Patent Document

BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to sensing devices, and more specifically to sensing circuits and related methods for sensing resistive memory cells.  
       BACKGROUND OF THE INVENTION  
       [0002]     Digital memories are widely used in computers, computer system components and computer processing systems. Resistive memories store digital information in the form of bits or binary digits as “0”s or “1”s based on the resistance of a memory element or cell.  
         [0003]     Resistive memory devices are configured in arrays where a resistive element or cell is at the intersection of a row line (or “word” line) and a column line (“digit” line or “bit” line). In order to read or sense the state of a memory cell, it is necessary to first select the desired memory cell by selecting the column line and row line, which intersect at the desired memory element. Once the desired memory element is isolated, the selected memory cell is then read by applying a read voltage to the cell. The applied voltage causes current flow through the selected cell which is sensed to determine the logic state of the cell. Sensing circuits often use digital counters which count a clock signal to establish a count value which is related to the current flow through the cell. An example of such an arrangement is illustrated in commonly-assigned U.S. Pat. No. 6,504,750, issued Jan. 7, 2003, titled “RESISTIVE MEMORY ELEMENT SENSING USING AVERAGING” which is incorporated by reference in its entirety herein.  
         [0004]     Current sensing circuits used to measure memory cell resistances use clocked comparators and counting circuits and have a tendency to saturate, providing a continuous string of ones or zeroes. Typically, each type of string is a result of an incompatible clock oscillator frequency for received voltages which are related to current flow through the cell.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides sensing methods and apparatus for adjusting the frequency of a clock oscillator used in a counting circuit for sensing resistive memory cells. In accordance with exemplary method and apparatus embodiments of the present invention, the output of a sensing circuit which produces “one” and “zero” counting pulses is processed to determine whether the sensing clock is operating at too low or too high a speed. If the speed is determined to be too low or too high, the clock speed is then adjusted as necessary to avoid saturation of the counting circuit.  
         [0006]     Other features and advantages of the present invention will become apparent when the following description is read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a schematic illustrating an exemplary sensing circuit, coupled to a resistive memory, in accordance with an exemplary embodiment of the invention;  
         [0008]      FIG. 2A -D are timing diagrams, illustrating operation of the circuit in  FIG. 1 ;  
         [0009]      FIG. 3  is a circuit that determines a saturation condition from the output of the  FIG. 1  circuit;  
         [0010]      FIG. 4  is a block diagram of an oscillator/clock adjustment circuit;  
         [0011]      FIG. 5  is a schematic of a non-overlapping clock generation circuit; and  
         [0012]      FIG. 6  depicts a block diagram of a processor system employing a resistive memory having a sensing and adjustment circuit in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIG. 1  shows the integrated charge sensing circuit  100  of an exemplary embodiment of the invention coupled to an exemplary memory cell  101  of an array of resistive memory cells arranged at the intersection of digit (column) line  105  and row line  112 . Exemplary memory cell  101  is shown, addressed by row line  112  and digit line  105 . Memory cell  101  includes an access transistor  102  and a resistor or resistive element  103  coupled to a voltage source of Vcc/2, where Vcc is a supply voltage. Voltage source Vcc/2 is also coupled to the non inverting input of comparator  107 .  
         [0014]     The digit line  105  is connected to an integrated memory element resistance measurement circuit  115 . It is understood that while  FIG. 1  only illustrates one resistive memory cell  101 , the same principles described herein are equally applicable to a multitude of resistive memory cells, arranged in an array. Also, although the invention has been described with a resistive random access memory element accessed with an access transistor, the invention will also operate with other types of memory and other access schemes, such as a crosspoint array.  
         [0015]     In accordance with the illustrated embodiment, digit line  105  is connected to a respective integrated memory element resistance measurement circuit  115  through a column select transistor  104 . Alternately, a single measurement circuit  115  may be multiplexed among a plurality of digit lines. Measurement circuit  115  includes a comparator  107  for measuring the current through memory cell  101 , which is stored as a voltage on a digit line capacitor  106 . Comparator  107  is provided with an offset voltage, Vos at the input receiving the Vcc/2 voltage as a reference input. In accordance with the  FIG. 1  embodiment, comparator  107  makes a comparison between the voltage on digit line  105  and the reference voltage V cc /2−V os  when the leading edge of clock signal Φ 2  from clock source  120  goes high. When the voltage on digit line  105  exceeds V cc /2−V os , the output of a comparator  107  switches high. The high output of comparator  107  closes a switch  109 , causing some of the charge stored on digit line capacitor  106  to be transferred onto a capacitor  108 . When the voltage on the digit line  105  falls below V cc /2−V os , and the comparator  107  is again operated by the clock signal Φ 2 , comparator  107  will switch low, opening switch  109  and closing switch  110 , causing charge on capacitor  108  to pass to ground. The comparator is also turned off (i.e., output goes low) by the falling edge of clock Φ 2 .  
