Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of and claims the benefit of priority under 35 U.S.C. §120, to U.S. patent application Ser. No. 12/017,297, entitled “BITLINE SELECTION CIRCUITRY FOR NONVOLATILE MEMORIES”, filed Jan. 21, 2008, which is a continuation of and claims the benefit of priority under 35 U.S.C. §120 to U.S. application Ser. No. 11/120,894, entitled “BITLINE SELECTION CIRCUITRY FOR NONVOLATILE MEMORIES”, filed on May 3, 2005, which claims the benefit of priority under 35 U.S.C. §119 to French Application No. 0501084, filed on Feb. 3, 2005, all of which are hereby incorporated by reference herein in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to read operations in nonvolatile memories. More specifically, the invention reduces delay in nonvolatile memory read operations by minimizing cross coupling voltage effects between bit lines. 
       BACKGROUND ART 
       [0003]    Nonvolatile memories, known as flash memory devices, have become very popular in a variety of uses including mobile phones, digital answering machines, and personal digital voice recorders. Low pin count, low cost, and ease-of-use are key factors for the wide utilization of flash memory. 
         [0004]    With respect to  FIG. 1 , a prior art flash memory device  100  is composed of a matrix of memory cells. An array of bit lines  110   a - 110   n  connect memory cells to a selection network  120 . An array of word lines  115   a - 115   n  carry selection signals for parallel memory locations. The selection network  120  controls which bit lines  110   a - 110   n  are connected to a sense amplifier  130  for reading. 
         [0005]    With respect to  FIG. 2 , the prior art selection network  120  ( FIG. 1 ) of the flash memory device  100  is a first array of select transistors  210   a - 210   g  connecting bit lines  110   a - 110   g  to a first bank select transistor  215  and a second array of select transistors  210   h - 210   n  connecting bit lines  110   h - 110   n  to a second bank select transistor  225 . Control signals applied to the first bit line select transistor  210   a  and the first bank select transistor  215  allow the sense amplifier  130  to read a memory cell on the first bit line  110   a . Remaining memory cells are selected similarly with the use of an array of word lines (not shown). 
         [0006]    With respect to  FIG. 3 , in a prior art bit line schematic diagram  300 , an array of memory cells  305   a - 305   g  connects to the array of bit lines  110   a - 110   g . Bit lines  110   a - 110   g  have an associated bit line loading capacitance  310   a - 310   g  to ground and a bit line coupling capacitance  320   a - 320   g  between adjacent lines. The bit line select transistors  210   a - 210   g  connect the bit lines  110   a - 110   g  to the first bank select transistor  215 . A control signal applied to the gate of the first bank select transistor  215  connects a selected bit line to the sense amplifier  130 . 
         [0007]    A bit line selection waveform diagram  400  of  FIG. 4  includes a first bit line select pulse  410  applied to a first bit line select transistor  210   a  ( FIG. 3 ) to begin a read operation. The first bit line  110   a  is precharged to a high-voltage level prior to reading a first memory cell  305   a . A first bank select pulse  430  activates the first bank select transistor  215 , connecting the sense amplifier  130  to the first bit line  110   a . If the first memory cell  305   a  is on, the sense amplifier  130  senses the current being drawn through the cell. 
         [0008]    A second bit line select pulse  420  applied to a second bit line select transistor  210   b  begins a path to the second memory cell  305   b . The second memory cell  305   b  is connected through the second bit line select transistor  210   b  and the first bank select transistor  215  to the sense amplifier  130 . Cross coupling between bit lines allows a cross coupling current  330  to flow through the first memory cell  305   a , the first bit line  110   a , the first bit line coupling capacitance  320   a , the second bit line select transistor  210   b , and the first bank select transistor  215  to the sense amplifier  130 . If the second memory cell  305   b  is off and a first memory cell  305   a  is on, this cross coupling path causes a cell-read problem. 
         [0009]    A precharged high-voltage level on the first bit line  110   a  is a remnant from the first read operation. The high-voltage level is discharged through the first memory cell  305   a  resulting in a first bit line voltage response  450 . The first bit line coupling capacitance  320   a  allows a second bit line current response  460  to be produced from the first bit line voltage response  450 . During the cross coupling activity of the second bit line current response  460 , the sense amplifier  130  detects the first memory cell  305   a  being on but the control signals are selecting the second memory cell  305   b  which is off. In this case, incorrect data are read. 
         [0010]    The length of time that the second bit line current response  460  remains above a sense amplifier threshold  464  defines a cross coupling delay  465 . The cross coupling delay  465  is that period of time necessary to delay a read operation for a second memory cell in order to avoid the sense amplifier  130  reading incorrect data. Therefore, reading of the prior art flash memory device  100  is significantly delayed due to a wait period inherent in the cross coupling delay  465  between each read operation. Waiting for the cross coupling delay  465  between each read operation slows down the overall reading of the flash memory device  100  significantly. 
