Abstract:
A system for reading data in a memory cell includes three comparators, each of which has two inputs. A first reference cell having a low reference voltage is coupled to one input of the first comparator. A second reference cell having a high reference voltage is coupled to one input of the second comparator. A memory cell having a memory cell voltage is coupled to the other input of the first and second comparators. One input of the third comparator is coupled to the first comparator&#39;s output signal, which includes a difference voltage between the memory cell voltage and the low reference voltage. The other input of the third comparator is coupled to the second comparator&#39;s output signal, which includes a difference voltage between the memory cell voltage and the high reference voltage. A method and apparatus for reading data in a memory cell also are described.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to digital memory systems and, more particularly, to methods and systems for reading data stored in a memory cell. 
     2. Description of the Related Art 
     Memory systems typically include an array of separate memory cells. Each memory cell stores one data bit (i.e., a “1” or a “0” state). In an EPROM such as an EEPROM, a flash EPROM, or a flash EEPROM, the data stored in each memory cell must be verified. One method of verifying the contents of the data stored in each memory cell is to compare a cell output voltage of the memory cell to a reference output voltage of a reference cell. The reference cell voltage may be the equivalent of a “1” or a “0” state. The cell output voltage is compared to the reference output voltage. If the cell output voltage is the same as the reference output voltage, then the memory cell is verified as having the same state as the reference cell. The state of the memory cell is then compared to the data that is intended to be stored in the memory cell. If the memory cell has the correct state, then a next memory cell is similarly tested. If the memory cell does not have the correct state, then the memory cell must be reprogrammed. 
     One of the problems of the above process is that, as semiconductor device structures have become smaller, the speeds of the devices have increased, and the operating voltages have been reduced. For example, in many early generations of semiconductor devices, a “1” state was represented by a 5 VDC output voltage and a “0” state was represented by a 0 VDC (i.e., ground) output voltage. In more recent device structures a “1” state has been represented by a 1 VDC or even less (e.g., 0.6 VDC) output voltage, while a “0” state still has been represented by a 0 VDC (e.g., ground) output voltage. Further, the 0 VDC can often be slightly above ground potential such as 0.1 VDC. As the voltage difference between a “1” state and a “0” state has decreased, the process of determining whether a given device is in a “1” or “0” state becomes more finite and typically slower. The process has become more finite because the voltage difference is small (e.g., less than 1 VDC) and therefore requires very specific measurement. Because the process is more finite and because the voltage is so small, the process also has become slower. The output voltage typically must be allowed to rise to a near maximum voltage before the output voltage can be accurately measured. 
     FIG. 1 is a schematic diagram of a prior art circuit  100  for comparing a single reference cell  20  to a memory cell  10 . The memory cell  10  generates a memory cell current when a gate potential is applied to the memory cell&#39;s word line. The memory cell current is compared to a current from a reference cell  20  by the comparator  30 . Typically, EPROMs employ a column of UV-erased cells, which are identical in structure to the memory cells and act as the reference cells. The comparator  30  determines whether the memory cell  10  being verified is drawing more or less current than the reference cell  20 , which is weighted in some relationship to the memory cell  10 . In doing so, the comparator  30  verifies the program state of the memory cell  10 . 
     As both the memory cell  10  and the reference cell  20  of the typical EPROM are UV-erased, each has a different distribution of currents. Normally, this difference in distribution prevents the currents from being compared directly because of the possibility that an erased memory cell being verified could appear to be programmed and vice versa. To resolve this problem, a resistive load (such as R ref ) is used to effectively divide or weight the reference current, I ref . The typical load used is one-half or one-third that of the load R cel  for the memory cell  10 , resulting in a 2 to 1 or 3 to 1 load ratio. Currents also have been compared using other load ratios. 
     In FIG. 1, memory cell  10  is a transistor that represents a typical array memory cell such as in a “flash” EPROM. The memory cell  10  is coupled to a positive input  31  of comparator  30  via line  41 . A potential applied to the gate of memory cell  10  puts the cell into conduction, provided the potential is greater than the cell&#39;s threshold potential, V t1(cel) . Reference cell  20  is the reference cell for memory cell  10  and is used to produce a reference current, I ref , which is used to determine the presence of a charge in the memory cell  10 . The reference cell  20  is coupled to the negative input  32  of comparator  30  via line  42 . A potential applied to the gate of reference cell  20  puts the reference cell into conduction if the potential is greater than its threshold potential, V t1(ref) . When the program state of memory cell  10  is being verified, a gate potential, V WL1(cel) , is applied to the memory cell  10  and a gate potential, V WL1(ref) , is applied to reference cell  20  to produce a memory cell current, I cel , and a reference cell current, I ref , respectively. When currents I cel  and I ref  are conducting, array side load resistance R cel    11  and reference cell side load resistance R ref    21  create voltages V+ and V−, respectively. Voltages V+ and V− represent the input voltages to comparator  30 . 
     If both cells  10  and  20  are conducting, then the input voltages to comparator  30  are depicted by the following approximate or first order equations: 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 V+ = I cel R cel  = (½) beta (V WL1(cel)  − V t1(cel) )R cel   
                 (1) 
               