         [0016]     This process of discharging and recharging capacitor  106  continues for a predetermined period of time. The time taken to recharge capacitor  106  during these cycles will depend on the resistance of memory cell  101  and is reflected in the time period of the “high” and “low” states of comparator  107 . It is noted that each digit line in an array has some inherent capacitance and can be charged by the current conducted through each respective memory cell. Accordingly, capacitor  106  may be a discrete capacitor, parasitic capacitance of the digit line  105  or a combination of the two. In the embodiment shown in  FIG. 1 , switch  110  may also be operated by a complementary non-overlapping clock signal Φ 1  having pulses which interleave those of clock signal Φ 2 .  
         [0017]     The switching and discharging of capacitor  108  is implemented with switches  109  and  110  which, as shown in  FIG. 1 , act together to either connect feedback capacitor  108  to digit line  105  or alternatively connect feedback capacitor  108  to ground (to discharge the capacitor between cycles), depending upon the output state of comparator  107 . Those skilled in the art will appreciate, with the benefit of the present description, that the switching function can be implemented in numerous different circuits and is not limited to two switches. Circuit  100  further includes a cycle counter  130 , controlled by an enable signal (ENABLE) that counts the number of times that the comparator  107  goes high during a predetermined period of time. The count is inversely proportional to the current and thus to the resistance of the memory element  103  being reead.  
         [0018]     A digital value comparison is performed on the value stored in cycle counter  130  by a digital value comparison device  131  to determine if the counted value (and thus the memory cell resistance) is within a specified range. Digital value comparison device  131  performs a comparison of the cycle counter  130  in any one of several ways, for example by comparing the value against a threshold or as a ratio of the number of times the comparator  107  goes high over a total number of sensing cycles. Counter  130  may alternately count the low state of comparator  107 . Counter  130  may also count the amount of time the output of the comparator  107  is high or low based on a sampling rate.  
         [0019]     Still further, the output of comparator  107  can supply “up” and “down”. signals as control inputs to an up/down counter, which counts a clock signal. Here again, the count value in the counter  130  represents the resistance of the memory cell, which can be compared to a threshold to determine if the count value is higher or lower than the threshold and thus provide an indication of the logical state of the memory element  103 . If the sensed resistance in the memory cell is above the threshold, a logic “high” signal is outputted. Conversely, if the sensed resistance is below the threshold, a logic “low” signal is outputted.  
         [0020]      FIG. 2  is a set of timing diagrams for the operation of the integrated charge sensing circuit  100  of  FIG. 1 .  FIG. 2A  shows Φ 1  and Φ 2  as two complementary and non-overlapping clock signals.  
         [0021]     Three distinct examples of the circuit operation are depicted in  FIGS. 2B, 2C  and  2 D. In the bottommost example of  FIG. 2D , the resistance of the memory element  103  is small. In this instance, the voltage on digit line  105  (bold line) is pulled quickly to Vcc/2 because there is very little resistance, which limits how fast digit line capacitor  106  charges. This causes comparator  107  output (COMP OUT) to go high frequently during the charges and discharges of capacitor  106  over a predetermined measurement period. The comparator  107  output, therefore, mimics the clock signal Φ 2 . If the resistance is very small, so low that digit line  105  can never be pulled below the V cc /2−V os  threshold, then the output of comparator  107  will go high every time the comparator is clocked.  
         [0022]     In the middle example of  FIG. 2C , the resistance in the memory cell is very large. In this instance, digit line  105  is quickly pulled low to below V cc /2−V os . Because of the high resistance the digit line charges very slowly back to Vcc/2, which causes comparator  107  output to remain low most of the time.  
