Memory devices for comparing input data to data stored in memory cells coupled to a data line

Abstract

A memory device might include control circuitry configured to cause the memory device to compare input data to data stored in memory cells connected to a data line, cause a first level of current to flow from the data line in response to a mismatch between one digit of the input data and data stored in a respective pair of memory cells, cause a second level of current to flow from the data line in response to a mismatch between a different digit of the input data and the data stored in a respective pair of memory cells, compare a representation of a level of current in the data line to a reference, and deem the input data to potentially match or not match the data stored in the plurality of memory cells in response to the comparison.

Description

FIELD

The present disclosure relates generally to memory devices and in particular the present disclosure relates to memory devices configured to apply different weights to different strings of memory cells coupled to a data line and methods.

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 charge storage structures, 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 may utilize architectures known as NOR flash and NAND flash. The designation is derived from the logic used to read the devices. In a NOR flash architecture, a column of memory cells are coupled in parallel with each memory cell coupled to a data line, such as a bit line. A “column” refers to a group of memory cells that are commonly coupled to a data line, such as a bit line. It does not require any particular orientation or linear relationship, but instead refers to the logical relationship between memory cell and data line.

Typically, the array of memory cells for NAND flash memory devices is arranged such that the control gate of each memory cell of a row of the array is connected together to form an access line, such as a word line. Columns of the array include strings (often termed NAND strings) of memory cells connected together in series, source to drain, between a pair of select lines, a source select line and a drain select line. The source select line includes a source select gate at each intersection between a NAND string and the source select line, and the drain select line includes a drain select gate at each intersection between a NAND string and the drain select line. Each source select gate is connected to a source line, while each drain select gate is connected to a data line, such as column 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 applications are often used in network routers for packet forwarding and the like. Each individual memory cell in a CAM usually requires its own comparison circuit in order to allow the CAM to detect a match between a bit of input data, such as an input feature vector (e.g., sometimes referred to as a key or key data) with a bit of data, such as a data feature vector, stored in the CAM. However, not all bits are the same. For example, binary expressions may include a most significant bit (MSB), a least significant bit (LSB), and bits between the MSB and LSB.

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 identifying and differentiating between MSBs, LSBs and bits between the MSB and LSB in comparisons between input data and data stored in memory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified block diagram of a memory device, according to an embodiment.

FIG. 1B is a schematic of a column of a memory array, according to another embodiment.

FIG. 1C shows a logical truth table and state definitions in accordance with the embodiment of FIG. 1B.

FIG. 2 is a block diagram that illustrates a portion of a memory device, according to another embodiment.

FIG. 3 is a schematic of a memory array, according to another embodiment.

FIG. 4 is an example of comparing a input data to data stored in a memory array, according to another embodiment.

FIG. 5 is an example of how memory blocks might be weighted for comparisons between an input feature vector having multiple components with multiple bits and a data feature vector having multiple components with multiple bits.

FIG. 6 is a simplified block diagram of a memory system, according to an embodiment.

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.

FIG. 1A is a block diagram of a memory device 100, e.g., a NAND, NOR, CAM memory device, etc. Memory device 100 may include a memory array 204. Memory array 204 may be organized into columns 212 of memory cells, such as columns 212₁ to 212_L, that may be respectively accessed by data lines (e.g., bit lines 215₁ to 215_L). A page buffer 110 may be coupled to bit lines 215. An input buffer 120 may also be coupled to memory array 204. The input buffer 120 can be used to temporarily store input data (e.g., an input feature vector), such as key data, for comparison (e.g., during a read operation performed on array 104) to data stored in memory array 104. Data stored in each column 212 of memory array 204 can be referred to as a data feature vector.

For example, the data feature vectors stored in columns 212 of memory array 104 may correspond to known entities, such as known patterns, and memory device 100 may be configured to determine whether an input feature vector at least partially matches a particular known entity represented by a data feature vector, thereby at least potentially identifying the input feature vector as being the particular known entity.

Memory device 100 may be configured to identify the input feature vector as being a particular known entity in response to determining that the input feature vector potentially matches the data feature vector representing the particular known entity. For example, memory device 100 may be configured to identify the data feature vectors most likely to match the input feature vector based on how closely the data feature vectors match the input feature vector.

Components of the input feature vector correspond to certain features (e.g., attributes) of the input feature vector, and thus the entity represented by the input feature vector. Similarly, components of the data feature vectors correspond to certain features (e.g., attributes) of the data feature vectors, and thus the entities represented by the data feature vectors. For example, each data feature vector and the input feature vector may be a pattern, such as of a person's face, fingerprint, etc., and the components may be the features of that pattern.

Components of the input feature vector may be compared to like components of the data feature vectors to determine which data feature vectors potentially match the input feature vector, thereby identifying the entity represented by the input feature vector. For some embodiments, an input feature vector may be determined to potentially match a data feature vector even though there might be a mismatch of certain components.

FIG. 1B illustrates an example of a column 212 of memory array 204, e.g., configured according to the NAND architecture. Column 212 comprises N strings (e.g., NAND strings) 310₁ to 310_N of series-coupled memory cells 315. One end of each of strings 310₁ to 310_N is coupled to the same bit line 215 that is coupled in turn to page buffer 110. The opposite end of each of strings 310₁ to 310_N is coupled to a source line SL. The bit line 215 acts as a summing node for the outputs from of each of strings 310₁ to 310_N. For some embodiments, strings 310₁ to 310_N may be respectively blocks 210₁ to 210_N of memory cells 315.

The h signals in FIG. 1B can be a pass signals. In an embodiment, the pass signal h has a voltage level high enough (e.g., about 4V in a particular embodiment) to operate a memory cell having its control gate coupled thereto as a pass-transistor, regardless of the programmed Vt of the memory cell. According to an embodiment, pass signals h might be applied to control gates of unselected memory cells of a string of memory cells (e.g., those memory cells corresponding to the at least a portion of a value of a feature not then being compared) to operate them as pass-transistors.

Memory cells 315₁ and 315₂ may be viewed as switches that selectively allow current to flow from a respective string 310 to bit line 215, or vice versa, when the remaining memory cells in the respective string are operating as pass-transistors. A mismatch between a component of a data feature vector stored in a string 310 and a like component of an input feature vector results in current flow in that string 310.

Resistors 340₁ and 340_N are respectively coupled in series with strings 310₁ and 310_N and respectively control the analog value of current through strings 310₁ and 310_N. For example, resistors 340₁ and 340_N may act as constant current sources of varying values.

The current value through a string can be set by setting the resistance of the respective resistor 340. For example, the resistance of a resistor 340 can be set by programming the threshold voltage of the resistor 340, setting the channel length of the resistor 340 to a specific value, setting the voltage VC applied to the control gate of the resistor 340 to a specific value, etc. Multiple resistors 340 may be in series for additional control (compensation) if appropriate.

The ratio of current may be binary. For example, for a unit of current through string 310₁, the currents through strings 310₂ to 310_N might respectively be ½ to 1/2^N-1 of a unit, where N is the block (e.g. string number), e.g., starting at 1. The string corresponding to the most significant bit, e.g., string 310₁, may have a weight of 1 and a unit current in the event of a mismatch, and the string corresponding to the least significant bit, e.g., string 310_N, may have the smallest weight, e.g., 1/2^N-1, and a current of 1/2^N-1 of a unit in the event of a mismatch.

In an example, a feature A1=(a₁, a₂, a₃, a₄, . . . , a_N) of a data feature vector may be programmed as 1011, e.g., for N=4. A feature B1=(b₁, b₂, b₃, b₄, . . . , b_N) of an input feature vector may be input as 1010, e.g., for N=4, for comparison to feature A1. Since a mismatch occurs in the fourth significant bit, corresponding to the least significant bit in this case, the value of the current flow will be 1/2^(4-1)=⅛ of a unit. This may be referred to as the least significant bit value of current. Note that bits a₁ through a_N may be respectively stored in blocks 210₁ through 210_N (e.g., strings 310₁ through 310_N) for some embodiments.

A feature B1=(b₁, b₂, b₃, b₄, . . . , b_N) of an input feature vector may input as 0011 for comparison to feature A1. Since a mismatch occurs in the first significant bit, corresponding to the most significant bit in this case, the value of the current flow will be the most significant bit value of current 1/2^(1-1)=1 unit.

For some embodiments, the input feature vector can be made up of multiple features (e.g., components of the input feature vector), e.g., IF(B)=(B1, B2, B3, . . . , BN) and each data feature vector can be made up of multiple features (e.g., components of the data feature vectors), e.g., DF(A)=(A1, A2, A3, . . . , AN)

The page buffer 110 may measure the analog current flow in bit line 215. For example, zero current is due to an exact match, and the smallest amount of current is due to a single feature mismatch, e.g., a least significant bit mismatch. A current of weight one results from a most significant bit mismatch. All mismatches in a column may be summed in an analog fashion. The threshold value of what current constitutes a match can be programmable.

In FIG. 1B, the bit values corresponding voltage signals applied to the control gates of memory cells 315₁ and 315₂ are respectively represented by d* and d, where d* is the complement of d and represents not d (e.g., see input data in the state definitions in FIG. 1C). In fact, a superscript * is used herein to denote a complement.

For example, if a corresponding digit of data of a received input feature vector has a binary bit value of b=0, a voltage of about 2V, corresponding to a bit value of d₁=0, may be applied to the control gate of memory cell 315₂ of string 310₁, and a voltage of about 4V, corresponding to a bit value of d₁*=1, may be applied to the control gate of memory cell 315₁ of string 310₁. That is, d₁*=1 and d₁=0 may correspond to a binary bit value of b=0 of a digit of data of the input feature vector, as shown in FIG. 1C. If a corresponding digit of data of a received input feature vector has a binary bit value of b=1, a voltage of about 4V, corresponding to a bit value of d₁=1, may be applied to the control gate of memory cell 315₂ of string 310₁, and a voltage of about 2V, corresponding to a bit value of d₁*=0, may be applied to the control gate of memory cell 315₁ of string 310₁. That is, d₁*=0 and d₁=1 may correspond to a binary value of b=1 of a digit of data of the input feature vector, as shown in FIG. 1C.

Memory cells 315₁ and 315₂ of string 310₁ might be respectively programmed to threshold voltages Vt1 and Vt1* to store at least a portion of a value of a feature of the data feature vector stored in string 310₁. For example, memory cells 315₁ and 315₂ of string 310₁ might be respectively programmed to threshold voltages Vt1 and Vt1* of about 1V and about 3V to store a binary value of a=0 for a digit of data of the stored data feature vector. For example, when memory cell 315₁ is programmed to a threshold voltage Vt1 of about 1V, memory cell 315₁ may store a bit value of z=0, and when memory cell 315₂ is programmed to a threshold voltage Vt1* of about 3V, memory cell 315₂ may store a bit value of z*=1. This means that when memory cells 315₁ and 315₂ respectively store bit values of z=0 and z*=1, memory-cell pair 315₁, 315₂ stores a binary value of a=0, as shown in FIG. 1C.

Memory cells 315₁ and 315₂ of string 310₁ might be respectively programmed to threshold voltages Vt1 and Vt1* of about 3V and about 1V to store a binary value of a=1 for the digit of data of the data feature vector. For example, when memory cell 315₁ is programmed to a threshold voltage Vt1 of about 3V, memory cell 315₁ may store a bit value of z=1, and when memory cell 315₂ is programmed to a threshold voltage Vt1* of about 1V, memory cell 315₂ may store a bit value of z*=0. This means that when memory cells 315₁ and 315₂ respectively store bit values of z=1 and z*=0, memory-cell pair 315₁, 315₂ stores a binary value of a=1, as shown in FIG. 1C. Similarly, memory cells 315₁ and 315₂ of string 310_N might be respectively programmed to threshold voltages VtN and VtN* to store at least a portion of a value of a feature of the data feature vector stored in string 310_N.

The logic truth table of FIG. 1C illustrates the results from comparing a digit b of input data, such as of a feature (e.g., component) B of an input feature vector, to a digit a of corresponding stored data, such as of a same feature (e.g., component) A of a stored data feature vector. With additional reference to FIG. 1B, for example, during a compare operation of an input data to stored data, for a digit a, signals, corresponding to the bit values d* and d, are respectively applied to the control gates of the memory cells 315₁ and 315₂ of string 310₁.

