Patent Publication Number: US-11024356-B2

Title: Apparatus for low power write and read operations for resistive memory

Description:
CLAIM OF PRIORITY 
     This application is a Divisional of, and claims the benefit of priority to U.S. patent application Ser. No. 16/052,552, filed Aug. 1, 2019, now issued as U.S. Pat. No. 10,438,640, on Oct. 8, 2019, which is a divisional of, and claims priority to U.S. patent application Ser. No. 14/129,277, filed Dec. 24, 2013, issued as U.S. Pat. No. 10,068,628 on Sep. 4, 2018, which is a National Phase Application of, and claims the benefit of priority to International Patent Application No. PCT/US2013/048753 filed Jun. 28, 2013, each titled “A PPARATUS FOR  L OW  P OWER  W RITE AND  R EAD  O PERATIONS FOR  R ESISTIVE  M EMORY ,” and which is incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     On chip embedded memory with non-volatility can enable energy and computational efficiency. Several new types of solid-state, high-density, non-volatile memories store information using a memory element with a variable resistance. The resistance of spin transfer torque-magnetic random access memory (STT-MRAM) depends on the relative magnetization polarities of two magnetic layers. Other variable resistance memories include resistive RAM (ReRAM) and conductive bridging RAM (CbRAM), whose resistances depend on the formation and elimination of conduction paths through a dielectric or an electrolyte. There is also phase change memory (PCM), for which the resistivity of a cell depends on the crystalline or amorphous state of a chalcogenide. 
       FIG. 1  illustrates a two terminal 1T-1MTJ (Magnetic Tunnel Junction) bit-cell  100  for STT-MRAM. Bit-cell  100  includes an access transistor M1 and an MTJ device. The MTJ device is the storage element of STT-MRAM, which includes a pinned magnetic layer and a free magnetic layer. The free magnetic layer magnetization orientation can change with the write current direction. If the write current flows from the free magnetic layer to the pinned magnetic layer, the free magnetic layer magnetization aligns with the pinned magnetic layer and the MTJ device is in the parallel state (R P ) with low resistance. If the write current flows from the pinned magnetic layer to the free magnetic layer, the free magnetic layer magnetization direction opposes the pinned magnetic layer and the MTJ device is in the anti-parallel (R AP ) state with high resistance. 
     For these resistive memories, read operation is generally faster than write operation, and the write current is generally larger than the read current. Unlike SRAM (Static Random Access Memory) and DRAM (Dynamic Random Access Memory), which consume transient write power, resistive memory still consumes static write power whether or not the cell is flipped during write operation. The read power of these resistive memories may also be static depending on the implementation of the read sensor. Reducing read and write energies are a challenge for resistive memory in order to meet the targets for high performance and low power applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the disclosure, which, however, should not be taken to limit the disclosure to the specific embodiments, but are for explanation and understanding only. 
         FIG. 1  illustrates a two terminal 1T-1MTJ bit-cell for STT-MRAM. 
         FIG. 2  is a conventional write path for a resistive memory. 
         FIG. 3  is a data driven write path in time domain, according to one embodiment of the disclosure. 
         FIG. 4  is a data driven write path in current domain, according to one embodiment of the disclosure. 
         FIG. 5  is a variable strength write driver for a data driven write path in current domain, according to one embodiment of the disclosure. 
         FIG. 6  is an array architecture that combines any or all embodiments of  FIGS. 3-5  with logic to perform read operation before write operation, according to one embodiment of the disclosure. 
         FIG. 7  is a write path architecture with self-controlled write operation, according to one embodiment of the disclosure. 
         FIG. 8  is a read path architecture with self-controlled read operation, according to one embodiment of the disclosure. 
         FIG. 9A  is a source line logic for the read path architecture with self-controlled read operation, according to one embodiment of the disclosure. 
         FIG. 9B  is a lead/lag detector for the read path architecture with self-controlled read operation, according to one embodiment of the disclosure. 
         FIG. 10  is a plot showing operation of the read path architecture with self-controlled read operation, according to one embodiment of the disclosure. 
         FIG. 11  is a smart device or a computer system or an SoC (system-on-chip) with any of the read and write design architectures described with reference to  FIGS. 3-10 , according to one embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments describe write and read operation and design techniques for resistive memories to lower power consumption. In one embodiment, a data-driven write apparatus is used to lower power of write operation in resistive memories. In such an embodiment, asymmetric write switch current between Write0 and Write1 is used to lower power of write operation. For example, power savings in write operation between 25% and 37% of total write energy are realized by the apparatus. 
     In the following embodiments, the terms “Write0” refers to write operation to write a logical low to the memory element, and “Write1” refers to write operation to write a logical high to the memory element. In one embodiment, an apparatus is used to perform read operation before write operation. In such an embodiment, unnecessary writes are avoided by using an initial low-power read operation. Power savings, for example, in the range of 30%, may be realized when the read/write ratio is 50/50. 
     In one embodiment, an apparatus is provided to perform self-controlled write operation. In this embodiment, write operation is stopped as soon as the bit-cell flips i.e., the bit-cell which is selected for being written to. In one embodiment, an apparatus is provided to perform self-controlled read operation. In this embodiment, read operation is stopped as soon as data is detected. Power savings, for example, in the range of 10%-25%, in read power may be realized by the apparatus to perform self-controlled read operation. 
     Although the underlying memory element for resistive memory technologies varies, methods for reading and writing are electrically similar and are encompassed by embodiments. The embodiments can be combined to achieve the best energy savings for resistive memories. 
     In the following description, numerous details are discussed to provide a more thorough explanation of embodiments of the present disclosure. It will be apparent, however, to one skilled in the art, that embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring embodiments of the present disclosure. 
     Note that in the corresponding drawings of the embodiments, signals are represented with lines. Some lines may be thicker, to indicate more constituent signal paths, and/or have arrows at one or more ends, to indicate primary information flow direction. Such indications are not intended to be limiting. Rather, the lines are used in connection with one or more exemplary embodiments to facilitate easier understanding of a circuit or a logical unit. Any represented signal, as dictated by design needs or preferences, may actually comprise one or more signals that may travel in either direction and may be implemented with any suitable type of signal scheme. 
     Throughout the specification, and in the claims, the term “connected” means a direct electrical connection between the things that are connected, without any intermediary devices. The term “coupled” means either a direct electrical connection between the things that are connected or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means one or more passive and/or active components that are arranged to cooperate with one another to provide a desired function. The term “signal” means at least one current signal, voltage signal or data/clock signal. The meaning of “a,” “an,” and “the” include plural references. The meaning of “in” includes “in” and “on.” 
