Abstract:
This invention discloses a write-sensing circuit for semiconductor memories comprising a first and a second local bit-lines (BLs) forming a complementary BL pair, a first and a second global bit-lines (GBLs) forming a complementary GBL pair, and at least one switching circuit controlled by the first and second GBLs and controllably coupling a predetermined power supply source to the first and second BLs, separately, wherein when the first and second GBLs are asserted during a write operation, the switching circuit couples only one of the first and second BLs to the predetermined voltage supply source.

Description:
BACKGROUND 
   The present invention relates generally to dynamic random access memory (DRAM) circuits, and, more particularly, to designs of DRAM write circuit. 
   Dynamic random access memory (DRAM) is a type of random access memory that stores each bit of data in a separate capacitor. A simplest DRAM cell comprises a single N-type metal-oxide-semiconductor (NMOS) transistor and a single capacitor. If charges are stored in the capacitor, the cell is said to store a logic HIGH, depending on the convention used. Then if no charge is present, the cell is said to store a logic LOW. Since the charges in the capacitor dissipate over time, DRAM systems require additional refreshing circuitries to periodically refresh the charges stored in the capacitors. Since a capacitor can store only a very limited amount of charges, to fast distinguish the difference between a logic ‘1’ and a logic ‘0’, two bit-lines (BLs) are typically used for each bit with the first in the bit line pair known as a bit line true (BLT) and the other being the bit line complement (BLC). Prior to a sensing, BLT and BLC are equalized to the same voltage. Upon a sensing, the voltages of the BL pair start to split oppositely, e.g., if BLT swings to a higher voltage, then BLC swings to lower voltage. A sense amplifier compares BLT and BLC voltages and outputs either a high or a low voltage to represent a logic value stored in the bit. 
   The single NMOS transistor&#39;s gate is controlled by a word-line (WL). When the WL is asserted a logic high voltage, the single NMOS transistor will be turned on and allow the cell capacitor to be accessed. 
   In a large DRAM bank, memory cells are organized in blocks, and a bank may contain a plurality of blocks. A BL pair is coupled to all the cells in a column of a block, and then a global bit-line (GBL) pair is coupled to all the BL pairs belonging to the same column of the plurality of blocks through select transistors, which is controlled, conventionally, by a single select signal for the entire bank. The GBL pairs are coupled to the data input/output through multiplexers. When writing or reading the DRAM cells, the select transistors are turned on, so that data can be written into the memory cells from the corresponding GBL pairs and through the corresponding BL pairs, or read from the memory cells through the corresponding BL pairs onto the corresponding GBL pairs. During memory cell refreshing, since the cells are not accessed, so that the select transistors need not be turned on. 
   In general, accessing a DRAM cell involves WL activation, signal development on the BL pairs, BL sensing, WL deactivation and BL equalization (EQ). BL sensing is a signal write back or refreshing process. When a DRAM cell is being written, outside voltage will charge the storage capacitor, there is no data loss issue. But if the memory cell is not being written and not fully refreshed before connecting its corresponding BL pair to its corresponding GBL pair, data stored in the memory cell may be lost due to large capacitive load of the GBL. 
   Assuming a 64-bit DRAM has 256 columns in a bank, then each time only ¼ of the BLs need to be accessed. However, because all the select transistors of an entire bank are controlled by a single signal in conventional DRAMs, all 256 BL pairs are connected to their corresponding GBL pairs during a write operation, even though only 64 GBL pairs are forced with outside voltage source. If the memory cells on the rest 192 BL pairs are not fully refreshed prior to the select transistor&#39;s turn-on, these memory cells may suffer data loss. For this reason, the write operation in conventional DRAM must be sequential, i.e., writing may only start after a sensing is completed. 
     FIG. 1  shows a timing sequence of writing in a conventional DRAM. Prior to the writing  108 , the BLs are equalized in time slot  102 . At the beginning of the writing, the WL starts to develop in time slot  104 , and maintains activated throughout the entire write cycle  100 . Sensing occurs in time slot  106 . During this time, all the cells coupled to the activated WL are refreshed to their full capacity. Writing to the selected cells happens in time slot  108  after the sensing period  106  is completed. Due to these sequential executions, the writing cycle  100  is particularly long, which limits the overall speed of the DRAM. 
