Patent Abstract:
A memory array of dual part cells has a pair of twisted write bitlines and a pair of twisted read bitlines for each column. The twist is made by alternating the vertical position of each bitline pair in each section of a column, with the result of generating common mode nose and of reducing differential mode noise.

Full Description:
FIELD OF THE INVENTION 
   This invention is related to Multi-port memory architecture. More particularly, this invention describes the improved architecture for a multi-port memory to enable a reliable differential sensing by creating a common noise while canceling the coupling level. 
   BACKGROUND OF THE INVENTION 
   Single port memory allows either a read or write operation for each cycle time. Typically, the single port memory uses either 6 transistor static memory cells (6 T SRAM) or a single 1-transistor dynamic cell ( 1 T).  FIG. 1-A  shows a transistor level schematic for the typical 6 T SRAM cell  0 . It consists of four NMOS transistors  1 , 2 , 5 , and  6 , and two PMOS transistors  3  and  4 . The PMOSs  3  and  4  and NMOSs  5  and  6  configure a CMOS cross-coupled latch, which maintains a data bit as a storage element. The NMOSs  1  and  2  are used to couple the nodes  7  and  8  to the bitlines (BL and bBL) when a wordline WL is activated. This allows the data bit to be read or written from the BL and bBL.  FIG. 1-B  shows a transistor level schematic for a dynamic memory cell  10 . It consists of one NMOS transistor  11  and capacitor  12  (1 T DRAM cell). When a WL is activated, the NMOS  11  couples the capacitor  12  to the BL. This allows a data bit stored in the capacitor  12  to be read or written from BL. 
   Regardless of the 6 T SRAM or 1 T DRAM, only one WL per array can be activated either for a read or write. This is because activating two or more WLs causes a data contention on the common BL. In order to improve the array utilization, a high performance memory system requires a simultaneous read and write operation. 
     FIG. 2-A  shows a transistor level schematic for a dual port static memory cell. It consists of four NMOS transistors  21 ,  22 ,  25  and  26 , and two PMOS transistors  3  and  4 . Unlike the 1 port SRAM cell, the gates of the NMOS switching transistors,  1 A and  1 B, couple to different wordlines WL 0  and WL 1 . By activating two word lines, WL 0  and WL 1 , a first memory cell coupling to the WL 0  and a second memory cell coupling to the WL 1  can be simultaneously read or written through BL 0  and BL 1  without having a data contention. In accordance with standard usage in the field, the phrase “simultaneously read or written” means ‘during the same clock cycle’. As those skilled in the art are aware, activating WL 0  (or WL 1 ) will turn on all the counterpart transistors  21  (or  22 ) in the row. If the cell to be read and the cell to be written are in the same row, the state of the data will be undefined until the voltages within the cell have stabilized. One of the read and write operations will therefore be delayed according to a convention to avoid contaminating the data. Preferably, the write operation will be done first so that the read operation produces the current data. 
     FIG. 2B  shows a transistor level schematic for a dual port dynamic memory cell. It consists of two NMOS switching transistors  14 A and  14 B, and one capacitor  16 . Similar to the dual port static memory cell, the gates of NMOS switching transistors,  14 A and  14 B couple to the different wordlines WL 0  and WL 1 . By activating two WL 0  and WL 1 , the memory cell coupling to the WL 0  and the memory cell coupling to the WL 1  can be simultaneously read or written through BL 0  and BL 1  without having a data contention. 
     FIG. 3A  shows a transistor level schematic of the 3 T gain cell. The NMOS transistor  34  couples the storage node  32  to the write bitline WBL for a write operation, when the write wordline WWL goes high. The storage node  32  may preferably have a capacitor  35  to store the data bit. The data bit stored in a storage node  32  can be read out to the read bitline RBL when a read wordline RWL goes high. If the storage node  32  contains a high level, two NMOS transistors  31  and  33  are both on, discharging the RBL. If the storage node keeps a low voltage, the NMOS transistor  33  is off, keeping the RBL at the precharged voltage. 
     FIG. 3-B  shows a transistor level schematic for the 2 T gain cell. Similar to the 3 T gain cell, the NMOS transistor  34  couples the storage node  32  to the write bitline WBL for a write operation, when the write wordline WWL goes high. The storage node  32  may preferably have a capacitor  35  to keep the data bit. Unlike the 3 T gain cell, the read NMOS switching transistors  31  are eliminated. The source of the NMOS transistor  32  couples to the read wordline RWL. This thus allows a data bit read operation by measuring a NMOS  33  transistor resistance. A typical method is to apply a voltage between RBL and RWL. They are both high unless they are selected. To read the data bits, RWL goes low. If the data bit is low, the NMOS  33  is off, keeping the RBL at high level. If the data bit is high, the NMOS  33  is on, making the RBL go low. Regardless of the 3 T gain cell or 2 T gain cell discussed above, these cells also allow simultaneous read and write operations. 
