Abstract:
A domino static random access memory (SRAM) having one or more SRAM memory cells connected with a local bit line is disclosed. The SRAM may include a global bit line, a first precharge device connected between a voltage supply and the local bit line, and a second precharge device connected between the voltage supply and the global bit line. In addition the SRAM may include a global bit line discharge logic connected with the global bit line and the local bit line. The global bit line discharge logic is adapted to draw the global bit line to a voltage below a precharge voltage and above a ground voltage during a read operation.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to a static random-access memory (SRAM). In particular, this disclosure relates to local evaluation logic for a domino-logic SRAM. 
       BACKGROUND 
       [0002]    SRAMs may be arranged in a domino-logic structure in which two or more SRAM cells are connected in parallel to two local bit lines. One of the two local bit lines is coupled to a global bit line through a global bit line discharge logic. Both the local and global bit lines in a domino SRAM are precharged high before a read operation, and may be discharged, or “pulled down” to ground during a read operation. A particular cell is selected for reading by activating the cell&#39;s word line. One of the two precharged local bit lines coupled to the selected cell is discharged during a read operation. If the discharged local bit line is the one that is coupled to the global bit line, the discharge of the local bit line activates logic that discharges the global bit line. 
         [0003]      FIG. 1  is a schematic representation of a portion of a prior art SRAM circuit  100  having a column of SRAM cells  110 , a global bit line (GBL)  106 , a local eval logic  122 , and a GBL discharge logic  120 . Each SRAM cell  110  includes a pair of cross-coupled inverters,  130 ,  132 . SRAM cells are connected to the local eval logic through a local bit line true (LBLT)  102  and a local bit line complement (LBLC)  104 . The LBLT and the LBLC are precharged by a local bit line precharge line (LBL_PCH)  114  and transistors  1 P 1  and  1 P 2 . A write true line (WT)  126  and its associated transistors  1 N 3  and  1 N 6 , a write complement line (WC)  124  and its associated transistors  1 N 4  and  1 N 5 , and a write enable line (WE)  118  and its associated transistor  1 N 7  control the write operations of the cell  110 . The GBL  106  is precharged by a GBL precharge line (GBL_PCH)  112  and a transistor  1 P 3 . 
         [0004]    A read operation employs the GBL discharge logic  120 . As an illustration, to read a “one” from the cell  110  in the SRAM column, a word line (WL)  108  turns on pass transistors  1 N 1  and  1 N 2 . When the “one” is read, the LBLT  102  remains high while the LBLC  104  is pulled down. The value on the LBLC is inverted to a “one” by an inverter  116 , which turns on an N-channel field-effect (NFET) transistor  1 N 8 . Transistor  1 N 8  pulls down the precharged GBL  106  to ground, effectively placing an inversion of the “one” on the GBL. 
       SUMMARY 
       [0005]    One embodiment is directed to a domino static random access memory (SRAM). The SRAM may include one or more SRAM memory cells connected with a local bit line. The SRAM may also include a global bit line, a first precharge device connected between a voltage supply and the local bit line, and a second precharge device connected between the voltage supply and the global bit line. In addition the SRAM may include a global bit line discharge logic connected between the global bit line and the local bit line. The global bit line discharge logic is adapted to draw the global bit line to a voltage below a precharge voltage and above a ground voltage during a read operation. 
