Patent Publication Number: US-10776362-B2

Title: Memory devices for pattern matching

Description:
RELATED APPLICATION 
     This application is a Continuation of U.S. application Ser. No. 15/253,965 titled “MEMORY DEVICES FOR PATTERN MATCHING,” filed Sep. 1, 2016, now U.S. Pat. No. 10,089,359 issued on Oct. 2, 2018, which is a Divisional of U.S. application Ser. No. 13/449,082 titled “METHODS AND APPARATUS FOR PATTERN MATCHING,” filed on Apr. 17, 2012, now U.S. Pat. No. 9,436,402 issued on Sep. 6, 2016, which is commonly assigned and incorporated herein by reference in its entirety and which claims priority to U.S. Provisional Application Ser. No. 61/476,574, filed Apr. 18, 2011, and entitled “METHODS AND APPARATUS FOR PATTERN MATCHING.” 
    
    
     FIELD 
     The present disclosure relates generally to memory and in particular, in one or more embodiments, the present disclosure relates to methods and apparatus for pattern matching. 
     BACKGROUND 
     Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory. 
     Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the cells, through programming of a charge storage structure, such as floating gates or trapping layers or other physical phenomena, determine the data state of each cell. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, cellular telephones, and removable memory modules, and the uses for flash memory continue to expand. 
     Flash memory typically utilizes one of two basic architectures known as NOR flash and NAND flash. The designation is derived from the logic used to read the devices. In NOR flash architecture, a logical column of memory cells is coupled in parallel with each memory cell coupled to a data line, such as those typically referred to as digit (e.g., bit) lines. In NAND flash architecture, a column of memory cells is coupled in series with only the first memory cell of the column coupled to a bit line. 
     Content addressable memories (CAM) are memories that implement a lookup table function in a single clock cycle. They use dedicated comparison circuitry to perform the lookups. CAM application are often used in network routers for packet forwarding and the like. Each individual memory bit in a CAM requires its own comparison circuit in order to allow the CAM to detect a match between a bit of the key word with a bit stored in the CAM. Typical CAM cells, then, use approximately nine to ten transistors for a static random access memory (SRAM)-based CAM, or four to five transistors for a dynamic random access memory (DRAM)-based CAM. 
     Current CAM technology allows for a density of approximately four to eight megabytes, with clock cycle times of approximately 10 nanoseconds. A key word is typically input to the CAM for matching. Key words range in size up to about 144 bits. While current CAM technology allows for fast searching, it is limited in total stored content size, as well as key word size. 
     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for increased CAM density and functionality. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a memory on which embodiments of the disclosure may be practiced; 
         FIG. 2  is a block diagram of more detail of a page buffer; 
         FIG. 3  is a block diagram of more detail of a detector; 
         FIGS. 4A, 4B, 4C, and 4D  are flow chart diagrams of methods according to various embodiments of the disclosure; 
         FIG. 5  is a circuit diagram of a portion of a memory; 
         FIG. 6  is a diagram of representative read and threshold voltages according to an embodiment of the disclosure; 
         FIG. 7  is a table showing outcomes of searches of search data versus stored data according to an embodiment of the disclosure; 
         FIG. 8A  is a diagram showing a partial view of a search function according to an embodiment of the disclosure; 
         FIG. 8B  is a diagram showing a partial view of another search function according to an embodiment of the disclosure; 
         FIG. 9  is a block diagram of a portion of a memory according to another embodiment of the disclosure; 
         FIG. 10  is a circuit diagram of a portion of a memory according to another embodiment of the disclosure; and 
         FIG. 11  is a functional block diagram of an electrical system having at least one memory device with a memory array configuration according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of the embodiments, reference is made to the accompanying drawings that form a part hereof. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. 
