Abstract:
A memory controller, interface (I/F) device and method for controlling data communication with a memory device are disclosed. The memory controller allows different types of memory devices to be supported. The memory controller has a first buffer for capturing data at a rising edge of a timing control signal and a second buffer for capturing data at a falling edge of the timing control signal. A mode controller controls or adjusts the timing control signal depending on which one of single data rate (SDR) synchronous dynamic random access memory (SDRAM) mode and double date rate (DDR) mode is selected in response to a mode selection or switch signal. In SDR SDRAM mode, a clock signal is supplied to only the first buffer. In DDR mode, a data strobe signal is supplied to both the first and second buffers. The memory controller may also include a level adjuster for adjusting voltage levels of signals transferred between the memory device and buffers.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a memory controller, and in particular to a memory control technique supporting a plurality of different specifications of random access memory (RAM). 
     2. Description of the Related Art 
     With the increasing speed of central processing units (CPUs), the demand for high-speed dynamic RAM (DRAM) is growing and thereby various types of DRAM have been developed and put in use. A well-known type of DRAM is a synchronous DRAM (SDRAM), which transfers data synchronously with an external clock signal. In SDRAM, the data transfer timing for read and write operations is synchronized with the rising edge of the external clock signal. Current information processing systems such as personal computer (PC) systems are typically designed to use SDRAM. 
     As next generation DRAM, there are considered a RAMBUS® DRAM for personal computers and a double data rate (DDR) SDRAM for servers. In DDR SDRAM, the data transfer timing for read and write operations is synchronized with the rising and falling edges of the clock signal or data strobe signal. Since SDRAM employs a rising edge of the clock signal, a memory controller designed for SDRAM is not applied to DDR SDRAM. Accordingly, it is desirable to provide compatibility for both SDRAM and DDR SDRAM within the same system. 
     To provide such compatibility, a semiconductor memory device selectively operating in a single data rate (SDR) mode and a DDR mode has been disclosed in Japanese Patent Application Unexamined Publication Kokai No. 10-302465. More specifically, the semiconductor memory device is provided with an operation controller which selects one of the SDR mode and the DDR mode depending on an external adjustment signal. When the SDR mode is selected, pulses are generated at timing corresponding to one edge of a system clock signal. In the case of the DDR mode selected, pulses are generated at timing corresponding to both edges of the system clock signal. 
     However, the conventional mode selection mechanism is incorporated within the semiconductor memory device. Accordingly, memory manufacturing steps become complicated, resulting in increased cost of manufacturing. Taking into consideration progression of technical innovation in the field of memory, it is necessary to enhance general versatility and extensibility in a memory controller to handle different types of memory which may be developed in the future. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a memory controller and control method having general versatility and enhanced extensibility, allowing different types of memory devices to be supported. 
     According to the present invention, a memory controller separate from a memory device to be controlled is provided with a function of supporting different types of memory devices. 
     According to an aspect of the present invention, a memory controller for controlling data communication with a memory device, includes: a timing adjuster for adjusting timing of data transfer between the memory device and a data bus depending on a timing control signal; and a mode controller for controlling the timing control signal to meet timing requirements of the memory device depending on a mode selection signal inputted from outside. 
     The memory controller may further include a level adjuster for adjusting a voltage level of data to be transferred between the memory device and the data bus depending on a type of the memory device. 
     According to another aspect of the present invention, a memory controller includes: a first buffer for capturing data at a rising edge of a timing control signal; a second buffer for capturing data at a falling edge of the timing control signal; and a mode controller for controlling the timing control signal to meet timing requirements of the memory device depending on a mode selection signal inputted from outside. 
     The mode controller may select one of a clock signal and a data strobe signal as the timing control signal depending on the mode selection signal. In the case where the memory device is a synchronous RAM (random access memory), the mode controller selects the clock signal to supply it as the timing control signal to the first buffer. In the case where the memory device is a double data rate (DDR) synchronous RAM, the mode controller selects the data strobe signal to supply it as the timing control signal to the first and second buffers. 
     The memory controller may further include a level adjuster for adjusting a voltage level of data to be transferred between the memory device and the data bus depending on a type of the memory device. 