     FIG. 4  shows a memory array architecture for the 3 T gain cell which allows simultaneous read and write operations. A memory  40  consists of an array of 3 T gain cells  42  arranged in a matrix. However, another memory cell, which has a read and write port, may be used. The memory cells are controlled by their corresponding read wordline RWL, write wordline WWL, read bitline RBL, and write bitline WBL. The data bit on the RBL is sensed by the corresponding sense amplifier  43 . The WBL is driven by the write driver circuit  44 . It is assumed that the memory cells  42 A and  42 C are in a write mode by activating WWL 0 , and the memory cells  42 B and  42 D are in a read mode by activating RWL 1 , while disabling WWL 1  and RWL 0 . The memory cell data bits in the cells  42 B and  42 D are read out to the RBL 0  and RBL 1 . They are sensed by the corresponding sense amplifiers  43 . A typical sense amplifier utilizes a reference voltage VREF, which allows discrimination between the voltage on the RBL corresponding to the case of reading either a 1 or a 0 from the memory cell. The memory cell data bits in the memory cells  42 A and  42 C are written through the WBL 0  and WBL 1 . The WBL 0  and WBL 1  are driven by the corresponding write bitline drivers  44 . Note that these read and write operation are simultaneously enabled, which causes a potential RBL and WBL coupling noise. 
     FIG. 5  shows a simplified RBL and WBL coupling noise model and simulated waveform. It is assumed that the data bit on the RBL 1  is being sensed by utilizing the sense amplifier  43 , while the adjacent WBL 0  and WBL 1  are driven by the write drivers  44  for a write mode. It is also assumed that a read bitline RBL 1  is precharged to VDD through the PMOS  55 . Assuming that the gain cell stores a low data bit, the RBL should maintain VDD. However, when the WBLs go high or low, due to the coupling capacitor between RBL and WBLs, the RBL goes high or low depending on the WBL voltage swing. As shown in a simulation, this coupling noise is as large as 250 mV even if the PMOS load device is not disabled during the sensing operation. Over 250 mV coupling noise makes a simultaneous read and write operation difficult or potentially impossible. 
   RBL shielding techniques may be used to eliminate this coupling noise. However, this would increase the cell area significantly. Note that this WBL coupling noise to the RBL is a unique problem that results from enabling a simultaneous read and write operation. Note that a conventional BL twisting method is not applicable to cancel the noise, because of a single ended RBL and WBL configuration used in this array. 
   SUMMARY OF THE INVENTION 
   The invention relates to an architecture for a memory array of multi-port cells that enables reliable differential sensing by creating common mode noise while simultaneously canceling the coupling level. 
   A feature of the invention is the use of a pair of twisted bitlines for both the read bitlines and the write bitlines. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  show prior art SRAM and DRAM cells. 
       FIGS. 2A and 2B  show prior art two-port SRAM and DRAM cells. 
       FIGS. 3A and 3B  show alternative prior art DRAM cells. 
       FIG. 4  shows a prior art array architecture. 
       FIG. 5  shows a model of noise generation in an array. 
       FIG. 6  shows an array with interleaved read bitlines for generation of common mode noise. 
       FIG. 7  shows an array with interleaved write bitlines for cancellation of common mode noise. 
       FIG. 8  shows an array according to the invention. 
       FIG. 9  shows a layout of an array. 
   

   DETAILED DESCRIPTION 
     FIG. 6  shows a first method to overcome the problem described in the copending application Ser. No. 10/604,994, filed Aug. 29, 2003 and incorporated by reference in its entirety. A single memory array  60  comprises a plurality of 3 T gain cells arranged in a matrix. However, other memory cells, which have a read and write port, may be used to configure a memory array. The single memory array is further divided into two memory sections  60 A and  60 B. Each WBL is extended into the two memory sections  60 A and  60 B. On the other hand, each RBL  67  in a same column is divided into two local RBL (LRBLA and LRBLB). Each memory column further contains hierarchical read bitlines  66  (HRBLA and HRBLB) that do not connect to the cells. They are arranged over the LRBLA and LRBLB in a different wiring layer. LRBLA and LRBLB (line  67 ) are coupled to the HRBLB and HRBLA (line  66 ) respectively by vertically twisting structure  68 . 