     The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present disclosure is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     Referring generally to  FIG. 1 , a NAND flash memory device  100  typically comprises a memory array  102  divided into columns (accessed by data lines  104 , such as those commonly referred to as bit lines) and rows (accessed by access lines  106 , such as those commonly referred to as word lines), a voltage generation system  108 , a controller  110 , input/output circuitry  112  for command and data transfers, and a plurality of page buffers  114 , each page buffer coupled to a bit line and which are used to latch data sensed from the array during a read operation, and to store data to be programmed into the array, and to a bank  116  of data detectors. 
     One example of a page buffer  114  is shown in greater detail in  FIG. 2 . Page buffer  114  is a circuit block comprising a number of memory elements and additional circuitry. The page buffer  114  stores data read from the array, or user data to be written to the array. Page buffer  114  also can perform basic logic functions, such as AND, OR, and NOT operations, on the data. Page buffer  114  comprises in one embodiment a pair of static memory elements, a first data cache  202  (sometimes referred to as a primary data cache or PDC), and a second data cache  204  (sometimes referred to as a secondary data cache or SDC). PDC  202  holds data that are used to keep the bit line at a voltage level sufficient to shift a threshold voltage of a memory cell during programming, or to sense the data from a bit line during a read operation. SDC  204  is a memory element accessible to a user, and is used as a data read/write buffer. The PDC and SDC are independent from one another. Page buffer  114  further comprises a sense amplifier  206  to read data from memory cells, and a number of dynamic memory elements  208 . The controller  110  can move data in the PDC  202  to the SDC  204  to allow it to be read by a user, or to any of the dynamic memory elements  208  for use in logical operations. 
     For example, an XNOR operation may be used during programming to verify whether all cells have been programmed to the value input by the user. In a verify operation, data are read from the array after each programming pulse, and loaded into the PDC. Then a logic operation PDC=PDC XNOR SDC is performed. After one or more programming pulses, the data read into the PDC from the array should eventually be equal to the user data that is to be programmed to the array, and the PDC will, after the PDC XNOR SDC operation, contain all logical 1 values. The controller detects the all logical 1 values in the PDC and identifies programming as complete. 
     Each page buffer  114  is coupled to one data detector circuit  116  of the bank plurality of data detector circuits as shown in greater detail in  FIG. 3 . Each data detector  300  comprises a current mirror  302  gated by the output  304  from the PDC  202  of its data cache. For each bit of the PDC, if the bit value is a logical 0, a current equal to reference current Iref is mirrored on the global verify line  306 . The plurality of data detectors are all coupled with an OR circuit to the global verify line  306 . For detecting, the global verify line  306  is initially pre-charged to a logic high value. The global verify line  306  is driven to logical 0 unless all data in the PDC are logic 1 value. 
     The detector bank  116  can also detect when all the PDC data are logic 1 values except a for a determined number N of them. To accomplish this, if the global verify line  306  is discharged, then it can be pre-charged again, and a reference current Iref  308  injected onto the global verify line  306 . The current Iref may be set at various levels. For example, Iref1 is a current sufficient to overcome the discharge of the global verify line  306  due to one bit value of logic 0 in the PDC data. A reference current Iref2, in one embodiment twice the current of Iref1, is sufficient to overcome the discharge of the global verify line  306  for up to two bit values of logic 0 in the PDC data. Increasing Iref currents (Iref*N) may be applied (e.g., injected) to determine a number of non-logic 1 values in the PDC data. The number N will depend upon available current, and may, in one embodiment, be set by a user. 
     Since the ability to determine whether a number of PDC bit values, up to N, are logic 0 values, this arrangement is used in one embodiment for pattern matching, such as in a CAM embodiment of the NAND array. If a user desires to know whether a certain pattern, for example, a page-wide pattern, is stored in a memory array, and also desires to know the address of the pattern, the arrangement of  FIGS. 1-3  may be used to determine that. 
     In one embodiment, shown in flow chart form in  FIG. 4A , a method  400  for pattern matching comprises providing a pattern to be searched in the memory in block  402 , and pattern checking for the pattern to be searched in the memory in block  404 . In one embodiment  420  shown in  FIG. 4B , pattern checking further comprises reading a page from the memory in block  422 , and comparing with an XNOR operation the read page and the pattern to be searched in block  424 . 