       DISCLOSURE OF INVENTION 
       [0011]    Bit lines of a memory device are arranged by an interleaving of even and odd bit lines and segregated into an even and odd bank. A discharge network discharges the banks alternately. A bit line selection network alternately connects the banks to a sense amplifier. The bank of odd bit lines is discharged just prior to a selection of the bank of even bit lines for reading and vice-versa. 
         [0012]    Interleaving of even and odd bit lines in combination with alternating selection and discharge of banks reduces a cross coupling voltage. A discharge delay ensures that a sense amplifier does not detect any signal during a discharge phase. The discharge delay is much shorter than the cross coupling delay required with no discharge scheme present. Discharging complementary banks of bit lines ensures that along with a short access time, correct data are detected by the sense amplifier. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a block diagram of a prior art flash memory device incorporating a selection network. 
           [0014]      FIG. 2  is a block diagram of the prior art selection network of  FIG. 1 . 
           [0015]      FIG. 3  is a block diagram of a single bank of the prior art selection network of  FIG. 2  indicating coupling capacitance and cross coupling current. 
           [0016]      FIG. 4  is a waveform diagram of a prior art bit line and bank selection process of the block diagram of  FIG. 3 . 
           [0017]      FIG. 5  is a block diagram of a selection network of the present invention. 
           [0018]      FIG. 6  is a waveform diagram of the present invention with a bit line and bank selection process of the block diagram of  FIG. 5 . 
           [0019]      FIG. 7  is an exemplary process flow diagram of the present invention in a sequential read operation incorporating an alternating discharge scheme. 
           [0020]      FIG. 8  is an exemplary process flow diagram of the present invention in a sequential read operation incorporating a previous location discharge scheme. 
           [0021]      FIG. 9  is an exemplary process flow diagram of the present invention in a sequential read operation incorporating an adjacent locations discharge scheme. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    With reference to  FIG. 5 , a bank of odd bit lines  505   a - 505   n  and a bank of even bit lines  515   a - 515   n  feed into an exemplary bit line selection network  500  of the present invention. Even and odd bit lines from the two banks are interleaved. Odd selection transistors  510   a - 510   n  connect the bank of odd bit lines  505   a - 505   n  to an odd junction bus  550 . Even select transistors  520   a - 520   n  connect the bank of even bit lines  515   a - 515   n  to an even junction bus  560 . An even bank select transistor  540  connects the even junction bus  560  to a sense amplifier  595 . An odd bank select transistor  530  connects the odd junction bus  550  to the sense amplifier  595 . An odd bank discharge transistor  575  connects the odd junction bus  550  to ground. The even junction bus  560  is connected to ground by an even bank discharge transistor  585 . 
         [0023]    With reference to  FIG. 6 , an even bank select pulse  640 , of an exemplary bit line selection waveform diagram  600 , controls selection of the even junction bus  560  ( FIG. 5 ). The bank of even bit lines  515   a - 515   n  is selectable when the even bank select pulse  640  is applied to the even bank select transistor  540 . An odd bank select pulse  630  applied to an odd bank select transistor  530  selects the odd junction bus  550 . A control signal (not shown) applied to the gates of the odd select transistors  510   a - 510   n  connects the bank of odd bit lines  505   a - 505   n  to the odd junction bus  550 . 
         [0024]    A control signal applied to the odd select transistors  510   a - 510   n  and an odd bank select-bar pulse  670  applied to the odd bank discharge transistor  575  discharges the bank of odd bit lines  505   a - 505   n . Alternatively, the adjacent two odd bit lines of an even bit line to be read may be selected for discharge. The odd bank select-bar pulse  670  is the complement of the odd bank select pulse  630 . Therefore, the bank of odd bit lines  505   a - 505   n  discharges when the bank of odd bit lines  505   a - 505   n  is not selected. An even bank select-bar pulse (not shown) operates similarly in comparison with the even bank select pulse  640 , the even select transistors  520   a - 520   n , and the bank of even bit lines  515   a - 515   n . 
         [0025]    The sense amplifier  595  ( FIG. 5 ) drives a first bit line voltage response  650  high during the time the bit line is selected for reading which is defined by a first bit line select pulse  610 . The sense amplifier  595  performs a read operation by sensing the current in the first bit line  505   a  while biased at a high voltage condition. At the end of the read operation, the odd bank select-bar pulse  670 , driving the odd bank discharge transistor  575  and a control signal to the odd select transistors  510   a - 510   n , connects the first bit line, along with the remainder of the bank of odd bit lines  505   a - 505   n , to ground. The falling edge of the first bit line voltage response  650  depicts the discharge transition for the bank of odd bit lines  505   a - 505   n . 