   The effect of the twisting structure  68  is that the two sections of the bitlines contribute equal and opposite signals to the sense amp; i.e. the two sections have opposite senses of reception in that the same field produces signals of opposite polarity in the two sections of the bitlines. The differential nature of reception of the sense amp input converts the opposite current flows to the same direction so that the received noise from the WBL cancels out on the two sides of the sense amp. The LRBLA and HRBLB in a row  0  are coupled to a differential sense amplifier  63 A. The HRBLA occupies the input to the sense amplifier that would be used by a reference cell in the layout of  FIG. 4 . 
   Similarly, the LRBLA and HRBLB in a row  1  are coupled to a differential sense amplifier  63 B. This interleaved sense amplifier arrangement makes it easy to accommodate sense amplifiers  63 . Optionally, both SA  63 A and  63 B may be arranged only at the A or B side (or without interleaving them). Similar to the sense amplifiers, write driver arrangement  64 A and  64 B is also preferably interleaved. Optionally, both write drivers  64 A and  64 B may be arranged only at the A or B side (or without interleaving them). Each memory section A and B further contains a reference wordline REFWL coupling to reference cells  69 . The reference cell located in this example is within the array not outside of the array. The crossing dot over refwl and LRBLA in  FIG. 6  represents the reference cell. The reference cell concept and its timing with regard to RWL is well known to those skilled in the art. The reference cells are connected to a reference voltage (VREF), so that when the reference cell is accessed, the RBL or RBLB which the reference cell is connected to is discharged to a level that is midway between that of the case of a cell storing a high or a low. Each read bitline couples to a PMOS load device  65 , which is always on. Alternatively, a PMOS device may be turned off when a read wordline RWL is selected. 
   An advantageous feature of this invention is that driving a WBL creates a coupling noise to both LRBLA and LRBLB by the same amount. Coupling between the WBL and the LRBLA and LRBLB is denoted by capacitor  62 A Coupling to the adjacent column is denoted by capacitor  62 B. This is because half of the read line adjacent to WBL is LRBLA and the other half of the read line adjacent to WBL is LRBLB. This generates the common noise, making differential sensing possible. 
   This first method creates a common noise environment to maintain the differential signal, but the level of the coupling noise is not cancelled. Canceling of common mode level shift is important to avoid a sensing speed dependency on the data pattern. In addition, a WBL couples to all the cells in a column, resulting in a large capacitive write load. 
     FIG. 7  shows a second method to overcome the problem described in the copending application. A single memory array  70  comprises a plurality of 3 T gain cells arranged in a matrix. However, other memory cells, which have a read and write port, may be used to configure a memory array. The single memory array  70  further is divided into two memory sections  70 A and  70 B. Unlike the first embodiment, each RBL is extended into two memory sections  70 A and  70 B. They are directly coupled to the corresponding sense amplifier  73  together with the other input VREF. On the other hand, each WBL in a column is divided into two local WBL (LWBLA and LWBLB). Each memory section of the column further contains a hierarchical write bitline (HWBLA and HWBLB). The HWBLA and HWLBLB are arranged over the LWBLA and LWBLB in another metal layer. LWBLA and LWBLB are coupled to the HWBLB and HWBLA respectively by vertically twisting them at the twisting area  78 . The LWBLA and HWBLB in a row  0  are coupled to write drivers  74 A and  741 A located at the edge of the memory section A. Similarly, the LWBLA and HWBLB in a row  1  are coupled to the write drivers  74 B and  741 B located at the edge of the memory section B. This interleaved circuit arrangement makes it easy to accommodate the write driver circuitry. Optionally, both all drivers  74 A,  741 A,  74 B and  741 B may be arranged only at either A or B side (or without interleaving them). Similar to the write drivers  74 , sense amplifiers  73 A and  73 B are arranged in an interleaving manner, with one input of the SA connected to (externally supplied) VREF. More particularly, the RBL in a row  0  and the RBL in a row  1  are coupled to the sense amplifiers  73 A located at the A section and  73 B located at the B section, respectively. Optionally, they may be arranged at either A or B side (or without interleaving them). Each read bitline is connected to a PMOS load device  65 , which is always on. Alternatively, a PMOS device may be turned off when a read wordline RWL is selected. An advantageous feature of this invention is that the write drivers  74  and  741  drive the corresponding HWBLA connecting to LWBLB through vertical twist  78  and drive LWLBLA connecting to HWLBLB in the opposite direction (with the opposite polarity). Thus the opposite polarity of the signals received in the adjacent RBL cancels the coupling noise to the adjacent RBL  173 . This is because half of the adjacent RBL  173  is close to LWBLA and the other half of the adjacent RBL  173  is close to LWBLB, where LWBLA and LWBLB swing in the opposite direction. This allows the RBL sensing operation by utilizing a constant RBL reference voltage VREF. The VREF voltage is set to a half level when the RBL discharges when the high data bit would be read out from the memory cell. This second embodiment has the following two disadvantages over the previous embodiment. Firstly, the number of the cells coupling to the RBL is 2× of that for the first method, because one RBL supports all the column. In addition, the differential signal on the RBL pair depends on the speed of the HWBL swing, which may have a skew. 