     The page is read from the memory in one embodiment into a first cache, such as PDC  202  of  FIG. 2 , and the pattern to be searched is loaded into a second cache, such as SDC  204  of  FIG. 2 , before the XNOR operation. Once the XNOR operation is complete, the result of the XNOR operation is applied to a bank of detectors such as detectors  116 , and a global verify line such as global verify line  306  is checked as in a verify operation. If the global verify line is still charged, that is it has a logic 1 value, then the data in the first and cache and the data in the second cache match. This result is obtained when the XNOR operation combining the first and second data caches results in all logic 1 values. In one embodiment, the result of the XNOR operation is written to the first data cache before applying it to the detectors. 
     If a match of the pattern to be searched (loaded into the second cache) and page data written to the first cache is found, the location of the start of the match may be stored. If no match of the pattern to be searched and page data is found, the page address is incremented, and the page corresponding to the incremented page address is read, the first cache and the second cache are combined in an XNOR operation, and another determination is made if a match has occurred. This method may be repeated until an end page address is reached or a match is found. 
     While a full page pattern is described for searching herein, it should also be understood that less than a full page pattern may also be searched without departing from the scope of the disclosure. 
     Reading a page in one embodiment comprises setting a starting page address and an ending page address and reading the page at the starting page address into the first data cache. 
     In another embodiment, a reference current such as that described above is injected onto the global verify line at a value of Iref*N, where N is an integer representing a number of bits less than the total number of bit errors in the pattern matching. In this embodiment, if the global verify line remains charged at a logic 1 value, then more than all the bits minus N are matched between the data read into the first cache and the pattern to be searched that is in the second cache. In one embodiment, the first cache is a PDC of the memory, and the second cache is a SDC of the memory. When the pattern to be searched is found, that is, when the XNOR operation results in a logical 1 value for all bits and therefore does not discharge the global verify line, or up to N bits are incorrect but the appropriate reference current is injected to the global verify line, one embodiment sets a status bit to indicate the pattern to be searched has been found. 
     Additionally, an address of the start of the pattern to be searched may also be stored. If additional reference current is injected onto the global verify line, for example Iref*3, before the global verify line remains charged to a logical 1 value, the number N (in this example 3) can also be stored to indicate how many bits between the pattern to be searched and the pattern in the memory do not match. This can be done in one embodiment on a page by page basis, or in another embodiment by whatever pattern is to be searched. Further, a number of failing bits can be stored for each page of the memory. 
     A flow chart for a more detailed method  430  for pattern matching is shown in greater detail in  FIG. 4C . Method  430  comprises setting a page end address in block  432 , loading a pattern to be searched and a starting page address in block  434 , reading a page from the memory array in block  436 , and combining the read page and the pattern to be searched with an XNOR operation in block  438 . If the read page and the pattern to be searched match, as determined in decision block  440 , the matching address is saved in block  442 , and the process is complete at block  444 . If the read page and the pattern to be searched to not match, the page address is incremented at block  446 , and process flow continues at block  436 , until the last page, as determined in decision block  448  by the previously set end page address, is reached. This may be checked after incrementing the page address, as shown in block  446 . 
     In another embodiment  460 , shown in greater detail in  FIG. 4D , pattern matching (block  404  of  FIG. 4A ) comprises programming the pattern to be searched into a plurality of registers in block  462 , and comparing selected programmed bits of the pattern to data stored in the array in block  464 . An error count for the comparison may also be determined. Comparing is performed in one embodiment by activating bit lines for specific word lines in the array. 