If the voltage of the signal corresponding to the respective d*/d bit value is greater than the programmed threshold voltage (e.g., Vt* or Vt) of the respective memory cell (e.g., 315₁ and 315₂), the respective memory cell will conduct, e.g., and will be ON. If the voltage of the signal corresponding to the respective d*/d bit value is less than the programmed threshold voltage (e.g., Vt* or Vt) of the respective memory cell (e.g., 315₁ and 315₂), the respective memory cell will not conduct, e.g., and will be OFF. According to an embodiment implementing the truth table of FIG. 1C, if the value of digit b matches the value of digit a, at least one of the memory cells (e.g., 315₁ and 315₂) will not conduct, e.g., and will be OFF, in response to receiving the signals corresponding to the bit values d* and d selected in accordance with the value of digit b.

Referring to FIG. 1C, the first column includes three possible values (binary 0, binary 1, and X) for digit b. For a particular digit b, for example, signals corresponding to the bit values d* and d, selected in accordance with the value of digit b, can be applied to the control gates of a corresponding pair of memory cells (e.g., memory cells 315₁ and 315₂ of string 310₁). For example, if the digit b has a value of binary 0, a signal having a voltage level of about 4V, e.g., corresponding to a bit value d₁*=1, might be applied to the control gate of a first memory cell (e.g., memory cell 315₁) of the pair, and a signal having a voltage level of about 2V, e.g., corresponding to a bit value d₁=0, might be applied to the control gate of a second memory cell of the pair (e.g., memory cell 315₂).

Continuing with such an example, if the digit b has a value of binary 1, a signal having a voltage level of about 2V, e.g., corresponding to a bit value d₁*=0, might be applied to the control gate of the first memory cell (e.g., memory cell 315₁) of the pair, and a signal having a voltage level of about 4V, e.g., corresponding to a bit value d₁=1, might be applied to the control gate of a second memory cell of the pair (e.g., memory cell 315₂).

Further continuing with the example, if do not care data X has been inserted into the digit d, a signal having a voltage level of about 0V might be applied to the control gate of the first memory cell (e.g., memory cell 315₁) of the pair, and a signal having a voltage level of about 0V might also be applied to the control gate of the second memory cell of the pair (e.g., memory cell memory cell 315₂), thus ensuring neither cell of the pair conducts, assuring a match no matter what actual data is stored by the pair of cells.

The F entry in the first column of FIG. 1C corresponds to an operation corresponding to memory cells in a pass-through mode, such as where their control gates are both biased with the pass through signal h (e.g., allowing other digits to be compared).

Each row of the second column of FIG. 1C illustrates four possible values for digit a (i.e., the digit of data being compared to the digit b). For a particular digit a, for example, a pair of memory cells can be programmed to threshold voltages (e.g., Vt1, Vt1*, corresponding to stored bit values z and z*) selected in accordance with the value of digit a.

For example, if the digit a has a value of binary 0, a first memory cell (e.g., memory cell 315₁ of string 310₁) of the pair might be programmed to a threshold voltage (e.g., Vt1) of about 1V, while the second memory cell of the pair (e.g., memory cell 315₂ of string 310₁) might be programmed to a threshold voltage (e.g., Vt1*) of about 3V. Continuing with such an example, if the digit a has a value of binary 1, a first memory cell (e.g., memory cell 315₁ of string 310₁) of the pair might be programmed to a threshold voltage (e.g., Vt1) of about 3V, while the second memory cell of the pair (e.g., memory cell 315₂ of string 310₁) might be programmed to a threshold voltage (e.g., Vt1*) of about 1V.

Further continuing with the example, to store do not care data X for digit a, a first memory cell (e.g., memory cell 315₁ of string 310₁) of the pair might be programmed to a threshold voltage (e.g., Vt1) of about 3V, and the second memory cell of the pair (e.g., memory cell 315₂ of string 310₁) might also be programmed to a threshold voltage (e.g., Vt1*) of about 3V, thus ensuring that at least one cell of the pair does not conduct regardless of the selected the voltage of the signal corresponding to the respective d*/d bit value, and thereby assuring a match regardless of the value of digit b. Still further, in a another variation, the memory cells have not been programmed to store either a binary 0, a binary 1, or do not care data X (e.g., they both have a threshold voltage of about 1V), in which case a match with digit b is only detected if do not care data has been inserted into digit b.

Each row of the third column of FIG. 1C illustrates a respective result of comparing the value of digit b to each of the four possible values for digit a. A binary 1 indicates no current conduction on the bit line coupled to the string. A binary 0 indicates that current is flowing. Thus, referring to the first row of the third column of FIG. 1C, when digit b has a value of binary 0 and the corresponding pair of memory cells store a binary 1, the result is binary 0—indicating a no match condition. When the digit b has a value of binary 0 and the corresponding pair of memory cells store a binary 0, the result is binary 1—indicating a match condition. When the digit b has a value of binary 0 and the corresponding pair of memory cells store do-not-care data X, the result is binary 1—indicating a match condition. When the digit b has a value of binary 0 and the pair of memory cells is erased F, the result is 0—indicating a no match condition.

FIG. 1C also includes examples of state definitions from the logic truth table. The input the bit values d and d*, selected in accordance with the value of digit b, have the illustrated logic highs H (logic 1) and logic lows L (logic 0) as indicated. The memory array stores z and z*, selected in accordance with the value of digit a, having the illustrated logic highs H and logic lows L as indicated.

FIG. 2 is a block diagram that illustrates a portion of memory device 100 that includes memory array 204, page buffer 110, and input buffer 120. For some embodiments, memory array 204 may be organized in blocks 210 of memory cells, such as blocks 210₁ to 210_N.

Each column 212 may include memory cells from each of blocks 210. Each block 210 may include access lines (e.g., word lines 220) respectively coupled to rows of memory cells in each block 210 so that each block 210 is organized in rows and columns of memory cells.

Bit lines 215₁ to 215_L may be respectively coupled, one-to-one, to current sense amplifiers, such as sense amps 225₁ to 225_L that may have analog-to-digital converter functionality. Sense amps 225₁ to 225_L may be respectively coupled, one-to-one, to comparators 228₁ to 228_L. A single register 230 (e.g., that may be referred to as difference register), in some embodiments, may be coupled to comparators 228₁ to 228_L. Sense amps 225, comparators 228, and register 230 may be part of page buffer 110 for some embodiments. For other embodiments, there may be a plurality of individual difference registers, where the individual difference registers are coupled one-to-one to comparators 228₁ to 228_L.

Word lines 220 may be coupled to respective ones of drivers 232 for receiving voltages therefrom. Mask registers 240 may be coupled to drivers 232, and may be used to mask certain blocks 210 or certain cells of blocks 210 when reading from or writing to array 204. For example, mask registers 240 may be coupled one-to-one to word lines 220. Drivers 232 and mask register 240 may be part of input buffer 120 for some embodiments. In other words, mask registers 240 may mask memory cells coupled thereto. For example, a mask register 240 may store a mask bit that causes a memory cell coupled thereto to be masked.

Input (e.g., key) registers 245 (e.g., which may also be referred to as input feature registers) may be coupled one-to-one to mask registers 240, and thus to word lines 220, and may be part of input buffer 120 for some embodiments. For other embodiments, the input feature registers 245 and mask registers 240 may be interchanged so that input feature registers 245 are between mask registers 240 and drivers 232.

Note that the set of input feature registers receive input data, such as input feature vectors, for comparison to data, such as data feature vectors, stored in columns 212 of memory array 204. For example, an input feature vector may be programmed into resisters 245.

Each component of a data feature vector may be stored in one or more blocks 210. For some embodiments, multiple components of a data feature vector may be stored in a single block. For other embodiments, components for multiple data feature vectors may be stored in a single block.

Each feature (e.g., component) of a feature vector may be expressed by a binary expression of data having a most significant bit, a least significant bit, and bits of differing significance between the most significant bit and the least significant bit, where bits of the input feature vector are compared to bits of each data feature vector having like significance. For example, the most significant bit of the input feature vector may be compared to the most significant bit of a data feature vector, the least significant bit of the input feature vector to the least significant bit of the data feature vector, etc. The bits of a feature (e.g., component) of a data feature vector may be respectively stored in different blocks 210 of a group of blocks 210; the bits of another feature (e.g., component) of the data feature vector may be respectively stored in different blocks 210 of another group of blocks 210; etc.

For some embodiments, more than one bit of a component of a data feature vector can be stored in a single block. For example, multiple bits for a single component of a data feature vector can be stored in a single block. In addition, bits for multiple components of a data feature vector can be stored in a single block.

For some embodiments, each sense amp 225 may sense a level of a current I_A on a respective one of bit lines 215 and may convert the sensed current level I_A to a digital representation I_D (e.g., a voltage level representing a sensed current level) of the level of the sensed current I_A. Each sense amp 225 may send the representation I_D to a respective comparator 228.

Register 230 may store a particular reference I_ref (e.g., a voltage level) to be compared to the representation I_D at the respective comparator 228. That is, the respective comparator 228 may receive the particular reference I_ref from register 230 and compare the particular reference I_ref to the representation I_D. For example, comparator 228₁ may receive the particular reference I_ref from register 230 and compare the particular reference I_ref to representation I_D1 that is a digital representation of the level of the sensed current I_A1 on bit line 215₁; comparator 228₂ may receive the particular reference I_ref from register 230 and compare the particular reference I_ref to representation I_D2 that is a digital representation of the level of the sensed current I_A2 on bit line 215₂; and so on, until comparator 228_L receives the particular reference I_ref from register 230 and compares the particular reference I_ref to representation I_DL that is a digital representation of the level of the sensed current I_AL on bit line 215_L.

For some embodiments, a representation I_D that is less than or equal to the particular reference I_ref may be indicative of a potential match, between data (e.g., a data feature vector) stored in a respective column 212 and input data (e.g., an input feature vector) stored in registers 245, and a representation ID that is greater than the particular reference I_ref may be indicative of no match between the data stored in the respective column 212 and the data stored in registers 245. For example, memory device 100 may be configured to determine that there is a potential match in response to the representation I_D being less than or equal to the particular reference I_ref and no match in response to the representation I_D being greater than the particular reference I_ref. For example, if the representation I_D1 is less than or equal to the particular reference I_ref, the input feature vector is potentially an exact match to the data feature vector stored in column 212₁, and if the number I_D2 is less than or equal to the particular reference I_ref, the input feature vector is potentially an exact match to the data feature vector stored in column 212₂, whereas if the representation I_DL is greater than the particular reference I_ref, the input feature vector is not a potential match to the data feature vector stored in column 212_L.

For other embodiments, to vary the number of potential matches and/or to identify data feature vectors that may have a greater or lesser potential of matching the input feature vector, register 230 may be programmable so that a user, for example, can program different particular references I_ref into register 230. For example, larger particular references I_ref, can potentially identify a larger number of data feature vectors that potentially match the input feature vector than smaller particular references I_ref, in that the larger particular reference I_ref allows for data feature vectors that are further away from matching the input feature vector than the smaller particular references I_ref. For example, larger particular references I_ref may be used when a smaller reference I_ref yields little or no data feature vectors potentially matching the input feature vector. Conversely, if there are too many data feature vectors potentially matching the input feature vector, the particular reference I_ref may be reduced, in that the smaller particular reference I_ref requires data feature vectors potentially matching the input feature vector to be closer to the input feature vector than a larger value of the particular reference I_ref.

For some embodiments, progressively reducing the particular reference I_ref can act to identify the data feature vectors that are closest to the input feature vector. For other embodiments, setting the particular reference I_ref to zero identifies data feature vectors that are an exact match to the input feature vector.

The level of a current I_A on a respective one of bit lines 215 might not be zero, in that there may be some current leakage through a memory cell that is turned off, and thus though the string containing that memory cell and the bit line 215 coupled to that string. Therefore, the digital representation I_D that is compared to I_ref might not be zero. Therefore, to compensate for such leakage, I_ref may be set to a certain value, e.g., corresponding to a predetermined current leakage that may occur, for some embodiments. Then, for some embodiments, if digital representation I_D is less than or equal to the certain value, digital representation I_D is taken to be zero. As such, register 230 is configured to store a value of I_ref that compensates for current leakage.

