Patent Publication Number: US-6715096-B2

Title: Interface circuit device for performing data sampling at optimum strobe timing by using stored data window information to determine the strobe timing

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an interface circuit such as a memory control device for controlling access to a memory device, and in particular to a bus interface circuit for rapidly taking in data in synchronization with a clock signal. 
     2. Description of the Background Art 
     FIG. 78 shows an example of a structure of a conventional data processing system. In FIG. 78, the conventional processing system includes a clock generator CG generating a system clock signal CLK, memory units MU 0 -MUn connected in parallel to a common bus CB and operating in synchronization with system clock signal CLK, a processor PC, such as a CPU (Central Processing Unit), for processing data stored in memory units MU 0 -MUn, and a memory controller MCR for transferring data requested of accessing by processor PC between memory units MU 0 -MUn and processor PC. Memory controller MCR operates in synchronization with system clock signal CLK as well. 
     In such processing system, for achieving fast data processing, a system clock rate has been increased, and a fast bus interface has been defined for fast data transfer via the common bus CB between memory controller MCR and memory units MU 0 -MUm. As typical examples of such a fast interface, there are an interface of DDR (Double Data Rate) specification that data are transferred in synchronization with both the rising and falling edges of system clock signal CLK, and an interface of Rambus specification that the clock signal and data are transferred in the same direction. By utilizing such a fast interface, the efficiency of data transfer on common bus CB is improved, and a waiting time of processor PC is reduced so that the system performance is improved. 
     FIG. 79 shows a relationship in timing between the data and the system clock signal in the DDR mode. In the DDR mode, the data is sampled in synchronization with the rising edge and the falling edge of clock signal CLK. Data DO sampled at the rising edge of clock signal CLK has a set-up time, tsu, and a hold time, th, with respect to clock signal CLK. Likewise, data D 1  sampled at the falling edge of clock signal CLK has set-up time, tsu, and hold time, th, with respect to the falling edge of clock signal CLK. The “sampling” means the timing at which memory controller MCR takes in the data, or the operation of taking the data into the memory unit. 
     Usually, data read from memory units MU 0 -MUn is transferred in synchronization with clock signal CLK. As clock signal CLK becomes faster and a cycle time T thereof becomes shorter, the set-up time, tsu, and hold time, th, of data D also become shorter. When clock signal CLK has a duty ratio of 50, a sum of set-up time, tsu, and hold time, th, can take the value of up to T/2. For example, if clock signal CLK has a frequency of 100 MHz, clock signal CLK has a period T of 10 ns (nano seconds) so that the set-up time, tsu, and hold time, th, can take the values of up to 5 ns. However, these specification values are of the order of pico-seconds. For ensuring such small specification values of the short set-up time and hold time, it is necessary to determine whether the specification values are accurately satisfied or not, and an expensive tester is required for such determination, resulting in increased cost of the memory unit. 
     According to the interface of the Rambus specification, the width and the data transfer rate of the data bus are fixedly determined. Therefore, a system designer cannot freely determine the transfer rate and the data bus width so that flexibility in design is considerably low. 
     Where memory controller MCR and memory units MU 0 -MUn as shown in FIG. 78 are assembled on-board, the impedance of common bus CB (data bus) changes in accordance with the number of on-board memory units, and may also change due to variations in arrangement of on-board wiring lines. If such changes occur in impedance of the common bus (data bus), no timing margin is present in receiving the data by memory controller MCR even if the set-up time and the hold time are set to the values within the specification values, respectively. Due to such less timing margin, it may be impossible to take in accurate data in many cases. As clock signal CLK becomes faster, an eye pattern (effective data period) of data D becomes shorter. 
     Therefore, as shown in FIG. 79, failures occur in the set-up margin and the hold margin for data when variations in propagation delay occur among the memory units in data transfer. For matching the impedances of the on-board wiring lines with each other, if wiring lines of equal lengths may be arranged between the memory units and the memory controller, the board must have a multi-layered structure, and the wiring must be made in a three-dimensional structure so that the circuit-board becomes expensive. 
     FIG. 80 schematically illustrates a timing relationship of data transfer between memory controller MCR and memory unit MU in the memory system shown in FIG.  78 . In FIG. 80, each of memory controller MCR and memory unit MU includes a delayed locked loop (DLL) for generating an internal clock signal in accordance with the clock signal sent from clock generator CG. The clock signal generated by clock generator CG is transmitted as a clock signal Cy to memory controller MCR, and to memory unit MU as a clock signal Ct with a delay by a period Tskew due to a propagation delay on the clock signal line. 
     In memory unit MU, an output circuit outputs data Dt in accordance with the internal clock signal from DLL. Memory unit MU outputs data Dt from its output circuit with a delay of time Tt relative to the rising of the internal clock signal from DLL. Data Dt from memory unit MU reaches memory controller MCR after elapsing of a propagation delay time Tf. In memory controller MCR, a register takes in supplied data Dr in accordance with the internal clock signal generated from its internal DLL. When the register takes in data Dr that is sent from memory unit MU and arrives at memory controller MCR, the data is sampled into the register upon elapsing of set-up time Ts. 
     FIG. 81 is a signal waveform diagram representing operations of the system shown in FIG.  80 . The clock signal generated from clock generator CG has a cycle period of Tcycle. Memory unit MU outputs effective data in accordance with clock signal Ct with a delay of time Tt. Data Dt from memory unit MU arrives at memory controller MCR via the data bus after time Tf. Data Dr arriving at memory controller MCR is sampled into the register in accordance with clock signal Cr upon elapsing of set-up time Ts. 
     Therefore, the propagation time Tf changes in accordance with the wiring length between memory unit MU and memory controller MCR. Propagation delay Tskew of the clock signal changes in accordance with the distance between clock generator CG and memory unit MU as well. Therefore, even with the DLL for outputting data Dt from memory unit MU in synchronization with the clock signal, set-up time Ts differs for a different length of the path from the output circuit of memory unit MU to the register of memory controller MCR. Thus, the timing margin for taking in the data becomes inadequate, and the memory system capable of accurate data transfer cannot be achieved. 
     In the structure with the memory modules assembled on-board, the impedance of the data bus locally changes depending on the number of the on-board memory modules. Therefore, it is extremely difficult to make a propagation delay on a signal transfer path, which extends from the clock generator through the memory module, data line and input pad to the register (receiver) of the controller, equal for all the memory modules (memory chips). Accordingly, it is difficult to achieve the on-board memory system, in which fast and accurate data transfer can be achieved under fast operation environments. 
     As shown in FIG. 80, memory unit MU is configured to output the data, synchronized in phase with clock signal CLK as much as possible, by using DLL for reducing the delay with respect to the supplied clock signal. However, provision of such DLL in memory unit MU increases the chip area of memory unit MU as well as power consumption, and in addition, the memory unit becomes expensive due to increase in chip area. 
     The following problems arise if the margin for data take-in timing at the memory controller changes in accordance with the position/number of the chips or modules in an on-board system. Specifically, if an error occurs in the data taken by the memory controller, it is difficult to determine whether the cause of this error is present in the data transfer path itself or in the memory unit (or chip). Thus, it is difficult to detect and eliminate the cause of error at the board level. 
     Further, in a multi-processor system in which a plurality of processors (CPUs) are connected in parallel to a data bus and access memory devices via a memory controller, such system is configured on-board. When data transfer is performed using a fast clock signal close to, e.g., 1 GHz in such on-board multi-processor system, margins of the set-up time and hold time for data sampling become extremely small and data cannot be transferred fast and accurately, resulting in a problem similar to that of the memory system already described above. 
     A similar problem occurs in a multiprocessor system, in which a plurality of processors are coupled in parallel to a common data bus, and access a common memory without control of a memory controller. Particularly, in the case where the system is expanded so that the number of processors increases or an impedance of the data bus increases, a skew of data with respect to a clock signal changes to impair the reliability of the system, resulting in low expandability of the system. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an interface circuit, which can accurately transfer data to any one of semiconductor devices in a system. 
     Another object of the invention is to provide an interface circuit, which can implement an on-board system allowing fast and accurate data transfer. 
     Still another object of the invention is to provide an interface circuit, which can ensure a sufficient set-up time and a sufficient hold time of data for each of on-board units when assembled on-board. 
     Yet another object of the invention is to provide an interface circuit, which can rapidly and accurately transfer data without increasing a cost of units in a system. 
     Further another object of the invention is to provide an interface circuit, which allows accurate identification of a cause of error even at the on-board level. 
     A further object of the invention is to provide an interface circuit for achieving an inexpensive system allowing fast and accurate data transfer regardless of the structure of the on-board system. 
     A particular object of the invention is to provide an inexpensive memory control device capable of fast and accurate data transfer. 
     An interface circuit according to the invention includes: production/storage circuitry for detecting a point of change in data on a data line, producing effective window information indicating an effective period of the data in accordance with a result of the detection, and storing the effective window information; strobe timing determining circuitry for determining a strobe timing for taking in data on the data line in accordance with the effective window information stored in the production/storage circuitry; and clock producing circuitry for producing a clock signal for taking in the data in accordance with the strobe timing determined by the strobe timing determining circuitry. 
     An interface circuit according to another aspect of the present invention includes: transition point detecting circuitry for detecting a point of transition in data on a data line; producing circuitry for producing effective window information indicating an effective period of the data in accordance with a transition point detected by the transition point detecting circuitry; determining circuitry for determining whether a width of the effective window satisfies a first condition; and storage circuitry for storing an address and a data pattern of the data when the determining circuitry determines that the first condition is satisfied. 
     An interface circuit according to still another aspect of the present invention includes: effective window extracting circuitry for detecting a point of change in data on a data line, and producing an effective window signal indicating an effective period of the data in accordance with a result of the detection; effective window width detecting circuitry for detecting an effective period width of the effective window signal using a reference clock signal, and storing a result of the detection; strobe timing storing circuitry for determining a strobe timing for the data on the data line from the effective window width information detected by the effective window width detecting circuitry, and storing the determined strobe timing; and strobe signal producing circuitry for producing a strobe signal for the data on the data line in accordance with the strobe timing stored in the strobe timing storing circuitry. 
     In the interface circuit, the effective window width of the data on the data line is detected, and the strobe timing for the data is determined in accordance with the detected effective window. Thereby, data strobe (sampling) can be performed at the optimum timing for the data on the data line. Even if a system constituent unit such as a memory device is assembled on a board and an impedance of a data transfer path changes in accordance with a system structure, the data can be accurately taken in. 
     When a detected effective window width does not satisfy predetermined conditions, conditions that would reduce the set-up/hold margins can be detected, so that the system can be made stable. By storing the pattern and address of the data when the margin is reduced, it can be determined whether the cause of failure is present in the data transfer device or on the data transfer path. When the cause of failure concentrates in the addresses of a specific memory chip, it can be determined that the cause of failure is present in that memory device (chip). 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows a structure of a memory system according to a first embodiment of the invention; 
     FIG. 2 illustrates an operation principle of the first embodiment of the invention; 
     FIG. 3 is a signal waveform diagram representing an operation of detecting an effective data window in the first embodiment of the invention; 
     FIG. 4 schematically shows a structure of a timing control circuit shown in FIG. 1; 
     FIG. 5 schematically shows a structure of an effective window detecting circuit shown in FIG. 4; 
     FIG. 6 shows, by way of example, a structure including a primitive window detecting circuit and a local window detecting circuit; 
     FIG. 7 is a signal waveform diagram representing operations of the circuits shown in FIG. 6; 
     FIG. 8 shows a structure of a final window detecting circuit shown in FIG. 5; 
     FIG. 9 schematically shows a structure of a strobe timing detecting circuit shown in FIG. 4; 
     FIG. 10 shows a structure including a delay line, a clock select circuit and a first stage of a latch circuit shown in FIG. 9; 
     FIG. 11 is a timing chart representing operations of the circuits shown in FIG. 10; 
     FIGS. 12A and 12B are signal waveform diagrams representing operations of a strobe timing detecting circuit shown in FIG. 9; 
     FIGS. 13A and 13B are signal waveform diagrams representing operations of the strobe timing detecting circuit shown in FIG. 9; 
     FIG. 14 schematically shows a structure of a latch timing signal generating portion shown in FIG. 10; 
     FIG. 15 shows, by way of example, contents stored in a result register and a strobe timing storing circuit; 
     FIG. 16 shows a timing relationship between an effective data window and a strobe clock signal; 
     FIG. 17 represents a principle of operation of determining the strobe timing; 
     FIG. 18A shows, by way of example, a structure of a storage controller in a register A, and FIG. 18B shows an example of a structure of a storage controller in a register B; 
     FIG. 19 schematically shows a structure of the strobe timing storing circuit shown in FIG. 9; 
     FIG. 20 shows another structure of a strobe timing storage circuit shown in FIG. 9; 
     FIG. 21 shows more specifically structures of a timing control circuit and a strobe clock generating circuit shown in FIG. 1; 
     FIG. 22 schematically shows a structure of an input circuit shown in FIG. 1; 
     FIG. 23 schematically shows alternative structures of the delay line and the result register shown in FIG. 9; 
     FIGS. 24A and 24B are timing charts representing operations of the circuits shown in FIG. 23; 
     FIGS. 25A and 25B are timing charts representing operations of the circuits shown in FIG. 23; 
     FIG. 26 schematically shows a structure of a modification of the first embodiment of the invention; 
     FIG. 27 represents an SSO push-out operation in a second embodiment of the invention; 
     FIG. 28 represents an operation principle of the second embodiment of the invention; 
     FIG. 29 shows, by way of example, a structure of a transition point detecting circuit in the second embodiment of the invention; 
     FIG. 30 is a signal waveform diagram representing operations of the circuits shown in FIG. 29; 
     FIG. 31 schematically shows a structure of a strobe timing calculating circuit in the second embodiment of the invention; 
     FIG. 32 shows signal qualities in a third embodiment of the invention; 
     FIG. 33 schematically shows a structure of the strobe timing storage circuit in the third embodiment of the invention; 
     FIG. 34 schematically shows a structure of an error register in the third embodiment of the invention; 
     FIG. 35 schematically shows a structure of a main portion of the third embodiment of the invention; 
     FIG. 36 schematically shows a modification of the third embodiment of the invention; 
     FIG. 37 schematically shows a structure of a bit change detecting circuit shown in FIG. 36; 
     FIG. 38 schematically shows a structure of a transfer circuit shown in FIG. 36; 
     FIG. 39 schematically shows a structure of a latch circuit shown in FIG. 36; 
     FIG. 40 is a timing chart representing operations of the circuits shown in FIGS. 37 to  39 ; 
     FIG. 41 schematically shows a structure of a slowest change detecting circuit shown in FIG. 36; 
     FIG. 42 schematically shows a structure of the transfer circuit shown in FIG. 36; 
     FIG. 43 schematically shows a structure of the latch circuit shown in FIG. 36; 
     FIG. 44 schematically shows a structure of a fastest change detecting circuit shown in FIG. 36; 
     FIG. 45 is a signal waveform diagram representing operations of the circuits shown in FIG. 36; 
     FIG. 46 schematically shows a structure of a falling edge data window detecting circuit shown in FIG. 36; 
     FIG. 47 is a signal waveform diagram representing a data window detecting operation in a modification of the third embodiment of the invention; 
     FIG. 48 shows a structure of an input portion of the rising edge data window detecting circuit shown in FIG. 36 in greater detail; 
     FIG. 49 shows, by way of example, signal qualities in a second modification of the third embodiment of the invention; 
     FIG. 50 shows, by way of example, a signal waveform with the signal quality shown in FIG. 49; 
     FIG. 51 shows a memory structure and a data transfer rate with the signal quality shown in FIG. 49; 
     FIG. 52 schematically shows a structure of a system including a memory control device according to a fourth embodiment of the invention; 
     FIG. 53 schematically shows a structure of a main portion of a memory control unit shown in FIG. 52; 
     FIG. 54 shows an effective data window detecting operation in the fourth embodiment of the invention; 
     FIG. 55 shows a structure of an effective data window detecting portion shown in FIG. 53; 
     FIG. 56 is a signal waveform diagram representing an operation of circuits shown in FIG. 55; 
     FIG. 57 shows, by way of example, a modification of the effective data window detecting portion shown in FIG. 53; 
     FIG. 58 schematically shows a structure of an output circuit of a memory device; 
     FIG. 59 shows more specifically a structure of a main portion of the memory control unit according to the fourth embodiment of the invention; 
     FIG. 60 is a signal waveform diagram representing an operation of the circuit shown in FIG. 59; 
     FIG. 61 schematically shows a structure of a monitor mask instructing signal generating portion shown in FIG. 59; 
     FIG. 62 schematically shows a structure of a transfer instructing signal generating portion shown in FIG. 59; 
     FIG. 63 schematically shows a structure of a register circuit of a strobe register shown in FIG. 59; 
     FIG. 64 is a timing chart representing operations of the strobe register shown in FIG. 59; 
     FIG. 65 schematically shows a structure of a temporary register shown in FIG. 59; 
     FIG. 66 schematically shows a structure of a strobe timing detecting circuit shown in FIG. 65; 
     FIG. 67 represents an operation of the strobe timing detecting circuit shown in FIG. 65; 
     FIG. 68 schematically shows structures of combining circuit and one stage of drive circuit shown in FIG. 59; 
     FIG. 69 is a signal waveform diagram representing an operation of the circuit shown in FIG. 68; 
     FIG. 70 show a structure of a first modification of a shift clock signal generating portion shown in FIG. 59; 
     FIG. 71 show a structure of a second modification of the shift clock signal generating portion shown in FIG. 59; 
     FIG. 72 schematically shows a structure of a processing system according to a fifth embodiment of the invention; 
     FIG. 73 schematically shows a structure of a first modification of the fifth embodiment; 
     FIG. 74 schematically shows a structure of a processing system according to a second modification of the fifth embodiment; 
     FIG. 75 schematically shows a structure of a processing system of a third modification of the fifth embodiment; 
     FIG. 76 schematically shows a structure of a processing system of a fourth modification of the fifth embodiment; 
     FIG. 77 schematically shows a structure of a processing system of a fifth modification of the fifth embodiment; 
     FIG. 78 schematically shows a structure of a conventional data processing system; 
     FIG. 79 is a timing chart representing operations of the data processing system shown in FIG. 78; 
     FIG. 80 shows signal transfer times in a conventional memory system; and 
     FIG. 81 is a timing chart representing operations of the memory system shown in FIG.  80 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     FIG. 1 schematically shows a structure of a memory system including a memory control device according to the present invention. In FIG. 1, the memory system includes: memory devices  1   a - 1   n  connected in parallel to a common data bus  4 ; a memory control unit  2  for accessing memory devices  1   a - 1   n ; and a clock generator  3  for applying a basic timing signal (a system clock referred to as a “clock signal” hereinafter) to memory devices  1   a - 1   n  and memory control unit. The memory devices  1   a - 1   n  may be chips, or may be modules. 