               
                   
                   
               
               
                   
                 V− = I ref R ref  = (½) beta (V WL1(ref)  − V t1(ref) )R ref   
                 (2) 
               
               
                   
                   
               
             
          
         
       
     
     The output signal of the comparator  30 , CPout, changes state or “trips” when: 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
             
               
                   
                 V+ and V− are equal: 
                   
               
               
                   
                 (V W1(cel)  − V t1(cel) )R (cel) = (V   WL1(ref)  − V t1(ref) )R (ref)   
                 (3) 
               
               
                   
                   
               
             
          
         
       
     
     As described above, the comparator  30  amplifies the difference between the V+ and V−. If the memory cell  10  is conducting and the reference cell  20  is not conducting, then the difference output from the comparator  30  can still be quite small and therefore slow to change state. As a result, verifying each of the many thousands of memory cells in an entire programmed memory array will require an excessive amount of time. 
     Therefore, in view of the foregoing, what is needed is a method and apparatus for quickly and accurately verifying the programmed state of each memory cell in a programmed memory array. 
     SUMMARY OF THE INVENTION 
     Broadly speaking, the present invention fills these needs by using dual reference cells to read or verify data in a memory cell. By way of example, the present invention may be implemented in the form of a system, an apparatus, a method, a device, or a computer readable media. 
     In accordance with one aspect of the present invention, a system for reading data in a memory cell is provided. This system includes first, second, and third comparators, each of which has a first input and a second input. A first reference cell having a low reference voltage is coupled to the first input of the first comparator. A second reference cell having a high reference voltage is coupled to the first input of the second comparator. A memory cell having a memory cell voltage is coupled to the second input of both the first comparator and also the second comparator. The first input of the third comparator is coupled to an output signal of the first comparator, which includes a difference voltage between the memory cell voltage and the low reference voltage. The second input of the third comparator is coupled to an output signal of the second comparator, which includes a difference voltage between the memory cell voltage and the high reference voltage. 
     In one embodiment, the output signal of the third comparator is a representation of data stored in the memory cell. In one embodiment, the low reference voltage is substantially equal to the memory cell voltage when the memory cell is in a low voltage state. In one embodiment, the high reference voltage is substantially equal to the memory cell voltage when the memory cell is in a high voltage state. In one embodiment, the first reference cell is coupled to ground such that the low reference voltage is substantially equal to the ground potential. In one embodiment, the first reference cell includes a first voltage divider circuit and the second reference cell includes a second voltage divider circuit. 
     In accordance with another aspect of the present invention, an apparatus for reading data contained in a memory cell is provided. This apparatus includes a first reference cell having a high threshold voltage for providing a low reference current. A second reference cell having a low threshold voltage provides a high reference current. A first load receives the high reference current and outputs a first reference voltage. A second load receives the low reference current and outputs a second reference voltage. A first comparator receives the first reference voltage and a memory cell voltage and generates an output signal. A second comparator receives the second reference voltage and the memory cell voltage and generates an output signal. A third comparator receives the output signals from the first and second comparators and generates an output signal. 
     In one embodiment, the output signal from the third comparator represents data contained in the memory cell. In one embodiment, the first and second loads include at least one resistor. In one embodiment, the first and second loads include at least one capacitor. 
     In accordance with yet another aspect of the present invention, a method for reading data in a memory cell is provided. In this method a first reference cell voltage is compared with a memory cell voltage from a memory cell to produce a first output signal. A second reference cell voltage is compared with the memory cell voltage to produce a second output signal. The first output signal is then compared with the second output signal to produce a third output signal. 
     In one embodiment, the first reference cell is a low reference voltage and the second reference cell is a high reference voltage. In one embodiment, the low reference voltage is substantially equal to the memory cell voltage when the memory cell is in a low voltage state. In one embodiment, the low reference voltage is substantially equal to a ground potential. In one embodiment, the third output signal is a representation of data in the memory cell. In one embodiment, the first reference cell includes a first voltage divider circuit and the second reference cell includes a second voltage divider circuit. 
     One advantage of the present invention is that the representation of the stored data is amplified by combining the differences between the memory cell state and both a reference “1” state and also a reference “0” state. The resulting representation of the stored data can be detected more accurately and more quickly than in conventional systems and methods that use only a single reference cell. 
    