     Referring now also to  FIGS. 5, 6, 7, and 8 , in one embodiment  800 , a key word  802  is identified as the basis for the pattern that is to be searched. The key word  802  comprises a number of bits, each bit being a logical 1 or logical 0 in one embodiment. The key word is in one embodiment programmed into key registers, with each bit of the key word using two register entries, wherein a specific pattern of voltages is applied to the gates of particular memory cells of the array for each bit of the key word. In one embodiment, register bit  0  causes a voltage of 2 volts to be applied to the gate of the first cell of the two cells, and register bit  1  causes a voltage of 4 volts to be applied to the gate of the second cell of the two cells. This combination of voltages at the gates of the cells corresponds to a bit having a value of 0. A bit having a value of 1 is stored in the key register as a register bit  0  causing a voltage of 4 volts to be applied to the gate of the first cell of the two cells, and register bit  1  causing a voltage of 2 volts to be applied to the gate of the second cell of the two cells. Therefore, programming a pattern programs each bit of the pattern into two register entries that identify the single key word bit. Then, comparison is made to a representation of data stored in the array, where each bit of stored data is also represented by two cells, each having its own programmed threshold voltage. 
     In the case of data stored in the array, a logical 0 is programmed into two cells and is represented by a first threshold voltage of 3 volts on the first cell and a second threshold voltage of 1 volt on the second cell, and a logical 1 is programmed into two cells and is represented by a first threshold voltage of 1 volt on the first cell and a second threshold voltage of 3 volts on the second cell. The cell gate voltages and threshold voltage values are used in a sensing operation, with the word line voltages applied through a multiplexer to blocks having the coded stored data, also coded to have threshold voltages of 1 or 3 volts, with a logical 0 represented by Vt 0 =3 volts and Vt 1 =1 volt and a logical 1 represented by Vt 0 =1 volt and Vt 1 =3 volts. 
     The basic string structure, threshold voltages and read voltages, and a table of stored data versus search data and resultant match or no-match outcomes are shown in  FIGS. 5, 6, and 7 .  FIG. 5  shows a portion  500  of a memory array, having a representative set of two memory cells  502  and  504  which are part of a string  506  of memory cells. A data line  508  is coupled to the string  506 . In  FIG. 5 , the gates of the cells  502  and  504  are coupled to access lines  510  and  512 . It is these access lines that have applied to them the voltages that are programmed into the key registers described herein. Representative threshold voltages programmed onto the memory cells are shown in  FIG. 6 . Threshold diagram  600  shows that cell  502  has a threshold voltage Vt 0  of 1 volt, and cell  504  has a threshold voltage Vt 1  of 3 volts. The two threshold voltages of Vt 0 =1 and Vt 1 =3 correspond to a stored value of 1.  FIG. 7  shows a table  700  having stored data values compared with search data values, in which X values indicate that the cell is off, and 0 values indicate that the cell is on. If a cell is on, a sensing operation results in a discharge of the bit line, indicating that no match is present between the voltages applied to the access lines  510  and  512  (from the key register) to the cells  502  and  504 . Each cell pair and each register bit pair of values allow for a matching or a non-matching determination for a bit of the key word. 
     While one set of threshold voltages and gate voltages are described herein, it should be understood that as cell structures vary, or array and threshold voltages change, different sets of voltages may be used without departing from the scope of the disclosure. 
       FIG. 8A  shows a key word having N bits, and selected with select signal 0, indicated at  801 . Each bit of the key word is programmed into two bits of a key register  804 . There are N key registers  804   0 ,  804   1 , . . .  804   N  in one embodiment, each key register representing a block of entries, corresponding therefore to 32 bits of the key word. Each key register  804   0 ,  804   1 , . . .  804   N  has a multiplexer  806   0 ,  806   1 , . . .  806   N  to select a key register pair to compare with a data cell pair. The multiplexer  806  for each key register  804  selects the two values of the respective bit in each block (bit 0  and bit 1  of key register 0 for first bit of the key word, bit 0  and bit 1  of key register 1 for the second bit of the key word, . . . bit 0  and bit 1  of key register N for the Nth bit of the key word, etc . . . ) The select signal for the multiplexer determines which pair of key register entries are used for applying to the current key word, in this case programming the key word into the key register in the first two entries of each key register. The stored coded voltages for the key register bits are applied to the selected cells that are at the position on the page which is being pattern matched. That is, bit 0  and bit 1  voltages of key register 0 are selected through multiplexer  806  and are applied as the gate voltages for the cells 0 and 1 in the string of the page being searched for a match. A pass voltage is applied to the gates of all remaining cells of the string. Match and non-match data are shown at the right side of  FIG. 8  as strings  808   0 ,  808   1 , . . .  808   N  and  810   0 ,  810   1 , . . .  810   N , respectively. 