     The term “scaling” generally refers to converting a design (schematic and layout) from one process technology to another process technology. The term “scaling” generally also refers to downsizing layout and devices within the same technology node. The term “scaling” may also refer to adjusting (e.g., slow down) of a signal frequency relative to another parameter, for example, power supply level. The terms “substantially,” “close,” “approximately,” “near,” and “about,” generally refer to being within +/−20% of a target value. 
     Unless otherwise specified the use of the ordinal adjectives “first,” “second,” and “third,” etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking or in any other manner. 
     For purposes of the embodiments, the transistors are metal oxide semiconductor (MOS) transistors, which include drain, source, gate, and bulk terminals. The transistors also include Tri-Gate and FinFet transistors, Gate All Around Cylindrical Transistors or other devices implementing transistor functionality like carbon nano tubes or spintronic devices. Source and drain terminals may be identical terminals and are interchangeably used herein. Those skilled in the art will appreciate that other transistors, for example, Bi-polar junction transistors—BJT PNP/NPN, BiCMOS, CMOS, eFET, etc., may be used without departing from the scope of the disclosure. The term “MN” indicates an n-type transistor (e.g., NMOS, NPN BJT, etc.) and the term “MP” indicates a p-type transistor (e.g., PMOS, PNP BJT, etc.). 
       FIG. 2  is a conventional write path  200  for a resistive memory. The write path  200  consists of write enable pass-gates  201  and  202 , write drive buffers  203  and  204 , and resistive memory elements coupled with their respective access (or select) transistors i.e., RME 0 -M1 0  to RME n -M1 n , where ‘n’ is an integer equal or greater than 1. Wordline (WL) is received by each transistor. For example, M1 0  receives WL&lt;0&gt;, and M1 n  receives WL&lt;n&gt;, where ‘n’ is an integer greater than zero. WREN (write enable) is used to control the pass-gates  201  and  202 . One terminal of the bit-cell is coupled to bit line (BL) and the other end of the bit-cell is coupled to source or select line (SL). Write data DATAIN is received by write drivers  203  and  204  that generate wrdata_b and wrdata, where wrdata_b is inverse of wrdata. Depending on the logical level of WREN, wrdata_b and wrdata are coupled to BL and SL, respectively. 
     When WREN is high and DATAIN is also high, the current flow across the selected bit-cell switches it from the R P  state to the R AP  state (i.e., Write1). When WREN is high and DATAIN is low, the selected bit-cell switches from R AP  to R P  (i.e., Write0). The required write energies between the R AP  and R P  states are asymmetric i.e., the energy required to switch from R AP  to R P  (i.e., Write0) can be substantially smaller than the energy required to switch R P  to R AP  (i.e., Write1). The conventional design  200  always consumes the worst case energy for Write1, and this wastes energy during Write0. 
       FIG. 3  is a data driven write path  300  in time domain, according to one embodiment of the disclosure. It is pointed out that those elements of  FIG. 3  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     For resistive memory, Write0 and Write1 switching energies are asymmetric. For example, the STT-MRAM switching energy from R AP  to R P  (i.e., Write0) can be substantially smaller than the switching energy from R P  to R AP  state (i.e., Write1). In addition, the source follower effect reduces the current seen by the cell for Write1 operation. Therefore, Write1 energy is much greater than the Write® energy. Existing designs (e.g., write path  200 ) expend the worst case write energy (i.e., the energy required for Write1) whether performing Write0 or Write1. This wastes energy during Write0 operation. 
     The embodiment of  FIG. 3  uses a “data-driven write” technique to significantly reduce overall write energy. This can be done either in the time domain or in the current domain. In the time domain as discussed with reference to  FIG. 3  and  FIG. 4 , a longer write pulse is applied for Write1 than for Write0, according to one embodiment. In the current domain as discussed with reference to  FIG. 5  and  FIG. 6 , write time is fixed and a larger write current is sourced for Write1 than for Write0, according to one embodiment. 
     Referring to  FIG. 3 , in one embodiment, write path  300  comprises a selection unit  301  which is operable by DATAIN. In one embodiment, selection unit  301  is a multiplexer which receives at least two write enable signals—wren1 for enabling writing of a logical one, and wren0 for enabling writing of a logical zero to a selected bit-cell of the resistive memory. So as not to obscure the embodiments, elements and features discussed previously may not be repeated. 
     In one embodiment, data-driven write architecture  300  reduces energy in the time domain with a write-enable pulse. In one embodiment, wren1 is a pulse with longer pulse duration than pulse duration of wren0. In this embodiment, wren1 results in longer duration of write current and wren0 results in shorter duration of write current compared to one another. In one embodiment, for Write0 operation, DATAIN is low and wren0 is selected. In one embodiment, for Write1 operation, DATAIN is high and wren1 is selected. In one embodiment, pulse widths for wren1 and wren0 are programmable by software or hardware. In one embodiment, pulse widths for wren1 and wren0 are predetermined by fuses. 
       FIG. 4  is a data driven write path  400  in current domain, according to one embodiment of the disclosure. It is pointed out that those elements of  FIG. 4  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     In one embodiment, write data drivers  401  and  402  have skewed current drive strengths relative to one another. In one embodiment, write data drivers  401  and  402  comprises inverters at their driving ends. In one embodiment, devices (e.g., p-type device MP1, n-type device MN1, p-type device MP2, and n-type device MN2) in each write buffer are sized such that Write0 current is less than Write1 current. In one embodiment, MN1 and MP2 are made larger in size (i.e., W/L) for Write1 operation compared to traditional drivers  203  and  204  to provide more current for writing a logical one to a selected bit-cell compared to current for Write0. In one embodiment, MN2 and MP1 are made smaller in size (i.e., W/L) for Write0 operation compared to the same transistors in traditional drivers  203  and  204 , to provide less current for writing a logical zero to a selected bit-cell compared to current for Write1. 
     In one embodiment, if the orientation of the resistive memory element (e.g., MJT) is changed or the positions of the MTJ and the access device are interchanged, the sizes for MP1, MN1, MP2, and MN2 in each write buffers are re-sized to accommodate for asymmetric power consumption by Write0 and Write1 e.g., Write0 current is less than Write1 current. 
     In one embodiment, adjustment to device sizes for write drivers  401  and  402  can be done dynamically (i.e., automatically) depending on DATAIN and DATAIN #. For example, when DATAIN is such that a logical zero is being written to a bit-cell, then the turn-on strengths of the devices MP1, MN1, MP2, and MN2 are adjusted i.e., MN2 and MP1 are made larger in size (i.e., W/L) compared to the same transistors in traditional drivers  203  and  204 . In one embodiment, write data drivers  401  and  402  (plus preceding inverter  403 ) replace write data drivers  203  and  204 , respectively, of write path architecture  200 . 