    
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the principles of the invention. 
     FIG. 1 is a schematic diagram of a prior art circuit for comparing a single reference cell to a memory cell. 
     FIG. 2 is a schematic diagram of a circuit for verifying data in a memory cell using two reference cells in accordance with one embodiment of the present invention. 
     FIG. 3 is a graph of the relationship of signals ΔCP 1  and ΔCP 2  when the memory cell has a high threshold voltage. 
     FIG. 4 is a graph of the relationship of signals ΔCP 1  and ΔCP 2  when the memory cell has a low threshold voltage. 
     FIG. 5 is a schematic diagram of a circuit for verifying data in a memory cell using two reference cells in accordance with another embodiment of the present invention. 
     FIG. 6 is a schematic diagram of a circuit for verifying data in a memory cell using two reference cells in accordance with yet another embodiment of the present invention. 
     FIG. 7 is a flowchart diagram that illustrates the method operations performed in reading a non-volatile memory with dual reference cells in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Several exemplary embodiments for using dual reference cells to read or verify data in a memory cell will now be described. It will be apparent to those skilled in the art that the present invention may be practiced without some or all of the specific details set forth herein. 
     Memory system manufacturers are under constant pressure to increase memory speed so that a memory access (e.g., read/write) does not delay or stall a computer system operation. One aspect of increasing memory access speed is the time required for storing and verifying data stored in non-volatile memory. Prior art systems and methods of verifying data are typically relatively slow, as discussed above with reference to FIG.  1 . 
     Increasing the verification speed can allow non-volatile memory systems, such as flash-type memory systems, to be used in applications that require faster overall memory access time. Thereby more applications can exploit the benefits of non-volatile memory (e.g., maintaining data integrity without requiring power, etc.) without suffering excessive memory access delays. For example, many devices such as digital cameras and personal digital assistants (PDAs) use portable memory cards. The portable memory cards typically include flash memory-type memory systems. A portable memory card with a faster access and verification speed allows a first device (e.g., a digital camera) to quickly store and verify data on the memory card and thereby respond more quickly and allow a user to use the camera more quickly (e.g., take pictures in rapid succession). When the user is finished taking pictures, the memory card can be removed from the digital camera and can be coupled to a second device (e.g., a personal computer). The personal computer can then access and manipulate the data (e.g., digital images) stored on the memory card. 
     FIG. 2 is a schematic diagram of a circuit  200  for verifying data in a memory cell using two reference cells in accordance with one embodiment of the present invention. Memory cell  202  outputs a cell current I ce , through output conductor  212  to node  222 . Capacitor C cel  is coupled between node  222  and ground. Node  222  couples cell voltage V cel  to node  228  through conductor  227 . Node  228  couples cell voltage V cel  to the positive input  242  of comparator CP 1  through conductor  232 . Node  228  also couples cell voltage V cel  to the negative input  248  of comparator CP 2  through conductor  238 . 
     A first reference cell  204  outputs a high reference cell current I ref     —     h  through output  214  to node  224 . Capacitor C 1  is coupled between node  224  and ground. Node  224  couples a low reference voltage V r1  to a negative input  244  of comparator CP 1  through conductor  234 . A second reference cell  206  outputs a low reference cell current I ref     —     1  through output  216  to node  226 . Capacitor C 2  is coupled between node  226  and ground. Node  226  couples a high reference voltage V rh  to a positive input  246  of comparator CP 2  through conductor  236 . The output  247  (ΔCP 1 ) of CP 1  is coupled to a first input  250  of a third comparator CP 3 . The output  249  (ΔCP 2 ) of CP 2  is coupled to a second input  251  of the comparator CP 3 . Comparator CP 3  outputs a signal through output  252 . 