   One way to overcome this limitation is to selectively turn on only those select transistors for the BL pairs being accessed, while keep the rest of the select transistors in the memory bank in off state. But this requires additional select signal lines, which will increase the memory die size. 
   As such, what is desired is a DRAM writing scheme that allows early writing, i.e., simultaneous writing and sensing, without increasing memory die size. 
   SUMMARY 
   This invention discloses a write-sensing circuit for semiconductor memories comprising a first and a second local bit-lines (BLs) forming a complementary BL pair, a first and a second global bit-lines (GBLs) forming a complementary GBL pair, and at least one switching circuit controlled by the first and second GBLs and controllably coupling a predetermined power supply source to the first and second BLs, separately, wherein when the first and second GBLs are asserted during a write operation, the switching circuit couples only one of the first and second BLs to the predetermined voltage supply source. 
   The construction and method of operation of the invention, however, together with additional objectives and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. A clearer conception of the invention, and of the components and operation of systems provided with the invention, will become more readily apparent by referring to the exemplary, and therefore non-limiting, embodiments illustrated in the drawings, wherein like reference numbers (if they occur in more than one view) designate the same elements. The invention may be better understood by reference to one or more of these drawings in combination with the description presented herein. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. 
       FIG. 1  illustrates a conventional write timing sequence. 
       FIG. 2  illustrates a write timing sequence of a concurrent writing and sensing scheme according to one embodiment of the present invention. 
       FIG. 3  is a schematic diagram illustrating an array structure of a DRAM according to the embodiment of the present invention. 
       FIGS. 4A˜4C  are schematic diagrams illustrating three implementations of a write-sensing block according to embodiments of the present invention. 
       FIG. 5  is a schematic diagram illustrating an implementation of a read/write data path employed in the DRAM of the present invention. 
   

   DESCRIPTION 
   The present invention discloses a concurrent writing and sensing scheme in dynamic random access memories (DRAM) that can shorten the write cycle of the DRAM. 
     FIG. 1  has already been described and discussed as the relevant background to the present invention. It requires no further discussion here. 
     FIG. 2  illustrates a write timing sequence of a concurrent writing and sensing scheme according to one embodiment of the present invention. Here bit-lines (BLs) are also first equalized in time slot  202 . At the beginning of a writing, a selected word-line (WL) develops in time slot  204 . Then sensing starts in time slot  206  as that of the conventional DRAM shown in  FIG. 1 . But the present invention starts writing in time slot  208  right after the WL is developed in time slot  204 , concurrent to the sensing in time slot  206 , therefore a write cycle  200  of the present invention is shortened comparing to the conventional write cycle  100  shown in  FIG. 1 . 
   A key aspect of the present invention is to keep select transistors that connect BLs to their corresponding GBLs in off state during write operation, so that those non-accessed cells in a memory bank will not face data loss issue if the writing starts before a sensing ends. Without turning on the select transistors, the present invention writes data to memory cells through separate paths which will be described in details in following paragraphs. 
     FIG. 3  is a schematic diagram illustrating a memory core structure  300  of a DRAM according to the embodiment of the present invention. For illustrative purpose, the DRAM core structure  300  is shown to comprise a plurality of blocks  310 [0:n], a plurality of blocks  340 [0:n+1], a block  370  and a plurality of global-bit-lines  360 [0:m−1] coupled to the block  370 , where n and m are integrals. 
   Referring to  FIG. 3 , the blocks  310 [0:n] may be implemented as DRAM banks. Each bank  310  may comprise a DRAM cell array  320 , and two write-sensing (WS) blocks  330   s , one  330 [Lx] on the left, and the other  330 [Rx] on the right, where x is an integral between 0 and n. A typical size of the array  320  is 32 rows (WLs) by 1024 columns (BLs). The block  340  may be implemented as a 2-to-1 multiplexer placed between two adjacent banks  310 [ 0 ] and  310 [ 1 ] as shown in  FIG. 3 . The multiplexer  340 [ 1 ] selects one of the two WS blocks  330 [R 0 ] and  330 [L 1 ]. The output of the multiplexer  340 [ 1 ] is coupled to the global-bit-line  360 [ 0 ]. A left and a right adjacent WS blocks  330   s  and a multiplexer  340  form a read-write block  350 . The block  370  may be implemented as a m-to-1 multiplexer to save line routings in the DRAM. Typically, m=8.  FIG. 3  shows only array structure of one bit, some DRAM may access 64 bits, or columns at a time. Then for a word-line having 1024 columns, only 64 out of the 1024 columns are written and the rest are sensed and hence refreshed during a write cycle. Conventionally, the global-bit-lines  360   s  are always constructed in pairs to supply a differential voltage or current to a data sense amplifier. 