       FIG. 5  is a variable strength write driver  500  for a data driven write path in current domain, according to one embodiment of the disclosure. It is pointed out that those elements of  FIG. 5  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     In one embodiment, variable strength write driver  500  comprises a differential driver  501 , selection unit  502 , and reference generator  503 . In one embodiment, differential driver  501  comprises a p-type current source PH with an adjustable current strength, n-type current source NF with an adjustable current strength, p-type devices MPcd1 and MPcd2, and n-type devices MNcd1 and MNcd2. In one embodiment, DATAIN (Din) is received by gate terminals of MPcd1 and MNcd1. In one embodiment, DATAIN # (Din # i.e., inverse of Din) is received by gate terminals of MPcd2 and MNcd2. 
     In one embodiment, MPcd1 and MNcd1 are coupled in series with a common node providing output i.e., wrdata_b (also referred to as wrdata #). In one embodiment, source terminal of MPcd1 is coupled to drain terminal (i.e., iVCC) of PH. In one embodiment, source terminal of MNcd1 is coupled to drain terminal (i.e., iVSS) of NF. In one embodiment, source terminal of PH is coupled to power supply VCC. In one embodiment, source terminal of NF is coupled to ground. In one embodiment, MPcd2 and MNcd2 are coupled in series with a common node providing output i.e., wrdata. In one embodiment, source terminal of MPcd2 is coupled to drain terminal (i.e., iVCC) of PH. In one embodiment, source terminal of MNcd2 is coupled to drain terminal (i.e., iVSS) of NF. 
     In one embodiment, DATAIN (i.e., Din) is used to determine bias voltages Vph for PH and/or Vnh for NF. In one embodiment, selection unit  502  is one or more multiplexers which receive a plurality of voltage references which are used to select bias voltage levels for Vph and/or Vnh in response to DATAIN and signal “settings.” In one embodiment, selection unit  502  is an analog multiplexer which comprises pass gates. In one embodiment, selection unit  502  receives reference voltages e.g., Vcc, v1, v2, v3, v4, and Vss (i.e., ground), where v1 is less than Vcc but greater than v2, v2 is greater than v3, v3 is greater than v4, v4 is greater than Vss. While the embodiment illustrates references, Vcc, v1, v2, v3, v4, and Vss, fewer or more references can be received by selection unit  502 . 
     In one embodiment, reference voltages e.g., Vcc, v1, v2, v3, v4, and Vss are generated by reference generator  503 . In one embodiment, reference generator  503  is a voltage divider. In one embodiment, voltage divider comprises a series of resistors R1-R5 coupled together to form a voltage divider to generate bias voltages v1, v2, v3, and v4. In other embodiments, reference generator is any known reference generator e.g., bandgap reference generator, etc. Vph and Vnh are adjusted via DATAIN to achieve optimal write currents for Write0 and Write1 operations. In one embodiment, Vph varies between 0V to 0.5V. In one embodiment, Vnh varies between 1V to 0.5V. 
     In one embodiment, for a Write1, Din is set to Vcc and Din # is set to Vss. In one embodiment, a relatively stronger bias is applied at Vph and Vnh. For example, Vph may be set to Vss, and Vnh may be set Vcc. In such an embodiment, wrdata # is coupled to Vss through a relatively strong footer in device NF, and wrdata is coupled to Vcc through a relatively strong header in device PH. In such an embodiment, larger write current is generated. 
     In one embodiment, for Write0, Din is set to Vss and Din # is set to Vcc. In one embodiment, a relatively weaker bias is applied at Vph and Vnh. For example, Vph may be set to v3 and Vnh may be set v2. In such an embodiment, wrdata # is coupled to Vcc through a relatively weak header in device PH, and wrdata is coupled to Vss through a relatively weak footer in device NF. In such an embodiment, a smaller write current is generated. In one embodiment, the setting for multiplexer  502  may be adjusted on the fly (i.e., dynamically) if it is found that a particular design, post-manufacturing, can be written with weaker bias settings, and thus lower current and lower power for either Write0 or Write1 is realized. In one embodiment, selection by multiplexer  502  can made to match whichever write current is highest, for example, in the case that the MTJ is flipped or interchanged in position with the access device M1. 
     Existing designs use a write current magnitude and write time duration that meet the worst case scenario of the Write1 operation—regardless of whether they are performing Write0 or Write1. For any given application, the power savings for the “data-driven write” technique varies depending on the distribution of Write0 and Write1 operations. A typical usage case may be defined as one where Write0 and Write1 operations are equally likely and each occurs 50% of the time. In this case, the energy savings is 25% better than existing designs. For computer architectures and applications with Write0 more likely than Write1, the power savings improve. For example, if 75% of the total write operations are Write0 operations, then the write energy savings improves to 37%. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Normalized write energy with different percentage of Write0 
               
               
                 operation and different write time or write current ratio 
               
            
           
           
               
               
               
            
               
                 write time or write 
                 percentage of write 0 operation 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 current ratio (wr0/wr1) 
                 0 
                 25% 
                 50% 
                 75% 
                 100% 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 0.25 
                 1 
                 0.81 
                 0.63 
                 0.44 
                 0.25 
               
               
                 0.5 
                 1 
                 0.88 
                 0.75 
                 0.63 
                 0.50 
               
               
                 0.75 
                 1 
                 0.94 
                 0.88 
                 0.81 
                 0.75 
               
               
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
            
           
         
       
     
     Table 1 shows exemplary normalized write energy results for different percentages of Write0 operations and different write times or write current ratios for Write0 and Write1. When Write0 makes up 50% of total write operations, the energy savings from the embodiment of  FIG. 5  is 25% better than a conventional design. For some computer architectures and applications, Write 0 is more likely than Write1, and the energy savings improves. For example, if Write0 operation occurs during 75% of total write operations, the energy savings is 37% from the embodiment of  FIG. 5  compared to conventional design. In one embodiment, write data drivers  203  and  204  of write path architecture  200  are replaced with write driver  500 . 
       FIG. 6  is an array architecture  600  that combines any or all embodiments of  FIGS. 3-5  with logic to perform read operation before write operation, according to one embodiment of the disclosure. It is pointed out that those elements of  FIG. 6  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     For resistive memory, read time is much faster than the write time. Hence, read energy is much smaller than write energy. For example, read energy is about 5% of write energy for STT-MRAM in contemporary process technologies. In one embodiment, during each write operation, architecture  600  first reads data. For example, if the output read data is identical to the input write data, there is no need to waste energy with a write. 
