Accordingly, it is possible to realize the preset layout without large alteration from the layout in the past. 
     (Embodiment 14) 
     (Configuration)  FIG. 47  illustrates a configuration of a content addressable memory device according to Embodiment 14. 
     The configuration illustrated in  FIG. 47  is different from the configuration according to Embodiment 13 illustrated in  FIG. 42  with respect to a ML switch  108 . 
     That is, in Embodiment 13, the ML switch  118  comprises the inverter INV 101  which receives the inverted switch activation signal SL 2  ML_n, and the N-channel MOS transistor TR 112  which receives the output of the inverter INV 101 . 
     As compared with this, in Embodiment 14, the ML switch  108  comprises a P-channel MOS transistor TR 102  which receives the inverted switch activation signal SL 2  ML_n. 
     When the inverted switch activation signal SL 2  ML_n is activated to an L level, the P-channel MOS transistor TR 102  is set to ON, and couples the match line ML and the common node VMA. 
     (Effects) As described above, according to the present embodiment, the same effect as in Embodiment 13 can be obtained by composing the ML switch of the P-channel MOS transistor, in place of the N-channel MOS transistor. 
     (Embodiment 15) 
     (Configuration)  FIG. 48  illustrates a configuration of a content addressable memory device according to Embodiment 15. 
     The configuration illustrated in  FIG. 48  is different from the configuration according to Embodiment 13 illustrated in  FIG. 42  with respect to a SL switch  122 . 
     That is, in Embodiment 13, the SL switch  112  comprises the inverted logical product circuit NAND 102  which receives the switch activation signal SL 2 ML and the search data GSL, and the P-channel MOS transistor TR 212  which receives the output of the inverted logical product circuit NAND 102 . 
     As compared with this, in Embodiment 15, the SL switch  122  comprises a logical product circuit AND 102  which receives the switch activation signal SL 2 ML and the search data GSL, and an N-channel MOS transistor TR 213  which receives an output of the logical product circuit AND 102 . 
     When the switch activation signal SL 2 ML and the search data GSL are activated to an H level, the N-channel MOS transistor TR 213  is set to ON, and couples the match line ML and the common node VMA. 
     (Effects) As described above, according to the present embodiment, the same effect as in Embodiment 13 can be obtained by composing the SL switch of the N-channel MOS transistor, in place of the P-channel MOS transistor. 
     (Embodiment 16) 
     (Configuration)  FIG. 49  illustrates a configuration of a switching control circuit according to Embodiment 16. 
     As illustrated in  FIG. 49 , the switching control circuit  162  comprises a delay circuit  163 , such as an inverter which delays the clock CAMCLK, and an inverted logical product circuit  164  which outputs an inverted logical product of an inverted signal of the clock CAMCLK and the clock CAMCLK, as an inverted switch activation signal SL 2  ML_n. The switching control circuit  162  comprises further an inverter INV 162  which reverses an output of the inverted logical product circuit  164 , and outputs a switch activation signal SL 2 ML. That is, for a delay time in the delay circuit  163  after the clock CAMCLK is set to an L level, the switching control circuit  162  activates the inverted switch activation signal SL 2  ML_n to an L level, and after some delay (since it passes through the inverter), the switching control circuit  162  activates the switch activation signal SL 2 ML to an H level. 
     Furthermore, the switching control circuit  162  comprises a logical product circuit  165  which outputs, as a pre-charge signal MLPRE, a logical product of an inverted signal of the switch activation signal SL 2 ML and the customary pre-charge signal MLPREM. 
     (Effects) As described above, according to the present embodiment, by replacing the switching control circuit illustrated in  FIG. 43  with the switching control circuit illustrated in  FIG. 49 , the same effect as in Embodiment 13 is obtained. 
     (Embodiment 17) 
     (Configuration)  FIG. 50  illustrates a configuration of a content addressable memory device according to Embodiment 17. 
     The configuration illustrated in  FIG. 50  is different from the configuration according to Embodiment 13 illustrated in  FIG. 42  with respect to a ML switch  128 . 
     The present ML switch  128  is coupled to the match line ML, and comprises an inverter INV 128  which receives the voltage of the match line ML. 
     The ML switch  128  comprises further a logical product circuit AND 128  which receives an inverted signal of the inverted switch activation signal SL 2  ML_n and an output of the inverter INV 128 , and an N-channel MOS transistor TR 112  which receives an output of the logical product circuit  128 . 
     The N-channel MOS transistor TR 112  is set to ON and couples the match line ML and the common node VMA, when the inverted switch activation signal SL 2  ML_n is activated to an L level and the voltage of the match line ML is at an L level (that is, when the search result is of Miss). 