     In operation, the cell current I cel  flows through node  222  and capacitor C cel  to ground, which produces the cell voltage V cel  across the capacitive load made up of capacitor C cel . The cell voltage V cel  is then coupled from node  222  to the positive input  242  of comparator CP 1  and the negative input  248  of comparator CP 2 . The reference cell  204  has a low threshold voltage V t     —     r1  and produces a high reference current I ref     —     h . The high reference cell current I ref     —     h  flows through node  224  and capacitor C 1  to ground, which produces the low reference voltage V r1  across the capacitive load of capacitor C 1 . The low reference voltage V r1  is coupled from node  224  to the negative input  244  of comparator CP 1 . The reference cell  206  has a high threshold voltage V t     —     rh  and produces a low reference current I ref     —     1 . The low reference cell current I ref     —     1  flows through node  226  and capacitor C 2  to ground, which produces the high reference voltage V rh  across the capacitive load of capacitor C 2 . The high reference voltage V rh  is coupled from node  226  to the positive input  246  of comparator CP 2 . Those skilled in the art will appreciate that, if desired, capacitive loads C cel , C 1 , and C 2  may include or be replaced by resistive loads. 
     CP 1  amplifies the difference voltage between the low reference voltage V r1  and the cell voltage V cel  to produce a difference output voltage ΔCP 1 . CP 2  amplifies the difference voltage between the high reference voltage V rh  and the cell voltage V cel  to produce a difference output voltage ΔCP 2 . ΔCP 1  and ΔCP 2  are then applied to inputs  250  and  251 , respectively, of comparator CP 3 . Comparator CP 3  amplifies the difference voltage between ΔCP 1  and ΔCP 2  to output a ΔCP 3  signal at the output  252  of comparator CP 3 . The ΔCP 3  signal represents an amplified data signal of the data stored in the memory cell  202 . 
     In one embodiment, the high reference voltage V rh  is substantially equal to a cell voltage V cel  from a memory cell in the high output voltage state (i.e., a binary “1” state, or a binary “0” state in inverted logic). In one embodiment, the low reference voltage V r1  is substantially equal to a cell voltage V cel  from a memory cell in the low output voltage state (i.e., a binary “0” state, or a binary “1” state in inverted logic). 
     FIG. 3 is a graph of the relationship of signals ΔCP 1  and ΔCP 2  when the memory cell  202  has a high threshold voltage. Referring to the elements shown in FIG. 2, when both memory cell  202  and reference cell  206  have a high threshold voltage, the difference between the two signals, as amplified in comparator CP 2  (i.e., ΔCP 2 ) is quite small, as shown by the line labeled ΔCP 2  in FIG.  3 . In contrast, when the memory cell  202  has a high threshold voltage and the reference cell  204  has a low threshold voltage, the difference voltage between the two signals, as amplified in comparator CP 1  (i.e., ΔCP 1 ) is quite large, as shown by the line labeled ΔCP 1  in FIG.  3 . 
     FIG. 4 is a graph of the relationship of signals ΔCP 1  and ΔCP 2  when the memory cell has a low threshold voltage. Referring to the elements shown in FIG. 2, when both memory cell  202  and reference cell  204  have a low threshold voltage, the difference between the two signals, as amplified in comparator CP 1  (i.e., ΔCP 1 ) is quite small, as shown by the line labeled ΔCP 1  in FIG.  4 . In contrast, when the memory cell  202  has a high threshold voltage and the reference cell  206  has a high threshold voltage, the difference voltage between the two signals, as amplified in comparator CP 2  (i.e., ΔCP 2 ) is quite large, as shown by the line labeled ΔCP 2  in FIG.  4 . 