     For resistive memory, read current is relatively small to avoid read disturbances, and read time is also typically much faster. Hence, the energy dissipated by read circuitry is much smaller than that dissipated during write. In addition, the time duration of a read operation is small compared with the time duration of a write operation. In one embodiment, during each write operation, data is read first with a small overhead in delay (e.g., 10%-20%) and energy (e.g., 5%). If the output read data is identical to the input write data, there is no need to waste time and energy on a write operation. The overall energy savings from skipping unnecessary write operations more than compensates for the small overhead incurred by the read operation. 
     The embodiments of “data driven write” of  FIG. 5  and “read before write” of  FIG. 6  can be combined to improve energy savings for resistive memories. Array architecture  600  is one embodiment that combines these techniques. Other techniques of  FIGS. 3 and 4  can also be combined with the embodiment of  FIG. 6 . 
     In one embodiment, architecture  600  comprises data array  601 , reference array  602 , compare unit  603 , logic gate  604 , write driver  605  (wrdrv), sense amplifier (SA)  606 , reference multiplexer  607 , write enable multiplexer  301 , column select unit  608 , write enable pass-gate M201 (e.g., a transistor of pass-gate  201  of  FIG. 2 ), reference enable pass-gate M601, read enable transistor controlled by rden, and reference read enable transistor controlled by refrden. 
     So as not to obscure the embodiments, a single transistor is used to represent multiplexors (e.g.,  202 ) for illustration purposes. In one embodiment, wren0 and wren1 pulses are provided to multiplexer  301 , and one of the pulses (according to the DATAIN) is applied to both data array  601  and reference array  602 . For reference write, a single wren0/1 signal is selected based on DATAIN. 
     In one embodiment, during read portion of the operation, both rden and refrden cause their respective transistors to be turned on. In such an embodiment, rddataout is generated through SA circuit  606 . In one embodiment, during data write operation, the read portion of the operation is performed first, and rddataout is compared with DATAIN by compare unit  603 . In one embodiment, compare unit  603  comprises an exclusive-OR (XOR) logic. In other embodiments, other logic units may be used to implement compare unit  603 . In one embodiment, when rddataout and DATAIN match, output “cmp” of compare unit  603  is ‘0’, wren is “0” and no write operation is needed. In this embodiment, wren0 and wren1 are not 0. In one embodiment, when rddataout and DATAIN do not match, output cmp of compare unit  603  is ‘1,’ and one of wren0/1 is selected depending on the polarity of DATAIN. 
     While the embodiment combines the apparatus of  FIG. 3 , architecture  600  may only have the apparatus needed for performing read operation before write operation. In one embodiment, architecture  600  combines (in any combination) all the embodiments discussed. 
       FIG. 7  is a write path architecture  700  with self-controlled write operation, according to one embodiment of the disclosure. It is pointed out that those elements of  FIG. 7  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     In one embodiment whether performing Write1 or Write0, the write time for any given bit-cell at any given time follows a distribution such that some bit-cells take longer to write than other bit-cells. Existing designs source a current through the memory element for a time duration that guarantees writing cells with write times well above the mean, regardless of the actual time necessary to write that particular cell. For example, in an MTJ memory element in contemporary process technologies, a nominal write time of 10 ns ensures a successful write for cells with a 5-sigma write time. With a self-controlled write implemented by write path architecture  700 , the aggregate write time is reduced to the time that it actually takes to write an average bit-cell. This reduction in write time translates to a reduction in the amount of time that current flows and, hence, a reduction in power consumption. 
     In one embodiment, write path architecture  700  comprises a current sensor  701  and logic unit  702 . So as not to obscure the embodiments, elements/features previously discussed are not repeated. In one embodiment, current sensor  701  senses current via sense line  703  of write drivers  203  and/or  204  to determine whether write drivers are writing the same data or different data i.e., the current through write drivers  203 ,  204 , and the memory cell should all be the same since they are coupled together in series. For example, write path architecture  700  monitors the bit-cell current (via write drivers  203  and/or  204 ) to detect any change due to a resistance change in the memory element. 
     If the bit-cell current changes significantly, it means that the bit-cell has flipped and the write operation can be stopped. Upon a significant increase or decrease in current, current sensor  701  block outputs a ‘0’ that disables the write pass-gates  201  and  202 . For example, resistance of MTJ may change by 2× when switching states; it should be at least ½ the total resistance in the whole path (write driver  204 →write pass-gate  202 →access device M1→MTJ→write pass-gate  201 →write driver  203 ), so in this example “significant” would be a change of 25% in the current magnitude. 
     In one embodiment, write pass-gates  201  and  202  are blocked via logic gate  702  if current sensor  701  indicates that write operation has been performed. In one embodiment, logic gate  702  is an AND gate. In another embodiment, logic gate  702  is a NAND gate. In other embodiments, other logic gates may be used to perform the function described. In one embodiment, write path architecture  700  avoids wasting power for bit-cells that flip before the worst-case 5-sigma cell, and it provides a complementary power savings method when used with other embodiments. In such an embodiment, the self-controlled write scheme reduces the aggregate write power dissipation. In one embodiment, architecture  700  combines (in any combination) all the embodiments discussed. 
       FIG. 8  is a read path architecture  800  with self-controlled read operation, according to one embodiment of the disclosure. It is pointed out that those elements of  FIG. 8  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     During a read operation (whether sensing current, voltage, or RC (resistor-capacitor) time constants, current flows across the memory element. Although the total read energy is generally less than the total write energy, read energy still has a significant and application-dependent impact on the system power consumption. Reducing read energy may also save write energy when using the “read before write” approach described with reference to  FIG. 8 . For the self-controlled read, read path architecture  800  implements a “read complete” function in the sensing element. This output stops the flow of current during the present read operation. 
     In one embodiment, when reading, the time needed to resolve the state of any given cell on any given chip may take longer or shorter than other cells on other chips. Existing designs source a current through the memory element for a time duration that guarantees a successful read for cells and sensors with read times well above the mean—regardless of the actual time necessary for a particular sensor to read a particular cell. For example, in an MTJ memory element in contemporary process technologies, current flows for up to 2 ns in order to successfully read cells and sensors with a 6-sigma read time. A self-controlled read as described by the embodiment of read path architecture  800  reduces the aggregate read time down to the read time for an average cell and sensor. This translates to a reduction in the amount of time that current flows during read and, hence, a reduction in power consumption. 
     Read time is usually determined by a 5-sigma cell, however, the average read time is smaller. Self-controlled read, as described by the embodiment read path architecture  800 , allows for a shorter read time in the aggregate than with the usual method of using 5-sigma read time for all cells. 