     If the key word data is a logical 0 (shown as bit A of the key word  802 ), and the stored data is a logical 0 (shown as Vt 0  and Vt 1  of string  808   0 , the gate of cell 0 of block  808   0  will have 2 volts applied thereto, the gate of cell 1 of block  808   0  will have 4 volts applied thereto, Vt 0  will be 3 volts, and Vt 1  will be 1 volt. In that circumstance, the gate voltage of 2 volts does not turn on the cell with a Vt 0  of 3 volts, the gate voltage of 4 volts does turn on the cell with Vt 1  of 1 volt, but the one cell that remains off prevents the bit line from discharging, and indicates a data match to that bit of stored data. If the key word data is a logical 0 (shown as bit A of the key word  802 ), and the stored data is a logical 1 (shown as Vt 0  and Vt 1  of string  810   0 , the gate of cell 0 of block  810   0  will have 2 volts applied thereto, the gate of cell 1 of block  810   0  will have 4 volts applied thereto, Vt 0  will be 1 volt, and Vt 1  will be 3 volts. In that circumstance, the gate voltage of 2 volts turns on the cell with a Vt 0  of 1 volt, the gate voltage of 4 volts turns on the cell with Vt 1  of 1 volt, and the bit line is discharged, indicating a data non-match to that bit of stored data. 
       FIG. 8B  shows operation for a select 1 signal to the multiplexers  806 , indicated at  851 . In this case, the key word is programmed into the key register in the third and fourth entries of each key register. If the key word data is a logical 0 (shown as bit A of the key word  802 ), and the stored data is a logical 0 (shown as Vt 2  and Vt 3  of string  808   0 , the gate of cell 2 of block  808   0  will have 2 volts applied thereto, the gate of cell 3 of block  808   0  will have 4 volts applied thereto, Vt 2  will be 3 volts, and Vt 3  will be 1 volt. In that circumstance, the gate voltage of 2 volts does not turn on the cell with a Vt 2  of 3 volts, the gate voltage of 4 volts does turn on the cell with Vt 3  of 1 volt, but the one cell that remains off prevents the bit line from discharging, and indicates a data match to that bit of stored data. If the key word data is a logical 0 (shown as bit A of the key word  802 ), and the stored data is a logical 1 (shown as Vt 2  and Vt 3  of string  810   0 , the gate of cell 2 of block  810   0  will have 2 volts applied thereto, the gate of cell 3 of block  810   0  will have 4 volts applied thereto, Vt 2  will be 1 volt, and Vt 3  will be 3 volts. In that circumstance, the gate voltage of 2 volts turns on the cell with a Vt 2  of 1 volt, the gate voltage of 4 volts turns on the cell with Vt 3  of 1 volt, and the bit line is discharged, indicating a data non-match to that bit of stored data. 
     All other cells have pass voltages applied to their gates, so that only the cells of the selected bit of the key word in each block are used in the match/no match decision. Similarly, each combination of search data versus stored data has a match or a non-match situation, with a match keeping the associated bit line charged, and a non-match discharging the associated bit line. 