     When each of memory devices  1   a - 1   n  is formed of a single chip, memory devices  1   a - 1   n  include internal circuits  8   a - 8   n  each including a memory cell array and a memory cell select circuit, and output circuits  9   a - 9   n  outputting data onto common data line  4  in synchronization with an internal clock signal. Memory devices  1   a - 1   n  do not contain an outputting DLL. Output circuits  9   a - 9   n  output data in synchronization with clock signal CLK applied from clock generator  3 . In the case where each of memory devices  1   a - 1   n  is formed of a module, the internal circuit includes a plurality of memory chips, and the output circuit is included in a bus interface circuit provided common to these memory chips. 
     Since a DLL for producing an output clock signal is not required in memory devices  1   a - 1   n , the power consumption and the chip area of memory devices  1   a - 1   n  are reduced, and the cost of each of memory devices  1   a - 1   n  is reduced. Each of internal circuits  8   a - 8   n , of which internal structures will be described later, includes a plurality of banks. The “banks” each are a memory circuit which can be driven to an active state (selected state of a memory cell) independently of other bank (s). In the case where each of memory devices  1   a - 1   n  is formed of a single chip, the memory circuit is a memory array or a memory mat. In the case where each of memory devices  1   a - 1   n  is a memory module, the memory circuit includes a predetermined number of memory chips. 
     Memory control unit  2  includes: a timing control circuit  5  which stores data take-in timing information for each of memory devices  1   a - 1   n  in a register circuit, and controls the data take-in timing in accordance with the stored timing information; a strobe clock generating circuit  6  which is controlled by timing control circuit  5  to generate a strobe clock signal for taking in data; and an input circuit  7  which takes in data applied on common data bus  4 , and transmits the taken-in data to an internal circuit in accordance with the strobe clock signal received from strobe clock generating circuit  6 . 
     Memory control unit  2  detects an effective window of the data for each of memory devices  1   a - 1   n , and so generates a strobe clock signal as to optimize the set-up time and the hold time based on the detected effective window information. 
     FIG. 2 conceptually represents operations of timing control circuit  5  shown in FIG.  1 . FIG. 2 shows the waveforms in the case where memory devices  1   a - 1   n  operate in a DDR (Double Data Rate) mode. However, the memory devices may operate in an SDR (Single Data Rate) mode to output data in synchronization with the rising of clock signal CLK, or may operate to output the data in an EDO (Extended Data Output) mode or a burst EDO mode. In the case of an EDO mode DRAM, the internal circuitry operates asynchronously with the clock signal. However, the control signals designating the operation mode of internal circuitry, i.e., a row address strobe signal/RAS and a column address strobe signal/CAS are applied from memory control unit  2  to memory devices  1   a - 1   n  in synchronization with clock signal CLK. Further, in memory control unit  2 , the sampling of data is performed in synchronization with the clock signal, and therefore, the set-up/hold times for the clock signal must likewise be optimized. Referring to FIG. 2, conceptual description will now be given on the operations of the memory control unit according to the first embodiment of the invention. 
     Among memory devices  1   a - 1   n , a memory device designated by memory control unit  2  outputs data in synchronization with clock signal CLK. This data is sent to memory control unit  2  via common data bus  4 . Data Dc which arrives at memory control unit  2  has a pattern of H- and L-levels in accordance with the logic levels of the data bits, and forms a series of eye patterns which change in synchronization with the rising and falling of the clock signal. In memory control unit  2 , timing control circuit  5  produces a signal EDW_R defining a window period for data Dc transmitted in synchronization with the rising of clock signal CLK, and a signal EDW_F defining an effective period of the data transmitted in synchronization with the falling of clock signal CLK. By using effective window defining signals EDW_R and EDW_F, the rising timing and falling timing of clock signal CLK are so adjusted by times Tar and Taf to optimize the set-up time and the hold time, respectively, whereby the strobing clock signals are produced. 
     The effective data window varies due to (1) a skew between data bits, (2) a difference in data bit transition time between H- and L-levels, (3) interference noises between data lines and (4) jitter of outputting clock signal. However, the DLL is not employed. Therefore, the influence in the above item (4) is not present, and a variation in effective window width due to jitter of the clock signal does not occur. 
     For easy detection of the effective window, data bits  1 ,  0 ,  1 ,  0  . . . are output to data pins numbered  0 ,  1 ,  2 ,  3 , . . . , respectively. An output test pattern of memory devices  1   a - 1   n  may be set in a test mode, or such data pattern may be written in the memory devices and then may be read out. By detecting a change (transition) point of the data pattern, effective window designating signals EDW_R and EDW_F are produced, and the timing providing the optimum set-up/hold times with respect to these valid or effective windows is stored in registers. Thereafter, the strobe clock signal is produced in accordance with the strobe timing stored in the registers. Specific structures will now be described. 
     FIG. 3 conceptually represents an operation of extracting the effective data window signals. In the data pattern outputted in synchronization with clock signal CLK, data D 0 , D 2 , . . . outputted from even-numbered pin terminals are at the same logical level as clock signal CLK, and data D 1 , D 3 , . . . outputted from odd-numbered pin terminals are at the logical level opposite to that of clock signal CLK. In the DDR mode, data reading is performed with the rising edge of the clock signal being a leading edge. Therefore, the data pattern is written in the memory device in advance, and then the data pattern is sequentially read out. Alternatively, such a configuration may be employed that a register circuit or the like is provided at an output stage in the memory device and data of “0101 . . .” or “1010 . . .” are stored in the register circuit in the test mode, and the stored data are read out in place of memory cell data. In this configuration, the data stored in the register circuit are read out in response to a test mode designation signal. Further, such a configuration may be employed that a register circuit for storing data “10” or “01,” for each data output terminal and the storage contents of the register circuit are alternately read out in the test mode. FIG. 3 shows data D 0 -D 3  of 4 bits as a representative example. In the case where the data of 64 bits is transferred via common data bus  4 , the effective data window signals which are detected for each four bits are successively compressed to produce an effective data window signal for the data of 64 bits. 
     In FIG. 3, the pattern of the data transferred when clock signal CLK rises to H-level is known in advance. After receiving this data pattern, the slowest change point among those of data bits D 0 -D 3  is detected, and then the fastest change point among those of data bits D 0 -D 3  is detected. In accordance with the data pattern transmitted upon rising of clock signal CLK, a time period from the slowest change point to the subsequent fastest change point is considered as the effective data window of the data signals transmitted in synchronization with the rising of clock signal CLK, and signal EDW_R is kept active for this time period. 
     At the time of falling of clock signal CLK, the data pattern to be transmitted is likewise known in advance. In this case, the slowest change point of the data bits and the subsequent fastest change point of data bits are likewise detected. A period between these change points is extracted as the effective data window of the data transmitted in synchronization with the falling of clock signal CLK, and effective data window signal EDW_F which is made active during this period is produced. By determining, in advance, a data bit pattern transmitted in synchronization with the rising and falling edges of clock signal CLK, it is possible to produce effective window signal EDW_R for the data transmitted in synchronization with the rising edge of clock signal CLK as well as effective window signal EDW_F for the data transmitted in synchronization with the falling of clock signal CLK. 
     FIG. 4 schematically shows a structure of timing control circuit  5  included in control unit  2  shown in FIG.  1 . In FIG. 4, timing control circuit  5  includes: an effective window detecting circuit  5   a  that receives data bits D 0 -Dm applied via common data bus  4 , detects effective windows for both the rising and falling of clock signal CLK, and produces match detection signals (final effective data window detection signals) MDC_R and MDC_F indicating the detected effective window; a strobe timing detecting circuit  5   b  that, in accordance with clock signal CLK and match detection signals MDC_R and MDC_F generated from effective window detecting circuit  5   a,  extracts the strobe timing for data transferred in synchronization with the rising and falling edges of clock signal CLK; and a strobe timing storage circuit  5   c  which stores the strobe timing detected by strobe timing detecting circuit  5   b,  and applies a data strobe trigger signal TRSEL to strobe clock generating circuit  6  shown in FIG.  1 . 
     Strobe timing storage circuit  5   c  stores the strobe timings at the rising edge and the falling edge for each of memory devices  1   a - 1   n . If memory devices  1   a - 1   n  have a bank structure, strobe timing storage circuit  5   c  stores the strobe timings for each bank. Data strobe timing storage circuit  5   c  is formed of, e.g., a register file, of which contents can be read out and written in accordance with an address signal. 
     FIG. 5 schematically shows a structure of effective window detecting circuit  5   a  shown in FIG.  4 . FIG. 5 shows data D 0 -D 63  of 64 bits as an example of data bits D 0 -Dm. In FIG. 5, effective window detecting circuit  5   a  includes: primitive window detecting circuits  10 - 0 - 10 - 31  provided for respective sets of data of 2 bits and each detecting a valid (effective) window of corresponding data bits; local window detecting circuits  11 - 0 - 11 - 15  each provided for a pair of adjacent two primitive window detecting circuits and detecting a period of matching of primitive windows detected by corresponding primitive window detecting circuits, for extracting a local window; and a final window detecting circuit  12  for producing final effective window detection signals (match signals) MDC_R and MDC_F for the data of 64 bits in accordance with the local window extraction signals produced from local window detecting circuits  11 - 0 - 11 - 15 . 
     Primitive window detecting circuit  10 - 0  receives data bits D 0  and D 1 , and primitive window detecting circuit  10 - 1  receives data bits D 2  and D 3 . Primitive window detecting circuit  10 - 30  receives data bits D 60  and D 61 , and primitive window detecting circuit  10 - 31  receives data bits D 62  and D 63 . Each of primitive window detecting circuits  10 - 0 - 10 - 31  receives a data bit set of complementary data bits. Even-numbered data bits have the same logical level as the clock signal, and odd-numbered data bits have the logical level opposite to the logical level of the clock signal. 
     By utilizing these pairs of complementary data bits, the effective window can be detected easily as will be described later. Further, the data bits change in random directions in a normal operation. Therefore, by detecting the effective windows with the pairs of complementary data bits, a window for providing an optimum strobe timing can be detected as an average value. 
     Each of primitive window detecting circuits  10 - 0 - 10 - 31  detects both the effective window of data transferred in synchronization with the rising edge of clock signal CLK and the effective window of data transferred in synchronization with the falling edge of clock signal CLK. 
     Each of local window detecting circuits  11 - 0 - 11 - 15  detects the effective window for 4-bit data. More specifically, each of local window detecting circuits  11 - 0 - 11 - 15  detects the matching region of the primitive windows detected for corresponding 2-bit data, to detect an effective window for data of 4 bits. Each of them detects the effective window of the data transferred in synchronization with each of the rising and falling edges of clock signal CLK. 
     Final window detecting circuit  12  produces, in accordance with the local window detection signals from local window detecting circuits  11 - 0 - 11 - 15 , final effective window detection signal (match detection signal) MDC_R for the data transferred in synchronization with the rising edge of clock signal CLK as well as final effective window detection signal (match detection signal) MDC_F for the data transferred in synchronization with the falling edge of clock signal CLK. 
     FIG. 6 shows, by way of example, the structures of the primitive window detecting circuit and the local window detecting circuit shown in FIG.  5 . FIG. 5 shows, as a representative example, a structure of primitive window detecting circuit  10 - 0  and local window detecting circuit  11 - 0  provided for data bits D 0  and D 1 . 
     In FIG. 6, primitive window detecting circuit  10 - 0  includes a current mirror type circuit  10   a  that detects an effective window of the bits of data transferred in synchronization with the falling edge of dock signal CLK, and a current mirror type circuit  10   b  that detects a valid (effective) window of the bits of data transferred in synchronization with the rising edge of clock signal CLK. 
     Current mirror circuit  10   a  includes: a P-channel MOS transistor (insulated gate type field effect transistor) Q 1  having a source coupled to a power supply node and a gate receiving data bit D 0 ; P-channel MOS transistors Q 2  and Q 3  supplied with a current from MOS transistor Q 1  and forming a current mirror stage supplying the currents of the same magnitude; and N-channel MOS transistors Q 4  and Q 5  forming a differential stage for comparing data bit D 1  with reference voltage Vref. A drain of MOS transistor Q 4  is connected to a drain and a gate of P-channel MOS transistor Q 2 . A primitive window signal ZN 01  is outputted from a connection node connecting MOS transistors Q 3  and Q 5 . This primitive window signal ZN 01  designates an effective window region of data bits D 0  and D 1  transferred in synchronization with the falling of clock signal CLK. 
     Current mirror type circuit  10   b  includes; a P-channel MOS transistor Q 6  having a source connected to the power supply node and a gate receiving data bit D 1 ; P-channel MOS transistors Q 7  and Q 8  supplied with a current from MOS transistor Q 6  to operate as a current mirror stage; and N-channel MOS transistors Q 9  and Q 10  coupled between ground node and P-channel MOS transistors Q 7  and Q 8 , respectively, to form a differential stage for comparing data bit D 1  with reference voltage Vref MOS transistor Q 9  has a drain connected to a drain and a gate of MOS transistor Q 7 , and receives data bit D 0  on its gate. MOS transistor Q 10  receives reference voltage Vref on its gate. A primitive window signal N 01  is outputted from a connection node connecting MOS transistors Q 8  and Q 10 . This primitive window signal N 01  designates an effective window region of data bits transferred in synchronization with the rising edge of clock signal CLK. 
     Local window detecting circuit  11 - 0  includes an NAND circuit  11   a  that receives primitive window signal ZN 01  from current mirror type circuit  10   a  and a primitive window signal ZN 23  from primitive window detecting circuit  10 - 1  shown in FIG. 5, to produce a local window detection signal ZEDW_F 03 , and an NAND circuit  11   b  that receives primitive window signal N 01  from current mirror type circuit  10   b  and a primitive window signal N 23  from primitive window detecting circuit  10 - 1  shown in FIG. 5, to produce a local window detection signal ZEDW_R 03 . 
     Local window detection signal ZEDW_F 03  indicates an effective window region of data bits D 0 -D 3  transferred in synchronization with the falling edge of clock signal CLK. Local window detection signal ZEDW_R 03  indicates an effective window region of data bits transferred in synchronization with the rising edge of clock signal CLK. Other primitive window detecting circuits  10 - 1 - 10 - 31  and other local window detecting circuits  11 - 1 - 11 - 15  shown in FIG. 5 have structures the same as those shown in FIG. 6 except for applied data bits and primitive window detection signals. Now, primitive window detecting circuit  10 - 0  and local window detecting circuit  11 - 0  shown in FIG. 6 will be described with reference to a signal waveform diagram of FIG.  7 . 
     It is now assumed that data bits D 0 -D 3  are transferred in synchronization with the rising of clock signal CLK. In the data transfer operation, data bits D 0  and D 2  are at the same logical level as clock signal CLK, and data bits D 1  and D 3  are at the logical level opposite to that of clock signal CLK. According to this data pattern, it is identified at which edge of clock signal CLK a data bit is transferred. 
     When data bit D 0  rises, current mirror circuit  10   a  is deactivated, and primitive window detection signal ZN 01  maintains L-level regardless of the voltage level of bit D 1 . While data bit D 1  is at H-level, current supply transistor Q 6  in current mirror type circuit  10   b  is off, and primitive window detection signal N 01  is at L-level during that H-level period. When data bit D 0  rises to H-level, and data bit D 1  falls to L-level, current supply transistor Q 6  in current mirror type circuit  10   b  is turned on, and primitive window detection signal N 01  rises to H-level because data bit D 0  is at a higher voltage level than reference voltage Vref. 
     For data bits D 2  and D 3 , the logical levels of the primitive window detection signals are determined with data bits D 2  and D 3  replacing data bits D 0  and D 1  in the structure corresponding to primitive window detecting circuit  10 - 0  shown in FIG.  6 . When data bit D 3  attains L-level, current supply transistor Q 6  in current mirror type circuit lob is turned on. When data bit D 2  rises to H-level, primitive window detection signal N 23  rises to H-level. Primitive window detection signal Z 23  is at L-level when data bit D 3  is at L-level. When data bit D 2  rises to H-level, current supply transistor Q 1  is turned off, and primitive window detection signal Z 23  attains and keeps L-level. 
     In the case where the data is transferred in synchronization with the falling edge of clock signal CLK, and bit D 0  falls to L-level, the output signal of current mirror type circuit  10   a  is at L-level. When bit D 1  rises from L-level to H-level, primitive window detection signal ZN 01  rises to H-level. When data bit D 1  is at L-level, data bit D 0  is at L-level, and primitive window detection signal N 01  falls to L-level. When data bit D 0  falls to L-level, and data bit D 1  attains H-level, current supply transistor Q 6  in current mirror type circuit  10   b  is turned off, and primitive window detection signal N 01  maintains L-level. 