     FIGS. 4A˜4C  are schematic diagrams illustrating three implementations of the read-write block  350  according to embodiments of the present invention. In general, the read-write block  350  comprises of the multiplexer  340  and two WS blocks  330   s . The multiplexer  340  may be implemented as N-type metal-semiconductor-oxide (NMOS) transistors,  342 ,  344 ,  346  and  348 , serving as pass gates between local bit-lines and global-bit-lines (GBLs). When signal SSL turns to a logic HIGH, while signal SSR turns to a logic LOW, the NMOS transistors  342  and  344  are turned on while the NMOS transistors  346  and  348  are turned off, so that the local bit-lines (BL) of the left write-sensing block  330  are selected to be coupled to the global-bit-lines ( 362  and  364 ), i.e., BL[ 0 ] is coupled to GBL  362 , and BLB[ 0 ] is coupled to GBLB  364 . On the other hand, when signal SSL turns to the logic LOW, while signal SSR turns to a logic HIGH, the NMOS transistors  342  and  344  are turned off while the NMOS transistors  346  and  348  are turned on, so that local bit-lines (BL) of the right write-sensing block  330  are selected to be coupled to the global-bit-lines ( 362  and  364 ), i.e., BL[ 1 ] is coupled to GBL  362 , and BLB[ 1 ] is coupled to GBLB  364 . 
   Referring to  FIGS. 4A˜4C , the WS block  330  comprises a local BL sense amplifier block  332  and a local BL switching block  334 . The sense amplifier block  332  may be implemented as a conventional cross-coupled inverter pair. To activate the sense amplifier block  332 , signal SP will be pulled up to a positive high supply voltage (Vcc), and signal SN will be pulled down to a complimentary low supply voltage (Vss). Differences among implementations shown in  FIGS. 4A˜4C  are in the formations and connections of the BL switching block  432  in  FIG. 4A ,  434  in  FIG. 4B and 436  in  FIG. 4C . 
   Referring to  FIG. 4A , the BL switching block  432  is formed by two NMOS transistors  402  and  404 . Both sources of the NMOS transistors  402  and  404  are coupled to the signal SN. A drain and a gate of the NMOS transistor  402  are coupled to the BLB and global-bit-line  362 , respectively. A drain and a gate of the NMOS transistor  404  are coupled to the BL and global-bit-line  364 , respectively. During both read and write operations, the global-bit-lines  362  and  364  are consistently coupled to GBL and GBLB, respectively. Note that in  FIG. 4A , the bit-lines at the drain of the NMOS transistor  402  and  404  and the bit-lines at the output of the sense amplifier  332  are twisted. 
   Referring to  FIG. 4B , the BL switching block  434  is also formed by two NMOS transistors  402  and  404 . Both sources of the NMOS transistors  402  and  404  are coupled to the signal SN. A drain and a gate of the NMOS transistor  402  are coupled to the BL and global-bit-line  362 , respectively. A drain and a gate of the NMOS transistor  404  are coupled to the BLB and global-bit-line  364 , respectively. During read operation, the global-bit-lines  362  and  364  are coupled to GBL and GBLB, respectively. During write operation, instead, the global-bit-lines  362  and  364  are coupled to GBLB and GBL, respectively. By swapping the global-bit-lines, the local bit-lines may avoid the twisting, therefore saving layout space and complexity. 
   Referring to  FIG. 4C , the BL switching block  436  is formed by two P-type metal-semiconductor-oxide (PMOS) transistors  412  and  414 . Both sources of the PMOS transistors  412  and  414  are coupled to the signal SP. A drain and a gate of the PMOS transistor  412  are coupled to the BL and global-bit-line  362 , respectively. A drain and a gate of the PMOS transistor  414  are coupled to the BLB and global-bit-line  364 , respectively. During read operation, the global-bit-lines  362  and  364  are coupled to GBL and GBLB, respectively. During write operation, instead, the global-bit-lines  362  and  364  are coupled to GBLB and GBL, respectively. By swapping the global-bit-lines, the local bit-lines may avoid the twisting, therefore save layout space and complexity. 
