Abstract:
A method and system are described for a two step precharging of bitlines in a memory array. In the first step a partial precharge of the bitline is accomplished with a lower power supply, the second step completes the bitline precharge with the higher power supply. Since the higher power supply must ultimately supply the final bitline precharge voltage achieving a partial bitline precharge with a lower power supply will result in lower sram and system power.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/882,914 filed Dec. 30, 2006, the disclosure of which is incorporated herein. 
     
    
     BACKGROUND 
       [0002]    A considerable portion of portable electronic devices, such as cellular telephones, are memory devices. Therefore, a goal in the semiconductor and electronics industry is to make memory devices in portable electronic devices, smaller and consume less power. For example, a challenge is to support trends to smaller sized memory devices, such as the trend from “65 nm” technology bit cells to “45 nm” technology bit cells. Since portable electronic devices rely almost exclusively on battery power, components such as memory devices should be power efficient, minimizing power consumption and power dissipation. 
         [0003]    Semiconductor memories or memory devices can be characterized as volatile random access memories (RAMs) or nonvolatile read only memories (ROMs), where RAMs can include static RAM (SRAM) and dynamic RAM (DRAM). In general, SRAM and DRAM differ in the way they store a state of a bit in a bit cell of the memory. In SRAM, each bit cell can include circuitry (typically a transistor circuit) that implements a bi-stable latch. Such a transistor circuit can rely on transistor gain and positive feedback, where one of two possible states are assumed, i.e., “ON” or state 1, or “OFF” or state 2. An application of voltage to the bi-stable latch induces the state to change from one to the other. This allows a state written to a bit cell to be retained until the bit cell is reprogrammed. 
         [0004]    An SRAM may be arranged as a matrix of memory or bit cells fabricated in an integrated circuit (IC) chip, where address decoding in the IC chip allows access to each bit cell for read/write functions. SRAM bit cells can include active feedback from cross-coupled inverters in the form of a latch to store or “latch” a bit of information. These SRAM bit cells can be arranged in rows, such that blocks of data (e.g., words, bytes, etc.) can be written or read simultaneously. 
         [0005]    A particular challenge in memory device technology in general, and SRAM in specific, is variability in process and manufacture of memory devices. For example, there can be significant variances in the bit cells of SRAM devices that affect performance. The variances may further be complicated due to actual operating temperature changes. 
       SUMMARY 
       [0006]    This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
         [0007]    In an embodiment, a static random access memory (SRAM) device includes multiple bit cells connected to bitlines and wordlines, and a first precharge device to perform partial precharging of the bitlines with a first power supply and a second precharge device to complete precharging of the bitlines with a second power supply. 
     
    
     
       BRIEF DESCRIPTION OF THE CONTENTS 
         [0008]      FIG. 1  is a block diagram illustrating an exemplary system that supports reduced power bitline precharge for a SRAM memory device. 
           [0009]      FIG. 2  is a block diagram illustrating an exemplary precharged bit cell array in a SRAM memory device. 
           [0010]      FIG. 3  are timing diagrams illustrating wordline, bitline, precharge 1 , precharge 2  activation in a SRAM memory device. 
           [0011]      FIG. 4  is a flowchart illustrating a process to support reduced power bitline precharge for a SRAM memory device. 
       
    
    
