Abstract:
A memory system is provided, comprising at least one memory unit and a source power supply circuit. Each memory unit is coupled between a source voltage and a ground voltage and accesses digital data according to a word line signal and a bit line signal. The source power supply circuit provides the source voltage to the memory units. When the memory unit is in a writing status, the source voltage is the first power voltage. When the memory unit is in a reading status, the source voltage is the second power voltage. The second power voltage equals to the first power voltage subtracted by a specific voltage for avoiding rewriting error.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the full benefit and priority of Taiwan Patent Application Serial No. 97110051, filed Mar. 21, 2008, and incorporates the entire contents of said application herein 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a memory unit with a bit line signal, and in particular relates to a memory unit receiving variable voltage supply to avoid data writing errors. 
         [0004]    2. Description of the Related Art 
         [0005]      FIG. 1  shows a conventional static random access memory (SRAM)  100  with five transistors  101 ,  102 ,  103 ,  104  and  105 . A switch  101  is an NMOS transistor. The NMOS transistor  101  is turned on or turned off according to a word line signal WL to transmit a bit line signal BL to a memory unit  110 . The memory  110  is formed by a latch circuit which comprises two inverters coupled cross over. The first inverter comprises an NMOS transistor  102  and a PMOS transistor  104 . The second inverter comprises an NMOS transistor  103  and a PMOS transistor  105 . Voltage levels of points B and C are opposite when storing digital data. 
         [0006]    When the memory  100  is written by data  1 , the voltage level of the bit line signal BL is pulled up to voltage Vdd and the word line signal WL turns on the NMOS transistor  101 . Thus, the point B is at a high voltage level and the point C is at the low voltage level. When the memory  100  is written by data  0 , the voltage level of the bit line signal BL is pulled down to voltage GND and the word line WL turns on the NMOS transistor  101 . Thus, the point B is at a low voltage level and the point C is at a high voltage level. 
         [0007]    When the memory storing data  1  is read, the bit line BL is precharged to voltage Vdd and then the word line WL turns on the NMOS transistor  101 . Then, a memory system detects the voltage level of the bit line BL. Since the point B is at a high voltage level, the voltage level of the bit line BL will not be pulled down. Thus, the memory system detects the memory storing data  1 . 
         [0008]    When the memory storing data  0  is read, the bit line BL is precharged to voltage Vdd and then the word line WL turns on the NMOS transistor  101 . Then, a memory system detects the voltage level of the bit line BL. Since the point C is at a high voltage level and the point B is at a low voltage level, the voltage level of the bit line BL will be pulled down. Thus, the memory system detects the memory storing data  0 . 
         [0009]    Because the memory comprises one bit line BL, when the memory is stored with data  1  (the point B is at high voltage level), the memory unit  110  can not be rewritten with data with a high voltage level. The conventional method is to adjust the beta ratio of transistors  102 ,  103 ,  104  and  105 . However, the conventional method will cause instability with the memory unit  110 . Thus, the objective of the invention is to solve the described problem. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
         [0011]    An embodiment of a memory system is provided. The memory system comprises at least one memory unit and a source power supply circuit. Each memory unit is coupled between a source voltage and a ground voltage and accesses digital data by using a word line signal and a bit line signal. The source power supply circuit provides the source voltage to the memory units. When the memory unit is in a writing status, the source voltage is the first power voltage. When the memory unit is in a reading status, the source voltage is the second power voltage. The second power voltage equals to the first power voltage subtracted by a specific voltage for avoiding rewriting error. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0013]      FIG. 1  shows a conventional static random access memory (SRAM) with five transistors; 
           [0014]      FIG. 2  shows a source power supply circuit according to an embodiment of the invention; 
           [0015]      FIG. 3  shows a layout diagram of a memory system according to an embodiment of the invention; 
           [0016]      FIG. 4  shows a source power supply circuit according to another embodiment of the invention; and 
           [0017]      FIG. 5  shows a layout diagram of a memory system according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0019]    Since the memory  100  only comprises one bit line BL, when the memory unit  110  stores data as 1 (the point B is at high voltage level) and a high voltage level is used to rewrite data in the memory unit  110 , a rewriting error will occur. A voltage difference between a source and a drain of an NMOS transistor is voltage V TN . When the memory unit  110  is rewritten with data with a high voltage level, the bit line BL is voltage V dd  and the voltage level of the point B is voltage V dd -V TN . The NMOS transistor  103  can not be turned on completely and the NMOS transistor  102  can not be turned off completely. Thus, the memory  100  can not be rewritten at the points B and C at the correct voltage levels, which causes the rewriting error when the memory unit  110  is rewritten with data with a high voltage level. 
         [0020]      FIG. 2  shows a source power supply circuit  200  according to an embodiment of the invention. The source power supply circuit  200  can provide different source voltages SL to the memory unit  110 . Referring to  FIG. 1 , when the memory unit  110  is in a writing status, the source power supply circuit  200  can provide voltage V dd -V TN . Thus, the NMOS transistor  103  of the memory unit  110  can be turned on completely to avoid the writing error. When the memory unit  110  is in a reading status, the source power supply circuit  200  can provide voltage V dd  and the memory unit  110  can be read correctly by using the NMOS transistor  101  and the bit line BL. 
         [0021]    The source power supply  200  determines the voltage levels of a point A and the source voltage SL according to the voltage levels of a write enable signal WEB, and word line signals WLB 1  and WLB 2 . When the memory unit is in the writing status, the point A is at a high voltage level and the voltage level of the source voltage is V dd -V TN . When the memory unit  110  is in the reading status, the point A is at a low voltage level and the voltage level of the source voltage SL is voltage V dd , as shown in Table 1. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 WLB1 
                 WLB2 
                 WEB 
                 A 
                 SL 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Read 
                 X 
                 X 
                 1 
                 GND 
                 V dd   
               