     According to still another aspect of the present invention, an interface device connecting a processor and a memory device through a bus, includes: a memory controller for controlling data communication with the memory device, wherein the memory controller comprises: a timing adjuster for adjusting timing of data transfer between the memory device and a data bus depending on a timing control signal; and a mode controller for controlling the timing control signal to meet timing requirements of the memory device depending on a mode selection signal inputted from outside. 
     According to another aspect of the present invention, a control method for controlling data communication with a memory device, includes the steps of: a) capturing data in a first buffer at a rising edge of a timing control signal; b) capturing the data in a second buffer at a falling edge of the timing control signal; and c) controlling the timing control signal to meet timing requirements of the memory device depending on a mode selection signal inputted from outside. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing a mode selection operation according to the present invention; 
     FIG. 2 is a block diagram showing an example of an information processing system employing a memory controller according to an embodiment of the present invention; 
     FIG. 3 is a block diagram showing internal circuits of the memory controller according to the embodiment shown in FIG. 2; 
     FIG. 4 is a block diagram showing internal circuits of a data buffer in the memory controller of FIG. 3; 
     FIGS. 5A-5D are time charts showing a DDR-mode read operation in accordance with an embodiment of the invention; 
     FIGS. 6A-6C are time charts showing an SDR SDRAM-mode read operation in accordance with an embodiment of the invention; 
     FIGS. 7A-7D are time charts showing a DDR-mode write operation in accordance with an embodiment of the invention; and 
     FIGS. 8A-8C are time charts showing an SDR SDRAM-mode write operation in accordance with an embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a memory controller according to the present invention is designed to support a plurality of memories A, B, which have different specifications, for example, single data rate (SDR) synchronous RAM such as SDR SDRAM, double data rate RAM such as DDR SDRAM, static RAM (SRAM), DDR SRAM, and so on. The memory controller can be set to a selected one of a plurality of internal states each corresponding to the different-type memories depending on a mode selection signal received from outside. The details of the memory controller will be described hereafter, taking as an example the case where both SDR SDRAM and DDR SDRAM are supported. 
     System Configuration 
     Referring to FIG. 2, an information processing system includes an interface section  1 , a CPU  2 , a memory  3 , and a flash memory  4 . The interface section  1  is composed of a memory controller  11  according to the present invention, device interfaces (I/F) to the CPU  2 , the flash RAM  4 , and PCI bus, and registers for function setting, memory form factor indication and the like. The CPU  2  performs data read and write of the memory  3  and flash RAM  4  through the interface section  1 . In this example, the memory  3  is one of SDR SDRAM and DDR SDRAM, which may be used as a work memory in the system. The flash RAM  4  may previously store necessary data such as programs and various control data. 
     The interface section  1  and the CPU  2  may be formed as an integrated circuit on a single semiconductor substrate. The interface section  1  and the CPU  2  may be formed individually as an integrated circuit. As described before, the memory controller  11  is supplied with the mode selection signal. For example, when the mode selection signal is high, the internal state of the memory controller  11  is changed to SDR SDRAM mode and, when low, it is changed to DDR mode. 
     Accordingly, even if the memory  3  is changed from SDR SDRAM to DDR SDRAM, the memory controller  11  allows data communication with the memory  3  without changing specifications of the CPU and DDR SDRAM. 
     Memory Controller 
     Referring to FIG. 3, a delay-locked loop circuit  12  supplies the memory controller  11  with a necessary internal clock signal. Further, a power supply circuit (not shown) supplies the memory controller  11  with a power supply voltage. 
     The memory controller  11  is provided with a control signal generator  31 , which supplies respective control signals to address/data switch  35 , address signal generator  34 , and data buffer  36  in response to control data received from selector  32  and refresh circuit  33 . The selector  32  determines an access mode from data and address data on an address/data bus to output access mode control data to the control signal generator  31 . The refresh circuit  33  generates refresh timing control data from data and address data on the address/data bus and outputs it to the control signal generator  31 . 
     The CPU  2  supplies data and address data to the selector  32 , the refresh circuit  33  and the address/data switch  35  through the address/data bus. The address/data switch  35  divides data and address data on the address/data bus respectively to the data buffer  36  and the address signal generator  34  depending on the control signal from the control signal generator  31 . The address signal generator  34  inputs the address data from the address/data switch  35  to generate an address signal based on the control signal received from the control signal generator  31 . The address signal is output to the memory  3 . 
