Abstract:
A flash memory using a pre-sensing amplifier coupled to receive differential inputs from a pair of memory cells of said flash memory array and to generate a differential output from the pre-sensing amplifier. The differential output is coupled to a bus, which is also coupled to a post-sensing amplifier. The differential configuration on the bus allows marginal voltage differences to be detected by the post-sensing amplifier so that logic states from the flash memory can be sensed without the bus transitioning to half of the supply voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of memories and more particularly to a technique of providing a differential signal output from a flash memory. 
     2. Description of the Related Art 
     One type of non-volatile electrically erasable and electrically programmable read-only semiconductor memory is commonly referred to as a flash memory. Once programmed, the flash memory retains the program data until the memory is erased. In a typical flash memory structure, several blocks of flash memory are configured together on a flash memory device. A variety of flash memory devices are known in the art, but generally, a flash cell is comprised of a metal oxide semiconductor (MOS) transistor which includes an isolated or floating gate that is programmed typically by electron injection from the channel. 
     In one typical configuration, a flash cell is programmed by applying a high voltage (such as 12 volts) on the control gate, 0 volts on the source and an intermediate voltage such as 6 volts on the drain. A channel-hot-electron injection causes the isolated or floating gate to be negatively charged. The charged floating gate causes the threshold voltage (V t x) of the device to increase. Thus, a programmed cell requires a higher threshold voltage to turn the transistor on as compared to an erased cell. In a read operation, generally, the source is grounded and a read voltage, such as 5 volts, is applied to the control gate and the output is determined at the drain. The amount of the read current at the drain determines if the device is programmed or not programmed. In order to erase the programmed cell, the drain is made to float while a voltage is impressed across the source and the control gate, such as 12 volts on the source with a grounded control gate or 5 volts on a source with a negative voltage on the control gate. When the cell is being erased, charges are removed from the floating gate to the source terminal so that the threshold voltage of the device is reduced. 
     In flash memories, sensing amplifiers are utilized to read the content of the flash memory arrays. These sensing amplifiers generally provide single-ended outputs onto an output bus. A common problem with single-ended outputs is that the output typically swings from ground to a rail voltage, such as V cc . When additional memory cells are introduced, which outputs are impressed on the output line, the loading increases the capacitance coupled to the output lines. This loading, along with the significant voltage swings impacts the performance of the memory. Thus, performance, such as the speed in reading an output from a flash cell and the circuit power requirements, can degrade as more load is placed on the output. 
     What is needed is a technique for addressing this performance factor in flash memories. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block schematic diagram showing a flash memory device implementing the present invention in which differential sensing outputs are obtained from multiple memory strips containing flash memory. 
     FIG. 2 illustrates one embodiment of the present invention in which a differential pre-sensing amplifier is used to drive differential output lines. 
     FIG. 3 illustrates an expanded circuit diagram for the embodiment of FIG.  2 . 
     FIG. 4 is a timing diagram showing the timing of the various signal nodes illustrated in FIG.  3 . 
     FIG. 5 is a block schematic diagram showing one system utilization for the flash memory device incorporating the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, one implementation of the present invention is shown in which the particular example involves a flash memory device  10 . The flash memory is a non-volatile electrically erasable and electrically programmable read-only-semiconductor memory. The example memory device  10  is comprised of a number of memory strips  11 , wherein each memory strip  11  includes one or more flash memory block(s)  12 . Generally, each memory block  12  is configured as an array or arrays and functions equivalently to various flash memory devices currently known in the art. The memory blocks  12  can be programmed, read and block erased according to practices known in the art pertaining to flash memory devices. In one particular embodiment, two blocks  12  are present on each strip  11  and are commonly referred to as an odd block and an even block. Each strip  11  also has a local logic circuit  13  which includes various circuitry required to access, program, read and erase memory cells of the memory block(s)  12  on each strip. A variety of circuits can be implemented to provide these functions, including those circuits known in the art. 