     A memory array comprises M-rows and N-columns of data bit-cells  840 , each with a resistive memory element  890  that assumes either a lower resistance state with resistance R L  or a higher resistance state with resistance R H . For every N columns of data bit cells  840 , there is a single M-row column of reference bit-cells fixed to the lower resistance state  850  and a single M-row column of reference bit-cells fixed to the higher resistance state  860 . During a read operation, decoder  805  selects one row and one column of the memory array to address a single data bit-cell  840 : the YSELECT signal selects the column, and the WORDLINE signal selects the row. 
     Likewise, for the reference bit-cells  850 / 860 , the WORDLINE selects the same row as the data cell  840 , and the REFSELECT signal selects both reference columns. The RDSEL signal enables read operation for both the reference and data columns. Selection activates access devices  830  that electrically connect the sourceline logic  810  to the data sensor  880  on the BITLINE. The access device  830  may be an NMOS transistor, a PMOS transistor, or a CMOS pass-gate. 
       FIG. 9A  are pulse drivers for source line logic  810  for the read path architecture  800  with self-controlled read operation, according to one embodiment of the disclosure.  FIG. 9B  is a time domain lead/lag detector (or data sensor)  880  for the read path architecture  800  with self-controlled read operation, according to one embodiment of the disclosure. 
     It is pointed out that those elements of  FIGS. 9A-B  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     In one embodiment, sourceline logic  810  comprises three buffers  911 - 913  that each sends a pulse through the data column and the reference columns. For a data cell  840  in the R H  (R AP ) state, the pulse will encounter a longer RC delay than the delay through the parallel combination of the reference columns. Similarly, for a data cell  840  in the R L  (R P ) state, the pulse will encounter a shorter RC delay than the delay through the parallel combination of the reference columns. In one embodiment, to detect the state of the data cell, the data sensor  880  senses the position in time of the data pulse relative to the reference pulse. In one embodiment, the lead/lag detector  880  comprises high-gain D flip-flops on the REFERENCE  981  and DATA  982  paths followed by high-gain cross-coupled NAND gates on the REFERENCE  983  and DATA  984  paths. 
     In one embodiment, after RESET, the flip-flops preserve the order of the rising edges of the DATA and REFERENCE pulses and prevent the falling edges from disturbing the DATAOUT output. In one embodiment, if a REFERENCE edge arrives first, the cross-coupled NAND gates latch a data ‘0’ on the DATAOUT signal until RESET. In one embodiment, if a DATA edge arrives first, the cross-coupled NAND gates latch a data ‘1’ on the DATAOUT signal until RESET. In one embodiment, when either DATAOUT or DATAOUTB rises, there is no longer a need to induce current through the data  840  and reference cells  850 / 860 . 
     The activation of the READ COMPLETE signal is a logical function (such as “exclusive-OR (XOR)”) of the DATAOUT and DATAOUTB signals. In one embodiment, READ COMPLETE is used as a DFT (Design-For-Test) feature to confirm that the sensor actually fired, instead of the sensor&#39;s initial DATAOUT state just happening to be the same as the read data. In one embodiment, the READ COMPLETE signal can then disconnect the SOURCELINE from the BITLINE by disabling the access devices  830 . This stops current from flowing through the data cell  840  and the reference cells  850 / 860 . In one embodiment, the READ COMPLETE signal can also tri-state the pulse driver in the SOURCELINE logic  810  and disable the input buffers  985 / 986  on the flip-flops  981 / 982  in the lead/lag detector  880 . This prevents any crowbar current flow in these blocks. 
       FIG. 10  is a plot  1000  showing operation of the read path architecture  800  with self-controlled read operation, according to one embodiment of the disclosure. It is pointed out that those elements of  FIG. 10  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     The first read in plot  1001  from 2 ns to 10 ns is a baseline read without the embodiment of  FIG. 8 , and the second read in plot  1001  from 10 ns to 18 ns uses a self-controlled read of  FIG. 8  to save read power. The READ COMPLETE signal is implemented as a logical Exclusive-OR (XOR) function of DATAOUT and DATAOUTB. The READ COMPLETE signal shown in plot  1002  tri-states the SOURCELINE LOGIC drivers  911 - 913  and the LEAD/LAG input buffers  985 / 986 , and it electrically disconnects the SOURCELINE from the BITLINE by disabling the RDEN signal as shown in plot  1003 . 
     This saves power. For example, it stops any crowbar current in the SOURCE LINE LOGIC drivers  911 - 913  and in the LEAD/LAG input buffers  985 / 986 ; the BITLINE is not discharged fully to Vss so the subsequent pre-charge uses less power; and the RDEN multiplexer (or pass-gate) prevents current from flowing across the memory element when the SOURCELINE and BITLINE are at different potentials. In one embodiment, architecture  800  combines (in any combination) all the embodiments discussed. 
       FIG. 11  is a smart device or a computer system or an SoC (system-on-chip) with any of the read and write design architectures described with reference to  FIGS. 3-10 , according to one embodiment of the disclosure. It is pointed out that those elements of  FIG. 11  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
       FIG. 11  illustrates a block diagram of an embodiment of a mobile device in which flat surface interface connectors could be used. In one embodiment, computing device  1600  represents a mobile computing device, such as a computing tablet, a mobile phone or smart-phone, a wireless-enabled e-reader, or other wireless mobile device. It will be understood that certain components are shown generally, and not all components of such a device are shown in computing device  1600 . 
     In one embodiment, computing device  1600  includes a first processor  1610  with any of the read and write design architectures described with reference to embodiments of  FIGS. 3-10 , according to the embodiments discussed. Other blocks of the computing device  1600  may also include any of the read and write design architectures described with reference to embodiments of  FIGS. 3-10 . The various embodiments of the present disclosure may also comprise a network interface within  1670  such as a wireless interface so that a system embodiment may be incorporated into a wireless device, for example, cell phone or personal digital assistant. 
     In one embodiment, processor  1610  (and processor  1690 ) can include one or more physical devices, such as microprocessors, application processors, microcontrollers, programmable logic devices, or other processing means. The processing operations performed by processor  1610  include the execution of an operating platform or operating system on which applications and/or device functions are executed. The processing operations include operations related to I/O (input/output) with a human user or with other devices, operations related to power management, and/or operations related to connecting the computing device  1600  to another device. The processing operations may also include operations related to audio I/O and/or display I/O. 
     In one embodiment, computing device  1600  includes audio subsystem  1620 , which represents hardware (e.g., audio hardware and audio circuits) and software (e.g., drivers, codecs) components associated with providing audio functions to the computing device. Audio functions can include speaker and/or headphone output, as well as microphone input. Devices for such functions can be integrated into computing device  1600 , or connected to the computing device  1600 . In one embodiment, a user interacts with the computing device  1600  by providing audio commands that are received and processed by processor  1610 . 