     Similar operations are performed on data bits D 2  and D 3 . When data bit D 3  falls to L-level, current supply transistor Q 6  is turned off so that primitive window detection signal N 23  falls to L-level. Even when data bit D 3  rises to H-level, if data bit D 2  is at H-level, current supply transistor Q 1  in current mirror type circuit  10   a  is still off, and primitive window detection signal N 23  maintains L-level. When data bit D 2  falls to L-level, current supply transistor Q 1  is turned on, and primitive window detection signal ZN 23  rises to H-level because data bit D 3  is at H-level. When data bit D 2  rises to H-level, current supply transistor Q 1  is turned off, and primitive window detection signal ZN 23  falls to L-level. Primitive window detection signal ZN 23  likewise falls to L-level owing to the differential amplification in current mirror type circuit  10   a  when data bit D 3  attains L-level. 
     Local window detection signal ZEDW_R 03  attains L-level when both primitive window detection signals N 01  and N 23  are at H-level. Local window detection signal ZEDW_F 03  attains L-level when both primitive window detection signals ZN 01  and ZN 23  are at H-level. 
     The primitive window detection signal rises in response to the slowest change in data bits, and falls to L-level in response to the fastest change in data bits. Likewise, primitive window detection signals ZN 01  and ZN 23  rise to H-level in response to the slowest change in data bits, and becomes inactive in response to the fastest change in data bits. Therefore, by combining these primitive window detection signals by NAND circuits  11   a  and  11   b,  window detection signals ZEDW_F 03  and ZEDW_R 03  can be held in the active state at L-level only while all the four bits of data D 0 -D 3  are in the definite state. 
     The local window detecting operation described above is executed on the remaining data bits in the circuits each processing four bits of data, and the respective local effective data windows are detected. 
     As described above, the complementary data bit pairs are used, and the logic levels of the data bits are determined in accordance with the trigger edges of the clock signal. Accordingly, the effective windows of the data bits transferred at both the rising and falling edges of the clock signal can be easily detected. 
     FIG. 8 shows a structure of a portion of final window detecting circuit  12  shown in FIG. 5, for generating final effective data window detection signal (match detection signal) MDC_R. FIG. 8 shows a structure of the effective data window detecting circuit for data bits D 0 -D 63 , as an example. The data bits are 64 bits, and 16 local effective data window detection signals ZEDW_R 03 -ZEDW_R 6063  are produced. The local valid window detection signals are grouped into sets each including four local valid window detection signals, and NOR gates GR 0 -GR 3  are provided for the respective sets. More specifically, NOR gate GR 0  receives local window detection signals ZEDW_R 03 -ZEDW_R 1215  for data bits D 0 -D 15 . NOR gate GR 3  receives local window detection signals ZEDW_R 4851 -ZEDW_R 6063  for data bits D 48 -D 63 . 
     Final window detecting circuit  12  further includes four-input NAND gate GD receiving the output signals of NOR gates GR 0 -GR 3 . NAND gate GD produces final window detection signal ZMDC_R, and an inverter GI receives the output signal of NAND gate GD to produce final effective (valid) data window detection signal (match detecting signal) MDC_R. 
     Each of NOR gates GR 0 -GR 3  generates a signal at H-level when all the received window detection signals attain L-level. Therefore, NOR gate GR 0  generates a signal indicating the effective (valid) window region of data bits D 0 -D 15 , and NOR gate GR 3  generates a signal indicating the effective (valid) window region of data bits D 48 -D 63 . 
     NAND gate GD generates a signal at L-level when all the received signals are at H-level. Therefore, final effective data window detection signal ZMDC_R outputted from NAND gate GD indicates the effective or valid data window region of data bits D 0 -D 63 . 
     By using signal ZEDW_F as the local window detection signal in the structure shown in FIG. 8, signals ZMDC_F and MDC_F indicating the final valid data window for the data, which in turn is transferred in synchronization with the falling edge of clock signal CLK, are produced. Thereby, the effective data window region of the applied data bits can be extracted. For the effective data window region thus extracted, the phase of clock signal CLK is so adjusted to produce the strobing clock signal to optimize the set-up/hold times. 
     FIG. 9 schematically shows a structure of strobe timing detecting circuit  5   b  shown in FIG.  4 . In FIG. 9, strobe timing detecting circuit  5   b  includes: a timing adjusting circuit  20  for adjusting the timing of clock signal CLK in correspondence to the signal propagation delay at effective window detecting circuit  5   a;  a delay line  22  including a plurality of cascaded delay stages  22   a  each having a delay time of, e.g., 200 ps (pico seconds), for successively transmitting the clock signal received from timing adjusting circuit  20 ; a result register circuit  24  including latch circuits  24   a  provided corresponding to delay stages  22   a  on delay line  22 , for comparing the phase of the final effective data window detection signal MDC_R (or MDC_F) and the phases of the output signals of corresponding delay stages  22   a  of delay line  22 , and latching the results of the comparison; and a strobe timing calculating circuit  26  for calculating strobe timing in accordance with the signals stored in the latch circuits  24   a.  The strobe timing calculated by strobe timing calculating circuit  26  is stored in a strobe timing storage circuit  5   c.    
     Strobe clock generating circuit  6  shown in FIGS. 1 and 9 includes select circuits  6   a  provided corresponding to respective delay stages  22   a  on delay line  22 , and produces a strobe clock signal CKST by selecting the input clock signal of a specific delay stage  22   a  on delay line  22  in accordance with a data trigger select signal TRSEL produced based on the strobe timing information stored in strobe timing storage circuit  5   c.    
     FIG. 10 shows, by way of example, structures of the delay stage on delay line  22 , select circuit  6   a  in strobe clock generating circuit  6  and latch stage  24   a  of latch circuit  24 . In FIG. 10, delay stage  22   a  includes delay inverter circuits  22   aa  and  22   ab  provided by even number of stages (two stages in FIG. 10) for delaying the clock signal received from a preceding stage. Delay stage  22   a  has a delay time of, e.g., 200 ps. 
     Latch stage  24   a  includes: a latch  24   aa  which latches an input clock signal of a corresponding delay stage  22   a  in response to final effective data window detection signal MDC 0 ; and a latch  24   ab  which latches the latch output of latch  24   aa  in response to a transfer instructing signal MDC 1  for application to strobe timing calculating circuit  26 . 
     Select circuit  6   a  includes a tri-state inverter buffer circuit  6   aa  that produces strobe clock signal CKST by inverting the input clock signal of a corresponding delay stage  22   a  in response to data trigger select signal TRSEL from strobe timing storage circuit  5   c.  The reason why strobe clock signal CKST is produced by tri-state inverter buffer circuit  6   aa  is that the corresponding sampling circuit (input circuit) attains the latching state in synchronization with the falling of the clock signal (strobe clock signal). Select circuit  6   a  has the outputs wired-connected with each other. 
     FIG. 11 illustrates generation timing of signals MDC 0  and MDC 1  defining the latch timing of latch stage  24   a  shown in FIG.  10 . As shown in FIG. 11, match detection signal MDC 0  is produced in response to a first (leading) final effective data window detection signal MDC_R (or MDC_F). Transfer control signal MDC 1  is produced in accordance with a second (subsequent) final effective data window detection signal MDC_R (or MDC_F). Based on match detection signal MDC 0 , the position of the effective data window is detected, and then the output of latch  24   aa  is transferred to latch  24   ab  in accordance with transfer control signal MDC 1 . Then, strobe timing calculating circuit  26  calculates the optimum strobe timing. 
     FIG. 12A is a waveform diagram showing a relationship in timing between final effective data window detection signal MDC_F and the delay line input clock signals. Input clock signals CLKda-CLKdw on the delay stages have phases shifted by a unit time (e.g., of 200 ps) from each other. It is now assumed that final effective data window detection signal MDC_F rises at a time between falling edges of delayed clock signals CLKda and CLKdb. In this case, latch stage  24   a  shown in FIG. 10 enters the latch state in synchronization with the rising of final effective data window detection signal MDC_F, and takes in and latches the corresponding delayed clock signal. Therefore, latch stage  24   a  corresponding to delayed clock signal CLKda latches “0”. Latch stages  24   a  corresponding to delayed clock signals CLKdb-CLKdv latch “1” because the corresponding delayed clock signals are at H-level. The latch stage corresponding to delayed clock signal CLKdw stores “0”. 
     The number of the latch stages storing “1” corresponds to the period for which clock signal CLK (CLKd) is at H-level as shown in FIG. 12B. A phase shift between the effective data window start and clock signal CLK is detected, and the strobe timing is determined such that set-up time, tsu, and hold time, th, are made optimum. It is assumed that the number of latch stages storing “1” corresponds to a period for which clock signal CLK is at H-level, and also corresponds to the L-level period (assuming that the duty of clock signal CLK is 50%), and that the effective data window width is equal to the period for which the clock signal is at H-level. Based on this assumption, the strobe timing is determined such that the select circuit corresponding to the latch stage at the central position among the latch stages storing “1” is activated. The operation of determining the strobe timing will be described later. 
     FIG. 13A shows a timing relationship between final effective data window detection signal MDC_R and the delayed clock signals. Delay line  22  generates delayed clock signals CLKd 1 -CLKd. It is now assumed that the final effective data window detection signal MDC_R rises to H-level at the time point between the rising edges of delayed clock signals CLKd 1  and CLKd 2 . In this condition, at a more downstream delay stage on delay line  22 , the rising of delayed clock signal is delayed more so that latch stage  24   a  provided corresponding to delayed clock signal CLKd 1  stores “1”, and delay stages  24   a  corresponding to respective delayed clock signals CLKd 2 -CLKdy store “0”. Latch stage  24   a  corresponding to delayed clock signal CLKdz stores “1”. 
     In this case, therefore, the number of latch stages storing “0” corresponds to the period for which delayed clock signal CLKd is at L-level, as shown in FIG.  13 B. This period of L-level of dock signal CLK is uniquely determined in the memory system assembled on a board. In view of this period, the data strobe timing is determined so as to optimize set-up time, tsu, and hold time, th. 
     FIG. 14 shows, by way of example, a structure of a portion generating control signals MDC 0  and MDC 1  shown in FIG.  10 . In FIG. 14, the match detecting signal generating portion includes: a T flip-flop  30  which changes its output state in response to the falling of final effective data window detection signal MDC_R (MDC_F); a transmission gate  31  which is turned on in accordance with complementary signals from outputs Q and /Q of T flip-flop  30 , to produce match detection signal MDC 0  from final effective data window detection signal MDC_R (or MDC_F); and a transmission gate  32  which is tuned on complementarily with transmission gate  31  in accordance with the complementary output signals from T flip-flop  30 , to pass final effective data window detection signal MDC_R (or MDC_F) therethrough for generating transfer control signal MDC 1 . 
     Initially, T flip-flop  30  is in the reset state, and the signal outputted from its output Q is at L-level. Accordingly, match detection signal MDC 0  is produced in accordance with final effective data window detection signal MDC_R (or MDC_F). When final effective data window detection signal MDC_R (or MDC_F) falls, the state of output signal of T flip-flop  30  changes, and transfer control signal MDC 1  is produced in accordance with final effective data window detection signal MDC_R (or MDC_F). Thus, the phase information taken into the latch stages can be successively transferred. 
     Transmission gates  31  and  32  attain the output high-impedance state when they are off. For reliable initialization, resetting MOS transistors, rest to the ground voltage level when corresponding transmission gates  31  and  32  are off, may be provided 
     FIG. 15 conceptually represents an operation of strobe timing calculating circuit  26  shown in FIG.  9 . In FIG. 15, result register circuit  24  stores the position of the leading edge of the final effective data window detection signal relative to a corresponding edge (rising or falling edge) of clock signal CLK. More specifically, the position where a transition of data from “0” to “1” occurs in result register circuit  24  is the leading edge of the effective data window. The position where a transition of data from “1” to “0” occurs in result register circuit  24  is the position of the trailing edge of the clock signal corresponding to this effective data window. 
     When the effective data window is substantially equal to half the cycle of clock signal CLK (when the data transfer is performed in the DDR mode), the region where “1” successively appears corresponds to the period for which the definite data bits are transferred. The central position of the latch stages storing the value of “1” in result register circuit  24  is detected and stored in strobe timing storage circuit  5   c.  If the effective data region is substantially equal to half the cycle of clock signal CLK, the margins for the set-up time and hold time can be maximized by obtaining the above central region (because the set-up time is made equal to the hold time). A data trigger select signal TRSEL is generated from the register at the bit position stored in strobe timing storage circuit  5   c.  Corresponding select circuit  6   a  shown in FIG. 10 is turned on, and the delayed clock signal corresponding to the register position stored in strobe timing storage circuit  5   c  is selected to produce strobe clock signal CKST. Therefore, the edge of strobe clock signal CKST changes substantially at the central region of the effective data, and the corresponding effective data is sampled. 
     When the position of final effective data window detection signal MDC is shifted from clock signal CLK as shown in FIG. 16, a position of such a clock signal that changes at the same position as the leading edge of final effective data window detection signal MDC is detected (although an error due to a delay time, e.g., of 200 ps of the delay stage is present). The window of this clock signal is indicated by CLKD. The position of this clock signal window CLKD is stored in result register circuit  24 . Data trigger select signal TRSEL is produced by calculating the strobe timing so that the edge may be present at the central position of clock signal window CLKD. Strobe clock signal CKST has an active (H-level) period substantially equal to the effective data window width, and the strobing of data can be performed substantially in the central region of the effective data, so that the margins for set-up time, tsu, and hold time, th, can be maximized. 
     FIG. 17 illustrates a specific manner of calculating the strobe timing. Numbers are assigned to the respective latch stages included in result register circuit  24 . It is now assumed that “1” is stored in and between latches LA and LB. In this case, register A stores the number LA of latch LA, and register B stores the number LB of last latch LB storing “1”. The strobe timing is obtained form these latch numbers LA and LB by a relation of (LA+LB)/2. The register number (i.e., the select circuit) indicating the strobe timing is stored in storage circuit  5   c.    
     In registers A and B, the leading and trailing positions of “1” may be stored utilizing hardware as described below. The output signals of the adjacent latches in result register  24  are detected, a point of change of “01” is detected, and a point of change of “10” is detected. Thus, the leading and trailing positions to be stored in registers A and B can be detected. In this case, each of registers A and B must have register circuits equal in number to the latch stages in result registers  24 . Also, it is necessary to employ a gate circuit for detecting a set of “01” and a gate circuit for detecting a set of “10”. 
     In this case, therefore, such a structure may be employed that a controller (not shown) merely obtains the numbers of the latches storing “1” in result register  24 , and the detected latch numbers are stored in registers A and B for detecting the strobe timing. Strobe timing storage circuit  5   c  must store the strobe timing for each bank in each memory device, and a circuit controlling the storage on a bank basis may also execute the operation of calculating the values to be stored in registers A and B. 
     FIG. 18A schematically shows a structure of a data storage control portion for register A, and FIG. 18B schematically shows a structure of a data storage control portion for register B. 
     In FIG. 18A, result register circuit  24  includes latch stages  22   ap  and  22   aq.  Gate circuits GAp and GAq are provided corresponding to latch stages  22   ap  and  22   aq,  respectively. The output signals of gate circuits GAp and GAq are applied to and stored in register circuits Aap and Aaq in register A, respectively. Gate circuit GAp receives the latch signal of corresponding latch stage  22   ap  and an inverted signal of the output signal of the latch in the preceding stage, and outputs a signal of “1” when both the received signals are “1”. Gate circuit GAq receives the output signal of latch stage  22   ap  and the output signal of corresponding latch stage  22   aq.  Therefore, when latch stages  22   ap  and  22   aq  store “0” and “1”, respectively, as shown in FIG. 18A, the output signal of gate circuit GAp attains L-level (“0”), and only the output signal of gate circuit GAq attains “1”. Accordingly, the leading edge of the final effective data window region can be detected and stored in register A. 
     In FIG. 18B, gate circuits GBs and GBt are provided for latch stages  22   as  and  22   at  in result register circuit  24 , respectively. Register circuits Bas and Bat are provided in register B for receiving the output signals of gate circuits GBs and GBt, respectively. Gate circuit GBt receives the output signal of corresponding latch stage  22   at  and an inverted signal of the output signal of the subsequent latch stage. Gate circuit GBs receives the output signal of corresponding latch stage  22   as  and an inverted signal of the output signal of subsequent latch stage  22   at.  Therefore, gate circuit GBs generates a signal of “1” when latch stages  22   as  and  22   at  store “1” and “0”, respectively. Gate circuit GBt generates a signal of “0” because the output signal of latch stage  22   at  is “0”. By utilizing the structure shown in FIG. 18B, the trailing edge of the final effective data window region can be detected and stored in register B. 
     A not shown control device detects the numbers of registers storing data of “1” in registers A and B. The strobe timing is determined based on the detected register numbers, and “1” is written into a corresponding register in storage circuit  5   c  under the control of the control circuit. 
     FIG. 19 shows, by way of example, the structures of strobe timing calculating circuit  26  and strobe timing storage circuit  5   c.  Referring to FIG. 19, strobe timing calculating circuit  26  includes an adder  26   a  for adding the leading edge latch number stored in register A and the trailing edge latch number stored in register B, a multiplier  26   b  for multiplying the sum obtained by adder  26   a  by ½, and a bit developing circuit  26   c  for performing bit development (expansion) of the output value of multiplier  26   b  in accordance with the number of delay stages on the delay line. These registers A and B store the numbers of latches corresponding to the leading edge position and the trailing edge position of the pulse width of the clock signal which in turn matches with the leading edge of the final effective data window detection signal, respectively. Adder  26   a  adds these latch numbers, and multiplier  26   b  multiplies the result by a factor of ½ so that the number of a latch stage corresponding to the clock signal having an edge at the central position is detected. Bit developing circuit  26   c  sets the register circuit, corresponding to the latch number applied from multiplier  26   b,  to 1, and also sets all the remaining register circuits to 0. Thus, it is possible to produce the select control signals for the clock select circuits provided corresponding to the respective delay stages on delay line  22 . 