     The data buffer  36  receives the mode switch signal from outside and further a clock signal and a data strobe signal from the control signal generator  31 . The data buffer  36  has a bidirectional data port and a bidirectional data strobe port, which are connected to the memory  3  through the level adjuster  37 . The data buffer  36  performs data input/output timing adjustment depending on which one of SDR SDRAM mode and DDR mode is selected, which will be described later. Therefore, the data buffer  36  may be referred to as a timing adjuster. Here, when the mode switch signal is high, the data buffer  36  is set to SDR SDRAM mode, allowing data read and write of SDR SDRAM. When the mode switch signal is low, the data buffer  36  is set to DDR mode, allowing data read and write of DDR SDRAM. 
     The level adjuster  37  includes two pairs of input and output amplifiers, the respective two pairs corresponding to the bidirectional data port and the bidirectional data strobe port of the data buffer  36 . Each pair of the input and output amplifiers is supplied with an input/output (I/O) power supply voltage depending on which one of SDR SDRAM and DDR SDRAM is connected as the memory  3 . In the case where the memory  3  is SDR SDRAM, the voltage relationship related to I/O signals is set to a level of LVTTL (Low Voltage Transistor Transistor Logic). On the other hand, in the case where the memory  3  is DDR SDRAM, the voltage relationship related to I/O signals is set to a level of SSTL-2 (Series Stub Termination Logic-2). Such a level interface circuit supporting both LVTTL and SSTL has been disclosed in Japanese Patent Application Unexamined Publication Kokai No. 11-88146. 
     In SDR SDRAM-mode write operation, the data is output from the address/data switch  35  to the data buffer  36 , in which the output timing of the data is adjusted so as to meet the SDR SDRAM-mode requirement. The voltage level of data outputted from the data buffer  36  is adjusted to the LVTTL level by the level adjuster  37  and then the data is written onto the memory  3  (here, SDR SDRAM). 
     In DDR-mode write operation, the data is output from the address/data switch  35  to the data buffer  36 , in which the output timing of the data is adjusted so as to meet the DDR-mode requirement. The voltage level of the data and data strobe outputted from the data buffer  36  is adjusted to the SSTL-2 level by the level adjuster  37 . Thereafter, the data and the data strobe signals are output to the memory  3  (here, DDR SDRAM). In this manner, the data is written into the memory  3 . 
     In SDR SDRAM-mode read operation, data read out from the memory  3  (here, SDR SDRAM) is adjusted in voltage to a CPU-side voltage level at the level adjuster  37  and then is adjusted in timing at the data buffer  36 . The timing-adjusted data is output from the data buffer  36  to the CPU  2  through the address/data switch  35  and the address/data bus. 
     In DDR-mode read operation, data and data strobe signals read out from the memory  3  (here, DDR SDRAM) are adjusted in voltage to a CPU-side voltage level at the level adjuster  37 . The data is adjusted in timing depending on the data strobe signal at the data buffer  36 . The timing-adjusted data is output from the data buffer  36  to the CPU  2  through the address/data switch  35  and the address/data bus. 
     Data Buffer 
     Referring to FIG. 4, the data buffer  36  includes a mode controller  41 , a rising-edge buffer  42 , and a falling-edge buffer  43 . The mode controller  41  inputs the clock signal and data strobe signal from the control signal generator  31  and further inputs the mode switch signal from outside. 
     When the mode switch signal is high, that is, in SDR SDRAM mode, the mode controller  41  supplies the clock signal to only the rising-edge buffer  42 . Accordingly, data received from the CPU  2  is captured and stored in the rising-edge buffer  42  at the rising edge of the clock signal, allowing transfer timing of the data to be adjusted so as to meet the SDR SDRAM requirement. 
     When the mode switch signal is low, that is, in DDR mode, the mode controller  41  supplies an input data strobe signal to both the rising-edge buffer  42  and the falling-edge buffer  43  and further outputs it as an output data strobe to the memory  3  through the level adjuster  37 . Accordingly, data received from the CPU  2  is captured and stored in the rising-edge buffer  42  at the rising edge of the data strobe signal and is captured and stored in the falling-edge buffer  43  at the falling edge of the data strobe signal, allowing transfer timing of the data to be adjusted so as to meet the DDR SDRAM requirement. 