     Also present on each memory strip  11  is a pre-sensing amplifier (pre-sense amp)  15  which includes circuitry for practicing the present invention. A pair of data lines  16  and  17  are also shown coupling outputs of the memory blocks  12  to the sense amplifier  15 . In typical flash configurations, data line  16  is typically comprised of a plurality of lines “n” and the lines are referred to as bit lines which emanate as column lines from an array of main memory cells, while lines  17  (line  17  is also comprised of multiple lines) are from reference cells of the flash memory array. The general configuration is not critical to the understanding of the present invention other than that two read lines pertaining to a given memory cell array are coupled to the pre-sense amplifier  15  to determine if the particular flash memory cell being read is either in a programmed state or an erased state. 
     In the flash memory device  10  shown in FIG. 1, the outputs of the pre-sense amplifier  15  are coupled on a pair of differential signal lines (also referred to as bus, bus lines, or sense output)  18  and  19  to a post-sensing amplifier (post-sense amp)  20 . It is appreciated that the memory device  10  may have only one memory strip  11  or it may have a plurality of memory strips  11 , in which the outputs from each pre-sensing amplifier  15  are coupled to the post-sensing amplifier  20  on the bus lines  18  and  19 . For example, in one embodiment, each of the memory strips includes eight megabits (8 M) of flash memory, such that the number of strips will determine the total memory included within the flash memory device  10 . For example, if four strips are present, then 32 M of memory would be available in device  10 . Similarly if eight such strips  11  are present in device  10 , then 64 M of memory would be available. 
     Also, it is to be noted that each strip will have a number of pre-sense amps  15 , which number depends on the number of data lines being read. For example, if the memory block is a 64 bit array (that is n=64), then there will be at least 64 pre-sense amps  15  on that strip  11 . Similarly, the number of differential output line pairs  18 ,  19  will also correspond to the number of data line pairs  16 ,  17  and the number of post-sense amps  20  will also correspond to the number of line pairs  18 ,  19 . 
     As will be described below, each pre-sense amp  15  receives the output of the memory block as a differential input and provides a differential output on the bus pairs  18  and  19 . Each post-sensing amplifier  20  receives differential inputs from the line pairs  18  and  19  and generally generates a single-ended output on bus  21  as an output from the memory device  10 . Again, the total number of data lines  21  is dependent on the number of post-sense amps  20  that are present. In many applications, the output on lines  21  from the memory device  10  are coupled to various other devices, including a processor which utilizes the data output from the memory device  10 . The output from the post sensing amplifier  20 , although could be a differential output, is shown in FIG. 1 as a single-ended output, so that device  10  can readily replace existing flash memory devices which provide a single-ended output to other chips which utilize the flash memory. 
     Also referring to FIG. 2, the circuitry utilized in the pre-sensing amplifier  15  and the post-sensing amplifier  20  are shown. Each pre-sensing amplifier  15  includes a differential amplifier  25  coupled to receive the outputs from the memory array as differential inputs when a given memory block  12  is sensed. The sensing of the memory array is essentially a read operation in which the content of the addressed memory cells are read. Flash memories generally are read to determine if the particular memory cell is programmed or erased. In one embodiment, the input to a given differential amplifier  25  is obtained by having an output from a memory cell being read on one of the input line (for example, line  16 ) while an output from a reference cell (having a known reference value) is read on the other corresponding line (for example, line  17 ). 
     The amplifier  25  outputs a differential signal on to the line pair  18 ,  19  through transistors  26 ,  27 . In the particular example, the outputs of the differential amplifier  25  are coupled to the gates of transistors  26 ,  27 , which in this example are P-type devices. Transistors  26 ,  27  operate as pull-up transistors to drive the differential read signal from the memory cells onto the output lines  18 ,  19 . As noted in FIG. 2, each of the pre-sense amps  15  has the output coupled onto line pairs  18 ,  19  through a pair of P-type transistors when the pre-sense amp  15  is activated by an enable signal, EnablePre#. 
     Each post-sensing amplifier  20  includes an amplifier  30  which has its inputs coupled to the line pairs  18 ,  19 . The output of the amplifier  30  is coupled through a driver  33  to provide an output on line  21 . It is appreciated that the output of the amplifier  30  could be made a differential output, but is shown as a single-ended output so that the flash device  10  could readily be substituted for a variety of single ended flash memory devices currently in use. 