FIG. 3 is a schematic of array 204, e.g., a NAND array. Each bit line 215 may be coupled to a plurality strings 310 (e.g., strings 310₁ to 310_N respectively of blocks 210₁ to 210_N) of memory cells 315₁ to 315_M. The memory cells 315 may represent non-volatile memory cells for storage of data. The memory cells 315 of each string 310 are connected in series, source to drain, between a source select gate 320 and a drain select gate 322. Therefore, a column 212 of memory cells 315 may include a plurality of strings 310 coupled to a given bit line 215. Each drain select gate 322 selectively couples an end of a corresponding string 310 to a corresponding bit line 215. Drain select gates 322 may be commonly coupled to a drain select line 321, and source select gates 320 may be commonly coupled to a source select line 323.

Typical construction of a memory cell 315 may include a source 325, a drain 328, a charge-storage structure 330 (e.g., a floating gate, charge trap, etc.) that can store a charge that determines a data value of the cell, and a control gate 332, as shown in FIG. 3. Memory cells 315 have their control gates 332 respectively coupled to (and in some cases form) word lines 220. For example, in each of blocks 210₁ to 210_N, memory cells 315₁ to 315_M may have their control gates 332 respectively coupled to (and in some cases from) word lines 220₁ to 220_M. A row of the memory cells 315 may be those memory cells commonly coupled to a given word line 220.

A resistor (e.g., one or more resistors) 340 may be coupled in series with each string 310 of memory cells 315. For example, one or more resistors 340 may be between and coupled in series with a source select gate 320 and a memory cell 315₁ located at an end of a respective string 310 that is opposite the end coupled to a drain select gate 322. Each source select gate 320 may selectively couple a resistor 340, and thus a respective string 310, to a source line (not shown). Although two series-coupled resistors 340 are shown in FIG. 3, one resistor 340 may be coupled in series with each string 310, or more than two series-coupled resistors 340 may be coupled in series with each string 310 for more control over the resistance. For example, the more resistors 340 coupled in series with a string 310, the more control over the resistance on that string.

For embodiments where more than one resistor 340 is coupled to a string 310, the resistances of the respective resistors may be same as each other or different than each other. In the example of FIG. 3, each string 310₁ in block 210₁ may be coupled to resistors 340_1,1 and 340_2,1; each string 310₂ in block 210₂ may be coupled to resistors 340_1,2 and 340_2,2; and . . . each string 310_N in block 210_N may be coupled to resistors 340_1,N and 340_2,N.

The overall resistance of the one or more resistors 340 coupled to each string 310₁ in block 210₁ may be different than the overall resistance of the one or more resistors 340 coupled to each string 310₂ in block 210₂, and the overall resistance of the one or more resistors 340 coupled to each string 310₂ in block 210₂ may be different than the overall resistance of the one or more resistors 340 coupled to each string 310_N in block 210_N. For example, for some embodiments, the overall resistance of the one or more resistors 340 coupled to each string in each block 210 may be different than the overall resistance of the one or more resistors 340 coupled to each string 310 in every other block 210.

The one or more resistors 340 respectively coupled in series with strings 310₁, 310₂, and . . . 310_N respectively cause different levels of current to flow through strings 310₁, 310₂, and . . . 310_N, e.g., when strings 310₁, 310₂, and . . . 310_N are discharged from a common voltage level to ground. For example, the strings may be discharged through an activated source select gate 320 to a source line that may be coupled to ground.

The one or more resistors 340 coupled to each string 310 act to weight the respective string 310. Accordingly, differently weighted strings 310₁ to 310_N make different contributions to the overall level of the current flowing through a precharged bit line 215 coupled to and discharging through the differently weighted strings 310₁ to 310_N. This allows the bits of data respectively stored in the memory cells 315 of strings 310₁ to 310_N to be weighted differently. For example, the bits of data respectively stored in the memory cells 315 of strings 310₁ to 310_N can be weighted according to their significance.

For example, for some embodiments, strings 310₁ to 310_N may be respectively coupled to increasing resistances, and the memory cells of strings 310₁ to 310_N may respectively store most significant bits to least significant bits. In other words, the lower the resistance coupled to a given string, the higher the potential current flow through that string and the higher the significance of the bits of data stored in the memory cells of that string. For some embodiments, each string 310₁ may be coupled to the lowest resistance and may store the most significant bits of data, and each string 310_N may be coupled to the highest resistance and may store the least significant bits of data.

For some embodiments, each resistor 340 may be a transistor configured to act as a resistor. For example, the resistance of a transistor may be related to the channel length of the transistor, where the greater the channel length the greater the resistance. Therefore, the resistances on each string may be preset, e.g., during fabrication of the memory array, by fabricating the transistor, acting as a resistor, to have a predetermined channel length.

For example, strings 310₁ to 310_N may be respectively coupled to increasing resistances by fabricating the transistors, acting as resistors, respectively coupled in series with strings 310₁ to 310_N to respectively have increasing channel lengths. For embodiments, where resistors with different resistances are coupled to a particular string, the different resistances may be respectively preset by respectively coupling transistors with different channel lengths to the particular string.

For other embodiments, each resistor 340 may be a programmable resistor. For example, each resistor 340 may be configured in a manner similar to a memory cell 315. In one example, each resistor 340 may be a charge-storage cell having a charge storage structure 342 and a control gate 344 coupled to (and in some cases forming) a control line 345. For example, the control gates of resistors 340_1,1, 340_2,1, 340_1,2, 340_2,2, 340_1,N, and 340_2,N may be respectively coupled to control lines 345_1,1, 345_2,1, 345_1,2, 345_2,2, 345_1,N, and 345_2,N.

The resistance of a charge-storage cell is related to a difference between the threshold voltage programmed into the charge-storage cell and a voltage VC applied to the control gate 344 (e.g., the voltage placed on a corresponding control line 345). For example, a small difference, e.g., corresponding to charge-storage cell being partially ON, may produce a large resistance, and progressively increasing the difference, may progressively decrease the resistance until the difference is large enough that the charge-storage cell is fully ON, e.g., is placed in a read mode. For other embodiments, the each resistor 340 may be programed to the same threshold voltage, and the voltages VC applied to the control gates may be adjusted to adjust the resistance. Control lines 345 may be coupled to drivers, such as drivers 232, that apply the voltages VC.

For some embodiments, the voltage VC that is to be applied to the control gate of a resistor 340 to effect a certain reduction in the current level may be determined from an iterative process. For example, the resistor is programmed to a certain threshold voltage and the voltage VC is adjusted until the desired current level is obtained. The voltage VC thus obtained may be subsequently used in conjunction with the threshold voltage to cause the resistor 340 to set the current flow through a corresponding string 310.

In one example, the voltages VC_1,1 and VC_2,1 respectively applied to control lines 345_1,1 and 345_2,1 and the threshold voltages of the resistors 340_1,1 and 340_2,1 respectively coupled to control lines 345_1,1 and 345_2,1 may cause the voltages VC_1,1 and VC_2,1 to act as pass voltages so that resistors 340_1,1 and 340_2,1 provide little resistance to any current flowing through the respective string 310₁. As such, the resistors 340_1,1 and 340_2,1 may be set to reduce the current level by substantially a factor of one, e.g., substantially no reduction, and thus the strings 310₁ coupled in series with resistors 340_1,1 and 340_2,1 may be said to have a weight factor of substantially one. For some embodiments, the strings 310₁ in columns 212 may store the most significant bits of data (e.g., data feature vectors) stored in those columns.

Continuing with the example, the voltages VC_1,2 and VC_2,2 respectively applied to control lines 345_1,2 and 345_2,2 and the threshold voltages of the resistors 340_1,2 and 340_2,2 respectively coupled to control lines 345_1,2 and 345_2,2 may cause the overall (e.g., combined) resistance of the resistors 340_1,2 and 340_2,2 to be, for example, substantially two times the combined resistance of resistors 340_1,1 and 340_2,1. As such, resistors 340_1,2 and 340_2,2 may act to reduce the current level by substantially a factor of 2, and thus the strings 310₂ coupled in series with resistors 340_1,2 and 340_2,2 may be said to have a weight factor of substantially ½. That is, the current flow through resistors 340_1,2 and 340_2,2 may be substantially a factor of two less than through resistors 340_1,1 and 340_2,1. For some embodiments, the strings 310₂ in columns 212 may store the next-most (e.g., the second-most) significant bits of data (e.g., data feature vectors) stored in those columns.

Continuing further with the example, the voltages VC_1,N and VC_2,N respectively applied to control lines 345_1,N and 345_2,N and the threshold voltages of the resistors 340_1,N and 340_2,N respectively coupled to control lines 345_1,N and 345_2,N may cause the combined resistance of the resistors 340_1,N and 340_2,N coupled thereto to be, for example, substantially 2^N-1 times the combined resistance of resistors 340_1,1 and 340_2,1. As such, resistors 340_1,N and 340_2,N may act to reduce the current level by substantially a factor of 2^N-1, and thus the strings 310_N coupled in series with resistors 340_1,N and 340_2,N may be said to have a weight factor of substantially 1/2^N-1. That is, the current flow through resistors 340_1,N and 340_2,N may be substantially a factor of 2^N-1 less than through resistors 340_1,1 and 340_2,1. For some embodiments, the strings 310_N in columns 212 may store the least significant bits of data (e.g., data feature vectors) stored in those columns.

For some embodiments, bits of data of a data feature vector to be compared to like bits of data of an input feature vector may be stored in pairs of memory cells 315 so that each string 310 may store M/2 bits of data. For example, a memory-cell pair 315₁, 315₂ of string 310₁ may store the most significant bit of a component of a data feature vector to be compared to the most significant bit of the same component of the input feature vector; a memory-cell pair 315₁, 315₂ of string 310_N may store the least significant bit of the component of the data feature vector to be compared to the least significant bit of the component of the input feature vector; and the memory-cell pair 315₁, 315₂, of string 310₂ may store a bit (e.g., the second-most significant bit) of the component of the data feature vector between the most significant bit and the least significant bit of the component of the data feature vector to be compared to a like bit (e.g., the second-most significant bit) of the component of the input feature vector between the most significant bit and the least significant bit of the component of the input feature vector.

Another memory-cell pair of string 310₁ may store the most significant bit of another component of the data feature vector stored in the respective column 212 to be compared to the most significant bit of another component of the input feature vector; another memory-cell pair of string 310₂ may store the second-most significant bit of the other component of the data feature vector stored in the respective column 212 to be compared to the second-most significant bit of the other component of the input feature vector; and another memory-cell pair of string 310_N may store the least significant bit of the other component of the data feature vector stored in the respective column 212 to be compared to the least significant bit of the other component of the input feature vector. Each bit of a input feature vector may correspond to the values (e.g., bit values d and d*) stored in two registers 245 (FIG. 2).

In the example of FIG. 4, an input feature vector 400 (e.g., IF(B)=(B1, B2, BN) is programmed into registers of memory device 100, such as input registers 245 of input buffer 120. Input feature vector 400 may have components B1, B2, and BN. For example, the input feature vector 400 may represent a pattern, and each component B may be a feature (e.g., attribute) of the pattern. For example, each component B may be a feature of an unknown person. To simplify the example of FIG. 4, each component B may correspond to a single binary bit that may have a value of binary 1 or binary 0. For example, the value of components B1, B2, and BN may be respectively, binary 0, (corresponding bit values d₁*=1 and d₁=0), binary 1 (corresponding bit values d₂*=0 and d₂=1), and binary 1 (corresponding bit values d_N*=0 and d_N=1), as shown in FIG. 4. However, for other embodiments, each feature B may have a value expressed by a plurality of binary bits.

Component B1 may be the most important feature in a comparison of input feature vector 400 to data feature vectors stored in memory array 204, component B2 the next-most (e.g., the second-most) important feature of input feature vector 400, and component BN the least important feature of input feature vector 400. Therefore, the bit of component B1 may be the most significant bit of a binary expression of input feature vector 400; the bit of component B2 may be the second-most significant bit of the binary expression of input feature vector 400; and the bit of component BN may be the least significant bit of the binary expression of input feature vector 400. As such, input feature vector 400 may be represented by the binary expression 011, e.g., IF(B)=011.

Note that each component (e.g., bit in this example) of input feature vector 400 may use two register bits d and d*. For example, a register bit 0 may cause a voltage V_WL=2V to be applied to the word line 220 corresponding to the register containing bit 0, and a register bit 1 may cause a voltage V_WL=4V to be applied to the word line 220 corresponding to the register containing bit 1.