         [0023]     In the topmost example of  FIG. 2B , the resistance of the memory cell is in an intermediate range. Comparator  107  performs comparison operations on the rising edge of Φ 2  and a comparison is made between digit line  105  and V cc /2−V os . If digit line  105  voltage is greater than V cc /2−V os , the output of comparator  107  goes high. If digit line  105  voltage is less than V cc /2−V os , the output of comparator  107  remains low. The output of comparator  107  feeds clocked counter  130 . The comparison operation on the rising edge of Φ 2  is indicated by the dotted lines on  FIG. 3 . That is, at the rising edge of the first three pulses of Φ 2 , a comparison is made and the digit line is greater than V cc /2−V os . At the rising edge of the next (fourth) Φ 2  pulse, another comparison is made, and the digit line is less than V cc /2−V os .  
         [0024]     It is noted, in all instances that when a comparator  107  output goes high, current through the digit line capacitor  106  is discharged to capacitor  108 , resulting in a voltage drop at the digit line input of comparator  107 . The current through the resistance of the memory element  103  then pulls the digit line voltage back up towards Vcc/2. As shown in  FIG. 2D , the voltage gets pulled back above V cc /2−V os  quickly, whereas in  FIG. 2C , the resistance is so great that it takes a very long time to pull the voltage up over the threshold level of V cc /2−V os .  
         [0025]     The continuous string of “high” outputs from comparator  107  in the  FIG. 2D  example and the continuous string of “low” outputs from the comparator  107  in the  FIG. 2C  example require a high capacity counting circuit  130 . Since the high and low outputs of comparator  107  are in response to the clock signal Φ 2 , a large number of continuous high and low states indicate that the clock Φ 2  frequency is not well matched to the resistance values of memory element  103 .  
         [0026]     Turning to  FIG. 3 , an exemplary embodiment of the invention is shown in which the output  111  from the  FIG. 1  comparator  107  is being input to 8-bit shift register  300 . Each output ( 303 ,  304 ) from the shift register is coupled to an 8-input AND logic gate  301  and an 8-input NOR logic gate  302 . It is understood that, while an 8-bit configuration is disclosed in the exemplary embodiment, other configurations (e.g., 4 it, 16 bit, etc.) are equally applicable.  
         [0027]     When shift register  300  receives the output  111  from the comparator  107 , the register stores the outputs and transmits them along inputs  304  and  303  to AND gate  301  and NOR gate  302 , respectively. AND gate  301  outputs a high logic signal (“ 1 ”) when all the lines are logic “high,” while NOR gate  302  outputs a high logic signal (“ 1 ”) when all the lines are logic “low.” A high output  305  from AND gate  301  indicates that the comparator  107  is producing a large number of successive high output states (as in  FIG. 2D ) and that the clock speed is too slow. Similarly, a logic “high” output from NOR gate  302  indicates that the comparator  107  is outputting a large number of successive low states (as in  FIG. 2C ) and that the clock speed is accordingly too fast.  
         [0028]     Turning to  FIG. 4 , the outputs ( 305 ,  306 ) of AND gate  301  and NOR gate  302  are sent to counter  400 , which stores an increment/decrement count of the AND and NOR gates  301  and  300 . Counter  400  transmits a count  403  to digital-to-analog (DAC) converter  401 , which converts the digital count value of counter  400  to a an analog control signal  404  which controls oscillator  402 . Oscillator  402  receives the output signal  404 , which indicates to the oscillator whether to increase or decrease the oscillator clock frequency. Oscillator  402  then appropriately adjusts the frequency of a clock output signal which is used to generate the complementary and non-overlapping Φ 1  and Φ 2  clock signals.  
         [0029]     Furthermore, as discussed above in connection with  FIG. 2A , Φ 1  and Φ 2  are two complementary and non-overlapping clock signals. In order to ensure that the clock signals Φ 1  and Φ 2  do not overlap, the output signal  405  is preferably processed through a non-overlapping clock signal generating circuit illustrated in  FIG. 5 .  
         [0030]     Referring to  FIG. 5 , the oscillator clock output  405  is coupled to one terminal of NAND gate  500 . The output signal  405  is also inverted via logic inverter  502  and connected to one terminal of NAND gate  501 . The outputs of NAND gates  500  and  501  are each dually inverted via inverters  503 ,  505  and  504 ,  506 , respectively. The outputs  511  and  512  of the dual inverters ( 503 ,  505  and  504 ,  506 ) are each coupled to a respective output inverter  507  and  508 , and are also fed back respectively, in a cross-coupled fashion, to second terminals of NAND gates  501  and  500 . Inverters  507  and  508  respectively output the non-overlapping clock signals Φ 1  and Φ 2  at each output. Thus, the exemplary embodiments of the invention provides a control of the frequency of clock signals Φ 1  and Φ 2  used to operate the resistive memory circuit  115  to reduce the number of successive high or low states of comparator  107  and thus reduce the required counting capacity of counter  130 .  
