Abstract:
A non-volatile memory device includes an array of non-volatile (NV) memory cells organized in pairs. Each pair is included with a transistor to form a memory unit. Each unit is coupled to a bit line, a word line, and a pair of source lines. The NV elements are programmable to either a relatively high resistance or relatively low resistance and the particularly resistance is established, by converting one resistance type to the other or maintaining the existing resistance type the direction of current through the NV element. A bit is formed from two NV cells in different memory units which are programmed to different resistance types and thereby provide a differential pair from which the logic state of the bit can be determined.

Description:
BACKGROUND 
     1. Field 
     This disclosure relates generally to integrated circuits, and more particularly, to non-volatile random access memories (NVRAMs). 
     2. Related Art 
     Non-volatile (NV) memories have become very important in a variety of applications but typically have characteristics that make them difficult to use as a random access memory. Some of the difficulties are very slow program and erase times, inability to erase one bit at a time, and high voltage requirements for program and erase. Some of the resistive NV memories, such as magnetic tunnel junction resistors (MTJs) overcome these difficulties making them a candidate for use as a random access memory (RAM). Other difficulties such as large space requirements make this less attractive. 
     Accordingly there is a need to provide further improvement in obtaining NVRAMs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention are illustrated by way of example and are not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. 
       Shown in  FIG. 1  is a block diagram of an embodiment of a non-volatile random access memory (NVRAM) device. 
       Shown in  FIG. 2  is a circuit diagram of an embodiment of a portion of an array of the NVRAM of  FIG. 1 . 
       Shown in  FIG. 3  is a timing diagram of various operating modes for one of the memory cells of the array shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of memory devices and methods for operating the memory device disclosed herein provide non-volatile storage using resistive elements that are read differentially to overcome problems reading memory cells in devices where low current changes and low voltages are used. Two resistive elements are paired across two bit lines to provide complementary data for differential sensing. The configuration allows the programming of paired bits without extra decoding. Since only one select transistor is used for each pair of resistive elements, the increased pitch of the cell will allow a larger select transistor width than an array where memory cells use two transistors and two resistive elements. This is better understood by reference to the drawings and the following written description. 
       FIG. 1  illustrates a block diagram of an embodiment of a processing system  100  that includes one or more processors  101 , and memory device  102  with control circuitry  103 , row circuitry  104  with word line control  110 , column circuitry  106  with source line control  112 , bit line control  114  and sense amplifiers  116 , and memory array  108  with non-volatile (NV) memory cells  118 - 148 . 
     Memory array  108  is coupled to column circuitry  106  and row circuitry  114 . Row circuitry  114  and column circuitry  106  are coupled to control circuitry  103  and can receive addresses for read and write requests from computer processor(s)  101 . Data to be written to memory array  104  is provided from a processor  101  to column circuitry  106 . True bit lines BLN, BLN+1 and complementary bit lines BLBN, BLBN+1 are coupled between bit line control  114  and memory cells  118 - 148 . Word lines (not shown) are coupled between word line control  110  and memory cells  118 - 148 . Source line control  112  or other suitable component can provide analog voltage source line signals SL 1 N−1, SL 1 N, SL 1 N+1, SL 1 N+2, SL 2 N−1, SL 2 N, SL 2 N+1, SL 2 N+2 to NV memory cells  118 - 148 . 
     Control circuitry  103  controls program and erase procedures of the memory array  108  through row circuitry  104  and column circuitry  106 , such as in response to requests from one or more processors  101 . Data is read from the memory array  108  via column circuitry  106  by sensing voltage levels on bit lines BLN, BLN+1, BLBN, BLBN+1, which are coupled between memory array  108  and sense amplifiers  116 . Sense amplifiers  116  provide data from respective columns of data in memory array  108  to one or more processors  101 . 
     Row circuitry  104  provides a row address that is used to select one row of memory array  108  for read or write operations. A power supply voltage VDD is also conducted on word lines. VDD can be any suitable voltage. Column circuitry  106  receives a column address and includes a plurality of input/output (I/O) terminals for receiving or providing data signals. Bit lines run in the column direction, and the word lines run in the row direction. 