     Strobe timing storage circuit  5   c  includes a register circuit  40  including registers  40   a - 40   m  provided corresponding to the respective memory devices and banks, a select circuit  42  for selecting a register from register circuit  40 , and a control circuit  41  for controlling the selecting operation of select circuit  42  in accordance with address signal AD. Each of registers  40   a - 40   m  includes unit registers corresponding to the respective clock select circuits included in strobe clock generating circuit  6 . The bit width of each of registers  40   a - 40   m  is equal to the bit width of data outputted from bit developing circuit  26   c.  Select circuit  42  selects a register, and the data applied from bit developing circuit  26   c  is written into the selected register. Thereby, the signal indicating the delayed clock select position for a bank in a memory device is stored. 
     In a normal operation, control circuit  41  controls, in accordance with address AD specifying a memory device and a bank, select circuit  42  to select the corresponding register in register circuit  40 . Select circuit  42  produces data trigger select signal TRSEL, in which only one bit is “1” and the other bits are “0”, and applies the bits of the data trigger select signal TRSEL in parallel to select circuit  6   a  of strobe clock generating circuit  6  shown in FIG.  9 . 
     Registers A and B store the numbers of latch stages corresponding to the positions of the register circuits storing “1” in registers A and B shown in FIG. 17, respectively. 
     Modification of Strobe Timing Storage Circuit 
     FIG. 20 shows, by way of example, a modification of strobe timing storage circuit  5   c.  In FIG. 20, strobe timing storage circuit  5   c  includes a register circuit  45  for storing the strobe timing information for the memory devices and banks, a control circuit  46  for controlling write/read for register circuit  45 , a decoder  47  for decoding the strobe timing information read from register circuit  45 , and a select signal generating circuit  48  for generating data trigger select signal TRSEL in accordance with a decode signal from decoder  47 . 
     Register circuit  45  stores a device name  45   aa,  a bank name  45   ab  and strobe timing information  45   b  in a linked fashion. With device name  45   aa  and bank name  45   ab  used as an address, a corresponding strobe timing (the position number of latch stage) may be stored. Register circuit  45  may be formed of a Content Addressable Memory (CAM) with device name  45   aa  and bank name  45   ab  used as a referring address for reading out the strobe timing information. 
     FIG. 20 shows, by way of example, a state where strobe timing information #a is stored for bank B 0  of memory device la, and strobe timing information #b and #c are stored for banks B 0  and B 1  of memory device  1   n,  respectively. Strobe timing information  45   b  merely contains the position number, ((LA+LB)/2), of the latch stage. This reduces the bit width of register circuit  45 . Decoder  47  decodes the strobe timing information to produce a signal specifying a plurality of clock select circuits. Select signal generating circuit  48  latches the output signal of decoder  47 , and produces data trigger select signal TRSEL. 
     According to the structure shown in FIG. 20, register circuit  45  and decoder  47  are activated when a memory device is to be accessed. When one memory device is successively accessed, select signal generating circuit  48  automatically generates select signal TRSEL. Register  45  and decoder  47  can prepare for the next access, and can accommodate even switching between memory devices fast. 
     Control circuit  46  can access register circuit  45  in accordance with address signal AD specifying a device name and a bank name. When storing strobe information in register circuit  45 , control circuit  46  enters the write mode, and writes the information ((LA+LB)/2) indicating the strobe timing into this register circuit in accordance with address signal AD. 
     FIG. 21 shows a schematic structure of the clock generating portion. In FIG. 21, the clock generating portion includes a rising clock generating circuit  6 R for producing a strobe clock signal CKSTR for data transferred in synchronization with the rising edge of clock signal CLK, a falling clock generating circuit  6 F for producing a strobe clock signal CKSTF for data transferred in synchronization with the falling edge of clock signal CLK, rising adjusting circuitry  5 R for producing data trigger select signal TRSELR for rising clock generating circuit  6 R, and falling adjusting circuitry  5 F for producing data trigger select signal TRSELF for falling clock generating circuit  6 F. 
     Rising adjusting circuitry  5 R includes a circuit for producing data trigger select signal TRSELR in accordance with the final effective data window detection signal shown in FIG. 9 et. seq. Therefore, rising adjusting circuitry  5 R includes a delay line  22 R for delaying clock signal CLK and a register circuit (not shown) for detecting the matching position in delay line  22 R to store the detected position. Likewise, falling adjusting circuitry  5 F includes strobe timing detecting circuit  5   b  and strobe timing storage circuit  5   c,  and further includes a delay line  22 F for delaying clock signal CLK, a latch stage for detecting and storing the strobe timing in accordance with a delayed clock signal on delay line  22 , and a register circuit for storing the detected latch stage position. 
     Rising clock generating circuit  6 R selects a delayed clock signal from delay line  22 R in accordance with data trigger select signal TRSELR, and produces strobe clock signal CKSTR. Falling clock generating circuit  6 F selects a delayed clock signal from delay line  22 F in accordance with data trigger select signal TRSELF, and produces strobe clock signal CKSTF. 
     FIG. 22 shows a structure of the input circuit for taking in the data. In FIG. 22, input circuit  7  includes an input circuit  7 R taking in (sampling) data bits D&lt;all&gt;, transmitted from a selected memory device, at the rising edge of strobe clock signal CKSTR, and an input circuit  7 F taking in data bits D&lt;all&gt; in synchronization with the falling edge of strobe clock signal CKSTF. Input circuit  7 F samples the data bits applied in synchronization with the rising edge of strobe clock signal CKSTF. In the case where data is formed of 64 bits D&lt;63:0&gt;, input circuits  7 R and  7 F produce internal data Din&lt;all&gt; and Din&lt;all&gt; each formed of 64 bits, respectively. The internal data bits produced from input circuits  7 R and  7 F may be successively and internally transferred in parallel or may be successively transferred in series via a common internal data bus, depending on the internal structure of the control unit. 
     In input circuit  7 , input circuits  7 R and  7 F are provided for strobe clock signals CKSTR and CKSTF, respectively. Owing to this arrangement, the data bits transferred in synchronization with the rising and falling edges of clock signal CLK can be sampled using input circuits  7 R and  7 F that operate slowly (i.e., at an alternate cycle of clock signal CLK). 
     The rising edge of each of strobe clock signals CKSTR and CKSTF determines the sampling timing (strobe timing). By determining the logic of the clock select circuit in accordance with the phase of the strobe clock signal, both input circuits  7 R and  7 F can perform sampling (strobing) of data in synchronization with the rising edge of the strobe clock signals as shown in FIGS. 12B and 13B. For example, in the case where the clock signal selected from the delay line falls in the central region of the effective data window region (clock cycle) as shown in FIG. 12B, the strobe clock signal may be produced using the tri-state inverter buffer circuit as shown in FIG. 10, whereby the strobe clock providing the strobe timing at the rising edge of this strobe clock signal can be produced. As shown in FIG. 13B, as for the strobe clock signal for the data transferred in synchronization with the rising edge, the select circuit is formed of a tri-state buffer because the rising edge of a corresponding clock signal selected from the delay line provides the strobe timing. 
     First Modification 
     FIG. 23 shows a structure of a first modification of the first embodiment of the invention. More specifically, FIG. 23 shows a structure of a main portion of the strobe timing detecting circuit. The structure shown in FIG. 23 differs from the structure shown in FIG. 10 in that latch  24   aa  in latch stage  24   a  latches received effective data window detection signal MDC 0  (MDC_R) at the rising edge of the input clock signal of corresponding delay stage  22   a.  Structures other than the above are the same as those shown in FIG.  10 . The structure shown in FIG. 23 detects the window for the data transferred in synchronization with the rising edge of clock signal CLK. For the data transferred in synchronization with the falling edge of clock signal CLK, latch  24   aa  latches final effective data window detection signal MDC_F in response to the falling of the input clock signal of a corresponding delay stage. 
     According to the structure shown in FIG. 23, final effective data window detection signal MDC_R is latched at the rising edge of a corresponding delayed clock signal CLKda. Therefore, in the case where final effective data window detection signal MDC_R rises at a time point between the rising edges of delayed clock signals CLKda and CLKdb, and final effective data window detection signal MDC_R falls at a time point between the falling edges of delayed clock signals CLKdu and CLKdv, “1” is stored in the latches corresponding to the H-level region of this effective data window detection signal MDC_R. The number of registers corresponds to the time width of the final effective data window. Therefore, as shown in FIG. 24B, the effective data window can be cut out from clock signal CLK. 
     Accordingly, the input clock signal of a delay stage corresponding to the latch in the central position among the latches storing “1”, is selected as strobe clock signal CKSTR, whereby strobe clock signal CKSTR providing a strobe timing at the central region of the effective data window can be obtained. In this case, even if the width of effective data windows may differs for a different memory device, and may be smaller than half the cycle of clock signal CLK, it is possible to provide the strobe timing which can accurately make the set-up time and the hold time equal to each other, to maximize the margins for the set-up time and the hold time. 
     FIG. 25A represents a strobe timing detecting operation for the data transferred in synchronization with the falling edge of dock signal CLK. As shown in FIG. 25A, “1” is stored in each of latch stages corresponding to the period, for which final effective data window detection signal MDC_F is active. In this case, as shown in FIG. 25B, the latch stages between delayed clock signals CLKdb and CLKdy store “1”, and the other latch stages store “0”. Accordingly, even for the data transferred in synchronization with the falling of clock signal CLK, it is possible to produce strobe clock signal CKSTF providing the strobe timing at the central position of the effective data window width. 
     The delay time in delay line  22  may be equal to one cycle period of clock signal CLK. This can prevent such a situation that the match detecting operations are simultaneously performed in a plurality of portions. 
     In the first modification, structures other than the above can be the same as the structures of the strobe timing detecting circuit and strobe timing storage circuit already described with reference to FIGS. 19 and 20. It is possible to set the strobe timing according to the effective data window width. 
     Second Modification 
     FIG. 26 schematically shows a structure of a second modification of the first embodiment of the invention. FIG. 26 shows, by way of example, a structure for the data of 64 bits D 0 -D 63 . The strobe timing is adjusted for each data of 4 bits. For this, timing control circuit  5  includes timing adjusting circuits each provided for data of 4 bits. Timing adjusting circuit  5 - 0  adjusts the strobe timing for data bits D 0 -D 3  in accordance with local effective data window detection signals ZEDWR 0  and ZEDWF 0 . Timing adjusting circuit  5 - 15  adjusts the strobe timing for data bits D 60 -D 63  in accordance with local effective data window detection signals ZEDWR 15  and ZEDWF 15 . In general, timing adjusting circuit  5 - i  adjusts the strobe timing for data bits D( 4   i )-D( 4   i+ 3) in accordance with local effective data windows ZEDWRi and ZEDWFi. 
     Strobe clock generating circuits  6 - 0 - 6 - 15  generating the strobe clock signals are provided for timing adjusting circuits  5 - 0 - 5 - 15 , respectively. Strobe clock generating circuit  6 - 0  produces strobe clock signals CKSTF 0  and CKSTR 0  for data bits D 0 -D 3 , and strobe clock generating circuit  6 - 15  generates strobe clock signals CKSTF 15  and CKSTR 15  for data bits D 60 -D 63 . As for the structure of timing adjusting circuits  5 - 0 - 5 - 15  as well as strobe clock generating circuits  6 - 0 - 6 - 15 , local window detection signal ZEDW has only to be used in the foregoing structures instead of final effective data window detection signals MDC_R and MDC_F. 
     As shown in FIG. 26, adjustment of the strobe timing is performed for each data unit formed of four bits, whereby the data can be taken in more accurately. 
     Adjustment of the strobe timing for the data bits may be performed on a byte by byte basis, or may be performed on a basis of an even number of bytes. The multi-bit data may be divided into a plurality of sets of bits each having a common power supply pin provided therefor, and the strobe timing may be adjusted for each set. 
     According to the first embodiment of the invention, as described above, an effective data window is detected, and the timing of the strobe clock signal is adjusted in accordance with the effective data window, so that the data can be accurately taken in even when a signal propagation delay occurs in an on-board assembly. The memory device is not provided with a DLL, and a circuit for adjusting the data strobe timing is merely provided in a memory control unit. Therefore, the cost and current consumption of the memory device can be reduced. 
     Second Embodiment 
     FIG. 27 shows another example of the transfer data waveforms. In FIG. 27, all data bits D 0 -D 3  rise from L-level to H-level in synchronization with the rising of clock signal CLK in SSO (Simultaneous Switching Output). In this SSO, rising of the data bits becomes slow. If the power supply of the memory device (i.e., power supply for driving an output circuit) does not have a sufficient current supply capability, the power supply voltage lowers when an output buffer for outputting the data bits operates. Therefore, fast rising of the data bits may become impossible. When all the data bits change from L-level to H-level, therefore, the point of change (transition) of the effective data window becomes retarded, and the window width becomes narrow (the effective data window is detected at the point of change of a complementary data pair). 
     FIG. 28 shows a data bit pattern which is applied when the SSO push-out is detected. FIG. 28 shows, by way of example, data of 4 bits. In the case where the SSO push-out is to be detected, data bits D 0 -D 3  are set to “0001” at a falling edge FB of clock signal CLK preceding a rising edge RA of clock signal CLK. Then, data bits D 0 -D 3  are set to “1110” at rising edge RA of clock signal CLK. The data bit pattern at the next falling edge of the clock signal is not restricted. This is because the trailing edge of the effective window is not required for detecting the SSO push-out, although the leading edge of the effective window must be detected. 
     Subsequently, data bits D 0 -D 3  are set to “0001” at a rising edge RB of clock signal CLK, and then data bits D 0 -D 3  are set to “1110” at a next falling edge FA of clock signal CLK. The data bit pattern at the next rising edge of clock signal CLK is not restricted. At rising edge RA and falling edge FA, the change positions of data bits are detected, and the SSO push-out is detected. In the case of 4-bit data as described above, only one bit is changed from H-level (“1”) to L-level (“0”), and the remaining three bits are changed from L-level (“0”) to H-level (“1”). This is done to cause the one data bit to change to L-level fastest to reliably detect the delay in rising. 
     FIG. 29 shows, by way of example, a structure of the SSO push-out detecting portion for 4-bit data, and shows the structure for the case where data bit D 3  changes from “1” to “0”, and remaining data bits D 0 -D 2  change from “0” to “1”. 
     In FIG. 29, the push-out detecting portion includes: a pull-up resistance Z 1  of a high resistance connected between the power supply node and a node NDA; N-channel MOS transistors Q 20 , Q 21  and Q 22  connected in series between node NDA and the ground node, and receiving data bits D 0 , D 1  and D 2  on their gates, respectively; P-channel MOS transistors Q 23  and Q 24  connected in series between the power supply node and a node NDB; and a pull-down resistance Z 2  of a high resistance connected between node NDB and the ground node. MOS transistor Q 23  receives data bit D 3  on its gate, and MOS transistor Q 24  has a gate connected to node NDA. A change detection signal POD at the time of push-out is generated from node NDB and applied to latch circuit  24  shown in FIG.  9 . 
     FIG. 30 is a signal waveform diagram representing an operation of the SSO push-out detecting portion shown in FIG.  29 . Referring to FIGS. 29 and 30, description will now be given on the SSO push-out detecting operation. 
     Before the rising edge of clock signal CLK at time ta, data bits D 0 -D 2  are set to L-level, and data bit D 3  is set to H-level. Then, data including data bits D 0 -D 2  at H-level and data bit D 3  at L-level is read from the memory device at the rising of clock signal CLK. In the SSO push-out operation, rising to H-level is slowed down so that data bits D 0 -D 2  reach H-level after data bit D 3  reaches L-level. At time ta, data bits D 0 -D 2  in the SSO push-out detecting portion are at L-level, and all MOS transistors Q 20 -Q 22  are off. Node NDA is already precharged to H-level by pull-up resistance Z 1 . Data bit D 3  is at H-level, MOS transistor Q 23  is off and MOS transistor Q 24  is also off so that node NDB is maintained at the ground voltage level by pull-down resistance Z 2 . 
     When the transferred data bits change in synchronization with the rising of clock signal CLK at time ta, MOS transistor Q 23  is first turned on, and then MOS transistors Q 20 -Q 22  are turned on in accordance with data bits D 0 -D 2 , and responsively node NDA is discharged to the ground voltage level. When the potential on node NDA lowers to L-level, MOS transistor Q 24  is turned on, and node NDB is charged via MOS transistors Q 23  and Q 24  so that push-out detection signal POD rises from L-level to H-level. Push-out detection signal POD falls to L-level when data bits D 0 -D 3  change into a pattern other than “1110” in response to the falling of clock signal CLK. 
     Push-out detection signal POD is applied to latch circuit  24  shown in FIG. 9 for detecting a timing relationship between the leading edge of push-out detection signal POD and the rising or falling of clock signal CLK. 
     FIG. 31 schematically represents an operation of correcting the effective data window. The latch stage in latch circuit  24  latches “1” or “0” depending on the timing relationship between the push-out detection signal POD and clock signal CLK. Register A stores “1” in latch stage LA in accordance with effective data window detection signal MDC (MDC_R or MDC_F), whereby the leading position of the effective data window is indicated. Register C stores “1” in latch stage LC corresponding to the leading edge of push-out detection signal POD. The other registers store “0”. Correction of the effective data window and correction of strobe timing are performed based on the numbers of latches LA and LC storing “1” in registers A and C. The effective data window is corrected by shifting the position of its leading edge backward by (LC-LA). The strobe timing based on the effective data window detection signal is expressed by (LA+LB)/2. Therefore, the corrected strobe timing is expressed by (LC+LB)/2. The position of the trailing edge of the effective data window is not corrected because a speed-down occurs in the changing speed of the leading edge according to this push-out. 
