Abstract:
A DRAM circuit with reduced power consumption and in some circumstances faster memory array access speed. Input/output lines connected to a memory array are sensed according to their capacitance/length in comparison to a threshold capacitance/length. The input/output lines that are shorter, or less capacitive, than the threshold are sensed sooner than those input/output lines that are longer, more capacitive, than the threshold. Since shorter input/output lines are sensed sooner, they require less power and may be accessed faster.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to memory circuits. In particular, the invention relates to improving power consumption and memory access speed in a dynamic random access memory (DRAM) circuit.  
       BACKGROUND OF THE INVENTION  
       [0002]     An essential semiconductor device is semiconductor memory, such as a random access memory (RAM) device. A RAM device allows the user to execute both read and write operations on its memory cells. Typical examples of RAM devices include dynamic random access memory (DRAM) and static random access memory (SRAM).  
         [0003]     DRAM is a specific category of RAM containing an array of individual memory cells, where each cell includes a capacitor for holding a charge and a transistor for accessing the charge held in the capacitor. The transistor is often referred to as the access transistor or the transfer device of the DRAM cell.  
         [0004]      FIG. 1  illustrates a portion of a DRAM memory circuit containing two neighboring DRAM cells  10 . Each cell  10  contains a storage capacitor  14  and an access field effect transistor or transfer device  12 . For each cell, one side of the storage capacitor  14  is connected to a reference voltage (illustrated as a ground potential for convenience purposes). The other side of the storage capacitor  14  is connected to the drain of the transfer device  12 . The gate of the transfer device  12  is connected to a signal line known in the art as a word line  18 . The source of the transfer device  12  is connected to a signal line known in the art as a bit line  16  (also known in the art as a digit line). With the memory cell  10  components connected in this manner, it is apparent that the word line  18  controls access to the storage capacitor  14  by allowing or preventing the signal (representing a logic “0” or a logic “1”) carried on the bit line  16  to be written to or read from the storage capacitor  14 . Thus, each cell  10  contains one bit of data (i.e., a logic “0”or logic “1”).  
         [0005]     A typical DRAM circuit has input/output (I/O) transistors that allow data to be read from and written to a memory array using specific I/O data lines. Due to the DRAM memory array structure, I/O data line lengths will vary. This occurs because a particular I/O data line is coupled to an individual memory module that can be located in one of various locations within the memory array. The capacitance on an individual I/O line varies with the length of the data line. The longer the I/O data line, the greater the capacitance of the I/O data line and the greater amount of time required before data transmitted on the I/O can be sensed.  
         [0006]      FIG. 2  illustrates a conventional DRAM circuit  100 . DRAM circuit  100  includes DRAM memory array  110 , datapath  120 , delay circuit  130 , combinatorial logic circuit  140  and output pads  150 . Memory array  110  includes individual DRAM memory modules  112 ,  114 ,  116  and  118  that possess a certain amount of memory, for example 512 Kb. The memory array  110  may contain more or less memory modules. Moreover, the size of each module may contain more or less memory than illustrated. Each memory module  112 ,  114 ,  116 ,  118  is connected to a data sense amplifier (DSA), such as for example, DSAs  122  and  124 , in datapath  120  by I/O data lines.  
         [0007]     As illustrated, due to an alignment of memory modules, the I/O data lines  113  for memory module  112  are longer than the I/O data lines  119  for memory module  118 . The difference in length is due to the fact that memory module  112  is farther away from the data sense amplifiers than memory module  118 . Consequently, the capacitance of the I/O data lines  113  (e.g., 0.8-1.2 pf, typically around 1 pf) connected to memory module  112  is greater than the capacitance of the I/O pair line  119  (e.g., 0.4-0.8 pf, typically around 0.6 pf) connected to memory module  118 . A threshold distance between I/O data lines which is considered short or long is dependent upon various factors that include e.g., speed, current, layout, process and voltage.  