     Memory cells  118 - 148  each include two programmable resistive elements and a select transistor. Notably, each bit of data is stored using one resistive element in one of the memory cells and another resistive element in an adjacent one of the memory cells, as indicated by shaded portion  150  of memory cell  118  and shaded portion  152  of memory cell  122 . The other resistive element in each memory cell can be used along with another resistive element of another corresponding memory cell to store another bit of data, as indicated by crossed portion  154  of memory cell  118  and crossed portion  156  of memory cell  122 . Likewise, other pairs of memory cells  120 / 124 ,  126 / 130 ,  128 / 132 ,  134 / 138 ,  136 / 140 ,  142 / 146 , and  144 / 148  can be used together to store two bits of data in each pair. 
     Although memory array  108  is shown with sixteen memory cells  118 - 148 , two word lines (e.g., WLM−1, WLM, WLM+1, WLM+2 in  FIG. 2 ), one true bit line (BLN), and one complementary bit line (BLBN), memory device  104  can include any suitable number of memory cells, word lines, and true and complementary bit lines. Processing system  100  can be implemented using CMOS (complementary metal-oxide semiconductor) transistors as a System On Chip (SOC) or other integrated circuit device which includes at least one processor  101  coupled to memory device  104  via an appropriate interface (not shown), such as a bus or the like with multiple signals or bits. The integrated circuit device may include other circuits, modules or devices, such as other memory devices (not shown), other functional modules (not shown), and external interfaces, such as input, output or input/output (I/O) ports or pins or the like (not shown). In one alternate embodiment, memory device  104  is implemented alone in an integrated circuit without any other devices. In another alternative embodiment, memory device  104  is part of a larger system on the integrated circuit. Additionally, NV memory cells  118 - 148  can all be implemented with similar components, such as shown for example in  FIG. 2 . 
     Shown in  FIG. 2  is an embodiment of a portion of array  108  having word lines WLM−1, WLM, WLM+1, WLM+2; bit line BLN; complementary bit line BLBN; source lines SL 1 N−1, SL 1 N, SL 1 N+1, SL 1 N+2, SL 2 N−1, SL 2 N, SL 2 N+1, SL 2 N+2; and NV memory cells  118 - 124  and  134 - 140 . Bit line BLN and complementary bit line BLBN are connected to respective current electrodes of equalizing transistor  204 . A gate electrode of equalizing transistor  204  is coupled to equalizing control signal VEQ. A non-negating input to differential sense amplifier  116  is coupled to bit line BLN at a first current electrode of equalizing transistor  204  and a negating input to differential sense amplifier  116  is coupled to bit line BLBN at a second current electrode of equalizing transistor  204 . 
     Memory cell  118  includes resistive element  206  having a first terminal coupled to source line SL 1 N−1 and a second terminal coupled to a first current electrode of select transistor  210 , and resistive element  208  having a first terminal coupled to source line SL 2 N-1 and a second terminal coupled to the first current electrode of select transistor  210 . A control electrode of select transistor  210  is coupled to word line WLM and a second current electrode of select transistor  210  is coupled to bit line BLN. 
     Memory cell  120  includes resistive element  214  having a first terminal coupled to source line SL 1 N and a second terminal coupled to a first current electrode of select transistor  212 , and resistive element  216  having a first terminal coupled to source line SL 2 N and a second terminal coupled to the first current electrode of select transistor  212 . A control electrode of select transistor  212  is coupled to word line WLM−1 and a second current electrode of select transistor  212  is coupled to bit line BLN. 
     Memory cell  122  includes resistive element  218  having a first terminal coupled to source line SL 2 N and a second terminal coupled to a first current electrode of select transistor  222 , and resistive element  220  having a first terminal coupled to source line SL 1 N and a second terminal coupled to the first current electrode of select transistor  222 . A control electrode of select transistor  222  is coupled to word line WLM and a second current electrode of select transistor  222  is coupled to complementary bit line BLBN. 
     Memory cell  124  includes resistive element  226  having a first terminal coupled to source line SL 1 N+1 and a second terminal coupled to a first current electrode of select transistor  224 , and resistive element  228  having a first terminal coupled to source line SL 2 N+1 and a second terminal coupled to the first current electrode of select transistor  224 . A control electrode of select transistor  224  is coupled to word line WLM−1 and a second current electrode of select transistor  224  is coupled to complementary bit line BLBN. 