     The difference between ΔCP 1  and ΔCP 2  is a representation of the data in the memory cell. For example, if the difference between ΔCP 1  and ΔCP 2  is a logic high, then the memory cell includes a logic high. Alternatively, if the difference between ΔCP 1  and ΔCP 2  is a logic low, then the memory cell includes a logic low. The difference between ΔCP 1  and ΔCP 2  is greater than the difference obtained when the level of the memory cell is compared to one reference cell in accordance with conventional practice. Consequently, the resulting output signal can be detected more easily and more quickly and, in turn, the state of the data cell can be verified more quickly. 
     FIG. 5 is a schematic diagram of a circuit  500  for verifying data in a memory cell using two reference cells in accordance with another embodiment of the present invention. Circuit  500  differs from circuit  200  shown in FIG. 2 in that the reference cells  204 ,  206  have been replaced by voltage dividers represented by resistors R 1  and R 2 , respectively. Resistors R 1  and R 2  produce a low reference voltage V r1  across resistor R 1  and a high reference voltage V rh  across resistor R 2  by passing the respective currents I ref     —     1  and I ref     —     h  through the resistors to ground. 
     With reference to FIG. 5, the memory cell draws a cell current I cel  through output conductor  512  to node  522 . Resistor R cel  is coupled between VDD and node  522 . Node  522  couples cell voltage V cel  to node  228  through conductor  227 . Node  228  couples cell voltage V cel  to the positive input  242  of comparator CP 1  through conductor  232 . Node  228  also couples cell voltage V cel  to the negative input  248  of comparator CP 2  through conductor  238 . 
     A first reference voltage divider R 1  outputs a low reference cell current I ref     —     1  to node  524  through output  514  to node  524 . Resistor R 1  is coupled between VDD and node  524 . Node  524  couples a low reference voltage V r1  to a negative input  244  of comparator CP 1  through conductor  234 . A second reference voltage divider R 2  outputs a high reference cell current I ref     —     h  through output  516  to node  526 . Resistor R 2  is coupled between VDD and node  526 . Node  526  couples a high reference voltage V rh  to a positive input  246  of comparator CP 2  through conductor  236 . The output  247  (ΔCP 1 ) of CP 1  is coupled to a first input  250  of a third comparator CP 3 . The output  249  (ΔCP 2 ) of CP 2  is coupled to a second input  251  of the comparator CP 3 . Comparator CP 3  outputs a signal through output  252 . 
     In operation, the cell current I cel  flows through resistor R cel  to node  522 , which produces the cell voltage V cel  across resistor R cel . The cell voltage V cel  is then coupled from node  522  to the positive input  242  of comparator CP 1  and the negative input  248  of comparator CP 2 . The first reference voltage divider R 1  has a high resistance and produces a low reference current I ref      —     1 . The low reference cell current I ref     —     1  flows from VDD through resistor R 1  and node  524  to ground, which produces the low reference voltage V r1  across resistor R 1 . The low reference voltage V r1  is coupled from node  524  to the negative input  244  of comparator CP 1 . The second reference voltage divider R 2  has a low resistance and produces a high reference current I ref     —     h . The high reference cell current I ref     —     h  flows from VDD through resistor R 2  and node  526  to ground, which produces the high reference voltage V rh  across resistor R 2 . The high reference voltage V rh  is coupled from node  526  to the positive input  246  of comparator CP 2 . 
     CP 1  amplifies the difference voltage of the low reference voltage V r1  and the cell voltage V cel  to produce a difference output voltage ΔCP 1 . CP 2  amplifies the difference voltage of the high reference voltage V rh  and the cell voltage V cel  to produce a difference output voltage ΔCP 2 . ΔCP 1  and ΔCP 2  are then applied to inputs  250  and  251 , respectively, of comparator CP 3 . Comparator CP 3  amplifies the difference voltage of the ΔCP 1  and ΔCP 2  to output a ΔCP 3  signal at the output  252  of comparator CP 3 . The ΔCP 3  signal represents an amplified data signal of the data stored in the memory cell. 