     The SSO push-out detecting information may be stored for each memory device, and it may be determined whether a memory device satisfies a predetermined push-out condition or not. This can ensure the reliability of the memory system. 
     The SSO push-out is detected for each four bits. The detection signals are applied to final window detecting circuit  12 . In a memory device operating in the above manner, the SSO push-out may be detected for each set of output circuits supplied with a power supply voltage from a common power supply line. Alternatively, the SSO push-out may be detected simultaneously for all the bits. In the case of, e.g., 64 bits, 63 bits are driven from “0” to “1”, and the remaining 1 bit is driven from “1” to “0”. 
     As described above, according to the second embodiment, the SSO push-out is detected to adjust the data strobing timing so that the reliability of the memory system is ensured (If the time of push-out is long, the data cannot be transferred in synchronization with a fast clock.) Further, the optimum strobe timing can be set for each memory device. 
     Third Embodiment 
     FIG. 32 shows classification of effective data windows according to a third embodiment of the invention. In this third embodiment, the effective data window information is classified into three levels QoS 1 , QoS 2  and QoS 3 . At level QoS 1 , both the set-up time and hold time are 400 ps. At level QoS 2 , both the set-up time and hold time are 600 ps. At level QoS 3 , both the set-up time and hold time are 800 ps. 
     By utilizing the structure shown in FIG. 23, the region of an effective data window can be cut out. With the structure shown in FIG. 23, the effective data window has a width equal to a distance between latch stages LB and LA in the latch circuit. For example, if the delay stage has a delay time of 200 ps, the effective data window has a width of 200·(LB-LA+1). The effective data window information corresponding to initialized level QoS 3  is stored in the register circuit together with the strobe timing information linked thereto. In the start-up of the system, the effective data window of a system memory device is detected in an initial diagnosis mode. Depending on the system operation conditions, deterioration of the memory device or the like, the rising and falling of the data bit may become slow, in which case the width of the effective data window is reduced. When the width of effective data window decreases to level QoS 2 , the address is stored in an error register. When it attains level QoS 1 , it is determined that accurate sampling of data is impossible, and the memory control unit informs the system (CPU) of this failure to trigger a system reset. This prevents destruction of program data in the memory system such as a main memory storing the program data. 
     FIG. 33 shows a structure of a strobe timing storage circuit  45 . Strobe timing storage circuit  45  includes a region  45   aa  for storing an address of a memory device, a region  45   ab  for storing a bank name (address) and a region  45   b  for storing a strobe timing. Effective data window EDW information at level QoS 3  is stored being linked to the device name, bank name and strobe timing information. The effective data window is detected in the initial diagnosis mode in the system start-up operation. The value at level QoS 3  is initialized to a fixed value. In each address (bank) of memory devices, a predetermined data pattern (the effective window detecting circuit detects an effective window width by using complementary bit pairs) is written and then is read out, and then the width of the effective data window for each address is detected utilizing the structure shown in FIG.  23 . The width of each effective data window is compared with the width of the effective data window at level QoS 3  or QoS 2  which is initially set, and an error register  50  shown in FIG. 34 is accessed in accordance with the result of comparison. In the case of level QoS 2  at which the effective data window is smaller than the width defined by level QoS 3 , address information #EAD including the address specifying the device and bank is stored in error register  50 . When check of all the devices is completed, if a faulty, i.e., an effective data window at level QoS 2  is found, the corresponding memory device is taken out from the board, and is solely tested. Then, the faulty address is accessed at a single chip level, and data writing/reading are performed. If this memory device operates accurately, it is determined that the cause of faulty is present in the data bus on the board. 
     FIG. 35 schematically shows a structure for detecting a margin of this data window. The structure shown in FIG. 35 includes: an effective window width calculating circuit  52  for calculating an effective window width in accordance with the latch signal from latch circuit  24  shown in FIG. 23; a comparator circuit  54  for comparing the effective window width calculated by effective window width calculating circuit  52  and the window width at level QoS 2  read from register circuit  57 ; and a comparator circuit  56  for comparing the effective window width calculated by effective window width calculating circuit  52  and the effective window width corresponding to level QoS 1  set in a register circuit  58 . 
     Effective window width calculating circuit  52  includes registers A and B, and calculates effective data window width (LB-LA+1) from the leading and trailing positions of the latch stages in the latch stages storing “1” outputted from latch circuit  24 . Comparator circuit  54  compares the window width corresponding to level QoS 2  and the effective data window width calculated by effective window width calculating circuit  52 . When comparator circuit  54  indicates that the effective data window width calculated by effective window width calculating circuit  52  is smaller than level QoS 2 , error register  50  stores a current address AD according to an error instructing signal generated from comparator circuit  54 . When the effective data window width calculated by effective window width calculating circuit  52  is smaller than the effective window width corresponding to level QoS 1 , it is determined that the correct data transfer cannot be performed due to a less margin of window, and comparator circuit  56  produces reset signal RST for triggering the system reset. 
     The effective data window width (the number of latch stages) corresponding to level QoS 1  applied to comparator circuit  56  is stored in register  58 . 
     When the effective data window width is equal to or larger than 600 ps of the set-up/hold times defined by level QoS 2 , it is determined that the accurate operation can be performed. The following structure may be employed: In accordance with level QoS 3 , the effective data window width is initially set. In previously described detection of the strobe timing, the effective data window region having the time width defined by level QoS 3  from the leading edge of the effective data window is detected, and the central point in this region is determined as the strobe timing. More specifically, the number of latch LA arranged corresponding to the leading position of the effective data window and the number of latch stages designated by level QoS 3  are added, an average value of a result of the addition is obtained, and thus the number of the register stage applying the strobe timing can be specified. 
     The set-up/hold times at these levels may take values other than the above, and are merely required to be determined appropriately in accordance with a data transfer rate. This level detection data pattern is set in a complementary bit pattern form for detecting a window. This is because the effective data window detecting circuit can detect an effective data window only when a complementary data bit pattern is applied. Description will now be given on a structure wherein the margin of the set-up/hold can always be detected even in a general, normal operation mode. 
     First Modification 
     FIG. 36 schematically shows, by way of example, a modification of the effective data window detecting portion. In FIG. 36, the effective data window detecting portion includes: a bit change detecting circuit  60  for detecting the transition time of each of data bits D 0 -D 3 ; a rising edge data window detecting circuit  65  for producing an effective data window detection signal DTD_R for data transferred in synchronization with the rising edge of the clock signal in accordance with the output signal of bit change detecting circuit  60 ; and a falling edge data window detecting circuit  70  for detecting the effective data window of data transferred in synchronization with the falling edge of the clock signal in accordance with the bit change detection signal generated from bit change detecting circuit  60 . 
     Rising edge window detecting circuit  65  includes: a transfer circuit  65   a  for transferring the change detection signal generated from bit change detecting circuit  60  when effective data window detection signal DTD_R of data (rising edge data) transferred in synchronization with the rising edge of the clock signal; a latch circuit  65   b  latching the output signals of transfer circuit  65   a;  a slowest change detecting circuit  65   c  for detecting the slowest change point from the data latched by latch circuit  65   b;  a transfer circuit  65   d  for transferring an output signal of bit change detecting circuit  60  when effective data window detection signal DTD_R of the rising edge data is at H-level; a fastest change detecting circuit  65   f  for detecting the fastest change point in data bits D 0 -D 63  in accordance with output signals of latch circuit  65   e;  and a set/reset flip-flop  65   g  set in response to the rising of the output signal of slowest change detecting circuit  65   c  and reset in response to the rising of the output signal of fastest change detecting circuit  65   f  for outputting effective data window detection signal DTD_R. 
     Falling edge data window detecting circuit  70  has a structure similar to rising edge data window detecting circuit  65 , but is different therefrom in activation timing of the transfer circuits for the slowest change detecting circuit and the fastest change detecting circuit. For controlling the operation of falling edge data window detecting circuit  70 , there is arranged a set/reset flip-flop  72  which is set in response to the rising edge of effective data window detection signal DTD_R, and is reset in response to the falling of effective data window detection signal DTD_F. The transfer circuits included in falling edge data window detecting circuit  70  are controlled in accordance with the signal generated at output Q of set/reset flip-flop  72  and effective data window detection signal DTD_F. 
     In the structure shown in FIG. 36, data is transferred in synchronization with the rising edge of the clock signal, and then data is transferred in synchronization with the falling edge of the clock signal. Transfer circuit  65   a  transfers the change detection signal, generated from bit change detecting circuit  60 , to latch circuit  65   b.  Slowest change detecting circuit  65   c  detects the slowest change point in the data bits, and sets a set/reset flip-flop  65   g.  Transfer circuit  65   d  is activated in response to the rising of effective data window detection signal DTD_R, to transfer the output signal of bit change detecting circuit  60 , to latch circuit  65   e.  Fastest change detecting circuit  65   f  detects the fastest change point in the bits of data latched by latch circuit  65   e,  and resets set/reset flip-flop  65   g.  Therefore, for the data transferred in synchronization with the rising edge of the clock signal, a period from the slowest change point in the data bits to the fastest change point in the same data bits is detected, and effective data window detection signal DTD_R is activated for this detected period. 
     In the falling edge data window detecting circuit  70 , when effective data window detection signal DTD_R for the data, transferred in synchronization with the rising edge, is activated, set/reset flip-flop  72  is set, and output signal DTR thereof rises, and responsively falling edge data window detecting circuit  70  takes in the output signal of bit change detecting circuit  60 . Then, the slowest change point and the fastest change point are detected in accordance with the bit change detection signals, and signal DTD_F is activated in accordance with the result of the detection. Thus, effective window for the data bits transferred in synchronization with the falling edge of the clock signal is detected. 
     FIG. 37 shows a structure of a bit change detector for data Di of one bit in bit change detecting circuit  60  shown in FIG.  36 . In FIG. 37, the bit change detector in bit change detecting circuit  60  includes a delay circuit  60   a  for delaying data bit Di by a predetermined time, and an EXOR gate  60   b  receiving an output signal of delay circuit  60   a  and data bit Di. EXOR gate  60   b  generates bit change detection signal BTDi. 
     In the bit change detector shown in FIG. 37, when data bit Di changes, signals at different logical levels are applied to EXOR gate  60   b  for a period equal to the delay time of delay circuit  60   a,  and bit change detection signal BTDi is at H-level for the delay time period. Thereby, the change point of data bit Di can be detected. 
     FIG. 38 shows a structure of transfer circuit  65   a  shown in FIG.  36 . In FIG. 38, transfer circuit  65   a  includes a tri-state inverter buffer circuit provided corresponding to each bit change detection signal. FIG. 38 shows, as a representative, a tri-state inverter buffer  65   aa  provided for bit change detection signal BDTi. Tri-state inverter buffer  65   aa  is activated when effective data window detection signal DTD_R is at L-level, to produce an internal bit change detection signal BTDii in accordance with bit change detection signal BTDi. 
     FIG. 39 shows, by way of example, a structure of latch circuit  65   b  shown in FIG.  36 . FIG. 39 shows a structure for the data of one bit. In FIG. 39, latch circuit  65   b  includes an inverter latch  65   ba  for latching internal bit change detection signal BTDii outputted from corresponding tri-state inverter buffer circuit  65   aa.  Inverter latch  65   ba  includes a diode-connected P-channel MOS transistor TRO for transmitting bit change detection signal BTDi when internal bit change detection signal BTDii is at L-level, an inverter VG 0  inverting a signal applied from MOS transistor TR 0 , and an inverter VG 1  for inverting an output signal of inverter VG 0 , for transmission to the input of inverter VG 0 . Inverter VG 0  outputs a latch bit change detection signal LBTDi. Inverter latch  65   ba  is reset in the initial state such that latch bit change detection signal LBTDi is at H-level (a reset transistor is not shown). Operations of the circuits shown in FIGS. 37 to  39  will now be described with reference to a signal waveform diagram. 
     When data bit Di changes, bit change detection signal BTDi rises from L-level to H-level in bit change detecting circuit  60  shown in FIG. 37 for a period equal to the delay time of delay circuit  60   a.  When bit change detection signal BTDi rises, internal bit change detection signal BTDii from tri-state inverter buffer circuit  65   aa  falls from H-level to L-level. When internal bit change detection signal BTDii falls to L-level, MOS transistor TRO in inverter latch  65   ba  is turned on, and latch bit change detection signal LBTDi generated from inverter VG 0  rises to H-level. After internal bit change detection signal BTDii rises to H-level, MOS transistor TR 0  maintains the off state, and latch bit change detection signal LBTDi maintains H-level. 
     FIG. 41 schematically shows a structure of slowest change detecting circuit  65   c  shown in FIG.  36 . In FIG. 41, slowest change detecting circuit  65   c  includes NAND circuits NG 0 -NG 15  provided for respective signal sets each including four bits of latch bit change detection signals LBTD, four 4-input NOR circuits NGR 0 -NGR 3 , provided corresponding to circuit sets each including four NAND circuits of NAND circuits NG 0 -NG 15 , for receiving the output signals of the corresponding sets of four NAND circuits, and a 4-input NAND circuit NGA receiving output signals of NOR circuits NGR 0 -NGR 3 . NAND circuit NGA generates leading window change detection signal LEWR via an inverter. 
     Each of NAND circuits NG 0 -NG 15  generates a signal at L-level when all the corresponding four inputs attain H-level. Each of NOR circuits NGR 0 -NGR 3  generates a signal at H-level when all the corresponding four inputs attain L-level. Therefore, the output signal of each of NAND circuits NG 0 -NG 15  falls to L-level in response to the slowest change in the corresponding bit change detection signals of four bits. Each of NOR circuits NGR 0 -NGR 3  raises its output signal to H-level in response to the slowest rising in the output signals of corresponding four NAND circuits. NAND circuit NGA lowers its output signal to L-level in response to the slowest falling in the output signals of NOR circuits NGR 0 -NGR 3 . By inverting the output signal of NAND circuit NGA, leading window change detection signal LEWR is raised to H-level in accordance with the bit change detection signal changing the most slowly. 
     FIG. 42 shows a structure of transfer circuit  65   d  shown in FIG.  36 . In FIG. 42, transfer circuit  65   d  includes a tri-state inverter buffer circuit  65   da  activated in response to H-level of data window detection signal DTD_R, to invert bit change detection signal BTDi for producing internal bit change detection signal BTDit. 
     The structure of transfer circuit  65   d  shown in FIG. 42 is the same as that of transfer circuit  65   a  shown in FIG. 38 except for that transfer circuit  65   d  is activated when transfer circuit  65   a  is deactivated. Thus, transfer circuit  65   d  is activated when the leading edge of the effective data window is detected, and the trailing edge of the effective data window is detected. 
     FIG. 43 schematically shows a structure of latch circuit  65   e  shown in FIG.  36 . In FIG. 43, latch circuit  65   e  includes: a diode-connected P-channel MOS transistor TR 1  which receives internal bit change detection signal BTDit from corresponding tri-state inverter buffer circuit  65   da;  an inverter circuit VG 2  which inverts a signal outputted from MOS transistor TR 1  to produce a latch bit change detection signal TBTDi; and an inverter circuit VG 3  which inverts an output signal of inverter circuit VG 2  for transmission to an input of inverter circuit VG 2 . Transfer circuit  65   ea  is provided for each of the data bits. 
     FIG. 44 shows a structure of fastest change detecting circuit  65   f  shown in FIG.  36 . Fastest change detecting circuit  65   f  in FIG. 44 includes: OR circuits OGR 0 -OGR 15  provided corresponding to signal sets each including latch bit change detection signals of 4 bits; OR circuits OGR 16 -OGR 19  provided for each four circuits of OR circuits OGR 0 -OGR 15 , and receiving an output signal of OR circuits of a corresponding set of four OR circuits; and a 4-input OR circuit OGR 20  receiving the output signals of OR circuits OGR 16 -OGR 20 . OR circuit OGR 20  generates a bit change detection signal TEWR 
     OR circuit OGR 0  receives latch bit change detection signals TBTD 0 -TBTD 3  of 4 bits, and OR circuit OGR 15  receives latch bit change detection signals TBTD 60 -TBTD 63  of 4 bits. These OR circuits each raise an output signal thereof to H-level when a signal at H-level is supplied thereto. Accordingly, each of OR circuits OGR 0 -OGR 15  detects the fastest bit change in the corresponding four data bits. Likewise, each of OR circuits OGR 16 -OGR 19  raises its output signal to H-level in response to the fastest rising in the output signals of the corresponding four OR circuits. OR circuit OGR 20  in the final stage raises its output signal TEWR to H-level in response to the fastest rising in the output signals of OR circuits OGR 16 -OGR 19 . Thereby, window change detection signal TEWR is driven to the active state at H-level in accordance with the fastest change in data bits D 0 -D 63 . 
     FIG. 45 is a signal waveform diagram representing an operation of rising edge data window detecting circuit  65  shown in FIGS. 37 to  44 . Referring to FIG. 45, description will be given on the operation of rising edge data window detecting circuit  65  shown in FIGS. 37 to  44 . 
     When data bit D changes, effective data window detection signal DTD_R is still at L-level, and transfer circuit  65   a  is turned on to transfer the bit change detection signal received from bit change detecting circuit  60  to latch circuit  65   b.  Leading window change detection signal LEWR from slowest change detecting circuit  65   c  rises to H-level in response to the slowest change of latch bit change detection signal LBTD (LBTDi). When the leading window change detection signal LEWR rises to H-level, set/reset flip-flop  65   g  shown in FIG. 36 is set, and effective data window detection signal DTD_R rises to H-level. 