     Memory cell  134  includes resistive element  230  having a first terminal coupled to source line SL 1 N−1 and a second terminal coupled to a first current electrode of select transistor  234 , and resistive element  232  having a first terminal coupled to source line SL 2 N-1 and a second terminal coupled to the first current electrode of select transistor  234 . A control electrode of select transistor  234  is coupled to word line WLM+2 and a second current electrode of select transistor  234  is coupled to bit line BLN. 
     Memory cell  136  includes resistive element  238  having a first terminal coupled to source line SL 2 N and a second terminal coupled to a first current electrode of select transistor  236 , and resistive element  240  having a first terminal coupled to source line SL 1 N and a second terminal coupled to the first current electrode of select transistor  236 . A control electrode of select transistor  236  is coupled to word line WLM+1 and a second current electrode of select transistor  236  is coupled to bit line BLN. 
     Memory cell  138  includes resistive element  242  having a first terminal coupled to source line SL 2 N and a second terminal coupled to a first current electrode of select transistor  246 , and resistive element  244  having a first terminal coupled to source line SL 1 N and a second terminal coupled to the first current electrode of select transistor  246 . A control electrode of select transistor  246  is coupled to word line WLM+2 and a second current electrode of select transistor  246  is coupled to complementary bit line BLBN. 
     Memory cell  140  includes resistive element  250  having a first terminal coupled to source line SL 1 N+1 and a second terminal coupled to a first current electrode of select transistor  248 , and resistive element  252  having a first terminal coupled to source line SL 2 N+1 and a second terminal coupled to the first current electrode of select transistor  248 . A control electrode of select transistor  248  is coupled to word line WLM+1 and a second current electrode of select transistor  248  is coupled to complementary bit line BLBN. 
     Resistive elements  206 ,  208 ,  214 ,  216 ,  218 ,  220 ,  226 ,  228 ,  230 ,  232 ,  238 ,  240 ,  242 ,  244 ,  250 ,  252  may be implemented using programmable magnetic tunneling junction resistors that change from a high resistance state to a low resistance state and vice versa, depending on the current passed through the resistor during the programming stage, as further described in connection with  FIG. 3 . Resistive elements  206 ,  208 ,  214 ,  216 ,  218 ,  220 ,  226 ,  228 ,  230 ,  232 ,  238 ,  240 ,  242 ,  244 ,  250 ,  252  remain in their programmed state until sufficient current is applied in an opposite direction to change the state. 
     Resistive elements  206 ,  208 ,  214 ,  216 ,  218 ,  220 ,  226 ,  228 ,  230 ,  232 ,  238 ,  240 ,  242 ,  244 ,  250 ,  252  (also referred to herein as non-volatile (NV) elements) are shown in  FIG. 2  with an arrow indicating the direction of current required to place the element in a high resistance state. In particular, resistive elements  206 ,  216 ,  218 ,  228 ,  230 ,  240 ,  242 ,  252  change to or remain in a high resistance state when current flows from a corresponding select transistor  210 - 248  to a respective source line SL 1 N−1, SL 1 N, SL 2 N, SL 1 N+1, SL 2 N+2. Resistive elements  208 ,  214 ,  220 ,  226 ,  232 ,  238 ,  244 ,  250  change to or remain in a high resistance state when current flows from a respective source line SL 1 N−1, SL 2 N−1, SL 1 N, SL 2 N, SL 1 N+1, SL 2 N+2 to a corresponding select transistor  210 - 248 . If current flows in a direction opposite to the direction of the arrow, the state will change to or remain in a low resistance state. 
     Each bit of data is stored using one resistive element in one of the memory cells coupled to a first of the word lines and one of the true bit lines, and a second resistive element in another one of the memory cells coupled to the same word line as the other memory cell and a complementary bit line corresponding to the true bit line. For example, resistive element  206  in memory cell  118  and resistive element  220  in memory cell  122  are used to store one bit of data, while resistive element  208  in memory cell  118  and resistive element  218  in memory cell  122  are used to store another bit of data. The combination of resistive elements  206 / 220  and resistive elements  208 / 218  are also referred to herein at non-volatile memory units. 