     (Effects) As described above, according to the present embodiment, when the search result is of Miss, only the match line ML which is discharged to 0V is coupled to the activated search line SL. Therefore, it is possible to enhance the distribution efficiency of a charge charged to the activated search line SL. 
     (Embodiment 18) 
     (Configuration)  FIG. 51  illustrates a configuration of a content addressable memory device according to Embodiment 18. 
     The configuration illustrated in  FIG. 51  is different from the configuration according to Embodiment 13 illustrated in  FIG. 42  with respect to a P-channel MOS transistor TR 191  which pre-charges the match line ML. 
     In  FIG. 51 , a P-channel MOS transistor TR 191  is provided to each match line ML, for pre-charging each match line ML. 
     The P-channel MOS transistor TR 191  has a source coupled to the power supply voltage (VDD/2), a drain coupled to the match line ML, and a gate to which an inverted pre-charge signal /MLPRE is inputted. The P-channel MOS transistor TR 191  pre-charges the match line ML to the power supply voltage (VDD/2), when the inverted pre-charge signal /MLPRE is activated to an L level. 
     (Effects) As described above, according to the present embodiment, it is possible to enhance the distribution efficiency of a charge when the search line SL and the match line ML are coupled; accordingly, it is possible to attain low power consumption. 
     (Embodiment 19) 
     (Configuration)  FIG. 52  illustrates a configuration of a content addressable memory device according to Embodiment 19. 
     The configuration illustrated in  FIG. 52  is different from the configuration according to Embodiment 13 illustrated in  FIG. 42  with respect to a SL driver  130 . 
     The SL driver  130  comprises an inverted logical product circuit NAND 101 , a P-channel MOS transistor TR 130  which receives a switch activation signal SL 2 ML, a P-channel MOS transistor TR 104 , an N-channel MOS transistor TR 105 , and an N-channel MOS transistor TR 106  which receives an inverted switch activation signal SL 2  ML_n. 
     The P-channel MOS transistor TR 104  and the N-channel MOS transistor TR 105  form an inverter. The P-channel MOS transistor TR 130 , the P-channel MOS transistor TR 104 , the N-channel MOS transistor TR 105 , and the N-channel MOS transistor TR 106  form a clocked inverter. 
     The inverted logical product circuit NAND 101  outputs an inverted logical product of a search line activation signal SLE and search data GSL, to the inverter which is comprised of the P-channel MOS transistor TR 104  and the N-channel MOS transistor TR 106 . 
     The P-channel MOS transistor TR 130  is set to ON when the switch activation signal SL 2 ML is deactivated to an L level. The P-channel MOS transistor TR 130  is set to OFF when the switch activation signal SL 2 ML is activated to an H level. 
     The N-channel MOS transistor TR 106  is set to OFF when the inverted switch activation signal SL 2  ML_n is activated to an L level. The N-channel MOS transistor TR 106  is set to ON when the inverted switch activation signal SL 2  ML_n is deactivated to an H level. 
     The SL driver  130  drives the search line SL to a Hi-Z state, when the switch activation signal SL 2 ML is at an H level and the inverted switch activation signal SL 2  ML_n is at an L level. 
     The SL driver  130  supplies a voltage of an H level to the search line SL (to activate the search line), when the switch activation signal SL 2 ML is at an L level, the inverted switch activation signal SL 2  ML_n is at an H level, the search line activation signal SLE is at an H level, and the search data GSL is at an H level. 
     The SL driver  130  supplies a voltage of L level to the search line SL (to deactivate the search line), when the switch activation signal SL 2 ML is at an L level, the inverted switch activation signal SL 2  ML_n is at an H level, the search line activation signal SLE is at an H level, and the search data GSL is at an L level. 
     (Effects) As described above, according to the present embodiment, it is possible to obtain the same effect as in Embodiment 13, by using the clocked inverter as the SL driver. Compared with Embodiment 13, it is possible to drive the search line SL to the Hi-Z state more reliably, before the search line SL is coupled to the common node VMA. 
     (Modification) (1) Match amplifier: Instead of the match amplifier A, the match amplifier B, and the match amplifier C, explained in Embodiment 3-Embodiment 6, it is also preferable to employ the match amplifier A plus the drive B, the match amplifier B plus the drive C, and the match amplifier C plus the drive D, as explained in Embodiment 2. The drive B, the drive C, and the drive D have the same configuration as the drive B according to Embodiment 2, and pre-charge or discharge the match lines MLB, MLC, and MLD, respectively, based on the voltage of match lines MLA, MLB, and MLC. In this case, the match amplifier B explained in Embodiment 2 is employed instead of the match amplifier D in Embodiment 3-Embodiment 6.