     When effective data window detection signal DTD_R rises to H-level, transfer circuit  65   a  is deactivated, and all the bit change detection signals attain or keep L-level, and the data bits no longer change. When the data bits change subsequently, trailing edge window change detection signal TEWR from fastest change detecting circuit  65   f  rises to H-level in response to the fastest change of bit change detection signal TBTD (TBTDi). In response to activation of trailing edge window change detection signal TEWR, set/reset flip-flop  65   g  is reset, and effective data window detection signal DTD_R falls to L-level. Thus, effective data window of the data bits transferred in synchronization with the clock edge can be detected. 
     FIG. 46 shows a structure of an input portion of falling edge data window detecting circuit  70  shown in FIG.  36 . In FIG. 46, falling edge data window detecting circuit  70  includes: a transmission gate  70   i  which transmits bit change detection signal BTDi generated from bit change detecting circuit  60  in response to period detection signal DTR and complementary period detection signal ZDTR from set/reset flip-flop  72  shown in FIG. 36; a transfer circuit  70   a  including a tri-state inverter buffer  70   aa  activated to invert a signal received through transmission gate  70   i  when effective data window detection signal DTD_F is at L-level; and a transfer circuit  70   d  which is activated to invert and transfer a bit change detection signal received through transmission gate  70   i  to a next latch stage when effective data window detection signal DTD_F is at H-level. Transfer circuit  70   d  includes a tri-state inverter buffer circuit  70   da  provided corresponding to data bit change detection signal BTDi and activated to invert the bit change detection signal when window detection signal DTD_F is at H-level. 
     The circuit structures of falling edge data window detecting circuits  70  arranged subsequent to transfer circuits  70   a  and  70   d  are the same as those of rising edge detecting circuit  65  shown in FIG.  36 . 
     FIG. 47 is a signal waveform diagram representing operations of the data window detecting circuit shown in FIG.  46 . Referring to FIG. 47, when the bits of data D change, effective data window detection signal DTD_R first rises to H-level in response to the slowest change of the data bits. When effective data window detection signal DTD_R rises to H-level, flip-flop  72  shown in FIG. 36 is set so that period detection signal DTR rises to H-level. When period detection signal DTR rises to H-level, transmission gate  70   i  shown in FIG. 46 is turned on to take bit change detection signal BTDi into falling edge data window detecting circuit  70 . At this time point, all bit change detection signals BDTi are already changed to L-level, and the output signals of falling edge window detecting circuit  70  do not change. In response to the fastest change of data bits, effective data window detection signal DTD_R falls to L-level, and the effective window of the data transferred at the rising edge of the clock signal is detected. At this time point, transmission gate  70   i  is already on, and falling edge data window detecting circuit  70  performs the detecting operation to raise effective data window detection signal DTD_F to H-level in response to the slowest change of data bit. 
     When effective data window detection signal DTD_F rises to H-level, transfer circuit  70   a  is deactivated, and transfer circuit  70   d  is activated to wait for a next change in data bits. When data bits change again, a fastest change detecting circuit provided downstream the transfer circuit  70   d  extracts the fastest change of data bits, and effective data window detection signal DTD_F falls to L-level in response to this fastest change of data bits. When effective data window detection signal DTD_F falls to L-level, set/reset flip-flop  72  (see FIG. 36) is reset, and transmission gate  70   i  is turned off so that falling edge data window detecting circuit  70  is isolated from bit change detecting circuit  60 . 
     In the detecting operation of falling edge data window detecting circuit  70 , such a situation may occur that effective data window detection signal DTD_R is at L-level, and transfer circuit  65   a  is activated to activate effective data window detection signal DTD_R. Therefore, as shown in FIG. 48, a flip-flop  75  is provided which is set at the falling of effective data window detection signal DTD_R, and is reset at the falling of period detection signal DTR. The output signal of flip-flop  75  is applied to a control gate of transmission gate  76  arranged at a stage preceding transfer circuits  65   a  and  66   b.  Thereby, the detecting operation of rising edge data window detecting circuit  65  can be stopped during the detecting operation of falling edge data detecting circuit  70 . 
     By utilizing the structure shown in FIGS. 36 et. seq., an effective data window can be detected in accordance with arbitrary change in data bit. The width of the effective data window region is normally detected and the level determination of levels QoS 1 -QoS 3  can always be performed so that the safety of the system can be ensured. 
     A desired, one clock cycle may be extracted for performing the window detecting operation. More specifically, effective data window detection signals DTD_R and DTD_F may be each activated once only in a period in which the effective window width is detected. The data strobe timing is already adjusted in the normal operation mode, and the detection of the effective data window is adapted to be performed in accordance with a test mode signal only for one clock cycle period, to implement such one clock cycle operation. 
     Second Modification 
     FIG. 49 shows a second modification of the third embodiment of the invention. In the second modification, as shown in FIG. 49, both the set-up time and the hold time at level QoS 0  are set to 100 ps for margins of the set-up/hold times. At levels QoS 1  and QoS 2 , the set-up/hold times are set to 200 ps and 400 ps, respectively, similarly to the foregoing third embodiment as shown in FIG.  32 . 
     According to level QoS 2 , an initial effective data window width is set. When effective data window width lowers to level QoS 1 , the data pattern and the associated address are stored in the error register shown in FIG.  34 . By utilizing the circuits shown in FIG. 36 et. seq., the effective data window of an arbitrary data pattern can be detected. When an effective data window lowers to level QoS 0 , the memory control unit signals the system (CPU) of the system reset. 
     When an operation is performed at level QoS 1 , data having a set-up or hold time between levels QoS 0  and QoS 1  is also transferred. In this case, erroneous determination of data “0” and “1” may occur. However, even such a memory system can be used for an application where a large amount of data is used, and partial error in determination of data “0” and “1” exerts only a small influence, such as graphic data. 
     The classes shown in FIGS. 32 and 49 determine the size of an effective data window, and the data transfer rate can be set based on the size of the determined effective data window. 
     FIG. 50 shows an upper limit transferable waveform of signal waveforms in the case where the signal quality shown in FIG. 49 is used. In FIG. 50, each of the rising time and falling time of the signal is 250 ps. At down to level QoS 1 , the signal quality is ensured, and the data transfer can be performed accurately. Accordingly, both the set-up time and the hold time are 200 ps. Therefore, the data window of this signal is equal to 900 ps. The transfer rate in this case is equal to {fraction (1/900)} ps=1.11 Gbits/sec. 
     The signal waveform shown in FIG. 50 is a waveform of an effective data window, and includes a signal skew between pins. Compared with this waveform, the set-up/hold times of the signal outputted from each pin are longer in a practical use, and the actual transfer rate may be lower than the foregoing value of 1.11 Gbits/sec. 
     FIG. 51 shows, by way of example, a structure of the memory system according to second modification of the third embodiment of the invention. In FIG. 51, memory control unit  2  and memory device  1  are coupled via common data bus  4 . Common data bus  4  has sub-data buses  4   a - 4   d  each transferring data of 64 bits. In the case where memory device  1  is formed of memory module including four memory chips, the memory chips are arranged corresponding to sub-data buses  4   a - 4   d,  respectively, and each memory chip transfers data of 64 bits at a time. 
     In this memory system, the data transfer rate is equal to 1.11·256=284 Gbits/sec if the signal waveform shown in FIG. 50 is used. Accordingly, by appropriately determining the set-up/hold times at level QoS indicating this signal quality by a system designer, the effective window width of the transfer data is determined, and accordingly the data transfer rate of the memory system can be determined. 
     According to the third embodiment of the invention, the signal quality can be desirably set based on its set-up/hold times, and the system reset or storage of an error address is performed when the signal quality lowers. Therefore, the data transfer rate can be kept high without lowering the signal quality. 
     Fourth Embodiment 
     FIG. 52 schematically shows a whole structure of a data processing system according to a fourth embodiment of the invention. In this data processing system shown in FIG. 52, memory control unit  2  is integrated with a logic (or processor)  104  on the same chip. A processing device  100  also includes a PLL (Phase-Locked Loop Circuit)  102 , which receives clock signal (system clock) CLK from clock generator  3 , and produces an internal operation clock signal PCLK through frequency multiplication of system clock CLK. Internal operation clock signal PCLK generated from PLL  102  has a frequency of, e.g., 1 GHz, and determines the operation speed of logic  104 . In other words, internal operation clock signal PCLK determines a processing cycle of logic  104 , and logic  104  executes the processing in synchronization with this operation clock signal PCLK. Operation clock signal PCLK for logic, produced by PLL  102 , is also supplied to a strobe timing adjusting circuit  110 . Strobe timing adjusting circuit  110  receives data Dc read from memory devices  1   a - 1   n , and detects the effective data period in accordance with operation clock signal PCLK for logic. Similarly to the first embodiment, strobe timing adjusting circuit  110  determines a strobe timing in accordance with the detected effective data period, and produces a strobe signal for taking data into input circuit  7 . 
     At the same time as sending of data Dc onto data bus  4 , output circuits  9   a - 9   n  in memory devices  1   a - 1   n  send therefrom a strobe signal STROBE (or STROBE#) designating the data taking-in timing. When data is outputted from memory device ( 1   a - 1   n ), strobe signal STROBE (or STROBE#) is produced by the same memory device ( 1   a - 1   n ) in synchronization with the output clock signal, and determines the timing at which the data are sampled. Accordingly, strobe signals STROBE (and STROBE#) transmitted onto a strobe signal line  105  and read data Dc are transmitted in the same direction to memory control unit  2 . However, when memory devices  1   a - 1   n  are inserted into sockets, impedances of signals lines vary depending on the state of insertion into the socket, and the impedance of each signal line varies depending on an arrangement of wires on the board. Accordingly, a skew is present even between read data Dc and strobe signals STROBE (and STROBE#). Strobe signals STROBE and STROBE# are complementary to each other. 
     FIG. 53 schematically shows structures of strobe timing adjusting circuit  110  and the input circuit shown in FIG.  52 . More specifically, FIG. 53 shows the structure of the input circuit and the strobe timing adjusting circuit for read data DQ of 4 bits. In the case where the memory device transfers data of 64 bits, the strobe timing may be adjusted for each 4 bits, as is done in the first embodiment already described. Also, the strobe timing may be adjusted a byte by byte basis, or on a unit of data bits associated with a common power supply pin. Further, strobe timing adjusting circuit  110  may adjust the timing commonly to the data of a total of 64 bits, similarly to the structure employing the final window detecting signal. The following description will be given on the case where the data is formed of four bits, for simplicity reason. 
     Input circuit  7  includes: an amplifier  107   a  which amplifies and converts data DQ transferred from the memory device into a signal at a CMOS level (digital value); and a data latch  107   b  which latches the output signal of amplifier  107   a  to produce an internal data bit in accordance with a modified strobe trigger signal CPTR supplied from strobe timing adjusting circuit  110 . Since data DQ read from the memory device is transferred via data bus  4 , data DQ is to be an analog signal due to distortion of its waveform or a small amplitude. Therefore, amplifier  107   a  is used to amplify and convert this analog signal of data DQ into a digital signal. 
     Strobe timing adjusting circuit  110  includes: a shifter/clock generating circuit  114  which receives output data bits of amplifier  107   a  and strobe signal STROBE to perform a shifting operation in accordance with processor internal operation clock signal PCLK, and produces modified strobe trigger signal CPTR in a normal data read mode; and a strobe timing calculating circuit  116  which determines an optimum strobe timing based on the effective window width of shifter/clock generating circuit  114  in a monitor mode, and sets the strobe timing in shifter/clock generating circuit  114  in accordance with the detected strobe timing. Strobe timing calculating circuit  116  includes a register circuit for determining the strobe timing. An operation of detecting the effective data window in this fourth embodiment will now be described. 
     In the effective data window detecting operation, data 010101 . . . is outputted from data terminals of pin numbers 0123 . . . , respectively, similarly to the first embodiment. These data bits are transferred at the rising and falling edges of clock signal CLK, and the logical level of data bit for each pin is inverted at every clock edge. Thus, the logical level of data bits outputted from the even-numbered pin terminals are always opposite to that of data bits outputted from the odd-numbered pin terminals. 
     FIG. 54 schematically shows a relationship between the effective data window and the strobe signal. In synchronization with strobe signal STROBE, the memory device applies a data bit group D&lt;gr&gt; to the memory control unit. A skew due to change in signal line impedance or the like is present between strobe signal STROBE and data bit group D&lt;gr&gt;. Data bit group D&lt;gr&gt; is data of 4 bits, e.g., in the structure shown in FIG. 53, and has an eye pattern similar to that of the first embodiment. Strobe signal STROBE is phase-synchronized with clock signal CLK at the time of output from the memory device. In the memory device, the output circuit is activated by an output enable signal for sending out the clock signal together with data bits when the data read operation is performed. 
     Accordingly, the data bit, transmitted when strobe signal STROBE is at H-level, is the data bit that is transferred in synchronization with the rising of clock signal CLK. Also, the data bit, transferred when strobe signal STROBE is at L-level (i.e., when strobe signal STROBE# is at H-level), is the data bit that is transferred in synchronization with the falling edge of clock signal CLK. Effective data window detection signals EDW_R and EDW_F are detected during the periods of H-level and L-level of strobe signal STROBE, respectively. The central point of each of effective data window detection signals EDW_R and EDW_F is determined as strobe timing. Thereby, strobing of the data bit can be performed at the central point in the eye pattern, which in turn is formed by the data bits transferred in synchronization with strobe signal STROBE, and set-up time Ts(tsu) and hold time Th(th) can be made equal, so that the operation of taking in data can be accurately performed with sufficient margins. 
     FIG. 55 shows an example of the structure of a window detecting circuit  112  shown in FIG.  53 . Referring to FIG. 55, window detecting circuit  112  includes an EXOR circuit  112   a  receiving data bits D 0  and D 1 , an EXOR circuit  112   b  receiving data bits D 2  and D 3 , an AND circuit  112   c  receiving strobe signal STROBE and the output signal of EXOR circuit  112   a  to produce a primitive window detection signal EDR 01 , a gate circuit  112   d  receiving strobe signal STROBE and the output signal of EXOR circuit  112   a  to produce a primitive window detection signal EDF 01 , an AND circuit  112   e  receiving strobe signal STROBE and the output signal of EXOR circuit  112   b  to produce a primitive window detection signal EDR 23 , and an AND circuit  112   f  receiving strobe signal STROBE and the output signal of EXOR circuit  112   b  to produce a primitive window detection signal EDF 23 . When strobe signal STROBE is at L-level, gate circuits  112   d  and  112   f  are enabled to drive corresponding primitive window detection signals EDF 01  and EDF 23  to H-level in accordance with the output signals of corresponding EXOR circuits  122   a  and  122   b,  respectively. 
     EXOR circuits  112   a  and  112   b  are non-coincidence detecting circuits, and generate signals at H-level when paired data bits D 0  and D 1  are different in logic level and so are paired data bits D 2  and D 3 , respectively. 
     Window detecting circuit  112  further includes an AND circuit  112   g  which receives primitive window detection signals EDR 01  and EDR 23  from AND circuits  112   c  and  112   e  to produce an effective window detection signal EDW_R, and an AND circuit  112   h  which receives primitive window detection signals EDF 01  and EDF 23  supplied from gate circuit  112   d  and  112   f,  respectively, to produce an effective window detection signal EDW_F. 
     Effective window detection signal EDW_R indicates the effective window region for the data bits transferred in synchronization with rising of clock signal CLK, and effective window detection signal EDW_F indicates the effective window region for the data bits transferred in synchronization with falling of clock signal CLK. An operation of window detecting circuit  112  shown in FIG. 55 will now be described with reference to a signal waveform diagram of FIG.  56 . 
     Even-numbered bits D 0  and D 2  are opposite in logical level to odd-numbered data bits D 1  and D 3 , and the logical levels thereof change at each of the rising and falling edges of the clock signal. This is the same as the structure of the first embodiment. 
     Each of EXOR circuits  112   a  and  112   b  outputs the signal at H-level when the received data bits are different in logical level from each other. Strobe signal STROBE for the data, which is transferred in synchronization with the rising of clock signal CLK, is at H-level. Therefore, effective window detection signal EDW_R is produced in accordance with primitive window detection signals EDR 01  and EDR 23  generated from AND circuits  112   c  and  112   e,  respectively. Strobe signal STROBE shown in FIG. 56 changes in the same manner as data bit D 0 . If strobe signal STROBE has the phase significantly shifted from all the data bits, the width of these effective windows are different correspondingly. In either case, the region where all the data bits are definite can be detected in a period of H-level of strobe signal STROBE. 
     When strobe signal STROBE is at L-level, gate circuits  112   d  and  112   f  produce primitive window detection signals EDF 01  and EDF 23  in accordance with the output signals of EXOR circuits  112   a  and  112   b,  respectively. In this case, effective window detection signal EDW_F supplied from AND circuit  112   h  is at H-level while all the data bits are in the definite state. In the monitor mode of determining the strobe timing, therefore, if the data bit pattern applied from the memory device is set to a checker-board pattern, the effective window detection signal can be kept active using the EXOR circuit while the data bits are in the definite state. By utilizing strobe signal STROBE, it is possible to detect the period, for which the data bits are in the definite state while strobe signal STROBE is active (at H-level or L-level). In the normal data reading mode, the optimum strobe timing can be set with reference to the strobe signal. 
     Modification of Window Detecting Circuit 
     FIG. 57 shows a structure of a modification of window detecting circuit  112 . Window detecting circuit  112  shown in FIG. 57 uses strobe signal STROBE and inverted signal STROBE# thereof AND circuits  112   i  and  112   j  each receiving complementary strobe signal STROBE# on its first input are employed instead of gate circuits  112   d  and  112   f  shown in FIG.  55 . On second inputs, AND circuits  112   i  and  112   j  receive the output signals of EXOR circuits  112   a  and  112   b,  respectively. Structures other than the above are the substantially same as those shown in FIG.  55 . 