     Shown in  FIG. 3  is a timing diagram of signals for memory cell  118 / 122  of array  108  shown in  FIG. 2  during write “1”, write “0”, and read modes of operation. To write and store “1” using resistive elements  206  and  220 , 
     SL 1 N−1 is low at VSS (or ground), 
     SL 2 N−1 is at voltage level VP/2, where VP is a programming or reference voltage, 
     SL 1 N is low at VSS (or ground), 
     SL 2 N is at VP/2, 
     bit lines BLN and BLBN are high at voltage VP, 
     word line WLM−1 is at VSS (or ground), 
     word line WLM is high at voltage VDD, where VDD is a supply voltage, 
     word lines WLM+1 and WLM+2 are low at VSS (or ground), and 
     equalizer transistor  204  is in conducting mode with voltage VEQ being high. 
     With the signals set as provided above, select transistors  210  and  222  are in conductive mode, and current flows from bit line BLN to source line SL 1 N−1 through select transistor  210  and resistive element  206  to place resistive element  206  in high resistance state. At the same time, current flows from bit line BLBN to source line SL 1 N through select transistor  222  and resistive element  220  to place resistive element  220  in low resistance state. 
     Although not shown in  FIG. 3 , to write and store “0” using resistive elements  206  and  220 , 
     SL 1 N−1 is high at VP, 
     SL 2 N−1 is at voltage level VP/2, 
     SL 1 N is high at VP, 
     SL 2 N is at VP/2, 
     bit lines BLN and BLBN are at voltage VSS (or ground), 
     word line WLM−1 is at VSS (or ground), 
     word line WLM is high at voltage VDD, 
     word lines WLM+1 and WLM+2 are low at VSS (or ground), and 
     equalizer transistor  204  is in conducting mode with voltage VEQ being high. 
     With the signals set as provided above, select transistors  210  and  222  are in conductive mode, and current flows from source line SL 1 N−1 to bit line BLN to through resistive element  206  and select transistor  210  to place resistive element  206  in low resistance state. At the same time, current flows from source line SL 1 N to bit line BLBN through resistive element  220  and select transistor  222  to place resistive element  220  in high resistance state. 
     To write and store “0” using resistive elements  208  and  218 , 
     SL 1 N−1 is voltage level VP/2, 
     SL 2 N−1 is at voltage level VP, 
     SL 1 N is low at VP/2, 
     SL 2 N is at VP, 
     bit lines BLN and BLBN are low at voltage VSS (or ground), 
     word line WLM−1 is at VSS (or ground), 
     word line WLM is high at voltage VDD, 
     word lines WLM+1 and WLM+2 are low at VSS (or ground), and 
     equalizer transistor  204  is in conducting mode with voltage VEQ being high. 
     With the signals set as provided above, select transistors  210  and  222  are in conductive mode, and current flows from source line SL 2 N−1 to bit line BLN through resistive element  208  and select transistor  210  to place resistive element  208  in high resistance state. At the same time, current flows from source line SL 2 N to bit line BLBN through resistive element  218  and select transistor  222  to place resistive element  218  in low resistance state. 
     Although not shown in  FIG. 3 , to write and store “1” using resistive elements  208  and  218 , 
     SL 1 N−1 is voltage level VP/2, where VP is a programming voltage, 
     SL 2 N−1 is at voltage level VSS 
     SL 1 N is at VP/2, 
     SL 2 N is at VSS, 
     bit lines BLN and BLBN are high at voltage VP, 
     word line WLM−1 is at VSS (or ground), 
     word line WLM is high at voltage VDD, 
     word lines WLM+1 and WLM+2 are low at VSS (or ground), and 
     equalizer transistor  204  is in conducting mode with voltage VEQ being high. 
     With the signals set as provided above, select transistors  210  and  222  are in conductive mode, and current flows from bit line BLN to source line SL 2 N−1 to through select transistor  210  and resistive element  208  to place resistive element  208  in low resistance state. At the same time, current flows from bit line BLBN to source line SL 2 N through select transistor  222  and resistive element  218  and to place resistive element  218  in high resistance state. 
     Note that during both write “1” and write “0” modes, resistive elements in both cells are programmed at the same time, eliminating additional decoding that would be required to address the resistive elements individually. 