For some embodiments, a 0 value (bit) of input feature vector 400 may be coded as a first voltage (e.g., 2V) in a first register bit and a second voltage (e.g., 4V) in a second register bit in a pair of registers 245, and a 1 value (bit) of input feature vector 400 may be coded as the second voltage (e.g., 4V) in a first register bit and the first voltage (e.g., 2V) in the second register bit in a pair of registers 245, as shown in FIG. 4.

In the example of FIG. 4, input feature vector 400 is to be compared to a data feature vector stored in each of columns 212₁, 212₂, and 212_L of array 204 of FIG. 3. For example, each data feature vector may be a pattern that is to be compared to the pattern of input feature vector 400. For example, each data feature vector may represent a known person having different attributes respectively stored in blocks 210₁ to 210_N.

For example, a data feature vector DF₁(A)=(A1, A2, AN)₁ may be stored in column 212₁; a data feature vector DF₂(A)=(A1, A2, AN)₂ may be stored in column 212₂; and a data feature vector DF_L(A)=(A1, A2, AN)_L may be stored in column 212_L. Data feature vector DF₁(A) may be equal to the binary expression 000; data feature vector DF₂(A) may be equal to the binary expression 011; and data feature vector DF_N(A) may be equal to the binary expression 111. Therefore, there is an exact match between input feature vector 400 (e.g., input feature vector IF(B)=011) and data feature vector DF₂(A), a mismatch between the most significant bits of input feature vector IF(B) and data feature vector DF_N(A), and a mismatch between the second-most significant bits of input feature vector IF(B) and data feature vector DF₁(A) and the least significant bits of input feature vector IF(B) and data feature vector DF₁(A). As discussed below, the most, second-most, and least significant bits may respectively have the weight factors of 1, ½, and 1/2^N-1.

For some embodiments, a plurality of bits may be stored in a string of memory cells. For example, each of a plurality of memory-cell pairs in a string may store a bit. The bits stored in a string of memory cells may be bits of different components of a data feature vector. Alternatively, the bits stored in a string of memory cells may be different bits of a single component of a data feature vector.

The data stored in columns 212₁, 212₂, and 212_L in block 210₁ may represent a component (e.g., a feature) A1 of the data feature vectors stored in columns 212₁, 212₂, and 212_L and may be in the same category (e.g., of the same type) as component B1 of input feature vector 400; the data stored in columns 212₁, 212₂, and 212_L in block 210₂ may represent a component (e.g., a feature) A2 of the data feature vectors stored in columns 212₁, 212₂, and 212_L and may be in the same category (e.g., of the same type) as component B2 of input feature vector 400; and the data stored in columns 212₁, 212₂, and 212_L in block 210_N may represent a component (e.g., a feature) AN of the data feature vectors stored in columns 212₁, 212₂, and 212_L and may be in the same category (e.g., of the same type) as component BN of input feature vector 400. For example input feature vector 400 and the data feature vectors stored in columns 212₁, 212₂, and 212_L may represent people, and like components A1 and B1 may be race, like components A2 and B2 may be height, and like components AN and BN may be eye color, i.e., race, height, and eye color may be different categories.

Therefore, component B1 of input feature vector 400 (e.g., input feature vector IF(B)) will be compared to component A1 of data feature vectors DF₁(A), DF₂(A), and DF_L(A). Component B2 of the input feature vector IF(B) will be compared to component A2 of data feature vectors DF₁(A), DF₂(A), and DF_L(A). Component BN of the input feature vector IF(B) will be compared to component AN of data feature vectors DF₁(A), DF₂(A), and DF_L(A).

Component A1 of the data feature vector stored in each of columns 212₁, 212₂, and 212_L may be stored in the memory-cell pair 315₁, 315₂ of string 310₁ in each of columns 212₁, 212₂, and 212_L; component A2 of the data feature vector stored in each of columns 212₁, 212₂, and 212_L may be stored in the memory-cell pair 315₁, 315₂ of string 310₂ in each of columns 212₁, 212₂, and 212_L; and component AN of the data feature vector stored in each of columns 212₁, 212₂, and 212_L may be stored in memory-cell pair 315₁, 315₂ of string 310_N in each of columns 212₁, 212₂, and 212_L. Note that strings 310₁, 310₂, and 310_N are respectively in blocks 210₁, 210₂, and 210_N (FIG. 3).

For some embodiments, the components A1, A2, and A3 of the data feature vector stored in each of columns 212₁, 212₂, and 212_L may respectively be the most, second-most, and least important features in the comparison to input data feature vector 400, and thus may respectively be the most, second-most, and least significant bits of the data feature vector stored in each of columns 212₁, 212₂, and 212_L. Component B1 of input data feature vector 400 is to be compared to data stored in the memory-cell pair 315₁, 315₂ of string 310₁ in each of columns 212₁, 212₂, and 212_L. Component B2 of input data feature vector 400 is to be compared to the data stored in the memory-cell pair 315₁, 315₂ of string 310₂ in each of columns 212₁, 212₂, and 212_L. Component AN of input data feature vector 400 is to be compared to the data stored in the memory-cell pair 315₁, 315₂ of string 310_N in each of columns 212₁, 212₂, and 212_L. That is, the most significant bit of the input data (e.g., corresponding to input feature vector 400) is compared to the most significant bit of the data stored in each of columns 212₁, 212₂, and 212_L; the second-most significant bit of the input data is compared to the second-most significant bit of the data stored in each of columns 212₁, 212₂, and 212_L; and the least significant bit of the input data is compared to the least significant bit of the data stored in each of columns 212₁, 212₂, and 212_L.

Each memory cell 315₁ is programed to a threshold voltage Vt, and each memory cell 315₂ is programed to a threshold voltage Vt*. The values of threshold voltages Vt* and Vt are shown in FIG. 4 and represent stored data. For example, memory cells 315₁ and 315₂ in block 210₁ are respectively programed to threshold voltages Vt1 and Vt1*; memory cells 315₁ and 315₂ in block 210₂ are respectively programed to threshold voltages Vt2 and Vt2*; and memory cells 315₁ and 315₂ in block 210_N are respectively programed to threshold voltages VtN and VtN*.

To compare component B1 of input feature vector 400 to the data stored in the memory-cell pair 315₁, 315₂ of string 310₁ in each of columns 212₁, 212₂, and 212_L, the register bits d₁* and d₁ corresponding to component B1 of input feature vector 400 may cause a voltage V_WL1=4V (e.g., corresponding to a bit value of d₁*=1) to be applied to the word line 220₁ (FIG. 3) in block 210₁ and a voltage V_WL2=2V (e.g., corresponding to a bit value of d₁=0) to be applied to the word line 220₂ (FIG. 3) in block 210₁. A pass voltage may be applied to the remaining word lines (e.g., the word lines other than word lines 220₁ and 220₂) in block 210₁. That is, the remaining word lines may receive a voltage that allows the memory cells coupled thereto in the string 310₁ in each of columns 212₁, 212₂, and 212_L to pass current with little resistance.

FIG. 4 shows the status of the memory cell 315₁ in block 210₁ in each of columns 212₁, 212₂, and 212_L in response applying the voltage V_WL1=4V to the word line 220₁ in block 210₁. The status of the memory cell 315₂ in block 210₁ in each of columns 212₁, 212₂, and 212_L in response to applying the voltage V_WL2=2V to the word line 220₂ in block 210₁ is also shown.

The memory cells 315₂ in the strings 310₁ of columns 212₁ and 212₂ are OFF, in that the voltage V_WL2=2V applied to the word line 220₂ in block 212₁ is less than the threshold voltage (e.g., Vt1*=3V) of these memory cells 315₂, and is thus insufficient to turn these memory cells 315₂ ON. However, the memory cell 315₂ in the string 310₁ in column 212_L is turned ON, in that the voltage V_WL2=2V applied to the word line 220₂ is greater than the threshold voltage (e.g., Vt1*=1V) on that memory cell 315₂, and is sufficient to turn that memory cell 315₂ ON. The memory cells 315₁ in the strings 310₁ of columns 212₁, 212₂, and 212_L are all ON, in that the voltage V_WL1=4V applied to the word line 220₁ in block 212₁ is greater than the threshold voltage on those memory cells 315₁ (e.g., Vt1=1V for the memory cells 315₁ in columns 212₁ and 212₂ and Vt1=3V for the memory cell 315₁ in column 212_L), and is sufficient to turn those memory cells 315₁ ON.

For some embodiments, a memory cell 315₂ having a threshold voltage of Vt1*=3V stores a bit value of 1, and a memory cell 315₁ having a threshold voltage of Vt1=1V stores a bit value of 0. Therefore, the memory-cell pair 315₁, 315₂ stores a value of binary 0 when memory cell 315₁ stores a bit value of 0 and memory cell 315₂ stores a bit value of 1. A memory cell 315₂ having a threshold voltage of Vt1*=1V stores a bit value of 0, and a memory cell 315₁ having a threshold voltage of Vt1=3V stores a bit value of 1. Therefore, the memory-cell pair 315₁, 315₂ stores a value of binary 1 when memory cell 315₂ stores a bit value of 0 and memory cell 315₁ stores a bit value of 1. As such, component A1 of the data feature vector stored in column 212₁ has a binary value of 0, which matches component B1 of the input feature vector. Component A1 of the data feature vector stored in column 212₂ has a binary value of 0, which matches component B1 of the input feature vector. Component A1 of the data feature vector stored in column 212_L has a binary value of 1, which does not match (e.g., mismatches) component B1 of the input feature vector.

Therefore, the ON, OFF status of the memory-cell pairs 315₁, 315₂ in the strings 310₁ of columns 212₁ and 212₂ represents a match between component B1 of input feature vector 400 and the data stored in the memory-cell pairs 315₁, 315₂ in the strings 310₁ of columns 212₁ and 212₂, i.e., the match between component B1 of input feature vector 400 and the component A1 of the data feature vectors stored in columns 212₁ and 212₂. The ON, ON status of the memory-cell pair 315₁, 315₂ in the string 310₁ of column 212_L represents a mismatch between component B1 of input feature vector 400 and the data stored in the memory-cell pair 315₁, 315₂ in the string 310₁ of column 212_L, i.e., the mismatch between component B1 of input feature vector 400 and the component A1 of the data feature vector stored in column 212_L.

For some embodiments, the bit lines 215₁, 215₂, and 215_L (FIG. 3) respectively corresponding to columns 212₁, 212₂, and 212_L may be charged. The OFF status of the memory cells 315₂ in the strings 310₁ of columns 212₁ and 212₂ acts to prevent the bit lines 215₁ and 215₂ from discharging through the strings 310₁ in columns 212₁ and 212₂. That is, current discharge through a string may be prevented in the event of a match. However, the ON status of both memory cell 315₁ and memory cell 315₂ in the string 310₁ of column 212_L allows the bit line 215_N to discharge through the string 310₁ in column 212_L. That is, current discharge through a string may be allowed in the event of a mismatch.

Since component A1 of the data feature vector stored in each of columns 212₁, 212₂, and 212_L is the most significant bit of the data stored in each of columns 212₁, 212₂, and 212_L and is stored in the memory-cell pair 315₁, 315₂ of string 310₁ in each of columns 212₁, 212₂, and 212_L, the string 310₁ in each of columns 212₁, 212₂, and 212_L, and thus block 210₁ containing string 310₁, may be weighted with the largest weight factor of the strings 310₁, 310₂, 310_N and thus of the blocks 210₁, 210₂, 210_N, e.g., a weight factor of 1. The weight factor of 1 may signify that component A1 of the data feature vector stored in each of columns 212₁, 212₂, and 212_L is the most significant bit of the data stored in each of columns 212₁, 212₂, and 212_L.

To set the weight factor of 1, the one or more resistors 340 coupled in series with the string 310₁ in each of columns 212₁, 212₂, and 212_L, may be adjusted so that there is substantially no added resistance due to one or more resistors 340. Therefore, the level of the current on charged bit line 215_L, that discharges through the string 310₁ in column 212_L, is reduced by a factor of substantially one, e.g., substantially no reduction.