   The DRAM cell and local bit-lines (not shown) are operated as conventional DRAMs, i.e., the local bit-lines may be pre-charged to Vcc/2, and during sensing, the state of a selected DRAM cell may cause the bit-line pair to develop a differential voltage, which will be amplified by the BL sense amplifier  332  to eventually rail-to-rail voltage levels. 
   During read operation, the global-bit-line  362  and  364  are pre-charged to Vss for read-write block  350  shown in  FIGS. 4A and 4B , and pre-charged to Vcc for read-write block  350  shown in  FIG. 4C . The rest of the read operation is identical to all three implementations shown in  FIGS. 4A˜4C . At the start of a read, the pull-downs and equalizers (not shown) of local bit-lines are de-asserted, and a selected word-line (not shown) is asserted. In order to activate the BL sense amplifier, the signals SN and SP are pulled to Vss and Vcc, respectively. After the BL is sensed, either signal SSL or signal SSR will be turned on. In case the global-bit-lines  362  and  364  are pre-charged to Vss, if BL has developed to the logic HIGH, then the global-bit-line  362  will be pulled to the logic HIGH, while the global-bit-line  364  remains low. Then the differential voltage between the global-bit-lines  362  and  364  can be detected by a sense amplifier (not shown) after the multiplexer  370  shown in  FIG. 3 . In case the global-bit-lines  362  and  364  are pre-charged to Vcc, a logic LOW bit-line will develop the differential voltage between the global-bit-line  362  and  364 . 
   Referring to  FIGS. 4A and 4B , since the global-bit-lines  362  and  364  are pre-charged to Vss, and their voltage swing during sensing will not be high enough to turn on the NMOS transistor  402  or  404  before the sensing ends, so the NMOS transistor  402  and  404  are always off during read operation. The implementations shown in  FIGS. 4A and 4B  work just as that in a conventional DRAM. 
   Referring to  FIG. 4C , since the global-bit-lines  362  and  364  are pre-charged to Vcc, and their voltage swing during sensing will not be lower enough to turn on the PMOS transistor  412  or  414  before the sensing ends, so the PMOS transistor  412  and  414  are always off during read operation. The implementations shown in  FIG. 4C  also works just as that in a conventional DRAM. 
   During the write operation for the implementations shown in  FIGS. 4A˜4C , both signals SSL and SSR remain de-asserted at the logic LOW, so that none of the inputs of multiplexer  340  are selected. At the start of a writing, as in the read case, the bit-line pull-up and equalizer are turned off, and a selected word-line is asserted, and then the bit-line sense amplifier  332  is activated by pulling signal SN to Vss and signal SP to Vcc. For only those selected columns, their corresponding global-bit-lines are driven to a complimentary one-HIGH-one-LOW voltages, hence writing data into the DRAM cells on the asserted word-line and the selected global-bit-lines. The rest of the global-bit-lines remains pre-charged, and the bit-line and sense amplifier conditions are just like that in a read operation, so the DRAM cells on the asserted word-line but the unselected global-bit-lines are sensed and hence refreshed. How data are exactly written into the DRAM cells are described in the following paragraphs. 
   Referring to  FIG. 4A , when intending to write a logic LOW into a selected DRAM cell, the corresponding global-bit-lines  362  and  364  or GBL and GBLB are driven to the logic LOW and HIGH, respectively. Then the NMOS transistors  402  and  404  in the switching block  432  are turned off and on, respectively. As signal SN has been pulled to Vss, so that the turned-on NMOS transistor  404  will force BL voltage down to Vss. With the amplification of the BL sense amplifier  332 , the BL and BLB will eventually be driven to Vss and Vcc, respectively, therefore, a logic LOW will be written into the DRAM cell on the BL. Oppositely, when intending to write a logic HIGH into a selected DRAM cell, the corresponding global-bit-lines  362  and  364  or GBL and GBLB are driven to the logic HIGH and LOW, respectively. In this case the BL and BLB are driven to logic HIGH and LOW, respectively, and hence a logic HIGH will be written into the DRAM cell on the BL. 