     Differential sensing of bit line BLN and complementary bit line BLBN is performed during read operations using bit lines BLN and BLBN coupled to sense amplifier  116 . To read data stored in the combination of resistive elements  206  and  220 , 
     SL 1 N−1 is voltage level VSS, 
     SL 2 N−1 is at a read voltage level VREAD, such as 150 mV, 
     SL 1 N is at VSS, 
     SL 2 N is at VREAD, 
     bit lines BLN and BLBN are at read voltage VREAD, 
     word line WLM−1 is at VSS (or ground), 
     word line WLM is high at voltage VDD, 
     word lines WLM+1 and WLM+2 are low at VSS (or ground), and 
     equalizer transistor  204  is in conducting mode with voltage VEQ being high. 
     With the signals set as provided above, select transistors  210  and  222  are in conductive mode, and voltage on bit lines BLN and BLBN is sensed differentially to determine whether the combination of resistive elements  206  and  220  store a “1” or a “0”. If resistive element  206  is in a low resistance state and resistive element  220  is in a high resistance state, bit line BLN will be pulled low faster than bit line BLBN due to the difference in the resistance of the resistive elements  206  and  220  (if the sense amplifier is sensing voltage) and if sense amplifier  116  is current sensing then the current on BLB is higher than BLBN, thereby causing sense amplifier  116  to read a “0”. If resistive element  206  is in a high resistance state and resistive element  220  is in a low resistance state, bit line BLN will either discharge faster or have higher current relative to bit line BLBN, thereby causing sense amplifier  116  to read a “1”. 
     To read data stored in the combination of resistive elements  208  and  218 , 
     SL 1 N−1 is at read voltage level VREAD, 
     SL 2 N−1 is at VSS, 
     SL 1 N is at VREAD, 
     SL 2 N is at VSS, 
     bit lines BLN and BLBN are at read voltage VREAD, 
     word line WLM−1 is at VSS (or ground), 
     word line WLM is high at voltage VDD, 
     word lines WLM+1 and WLM+2 are low at VSS (or ground), and 
     equalizer transistor  204  is in conducting mode with voltage VEQ being high. 
     With the signals set as provided above, select transistors  210  and  222  are in conductive mode, and voltage on bit lines BLN and BLBN is sensed differentially to determine whether the combination of resistive elements  208  and  218  store a “1” or a “0”. If resistive element  208  is in a low resistance state and resistive element  218  is in a high resistance state, bit line BLN will either discharge slower or have lower current than bit line BLBN, thereby causing sense amplifier to read a “1”. If resistive element  208  is in a high resistance state and resistive element  218  is in a low resistance state, bit line BLN will discharge faster or have more current than bit line BLBN, thereby causing sense amplifier to read a “0”. 
     By now it should be appreciated that in some embodiments, there has been provided a non-volatile (NV) memory device ( 108 ) that includes a true bit line (BLN) and a complementary bit line (BLBN), a first word line (WLM), and a plurality of NV memory elements comprising a first NV memory element ( 206 ), a second NV memory element ( 208 ), a third NV memory element ( 218 ), and a fourth NV memory element ( 220 ). Each of the first, second, third, and fourth NV memory elements can have a first terminal and second terminal, wherein a higher resistance state is established by providing a write current from the first terminal to the second terminal and a lower resistance state is established by providing a write current from the second terminal to the first terminal. A first transistor ( 210 ) can have a first current electrode coupled to the true bit line (BLN), a control electrode coupled to the first word line, and a second current electrode coupled to the first terminal of the first NV memory element and the second terminal of the second NV memory element. A first source line (SL 1 N−1) can be coupled to the second terminal of the first NV memory element. A second source line (SL 2 N−1) can be coupled to the first terminal of the second NV memory element. A second transistor ( 222 ) can have a first current electrode coupled to the complementary bit line (BLBN), a control electrode coupled to the first word line, and a second current electrode coupled to the first terminal of the third NV memory element and the second terminal of the fourth NV memory element. A third source line (SL 2 N) can be coupled to the second terminal of the third NV memory element. A fourth source line (SL 1 N) can be coupled to the first terminal of the fourth NV memory element. 
     In another aspect, the first transistor, the second transistor and the first, second, third, and fourth NV memory elements ( 118  and  122 ) can comprise a two bit NV memory unit. 
     In another aspect, the first and fourth NV memory elements can comprise a first bit of the two bit memory unit and the second and third NV memory elements can comprise a second bit of the two bit memory unit. 