For embodiments where the one or more resistors 340 are configured as charge-storage cells, the voltage applied to the control gates thereof may cause the one or more resistors 340 to operate as pass transistors to produce the weight factor of 1. For example, the series-coupled resistors 340_1,1 and 340_1,2 (FIG. 3) coupled in series with the string 310₁ in each of columns 212₁, 212₂, and 212_L, may be programmed to a particular threshold voltage, and the voltages VC_1,1 and VC_2,1 respectively applied to control lines 345_1,1 and 345_1,2 (FIG. 3) respectively coupled to resistors 340_1,1 and 340_2,1 may be set to a value sufficiently above the particular threshold voltage so that the resistors 340_1,1 and 340_2,1 are fully ON and act as pass transistors with substantially no resistance added to the strings 310₁ in columns 212₁, 212₂, and 212_L. For some embodiments, the voltages VC_1,1 and VC_2,1 may be respectively applied to control lines 345_1,1 and 345_2,1 substantially concurrently (e.g., concurrently) with applying the voltage V_WL2=2V to the word line 220₂ in block 212₁ and applying the voltage V_WL1=4V to the word line 220₁ in block 212₁.

Continuing with the example of FIG. 4, to compare component B2 of input feature vector 400 to the data stored in the memory-cell pair 315₁, 315₂ of string 310₂ in each of columns 212₁, 212₂, and 212_L, the register bits d₂ and d₂* corresponding to component B2 of input feature vector 400 may cause a voltage V_WL2=4V (e.g., corresponding to a bit value of d₂=1) to be applied to the word line 220₂ in block 210₂ and a voltage V_WL1=2V (e.g., corresponding to a bit value of d₂*=0) to be applied to the word line 220₁ in block 210₂. A pass voltage may be applied to the remaining word lines (e.g., the word lines other than word lines 220₁ and 220₂) in block 210₂. That is, the remaining word lines may receive a voltage that allows the memory cells coupled thereto in the string 310₂ in each of columns 212₁, 212₂, and 212_L, to pass current with little resistance.

FIG. 4 shows the status of the memory cell 315₁ in block 210₂ in each of columns 212₁, 212₂, and 212_L in response to applying the voltage V_WL1=2V to the word line 220₁ in block 210₂. The status of the memory cell 315₂ in block 210₂ in each of columns 212₁, 212₂, and 212_L in response to applying the voltage V_WL2=4V to the word line 220₂ in block 210₂ is also shown.

The memory cells 315₂ in the strings 310₂ of columns 212₁, 212₂, and 212_L are all ON, in that the voltage V_WL2=4V applied to the word line 220₂ in block 212₂ is greater than the threshold voltage on those memory cells 315₂ (e.g., Vt2*=1V for the memory cells 315₂ in columns 212₂ and 212_L and Vt2*=3V for the memory cell 315₂ in column 212₁), and is sufficient to turn those memory cells 315₂ ON. The memory cells 315₁ in the strings 310₂ of columns 212₂ and 212_L are OFF, in that the voltage V_WL1=2V applied to the word line 220₁ in block 212₂ is less than the threshold voltage of these memory cells 315₂ (e.g., Vt2=3V), and is thus insufficient to turn these memory cells 315₁ ON. However, the memory cell 315₁ in the string 310₂ in column 212₁ is turned ON, in that the voltage V_WL1=2V applied to the word line 220₁ in block 212₂ is greater than the threshold voltage on that memory cell 315₁ (e.g., Vt2=1V), and is sufficient to turn that memory cell 315₁ ON.

For some embodiments, a memory cell 315₂ having a threshold voltage of Vt2*=3V stores a bit value of 1, and a memory cell 315₁ having a threshold voltage of Vt2=1V stores a bit value of 0. Therefore, the memory-cell pair 315₁, 315₂ stores a value of binary 0 when memory cell 315₂ stores a bit value of 1 and memory cell 315₁ stores a bit value of 0. A memory cell 315₂ having a threshold voltage of Vt2*=1V stores a bit value of 0, and a memory cell 315₁ having a threshold voltage of Vt2=3V stores a bit value of 1. Therefore, the memory-cell pair 315₁, 315₂ stores a value of binary 1 when memory cell 315₂ stores a bit value of 0 and memory cell 315₁ stores a bit value of 1. As such, components A2 of the data feature vectors stored in columns 212₂ and 212_L have binary values of 1, which match component B2 of the input feature vector 400. Component A2 of the data feature vector stored in column 212₁ has a binary value of 0, which does not match (e.g., mismatches) component B2 of the input feature vector 400.

Therefore, the OFF, ON status of the memory-cell pairs 315₁, 315₂ in the strings 310₂ of columns 212₂ and 212_L represents a match between component B2 of the input feature vector 400 and the data stored in the memory-cell pairs 315₁, 315₂ in the strings 310₂ of columns 212₂ and 212_L, i.e., the match between component B2 of input feature vector 400 and the component A2 of the data feature vectors stored in columns 212₂ and 212_L. The ON, ON status of the memory-cell pair 315₁, 315₂ in the string 310₂ of column 212₁ represents a mismatch between component B2 of the input feature vector 400 and the data stored in the memory-cell pair 315₁, 315₂ in the string 310₂ of column 212₁, i.e., the mismatch between component B2 of the input feature vector 400 and the component A2 of the data feature vector stored in column 212₁.

The OFF status of the memory cells 315₁ in the strings 310₂ of columns 212₂ and 212_L acts to prevent the charged bit lines 215₂ and 215_L from discharging through the strings 310₂ in columns 212₂ and 212_L, signaling the match. However, the ON status of both memory cell 315₁ and memory cell 315₂ in the string 310₂ of column 212₁ allows the charged bit line 215₁ to discharge through the string 310₂ in column 212_L, signaling the mismatch.

Since component A2 of the data feature vector stored in each of columns 212₁, 212₂, and 212_L is the second-most significant bit of the data stored in each of columns 212₁, 212₂, and 212_L and is stored in the memory-cell pair 315₁, 315₂ of string 310₂ in each of columns 212₁, 212₂, and 212_L, the string 310₂ in each of columns 212₁, 212₂, and 212_L, and thus block 210₂, may be weighted with second largest weight factor of the strings 310₁, 310₂, 310_N, and thus of the blocks 210₁, 210₂, 210_N, e.g., a weight factor of ½. The weight factor of ½ may signify that component A2 of the data feature vector stored in each of columns 212₁, 212₂, and 212_L is the second-most significant bit of the data stored in each of columns 212₁, 212₂, and 212.

To set the weight factor of ½, the one or more resistors 340 coupled in series with the string 310₂ in each of columns 212₁, 212₂, and 212_L may be adjusted so that the level of any current discharging through strings 310₂ in columns 212₁, 212₂, and 212_L is reduced by substantially a factor of 2. Therefore, the level of the current on the charged bit line 215₁ that discharges through the string 310₂ in column 212₁ is reduced by a factor of substantially 2.

For embodiments where the one or more resistors 340 are configured as charge-storage cells, the voltage applied to the control gates thereof in conjunction with the threshold voltage programmed in the one or more resistors 340 may cause the one or more resistors 340 to turn partially on so as to reduce the current flow by substantially the factor of 2 and thus produce the weight factor of ½. For example, the difference between the voltage applied to the control gate of a resistor coupled in series with the string 310₂ in each of columns 212₁, 212₂, and 212_L and the threshold voltage of that resistor may be different (e.g., less) than the difference between the voltage applied to the control gate of a resistor coupled in series with the string 310₁ in each of columns 212₁, 212₂, and 212_L and the threshold voltage of that resistor.

In embodiments where the series-coupled resistors 340_1,2 and 340_2,2 (FIG. 3) coupled in series with the string 310₂ in each of columns 212₁, 212₂, and 212_L may be programmed to a particular threshold voltage, the voltages VC_1,2 and VC_2,2 respectively applied to control lines 345_1,2 and 345_2,2 (FIG. 3) respectively coupled to resistors 340_1,2 and 340_2,2 may be set to a value that causes the resistors 340_1,2 and 340_2,2 to be partially ON, thereby reducing the current flow by substantially the factor of 2. Note that since current only discharges through string 310₂ of column 212₁, it is only this current that would be affected by the reduced resistance. For some embodiments, the voltages VC_1,2 and VC_2,2 may be respectively applied to control lines 345_1,2 and 345_2,2 substantially concurrently (e.g., concurrently) with applying the voltage V_WL1=2V to the word line 220₁ in block 210₂ and applying the voltage V_WL2=4V to the word line 220₂ in block 210₂ as well as with respectively applying the voltages VC_1,1 and VC_2,1 to control lines 345_1,1 and 345_2,1 in block 210₁ and with applying the voltage V_WL1=4V to the word line 220₁ in block 210₁ and applying the voltage V_WL2=2V the word line 220₂ in block 210₁.

Continuing with the example of FIG. 4, to compare component BN of the input feature vector 400 to the data stored in the memory-cell pair 315₁, 315₂ of string 310_N in each of columns 212₁, 212₂, and 212_L, the register bits d_N* and d_N corresponding to component BN may cause a voltage V_WL1=2V (e.g., corresponding to a bit value of d_N*=0) to be applied to the word line 220₁ in block 210_N and a voltage V_WL2=4V (e.g., corresponding to a bit value of d_N=1) to be applied to the word line 220₂ in block 210_N. A pass voltage may be applied to the remaining word lines (e.g., the word lines other than word lines 220₁ and 220₂) in block 210_N. That is, the remaining word lines may receive a voltage that allows the memory cells coupled thereto in the string 310_N in each of columns 212₁, 212₂, and 212_L to pass current with little resistance.

Note that the blocks 210₃ to 210_N-1 between blocks 210₂ and 210_N may be masked, e.g., by mask bits stored in mask registers 240 (FIG. 2). For example, a mask bit may cause the memory cells coupled to a mask register 240 by a word line to be masked when the mask register 240 stores the mask bit. Also, current from the bit lines coupled to the strings in these blocks may be prevented from discharging through these strings, e.g., keeping the drain select gates 322 coupled to these strings OFF.

FIG. 4 shows the status of the memory cell 315₁ in block 210_N in each of columns 212₁, 212₂, and 212_L in response to applying the voltage V_WL1=2V to the word line 2201. The status of the memory cell 315₂ in block 210_N in each of columns 212₁, 212₂, and 212_L in block 210_N in response to applying the voltage V_WL2=4V to the word line 220₂ is also shown.

The memory cells 315₂ in the strings 310_N of columns 212₁, 212₂, and 212_L are all ON, in that the voltage V_WL2=4V applied to the word line 220₂ in block 212_N is greater than the threshold voltage on those memory cells 315₂ (e.g., VtN*=1V on the memory cells 315₂ in columns 212₂ and 212_L and VtN*=3V on the memory cell 315₂ in column 212₁), and is sufficient to turn those memory cells 315₂ ON. The memory cells 315₁ in the strings 310_N of columns 212₂ and 212_L are OFF, in that the voltage V_WL1=2V applied to the word line 220₁ in block 212_N is less than the threshold voltage of these memory cells 315₁ (e.g., VtN=3V), and is thus insufficient to turn these memory cells 315₂ ON. However, the memory cell 315₁ in the string 310_N in column 212₁ is turned ON, in that the voltage V_WL1=2V applied to the word line 220₁ in block 212_N is greater than the threshold voltage on that memory cell 315₁ (e.g., VtN=1V), and is sufficient to turn that memory cell 315₁ ON.

For some embodiments, a memory cell 315₂ having a threshold voltage of VtN*=3V stores a bit value of 1, and a memory cell 315₁ having a threshold voltage of VtN=1V stores a bit value of 0. Therefore, the memory-cell pair 315₁, 315₂ stores a value of binary 0 when memory cell 315₁ stores a bit value of 1 and memory cell 315₂ stores a bit value of 0. A memory cell 315₂ having a threshold voltage of VtN*=1V stores a bit value of 0, and a memory cell 315₁ having a threshold voltage of VtN=3V stores a bit value of 1. Therefore, the memory-cell pair 315₁, 315₂ stores a value of binary 1 when memory cell 315₂ stores a bit value of 0 and memory cell 315₁ stores a bit value of 1. As such, components AN of the data feature vectors stored in columns 212₂ and 212_L have binary values of 1, which match component BN of the input feature vector 400. Component AN of the data feature vector stored in column 212₁ has a binary value of 0, which does not match (e.g., mismatches) component BN of the input feature vector 400.