     Display subsystem  1630  represents hardware (e.g., display devices) and software (e.g., drivers) components that provide a visual and/or tactile display for a user to interact with the computing device  1600 . Display subsystem  1630  includes display interface  1632 , which includes the particular screen or hardware device used to provide a display to a user. In one embodiment, display interface  1632  includes logic separate from processor  1610  to perform at least some processing related to the display. In one embodiment, display subsystem  1630  includes a touch screen (or touch pad) device that provides both output and input to a user. 
     I/O controller  1640  represents hardware devices and software components related to interaction with a user. I/O controller  1640  is operable to manage hardware that is part of audio subsystem  1620  and/or display subsystem  1630 . Additionally, I/O controller  1640  illustrates a connection point for additional devices that connect to computing device  1600  through which a user might interact with the system. For example, devices that can be attached to the computing device  1600  might include microphone devices, speaker or stereo systems, video systems or other display devices, keyboard or keypad devices, or other I/O devices for use with specific applications such as card readers or other devices. 
     As mentioned above, I/O controller  1640  can interact with audio subsystem  1620  and/or display subsystem  1630 . For example, input through a microphone or other audio device can provide input or commands for one or more applications or functions of the computing device  1600 . Additionally, audio output can be provided instead of, or in addition to display output. In another example, if display subsystem  1630  includes a touch screen, the display device also acts as an input device, which can be at least partially managed by I/O controller  1640 . There can also be additional buttons or switches on the computing device  1600  to provide I/O functions managed by I/O controller  1640 . 
     In one embodiment, I/O controller  1640  manages devices such as accelerometers, cameras, light sensors or other environmental sensors, or other hardware that can be included in the computing device  1600 . The input can be part of direct user interaction, as well as providing environmental input to the system to influence its operations (such as filtering for noise, adjusting displays for brightness detection, applying a flash for a camera, or other features). 
     In one embodiment, computing device  1600  includes power management  1650  that manages battery power usage, charging of the battery, and features related to power saving operation. Memory subsystem  1660  includes memory devices for storing information in computing device  1600 . Memory can include nonvolatile (state does not change if power to the memory device is interrupted) and/or volatile (state is indeterminate if power to the memory device is interrupted) memory devices. Memory subsystem  1660  can store application data, user data, music, photos, documents, or other data, as well as system data (whether long-term or temporary) related to the execution of the applications and functions of the computing device  1600 . 
     Elements of embodiments are also provided as a machine-readable medium (e.g., memory  1660 ) for storing the computer-executable instructions (e.g., instructions to implement any other processes discussed herein). The machine-readable medium (e.g., memory  1660 ) may include, but is not limited to, flash memory, optical disks, CD-ROMs, DVD ROMs, RAMs, EPROMs, EEPROMs, magnetic or optical cards, phase change memory (PCM), or other types of machine-readable media suitable for storing electronic or computer-executable instructions. For example, embodiments of the disclosure may be downloaded as a computer program (e.g., BIOS) which may be transferred from a remote computer (e.g., a server) to a requesting computer (e.g., a client) by way of data signals via a communication link (e.g., a modem or network connection). 
     Connectivity  1670  includes hardware devices (e.g., wireless and/or wired connectors and communication hardware) and software components (e.g., drivers, protocol stacks) to enable the computing device  1600  to communicate with external devices. The computing device  1600  could be separate devices, such as other computing devices, wireless access points or base stations, as well as peripherals such as headsets, printers, or other devices. 
     Connectivity  1670  can include multiple different types of connectivity. To generalize, the computing device  1600  is illustrated with cellular connectivity  1672  and wireless connectivity  1674 . Cellular connectivity  1672  refers generally to cellular network connectivity provided by wireless carriers, such as provided via GSM (global system for mobile communications) or variations or derivatives, CDMA (code division multiple access) or variations or derivatives, TDM (time division multiplexing) or variations or derivatives, or other cellular service standards. Wireless connectivity (or wireless interface)  1674  refers to wireless connectivity that is not cellular, and can include personal area networks (such as Bluetooth, Near Field, etc.), local area networks (such as Wi-Fi), and/or wide area networks (such as WiMax), or other wireless communication. 
     Peripheral connections  1680  include hardware interfaces and connectors, as well as software components (e.g., drivers, protocol stacks) to make peripheral connections. It will be understood that the computing device  1600  could both be a peripheral device (“to”  1682 ) to other computing devices, as well as have peripheral devices (“from”  1684 ) connected to it. The computing device  1600  commonly has a “docking” connector to connect to other computing devices for purposes such as managing (e.g., downloading and/or uploading, changing, synchronizing) content on computing device  1600 . Additionally, a docking connector can allow computing device  1600  to connect to certain peripherals that allow the computing device  1600  to control content output, for example, to audiovisual or other systems. 
     In addition to a proprietary docking connector or other proprietary connection hardware, the computing device  1600  can make peripheral connections  1680  via common or standards-based connectors. Common types can include a Universal Serial Bus (USB) connector (which can include any of a number of different hardware interfaces), DisplayPort including MiniDisplayPort (MDP), High Definition Multimedia Interface (HDMI), Firewire, or other types. 
     Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments. The various appearances of “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments. If the specification states a component, feature, structure, or characteristic “may,” “might,” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the elements. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
     Furthermore, the particular features, structures, functions, or characteristics may be combined in any suitable manner in one or more embodiments. For example, a first embodiment may be combined with a second embodiment anywhere the particular features, structures, functions, or characteristics associated with the two embodiments are not mutually exclusive. 
     While the disclosure has been described in conjunction with specific embodiments thereof, many alternatives, modifications and variations of such embodiments will be apparent to those of ordinary skill in the art in light of the foregoing description. For example, other memory architectures e.g., Dynamic RAM (DRAM) may use the embodiments discussed. The embodiments of the disclosure are intended to embrace all such alternatives, modifications, and variations as to fall within the broad scope of the appended claims. 
     In addition, well known power/ground connections to integrated circuit (IC) chips and other components may or may not be shown within the presented figures, for simplicity of illustration and discussion, and so as not to obscure the disclosure. Further, arrangements may be shown in block diagram form in order to avoid obscuring the disclosure, and also in view of the fact that specifics with respect to implementation of such block diagram arrangements are highly dependent upon the platform within which the present disclosure is to be implemented (i.e., such specifics should be well within purview of one skilled in the art). Where specific details (e.g., circuits) are set forth in order to describe example embodiments of the disclosure, it should be apparent to one skilled in the art that the disclosure can be practiced without, or with variation of, these specific details. The description is thus to be regarded as illustrative instead of limiting. 
