Abstract:
A memory device having a mode register with a selectable bit which sets the memory device to operate with a selected one of a plurality of possible clock input signals, for example, a single clock input or differential clock input.

Description:
This is a continuation application of U.S. patent application Ser. No. 09/939,653, filed on Aug. 28, 2001, now U.S. Pat. No. 6,687,184, issued on Feb. 03, 2004, the disclosure of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to memory devices, and more particularly, to a memory device having a selectable clock input. 
     BACKGROUND OF THE INVENTION 
     Memory devices such as Dynamic Random Access Memories (DRAM) and Synchronous Dynamic Random Access Memories (SDRAM) are regularly used in computing systems for applications ranging from video games to personal computers. 
     An SDRAM usually includes components such as memory arrays, row and column decoders, and control logic. Additionally, an SDRAM typically includes a mode register for setting an operation mode so that the SDRAM can perform various functions that are optimally selected for the system containing the SDRAM. The mode register may allow external setting of operation modes; that is, it may have its set values changed in response to an externally supplied signal. An external clock signal is also used for memory devices to synchronize the operation of the memory device with other components of the computing system. 
     The computing systems within which SDRAMs function usually operate with a predetermined clock input which can be a single clock input or a differential clock input. While a differential clock input system may be preferable for characteristics such as low noise, some point to point systems exist where a single clock input is preferred. 
     To accommodate a single clock input system and a differential clock input system, SDRAMs have to be selected according to, among other features, whether or not the SDRAM&#39;s components can accommodate the clock in the system with which the SDRAM is to be used. This need for multiple types of SDRAMs imposes not only additional manufacturing costs to produce different types of SDRAMs for various systems, but also storage, distribution, and other logistical costs. 
     What is needed is a memory device capable of accommodating more than one clock input system, for example, a single clock input system and a differential clock input system. 
     SUMMARY OF THE INVENTION 
     The shortcomings discussed above are largely overcome by the present invention which in one aspect provides a synchronous memory device with a mode register having a user selectable bit, the state of which internally configures the memory device to operate with either a single clock input or a differential clock input. 
     In another aspect, the present invention provides a memory device which has a mode register in its control logic which has a user selectable bit position which can be set to enable the control logic to appropriately control the operation of the memory device with different types of applied clock input signals. 
     In another aspect, the present invention provides a method for operating a memory system by providing a memory controller and a memory device having a mode register, initializing the memory system to operate with a first clock input signal by sending a signal from the memory controller to the mode register setting the memory device to operate at the first clock input signal, and changing the memory system to operate at a second clock input signal by sending a signal from the memory controller to the mode register to operate the memory device at the second clock input signal, wherein the second clock input signal is different from the first clock input signal. 
     These and other features and advantages of the present invention will be more clearly apparent from the detailed description which is provided in connection with accompanying drawings which illustrate exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a portion of an SDRAM in accordance with an embodiment of the present invention; 
         FIG. 2  is a diagram of a computer and memory system using the SDRAM illustrated in  FIG. 1 ; 
         FIG. 3  is a diagram of a control bus which may be used with the SDRAM illustrated in  FIG. 1 ; 
         FIG. 4  is a diagram of a clock input; 
         FIG. 5  is a diagram of a mode register employed in the SDRAM shown in  FIG. 1 ; and 
         FIG. 6  is diagram of another computer system in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, where like parts are designated by like reference numbers throughout, there is shown in  FIG. 1  a simplified block diagram of an SDRAM  10  in accordance with an embodiment of the present invention. Although the term SDRAM is used throughout this specification, the present invention is applicable to any DRAM technology which uses a clock, and for use in any computing system such as a game, a video card, and a computer system. 
     The SDRAM  10  has a control logic  20  that contains a mode register  22  and a command decoder  24 . The SDRAM  10  also has a memory array  30 , a row decoder  32  and a column decoder  34 , and an address register  36 . Multiple memory arrays  30  may be provided in the SDRAM  10 , along with multiple row decoders  32  and column decoders  34 . A row address multiplexer  38 , a column address counter/latch  37 , and read/write data path components are also provided within the SDRAM  10 . The SDRAM  10  interfaces with external components through a control bus  50 , an address bus  52 , and a data bus  54 . 