         [0008]     Delay circuit  130 , which includes delay device  132 , is coupled to an enable line of each data sense amplifier and controls the timing of when data is received by the data sense amplifiers from an I/O data line. The length of delay produced by delay circuit  130  before enabling all data sense amplifiers is associated with the memory module with the longest I/O data lines, in this case memory module  112 . Thus, each I/O data line, regardless of its length, has the same delay (i.e., the delay associated with memory module  112  and I/O data lines  113 ).  
         [0009]     Because transmissions on all the I/O data lines are given the same amount of delay, longer I/O data lines, i.e.,  113 , experience an acceptable change in voltage (delta V) of approximately 300 mV as illustrated in  FIG. 5 . Shorter I/O data lines; however, experience a delta V equal to a full rail voltage, which results in unnecessary power consumption.  
         [0010]     Once data is sensed by the sense amplifiers, i.e., DSAs  122  and  124 , the sensed data is transmitted to combinatorial logic circuit  140  via data lines. The data is subsequently sent to output pads  150  for use by a requesting device.  
         [0011]      FIG. 5  illustrates the signal timing for DRAM circuit  100 . At time t 1 , a chip select signal CS for all data sense amplifiers transitions from low to high. At time t 2 , the delay signal Hfflat produced by delay circuit  130  transitions from low to high, enabling all data sense amplifiers in datapath  120 . Delay signal Hfflat is associated with and generated in accordance with the time required for the most capacitive I/O data lines, in this case the I/O data line  113 . Delay signal Hfflat is used to transfer data from memory module  112  to DSA  124  within a given time period, for example 2 ns. At time t 3 , the delay signal Hfflat transitions from high to low. While the delay signal Hfflat is high, the delta V for the more capacitive I/O data lines is approximately 300 mv. However, the delta V for the less capacitive I/O data lines is a full rail voltage, which produces an unnecessary current draw for the less capacitive I/O data lines. At time t 4 , I/O pull up signal IOPU transitions from low to high in order to pull the I/O lines high.  
         [0012]     As discussed above, in current designs all I/O data lines coming from a memory array are given equal separation time before being sensed by a datapath sense amplifier. The delay for transmission on the I/O lines affects the memory access time for the memory array. In addition, I/Os with a lower capacitance must remain on longer to accommodate the timing of more capacitive I/Os, resulting in excessive power consumption.  
         [0013]     Thus, it is desirable to produce a memory device with reduced power consumption.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     The present invention provides a DRAM circuit that consumes less power during memory array access. The sense timing for an individual I/O data line connected to a memory array is dependent upon its length/capacitance. I/O data lines that are smaller in comparison to a predetermined length/capacitance are sensed before I/O data lines that are larger than the predetermined length/capacitance. This allows faster access from parts of the memory array connected with a smaller I/O data line.  
         [0015]     By sensing an I/O data line based on its length/capacitance only the minimum required separation time for the I/O data line is utilized, current during array access and overall power consumption are both reduced. This sensing technique also permits faster back-to-back array accesses on less capacitive I/O data lines because the sensing of the I/O data lines are controlled independently of the sensing of the other I/O data lines. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The foregoing and other advantages and features of the invention will become more apparent from the detailed description of exemplary embodiments provided below with reference to the accompanying drawings in which:  
         [0017]      FIG. 1  is a circuit diagram illustrating conventional dynamic random access memory (DRAM) cells;  
         [0018]      FIG. 2  is a conventional DRAM circuit showing input/output lines;  
         [0019]      FIG. 3  is an exemplary DRAM circuit according to an embodiment of the present invention;  
         [0020]      FIG. 4  is a timing diagram for sensing I/O lines in the  FIG. 3  circuit according to the present invention;  
         [0021]      FIG. 5  is a timing diagram for sensing I/O lines according for a conventional DRAM circuit; and  
         [0022]      FIG. 6  is a processor system using a  FIG. 3  DRAM circuit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     In the following detailed description, reference is made to the accompanying drawings, which are a part of the specification, and in which is shown by way of illustration various embodiments whereby the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to make and use of the invention. It is to be understood that other embodiments may be utilized, and that structural, logical, and electrical changes, as well as changes in the materials used, may be made without departing from the spirit and scope of the present invention.  
         [0024]      FIG. 3  illustrates an exemplary DRAM circuit  200  according to an embodiment of the present invention. DRAM circuit  200  is similar to DRAM circuit  100  and includes memory array  110 , datapath  120 , combinatorial logic circuit  140  and output pads  150 . However delay circuit  130  ( FIG. 1 ) is replaced with delay circuit  210  constructed in accordance with the invention.  
         [0025]     Delay circuit  210  has multiple delay devices, i.e., delay devices  212  and  214 , which are used to control the timing of when data is received and sensed by the data sense amplifiers from respective I/O data lines. Instead of delaying all data sense amplifiers based on the longest I/O data lines, the delay timing for the sense amplifiers are divided into stages that allow shorter I/O data lines to be sensed sooner than longer I/O data lines.  
         [0026]     With staged delays, delay circuit  210  produces multiple delay times. For example, because memory module  112  has long I/O data lines (in comparison to the other I/O data lines), a longer delay is required before its associated data sense amplifier  124  should be enabled. Accordingly, the enable line for data sense amplifier  124  receives a delay signal HffLong that enables data sense amplifier  124  according to a timing delay necessary for longer, more capacitive I/O data lines. The delay signal HffLong is created by combining the timing delay produced by both delay devices  212  and  214 . The determination of which data line receives which delay signal (HffLong or HffShort) can be determined at various stages in design, for example fabrication, testing, etc.  
         [0027]     Because memory module  118  has short I/O data lines, a shorter delay (in comparison to the other I/O data lines), is required before the data sense amplifier  122  is enabled. Accordingly, the enable line for data sense amplifier  122  receives a delay signal HffShort, which enables data sense amplifier  122  according to a timing delay necessary for shorter, less capacitive I/O lines. The delay signal HffShort is created by delay device  212  only. Data is output from the data sense amplifiers  122  and  124  to output pads  150  as previously discussed.  
         [0028]      FIG. 4  illustrates exemplary signal timing for DRAM circuit  200 . At time t 1 , chip select signals CS Short and CS Long transition from low to high. At time t 2 , the delay signal HffShort produced by delay device  212  of delay circuit  210  transitions from low to high, enabling the sense amplifiers in datapath  120  connected to the shorter, less capacitive I/O lines pairs. At time t 3 , the delay signal HffLong produced by delay devices  212  and  214  of delay circuit  210  transitions from low to high, enabling the sense amplifiers in datapath  120  connected to the longer, more capacitive I/O lines pairs. At time t 4 , the delay signal HffShort transitions from high to low since data transfer to data sense amplifier  122  has completed. Also at time t 4 , the chip select signal CS Short for the data sense amplifiers coupled to delay circuit  210  by delay signal HffShort, i.e., data sense amplifier  122 , transitions from high to low, and I/O pull up signal IOPU Short transitions from low to high in order to pull the shorter, less capacitive I/O lines high. At time t 5 , the delay signal HffLong transitions from high to low once data transfer to data sense amplifier  124  is complete. Also at time t 5 , the chip select signal CS Long for the data sense amplifiers coupled to delay circuit  210  by delay signal HffLong, i.e., data sense amplifier  124 , transitions from high to low. At time t 6 , I/O pull up signal IOPU Long transitions from low to high in order to pull the longer, more capacitive I/O data lines high.  
         [0029]     In utilizing multiple delay signals, HffShort and HffLong, sensing of those I/O data lines that are shorter is not delayed for an unnecessary amount of time (which as discussed above with regard to  FIG. 5  leads to an increased current draw during memory access and increased power consumption). By sensing shorter, less capacitive I/O data lines independently of the longer, more capacitive I/O data lines, the delta V for the more capacitive I/O data lines and less capacitive I/O data lines are both approximately 300 mv, which is desirable (in comparison to the prior art). Consequently, the current draw for the less capacitive I/O data lines is thereby reduced.  
         [0030]      FIG. 6  illustrates an exemplary processing system  500  that utilizes a DRAM memory device  200  in accordance with the embodiments of the present invention disclosed above in  FIGS. 1-3 .  FIG. 4  depicts an exemplary personal computer or work station architecture. The processing system  500  includes one or more processors  501  coupled to a local bus  504 . A memory controller  502  and a primary bus bridge  503  are also coupled to the local bus  504 . The processing system  500  may include multiple memory controllers  502  and/or multiple primary bus bridges  503 . The memory controller  502  and the primary bus bridge  503  may be integrated as a single device  506 .  
         [0031]     The memory controller  502  is also coupled to one or more memory buses  507 . Each memory bus accepts memory components  508  that include at least one memory device  200 . The memory components  508  may be a memory card or a memory module. Examples of memory modules include single inline memory modules SIMMs and dual inline memory modules DIMMs. The memory components  508  may include one or more additional devices  509 . For example, in a SIMM or DIMM, the additional device  509  might be a configuration memory, such as serial presences detect SPD memory. The memory controller  502  may also be coupled to a cache memory  505 . The cache memory  505  may be the only cache memory in the processing system. Alternatively, other devices, for example, processors  501  may also include cache memories, which may form a cache hierarchy with cache memory  505 . If the processing system  500  includes peripherals or controllers, which are bus masters or which support direct memory access DMA, the memory controller  502  may implement a cache coherency protocol. If the memory controller  502  is coupled to a plurality of memory buses  516 , each memory bus  516  may be operated in parallel, or different address ranges may be mapped to different memory buses  507 .  
         [0032]     The primary bus bridge  503  is coupled to at least one peripheral bus  510 . Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus  510 . These devices may include a storage controller  511 , a miscellaneous I/O device  514 , a secondary bus bridge  515 , a multimedia processor  518 , and a legacy device interface  520 . The primary bus bridge  503  may also be coupled to one or more special purpose high speed ports  522 . In a personal computer, for example, the special purpose port might be the Accelerated Graphics Port AGP, used to couple a high performance video card to the processing system  500 .  
         [0033]     The storage controller  511  couples one or more storage devices  513 , via a storage bus  512 , to the peripheral bus  510 . For example, the storage controller  511  may be a SCSI controller and storage devices  513  may be SCSI discs. The I/O device  514  may be any type of peripheral. For example, the I/O device  514  may be a local area network interface, such as an Ethernet card. The secondary bus bridge may be used to interface additional devices via another bus to the processing system. For example, the secondary bus bridge may be a universal serial port USB controller used to couple USB devices  517  via to the processing system  500 . The multimedia processor  518  may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to one additional device such as speakers  519 . The legacy device interface  520  is used to couple legacy devices, for example, older style keyboards and mice, to the processing system  500 .  
         [0034]     The processing system  500  illustrated in  FIG. 6  is only an exemplary processing system with which the invention may be used. While  FIG. 3  illustrates a processing architecture especially suitable for a general purpose computer, such as a personal computer or a workstation, it should be recognized that well known modifications could be made to configure the processing system  500  to become more suitable for use in a variety of applications. For example, many electronic devices that require processing may be implemented using a simpler architecture that relies on a CPU  501  coupled to memory components  508  and/or memory buffer devices  504 . These electronic devices may include, but are not limited to audio/video processors and recorders, gaming consoles, digital television sets, wired or wireless telephones, navigation devices (including system based on the global positioning system GPS and/or inertial navigation), and digital cameras and/or recorders. The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices.  
         [0035]     While the invention has been described and illustrated with reference to specific exemplary embodiments, it should be understood that many modifications and substitutions could be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.