     The following examples pertain to further embodiments. Specifics in the examples may be used anywhere in one or more embodiments. All optional features of the apparatus described herein may also be implemented with respect to a method or process. 
     For example, in one embodiment, an apparatus comprises: a resistive memory cell coupled to a bit line and a select line; a first pass-gate coupled to the bit line; a second pass-gate coupled to the select line; and a multiplexer operable by input data, the multiplexer to provide a control signal to the first and second pass-gates according to logic level of the input data. In one embodiment, the multiplexer to receive at least two inputs of different pulse widths. In one embodiment, apparatus further comprises logic to adjust pulse widths of the at least two inputs. In one embodiment, the at least two inputs are first and second write enable pulses, the first write enable pulse for controlling duration of writing a logical high to the resistive memory, and the second write enable pulse for controlling duration of writing a logical low to the resistive memory. 
     In one embodiment, the resistive memory is at least one of: STT-MRAM; ReRAM; or CBRAM. In one embodiment, the apparatus further comprises: a first write driver to drive input data to the first pass-gate; and a second write driver to drive an inverse of the input data to the second pass-gate. In one embodiment, the resistive memory is an STT-MRAM bit-cell which comprises: a select transistor controllable by a word line; and a magnetic tunnel junction (MTJ) device coupled in series with the select transistor. 
     In another example, an apparatus comprises: a resistive memory cell coupled to a bit line and a select line; a first pass-gate coupled to the bit line; a second pass-gate coupled to the select line; a first write driver to drive an input data to the first pass-gate, the first write driver having a first drive skew; and a second write driver to drive an inverse of the input data to the second pass-gate, the second write driver having a second drive skew, wherein the first drive skew is different than the second drive skew. 
     In one embodiment, the apparatus further comprises logic to adjust first and second drive skews of the first and second write drivers. In one embodiment, the logic to dynamically adjust first and second drive skews of the first and second write drivers according to the input data. In one embodiment, the apparatus further comprises: a multiplexer operable by the input data, the multiplexer to provide a control signal to the first and second pass-gates according to logic level of the input data. In one embodiment, the multiplexer receives at least two inputs of different pulse widths. In one embodiment, the apparatus further comprises logic to adjust pulse widths of the at least two inputs. 
     In one embodiment, the at least two inputs are first and second write enable pulses, the first write enable pulse for controlling duration of writing a logical high to the resistive memory, the second write enable pulse for controlling duration of writing a logical low to the resistive memory. In one embodiment, the resistive memory is at least one of: STT-MRAM; ReRAM; PCM; or CBRAM. In one embodiment, the resistive memory is an STT-MRAM bit-cell which comprises: a select transistor controllable by a word line; and a magnetic tunnel junction (MTJ) device coupled in series with the select transistor. 
     In another example, in one embodiment, an apparatus comprises: a resistive memory cell coupled to a bit line and a select line; a first pass-gate coupled to the bit line; a second pass-gate coupled to the select line; and a differential write driver to receive a differential input and to drive a differential output to the first and second pass-gates, wherein the differential write driver to cause a first output of the differential output to have a different drive strength than a second output of the differential output. In one embodiment, the differential write driver comprises an adjustable p-type current source. 
     In one embodiment, the apparatus further comprises a variable voltage generator to provide a bias for the adjustable p-type current source according to the first or second outputs or both. In one embodiment, the differential write driver comprises an adjustable n-type current source. In one embodiment, the apparatus further comprises a variable voltage generator to provide a bias for the adjustable n-type current source according to the first or second outputs or both. In one embodiment, the variable voltage generator comprises: a multiplexer controllable by the first or second outputs or both; and a voltage divider to provide a plurality of voltages of different levels to the multiplexer. 
     In one embodiment, the apparatus further comprises a multiplexer operable by the input data, the multiplexer to provide a control signal to the first and second pass-gates according to logic level of the input data. In one embodiment, the multiplexer to receive at least two inputs of different pulse widths. In one embodiment, the apparatus further comprises logic to adjust pulse widths of the at least two inputs. In one embodiment, the at least two inputs are first and second write enable pulses, the first write enable pulse for controlling duration of writing a logical high to the resistive memory, and the second write enable pulse for controlling duration of writing a logical low to the resistive memory. In one embodiment, the resistive memory is at least one of: STT-MRAM; ReRAM; PCM; or CBRAM. In one embodiment, the resistive memory is an STT-MRAM bit-cell which comprises: a select transistor controllable by a word line; and a magnetic tunnel junction (MTJ) device coupled in series with the select transistor. 
     In another example, in one embodiment, a resistive memory cell; a comparator to compare read data and write data, and to generate an output indicating whether read data is same as write data; and logic to receive output from the comparator, the logic to generate a write enable for controlling a pass-gate coupled directly or indirectly to the resistive memory cell. In one embodiment, the logic to enable write operation if the output of the comparator indicates that the read data is different from the write data. In one embodiment, the logic to disable write operation if the output of the comparator indicates that the read data is same as the write data. 
     In one embodiment, the apparatus further comprises logic to cause read operation before write operation. In one embodiment, the pass-gate is coupled to a bit line which is coupled to the resistive memory, wherein the pass-gate is a first pass gate, and wherein the apparatus further comprises: a second pass-gate coupled to a select line which is coupled to the resistive memory; and a differential write driver to receive a differential input and to drive a differential output to the first and second pass-gates, wherein the differential write driver to cause a first output of the differential output to have a different drive strength than a second output of the differential output. 
     In one embodiment, the apparatus further comprises: a multiplexer operable by the input data, the multiplexer to provide a control signal to the logic unit according to logic level of the write data. In one embodiment, the multiplexer to receive at least two inputs of different pulse widths. In one embodiment, the apparatus further comprises logic to adjust pulse widths of the at least two inputs. In one embodiment, the at least two inputs are first and second write enable pulses, the first write enable pulse for controlling duration of writing a logical high to the resistive memory, and the second write enable pulse for controlling duration of writing a logical low to the resistive memory. 
     In one embodiment, the pass-gate is coupled to a bit line which is coupled to the resistive memory, wherein the pass-gate is a first pass gate, and wherein the apparatus further comprises: a second pass-gate coupled to a select line which is coupled to the resistive memory; a first write driver to drive an input data to the first pass-gate, the first write driver having a first drive skew; and a second write driver to drive an inverse of the input data to the second pass-gate, the second write driver having a second drive skew, wherein the first drive skew is different than the second drive skew. 