The OFF, ON status of the memory-cell pairs 315₁, 315₂ in the strings 310_N of columns 212₂ and 212_L represents a match between component AN of input feature vector 400 and the data stored in the memory-cell pairs 315₁, 315₂ in the strings 310_N of columns 212₂ and 212_L, i.e., the match between component BN of input feature vector 400 and the component AN of the data feature vectors stored in columns 212₂ and 212_L. The ON, ON status of the memory-cell pair 315₁, 315₂ in the string 310_N of column 212₁ represents a mismatch between component BN of input feature vector 400 and the data stored in the memory-cell pair 315₁, 315₂ in the string 310_N of column 212₁, i.e., a mismatch between component BN of input feature vector 400 and the component AN of the data feature vector stored in column 212₁.

The OFF status of the memory cells 315₁ in the strings 310_N of columns 212₂ and 212_L acts to prevent the charged bit lines 215₂ and 215_L from discharging through the strings 310_N in columns 212₂ and 212_L, signaling the match. However, the ON status of both memory cell 315₁ and memory cell 315₂ in the string 310_N of column 212₁ allows the charged bit line 215₁ to discharge through the string 310_N in column 2121_L, signaling the mismatch.

Since component AN of the data feature vector stored in each of columns 212₁, 212₂, and 212_L is the least significant bit of the data stored in each of columns 212₁, 212₂, and 212_L and is stored in the memory-cell pair 315₁, 315₂ of string 310_N in each of columns 212₁, 212₂, and 212_L, the string 310_N in each of columns 212₁, 212₂, and 212_L, and thus block 210_N, may be weighted with smallest weight factor of the strings 310₁, 310₂, 310_N, and thus of the blocks 210₁, 210₂, 210_N, e.g., a weight factor of 1/2^N-1, where N is greater than 2. The weight factor of 1/2^N-1 may signify that component AN of the data stored in each of columns 212₁, 212₂, and 212_L is the least significant bit of the data stored in each of columns 212₁, 212₂, and 212.

To set the weight factor of 1/2^N-1, the one or more resistors 340 coupled in series with the string 310_N in each of columns 212₁, 212₂, and 212_L may be adjusted so that the level of any current discharging through strings 310_N in columns 212₁, 212₂, and 212_L is reduced by substantially a factor of 2^N-1. Therefore, the level of the current on charged bit line 215₁ that discharges through the string 310_N in column 212₁ is reduced by a factor of substantially 2^N-1.

For embodiments where the one or more resistors 340 are configured as charge-storage cells, the voltage applied to the control gates thereof in conjunction with the threshold voltage programmed in the one or more resistors 340 may cause the one or more resistors 340 to turn partially on so as to reduce the current flow by substantially the factor of 2^N-1 and thus produce the weight factor of 1/2^N-1. For example, the difference between the voltage applied to the control gate of a resistor coupled in series with the string 310_N in each of columns 212₁, 212₂, and 212_L and the threshold voltage of that resistor may be different (e.g., less) than the difference between the voltage applied to the control gate of a resistor coupled in series with the string 310₁ in each of columns 212₁, 212₂, and 212_L and the threshold voltage of that resistor and different (e.g., less) than the difference between the voltage applied to the control gate of a resistor coupled in series with the string 310₂ in each of columns 212₁, 212₂, and 212_L and the threshold voltage of that resistor.

In embodiments where the series-coupled resistors 340_1,N and 340_2,N (FIG. 3) coupled in series with the string 310_N in each of columns 212₁, 212₂, and 212_L may be programmed to a particular threshold voltage, the voltages VC_1,N and VC_2,N respectively applied to control lines 345_1,N and 345_2,N (FIG. 3) respectively coupled to resistors 340_1,N and 340_2,N may be set to a value that causes the resistors 340_1,N and 340_2,N to be partially ON, thereby reducing the current flow by substantially the factor of 2^N-1. Note that since current only discharges through string 310_N of column 212₁, it is only this current that would be affected by the reduced resistance.

For some embodiments, the voltages VC_L1,N and VC_2,N may be respectively applied to control lines 345_1,N and 345_2,N substantially concurrently (e.g., concurrently) with applying the voltage V_WL1=2V the word line 220₁ in block 212_N and applying the voltage V_WL2=4V the word line 220₂ in block 212_N, as well as substantially concurrently (e.g., concurrently) with applying the respective the voltages VC_1,2 and VC_2,2 to control lines 345_1,2 and 345_2,2 in block 210₂ and with respectively applying the respective the voltages VC_1,1 and VC_2,1 to control lines 345_1,1 and 345_2,1 in block 210₁, and thus substantially concurrently (e.g., concurrently) with applying the voltage V_WL1=2V to the word line 220₁ in block 210₂ and applying the voltage V_WL2=4V to the word line 220₂ in block 210₂ and with applying the voltage V_WL1=4V to the word line 220₁ in block 210₁ and applying the voltage V_WL2=2V to the word line 220₂ in block 210₁. Therefore, the weight factors may applied concurrently to the blocks 210₁, 210₂, and 210_N and thus to strings 310₁, 310₂, and 310_N.

The data stored in the memory-cell pairs 315₁, 315₂ in the strings 310₁, 310₂, and 310_N in column 212₂ respectively matches components B1, B2, and BN of input feature vector 400, i.e., the components A1, A2, and AN of the data feature vector DF₂(A) stored in column 212₂ respectively match the components B1, B2, and BN of input feature vector 400. Therefore, there is an exact match between input feature vector 400 and the data feature vector DF₂(A) stored in the memory-cell pairs 315₁, 315₂ in column 212₂. This is reflected by the fact that a memory cell of the pairs 315₁, 315₂ in each of the strings 310₁, 310₂, and 310_N in column 212₂ is OFF (i.e., memory cell 315₂ in string 310₁, memory cell 315₁ in string 310₂, and memory cell 315₁ in string 310_N are OFF). This means that charged bit line 215₂ coupled to the strings 310₁, 310₂, and 310_N in column 212₂ is prevented from being discharged through the strings 310₁, 310₂, and 310_N in column 212₂, meaning there is no current flow through bit line 215₂, and the level of the current I_A2 (FIGS. 2 and 3) on bit line 215₂ is zero.

Therefore, in the event of an exact match between an input feature vector and a data feature vector stored in the memory cells of a column, there is no current flow on the charged bit line coupled to those memory cells. That is, an exact match can be determined by detecting zero current flow through a charged bit line coupled memory cells whose data are compared with an input feature vector.

The components A2 and AN of the data feature vector DF_L(A) stored in column 212_L respectively match the components B2 and BN of input feature vector 400. That is, the data stored in the memory-cell pairs 315₁, 315₂ in the strings 310₂ and 310_N in column 212_L respectively match components B2 and BN of input feature vector 400, and current is prevented from flowing from charged bit line 215_L though the strings 310₂ and 310_N in column 212_L by the turned-off memory cells 315₁ in the strings 310₂ and 310_N in column 212_L.

However, component A1 of the data feature vector DF_L(A) stored in column 212_L does not match component B1 of input feature vector 400. That is, the data stored in the memory-cell pair 315₁, 315₂ in the string 310₁ in column 212_L does not match component B1 of input feature vector 400, and current I_AL (FIGS. 2 and 3) flows from charged bit line 215_L through the string 310₁ in column 212_L. Therefore, in the event of a mismatch between an input feature vector and data a feature vector stored in the memory cells of a column, there is current flow on the charged bit line coupled to those memory cells. That is, a mismatch can be determined by detecting current flow through a charged bit line coupled to memory cells whose data are compared with an input feature vector.

Component A1 of the data feature vector DF₁(A) stored in column 212₁ matches component B1 of input feature vector 400. That is, the data stored in the memory-cell pairs 315₁, 315₂ in the string 310₁ in column 212₁ matches component B1 of input feature vector 400, and current is prevented from flowing from charged bit line 215₁ though the string 310₁ in column 212₁ by the turned-off memory cell 315₂ in the string 310₁ in column 212₁.

Component A2 of the data feature vector DF₁(A) stored in column 212₁ does not match component B2 of input feature vector 400. That is, the data stored in the memory-cell pair 315₁, 315₂ in the string 310₂ in column 212₁ does not match component B2 of input feature vector 400, and current flows from charged bit line 215₁ through the string 310₂ in column 212₁. Component AN of the data feature vector DF₁(A) stored in column 212₁ does not match component BN of input feature vector 400. That is, the data stored in the memory-cell pair 315₁, 315₂ in the string 310_N in column 212₁ does not match component BN of input feature vector 400, and current also flows from charged bit line 215₁ through the string 310_N in column 212₁. The total current I_A1 (FIGS. 2 and 3) on bit line 215₁ is the sum of the current flowing through strings 310₂ and 310_N in column 212₁, in that strings 310₂ and 310_N are coupled in parallel to bit line 215₁.

The level of the current I_AL on bit line 215_L is greater than the level of the current I_A1 on bit line 215₁ when N is greater than 2. The current I_AL on bit line 215_L is the current through string 310₁ in column 212_L, and the level of this current is substantially unreduced due to the weight factor of 1 for block 210₁ and string 310₁. Due to the weight factor of ½ for block 210₂, and thus string 310₂, the level of the current flowing through string 310₂ is about ½ the current on bit line 215₁. Due to the weight factor of 1/2^N-1 for block 210_N, and thus string 310_N, the level of the current flowing through string 310_N is about 1/2^N-1 the current on bit line 215₁. Therefore, the level of the total current I_A1 on bit line 215₁ (the sum of the current flowing through strings 310₂ and 310_N) is about (½+1/2^N-1) of the level of the current on bit line 215_L.

The level of the current on a bit line is indicative of the degree (e.g., closeness) of a match between the data (e.g., data feature vectors) stored in memory cells coupled to the bit line and input data (e.g., a data feature vector), e.g., the lower the level the better (e.g., closer) the match. The level of the current I_A2 on bit line 215₂ is zero, in that there is an exact match between the data stored in the memory-cell pairs coupled to bit line 215₂ and input feature vector 400.

The mismatch between the data feature vector DF_L(A) stored in memory-cell pairs coupled to bit line 215_L and input feature vector 400 occurred between the component B1 of input feature vector 400, i.e., the most important component of input feature vector 400 to the comparison, and the component A1 of the data feature vector DF_L(A) stored in a memory-cell pair in block 210₁ with a weight factor of 1, i.e., the component A1 of the data feature vector DF_L(A) in column 212_L that is most important to the comparison.

The mismatch between the data feature vector DF₁(A) stored in memory-cell pairs coupled to bit line 215₁ and input feature vector 400 occurred between component B2 of input feature vector 400, i.e., the second most important component of input feature vector 400 to the comparison, and the component A2 of the data feature vector DF₁(A) stored in a memory-cell pair in block 210₂ with a weight factor of ½, i.e., the component A2 of the data feature vector DF₁(A) in column 212₁ that is second most important to the comparison.

The mismatch between the data feature vector DF₁(A) stored in memory-cell pairs coupled to bit line 215₁ and input feature vector 400 further occurred between the component BN of input feature vector 400, i.e., the least important component of input feature vector 400 to the comparison, and the component AN of the data feature vector DF₁(A) stored in a memory-cell pair in block 210_N with a weight factor of 1/2^N-1, i.e., the component AN of the data feature vector DF₁(N) stored in column 212₁ least important to the comparison.

Therefore, the data feature vector DF₁(A) stored in the memory-cell pairs in column 212₁ coupled to bit line 215₁ is closer to matching (e.g., more likely to match) input feature vector 400 than the data feature vector DF_L(A) stored in the memory-cell pairs in column 212_L coupled to bit line 215_L. This is evident by the fact that the current level in bit line 215₁ is lower than the current level in bit line 215_L.

The current levels I_A1, I_A2, and I_AL, respectively in bit lines 215₁, 215₂, and 215_L may be respectively converted to the digital representations I_D1, I_D2, and I_DL respectively at sense amps 225₁, 225₂, and 225_L, as described above in conjunction with FIG. 2. The digital representations I_D1, I_D2, and I_DL may then be compared to the particular reference I_ref respectively at comparators 228₁, 228₂, and 228_L, as further described above in conjunction with FIG. 2.