     DETAILED DESCRIPTION  
       [0012]    An exemplary system and methods for implementing reduced power consumption in a static random access memory (SRAM) device are described. The exemplary system and methods include providing separate precharge signals to bitlines connected to bit cells arranged in matrix or array of a SRAM device. The system and methods may be included in or part of a portable electronic device, for example a wireless communication device, such as a cellular telephone. 
         [0013]      FIG. 1  shows an exemplary system  100 . In this example, system  100  is a system on an integrated chip or SOIC. Although, the system  100  is discussed in reference to distinct blocks or components, it is to be appreciated that other implementations may combine such components or functions of such components, rely on functionality from other components (either internal or external to system  100 ), forego particular components and/or functionality, and so on. 
         [0014]    System  100  includes one or more processors or controllers  102 . An example of such controllers  102  is SmartReflex™ technology offered by the Texas Instruments® Corporation. Controllers  102 , such as SmartReflex™ controllers may implement a feedback and control system to monitor temperature and operation of system  100  and its components. In other words, controllers  102  may include intelligent and adaptive hardware and software techniques that dynamically control voltage, frequency, and power based on device activity, modes of operation and temperature. Furthermore, controllers  102  are coupled to and may be configured to monitor and provide intra and inter communications, and to regulate power in the system  100 . Interfaces  104  may be provided to support such communications. Interfaces  104  may include various communication input/output interfaces and communication busses or lines. 
         [0015]    The exemplary system  100  includes a power supply  106  which may be a component that receives power from an external source and stores the power to be used by system  100 . Power supply  106  can include a regulated voltage or current supply. The system  100  can include a clock  108  used for various timing operations by system  100 . 
         [0016]    System  100  includes a memory component or memory  100 . Memory  100  can include volatile and non volatile memory, such as ROM and RAM memory. Memory  100  is particularly accessed and controlled by controllers  102 , and interfaces with other components in system  100 . In particular, memory  110  receives power from power supply  106 , communicates with or through interfaces  106 , and receives clock or timing signals from clock  108 . Memory  100  includes a static random access memory (SRAM) device or component, hereinafter referred to as SRAM  112 . 
         [0017]    As discussed below, SRAM  112  may be configured as an array of bit cells. SRAM  112  may implement a particular size technology, such as 65 nm technology as known in the industry. 
         [0018]      FIG. 2  shows an exemplary SRAM  112  configuration. SRAM  112  includes an array of bit cells  200 . It is to be understood, that the array of bit cells  200  includes other bit cells not shown, and that SRAM  112  configuration includes other components. The components that are shown are illustrative of other like components that are not shown. The bit cells  200  are connected by bitline  202  and complementary or NOT bitline  204 , and various wordlines  206 . The array of bit cells also includes a column select line  208  that selects bit lines (e.g., bitline  202  and bitline (NOT)  204 ). 
         [0019]    For 65 nm technology node and future nodes (i.e., 45 nm technology and beyond), a SRAM bit cell  200  may not be able to operate reliably at voltages below 1.0 volt. A solution can be to operate the SRAM  112  with two different supply voltages (i.e., split-rail or dual-rail). In split-rail operation the chip logic (e.g., controllers  102 ) and the SRAM  112  periphery are allowed to operate from a power supply (e.g., power supply  106 ) which can go well below 1.0 volt. The SRAM  112  array of bit cells  200 , operates from a supply which will not go below 1.0 volt, or whatever the minimum operating voltage of the SRAM bit cell  200  happens to be. However, many wireless applications cannot afford two supplies, so the SRAM array voltage is typically generated on chip from an available 1.8 volt supply by use of a low dropout regulator. In other words, a 1.8 volt supply is used for the I/O circuits. 
         [0020]    Corresponding to a power point of view, current sourced from the SRAM  112  array supply is multiplied by 1.8 volts, whereas current sourced from the lower 1.0 volt or lower supply is multiplied by 1.0 volt or lower, depending on the value of this power supply. For low power applications as much of current load as possible is moved to the chip/SRAM periphery supply. 
         [0021]    A precharge voltage is required to precharge the bitlines  202  and  204 . As size of SRAM  112  decreases to a 45 nm technology node, the SRAM bit cell  200  may need very precise control of biases applied to terminals of the bit cell  200 . In the system we have described, the bit cell array uses a power supply V DD1    210  which may not go below 1.0v for example so that proper operation of the SRAM bit cell  200  can be guaranteed. On the other hand, the rest of the SRAM periphery may use a power supply V DD2    212  which may be equal to the power supply V DD1    210  or well below the power supply V DD1    210 . The bitlines  202  and  204  may not be precharged with the power supply V DD2    212  due to the necessity of precise bit cell bias control and the wide range of values of power supply V DD2    212 . 