     The SDRAM  10  may be used as part of a memory system  51  which in turn is used in a computing system, such as computer system  61  shown in  FIG. 2 . The computer system  61 , which may employ multiple SDRAMs  10 , has a CPU  60  and a memory controller  64  which is part of memory system  51 . Alternatively, the CPU  60  may provide the memory controller functions. The CPU  60  and the memory controller  64  communicate via a local bus  62 . The memory controller  64 , in turn, communicates with the SDRAM(s)  10  via the control bus  50 , address bus  52 , and data bus  54 . As illustrated in  FIG. 3 , the control bus  50  may include multiple signal lines, including a row address strobe line RAS#, a clock enable line CKE, a chip select line CS#, a write enable line WE#, and a column address strobe line CAS#. 
     The control bus  50  also includes a clock signal line CK, and may include a complimentary clock signal line CK#. In a single clock input system only the CK signal would be present, while in a differential clock input system both the CK and CK# signals would be present. Clock signals CK  72  and CK#  70  are graphically represented in  FIG. 4 . The SDRAM  10  synchronizes the output of read data with the rising edges  74  and falling edges  76  of a single clock input system, and with the crossing points  78  of a differential clock input system. Differential clock input systems are also known as double data rate systems. The clock signals are typically generated by a device such as an oscillator, which can be located in a processor, in a memory controller, or anywhere else in a computer system. 
     In a typical operation of the SDRAM  10 , row address and column address signals are asserted by the memory controller  64  on the address bus  52 , and latched into the address register  36 . The row address signals are then supplied to the row address multiplexer  38  which transmits the row address signal to the row decoder  32 , which appropriately accesses a row of the memory array  30 . The column address signals are supplied from the address register  36  to the column address counter/latch  37  which transmits the column address to the column decoder  34 , which appropriately accesses a column of the memory array  30 . As stated above, if the SDRAM contains multiple memory arrays  30 , multiple row decoders  32  and column decoders  34  would likewise be provided. 
     The memory array  30  is coupled to the data bus  54  via read/write data path circuitry  35 . The read data path portion of the read/write data path circuitry  35  comprises circuits which store output addressed data and ensures that the proper signal levels are delivered to the data bus  54 . The write data path portion of the read/write data path circuitry  35  comprises circuits which accept write data from the data bus  54 , hold data to be written, and drive the write data to the addressed areas of memory array  30 . 
     Read and write accesses to the SDRAM  10  are burst oriented, where the burst length determines the maximum number of column locations that can be accessed for a given read or write command. In order to write data, the memory controller  64  asserts a write command on the control bus  50  and subsequently supplies write data to the SDRAM  10  via the data bus  54 . In order to read data, the memory controller  64  asserts a read command on the control bus  50  while simultaneously asserting column and row addresses on the address bus  52 . The preceding is a cursory description of the SDRAM&#39;s  10  operation; the operation may involve numerous additional well known steps involving known components, the descriptions of which are not provided herein for the sake of brevity. 
     The overall operation of the SDRAM  10  is controlled by the control logic  20  which includes the command decoder  24  and the mode register  22 . The command decoder  24  interprets various signal combinations present on the control bus  50  as high level commands asserted by the memory controller  64 , thereby allowing the control logic  20  to carry out internal operations of the SDRAM  10  by implementing the asserted commands. The operation of the control logic  20  is further defined by the settings of the mode register  22 , which is loaded with values which control various SDRAM operational parameters. 
     The mode register  22  has bit positions which are used to define specific modes of operation of the SDRAM  10 . Binary values are set in the mode register  22  by the memory controller  64 . Typical operational states which can be set by binary values set in the mode register  22  include, for example, the selection of a burst length, a burst type, and a CAS latency. The mode register  22  is typically programmed by a command from the memory controller  64  at initialization of the computer system  61 , and will retain the stored information until it is programmed again or the SDRAM  10  loses power. Reprogramming the mode register  22  usually does not change the stored contents of the memory array(s)  30 . 