     With a key word having a number of bits equal to or smaller than the number of blocks N, an entire key word can be checked in parallel at one time. With a key word having a number of bits greater than the number of blocks N, N bits of the key word can be checked in parallel at one time, and a new select signal can be issued for additional bits of the key word above the number of blocks N. For example, for bits of the key word from bit N+1 to bit 2N, the select signal (such as signals  801  and  851 ) is set to 1, and the multiplexers  806  select the key word register third and fourth entries, and so on. 
     If a near-match is to be searched for, such as for example, a pattern stored in the array that matches the pattern to be searched but for a determined number of bits, for example, E, a counter for the number of errors is used in another embodiment.  FIG. 9  is a block diagram of a system  900  with a counter  902  for determining a number of non-match bits in a string or series of strings. The system  900  uses key registers  904  as in the systems described above. In the example, suppose K is the string size, N is a number of blocks, and M is a number of bit lines in the system. The key word to be searched, i.e., matched, in the array can have K/2*N bits. Stored data comprise M key words stored in an array of M*K*N cells in the array. A set of multiplexers  906 , a multiplexer for each block, selects a pair of voltages from its respective key register  904  to apply to the gates of the selected memory cells that are to be checked for a match. All other cell gates of the block are provided with an appropriate pass voltage. 
     For sensing to determine whether a match has been found, the bit lines are precharged, an error counter  902  value is set to zero, and a sense operation on the bit lines is performed. If a bit line does not discharge, there is a match of all selected bits on that string, and the counter  902  stores an error count of zero, in one embodiment into page buffer  908 . If the bit line does discharge, that indicates at least one bit that does not match. The error count value is incremented up one, the bit lines are again precharged, an increased reference current selected by multiplexer  910  is injected onto the bit lines, and the sense operation is again performed. This is repeated, summing the total number of errors, up to a maximum number of errors set by the number of available increased reference currents. When the number of errors in the selected cells is determined, that number is summed into a total error buffer  912 , the array address is incremented, and the next set of cell pairs in the array are selected for passing values from the key registers  904  to be searched as is described above. This is performed until the end of the key register data is reached or the end of the string is reached. A total number of errors for the entire keyword is stored in error buffer  912 . If the total number of errors is zero for the key word, the key word is matched. If the total number of errors is greater than zero, then that total is indicative of a number of detected errors, subject to a maximum number of counted errors per string, determined as described elsewhere herein. 
     The methods described above are, in one embodiment, performed in a smaller amount of time than a typical CAM memory operation, and are capable of using far larger key words and searching far larger arrays. For example, a typical load time for current key word registers is approximately 20 nanoseconds, which is performed K/2*N times. For 64 cell strings and 1024 blocks, a key word will load in about 650 microseconds. Sensing takes approximately 10 microseconds, and will be performed K/2 times (32 for 64 cell string) with 20 nanoseconds per bit line (typically 64000). So, for searching a 32000 bit pattern in a database of 64000 patterns, an expected time is about 2.3 milliseconds. As read, write and sense times change, that is only an example, and different read, write and/or sense times will affect the overall time for completion of the matching operation, as will different sized patterns (e.g., key words) to be searched, and such changes are within the scope of the disclosure. 
     In another embodiment  1000 , shown in greater detail in  FIG. 10 , error counting is performed in a different way. The counter circuit  1008  counts errors between stored data and data to be searched. An error count value is determined and stored as follows. The key word to be searched is stored in the key registers as before, and data in the array is stored as before. A sense operation is performed. If the bit line stays charged, then there are no errors, and the enable signal, from sense amplifier block  1002 , is a logical 0. No error count of any type is passed to the counter  1008 , as there are no errors. If the bit line discharges, an increasing series of reference currents are injected onto the bit line as described above. Once a number of errors for the string is determined, or a maximum error value has been reached, the enable signal from sense amplifier block  1002  is set to logical 1, and pulse line  1004  applies a number of pulses  1006  to the counter  1008 . The number of pulses on the pulse line  1004  is equal to the error count value. The counter accumulates the number of non-match bits between the key word and the data to be searched, subject to the maximum errors countable per string. 