         [0026]    During the discharge of the bank of odd bit lines  505   a - 505   n , a second bit line current response  660  is detected if the sense amplifier  595  is enabled during this discharge period. The second bit line current response  660  may ascend through a sense amplifier threshold  664 . Detection of this condition by the sense amplifier  595  indicates a conducting condition in the memory cell addressed on the second bit line. The width of this pulse in the second bit line current response  660  is a discharge delay  665  that defines an amount of time necessary to discharge any bit lines which may cause a cross coupling problem with the bit line about to be read. The discharge delay  665  is also a minimum of time required for delaying a second bit line select pulse  620  and for delaying activation of the sense amplifier  595  to read a succeeding location. 
         [0027]    A bit line select delay  625  is defined to be greater than a worst-case value expected for the discharge delay  665 . The bit line select delay  625  defines an amount of time the second bit line select pulse  620  (or any even bit line select pulse) is offset from application of the even bank select pulse  640 . The bit line select delay  625  identically defines an amount of time the first bit line select pulse  610  (or any odd bit line select pulse) is offset from the odd bank select pulse  630 . After the bit line select delay  625  has elapsed and the second bit line select pulse  620  is applied, the sense amplifier  595  is activated and reads the correct value within a memory cell on the second bit line  515   a . 
         [0028]    With reference to  FIG. 7 , an exemplary process flow diagram of an alternating bit line reading process  700  begins  705  a read operation at an even address with discharging  710  the bank of odd bit lines before selecting  720  the bank of even memory locations. The process  700  continues with selecting  730  an even bit line and reading  740  an even location memory cell. A determination  745  is made whether any additional memory location is to be read. If no additional memory location is to be read, the process  700  ends. 
         [0029]    If a succeeding memory location is to be read the process continues with discharging  750  the bank of even bit lines and selecting  760  the bank of odd memory locations. The process continues with selecting  770  an odd bit line and reading  780  an odd location memory cell. A determination is made whether there is an additional memory location to read  785 . If an additional memory location is to be read, the process iterates beginning with the discharging  710  of the bank of odd bit lines. Otherwise the process ends. For beginning  747  a read operation at an odd address the process commences with discharging  750  the bank of even bit lines and continues as discussed supra. 
         [0030]    With reference to  FIG. 8 , an exemplary process flow diagram of a sequential read process  800  begins with reading  810  a first memory location on a first bit line and determining  820  whether an additional memory location is to be read. If there is no further memory location to be read the process ends. If there is a further memory location to be read, the process continues with selecting  830  a subsequent bit line and discharging  840  a bit line that immediately precedes the selection in time. The process proceeds with reading  860  the additional memory location. The process resumes with again making the determination  820  whether an additional memory location is to be read and proceeding accordingly. 
         [0031]    With reference to  FIG. 9 , an exemplary process flow diagram of a sequential read process  900  begins with reading  910  a first memory location on a first bit line and determining  920  whether an additional memory location is to be read. If there is no further memory location to be read the process ends. If there is a further memory location to be read, the process continues with selecting  930  a subsequent bit line and discharging  940  an immediately preceding bit line position and an immediately succeeding bit line position. The process proceeds with reading  960  the additional memory location. The process resumes with again making the determination  920  whether an additional memory location is to be read and proceeding accordingly. 
         [0032]    In further regard to the exemplary process flow diagram of  FIG. 9 , a characterization is made by two even select transistors  520   b ,  520   c  ( FIG. 5 ) being selected to discharge two even bit lines  515   b ,  515   c  adjacent to an odd bit line  505   c  before the odd bit line  505   c  is read. An analogous situation is true for reading an even bit line. 
         [0033]    In an exemplary read process where two consecutive addresses to be read (not shown) are even (or odd), the first bit line read does not need discharging before reading the second bit line since the interleaved layout of even and odd bit lines prevents any coupling effects from causing a problem. 
         [0034]    The use of segregation of bit lines into banks of even and odd bit lines and alternating the reading and discharging of the banks reduces the voltage potential for coupling on adjacent bit lines. This ensures that the magnitude of the bit line select delay  625  with the present invention is significantly reduced from the cross coupling delay  465  ( FIG. 4 ) in the prior art bit line selection network where discharging is not incorporated. A similar reasoning holds for discharging the just prior memory location from the location to be read. 
         [0035]    While the present invention has been described in terms of the use of a sensing means for reading operations, a skilled artisan in this field would readily identify the suitability of using a voltage comparator circuit, latch, sense amplifier, or cross coupled inverters to provide similar sensing capabilities. An apparatus for selection of bit lines has been described using single transistor devices in series between points to be coupled electrically. A person of skill in the art would also consider the use of a matrix of transmission gates, a crossbar switch, or a multiplexer for the same coupling purposes.

Technology Category: g