   Referring to  FIG. 4B , when intending to write a logic LOW into a selected DRAM cell, the corresponding global-bit-lines  362  and  364  or GBLB and GBL are driven to the logic HIGH and LOW, respectively. Note that the global-bit-lines  362  and  364  are coupled to GBLB and GBL, respectively, during write operation while they are coupled to GBL and GBLB, respectively, during read operation. Then the NMOS transistors  402  and  404  in the switching block  434  are turned on and off, respectively. As signal SN has been pulled to Vss, so that the turned-on NMOS transistor  402  will force BL voltage down to Vss. With the amplification of the BL sense amplifier  332 , the BL and BLB will eventually be driven to Vss and Vcc, respectively, therefore, a logic LOW will be written into the DRAM cell on the BL. Oppositely, when intending to write a logic HIGH into a selected DRAM cell, the corresponding global-bit-lines  362  and  364  or GBLB and GBL are driven to the logic LOW and HIGH, respectively. In this case the BL and BLB are driven to the logic HIGH and LOW, respectively, and hence a logic HIGH will be written into the DRAM cell on the BL. 
   Referring to  FIG. 4C , when intending to write a logic LOW into a selected DRAM cell, the corresponding global-bit-lines  362  and  364  or GBLB and GBL are driven to the logic HIGH and LOW, respectively. Note that the global-bit-lines  362  and  364  are coupled to GBLB and GBL, respectively, during write operation while they are coupled to GBL and GBLB, respectively, during read operation. Then the PMOS transistors  412  and  414  in the switching block  436  are turned off and on, respectively. As signal SP has been pulled to Vcc, so that the turned-on PMOS transistor  402  will force BLB voltage up to Vcc. With the amplification of the BL sense amplifier  332 , the BL and BLB will eventually be driven to Vss and Vcc, respectively, therefore, a logic LOW will be written into the DRAM cell on the BL. Oppositely, when intending to write a logic HIGH into a selected DRAM cell, the corresponding global-bit-lines  362  and  364  or GBLB and GBL are driven to the logic LOW and HIGH, respectively. In this case the BL and BLB are driven to the logic HIGH and LOW, respectively, and hence a logic HIGH will be written into the DRAM cell on the BL. 
   Although only one example employing PMOS transistors in the switching block  436  is shown in  FIG. 4C , one having skill in the art would recognize that PMOS transistors may also be used to construct a circuit similar to the one shown in  FIG. 4A  with bit-lines twisted, yet, global-word-line polarities need not be swapped when operations changing from read to write. 
   In essence, the present invention utilizes the data themselves on the GBLs to selectively pull the BLs to their desired states for writing their corresponding memory cells without turning on the select transistors. Also note that in all three implementations of the read-write block  350  shown in  FIGS. 4A˜4C , writing to selected DRAM cells happens concurrently with the sensing, i.e., refreshing of the none selected DRAM cells in the same asserted word-line, therefore, the write cycle according to the embodiments of the present invention takes approximately the same amount of time, and the overall speed of the DRAM may be increased. Since there are only two transistors ( 432 ,  434  or  436 ) are added to the BL sense amplifier  332 , they can be inserted in the layout pitch of the existing BL sense amplifier  332 , so that the memory die size do not need to be increased. 
     FIG. 5  is a schematic diagram illustrating an implementation of a read/write data path  500  employed in the DRAM of the present invention. The data path  500 , which comprises a write path  510  and a read path  520 , shows data paths before the multiplexer block  370  shown in  FIG. 3 . The write path  510  may be implemented as a series of inverter drivers  512 ,  514  and  516 . The read path  520  may be implemented as a global-bit-line sense amplifier  522  and a driver  524 . Whether the write and read paths  510  and  520  inverting data or not, depends on whether a logic HIGH is represented by Vss or Vcc stored in a DRAM cell. But the write and read paths  510  and  520  must be designed in the same polarity characteristic, i.e., both must be either inverting or not inverting at the same time. The data path  500  shown in  FIG. 5  is adopted from conventional designs. 
   The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
   Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.