     In another aspect, during a read of the first bit of the two bit memory unit, the first word line can be for causing the first and second transistors to couple the first terminal of the first NV memory element to the true bit line and the second terminal of the fourth NV memory element to the complementary bit line and the first and fourth source lines are for providing a current sink. 
     In another aspect, during a write of a logic high to the first bit, the first word line can be for causing the first and second transistors to simultaneously couple the first terminal of the first NV memory element to the true bit line and the second terminal of the fourth NV memory element to the complementary bit line, the first source line can be for receiving a low potential, the true bit line can be for receiving a write voltage, the complementary bit line can be for receiving the low potential, and the fourth source line can be for receiving the write voltage. 
     In another aspect, during the write of a logic high to the first bit, the second source line and the third source line can be for receiving a reference voltage. 
     In another aspect, the reference voltage can be between the low potential and the logic high. 
     In another aspect, the reference voltage can be halfway between the low potential and the logic high. 
     In another aspect, the memory device can further comprise a second word line (WLM). The plurality of NV memory elements can further comprise a fifth NV memory element ( 214 ), a sixth NV memory element ( 216 ), a seventh NV memory element ( 226 ), and an eighth NV memory element ( 228 ), wherein each of the fifth, sixth, seventh, and eighth NV memory elements can be characterized as having a first terminal and second terminal. The higher resistance state can be established by providing the write current from the first terminal to the second terminal and the lower resistance state can be established by providing the write current from the second terminal to the first terminal. A third transistor ( 212 ) can have a first current electrode coupled to the true bit line (BLN), a control electrode coupled to the second word line, and a second current electrode coupled to the second terminal of the fifth NV memory element ( 214 ) and the first terminal of the sixth NV memory element ( 216 ). The third source line can be coupled to the first terminal of the fifth NV memory element, and the fourth source line can be coupled to the second terminal of the sixth NV memory element. A fourth transistor ( 224 ) can have a first current electrode coupled to the complementary bit line, a control electrode coupled to the second word line, and a second current electrode coupled to the second terminal of the seventh NV memory element ( 226 ) and the first terminal of the eighth NV memory element ( 228 ). A fifth source line (SL 2 N+1) can be coupled to the first terminal of the seventh NV memory element; and a sixth source line (SL 1 N+1) can be coupled to the second terminal of the eighth NV memory element. 
     In another aspect, the memory device can further comprise a second word line (WLM+2). The plurality of NV memory elements can further comprise a fifth NV memory element ( 230 ), a sixth NV memory element ( 232 ), a seventh NV memory element ( 242 ), and an eighth NV memory element ( 244 ). Each of the fifth, sixth, seventh, and eighth NV memory elements are characterized as having a first terminal and second terminal. The higher resistance state can be established by providing the write current from the first terminal to the second terminal and the lower resistance state can be established by providing the write current from the second terminal to the first terminal. A third transistor ( 234 ) can have a first current electrode coupled to the true bit line (BLN), a control electrode coupled to the second word line, and a second current electrode coupled to the first terminal of the fifth NV memory element ( 214 ) and the second terminal of the sixth NV memory element ( 216 ). The first source line can be coupled to the second terminal of the fifth NV memory element, and the second source line can be coupled to the first terminal of the sixth NV memory element. A fourth transistor ( 246 ) can have a first current electrode coupled to the complementary bit line, a control electrode coupled to the second word line, and a second current electrode coupled to the first terminal of the seventh NV memory element ( 226 ) and the second terminal of the eighth NV memory element ( 228 ). The third source line can be coupled to the first terminal of the seventh NV memory element and the fourth source line can be coupled to the second terminal of the first terminal of the eighth NV memory element. 