     It should be understood that while the embodiments disclosed herein are shown as implemented in a NAND memory, other memories, such as NOR and the like may be employed without departing from the scope of the disclosure. 
       FIG. 11  is a simplified block diagram of a memory device  1101  according to an embodiment of the disclosure, and on which various embodiments of the disclosure can be practiced. Memory device  1101  includes an array of memory cells  1104  arranged in rows and columns. Although the various embodiments will be described primarily with reference to NAND memory arrays, the various embodiments are not limited to a specific architecture of the memory array  1104 . Some examples of other array architectures suitable for the present embodiments include NOR arrays, AND arrays, and virtual ground arrays. In general, however, the embodiments described herein are adaptable to any array architecture permitting generation of a data signal indicative of the threshold voltage of each memory cell. Control logic  1116  is one example of a controller, such as controller  110 . 
     A row decode circuitry  1108  and a column decode circuitry  1110  are provided to decode address signals provided to the memory device  1101 . Address signals are received and decoded to access memory array  1104 . Memory device  1101  also includes input/output (I/O) control circuitry  1112  to manage input of commands, addresses and data to the memory device  1101  as well as output of data and status information from the memory device  1101 . An address register  1114  is coupled between I/O control circuitry  1112  and row decode circuitry  1108  and column decode circuitry  1110  to latch the address signals prior to decoding. A command register  1124  is coupled between I/O control circuitry  1112  and control logic  1116  to latch incoming commands. Control logic  1116  controls access to the memory array  1104  in response to the commands and generates status information for the external processor  1130 . The control logic  1116  is coupled to row decode circuitry  1108  and column decode circuitry  1110  to control the row decode circuitry  1108  and column decode circuitry  1110  in response to the addresses. 
     Control logic  1116  can be coupled to a sample and hold circuitry  1118 . The sample and hold circuitry  1118  latches data, either incoming or outgoing, in the form of analog data signals. For example, the sample and hold circuitry could contain capacitors or other analog storage devices for sampling either an incoming data signal representing data to be written to a memory cell or an outgoing data signal indicative of the threshold voltage sensed from a memory cell. The sample and hold circuitry  1118  may further provide for amplification and/or buffering of the sampled signal to provide a stronger data signal to an external device. 
     The handling of analog data signals may take an approach similar to an approach well known in the area of CMOS imager technology, where charge levels generated at pixels of the imager in response to incident illumination are stored on capacitors. These charge levels are then converted to signals using a differential amplifier with a reference capacitor as a second input to the differential amplifier. The output of the differential amplifier is then passed to analog-to-digital conversion (ADC) devices to obtain a digital value representative of an intensity of the illumination. In the present embodiments, a charge may be stored on a capacitor in response to subjecting it to a data signal indicative of an actual or target threshold voltage of a memory cell for reading or programming, respectively, the memory cell. This charge could then be converted to an analog data signal using a differential amplifier having a grounded input or other reference signal as a second input. The output of the differential amplifier could then be passed to the I/O control circuitry  1112  for output from the memory device, in the case of a read operation, or used for comparison during one or more verify operations in programming the memory device. It is noted that the I/O control circuitry  1112  could optionally include analog-to-digital conversion functionality and digital-to-analog conversion (DAC) functionality to convert read data from an analog data signal to a digital bit pattern and to convert write data from a digital bit pattern to an analog signal such that the memory device  1101  could be adapted for communication with either an analog or digital data interface. 
     During a write operation, target memory cells of the memory array  1104  are programmed until voltages indicative of their Vt levels match the levels held in the sample and hold circuitry  1118 . This can be accomplished, as one example, using differential sensing devices to compare the held voltage level to a threshold voltage of the target memory cell. Much like traditional memory programming, programming pulses could be applied to a target memory cell to increase its threshold voltage until reaching or exceeding the desired value. In a sense operation, the Vt levels of the target memory cells are passed to the sample and hold circuitry  1118  for transfer to an external processor (not shown in  FIG. 11 ) either directly as analog signals or as digitized representations of the analog signals depending upon whether ADC/DAC functionality is provided external to, or within, the memory device. 