         [0022]    In this example, V DD1    210  may be a generated 1.8 volt power supply, such as power supply  106 . In addition to V DD1    210 , a lower value power supply V DD2    212  is provided which may be 1.0 volts. Therefore, in this implementation there is a higher value (1.8 volt) power supply in V DD1    210  and a lower value (≦1.0 volt) power supply in V DD2    212 . V DD2    212  may be a power supply provided or regulated by controller(s)  102 . In particular applications, a SmartReflex™ controller may provide such regulated lower value power. 
         [0023]    V DD2    212  may be used to provide an initial precharge voltage to the bitlines  202  and  204 , and V DD1    210  is used to complete the final precharge of said bitlines. The total precharge is made to the bitlines  202  and  204  to a level which is precisely the array supply V DD1    210 . If only the array supply V DD1    210  supplies the bitline  202  and  204  precharge current, higher power dissipation may result. 
         [0024]    In this example, an additional bitline precharge devices may be added to each bitline  202  and  204 . These additional bitline precharge devices can be connected to an IC or SRAM periphery power supply (i.e., power supply  106 ), and these devices will pulse on after the wordline has shut off. After the bitlines  202  and  204  or bit cells  200  have been precharged with the chip/SRAM periphery voltage or V DD2    212 , the other precharge devices and equalize device turn on as normal. 
         [0025]    In this implementation, the precharge devices for a particular pair of bitlines  202  and  204  are made up of MOSFETs  214 ,  216 ,  218 ,  220 , and  230 . Particular MOSFETs will be described as making up exemplary precharge devices in the SRAM  112 . It will be apparent, that other MOSFETs shown in  FIG. 2  make up similar precharge devices and perform similar functions. In this example, MOSFETs  234 ,  236 ,  238 , and  240  are used to provide column select line  208  to select a particular bitline pair  202  and  204 . 
         [0026]    For example, a higher voltage precharge device, to provide a PreCharge 1  current or PreCharge 1   242 , includes MOSFETs  216  and  218 . MOSFETs  216  and  218  are particularly charged by higher value V DD1    210 . A lower voltage precharge device, to provide a PreCharge 2  current or PreCharge 2   244  includes MOSFETs  214  and  220 . MOSFETs  214  and  220  are particularly charged by lower value V DD2    212 . In operation, the lower voltage precharge device is activated first to provide a current to enable initial bitline precharge, and the higher voltage precharge device is activated to supply whatever remaining current may be needed for precharging the bitlines  202  and  204 . MOSFET  230  is used as an equalize device to ensure bitline  202  and bitline (NOT)  204  have exactly the same final precharge voltage. 
         [0027]      FIG. 3  shows timing diagrams for activation of Wordline  206 , Bitline  202 , PreCharge 1   242 , and PreCharge 2   244 . In particular, timing diagram  300  represents Wordline  206 ; timing diagram  302  represents Bitline  202 ; timing diagram  304  represents PreCharge 1   242 ; and timing diagram  306  represents PreCharge 2   244 . As illustrated by the timing diagrams of  FIG. 3 , PreCharge 1  is low or “ON” until the bitlines are accessed and made high or “OFF”. This turns off the precharge devices, and allows the Wordline  206  to turn “ON”, and develop a signal differential on Bitlines  202  and  204 . After the Wordline  206  is turned off, the discharged Bitline  202  or  204  is precharged back up. As illustrated, PreCharge 2   244  pulses or turns “ON” to provide a majority of the current injecting for precharging, and PreCharge 1   242  is used to complete the precharging and maintain the precharge level on Bitlines  202  and  204  until the next Wordline  206  activation. 
         [0028]      FIG. 4  shows a process  400  that provides for precharging bitlines in SRAM memory device. 
         [0029]    The process  400  is illustrated as a collection of blocks in a logical flow graph, which represent a sequence of operations that can be implemented in hardware such as described above. Although described as a flowchart, it is contemplated that certain blocks may take place concurrently or in a different order. 
         [0030]    At block  402 , a determination is made as to a precharge value for bitlines in bit cell array of a SRAM memory device. This value may be an existing array V DD  voltage value or a value below the array V DD  voltage. Consideration may also be made as to temperature, speed, and efficiency in determining the precharge value. 
         [0031]    At block  404 , determination is made as to a value that a first precharge device can provide. The first precharge device derives a precharge value from a voltage supply that may be lower than the array V DD  voltage value. The voltage supply may be directed or from a controller, such as SmartReflex™ controller that may take into account process variations of the physical SRAM memory device, along with temperature during operation. 
         [0032]    At block  406 , the first precharge value is applied to all of the bitlines in the SRAM memory device. Every bitline will have a corresponding first precharge device made up of MOSFETs as described above. 
         [0033]    At block  408 , to complete the bitline precharge, a second precharge and an equalize operation is applied or performed on the bitlines. This second precharge is derived from a second precharge device that relies on a higher voltage value, such as existing array V DD  voltage value. Only the necessary voltage is used to make up for the precharge deficiency provided by the first precharge device. 
       CONCLUSION 
       [0034]    The above-described systems and methods describe precharging bitlines of array bit cells in a SRAM memory device. Although the invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as exemplary forms of implementing the claimed invention.