If it is deemed that a mismatch between input feature vector 400 and the data feature vectors stored in columns 212 occurs when component B1 of input feature vector 400 mismatches the component A1 of the data feature vectors stored in in strings 310₁ of columns 212, the particular reference I_ref may be selected to be less than a digital representation of the level of the unweighted current in strings 310₁ of columns 212. For example, the particular reference I_ref may be selected to be less than a digital representation of the level of the current through string 310₁ in column 212_L, e.g., less than the representation I_DL of the level of the current I_AL on bit line 215_L, due to both of memory cells 315₁ and 315₂ in string 310₁ of column 212_L being ON and block 210₁ and string 310₁ of column 212_L having a weight factor of one. That is, comparator 228_L will indicate that the representation I_DL exceeds the particular reference I_ref, thereby indicating a mismatch, as indicated above in conjunction with FIG. 2.

Selecting the particular reference I_ref to be less than representation I_DL and greater than the representation I_D1, representing the level of the current of I_A1, will cause the data feature vector DF_L(A) stored in column 212_L to be identified as mismatching input feature vector 400 and the data feature vectors DF₁(A) and DF₂(A) stored in columns 212₁ and 212₂ to be identified as potentially matching input feature vector 400. This differentiates the data feature vector stored in column 212_L from the data feature vectors stored in columns 212₁ and 212₂. The differentiation of the data feature vector stored in column 212_L from the data feature vector stored in column 212₁ is enabled by the weighting of the strings in the respective columns.

Note that the data feature vectors stored in both of columns 212₁ and 212_L mismatches the data stored in input feature vector 400. However, the mismatch between the data feature vector stored in column 212_L and input feature vector 400 is more critical to the comparison between the data feature vectors stored in columns 212 and input feature vector 400 than the mismatch between the data feature vector stored in column 212₁ and input feature vector 400. The mismatch between the data feature vector stored in column 212_L and input feature vector 400 is due to a mismatch between component B1 of input feature vector 400 and the component A1 of the data feature vector stored in column 212_L that has a weight factor of 1. The mismatch between the data feature vector stored in column 212₁ and input feature vector 400 is due to a mismatch between component B2 of input feature vector 400 and the component A2 of the data feature vector stored in column 212₁ that has a weight factor of ½ and a mismatch between component BN of input feature vector 400 and the component AN of the data feature vector stored in column 212₁ that has a weight factor of 1/2^N-1.

Selecting the particular reference I_ref to be less than representation hi and greater than the representation I_D2, representing the level of the current of I_A2 (=0), will cause the data feature vectors stored in columns 212₁ and 212_L to be identified as mismatching input feature vector 400 and the data feature vector stored in column 212₂ to be identified as potentially matching input feature vector 400. Note that the data feature vector stored in column 212₂ matches input feature vector 400 exactly, but an exact match can only be identified by selecting the particular reference I_ref to be zero and the representation I_D2 being equal thereto.

Note that there may be some current in a string even though a memory cell in that string is turned off, owing to current leakage through the turned-off memory cell. As such, the current of I_A2 may be non-zero in spite of the memory cell in the string being turned off. Therefore, an exact match will not be indicated by comparing the representation I_D2, representing the level of the current of I_A2, to I_ref=zero. Therefore, register 230 may configured to store a certain value of I_ref that compensates for the leakage, and memory device 100 may be configured to determine that the representation I_D2 is zero when I_D2 is less than or equal to the certain value.

To further illustrate that lower bit-line currents indicate closer matches, it is worthwhile to consider an example where the memory cell 315₂ in string 310₂ in column 212₁ has a threshold voltage of 1V instead 3V, as shown in brackets in FIG. 4, and the memory cell 315₁ in string 310₂ in column 212₁ has a threshold voltage of 3V instead 1V, as shown in brackets in FIG. 4. Note that this corresponds to a component A2′ having a binary value of 1. Therefore, the data feature vector DF₁′(A)=(A1, A2′, AN)₁ stored in column 212₁ has a binary value of 010. Therefore, data feature vector DF₁′(A) mismatches input feature vector 400 in the least significant bit, i.e., component BN of input feature vector 400 mismatches component AN of data feature vector DF₁′(A).

In this example, memory cell 315₂ remains ON in response to applying the voltage V_WL2=4V to the word line 220₂ in block 210₂, but memory cell 315₁ is OFF in response to applying the voltage V_WL1=2V to the word line 220₁ in block 210₂.

The data feature vector DF₁′(A)=(A1, A2′, AN)₁=010 matches input feature vector 400 (IF(B)=(B1, B2, BN)=011) more closely than data feature vector DF₁(A)=(A1, A2, AN)₁=000 that was previously stored in column 212₁ and was discussed above. This is because the mismatch between data feature vector DF₁′(A) and input feature vector IF(B) occurs only in the least significant bit, whereas the mismatch between data feature vector DF₁(A) and input feature vector IF(B) occurs both in the second-most significant bit and the in the least significant bit.

In the case of data feature vector DF₁′(A), the current from charged bit line 215₁ is prevented from flowing through string 310₁ of column 212₁, in that memory cell 315₂ in string 310₁ is OFF. Current from charged bit line 215₁ is also prevented from flowing through string 310₂ of column 212₁, in that memory cell 315₁ in string 310₂ is OFF. Therefore, the current from charged bit line 215₁ only flows through string 310_N, in that both memory cell 315₁ and memory cell 315₂ in string 310_N of column 212₁ are ON.

However, in the case of data feature vector DF₁′(A), the current from charged bit line 215₁ that flows through string 310_N is reduced by a factor of 2^N-1, owing to the weight factor of 1/2^N-1 on block 210_N and string 310_N. This current is now the only contribution to the current I_A1′ (FIGS. 2 and 3) on bit line 215₁, so the level of the current I′_A1 on bit line 215₁ is about 1/2^N-1 of the level of the substantially unreduced current on bit line 215_L, as compared to the previous case of the data feature vector DF₁(A), where the level of total current I_A1 on bit line 215₁ was about (½+1/2^N-1) of the level of the current on bit line on bit line 215_L. The reduced current is indicative that the mismatch between data feature vector DF₁′(A)=010 and input feature vector IF(B)=011 occurs only in the least significant bit as opposed to the mismatch between data feature vector DF₁(A)=000 and input feature vector IF(B)=011 occurring in both the second-most significant bit and the least significant bit.

Current level I′_A1 may be subsequently converted to a representation I′_D1, representing the level of the current I′_A1, at sense amp 225₁ and compared to the particular reference I_ref at comparator 228₁, as shown in FIG. 2. The particular reference I_ref may be selected so that the respective comparators identify exact matches between input feature vector 400 and the data feature vectors stored in columns 212 and/or mismatches only in the least significant bits of input feature vector 400 and the data feature vectors stored in columns 212. Note that the smaller the current on a bit line the closer the match, where zero current corresponds to an exact match.

FIG. 5 is an example of how memory blocks might be weighted for comparisons between an input feature vector having multiple components with multiple bits and a data feature vector having multiple components with multiple bits. For some embodiments, an input feature vector IF(B)=(B1, B2, B3) may be temporarily stored in input buffer 120. Component B1 may have bits b₁₁, b₁₂, and b₁₃, where the first number in the subscript denotes the component number and the second number in the subscript denotes the bit number, e.g., j. Bits b₁₁, b₁₂, and b₁₃ may be respectively the most significant bit (e.g., bit number j=1), the second-most significant bit (e.g., bit number j=2), and the least significant bit (e.g., bit number j=3) of component B1. Component B2 may have bits b₂₁, b₂₂, and b₂₃. Bits b₂₁, b₂₂, and b₂₃ may be respectively the most significant bit (e.g., bit number j=1), the second-most significant bit (e.g., bit number j=2), and the least significant bit (e.g., bit number j=3) of component B2. Component B3 may have bits b₃₁, b₃₂, and b₂₃. Bits b₃₁, b₃₂, and b₃₃ may be respectively the most significant bit (e.g., bit number j=1), the second-most significant bit (e.g., bit number j=2), and the least significant bit (e.g., bit number j=3) of component B3.

Bits b₁₁, b₁₂, b₁₃, b₂₁, b₂₂, b₂₃, b₃₁, b₃₂, and b₂₃ may be stored in input buffer 120, as shown in FIG. 5. Each of the bits b₁₁, b₁₂, b₁₃, b₂₁, b₂₂, b₂₃, b₃₁, b₃₂, and b₂₃ may use two register bits (e.g., d and d*, FIG. 1C), as described above in conjunction with FIG. 4.

Input feature vector IF(B)=(B1, B2, B3) is to be compared to data feature vector DF(A)=(A1, A2, A3) that is stored in a column of blocks of memory cells in array 204. Component A1 may have bits a₁₁, a₁₂, and a₁₃, where the first number in the subscript denotes the component number and the second number in the subscript denotes the bit number, e.g., j. Bits a₁₁, a₁₂, and a₁₃ may be respectively the most significant bit (e.g., bit number j=1), the second-most significant bit (e.g., bit number j=2), and the least significant bit (e.g., bit number j=3) of component A1. Component A2 may have bits a₂₁, a₂₂, and a₂₃. Bits a₂₁, a₂₂, and a₂₃ may be respectively the most significant bit (e.g., bit number j=1), the second-most significant bit (e.g., bit number j=2), and the least significant bit (e.g., bit number j=3) of component A2. Component A3 may have bits a₃₁, a₃₂, and a₂₃. Bits a₃₁, a₃₂, and a₃₃ may be respectively the most significant bit (e.g., bit number j=1), the second-most significant bit (e.g., bit number j=2), and the least significant bit (e.g., bit number j=3) of component A3.

Bits a₁₁, a₁₂, a₁₃, a₂₁, a₂₂, a₂₃, a₃₁, a₃₂, and a₂₃ are respectively stored on memory blocks 1 to 9 of the column, as shown in FIG. 5. Each of bits a₁₁, a₁₂, a₁₃, a₂₁, a₂₂, a₂₃, a₃₁, a₃₂, and a₂₃ may use two bits (e.g., z and z*, FIG. 1C) respectively stored in first and second memory cells of a respective block, as described above in conjunction with FIG. 4.

Bits b₁₁, b₁₂, b₁₃, b₂₁, b₂₂, b₂₃, b₃₁, b₃₂, and b₂₃ are to be compared one-to-one to bits a₁₁, a₁₂, a₁₃, a₂₁, a₂₂, a₂₃, a₃₁, a₃₂, and a₂₃. The bits a may be weighted according to their position within a specific component, e.g., according to their bit number j. For example, the weight factor may be 1/2^j-1, starting with j=1 for the most significant bit. Therefore, bits a₁₁, a₁₂, and a₁₃ of component A1 may respectively have weight factors of 1, ½, and ¼; bits a₂₁, a₂₂, and a₂₃ of component A2 may respectively have weight factors of 1, ½, and ¼; and bits a₃₁, a₃₂, and a₃₃ of component A3 may respectively have weight factors of 1, ½, and ¼, as shown in FIG. 5.

Alternatively, the bits a may be weighted according to the block number in which they are stored, e.g., the weight factor may be 1/2^N-1, starting with N=1. Therefore, bits a₁₁, a₁₂, a₁₃, a₂₁, a₂₂, a₂₃, a₃₁, a₃₂, and a₃₃ respectively have weight factors of 1, ½, ¼, ⅛, 1/16, 1/32, 1/64, 1/128, and 1/256, as shown in FIG. 5. Note that for such embodiments, block 1 would store the most significant bit while block N would store the least significant bit.

Although the examples of FIGS. 1-5 were discussed in conjunction with NAND flash, the embodiments described herein are not limited to NAND flash, but can include other flash architectures, such as NOR flash, phase-change memory, resistive RAM, magnetic RAM, etc.

FIG. 6 is a simplified block diagram of an electronic system, such as a memory system 600, that includes a memory device 602. Memory device 602 may be a flash memory device, e.g., a NAND memory device, NOR memory device, etc. For some embodiments, memory device 602 may be an embodiment of memory device 100 in FIG. 1.

Memory device 602 includes an array of memory cells 204, row access circuitry 608 (e.g., that may include an input buffer, such as input buffer 120 in FIG. 1A), column access circuitry 610 (e.g., that may include a page buffer, such as page buffer 110 in FIG. 1A), control circuitry 612, input/output (I/O) circuitry 614, and an address buffer 616. Memory system 600 includes an external microprocessor 620, such as a memory controller or other external host device, electrically connected to memory device 602 for memory accessing as part of the electronic system. For some embodiments, memory device 602 may be a content addressable memory (CAM) device and may include CAM circuitry 605 that may include registers for receiving input feature vectors, multiplexers, and/or counters, etc.