     In other embodiments, a method of operating a memory device ( 108 ) in which the memory device can comprise a first bit having a first portion ( 206 ) in a first memory unit ( 118 ) and a second portion ( 220 ) in a second memory unit ( 122 ) and a second bit having a first portion ( 208 ) in the first memory unit and a second portion ( 218 ) in the second memory unit, the first portions are complementary to the second portions, wherein the first unit is coupled to a word line (WLM), a true bit line (BLN), a first source line (SL 1 N−1), and a second source line (SL 2 N−1), the second unit is coupled to the word line, a complementary bit line (BLBN), a third source line (SL 2 N) and a fourth source line (SL 1 N), the first and second portions comprise non-volatile (NV) memory units ( 206 , 208 , 218 , 220 ) which are written to a relatively high resistance with a write current passing from a first terminal to a second terminal and to a relatively low resistance with a write current passing from the second terminal to the first terminal. The method can include writing a first state into the first bit by writing a relatively high resistance into the first portion of the first unit using the true bit line and a relatively low resistance into the first portion of the second unit using the complementary bit line, and sensing a logic state of the first bit by coupling the first portion of the first unit to the true bit line and the first portion of the second unit to the complementary bit line by enabling the word line and using differential sensing on the true bit line and the complementary bit line. 
     In another aspect, the differential sensing comprises sensing a voltage differential between the true bit line and the complementary bit line. 
     In another aspect, the differential sensing comprises sensing a current differential between the true bit line and the complementary bit line. 
     In another aspect, the writing a relatively high resistance into the first portion of the first unit using the true bit line and a relatively low resistance into the first portion of the second unit is performed simultaneously. 
     In another aspect, the method can further comprise sensing a logic state of the second bit by coupling the second portion of the first unit to the true bit line and the second portion of the second unit to the complementary bit line by enabling the word line and using differential sensing on the true bit line and the complementary bit line. 
     In another aspect, the method can further comprise enabling the first and fourth source lines during the writing the first logic state into the first portions of the first and second units. 
     In another aspect, the enabling the first and fourth source lines comprises coupling the first and fourth source lines to ground. 
     In another aspect, the method can further comprise writing a second state into the first bit by writing a relatively high resistance into the second portion of the first unit using the true bit line and the second source line and a relatively low resistance into the second portion of the second unit using the complementary bit line and the third source line. 
     In another aspect, the writing the second state comprises flowing current from the second source to the true bit line through the second portion of the first unit and from the third source line to the complementary bit line through the second portion of the second unit. 
     In further embodiments, a memory array ( 108 ) can comprise a first bit line (BLN) and a second bit line (BLBN), a first word line (WLM) and a second word line (WLM−1), a first source line (SL 1 N−1), a second source line (SL 2 N−1), a third source line (SL 2 N), and a fourth source line (SL 1 N), and a plurality of non-volatile (NV) memory elements ( 206 ,  208 ,  218 ,  220 ,  214 ,  216 ) each characterized as having a first terminal and second terminal. A higher resistance state can be established by providing a write current from the first terminal to the second terminal and a lower resistance state is established by providing a write current from the second terminal to the first terminal. A first NV unit ( 118 ) can comprise a first NV memory element ( 206 ) and a second NV memory element ( 208 ) of the plurality of NV memory elements. A first transistor ( 210 ) can have a control electrode coupled to the first word line, a first current electrode coupled to the first bit line, and a second current electrode coupled to the first terminal of the first NV memory element and the second terminal of the second NV memory element. The first source line can be coupled to the second terminal of the first NV memory element and the second source line can be coupled to the first terminal of the second NV memory element. A second NV unit ( 122 ) can comprise a third NV memory element ( 218 ) and a fourth NV memory element ( 220 ) of the plurality of NV memory elements, and a second transistor ( 224 ) having a control electrode coupled to the first word line, a first current electrode coupled to the second bit line, and a second current electrode coupled to the first terminal of the third NV memory element and the second terminal of the fourth NV memory element. The third source line can be coupled to the second terminal of the third NV memory element and the fourth source line can be coupled to the first terminal of the fourth NV memory element. A third NV unit ( 120 ) can comprise a fifth NV memory element ( 214 ) and a sixth NV memory element ( 216 ) of the plurality of NV memory elements, and a third transistor ( 212 ) having a control electrode coupled to the second word line, a first current electrode coupled to the first bit line, and a second current electrode coupled to the second terminal of the fifth NV memory element and the first terminal of the fourth NV memory element. The third source line can be coupled to the first terminal of the fifth NV memory element and the fourth source line can be coupled to the second terminal of the sixth NV memory element. 
     Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims. 
     The term “coupled,” as used herein, is not intended to be limited to a direct coupling or a mechanical coupling. 
     Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. 
     Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements.