Abstract:
A memory and method for operating it provide for increased data access speed. In an implementation, a synchronous memory or SDRAM includes a central memory region with memory blocks arranged in sets on respective opposite sides. A number of primary sense amplifier sets are provided, each set being associated with a respective set of the memory blocks and located adjacent. A row cache is provided in the central memory region, and row decoders decode a row address in response to a “bank activate” command and move data from a decoded row address into a primary sense amplifier set associated with a memory block containing the decoded row address and into the row cache, before application of a “read” command to the SDRAM. Column decoders decode a column address in response to a “read” command and for reading data from the cache in accordance with the decoded column address.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. application Ser. No. 10/178,072, filed on Jun. 20, 2002, which is incorporated by referenced. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to improvements in memory architectures and methods for operating same, and more particularly, to improvements in memory architectures and operations thereof for increasing the speed of data transfers between the memory array and a cache memory associated therewith. 
     2. Relevant Background 
     During recent years, the memory industry has begun to widely use synchronous DRAMs (SDRAMs). SDRAMs have begun to be so commonplace, that often the reference to its synchronous nature is dropped, and they are referred to simply as DRAMs. However, to avoid confusion, synchronous DRAMs will be consistently referred to herein as “SDRAMs”. 
     As SDRAMs have developed, they have been operated at ever increasing clock speeds. The SDRAMS of the second generation were called double data rate (DDR) SDRAMs, and are now often referred to as “DDR-I” SDRAMs. In the second generation of synchronous SDRAMs, clock frequencies of 133 megahertz were common. Recently, a third new generation of synchronous SDRAMs are being defined to operate at speeds of two times or greater than DDR-I SDRAMs, and are sometimes referred to as “DDR-II” SDRAMs. 
     The clock frequencies presently being investigated for DDR-II SDRAMs are on the order of about 200 megahertz to 400 megahertz. Although the clock frequencies of the SDRAMs have been increasing, the actual signal delays as the signals propagate through the memory circuitry have not been concomitantly decreased. Thus, one might expect that an increased clock speed applied to an SDRAM might increase the data rate available from the SDRAM, or, alternatively, reduce the access time to the data in the SDRAM. However, the memory elements themselves in the SDRAMs are generally substantially the same, and therefore, require the same amount of access time, regardless of the clock speed. Thus, an increased clock speed generally only enables a random bit of data to be accessed in about the same time frame as devices with lower clock speeds. As a result, devices that operate at higher clock speeds merely require a larger number of clock cycles to access the data in the same time frame. 
     To address the goal in the design of memory devices of achieving increased random access speeds, it has been proposed to include cache memory elements in the device into which the contents of the SDRAM array may be temporarily stored prior to being delivered to the output of the memory. As used herein, the term “cache” or “cache memory” is used to refer to a data latch, register, memory, or other suitable circuit that can temporarily hold or store data read from a memory array prior to being delivered to the output of the memory. Among other things, the cache memory serves to reduce the overhead associated with the SDRAM array by allowing data access to occur while the precharge and next activation of the array is underway. This effectively speeds up the overall data rate by eliminating otherwise dead periods. 
     Thus, when an element from the memory array is read, it is detected by a sense amplifier that is associated with the memory cell being read, then subsequently delivered from the sense amplifier to the cache memory element that is at least temporarily associated therewith. One example of a memory array having a cache memory is shown in copending patent application Ser. No. 09/689,219, filed Oct. 11, 2000, the application being assigned to the assignee of this application, and incorporated by reference. 
     Today, in memory architectures, in general, and SDRAM architectures, in particular, one physical circuit layout that has been suggested includes sets of sense amplifiers alternating with memory array blocks serviced by the sense amplifiers. See, for example, U.S. Pat. No. 5,887,272, which is assigned to the assignee of this application, and which is incorporated by reference. The sense amplifiers are arranged in stripes between adjacent SDRAM array blocks. Each sense amplifier stripe may be connected to selectively service the SDRAM cells on both sides of the stripe. Thus, the sense amplifiers in a particular sense amplifier stripe may be selectively connected to selected memory cells on either the memory array located on left of the stripe, or to selected memory cells located on the right of the stripe. 
     Additionally, memory arrays are becoming increasingly dense. For example, SDRAM designers are under constant tension to design SDRAM circuits more densely, but at the same time, to include larger amounts of functionality in the circuit. One of the techniques that integrated circuit manufacturers have used to address these problems is to place greater and greater emphasis on multilayered structures. For example, above the active regions of the device, one or more layers of interconnecting metal or other conducting material, such as polysilicon, or the like, may be used. However, as the number of the layers increases, the planarity of the surface on which subsequent layers are formed becomes increasingly uneven. As a result, the overlying or subsequently formed structures have a tendency to be susceptible to discontinuities, due to step-like structures that form at the surface. As a result, the pitch of the interconnect structures generally cannot be designed at too low a level. (The pitch of an interconnect is regarded as the distance between an interconnect structure and its closest neighbor, plus the dimension of the interconnect itself.) 
     One SDRAM example is class of SDRAM devices, called the “Enhanced SDRAM,” or “ESDRAM,” which has been recently introduced by Enhanced Memory Systems, Inc of Colorado Springs, Colo. An example is the SM2603 and SM2604 Enhanced SDRAM (ESDRAM) devices, which are a 64 megabit JEDEC superset standard SDRAM. While pin, function, and timing are compatible with standard SDRAMs, they have a speed and architecture that optimizes system price-to-performance in high performance main memory, video graphics, and embedded systems. 
     This 64 megabit ESDRAM is a high-speed SDRAM configured as four banks of SDRAM with an SRAM row cache per bank and a synchronous interface. All inputs are registered and all outputs are driven on rising clock edges. Within each bank, the devices are organized as 4096 rows of 4096 bits each. Within each row, the 8M×8 device has 512 column address locations and the 4M×16 device has 256 column locations. Read and write accesses are accomplished by opening a row and selecting a column address location for the transaction. A “bank activate” (“ACT”) command instructs the device to open a row in one of the four banks, though all four banks may be active simultaneously. A subsequent “read” or “write” command instructs the device to read data from or write data to a specified column address location. 
     On a random read access, an SDRAM bank is activated and data is latched into the sense amplifiers. The sense amplifiers now hold a row of data and the row is considered open. A “read” command now causes the entire row to latch into the SRAM row cache, and the data at a specified column address location is driven out. Since the row data is latched into the SRAM row cache, the SDRAM sense amplifiers are decoupled from the data. Therefore, the SDRAM precharge time can be hidden behind a burst read from the row cache. This minimizes subsequent page miss latency. Since both precharge and row address strobe (RAS) to column address strobe (CAS) delays are hidden, the device supports an industry leading CAS latency of one at clock frequencies up to 83 megahertz, and CAS latency of two up to 166 megahertz. At 166 megahertz, all but one cycle of the next random access to any location in the same bank can be hidden. This SDRAM dramatically increases sustained bandwidth by up to two times over standard SDRAM. For interleaved burst read accesses, the entire precharge time is hidden and output data can be driven without any wait states. 
     Nevertheless, it may be possible to increase still further the data access time of the device. 
     SUMMARY OF THE INVENTION 
     In light of the above, therefore, in an implementation, the invention provides an improved memory integrated circuit device, especially a synchronous memory integrated circuit device such as an SDRAM. 
     In an implementation, the invention provides a synchronous memory array in which a row, or other portion, of the synchronous memory can be read and transferred to a cache memory upon receipt of a “bank activate” command to increase the access time of the device. 
     These and other objects, features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings and appended claims. 
     According to a broad aspect of the invention, a method is presented for reading data from a synchronous memory of the type having data cells arranged in rows and columns and having a row cache. The method includes receiving an initial command and row address data for reading contents of a row of the memory selected by the row address data in response thereto. The contents of the row selected by the row address data are moved into the row cache, without an intervening command, such as a “read” command. After the contents of the row have been moved into the row cache, a “read” command and column address data are received. In response to the “read” command, data is read from the row cache at a column address specified by the column address data for output by the memory. 
     According to another broad aspect of the invention, a synchronous memory is presented. The synchronous memory includes a row cache and means for receiving an initial command substantially concurrently with row address data and activating for reading a row of the memory selected by the row address data in response thereto. Means for moving the contents of the row of the memory selected by the row address into the row cache are provided, and means for receiving a “read” command substantially concurrently with column address data after the contents of the row has been moved into the row cache are also provided. Means for reading data from the row cache at a column address specified by the column address data in response to the “read” command are provided as well as means for moving the data read from the row cache to an output of the memory after a predetermined number of clock cycles thereafter. 
     According to yet another broad aspect of the invention, an SDRAM adapted to receive “bank activate” and “read” commands is provided. The SDRAM includes a central memory region and a number of memory blocks arranged in first and second sets on respective opposite sides of the central memory region. A number of primary sense amplifier sets are provided, each set associated with a respective pair of the memory blocks and located adjacent thereto. A row cache is provided in the central memory region, and row decoders decode a row address in response to a “bank activate” command and move data from a decoded row address into a primary sense amplifier set associated with a memory block containing the decoded row address and into the row cache, prior to application of a “read” command to the SDRAM. Column decoders decode a column address in response to a “read” command and for reading data from the row cache in accordance with the decoded column address. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is illustrated in the accompanying drawings, in which: 
         FIG. 1  is a diagram showing a physical SDRAM layout illustrating the relationship between the various SDRAM arrays, the sense amplifier stripes, and interconnecting buses, and illustrating the data movement patterns therein, in accordance with a typical embodiment of the prior art. 
         FIG. 2  is a diagram showing a physical SDRAM layout illustrating the relationship between the various SDRAM arrays, the sense amplifier stripes, the cache memory stripes, and the interconnecting buses, and illustrating the data movement patterns therein, in accordance with a preferred embodiment of the invention. 
         FIG. 3  shows timing diagrams and event occurrence timings in the operation of the memory of  FIG. 1 , without a row data cache in accordance with the prior art; and 
         FIG. 4  shows timing diagrams and event occurrence timings in the operation of the memory of  FIG. 2 , in accordance with a preferred embodiment of the invention. In the various figures of the drawing, like reference numerals are used to denote like or similar parts. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As mentioned above, although the clock frequencies of synchronous SDRAMs (SDRAMs) have been increasing, the actual signal delays as the signals propagate through the memory circuitry have not been concomitantly decreased. Typically, the outputs of an SDRAM are driven on the rising edge of the clock. Generally, four memory banks are organized, as desired, for instance, in four blocks, each containing 4096 rows of 4096 bits in a 64 Mbit device, or in some other convenient arrangement to form a memory device of other size. The blocks of a 64 megabit device may be arranged, for example, as an 8M×8 or 4M×16 SDRAM. For instance, in an 8M×8 device, 512 column address locations may be provided, and in a 4M×16 device, 256 column locations may be provided. 
     Typically, read accesses are accomplished by opening a row and subsequently selecting a column address location for the transaction. A “bank activate” command instructs the device to open a row in one of the four banks. A subsequent “read” command instructs the device to read data from a specified column address location. 
     Thus, in typical operation of a prior art device, when a row of data is desired to be accessed, a row selection is made, for example, concurrently with the “bank activate” command. The data at the selected row is transferred to primary sense amplifiers that are closely located to the block in which the selected row is contained. Then, when a subsequent “read” command is issued, column address data is applied to the SDRAM, and is propagated to the remotely located primary sense amplifiers. The data at the selected column address is then transferred back to secondary sense amplifiers in a central block of the device. 
     These transfers of selected row and column data all of the way out to the remote primary sense amplifier locations and the subsequent propagation of the data back from the primary sense amplifiers to the central secondary sense amplifiers or latches adds unnecessary time to the access time of the device. These propagation delays can be significant in terms of the recent speed demands that are placed on the SDRAMs. However, utilizing the cache memory embodiments described below and in the copending patent application Ser. No. 09/689,219, some of these delays can be significantly reduced. 
     One prior art DDR-II SDRAM memory arrangement  10  that has been proposed is illustrated in the diagram of  FIG. 1  to which reference is now made. The diagram of  FIG. 1  shows a block diagram of a DDR-II SDRAM and the signal flow paths of interest therein. The SDRAM architecture  10  includes a number of memory blocks  12 - 16  arranged on either side of a central block  28  that contains central column decoders and secondary sense amplifiers. Beneath each of the sections of memory blocks  12 - 16  are respective row decoders  30 . The memory blocks  12 - 16  are arranged in respective sections on each side of the central block  28 , and a number of such memory arrangements may be included in a single device. 
     Typically, primary row sense amplifiers are located in spaces  32 - 36  between respective pairs of the memory blocks  12 - 16 , so that they may be shared in operation. For example, a set of primary row sense amplifiers may be located in the space  32  between memory blocks  12  and  13 , another in the space  33  between memory blocks  13  and  14 , and so on. 
     The primary row sense amplifiers to the right of the memory block  12  at the furthest distance from the block  28  represents the worst case column decode delay, since the column identification data must travel the furthest to reach the primary sense amplifiers at this location. However, delays are also inherent with respect to the primary sense amplifiers at the other locations as well. It is important to note that, while sense amplifiers are shown shared between adjacent memory blocks, the sense amplifiers need not be shared or staggered in any specific implementation. 
     Thus, in operation, first when a “bank activate” signal is applied to the device, row address data is applied substantially concomitantly, or in association therewith. The row address is transmitted to the desired row location, and the data contained at the selected row is moved from the memory block in which it is contained to the primary row sense amplifiers associated therewith. For example, if the row selected is contained in block  13 , the data in the selected row is transferred to the primary sense amplifiers in spaces  32  and  33  adjacent to and associated with. 
     Subsequently, when a “read” command is applied to the device  10 , substantially concomitantly or in association with column address, the column address is transmitted to the location of the primary sense amplifiers that contain data at the previously selected row address, for example along path  41 . The data at the column address in the selected row is then returned along bus path  43  to the secondary sense amplifiers within the central block  28  for subsequent transfer to the input/output or I/O pins of the device. If desired, a burst length, for example, of four or more words, may be driven back across the array to the secondary sense amplifiers, prefetched into a buffer, and pipelined to the output pins. 
     It can be seen that this results in a signal propagation delay, at worst, of two times one-half of the memory distance of the memory banks of the array if the sense amplifiers of the selected row are located between the outermost memory blocks. Memory requests in most computer systems occur in multiple in-page (within same row) read requests. A standard DDR-II device should keep the row active in order to service more than one “read” command. Therefore, the device must incur the path delays of lines  41  and  43  for every in-page read cycle. 
     In contrast, according to a preferred embodiment of the invention, the propagation delay of an SDRAM is significantly reduced by taking advantage of the existence of an on-board or on-chip cache to receive the entire selected row of data immediately upon the receipt of the “bank activate” signal. According to the invention, the data contained in the selected row is entirely and immediately moved to the cache, although more than one bus transfer may be used. Thus, upon the subsequent receipt of the “read” command, only the column data needs to be decoded. This can be done locally in the cache, without requiring the column addresses to be propagated out into the memory array to retrieve the data thereat. This reduces the signal propagation delay by one-half of the memory distance to the selected memory banks of the array. This results in increasing the read and write operating speed of the device, and reducing the read latency to subsequent column address locations. 
     A diagram of an SDRAM architecture  50 , according to a preferred embodiment of the invention, is shown in  FIG. 2 , to which reference is now additionally made. The SDRAM architecture  50  includes a number of memory blocks  12 - 16  arranged on either side of a block  52  that contains row decode, secondary sense amplifiers, and a data cache. Beneath each of the sections of memory blocks  12 - 16  are respective row decoders  30 . The memory blocks  12 - 16  are arranged in respective sections on each side of the decoder, sense amplifier, and cache block  52 . 
     In contrast to the prior art device of  FIG. 1 , the enhanced architecture includes a data cache in the central circuit block  52 . The cache is used to latch the entire selected row data, and allows the SDRAM array to be precharged or refreshed while concurrently enabling access to the selected data. 
     In operation, upon receipt of the “bank activate” command, the device  50  decodes a row of data within a specified bank. The data contained in the decoded memory elements in a respective one of the memory blocks  12 - 16  are sensed in the respective primary sense amplifiers in the interspaces  32 - 36  between the blocks  12 - 16 , and immediately conducted on buses  44 - 45  to the cache in the block  52 . 
     Thereafter, when the “read” command and column address is received, the data contained in the cache in block  52  is decoded to select data cells at the selected column within the cache for output from the device. Since the data selected by the row address is entirely contained in the cache, the device can be operated in burst or page mode, with the particularly selected data being immediately ready for pipelined output from the cache. Thus, the secondary sense amps are utilized only during the initial bank activation (row decoding and cache loading) time. Multiple in-page (within same row) read requests may then be serviced out of the cache. 
     The performance improvement of the method and architecture of the invention can be appreciated from a comparison of the prior art timing waveforms shown in  FIG. 3  with the timing waveforms according to the present invention shown in  FIG. 4 . 
     As can be seen from the timing diagrams of  FIG. 3 , a first data latency of 5 and a CAS latency of 3 occurs due to the data transfer delays described above with respect to the operation of the memory architecture of  FIG. 1 . If a data cache is added, as shown in  FIG. 4 , the first data access still has a first data latency of 5, but with subsequent accesses having a CAS latency of 2. 
     Currently, DDR-II SDRAMs start at a 256 megabit density and a data rate of 400 megabits per second at a 200 megahertz clock frequency. Clock rates are expected to increase to 400 megahertz and beyond, which will likely increase the latencies in clock cycles to 4 or more. One clock cycle at 500 megahertz is only 2 nanoseconds. By caching the entire row and avoiding the column decode/data transfer times across the array, the CAS latency of the device is reduced by a clock cycle. Today&#39;s system benchmarks clearly demonstrate that reducing the data read latency yields up to 10 percent higher system performance. 
     Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed. 
     For example, in a nonmultiplexed device, the activate and read commands may be considered concurrent. Furthermore, while the drawings illustrate single data rate waveforms, the method and structure of the present invention can be adapted to double data rate operation as well.