         [0006]    Another embodiment is directed to a method for operating a domino static random access memory (SRAM). The method may include precharging a local bit line, the local bit line being connected to an SRAM cell and precharging a global bit line to a precharge voltage. In addition, the method may include reading data from the SRAM. A “zero” data value stored in one of the SRAM cells drives the local bit line to a discharged level. The local bit line discharge level enables the global bit line discharge logic to discharge the global bit line to a voltage level between the precharge voltage level and a voltage above ground. 
         [0007]    A further embodiment is directed to a design structure for producing an SRAM. Aspects of the various embodiments may allow circuit size and power consumption in an SRAM to be reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The drawings included in the present application are incorporated into, and form part of, the specification. They illustrate embodiments of the present invention and, along with the description, serve to explain the principles of the invention. The drawings are only illustrative of embodiments of the invention and do not limit the invention. 
           [0009]      FIG. 1  is a schematic representation of a portion of a prior art SRAM circuit having global bit line discharge logic. 
           [0010]      FIG. 2  is a diagrammatic representation of a portion of an SRAM including a column of SRAM cells, and global bit line discharge logic and a global bit line according to embodiments of the invention. 
           [0011]      FIG. 3  is a schematic representation of a portion of an SRAM circuit having a global bit line discharge logic PFET to discharge a global bit line, according to an embodiment of the invention. 
           [0012]      FIG. 4  is a waveform diagram of a discharge operation of an SRAM global bit line, according to embodiments of the invention. 
           [0013]      FIG. 5  is a flow diagram of a design process used in semiconductor design, manufacture, and/or test of the inventive SRAM depicted in  FIGS. 2 ,  3  and  4 , according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    According to embodiments of the invention, an SRAM global bit line (GBL) may be discharged through GBL discharge logic to a voltage level below a precharge voltage, but above a ground voltage. An SRAM column may contain local evaluation logic and one or more SRAM cells, each cell storing a “one” or a “zero.” The SRAM column may be coupled to the global bit line through a global discharge logic. The global discharge logic may include a P-channel field effect transistor (PFET). During an SRAM read operation, a local bit line true (LBLT) or a local bit line compliment (LBLC) may receive a “zero” from an SRAM cell. The LBLT or LBLC may be connected to the input of the global discharge logic. The “zero” on the input to the global discharge logic may cause the GBL to be partially discharged, thus transferring the “zero” to the GBL. The GBL is subsequently precharged to Vdd for a next operation. 
         [0015]    Power and space reduction and management are becoming increasingly important as circuit technology advances. Fully discharging GBLs to ground in SRAM circuits requires energy and resources. Decreasing SRAM circuit size and power consumption may enable higher density chips with less overall power consumption. Utilizing a single PFET device to discharge a GBL may eliminate a need for an inverter to drive an NFET device within the GBL discharge logic. By reducing the number of transistors in the GBL discharge logic, the circuit area required to discharge the global bit lines may be reduced. A reduction in circuit area may result in shorter GBLs, resulting in less energy consumption to charge and discharge them. Additionally, the PFET device electrical characteristics only allow the GBL to be discharged to a voltage level of one PFET threshold voltage (Vt) above ground, according to embodiments of the invention. 
         [0016]    As may be seen from the following equation, the reduction of overall GBL voltage swing may reduce the energy consumed to charge and discharge the GBL: 
         [0000]    
       