The memory device 602 receives control signals (which represent commands) from the processor 620 over a control link 622. Memory device 602 receives data signals (which represent data) over a data (DQ) link 624. Address signals (which represent addresses) are received via an address link 626 that are decoded to access the memory array 204. Address buffer circuit 616 latches the address signals. The memory cells in array 204 are accessed in response to the control signals and the address signals.

For some embodiments, control circuitry 612 may be configured to determine whether a input data at least partially matches a data stored in memory array 204, as described above in conjunction with FIGS. 1-5. That is, control circuitry 612 is configured to cause memory device 602 to operate (e.g., to perform the comparisons) as described above in conjunction with FIGS. 1-5.

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. 6 has been simplified. It should be recognized that the functionality of the various block components described with reference to FIG. 6 may not necessarily be segregated to distinct components or component portions of an integrated circuit device. For example, a single component or component portion of memory device 602 could be adapted to perform the functionality of more than one block component of FIG. 6. Alternatively, one or more components or component portions of memory device 602 could be combined to perform the functionality of a single block component of FIG. 6.

CONCLUSION

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the embodiments will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the embodiments.

Claims

1. A memory device, comprising: an array of memory cells; andcontrol circuitry configured to cause the memory device to: compare input data to data stored in a plurality of memory cells of the array of memory cells connected to a data line, wherein the input data comprises a plurality of digits, wherein each digit of the input data corresponds to a respective pair of series-connected memory cells of the plurality of memory cells connected in series with a respective resistor of a plurality of resistors, and wherein a resistance value of one resistor of the plurality of resistors is different than a respective resistance value of each remaining resistor of the plurality of resistors;in response to a mismatch between one digit of the input data and the data stored in the respective pair of series-connected memory cells for the one digit of the input data, cause a first level of current to flow from the data line through the respective pair of series-connected memory cells for the one digit of the input data;in response to a mismatch between a different digit of the input data and the data stored in the respective pair of series-connected memory cells for the different digit of the input data, cause a second level of current, different than the first level of current, to flow from the data line through the respective pair of series-connected memory cells for the different digit of the input data;compare a representation of a level of current in the data line to a reference;deem the input data to potentially match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being less than the reference; anddeem the input data to not match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being greater than the reference.

2. The memory device of claim 1, wherein the control circuitry being configured to cause the memory device to deem the input data to potentially match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being less than the reference comprises the control circuitry being configured to cause the memory device to deem the input data to potentially match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being less than or equal to the reference.

3. The memory device of claim 1, wherein the control circuitry being configured to cause the memory device to compare the input data to the data stored in the plurality of memory cells connected to the data line comprises the control circuitry being configured to cause the memory device to respectively compare different digits of the input data to data stored in different strings of series-connected memory cells while each of the different strings of series-connected memory cells is connected to the data line.

4. The memory device of claim 3, wherein the control circuitry is further configured to cause the memory device to cause different levels of current to respectively flow through the different strings of series-connected memory cells from the data line in response to the different digits of the input data respectively mismatching the data stored in the different strings of series-connected memory cells.

5. The memory device of claim 1, wherein the control circuitry being configured to cause the memory device to deem the input data to potentially match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being less than the reference comprises the control circuitry being configured to cause the memory device to deem the input data to match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being less than the reference.

6. The memory device of claim 1, wherein each digit of stored data is stored in a respective pair of series-connected memory cells of a respective string of series-connected memory cells of a plurality of strings of series-connected memory cells of the array of memory cells connected to a data line, wherein each digit of the input data corresponds to a respective digit of the stored data, and wherein the control circuitry being configured to cause the memory device to compare the input data to the stored data comprises the control circuitry being configured to cause the memory device to compare a particular digit of the input data to its respective digit of the stored data by applying a pair of voltage levels representative of a value of the particular digit of the input data to control gates of the respective pair of series-connected memory cells of the respective string of series-connected memory cells for the respective digit of the stored data.

7. The memory device of claim 6, wherein the control circuitry is further configured to cause the memory device to: program a first memory cell of the respective pair of series-connected memory cells of the respective string of series-connected memory cells for the particular digit of the stored data to a first threshold voltage level in response to the particular digit of the stored data having a first data value, and program the first memory cell to a second threshold voltage level, different than the first threshold voltage level, in response to the particular digit of the stored data having a second data value different than the first data value; andprogram a second memory cell of the respective pair of series-connected memory cells of the respective string of series-connected memory cells for the particular digit of the stored data to the second threshold voltage level in response to the particular digit of the stored data having the first data value, and program the second memory cell to the first threshold voltage level in response to the particular digit of the stored data having the second data value.

8. The memory device of claim 7, wherein the first threshold voltage level is higher than the second threshold voltage level, and wherein applying the pair of voltage levels representative of the value of the particular digit of the input data to the control gates of the particular pair of series-connected memory cells comprises the control circuitry being configured to: apply a first voltage level to the control gate of the first memory cell and apply a second voltage level, different than the first voltage level, to the control gate of the second memory cell in response to the particular digit of the input data having the first data value; andapply the second voltage level to the control gate of the first memory cell and apply the first voltage level to the control gate of the second memory cell in response to the particular digit of the input data having the second data value;wherein the first voltage level is configured to activate a memory cell having the second threshold voltage level and to deactivate a memory cell having the first threshold voltage level; andwherein the second voltage level is configured to activate a memory cell having the first threshold voltage level and to activate a memory cell having the second threshold voltage level.

9. A memory device, comprising: an array of memory cells; andcontrol circuitry configured to cause the memory device to: compare input data to data stored in a plurality of memory cells of the array of memory cells connected to a data line, wherein the input data comprises a plurality of digits, and wherein each digit of the input data corresponds to a respective pair of memory cells of the plurality of memory cells;in response to a mismatch between one digit of the input data and the data stored in the respective pair of memory cells for the one digit of the input data, cause a first level of current to flow from the data line through the respective pair of memory cells for the one digit of the input data;in response to a mismatch between a different digit of the input data and the data stored in the respective pair of memory cells for the different digit of the input data, cause a second level of current, different than the first level of current, to flow from the data line through the respective pair of memory cells for the different digit of the input data;compare a representation of a level of current in the data line to a reference;deem the input data to potentially match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being less than the reference; anddeem the input data to not match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being greater than the reference;wherein the control circuitry being configured to cause the memory device to compare the input data to the data stored in the plurality of memory cells connected to the data line comprises the control circuitry being configured to cause the memory device to respectively compare different digits of the input data to data stored in different strings of series-connected memory cells of the plurality of memory cells while each of the different strings of series-connected memory cells is connected to the data line;wherein the control circuitry is further configured to cause the memory device to cause different levels of current to respectively flow through the different strings of series-connected memory cells from the data line in response to the different digits of the input data respectively mismatching the data stored in the different strings of series-connected memory cells; andwherein the control circuitry being configured to cause the memory device to cause the different levels of current to respectively flow through the different strings of series-connected memory cells from the data line comprises the control circuitry being configured to cause the memory device to set a first resistor connected in series with a first string of series-connected memory cells of the different strings of series-connected memory cells to a first resistance, and to set a second resistor connected in series with a second string of series-connected memory cells of the different strings of series-connected memory cells to a second resistance different than the first resistance.

10. The memory device of claim 9, wherein the first resistor has a different channel length than the second resistor.

11. The memory device of claim 9, wherein the first resistor has a different threshold voltage level than the second resistor.

12. A memory device, comprising: a data line;an array of memory cells comprising a plurality of strings of series-connected memory cells, wherein each string of series-connected memory cells of the plurality of strings of series-connected memory cells is selectively connected to the data line; andcontrol circuitry configured to cause the memory device to: compare input data to data stored in the plurality of strings of series-connected memory cells connected to the data line, wherein the input data comprises a plurality of digits, wherein each digit of the input data corresponds to a respective pair of series-connected memory cells of a respective string of series-connected memory cells of the plurality of strings of series-connected memory cells, connected in series with a respective resistor of a plurality of resistors, and wherein a resistance value of one resistor of the plurality of resistors is different than a respective resistance value of each remaining resistor of the plurality of resistors;in response to a mismatch between one digit of the input data and the data stored in the respective pair of series-connected memory cells for the one digit of the input data, cause a first level of current to flow from the data line through the respective string of series-connected memory cells for the one digit of the input data;in response to a mismatch between a different digit of the input data and the data stored in the respective pair of series-connected memory cells for the different digit of the input data, cause a second level of current, different than the first level of current, to flow from the data line through the respective string of series-connected memory cells for the different digit of the input data;compare a representation of a level of current in the data line to a reference;deem the input data to potentially match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being less than the reference; anddeem the input data to not match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being greater than the reference.

13. The memory device of claim 12, wherein the second level of current is higher than the first level of current.

14. The memory device of claim 13, wherein the different digit of data corresponds to a less significant digit of the input data than the one digit of data.

15. The memory device of claim 12, wherein the plurality of resistors contains N resistors, wherein a first resistor of the plurality of resistors has a particular resistance, and wherein, for each integer value of i from 2 to N, the ith resistor of the plurality of resistors has a resistance of 1/2i-1 of the particular resistance.

16. The memory device of claim 12, wherein each resistor of the plurality of resistors comprises a respective transistor of a plurality of transistors, and wherein each transistor of the plurality of transistors has a different channel length than each remaining transistor of the plurality of transistors.

17. The memory device of claim 12, wherein each resistor of the plurality of resistors comprises a respective transistor of a plurality of transistors, and wherein each transistor of the plurality of transistors has a different threshold voltage than each remaining transistor of the plurality of transistors.

18. A memory device, comprising: an array of memory cells; andcontrol circuitry configured to cause the memory device to: compare input data to data stored in a plurality of memory cells of the array of memory cells connected to a data line, wherein the input data comprises a plurality of digits, and wherein each digit of the input data corresponds to a respective pair of series-connected memory cells of the plurality of memory cells connected in series with a respective resistor of a plurality of resistors;in response to a mismatch between one digit of the input data and the data stored in the respective pair of series-connected memory cells for the one digit of the input data, cause a first level of current to flow from the data line through the respective pair of series-connected memory cells and the respective resistor for the one digit of the input data having a first resistance;in response to a mismatch between a different digit of the input data and the data stored in the respective pair of series-connected memory cells for the different digit of the input data, cause a second level of current, lower than the first level of current, to flow from the data line through the respective pair of series-connected memory cells and the respective resistor for the different digit of the input data having a second resistance higher than the first resistance;compare a representation of a level of current in the data line to a reference;deem the input data to potentially match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being less than the reference; anddeem the input data to not match the data stored in the plurality of memory cells in response to the representation of the level of current in the data line being greater than the reference.

19. The memory device of claim 18, wherein the respective resistor for the one digit of the input data comprises a first transistor having a first control gate configured to receive a control signal, wherein the respective resistor for the different digit of the input data comprises a second transistor having a second control gate connected to the first control gate, wherein the respective resistor for the one digit of the input data is configured to have the first resistance in response to receiving the control signal at the first control gate, and wherein the respective resistor for the different digit of the input data is configured to have the second resistance in response to receiving the control signal at the first control gate.

20. The memory device of claim 18, wherein the respective resistor for the one digit of the input data comprises a first plurality of series-connected transistors, wherein the respective resistor for the different digit of the input data comprises a second plurality of series-connected transistors, wherein the respective resistor for the one digit of the input data is configured to have the first resistance in response to receiving a particular set of control signals on control gates of the first plurality of series-connected transistors, and wherein the respective resistor for the different digit of the input data is configured to have the second resistance in response to receiving the particular set of control signals on control gates of the second plurality of series-connected transistors.

21. The memory device of claim 18, wherein the respective resistor for the one digit of the input data comprises a first transistor having a first channel length, and wherein the respective resistor for the different digit of the input data comprises a second transistor having a second channel length longer than the first channel length.

22. The memory device of claim 18, wherein each resistor of the plurality of resistors comprises a respective transistor of a plurality of transistors, and wherein control gates of the plurality of transistors are commonly connected to receive a control signal.

23. The memory device of claim 22, wherein each transistor of the plurality of transistors is configured to generate a respective resistance in response to the control signal, and wherein, for each transistor of the plurality of transistors, its respective resistance is different than the respective resistance of each remaining transistor of the plurality of transistors.