               
                   
                 Write 
                 1 
                 0 
                 0 
                 V dd   
                 V dd -V TN   
               
               
                   
                 Write 
                 0 
                 1 
                 0 
                 V dd   
                 V dd -V TN   
               
               
                   
                   
               
             
          
         
       
     
         [0022]    The source power supply circuit  200  comprises a control circuit  210 , a CMOS inverter  220  and an NMOS transistor  201 . The control circuit  210  determines the voltage levels of the point A and the source voltage SL according to the voltage levels of a write enable signal WEB, and word line signals WLB 1  and WLB 2 . Thus, the memory  110  can receive a lower voltage to avoid the writing error in the writing status. When the voltage level of the write enable signal WEB is at low voltage level (logic 0), the memory unit  110  is in the writing status. When the voltage level of the write enable signal WEB is at high voltage level (logic 1), the memory unit  110  is in the reading status. The word line signals WLB 1  and WLB 2  correspond to different columns or rows of the memory unit. 
         [0023]      FIG. 3  shows a layout diagram of a memory system  300  according to an embodiment of the invention.  FIG. 3  only shows six transistors in the memory system  300 . However, the memory system is not limited to comprise only six memory units. Word line driving circuits  310  and  320 , respectively, transmit word line signals WL 1  and WL 2  to corresponding memory unit rows, as shown in  FIG. 3 . The word line signals WLB 1  and WLB 2  are respectively generated by the inverters  331  and  332  and transmitted to the source power supply circuit  200 . The source power supply circuit  200  can provide the source voltage SL to two different columns of the memory unit. 
         [0024]      FIG. 4  shows a source power supply circuit  400  according to another embodiment of the invention. The source power supply circuit  400  can provide different source voltages SL to the memory unit  110 . When the memory unit  110  is in the writing status, the source power supply circuit  400  can provide voltage V dd -V TN . Thus, the transistor  103  of the memory unit  110  can be turned on completely to avoid the writing error. When the memory unit  110  is in the reading status, the source power supply circuit  400  can provide voltage V dd  and the memory unit  110  can be read correctly by using the NMOS transistor  101  and the bit line BL. 
         [0025]    The source power supply circuit  400  determines the voltage levels of a point A and the source voltage SL according to the voltage levels of a write enable signal WEB and a bit line column control signal COLB. When the memory unit  110  is in the writing status, the point A is at high voltage level and the source voltage SL is voltage V dd -V TN . When the memory unit  110  is in the reading status, the point A is at low voltage level and the source voltage SL is voltage V dd , as shown in Table 2. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 COLB 
                 WEB 
                 A 
                 SL 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Read 
                 X 
                 1 
                 GND 
                 V dd   
               
               
                 Write 
                 0 
                 0 
                 V dd   
                 V dd -V TN   
               
               
                   
               
             
          
         
       
     
         [0026]    The source power supply circuit  400  comprises a control circuit (NOR gate)  410 , a CMOS inverter  420  and an NMOS transistor  401 . The control circuit  410  determines the voltage levels of the point A and the source voltage SL according to the voltage levels of the write enable signal WEB and the bit line column control signal COLB. Thus, the memory  110  can receive a lower voltage power to avoid the writing error in the writing status. When the voltage levels of the write enable signal WEB and the bit line column control signal are at low voltage levels (logic 0), the memory unit  110  is in the writing status. When the voltage level of the write enable signal WEB is at a high voltage level (logic 1), the memory unit  110  is in the reading status. 
         [0027]      FIG. 5  shows a layout diagram of a memory system  500  according to another embodiment of the invention.  FIG. 5  only shows six memory units in the memory system  500 . However, the memory system  500  is not limited to comprise only six transistors. Word line driving circuits  510  and  520 , respectively, transmit word line signals WL 1  and WL 2  to corresponding memory unit columns, as shown in  FIG. 5 . The bit line column control signals COLB 1  and COLB 2  are respectively transmitted to the source power supply circuits  200 - 1  and  200 - 2 . The source power supply circuits  200 - 1  and  200 - 2 , respectively, provide the source voltages SL 1  and SL 2  to different memory unit rows. 
         [0028]    With regard to the memory systems of  FIGS. 3 and 5 , since the length of the memory system  300  in the Y direction is shorter, the memory system  300  can be used for specific applications. Because the size of the memory system  500  is smaller, using the memory system  500  of the invention can reduce layout size and cost. 
         [0029]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited to thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.