     As shown in FIG. 2, the enable signal EnablePre# is used to enable the differential amplifier  25  in the pre-sense amp  15 . Equivalently, an enable signal, EnablePost#, is used to enable amplifier  30  in the post-sense amp  20 . The two enable signals, which are described in more detail below, are used to enable each of the sense amps  15 ,  20  respectively, so that these amplifiers  15 ,  20  operate as a latch in latching the memory output onto the line pairs  18 ,  19  and then to the output line  21 . It is also appreciated that the “#” sign is utilized herein to denote either a compliment condition (such as activation on low signal condition) and/or the complimentary line of the differential pair. Also referring to FIGS. 3 and 4, FIG. 3 shows a more detailed circuit diagram for the pre-sense amplifier  15  and the post-sense amplifier  20 . FIG. 4 shows waveform diagrams pertaining to signals on nodes associated with the sense amplifiers  15 ,  20 . 
     The embodiment shown in FIG. 3 is better understood when referenced with the timing diagram of the various signals (shown in FIG.  4 ). A particular flash memory cell which is to be read for its content is shown as transistor  40 . Transistor  40  is a typical flash memory cell having a floating gate, control gate, drain and source. Transistor  41  depicts a reference cell for setting a reference level for reading transistor  40 . The drains of both of the transistors (also referred to herein as cells)  40 ,  41  are coupled to respective differential inputs of the amplifier  25  in the pre-sense amp  15 . In the timing diagram, a read access commences at time T 1 . 
     In the particular embodiment shown, during time T 1 , the local bitlines are discharged, in which SLSOUT and SLSOUT# signal nodes are grounded. Both of the sense amplifiers  15 ,  20  are disabled. During time T 2 , the differential input lines  16 ,  17  noted by signal SIN (sense in) and its differential counterpart RIN (reference in) are precharged to V cc  through resisters  42 ,  43  each respectively coupled to the drains of the memory cells  40 ,  41 . The SIN and RIN signals are respectively coupled through P-type transistors  44 ,  45  to provide SLLAT and SLLAT# signals, which are respectively coupled to the gates of the P-type transistors  26 ,  27 , which outputs are then coupled to the line pairs  18 ,  19 . The signal onto line pairs  18 ,  19  are denoted as SLSOUT and SLSOUT#, respectively. 
     The enabling and disabling of each of the sense amplifiers  15 ,  20  are controlled by the EnablePre# and EnablePost#. The EnablePre# signal activates N-type transistor  50  which allows N-type transistors  51  and  52  to conduct. Thus, as shown in FIG. 4, through time T 2 , the signal nodes for SIN, RIN, SLLAT and SLLAT# are precharged to a V cc  level, while signal nodes for SLSOUT and SLSOUT# are at or near zero. 
     At some point after the commencement of the read sequence during time T 3 , a differential voltage develops between the SIN and RIN signals. A difference voltage (or margin) is noted due to the conduction of transistors  40  and  41  during T 3  when the control gates of these respective transistors turn on the transistors  40  and  41 . The difference in the threshold voltages of the transistors  40 ,  41  cause the transistors to have different voltages at the SIN/RIN differential inputs to the pre-sense amplifier  15 . This difference in the voltage is exemplified in the SIN/RIN signals at time T 3 . Since the SIN/RIN inputs are shorted through the transistors  44 ,  45  to the SLLAT/SLLAT# nodes respectively, when the EnablePre# goes low at the end of time T 3 , the SLLAT and the SLLAT# nodes are disconnected from the SIN/RIN inputs and the sense amplifier  15  amplifies the difference between the SLLAT and SLLAT# differential nodes. 
     At the same time that the pre-sensing amplifier  15  is enabled by EnablePre#, the output lines  18 ,  19  are removed from a grounded state to a tri-state condition. Although not shown, generally a device grounds lines  18 ,  19  to maintain the lines  18  and  19  at a disabled or zero volt condition until the end of period T 2 . Once the lines  18 ,  19  are tri-stated, they are now available to respond to the differential signal from the activated pre-sense amp  15 . The voltages on SLLAT/SLLAT# turn on the P-device output drivers  26 ,  27  to provide a differential output on lines  18 ,  19 . Generally one of the P-devices  26  or  27  will be barely on while the other will have full VCC potential across its gate. This difference in drive causes a different ramp rate on SLSOUT and SLSOUT#, allowing a differential voltage to develop across and latch onto these nodes, as shown in time T 4 . The differential signal is sensed and latched during time period T 5  by the post-sense amp  20  when Enable Post# is driven to ground. The latched output of the post-sense amp  20  is illustrated by the signals SLLATX and SLLATY during time T 5 . 