     The mode register  22  of an SDRAM constructed in accordance with an embodiment of the present invention is illustrated in  FIG. 5 . The mode register  22  has selectable bits A 0 –A 10 , BA 0 , and BA 1  used to define the various modes of operation discussed above. Bit A 0  defines the enable or disable state of a delay lock loop used to synchronize initial memory operations, bit A 1  defines the drive strength for all outputs as normal or reduced, and bits A 2 –A 10  define various operating modes such as load mode register, read, or write. Alternatively, bits A 0 –A 2  may be used to define the burst length, bit A 3  may be used to define the burst type, bits A 4 –A 6  may be used to define the CAS latency, and bits A 7 –A 10  may be used to define the operating mode such as normal or reset operation. Although the mode register  22  illustrated in  FIG. 5  contains multiple bits and sections, including an extended mode register section, the mode register  22  in accordance with the present invention need not incorporate all those sections. These combinations and operations of mode register bits are illustrative only and are not meant to be restrictive in order to practice the present invention. 
     A unique feature of mode register  22  in an SDRAM of the present invention is that a selectable bit may be used to define whether or not the control logic, and therefore the SDRAM  10 , will operate in a single clock input mode or a differential clock input mode. For the exemplary mode register  22  illustrated in  FIG. 5 , mode register bit A 10  can be set to 0 to enable the control logic  20  to operate the SDRAM  10  with a single clock input, or set to 1 to enable the control logic  20  to operate the SDRAM  10  within a differential clock input. As discussed above, the SDRAM  10  synchronizes the output of read data with the rising edges  74  and falling edges  76  of a single clock input system, and with the crossing point  78  of a differential clock input system. Either a differential clock input or a single clock input can be the default setting for the mode register  22 . Although bit A 10  controls the clock input signal setting in the illustrated mode register  22 , any bit may be used for this function. 
     The mode register  22  bit controlling the clock signal input setting for the operation of the SDRAM  10  is usually set at initialization of the computer system  61  to operate in agreement with other components of the system. Alternatively, the mode register  22  can be switched to operate between different types of clock inputs after initialization of the computer system  61 . For example, the computer system  61  may switch from operating at differential clock input to operating at a single clock input when switching from a regular mode to a power savings mode, or from a regular mode to a test mode. During a power savings mode, it may be advantageous to shut down several components of computer system  61 , but to retain the memory stored in the SDRAM  10 . For this purpose, a signal may be sent to the memory controller  64  to set the mode register  22 , and thereby the control logic  20 , to operate at a single clock input, thereby decreasing the amount of power consumed by the computer system  61  and SDRAM memory device. When the computer system  61  is to return to a normal operating mode, a signal would be sent to the memory controller  64  to reset the mode register  22  and control logic  20  to operate with a differential clock input system. 
       FIG. 6  illustrates an example of a computer system  80  that may incorporate an SDRAM  10  containing a mode register  22  in accordance with the present invention. The system  80  has a memory circuit  82  including an SDRAM  10  constructed in accordance with the present invention. The computer system  80  includes a central processing unit (CPU)  84  for performing computer functions, such as executing software to perform desired tasks and calculations. One or more input/output devices  86 ,  88 , such as a keypad or a mouse, are coupled to the CPU  84  and allow an operator to manually input data thereto or to display or otherwise output data generated by the CPU  84 . One or more peripheral devices such as a floppy disk drive  90  or a CD ROM drive  92  may also be coupled to the CPU  84 . The computer system  80  also includes a bus  94  that couples the input/output devices  86 ,  88  and the memory circuit  82  to the CPU  84 . 
     Thus, the present invention provides a mode register  22  that can enable one SDRAM  10  to operate with both single clock and differential clock input systems. This reduces the need to stock multiple types of SDRAMs, thereby reducing costs associated with manufacturing and stocking multiple types of components. Additionally, the mode register  22  in accordance with the present invention allows for the SDRAM  10  to operate in a computing system designed to switch between differential clock and single clock input signals. 
     While the foregoing has described in detail preferred embodiments known at the time, it should be readily understood that the invention is not limited to the disclosed embodiments. For example, although the invention has been described with respect to switching SDRAM operation between a single clock input or a differential clock input, it should be apparent that the invention can also be implemented with a suitable mode register input to select among any two or more different types of clock inputs. In addition, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not limited to the embodiment specifically described but is only limited by the scope of the appended claims.