     FIG. 6 is a schematic diagram of a circuit  600  for verifying data in a memory cell using two reference cells in accordance with yet another embodiment of the present invention. Circuit  600  differs from circuit  500  shown in FIG. 5 in that the first voltage divider R 1  has been replaced by a direct connection to ground potential. 
     With reference to FIG. 6, memory cell draws a cell current I cel  through output conductor  512  to node  522 . Resistor R cel , is coupled between VDD and node  522 . Node  522  couples cell voltage V cel  to node  628 . Node  528  couples cell voltage V cel  to the positive input  242  of comparator CP 1  and the negative input  248  of comparator  5 P 2 . The negative input  244  of comparator CP 1  is tied to a low reference voltage such as a ground potential through node  624 . A second reference voltage divider R 2  outputs a high reference cell current I ref—   h  through output  516  to node  526 . Resistor R 2  is coupled between VDD and node  526 . Node  526  couples a high reference voltage V rh  to a positive input  246  of comparator CP 2 . The output  247  (ΔCP 1 ) of CP 1  is coupled to a first input  250  of a third comparator CP 3 . The output  249  (ΔCP 2 ) of CP 2  is coupled to a second input  251  of the comparator CP 3 . Comparator CP 3  outputs a signal through output  252 . 
     In operation, the cell current I cel  flows through resistor R cel  to node  522 , which produces the cell voltage V cel  across resistor R cel . The cell voltage V cel  is then coupled from node  522  to the positive input  242  of comparator CP 1  and the negative input  248  of comparator CP 2 . The low reference voltage (e.g., ground potential) V r1 is  coupled from node  624  to the negative input  244  of comparator CP 1 . The second reference voltage divider R 2  has a low resistance and produces a high reference current I ref—   h . The high reference cell current I ref     —     h  flows from VDD through resistor R 2  and node  526  to ground, which produces the high reference voltage V rh  across resistor R 2 . The high reference voltage V rh  is coupled from node  526  to the positive input  246  of comparator CP 2 . 
     CP 1  amplifies the difference voltage of the low reference voltage V r1  and the cell voltage V cel  to produce a difference output voltage ΔCP 1 . CP 2  amplifies the difference voltage of the high reference voltage V rh  and the cell voltage V cel  to produce a difference output voltage ΔCP 2 . ΔCP 1  and ΔCP 2  are then applied to inputs  250  and  251 , respectively, of comparator CP 3 . Comparator CP 3  amplifies the difference voltage of the ΔCP 1  and ΔCP 2  to output a ΔCP 3  signal at the output  252  of comparator CP 3 . The ΔCP 3  signal represents an amplified data signal of the data stored in the memory cell. 
     FIG. 7 is a flowchart diagram  700  that illustrates the method operations performed in reading a non-volatile memory with dual reference cells in accordance with one embodiment of the present invention. The method begins in operation  702  in which the output voltage of the memory cell (e.g., memory cell voltage V cel ) is compared to a first reference voltage output from a first reference cell to produce a first difference voltage (e.g., ΔCP 1 ). In one embodiment, tThe first reference voltage is a low voltage. In operation  704 , the output voltage of the memory cell (e.g., memory cell voltage V cel ) is compared to a second reference voltage output from a second reference cell to produce a second difference voltage (e.g., ΔCP 2 ). In one embodiment, the second reference voltage is a high voltage. In operation  706 , the first and second difference voltages (e.g., ΔCP 1  and ΔCP 2 ) are compared. This comparison produces an output signal, which is output in operation  708 . In one embodiment, the output signal includes an amplified representation of the data stored in the memory cell. Once the representation of the data stored in the memory cell is output, the method is done. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the exemplary embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the specific details shown and described herein, but may be modified within the scope and equivalents of the appended claims.