     The mode switch operation as described above may be implemented by a mode switch program running on a program-controlled processor. 
     Operation 
     Hereafter, read and write operations and mode switch operation will be described with reference to FIGS. 5-8, taking as an example the case where CAS (column address strobe) latency CL is 2 and burst length BL is 4. The CAS latency CL is defined as the number of clocks needed until data is issued at a point of time when a read command has issued. The burst length BL is defined as the number of consecutive data. 
     DDR-Mode Read Operation 
     Referring to FIGS. 5A-5D, when a read (R) command is issued at clock timing t 0 , a data strobe signal from the DDR memory  3  goes low during an initial or “preamble” portion of time at clock timing t 2 , which is two clocks after the read command is issued, because of CL=2. After a lapse of one clock, the data strobe signal goes high for clock timing t 3 . At the rising edge of the data strobe signal, first data D 0  from the DDR memory  3  is captured and stored in the rising-edge buffer  42  of the data buffer  36 . Subsequently, when the data strobe signal goes low between t 3  and t 4 , second data D 1  from the DDR memory  3  is captured and stored in the falling-edge buffer  43  of the data buffer  36 . 
     Similarly, at the next rising edge of the data strobe signal for clock timing t 4 , third data D 2  from the DDR memory  3  is captured and stored in the rising-edge buffer  42  of the data buffer  36 . Subsequently, at the next falling edge of the data strobe signal between t 4  and t 5 , fourth data D 3  from the DDR memory  3  is captured and stored in the falling-edge buffer  43  of the data buffer  36 . 
     SDR SDRAM-Mode Read Operation 
     Referring to FIGS. 6A-6C, in SDR SDRAM mode, the data strobe signal is not used. Only a rising edge of the clock signal is used to read data from the SDRAM memory  3 . Accordingly, the falling-edge buffer  43  is not used in the SDR SDRAM mode. 
     More specifically, when a read (R) command is issued at clock timing t 0 , first data D 0  is output from the DDR memory  3  at clock timing t 2 , which is two clocks after the read command is issued, because of CL=2. The first data D 0  is captured and stored in the rising-edge buffer  42  at clock timing t 3 . Similarly, second data D 1  is captured and stored in the rising-edge buffer  42  at clock timing t 4 , and subsequently third data D 2  and fourth data D 3  are captured and stored in the rising-edge buffer  42  at clock timing t 5  and t 6 , respectively. 
     DDR-Mode Write Operation 
     Referring to FIGS. 7A-7D, when a write (W) command is issued at clock timing t 0 , an input data strobe signal goes low during an initial or “preamble” portion of time at clock timing t 1 . After a lapse of one clock, the data strobe signal goes high synchronously with clock timing t 2  and first data D 0  is output from the rising-edge buffer  42  of the data buffer  36  to the DDR memory  3 . Subsequently, when the data strobe signal goes low between t 2  and t 3 , second data D 1  is output from the falling-edge buffer  43  of the data buffer  36  to the DDR memory  3 . In this manner, a predetermined number of data D 0 -D 3  (here, BL=4) are written into the DDR memory  3  while synchronizing the data strobe signal with the clock signal. 
     SDR SDRAM-Mode Write Operation 
     Referring to FIGS. 8A-8C, in SDR SDRAM mode, the data strobe signal is not used. Accordingly, when a write (W) command is issued at clock timing t 0 , first data D 0  is output from the rising-edge buffer  42  of the data buffer  36  to the DDR memory  3 . Similarly, second to fourth data D 1 -D 3  are sequentially written into the DDR memory  3  synchronously with clock timing t 1  to t 3  of the clock signal. 
     As described above, in SDR SDRAM mode, no data strobe signal is used and therefore the mode controller  41  does not supply the data strobe signal to the rising-edge and falling edge buffers  42  and  43 . Accordingly, there is no need of masking control of the data strobe signal. 
     In the above embodiment, the case of DRAM was described. However, the present invention can be also applied to the case of SRAM because DDR-SRAM is available. Further, the present invention can be also applied to the case of three or more types of memories.