     The structure shown in FIG. 57 utilizes strobe signals STROBE and STROBE# which are complementary to each other. For primitive window detection signals EDF 01  and EDF 23 , complementary strobe signal STROBE# is utilized instead of strobe signal STROBE, whereby it is possible to provide equal gate delays to all the primitive window detection signals, so that the effective window detection can be performed accurately (because a delay corresponding to a gate of one stage for signal inversion in gate circuits  112   d  and  112   f  can be eliminated, and the influence by such a delay can be suppressed). 
     FIG. 58 schematically shows a structure of an output circuit  9  of the memory device. In FIG. 58, the memory device includes a clock circuit  118  for producing two-phase clock signals CLKE and CLKO (CLKE/O) changing in synchronization with the rising and falling of clock signal CLK, an output circuit  120  for successively transferring internal read data Din to produce external read data Q in accordance with clock signals CLKE/O generated from clock circuit  118  and an output enable signal OE, and a clock output circuit  122  for externally sending clock signal CLK to produce strobe signal STROBE when output enable signal OE is active. 
     Clock signals CLKE/O (CLKE, CLKO) attain H-level at the rising edge and the falling edge of clock signal CLK, respectively, and output circuit  120  serially outputs data of two bits, which are internally read in parallel, in accordance with complementary clock signals CLKE and CLKO. Thereby, data bits Q are outputted in synchronization with the rising and falling edges of clock signal CLK. 
     Clock output circuit  122  buffers and externally outputs clock signal CLK when output enable signal OE is active. Therefore, strobe signal STROBE is a signal synchronized, on the memory device side, with data bits Q. A skew occurs between strobe signal STROBE and data bits Q which arrive at the memory control unit due to a difference in impedance between signal wiring lines transmitting them. 
     The effective data window region is detected using strobe signal STROBE, so that it is possible to detect the region where the effective data is present within a period specified by the strobe signal. Thus, the effective data window region and the strobe timing can be determined more accurately. Also, it is possible to set the strobe timing with reference to the strobe signal in the normal data read mode. 
     FIG. 59 schematically shows structures of a strobe trigger signal generating portion and a strobe timing calculating portion. In FIG. 59, the strobe timing calculating portion includes an AND circuit  125  receiving a monitor mask instructing signal MMSK, which is made active in the test mode, and effective window detection signal EDW_R, and an AND circuit  126  receiving a normal read instructing signal NREAD and strobe signal STROBE. The outputs of AND circuits  125  and  126  are wired-OR connected. AND circuit  125  produces a final effective window detection signal MDC. 
     The strobe timing calculating portion further includes an AND circuit  127  receiving processor internal operation clock signal PCLK and the output signal of AND circuit  125  or  126 , to produce shift clock signal SFCK, and a strobe register  130  performing a shifting operation in synchronization with the rising and falling edges of shift clock signal SFCK. Strobe register  130  includes a plurality of shift circuits  130   a  which perform the shifting operation at the rising and falling edges of shift clock signal SFCK. 
     The strobe timing calculating portion further includes a temporary register  132  which receives and stores the data bit stored in each shift circuit  130   a  of strobe register  130  in response to a transfer instructing signal XFR. Temporary register  132  includes register circuits  132   a  provided corresponding to the respective shift circuits  130   a  of strobe register  130 . The strobe timing is calculated using the data stored in temporary register  132 . This calculation of the strobe timing may be performed utilizing software contained in a BIOS (basic I/O system) of the above-described processor, or dedicated hardware may be employed for the calculation. 
     The strobe timing calculating portion further includes a result register  134  for storing result data indicating the strobe timing, which is calculated from the data stored in temporary register  132 , and a combining circuit  136  which combines the result data stored in result register  134  with the data stored in strobe register  130 . Combining circuit  136  combines the bits stored in the respective shift circuits  130   a  of strobe register  130  with the corresponding data stored in register circuits  134   a  of result register  134  for application to a drive circuit  138 . Drive circuit  138  includes tri-state drivers  138   a  provided corresponding to register circuits  134   a  of result register  134 , respectively. The outputs of tri-state drivers  138   a  are wired-connected, and only one driver  138   a  produces modified strobe trigger signal CPTR. The other drivers  138   a  are kept in the output high-impedance state. In the structure shown in FIG. 59, strobe register  130  and temporary register  132  correspond to strobe timing calculating circuit  116 . Result register  134 , combining circuit  136  and drive circuit  138  correspond to shifter and clock signal generating circuit  114 . 
     FIG. 60 illustrates timing of the operations of AND circuits  125  and  125  shown in FIG.  59 . The operations of these AND circuits will now be described with reference to FIG.  60 . 
     For setting the strobe timing in accordance with the effective data window, the test mode is started. In this test mode, a timing set mode instructing signal TIMSM is activated and attains H-level. When strobe signal STROBE is supplied after timing set mode instructing signal TIMSM is activated, a monitor mask instructing signal MMSK becomes active in response to the second rising of strobe signal STROBE. Thereby, AND circuit  125  produces the final effective window detection signal MDC in accordance with effective window detection signal EDW_R. Final effective window detection signal MDC is applied to strobe register  130 , and is successively shifted in accordance with a shift clock signal SFCK applied from AND circuit  127 . This monitor mask instructing signal MMSK falls to L-level in response to the next rising of effective window detection signal EDW_R. Accordingly, only one effective window detection signal EDW_R is extracted by monitor mask instructing signal MMSK. Only while final effective window detection signal MDC is produced in accordance with effective window detection signal EDW_R, shift clock signal SFCK is generated, and strobe register  130  performs the shifting operation. Only while final effective window detection signal MDC is at H-level, strobe register  130  performs the shift operation, and therefore the time width of the effective window region is detected based on the number of clock cycles of shift clock signal SFCK. 
     When the effective window detecting operation is completed, transfer instructing signal XFR is activated in accordance with effective window detection signal EDW_R, and the effective window region instruction bits stored in strobe register  130  are transferred to temporary register  132 . The strobe timing calculation is performed using the data bits stored in temporary register  132 . 
     In FIG. 59, the effective window is detected for the data transferred in synchronization with the rising edge of clock signal CLK. However, the detection of the effective window and the detection of the strobe timing are likewise performed for the data bits transferred in response to the falling of the clock signal. For the data bits transferred at the falling edge, effective window detection signal EDW_F is used instead of effective window detection signal EDW_R. Also, complementary strobe signal STROBE# is used instead of strobe signal STROBE. 
     In the test mode, normal read instructing signal NREAD is set to the active state of H-level and monitor mask instructing signal MMSK is set to L-level and a signal Sin is produced from strobe signal STROBE, and the strobe timing is determined by one strobe signal STROBE. In this state, data is read. If accurate data reading is performed, further detection of the effective window is no longer required. 
     If data cannot be effectively taken in according to strobe signal STROBE, or if it is required to determine more accurately the central position of the data window, the effective data window detecting operation is performed by setting timing set mode instructing signal TIMSM to H-level, and the strobe timing is modified to detect the optimum strobe timing. Accordingly, if the data cannot be accurately taken in even when the strobe timing is set in accordance with strobe signal STROBE, it can be determined that the cause of failure is not present in the memory, but is present in on-board wiring lines, because the data and the strobe signal are outputted in synchronization with each other. 
     FIG. 61 shows an example of a structure of the monitor mask instructing signal generating portion shown in FIG.  59 . In FIG. 61, the monitor mask instructing signal generating portion includes: a counter  140  which is activated to count strobe signal STROBE when timing set mode instructing signal TIMSM is active; a set/reset flip-flop  141  which is set in response to a count-up instructing signal UP generated from counter  140 ; and a transfer circuit  142  which transfers monitor mask instructing signal MMSK generated from set/reset flip-flop  141  in accordance with effective window detection signal EDW_R. Transfer circuit  142  applies the signal to a reset input of set/reset flip-flop  141 . Transfer circuit  142  delays mask monitor instructing signal MMSK by a period of one effective window detection signal EDW_R. 
     Counter  140  counts the rising of strobe signal STROBE. When the count goes to 2, counter  140  activates count-up signal UP to set set/reset flip-flop  141 . Responsively, monitor instructing signal MMSK is activated. When monitor mask instructing signal MMSK is activated, one effective window detection signal EDW_R is extracted, and final effective window detection signal MDC is produced. When third effective window detection signal EDW_R rises, the output signal of transfer circuit  142  attains H-level, and set/reset flip-flop  141  is reset. Thereby, only the second effective window detection signal EDW_R can be extracted. 
     In the above operation, second effective window detection signal EDW_R is extracted. This is for the following reason. In the data read operation, read data is transferred from the memory device in synchronization with strobe signal STROBE. Before this transfer of read data, the data bus is in the high-impedance state. For the first data, therefore, strobe signal STROBE becomes active, and the data bus line changes from the high impedance state to H- or L-level potential. For the subsequent data, the data bus line is driven to H- or L-level depending on the value of data bit from the definite state already driven to H- or L-level in accordance with the strobe signal. Thus, set-up time Ts for the initial data bit is longer than those for subsequent data bits. Since effective window detection signal EDW_R for the initial data bit has a long set-up time for the above reason, this signal is discarded, and second effective window detection signal EDW_R is extracted and is utilized as final effective window detection signal MDC. 
     Monitor mask instructing signal MMSK may be a signal which attains H-level at the first falling of strobe signal STROBE, and attains L-level at the next falling of strobe signal STROBE, as indicated by broken line shown in FIG.  60 . 
     Counter  140  may be a frequency dividing circuit which halves the frequency of strobe signal STROBE. 
     FIG. 62 schematically shows a structure of a transfer instruction signal generating portion. In FIG. 62, the transfer instructing signal generating portion includes: a counter  145  which is activated in response to activation of a timing set mode instructing signal TIMSM, and counts the rising of effective window detection signal EDW_R when activated; and a one-shot pulse generating circuit  146  which generates a transfer instructing signal XFR in a one-shot pulse signal form in accordance with the count-up instructing signal generated from counter  145 . Counter  145  generates (activates) the count-up instructing signal when it counts the rising of effective window detection signal EDW_R four times when timing set mode instructing signal TIMSM is active. Therefore, information indicating the effective window can be transferred to temporary register  132  with a sufficient margin from strobe register  130 , under the condition that the shift-in operation of final effective window detection signal MDC is completed in strobe register  130  and the contents held in strobe register  130  are fixed. 
     FIG. 63 schematically shows a structure of shift circuit  130   a  of strobe register  130  shown in FIG.  59 . In FIG. 63, shift circuit  130   a  includes a latch circuit  130   aa  which takes in and latches an input signal IN in response to rising of shift clock signal SFCK, and a latch circuit  130   ab  which takes in and latches an output signal of latch circuit  130   aa  in response to the falling of shift clock signal SFCK. The output signals of these latch circuits  130   aa  and  130   ab  are both applied to temporary register  132  and combining circuit  136 . 
     FIG. 64 is a signal waveform diagram representing an operation of strobe register  130  including shift circuit  130   a  shown in FIG.  63 . Referring to FIG. 64, the operation of the strobe register shown in FIGS. 59 and  63  will now be described. 
     When effective window detection signal MDC attains H-level, shift clock signal SFCK generated from AND circuit  127  shown in FIG. 59 changes in synchronization with the processor internal operation clock signal PCLK. In synchronization with the rising and falling of shift clock signal SFCK, shift register circuits  130   aa  and  130   ab  of strobe register  130  successively transfer final effective window detection signal MDC. When final effective window detection signal MDC is at H-level, e.g., for a period of 4.5 clock cycles (4.5 Tcy) of the internal operation clock signal PCLK as shown in FIG. 64, shift clock signal SFCK also changes during a period of 4.5 cycles of operation clock signal PCLK. In strobe register  130 , therefore, latch circuits  130   aa  and  130   ab  perform the shifting operation in synchronization with the falling and rising of shift clock signal SFCK, so that the output signals of nine latch circuits in total attain H-level. When effective window detection signal MDC falls to L-level, shift clock signal SFCK attains L-level, so that strobe register  130  does not perform the shifting operation. Therefore, the time width of effective window detection signal MDC can be detected by counting the number of “1” in the output signals of shift register circuits  130   a  included in strobe register  130 . When normal read instructing signal NREAD is at H-level, the effective period of strobe signal STROBE is detected instead of that of effective window detection signal MDC. 
     FIG. 65 schematically shows a structure of temporary register  132  shown in FIG.  59 . This temporary register  132  includes an A-register  132   a  receiving data bits stored in the strobe register and a B-register  132   b  storing information indicating the strobe timing for application to the result register. A strobe timing detecting circuit  150  is coupled to A- and B-registers  132   a  and  132   b.  Strobe timing detecting circuit  150  obtains the central value of the effective window from the data bit values stored in A-register  132   a,  and stores the information indicating the central value in B-register  132   b.    
     FIG. 66 schematically shows a structure of strobe timing detecting circuit  150  shown in FIG.  65 . Referring to FIG. 66, strobe timing detecting circuit  150  includes: a decimal-to-binary converter  150   a  for performing decimal-to-binary conversion of the data bits stored in A-register  132   a;  a divider  150   b  performing division of binary effective data window information stored in A-register  132   a  to store the result of division in B-register  132   b;  and a decoder  150   c  for decoding the result of division stored in B-register  132   b,  and setting the bit indicating the central position of the effective data window to “1” for storage in B-register  132   b.    
     Divider  150   b  performs the division by a factor of 2 for detecting the central position of the effective data window. Strobe timing detecting circuit  150  may be configured to perform the arithmetic operation utilizing an arithmetic function (e.g., BIOS) of a logic (processor) integrated on the same chip with the memory controller. 
     FIG. 67 shows an operation sequence of strobe timing detecting circuit  150  shown in FIG.  66 . In FIG. 67, each of strobe register  130  and result register  134  includes a register circuit of 16 bits. The bit numbers of registers  130  and  134  may be appropriately determined depending on the frequency of the processor operation clock signal PCLK and the effective data window width. 
     First, strobe register  130  stores “1” according to the effective data window region. FIG. 67 shows the state where nine “1”s are stored. This state corresponds to the operation represent in FIG. 64, and therefore, the effective data window region corresponds to 4.5 cycles of operation clock signal PCLK. 
     The data bits stored in strobe register  130  are stored in A-register  132   a  in accordance with transfer instructing signal XFR. The data bits stored in A-register  132   a  are encoded by decimal-to-binary converter  150   a  for binary representation. The result (9) of this binary representation is stored in A-register  132   a  again. Subsequently, divider  150   b  divides the binary number (9) stored in A-register  132   a.  If the effective data window region is odd, the division result contains a fraction (0.5 smaller than 1). In this case, the fraction may be rounded up to 1 or down to 0. Such a structure may also be employed that divider  150   b  merely performs the shifting operation for rounding down the fraction. The division result (“5” in FIG. 67) of divider  150   b  is stored in B-register  132   b.  Thereby, B-register  132   b  stores binary data (5)2 indicating the vicinity of the central value of effective data window region. Data (5)2 stored in B-register  132   b  is decoded by decoder  150   c  to set the corresponding bit position to “1” and set all the remaining positions to “0”. This decode result (5)10 is transferred to and stored in result register  134 . In FIG. 67, therefore, the fifth bit in result register  134  takes the value of “1”, and all the other bits take the value of “0”. The position of “1” stored in result register  134  indicates the strobe timing. The data stored in result register  134  indicates the central position of the effective data window region. By using this, the strobe trigger signal is produced. 
     FIG. 68 shows structures of one stage of each of combining circuit  136  and drive circuit  138  shown in FIG.  59 . In FIG. 68, combining circuit  136  includes a combining stage  136   a  which is provided corresponding to register circuits  134   a  and  130   a  of result register  134  and strobe register  130 . Combining stage  136   a  includes; an inverter  151  which inverts a storage bit DMMi received from the corresponding register circuit in result register  134 ; an NAND circuit  152  which receives an output bit SSBi of the corresponding register circuit in strobe register  130  and corresponding bit DDMi of result register  134 ; and an NOR circuit  153  which receives the output bit of inverter circuit  151  and corresponding data bit SSBi received from strobe register  130 . In this combining stage  136   a,  when both the bits DMMi and SSBi are at H-level, the output signal of NAND circuit  152  attains L-level, and the output signal of NOR circuit  153  attains L-level. When the bit DMMi is at H-level and the bit SSBi is at L-level, the output signal of NOR circuit  153  attains H-level. 
     Drive circuit  138  includes a tri-state buffer (driver)  138   a  provided corresponding to each combining stage  136   a.  Tri-state buffer  138   a  includes a P-channel MOS transistor  155  receiving, on its gate, the output signal of NAND circuit  152 , and an N-channel MOS transistor  156  receiving, on its gate, the output signal of NOR circuit  153 . MOS transistors  155  and  156  are connected in series between a power supply node and a ground node. The output of tri-state buffer  138   a  is wired-OR connected, and generates modified strobe trigger signal CPTR. The operation of the circuits shown in FIG. 68 will now be described with reference to a signal waveform diagram of FIG.  69 . 
     In the normal operation mode, normal read instructing signal NREAD is at H-level, and input signal Sin to be supplied to strobe register  130  is produced in accordance with the signal generated from AND circuit  126  shown in FIG.  59 . When strobe signal STROBE rises to H-level, strobe signal STROBE is successively shifted into strobe register  130  shown in FIG.  59 . Effective window detection signal EDW_R is already produced at the rising of strobe signal STROBE, and the width thereof does not exceed the width of the strobe signal (see FIGS. 57 or  55 ). In the normal operation mode, it is not particularly required to produce effective data window detection signal EDW_R. In accordance with the position indicating the central region of the effective data window, which was detected in the test mode, “1” is already set in the corresponding register circuit in result register  134 . The structure for setting “1” in the result register can be similar to that of the first embodiment. 