     Threshold voltages of cells may be determined in a variety of manners. For example, an access line, such as those typically referred to as word lines, voltage could be sampled at the point when the target memory cell becomes activated. Alternatively, a boosted voltage could be applied to a first source/drain side of a target memory cell, and the threshold voltage could be taken as a difference between its control gate voltage and the voltage at its other source/drain side. By coupling the voltage to a capacitor, charge would be shared with the capacitor to store the sampled voltage. Note that the sampled voltage need not be equal to the threshold voltage, but merely indicative of that voltage. For example, in the case of applying a boosted voltage to a first source/drain side of the memory cell and a known voltage to its control gate, the voltage developed at the second source/drain side of the memory cell could be taken as the data signal as the developed voltage is indicative of the threshold voltage of the memory cell. 
     Sample and hold circuitry  1118  may include caching, i.e., multiple storage locations for each data value, such that the memory device  1101  may be reading a next data value while passing a first data value to the external processor, or receiving a next data value while writing a first data value to the memory array  1104 . A status register  1122  is coupled between I/O control circuitry  1112  and control logic  1116  to latch the status information for output to the external processor. 
     Memory device  1101  receives control signals at control logic  1116  over a control link  1132 . The control signals may include a chip enable CE #, a command latch enable CLE, an address latch enable ALE, and a write enable WE #. Memory device  1101  may receive commands (in the form of command signals), addresses (in the form of address signals), and data (in the form of data signals) from an external processor over a multiplexed input/output (I/O) bus  1134  and output data to the external processor over I/O bus  1134 . 
     In a specific example, commands are received over input/output (I/O) pins [7:0] of I/O bus  1134  at I/O control circuitry  1112  and are written into command register  1124 . The addresses are received over input/output (I/O) pins [7:0] of bus  1134  at I/O control circuitry  1112  and are written into address register  1114 . The data may be received over input/output (I/O) pins [7:0] for a device capable of receiving eight parallel signals, or input/output (I/O) pins [15:0] for a device capable of receiving sixteen parallel signals, at I/O control circuitry  1112  and are transferred to sample and hold circuitry  1118 . Data also may be output over input/output (I/O) pins [7:0] for a device capable of transmitting eight parallel signals or input/output (I/O) pins [15:0] for a device capable of transmitting sixteen parallel signals. It will be appreciated by those skilled in the art that additional circuitry and signals can be provided, and that the memory device of  FIG. 11  has been simplified to help focus on the embodiments of the disclosure. 
     Methods for pattern matching of a key word to data stored in the array  1104  may be performed in various embodiments on a memory such as memory  1100 . In some embodiments, CAM circuitry  1136 , which in various embodiments includes key registers, multiplexers, and/or counters, is coupled between the address register  1114  and the array  1104  for implementing methods such as those described above. Such methods are shown and described herein with reference to  FIGS. 1-10 . 
     While the memory device of  FIG. 11  has been described in accordance with popular conventions for receipt and output of the various signals, it is noted that the various embodiments are not limited by the specific signals and I/O configurations described. For example, command and address signals could be received at inputs separate from those receiving the data signals, or data signals could be transmitted serially over a single I/O line of I/O bus  1134 . Because the data signals represent bit patterns instead of individual bits, serial communication of an 8-bit data signal could be as efficient as parallel communication of eight signals representing individual bits. 
     CONCLUSION 
     Methods of matching a pattern to be searched with data in memories, and memories using the methods have been described. In particular, some embodiments store key words and array data so that two bits of a key register and two cells are used for matching, with a bit of a key word identified by two key word register entries and a bit of stored data identified with the two cells. Patterns are matched by comparing the representations of key word data and stored array data, and a number of errors can be counted. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.