      
       P=A*C*V 
       2 
       *F.  
      
     
       Where: 
       [0017]    P=chip dynamic power consumption (W)
 
A=activity factor (coefficient with values between 0 and 1 indicating signal activity level)
 
C=capacitance of nodes being charged and discharged (F)
 
V=signal voltage swing (V)
 
F=switching frequency (Hz)
 
Reducing the difference in voltage between the precharge voltage and the discharge voltage reduces V in the equation, which in turn may exponentially reduce power used in a read operation. Reduction of read operation power may provide opportunities for SRAM and overall chip power management.
 
         [0018]      FIG. 2  is a diagrammatic representation of a portion of an SRAM circuit having a local bit line compliment (LBLC) line  204  coupled to a write precharge logic  202  and a GBL discharge logic  220 , according to embodiments of the invention. The terms “true” and “complement” are used only to distinguish two similar but inversely operable lines. 
         [0019]    An SRAM circuit  200  may contain a column  201  coupled to a global bit line  206 . The column may have a plurality of groups of SRAM cells  210  and a local evaluation logic  222  for each group. A global bit line discharge line  205  may couple the local evaluation logic to the global bit line. If a “zero” is read from a cell, the LBLC  204  communicates with the GBL discharge logic  220  to partially discharge the GBL. 
         [0020]    In embodiments of the invention, partial GBL discharge may be carried out through the GBL discharge logic  220  which may use a switch, such as a P-channel field effect transistor (PFET), operated by the LBLC or the LBLT line. So long as there is no discharge path for the global bit line, the global bit line may not be pulled down. Once the LBLC or LBLT line enables a discharge path for the global bit line, the global bit line may be partially discharged by the column&#39;s GBL discharge logic. The GBL discharge logic may have a gate operated by a local bit line. 
         [0021]      FIG. 3  is a schematic representation of a portion of an SRAM circuit  300  having the GBL discharge logic  220  that may discharge the global bit line  206 , according to an embodiment of the invention. The LBLC  204  may contain a compliment value of read data in a read operation. The compliment value enables the GBL discharge logic  220  to discharge the shared global bit line  206 . A local bit line true (LBLT)  302  and the local bit line complement (LBLC)  204  are precharged prior to read and write operations by a local bit line precharge line  314  and transistors  3 P 1  and  3 P 2 . The global bit line  206  is precharged prior to read operations by a global bit line precharge line  312  and a transistor  3 P 3 . 
         [0022]    A write enable line  318  and its associated transistor  3 N 7  controls whether a read or write operation of the cell  210  is performed. A write operation employs a write true line  326  and its associated transistors  3 N 3  and  3 N 6 , and a write complement line  324  and its associated transistors  3 N 4  and  3 N 5  for inputting data to the cell  210 . 
         [0023]    A read operation employs the LBLC  204  and the GBL discharge logic  220 . In one embodiment, the GBL discharge logic  220  may include a PFET transistor  3 P 4 . The gate of PFET  3 P 4  may be coupled to the LBLC  204 . The source of PFET  3 P 4  may be coupled to the GBL  206 . The drain of PFET  3 P 4  may be coupled to the ground. 
         [0024]    As an illustration, to read a “one” from the cell  210 , a word line  308  turns on pass transistors  3 N 1  and  3 N 2 . A local bit line true  302  remains high, as a one is stored on the left node of the cell, while the local bit line complement (LBLC)  204  is pulled down. The LBLC is coupled to a global bit line discharge transistor  3 P 4 . The low on the LBLC turns on the global bit line discharge transistor  3 P 4 . When turned on, PFET transistor  3 P 4  provides a discharge path for the precharged GBL  206  to be pulled down, outputting the compliment of the “one” stored in the cell  210  on the GBL. GBL is discharged to a voltage level above ground. A stored “zero” may be read in a similar operation as described above. 
         [0025]      FIG. 4  is waveform diagram of the global bit line (GBL)  206  voltage level during a read operation of an SRAM cell according to embodiments of the invention.  FIG. 4  is an exemplary depiction that illustrates a typical GBL voltage  408  swing between the GBL precharge level  403  and the GBL discharge level  412 . In this example, the GBL precharge level is identical to supply voltage (Vdd)  402 . Other embodiments may employ GBL precharge levels that differ from supply voltage (Vdd). 
         [0026]    Prior to a read operation, the GBL  206  is precharged to Vdd. When a “one” is read from a cell, the GBL discharge logic ( 220 ) discharges the GBL  206  to a GBL discharge level  412 . The GBL discharge level is one PFET threshold voltage  401  above GND  410 . 
         [0027]    In an exemplary embodiment of the invention, the PFET thresholds Vt  401  may be approximately ten percent of the supply voltage Vdd  402 . In another exemplary embodiment of the invention, the PFET threshold Vt may be approximately twenty percent of the supply voltage Vdd. A generally accepted range of Vt values lies between approximately ten and thirty percent of supply voltage Vdd, but this range does not limit possible Vt values in any way. PFET threshold voltages may vary according to several factors, not limited to various design parameters and semiconductor process variations. One skilled in the art of SRAM design will understand how various design parameters can be determined to effect a change in PFET threshold voltages (Vt). 