     It is now assumed that the bit DMMi is set to H-level. Strobe register  130  shifts strobe signal STROBE in accordance with clock signal PCLK. When strobe signal STROBE is shifted into register circuit  130   a  corresponding to bit DMMi as a result of the shifting operation of strobe register  130 , corresponding bit SSBi attains H-level. When bit SSBi is at L-level and bit DDMi is at H-level, tri-state buffer  138   a  is in the output high-impedance state. Likewise, when bit DDMi is at L-level, corresponding tri-state buffer  138   a  is in the output high-impedance state. 
     When bit SSBi rises to H-level and bit DMMi is at H-level, the output signal of NAND circuit  152  attains L-level. Therefore, P-channel MOS transistor  155  of tri-state buffer  138   a  is turned on, and modified strobe trigger signal CPTR rises to H-level. The timing of rising of modified strobe trigger signal CPTR corresponds to the central position region of the effective data window detection signal, and therefore the strobe signal can be produced at optimum timing. 
     For the data transferred in synchronization with the falling of clock signal CLK, the strobe timing is detected by similar operations using strobe signal STROBE# and effective window detection signal EDW_F. This is similar to the first embodiment. By the structure similar to that of the first embodiment, the strobe timing is set in the result register in correspondence to each bank of each memory device. 
     This strobe timing is detected for each bank of the memory device, and is stored as shown in FIGS. 19 and 20. For accessing a memory device, a signal indicating the corresponding strobe timing is read out from the timing storage region shown in FIG. 19 or  20 , and is stored in the result register. Thereby, the data taking-in can be performed for each bank of each memory device at the optimum timing. Further, as shown in FIG. 26, the strobe timing may be adjusted on a four-bit basis, or on a basis of one or several byte(s), or may be adjusted collectively to all bits (i.e., 64 bits). 
     First Modification 
     In the structure shown in FIG. 59, generation of shift clock signal SFCK stops when strobe signal STROBE or effective window detection signal MDC attains L-level. Thereby, strobe register  130  stops the shifting operation. In the normal operation mode, when strobe register  130  does not perform the shifting operation, the bit SSBi may always be at H-level, and therefore modified strobe trigger signal CPTR may maintain H-level after strobe signal STROBE is shifted into strobe register  130 . In view of this, a structure shown in FIG. 70 is utilized so that the activation timing of modified strobe trigger signal CPTR may be equal to that of strobe signal STROBE. 
     FIG. 70 shows a structure of a modification of the shift clock generating portion. In FIG. 70, the shift clock generating portion includes; an AND circuit  160   a  receiving internal operation clock signal PCLK and shift-in signal Sin shown in FIG. 59; an AND circuit  160   d  receiving a timing set mode instructing signal TIMM and the output signal of AND circuit  160   a;  an AND circuit  160   b  receiving internal operation clock signal PCLK and normal read mode instructing signal NREAD; an inverter  160   c  inverting timing set mode instructing signal TIMM; an AND circuit  160   e  receiving the output signal of inverter  160   c  and the output signal of AND circuit  160   b;  and an OR circuit  160   f  receiving the output signals of AND circuits  160   d  and  160   e,  and produces shift clock signal SFCK. 
     When timing set mode instructing signal TIMM is at H-level, AND circuit  160   d  is enabled to produce shift clock signal SFCK through OR circuit  160   f  in accordance with the output signal of AND circuit  160   a.  In this case, therefore, generation of shift clock signal SFCK stops after signal Sin is shifted into the strobe register while strobe signal STROBE or effective window detection signal EDW_R (and EDW_F) is at H-level. In this state, the strobe timing is calculated. 
     In the normal operation mode, when data transferred from a memory device is to be taken in, timing mode set signal TIMM is at L-level, and normal read mode instructing signal NREAD is at H-level. Therefore, AND circuit  160   e  produces shift clock signal SFCK through OR circuit  160   f  in accordance with processor operation clock signal PCLK. Upon taking in data in the normal operation mode, the strobe register always performs the shifting operation in the data reading, and bit SSBi is at H-level for a period substantially equal to the period during which strobe signal STROBE is at H-level. Thereby, modified strobe trigger signal CPTR has a pulse width nearly equal to that of strobe signal STROBE. 
     For obtaining the central position of strobe signal STROBE, timing set mode instructing signal TIMM and normal read instructing signal NREAD are both set to H-level, and the strobe timing is detected similarly to the case of using the effective window detection signal. 
     Second Modification 
     FIG. 71 shows a structure of a second modification of the shift clock generating portion. Similarly to the structure shown in FIG. 59, the shift clock generating portion shown in FIG. 71 includes AND circuit  127  which receives signal Sin and processor operation clock signal PCLK, and produces shift clock signal SFCK. When normal read instructing signal NREAD is active, shift clock signal SFCK is generated (activated) for a period of strobe signal STROBE. 
     This shift clock generating portion further includes: a one-shot pulse generating portion  162   a  which generates a one shot pulse signal in response to falling of strobe signal STROBE; an NAND circuit  160   b  which receives the output signal of one-shot pulse generating circuit  162   a  and normal read instructing signal NREAD; and an NOR circuit  160   c  which receives timing set mode instructing signal TIMM and the output signal of NAND circuit  160   c.  NOR circuit  160   c  produces reset signal RST, which is in turn applied to strobe register  130 . 
     When timing mode set signal TIMM is at H-level, reset signal RST generated from NOR circuit  160   c  is fixed to L-level. Therefore, strobe register  130  is reset by a controller or a processing unit (both not shown) after completion of the timing detecting operation. When timing mode set signal TIMM is at L-level, NOR circuit  160   c  operates as an inverter. When strobe signal STROBE falls to L-level while normal read instructing signal NREAD is at H-level, one-shot pulse generating circuit  160   a  generates a one shot pulse signal, and the output signal of NAND circuit  160   b  attains L-level. Thereby, reset signal RST generated from NOR circuit  160   c  attains H-level, so that all the data stored in strobe register  130  are reset to L-level. Therefore, reset signal RST attains H-level after generation of shift clock signal SFCK stops in accordance with strobe signal STROBE. The period for which modified strobe trigger signal CPTR is at H-level coincides with the period starting from strobe timing for the H-level period of strobe signal STROBE, and is equal, at maximum, to ½ times the period for which strobe signal STROBE is at H-level. When strobe signal STROBE falls to L-level, the effective data window detection signal is already at L-level, and the strobe trigger signal CPTR is kept at H-level for a period of the maximum effective data window region. Therefore, the foregoing structure can likewise provide the set-up time and the hold time for the data to be taken in, which are equal to each other. 
     The structure shown in FIGS. 70 and 71 is provided also for the data transferred in synchronization with the falling of clock signal CLK. 
     Instead of strobe signal STROBE, clock signal CLK may be used for determining the strobe timing. In the case of a burst EDODRAM or the like not outputting the strobe signal, data strobe timing can be determined with reference to the clock signal. 
     For storing the data indicating the strobe timing in the result data in correspondence to a selected memory device and bank in the normal mode, a structure similar to that of the first embodiment can be utilized. 
     According to the fourth embodiment of the invention, as described above, the strobe signal is use for detecting the definite periods of the strobe signal and the transferred data, and the transferred data can be taken at an accurate timing determined on the memory controller side without provision of a DLL in a memory device. Accordingly, it is not necessary to provide, in the memory device, a DLL which usually consumes a current ranging from 10 mA to 100 mA. Therefore, the current consumption of the system can be remarkably reduced. In particular, a current consumption can be significantly reduced in a system such as a server using thousands of memory devices. 
     Fifth Embodiment 
     FIG. 72 schematically shows a structure of a processing system according to a fifth embodiment of the invention. In FIG. 72, the processing system includes a plurality of processors (CPUs)  210   a - 210   n  coupled in parallel to a common data bus  202 , and a memory control device  200  for transferring data between processors  210   a - 210   n  and a main storage  215 . Clock generator  3  commonly applies clock signal CLK to processors  210   a - 210   n  and memory control device  200 . 
     Processors  210   a - 210   n  and memory control device  200  operate in synchronization with clock signal CLK, and therefore data is transferred onto common data bus  202  in synchronization with clock signal CLK. However, the impedance of data bus  202  changes in accordance with the number of processors connected thereto, and the distances from processors  210   a - 210   n  to memory controller  200  depend on the positions of the processors. Accordingly, upon taking-in of data sent from processors  210   a - 210   n  by memory control device  200 , a skew occurs with respect to clock signal CLK, as already described in the foregoing embodiments. Particularly, in the case of system expansion, since the number of processors and the length of data bus increase, the impedance of the data bus (data transmission path) changes. In the case of system expansion, therefore, a large deviation occurs in data strobe timing, resulting in a problem that the system expandability is low. 
     Accordingly, memory control device  200  is provided with a strobe timing adjusting circuit  200   a,  which in turn adjusts the data strobe timing in input circuit  200   b  in accordance with data Dc sent from common data line  202  and the clock signal. Strobe timing adjusting circuit  200   a  corresponds to the combination of timing control circuit  5  and strobe clock generating circuit  6  shown in FIG.  1 . At the time of test or initialization, a data window is detected in accordance with the input data and the clock signal CLK, and the strobe signal is produced from this data window. 
     Accordingly, a sufficient set-up margin and a sufficient hold margin can be ensured regardless of the position of the processor on a board, as can be done in the memory system already described with reference to FIG. 1, and the data from the processors can be accurately taken and written into main storage  215 . 
     The data taken in by input circuit  210   b  is applied to internal circuit  210   c,  in which a processing such as conversion of the bit width of data is performed, and then is transferred to main storage  225 . 
     Strobe timing adjusting circuit  200   a  has a structure corresponding to the timing adjusting circuits of the first to third embodiments shown in FIGS. 1 to  50 , and therefore, has a function of evaluating a quality of the received data. Instead of the address specifying a memory device, a processor address specifying a process is used. 
     First Modification 
     FIG. 73 schematically shows a first modification of the fifth embodiment of the invention, and particularly shows a structure of a processing system. In FIG. 73, the processing system includes a plurality of processors  220   a - 220   n  coupled in parallel to common data bus  202 , and a memory control device  230  for controlling data transfer between processors  220   a - 220   n  and a main storage  225 . Processors  220   a - 220   n  and control device  230  commonly receive clock signal CLK from clock generator  3 , and operate in synchronization with clock signal CLK. 
     In the structure shown in FIG. 73, each of processors  220   a - 220   n  is provided with a strobe timing adjusting circuit  221  for adjusting data strobe timing. Strobe timing adjusting circuit  221  has a structure similar to that of timing adjusting circuit  200   a  shown in FIG. 72, and strobes the data transferred from memory control device  230  via data bus  202  at an optimum timing. Accordingly, even in the case where distances from memory control device  230  to processors  220   a - 220   n  are varied and therefore the skew of data with respect to clock signal CLK is also varied, the data can be taken in accurately. In addition, in detecting the quality of data, processor  220  is not particularly required to store the addresses. When a data quality failure is detected, it is merely required to execute the system reset. 
     In the multiprocessor system, the processor is provided with a function of adjusting the strobe timing. Thereby, a processor added for system expansion can take in the data at the optimum timing so that the system expansion can be easily implemented. 
     Second Modification 
     FIG. 74 schematically shows another modification of the fifth embodiment of the invention, and particularly a structure of a processing system. In FIG. 74, the processing system includes a plurality of processors  240   a - 240   n  coupled in parallel to common bus  202 , and memory control device  200  for controlling the data transfer between processors  240   a - 240   n  and main storage  215 . Processors  240   a - 240   n  commonly receive clock signal CLK from clock generator  3 . 
     In this structure of the second modification, processors  240   a - 240   n  apply strobe signal STROBE (and complementary strobe signal STROBE#) to memory control device  200  via a signal line  235 . In memory control device  200 , therefore, strobe timing adjusting circuit  200   d  produces data window indicating the effective data period in accordance with strobe signal STROBE and clock signal CLK. The structure of strobe timing adjusting circuit  200   d  is the same as that of timing adjusting circuit  110  shown in FIG.  52 . 
     In the structure shown in FIG. 74, even if the distances from the processors to the memory control device are varied, strobe timing adjusting circuit  220   d  can strobe the data at the timings according to the distances to respective processors  240   a - 240   n,  and therefore the data transfer can be performed accurately. 
     Structures other than the above are substantially the same as those shown in FIG. 72, and the corresponding portions bear the same reference numbers. 
     Third Modification 
     FIG. 75 shows a third modification of a fifth embodiment of the invention, and particularly a schematic structure of a processing system according to the third modification. The structure shown in FIG. 75 differs from the structure shown in FIG. 73 in the following point. Each of processors  250   a - 250   n  receives strobe signals STROBE and STROBE# from memory control device  245  via a signal line  235 . In each of processors  250   a - 250   n,  a strobe timing adjusting circuit  221  adjusts the strobe timing for the data sent from memory control device  245  in accordance with clock signal CLK sent from clock generator  3  and strobe signals STROBE and STROBE#. 
     Accordingly, in the structure shown in FIG. 75, data can be taken in at accurate timing even if the distances from the processors to the memory controller are varied. 
     The structures shown in FIGS. 74 and 75 may be combined to implement accurate, bi-directional data transfer between the memory control device and the processors. 
     Fourth Modification 
     FIG. 76 schematically shows a structure of a fourth modification of the fifth embodiment according to the invention. In the structure shown in FIG. 76, a common memory  260  is connected to common data bus  202 . In the data transfer operation, common memory  260  transfers strobe signals STROBE and STROBE# to processors  250   a - 250   n  via signal line  252  together with data. Common memory  260  operates in synchronization with the clock signal sent from clock generator  3 . 
     The structure shown in FIG. 76 is equivalent to a structure obtained by eliminating the memory control device from the structure shown in FIG. 75, and access conflict on common memory  260  can be avoided by a bus arbiter. 
     Accordingly, in the structure shown in FIG. 76, strobe timing adjusting circuit  221  in each of processors  250   a - 250   n  can accurately compensate for deviation in strobe timing even when the distances between the common memory and the processors are varied, and accurate data taking in operation can be implemented. Thereby, each processor can take in the data without an influence of change in impedance of the data bus even after system expansion, and the system capable of fast operation can be implemented. 
     In the structure shown in FIG. 76, the strobe timing adjusting circuit may be configured to detect the change point of data for detecting the window indicating the effective data period, as shown in FIG.  72 . 
     Fifth Modification 
     FIG. 77 shows a fifth modification of the fifth embodiment of the invention, and particularly a schematic structure of the processing system. In FIG. 77, the processing system includes a plurality of processors  300   a - 300   m  coupled in parallel to a processor bus  305 , a memory control device  310  coupled commonly to processors  300   a - 300   m  via processor bus  305 , and memory devices  320   a - 320   n  coupled in parallel to a memory bus  315  for transferring data to and from memory control device  310 . 
     Each of processors  300   a - 300   m  includes an interface circuit (I/F) circuit  301  with a strobe adjusting function for transferring strobe signal STROBE via a signal line  307  and transferring data via processor bus  305 . Likewise, memory control device  310  includes an interface circuit  311  with a strobe adjusting function for transferring strobe signal STROBE via signal line  307  and transferring data via processor bus  305 . Memory control device  310  further includes an interface circuit  312  with a strobe adjusting function for receiving strobe signal STROBE outputted from any of memory devices  320   a - 320   n,  and for transferring the data to or from a selected memory device via memory data bus  315 . 
     For processors  300   a - 300   m,  strobe signal STROBE is transferred bidirectionally. When processors  300   a - 300   m  receive data from memory control device  310 , strobe signal STROBE is transferred from memory control device  310  to processors  300   a - 300   m  via signal line  307 . When processors  300   a - 300   m  each transfer data to memory control device  310 , an associated interface circuit  301  transfers strobe signal STROBE to interface circuit  311  of memory control device  310  via signal line  307 . Therefore, processors  300   a - 300   m  can accurately perform both the loading and storing of data. 
     In memory control device  310 , internal circuit  313  determines the data transfer direction for transferring the data in the determined direction. Interface circuit  312  coupled to memory bus  315  receives strobe signal STROBE via signal line  317  from output circuit  322  of a selected (i.e., access-requested) memory device, and produces the effective data window for taking in (strobing) the data. 
     According to the structure shown in FIG. 77, data transfer can be accurately performed between the memory devices and the memory control device and between the memory control device and the processors. Thereby, the fast processing system having high expandability can be implemented. 
     The data transfer between the processors and the memory control device may be performed in a burst mode in synchronization with both the falling and rising edges of clock signal CLK. In this case, each interface circuit likewise adjust the strobe timing, and the data taking-in operation having a sufficient timing margin can be accurately performed even with a fast clock signal. 
     In the structure shown in FIG. 77, such a structure may be employed in which the edge of change of transferred data (received data) is detected for detection/production of the effective data window, similarly to the structure shown in FIG.  1 . 
     According to the fifth embodiment, as described above, the strobe timing is adjusted for the data transferred between the memory control device and the processors or between the processors and the memory. Therefore, accurate strobe can be ensured for the processor data so that the system that is capable of fast operation and has high expandability can be implemented. 
     Other Examples of Application 
     Description has been given on the memory devices that perform the data transfer at the double data rate (DDR) mode. However, the memory device employed in the present invention may be a device, which in turn transfers the data in synchronization with one of the rising and falling edges of the clock signal, or in an EDO mode. Further, the memory device may be a flash memory. 
     According to the present invention, as described above, the data window is detected from the data on the data line for optimizing the data strobe timing, and therefore the data transfer can be performed accurately regardless of the system structure. In the case of a memory system, a DLL for producing a data outputting clock is not required, and therefore the chip area and the power consumption can be reduced. Further, the data window width is monitored on a data receiving side, so that transfer of erroneous data can be prevented, and the reliability of the system can be ensured. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.