     In one embodiment, the apparatus further comprises logic to adjust first and second drive skews of the first and second write drivers. In one embodiment, the logic to dynamically adjust first and second drive skews of the first and second write drivers according to the input data. In one embodiment, the pass-gate is coupled to a bit line which is coupled to the resistive memory, wherein the pass-gate is a first pass gate, and wherein the apparatus further comprises: a second pass-gate coupled to a select line which is coupled to the resistive memory; and a differential write driver to receive a differential input and to drive a differential output to the first and second pass-gates, wherein the differential write driver to cause a first output of the differential output to have a different drive strength than a second output of the differential output. 
     In one embodiment, the differential write driver comprises an adjustable p-type current source. In one embodiment, the apparatus further comprises a variable voltage generator to provide a bias for the adjustable p-type current source according to the first or second outputs or both. In one embodiment, the differential write driver comprises an adjustable n-type current source. In one embodiment, the apparatus further comprises a variable voltage generator to provide a bias for the adjustable n-type current source according to the first or second outputs or both. 
     In one embodiment, the variable voltage generator comprises: a multiplexer controllable by the first or second outputs or both; and a voltage divider to provide a plurality of voltages of different levels to the multiplexer. In one embodiment, the resistive memory is at least one of: STT-MRAM; ReRAM; PCM; or CBRAM. In one embodiment, the resistive memory is an STT-MRAM bit-cell which comprises: a select transistor controllable by a word line; and a magnetic tunnel junction (MTJ) device coupled in series with the select transistor. 
     In another example, an apparatus comprises: a resistive memory cell; a write driver to receive input data for writing to the resistive memory cell; a current sensor to sense current in the write driver; and logic to receive output from the current sensor, the logic to generate a write enable for controlling a pass-gate coupled directly or indirectly to the resistive memory cell. In one embodiment, the logic to enable write operation if the output of the current sensor indicates that a write current change is within a threshold. 
     In one embodiment, the logic to disable write operation if the output of the current sensor indicates that a write current change is above a threshold. In one embodiment, the apparatus further comprises: a comparator to compare read data and write data, and to generate an output indicating whether read data is same as write data, wherein the input data is the same as write data; and logic to receive output from the comparator, the logic to generate a write enable for controlling a pass-gate coupled directly or indirectly to the resistive memory cell. 
     In one embodiment, the logic to enable write operation if the output of the comparator indicates that the read data is different from the write data. In one embodiment, the logic to disable write operation if the output of the comparator indicates that the read data is same as the write data. In one embodiment, the apparatus further comprises logic to cause read operation before write operation. 
     In one embodiment, the apparatus further comprises a pass-gate coupled to a bit line which is coupled to the resistive memory, wherein the pass-gate is a first pass gate, and wherein the apparatus further comprises: a second pass-gate coupled to a select line which is coupled to the resistive memory; and a differential write driver to receive a differential input and to drive a differential output to the first and second pass-gates, wherein the differential write driver to cause a first output of the differential output to have a different drive strength than a second output of the differential output. 
     In one embodiment, the apparatus further comprises: a multiplexer operable by the input data, the multiplexer to provide a control signal to the logic unit according to logic level of the write data. In one embodiment, the multiplexer to receive at least two inputs of different pulse widths. In one embodiment, the apparatus further comprises logic to adjust pulse widths of the at least two inputs. In one embodiment, the at least two inputs are first and second write enable pulses, the first write enable pulse for controlling duration of writing a logical high to the resistive memory, and the second write enable pulse for controlling duration of writing a logical low to the resistive memory. 
     In one embodiment, the apparatus further comprises a pass-gate coupled to a bit line which is coupled to the resistive memory, wherein the pass-gate is a first pass gate, and wherein the apparatus further comprises: a second pass-gate coupled to a select line which is coupled to the resistive memory; a first write driver to drive an input data to the first pass-gate, the first write driver having a first drive skew; and a second write driver to drive an inverse of the input data to the second pass-gate, the second write driver having a second drive skew, wherein the first drive skew is different than the second drive skew. 
     In one embodiment, the apparatus further comprises logic to adjust first and second drive skews of the first and second write drivers. In one embodiment, the logic to dynamically adjust first and second drive skews of the first and second write drivers according to the input data. In one embodiment, the apparatus further comprises a pass-gate coupled to a bit line which is coupled to the resistive memory, wherein the pass-gate is a first pass gate, and wherein the apparatus further comprises: a second pass-gate coupled to a select line which is coupled to the resistive memory; and a differential write driver to receive a differential input and to drive a differential output to the first and second pass-gates, wherein the differential write driver to cause a first output of the differential output to have a different drive strength than a second output of the differential output. 
     In one embodiment, the differential write driver comprises an adjustable p-type current source. In one embodiment, the apparatus further comprises a variable voltage generator to provide a bias for the adjustable p-type current source according to the first or second outputs or both. In one embodiment, wherein the differential write driver comprises an adjustable n-type current source. In one embodiment, the apparatus further comprises a variable voltage generator to provide a bias for the adjustable n-type current source according to the first or second outputs or both. 
     In one embodiment, the variable voltage generator comprises: a multiplexer controllable by the first or second outputs or both; and a voltage divider to provide a plurality of voltages of different levels to the multiplexer. In one embodiment, the resistive memory is at least one of: STT-MRAM; ReRAM; PCM; or CBRAM. In one embodiment, the resistive memory is an STT-MRAM bit-cell which comprises: a select transistor controllable by a word line; and a magnetic tunnel junction (MTJ) device coupled in series with the select transistor. 
     In another example, in one embodiment, an apparatus comprises: an array of resistive memory cells for containing data; reference resistive memory bit-cells; a detector for comparing data read from a reference bit-cell, from among the reference resistive memory bit-cells, and data read a resistive memory cell from the array of resistive memory cells; and logic to disable read operation when output of detector indicates that data has been read from the resistive memory cell from the array of resistive memory cells. 
     In one embodiment, the logic to further disable the detector when output of detector indicates that data has been read from the resistive memory cell from the array of resistive memory cells. In one embodiment, the logic is operable to tri-state source line which is coupled to the resistive memory cell from the array of resistive memory cells. In one embodiment, the resistive memory is at least one of: STT-MRAM; ReRAM; PCM; or CBRAM. 
     In one embodiment, the resistive memory is an STT-MRAM bit-cell which comprises: a select transistor controllable by a word line; and a magnetic tunnel junction (MTJ) device coupled in series with the select transistor. 
     In another example, in one embodiment, a system comprises, a processor; a wireless interface for allowing the processor to communicate with another device; a memory coupled to the processor, the memory according to any of the apparatus discussed above; and a display unit for displaying content processed by the processor. In one embodiment, the display unit is a touch screen. 
     An abstract is provided that will allow the reader to ascertain the nature and gist of the technical disclosure. The abstract is submitted with the understanding that it will not be used to limit the scope or meaning of the claims. The following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate embodiment.