         [0031]      FIG. 6  illustrates an exemplary processing system  2000  which utilizes a resistive sensing circuit such having clock frequency control as described in connection with  FIGS. 1-5 . The processing system  2000  includes one or more processors  2001  coupled to a local bus  2004 . A memory controller  2002  and a primary bus bridge  2003  are also coupled the local bus  2004 . The processing system  2000  may include multiple memory controllers  2002  and/or multiple primary bus bridges  2003 . The memory controller  2002  and the primary bus bridge  2003  may be integrated as a single device  2006 .  
         [0032]     The memory controller  2002  is also coupled to one or more memory buses  2007 . Each memory bus accepts memory components  2008 . Any one of memory components  2008  may contain the clock-controlled resistive sensing circuit as described in connection with  FIGS. 1-5 .  
         [0033]     The memory components  2008  may be a memory card or a memory module. The memory components  2008  may include one or more additional devices  2009 . For example, in a SIMM or DIMM, the additional device  2009  might be a configuration memory, such as a serial presence detect (SPD) memory. The memory controller  2002  may also be coupled to a cache memory  2005 . The cache memory  2005  may be the only cache memory in the processing system. Alternatively, other devices, for example, processors  2001  may also include cache memories, which may form a cache hierarchy with cache memory  2005 . If the processing system  2000  include peripherals or controllers which are bus masters or which support direct memory access (DMA), the memory controller  2002  may implement a cache coherency protocol. If the memory controller  2002  is coupled to a plurality of memory buses  2007 , each memory bus  2007  may be operated in parallel, or different address ranges may be mapped to different memory buses  2007 .  
         [0034]     The primary bus bridge  2003  is coupled to at least one peripheral bus  2010 . Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus  2010 . These devices may include a storage controller  2011 , a miscellaneous I/O device  2014 , a secondary bus bridge  2015 , a multimedia processor  2018 , and a legacy device interface  2020 . The primary bus bridge  2003  may also be coupled to one or more special purpose high speed ports  2022 . In a personal computer, for example, the special purpose port might be the Accelerated Graphics Port (AGP), used to couple a high performance video card to the processing system  2000 .  
         [0035]     The storage controller  2011  couples one or more storage devices  2013 , via a storage bus  2020 , to the peripheral bus  2010 . For example, the storage controller  2011  may be a SCSI controller and storage devices  2013  may be SCSI discs. The I/O device  2014  may be any sort of peripheral. For example, the I/O device  2014  may be a local area network interface, such as an Ethernet card. The secondary bus bridge may be used to interface additional devices via a secondary bus  2024  to the processing system. For example, the secondary bus bridge may be an universal serial port (USB) controller used to couple USB devices  2017  via to the processing system  2000 . The multimedia processor  2018  may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to one additional device such as speakers  2019 . The legacy device interface  2020  is used to couple legacy devices  2025 , for example, older styled keyboards and mice, to the processing system  2000 .  
         [0036]     The processing system  2000  illustrated in  FIG. 6  is only an exemplary processing system with which the invention may be used. While  FIG. 6  illustrates a processing architecture especially suitable for a general purpose computer, such as a personal computer or a workstation, it should be recognized that well known modifications can be made to configure the processing system  2000  to become more suitable for use in a variety of applications. For example, many electronic devices which require processing may be implemented using a simpler architecture which relies on a CPU  2001  coupled to memory components  2008  and/or memory devices  2009 . The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices.  
         [0037]     While the invention has been described in detail in connection with preferred embodiments known at the time, it should be readily understood that the invention is not limited to the disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, although the invention has been described in connection with specific circuits employing different configurations of transistor circuits, the invention may be practiced with many other configurations without departing from the spirit and scope of the invention. Furthermore, the circuits of  FIGS. 1 and 3 - 5  could be integrated on a single substrate, together with other appropriate circuitry. It is also understood that the logic structures described in the embodiments above can be replaced with equivalent logic structures to perform the disclosed methods and processes. Accordingly, the invention is not limited by the foregoing description or drawings, but is only limited by the scope of the appended claims.

Technology Category: 3