     As is described above, during time T 4  when the pre-sense amp  15  is activated, the difference in voltage sensed by the differential inputs SIN and RIN are amplified as noted by signals SLLAT and SLLAT#. The bus lines  18 ,  19  coupling the output of all of the sense amplifiers  15  will respond by noting the difference of the differential voltage SLLAT and SLLAT#. However, since the SLSOUT signal is referenced to its compliment and sensed differentially, the differential lines  18 ,  19  need only transition a smaller voltage before it is sensed by the post sensing amplifier  20 . This is exemplified by the difference in the amplitude of the SLSOUT and SLSOUT# signal during time T 4 . 
     Generally, a difference in the range of 50-200 millivolts between SLSOUT and SLSOUT# is adequate for providing the sensing output. The sensing could be available at a lower limit such as at 20 millivolts, but care must be taken that at the time of sensing the signal, an adequately measurable amplitude difference is present to ensure that a valid read is obtained from the memory cell. Furthermore, the sensing can be obtained at a higher voltage beyond 200 millivolts, but it could detract from the performance of the device if longer time is required to sense the voltage. Accordingly, a range of 50-200 millivolts provides adequate differential voltage to obtain proper reading of the memory cell, but with a minimal time period once the signals difference is noted. 
     As noted, since the power required to charge the output node of the pre-sense amp  15  is proportional to the magnitude of the transition voltage that it transitions and the capacitance of the node, limiting the voltage swing on lines  18 ,  19  reduces the amount of power required to put an output onto lines  18  and  19 . 
     In the example illustrated, the memory cells provide a 64 bit output such that there will be 64 such pre-sensing amplifiers  15  for each of the memory strips  11 . Accordingly, when small transitions are encountered with the differential sense amp of the present invention, smaller power requirements per line will make a significant difference in the total power when 64 lines are involved. In the particular example of a 64 bit data bus, the differential output onto bus lines  18 ,  19  can provide approximately 2 milliamp of read power savings which in some instances may be 10 percent of the total read power required for the device. 
     Furthermore, the limited voltage swing of the differential bus  18 ,  19  can also provide a performance benefit in the area of speed performance. Since the signal is referenced to its differential compliment it can be timed to be sensed when it has transitioned approximately 50-200 millivolts. This is in contrast to a digital signal which typically transitions at one half of the power supply voltage (or rail voltage) on average in order to sense valid data from a memory cell. The reduced transition time for sensing the memory cell provides considerable improvement in the performance of the memory device  10 . 
     Furthermore, since the sense amplifiers are placed on each individual strip  11 , only one overall post-sensing amplifier  20  per line is required for all of the memory strips  11 . Since the output driver of the pre-sensing amplifier  15  is a single P-device for each line (shown by transistors  26  and  27 ) the circuitry is considerably smaller than the alternative implementation known in the prior art, which typically uses a tri-state inverter which is adequately large to drive several picofarads of load. In the present case, the P-type transistors  26  and  27  are of smaller capacitance and provides a much smaller load on each of the sense amplifiers  15 . 
     It is also to be noted that in the embodiment shown in FIG. 3, a multiplexor  60  is utilized to select one of the SLLATX or SLLATY from the post-sense amp  20  to provide a single-ended output from amp  20 . A Select signal is used to select the output. Although a particular circuit is shown for the post-sense amp  20 , a variety of prior art sense amps utilized for sensing outputs from memories can be used for post-sense-amp  20 . 
     Referring to FIG. 5, it is appreciated that the flash memory device  10  of the present invention can be utilized in a variety of different capacities. One system is shown in FIG. 5 in which the flash memory device  10  is coupled to a processor  50 . The processor controls the operation of the flash memory  10  as well as utilizing the data present in the flash memory  10 . In general application, the processor along with the flash memory is coupled to other devices (here shown as I/O, input/output) to exemplify a system in which a processor  50  and flash memory  10  are utilized. A variety of other configurations and systems can be devised in order to utilize the flash memory device  10  incorporating the present invention. 
     Thus, a differential signal path for high speed data transmission in flash memory is described.