   This invention overcomes the disadvantages of the first and second methods for multi-port memory by utilizing a 3 dimensional twisted bitline architecture for both read and write bitline. 
   A 3 dimensional twisted bitline architecture is shown in  FIG. 8 , where each column for both RBL and WBL Pair is divided into 4 segments as shown in  FIG. 8 . A section  80 A of the memory array comprises rows  0  and  1  and a section  80 B comprises the remaining rows  2  and  3 . Each row has a Read Word Line RWL and a Write Word Line WWL. Rows  0  and  3  also have a Reference Word Line. REFWL that supplies the reference voltage to the sense amp to determine the data status of the accessed cell by RWL. 
   On the left of the figure, Column N has corresponding peripheral circuits comprising: a) a sense amp  83 B normally connected to RBLB and RBL and controllably connected by control means shown in  FIG. 9  to the reference word line; and b) a pair of write bitline drivers  84 B to drive WBL and WBLB. We need only a single side SA, write bitline driver for column N. The duplication of sense amplifiers and write bitline drivers is an option that provides reduced capacitance between the cells and the sense and drivers. 
   The sense amplifiers and drivers are activated if the address of the cell to be read from or written to lies in the corresponding half of the array; i.e. if the cell to be read is in section  80 B, the upper sense amp will be activated and the lower sense amp will be disabled. Similarly, for the write bitline drivers. A pair of drivers will be activated if the cell to be written to lies in the adjacent half of the array. 
   By using two levels of metal, true and complement read bitlines (RBL, RBLB) are formed by vertically twisting the read lines at a point between rows  1  and  2 . The vertical connection may be implemented simply by a pair of vias. Similarly, the write bitlines (WBL and WBLB) are also vertically twisted at points between rows  0  and  1  and between rows  2  and  3 . By twisting the RBLs, voltage coupled from an adjacent active WBL will appear as common mode noise on the RBL. Furthermore by providing complementary twisted write bitlines, rail-to-rail swings on the write bitlines result in minimal common mode voltage disturbance on the RBL. 
   An array according to the invention thus has an arrangement of read bitlines to suppress the common mode noise and an arrangement of write bitlines to suppress the differential mode noise. As an example, the common mode noise on the read lines has been simulated to be as much as 0.25V in the embodiment of  FIG. 6 . The embodiment of  FIG. 7  eliminates the noise level coupled to the read bitlines for the same parameters. Creating the common noise and canceling of the common noise level shift is important to avoid a sensing speed dependency on the data pattern. In addition, both RBL and WBL are coupled to half of the cells in the same column, resulting in a fast read and write operation. 
     FIG. 9  illustrates a layout of an array according to the invention. At the center, areas  92  and  92 A contain WWL drivers and RWL drivers. Illustratively, area  90 A contains columns in the lower half of the figure and area  90 B contains corresponding columns in the upper half. 
   Areas  93 A,  94 A and  95 A in the lower half of the figure contain peripheral circuits as described above, containing the sense amplifiers, write drivers and RPBUF that are located near the columns that they write to or read from. At the bottom, a set of I/O circuits  96 A interface with off-chip portions of the system that the chip is part of. In this case, there are 512 columns in the block illustrated. The number of columns per block will vary with different designs. A corresponding set of areas  93 B,  94 B,  95 B and  96 B contain corresponding circuits for the upper half. 
   At the lower left, the write addresses come in from external sources and processed by the logic in write control  91 , which recognize whether the write address is in the upper or lower half of the array and activate circuits accordingly. Similarly, the read addresses enter in the lower right and are processed by read control  91 A. For example, if a read operation is to be performed in the lower half, the correct sense amp in the lower portion will be activated and the sense amp on the upper portion of that column will be disabled, so as to reduce the capacitive load during the read operation. 
   While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.

Technology Classification (CPC): 7