         [0028]      FIG. 5  illustrates multiple design structures  500  including an input design structure  520  that is preferably processed by a design process. Design structure  520  may be a logical simulation design structure generated and processed by design process  510  to produce a logically equivalent functional representation of a hardware device. Design structure  520  may alternatively include data or program instructions that, when processed by design process  510 , generate a functional representation of the physical structure of a hardware device. Whether representing functional or structural design features, design structure  520  may be generated using electronic computer-aided design, such as that implemented by a core developer/designer. When encoded on a machine-readable data transmission, gate array, or storage medium, design structure  520  may be accessed and processed by one or more hardware or software modules within design process  510  to simulate or otherwise functionally represent an electronic component, circuit, electronic or logic module, apparatus, device, or system such as those shown in  FIGS. 2 and 3 . As such, design structure  520  may include files or other data structures including human or machine-readable source code, complied structures, and computer-executable code structures that, when processed by a design or simulation data processing system, functionally simulate or otherwise represent circuits or other levels of hardware logic design. Such data structures may include hardware-description language design entities or other data structures conforming to or compatible with lower-level HDL design languages such as Verilog and VHDL, or higher level design languages such as C or C++. 
         [0029]    Design process  510  preferably employs and incorporates hardware or software modules for synthesizing, translating, or otherwise processing a design/simulation functional equivalent of the components, circuits, devices, or logic structures shown in  FIGS. 2 and 3  to generate a Netlist  580  which may contain design structures such as design structure  520 . Netlist  580  may comprise, for example, compiled or otherwise processed data structures representing a list of wires, discrete components, logic gates, control circuits, I/O devices, models, etc. that describe the connections to other elements and circuits in an integrated circuit design. Netlist  580  may be synthesized using an iterative process in which Netlist  580  is resynthesized one or more times depending on design specifications and parameters for the device. As with other design structure types described herein, Netlist  580  may be recorded on a machine-readable data storage medium or programmed into a programmable gate array. The medium may be a non-volatile storage medium such as a magnetic or optical disk drive, a programmable gate array, a compact flash, or other flash memory. Additionally, the medium may be a system or cache memory, buffer space, or electrically or optically conductive devices and materials on which data packets may be transmitted and intermediately stored via the internet, or other suitable networking means. 
         [0030]    Design process  510  may include hardware and software modules for processing a variety of input data structure types including Netlist  580 . Such data structure types may reside, for example, within library elements  530  and include a set of commonly used elements, circuits, and devices, including models, layouts, and symbolic representations, for a given manufacturing technology (e.g., different technology nodes, 32 nm, 45 nm, 90 nm, etc.). The data structure types may further include design specifications  540 , characterization data  550 , verification data  560 , design rules  550 , and test data files  585  which may include input test patterns, output test results, and other testing information. Design process  510  may further include, for example, standard mechanical design processes such as stress analysis, thermal analysis, mechanical event simulation, process simulation for operations such as casting, molding, and die press forming, etc. One of ordinary skill in the art of mechanical design can appreciate the extent of possible mechanical design tools and applications used in design process  510 , without deviating from the scope and spirit of the invention. Design process  510  may also include modules for performing standard circuit design processes such as timing analysis, verification, design rule checking, place and route operations, etc. 
         [0031]    Design process  510  employs and incorporates logic and physical design tools such as HDL compilers and simulation model build tools to process design structure  520  together with some or all of the depicted supporting data structures, along with any additional mechanical design or data, to generate a second design structure  590 . Design structure  590  resides on a storage medium or programmable gate array in a data format used for the exchange of data of mechanical devices and structures (e.g., information stored on an IGES, DXF, Parasolid XT, JT, DRG, or any other suitable format for storing or rendering such mechanical design structures). Similar to design structure  520 , design structure  590  preferably comprises one or more files, data structures, or other computer-encoded data or instructions that reside on transmission or data storage media and that, when processed by an ECAD system, generate a logically or otherwise functionally equivalent form of one or more of the embodiments of the invention shown in  FIGS. 2 and 3 . In one embodiment, design structure  590  may comprise a compiled, executable HDL simulation model that functionally simulates the devices shown in  FIGS. 2 and 3 . 
         [0032]    Design structure  590  may also employ a data format used for the exchange of layout data of integrated circuits and/or symbolic data format (e.g., information stored in a GDSII, GL1, OASIS, map files, or any other suitable format for storing such design data structures). Design structure  590  may comprise information such as symbolic data, map files, test data files, design content files, manufacturing data, layout parameters, wires, levels of metal, vias, shapes, data for routing through the manufacturing line, and any other data required by a manufacturer or other designer/developer to produce a device or structure as described above and shown in  FIGS. 2 and 3 . Design structure  590  may then proceed to a state  595  where, for example, design structure  590  proceeds to tape-out, is released to manufacturing, is released to a mask house, is sent to another design house, is sent back to the customer, etc. 
         [0033]    Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will become apparent to those skilled in the art. Therefore, it is intended that the following claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.