Patent Publication Number: US-7721135-B2

Title: Method of timing calibration using slower data rate pattern

Description:
This application is a continuation of application Ser. No. 11/018,384, filed Dec. 22, 2004 now U.S. Pat. No. 7,376,857, which is a continuation of application Ser. No. 10/767,273, filed Jan. 30, 2004, now U.S. Pat. No. 6,845,460, which is a continuation of application Ser. No. 09/708,440, filed Nov. 9, 2000, now U.S. Pat. No. 6,735,709, issued May 11, 2004, the subject matter of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to calibrating timing of command and data signals on data paths of logic devices, e.g. memory devices, and in particular to using a first data rate slower than the devices&#39; normal operating rate to transfer a calibration bit pattern between logic devices during calibration. 
     BACKGROUND OF THE INVENTION 
     Memory devices are constantly evolving in the directions of faster speed and higher memory density. To this end, dynamic random access memory (DRAM) devices have evolved from simple DRAM devices to EDO to SDRAM to DDR SDRAM. 
     One characteristic of modern memory technology is that it may use both the positive- and negative-going edges of a clock cycle to READ and WRITE data to the memory cells and to receive command data from a memory controller. DDR SDRAM represents one example of a modern memory technology that utilizes both positive- and negative-going edges of a clock cycle. 
     Because of the required high speed operation of contemporary memory devices, system timing and output signal drive level calibration at start-up or reset is a very important aspect of the operation of such devices to compensate for wide variations in individual device parameters or within the system design itself. 
     One of the several calibration procedures which is performed in current memory devices is a timing synchronization of a clock signal with data provided on an incoming command/address path and on a data path DQ so that incoming data is correctly sampled and outgoing data is correctly timed. Currently, a memory controller achieves this timing calibration at system initialization (start-up or reset) by sending continuous transitions on the clock path and transmitting a 15 bit repeating pseudo random SYNC sequence “111101011001000” on the READ/WRITE data path DQ and the command/address path. The memory device determines an optimal internal delay for the dock path relative to arriving command/address and data signals to optimally sample the known bit pattern. This optimal delay is achieved by adjusting the timing of the received data bits to achieve a desired bit alignment relative to the clock. This is accomplished by adjusting a relative timing of clock and data signals, for example setting delay values in the clock or data signal paths, until the received data is properly sampled by the clock and recognized internally. Once synchronization has been achieved, that is, the proper timing on the receiving data or clock paths have been set, the memory controller stops sending the SYNC pattern and the memory device, after all calibrations are completed, can be used for normal memory READ and WRITE access. 
     To perform the above-described calibration operations, each memory device in a current memory system typically contains means for generating the calibration bit pattern internally, independent of the memory controller. During calibration, the data incoming on a data path under calibration is compared to the internally-generated calibration pattern at each memory device. Internal generation of the calibration bit pattern requires that each memory device contain the additional circuitry needed for pattern generation. For example, each memory device may include the four-bit shift register circuit illustrated in  FIG. 6 . Because of circuit die size constraints, it would be preferable to simplify the circuitry by avoiding the requirement of generating the calibration bit pattern at every memory device. 
     While the timing calibration described above, which is conducted at start-up and reset, has been found to perform adequately in most circumstances, there is a problem in that as the data rate of memory devices is increased, the timing margin for data capture is decreased. The timing margin for data capture is the amount of time that valid data is available on the bus or at a device for use during system operations after practical system effects are introduced. For example, for a data transfer rate of 1 GHz (1×10 9  Hz), the maximum possible data valid time is only 1 ns (1×10 −9  seconds). When practical system effects are introduced, such as accounting for device setup and hold, the timing margin for data capture is even less, for example, a timing margin of less than 100 ps (100×10 −12  seconds) is typical. 
     This timing margin can be increased by precisely calibrating and aligning the received data with the data capture clock. Improved calibration techniques can reduce the total data arrival time uncertainty that must be accounted for from a theoretical maximum to the actual timing uncertainty observed at the device under actual operating conditions. This reduction in uncertainty results in a corresponding decrease in the timing budget allocated to uncertainty and thus an increase in the timing margin for data capture. 
     One area in which calibration and alignment with the data capture clock may be improved is in the bit composition of the calibration test pattern. Current systems customarily use a single calibration bit pattern for all calibration operations. Because the same calibration bit pattern may be used for multiple circuit configurations, the bit pattern is designed to test a variety of circuit conditions, although the exact conditions to be encountered are largely unknown at the time of circuit design. The bit pattern may be selected to test a variety of circuit characteristics that affect timing, including static layout length differences, input path delay differences, intersymbol interference, and simultaneously switching outputs. 
     Moreover, the calibration bit pattern currently in use, for example the 15-bit pseudo random pattern, may not perform optimally for modern high performance memory systems. Because current memory devices capture incoming data on both positive and negative going transitions of the clock signal, even when timing calibration is achieved it may not be clear if alignment was achieved on a positive going or negative going clock edge. That is, the 15-bit synchronization pattern lacks any timing signature. If, for example, synchronization was achieved on the negative going edge of a clock signal when the circuitry is designed on the assumption that synchronization is achieved on a positive going edge, when data is later sampled during memory access the data sampling may be off by one bit. Thus, calibration may be achieved in the wrong phase of the clock signal, leading to incorrect sampling of the data during memory access operations, or requiring additional complicated circuitry to ensure that incoming data is synchronized to the proper phase of the clock. 
     Therefore, there is a need and desire for an improved calibration technique implementable in logic using simplified circuitry that is capable of compensating for a variety of circuit characteristics that may affect timing. 
     SUMMARY OF THE INVENTION 
     An improved technique and associated apparatus for timing calibration of a logic device, e.g. a memory device, is provided. A calibration bit pattern is transferred to a logic device first at a data rate slower than normal operating speed to ensure correct capture of the bit pattern at the device to be calibrated. Once the pattern is correctly captured and stored, a calibration signal containing the calibration bit pattern is transmitted to the logic device at the normal operating data rate and timing calibration at the logic device can occur. 
     The improved technique and apparatus permits the use of any pattern of bits as a calibration test pattern, programmable by the user or using easily-interchangeable hardware. The test bit pattern can thus be adapted to provide a better stimulus for characterizing the timing performance of a particular system or the expected data patterns that the data paths are expected to encounter. 
     Because the logic for generating the calibration test pattern is no longer required to be included in each system device (i.e., each logic device no longer requires a shift register for generating a local pseudo-random pattern), the invention simplifies the logic and thus reduces die size overhead for many logic devices. The resulting die size savings desirably reduces the cost and complexity of system devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other advantages and features of the invention will be more clearly understood from the following detailed description of the invention which is provided in connection with the accompanying drawings in which: 
         FIG. 1  illustrates a memory circuit topology with which the invention is used; 
         FIG. 2  illustrates a portion of the memory circuit shown in  FIG. 1 ; 
         FIG. 3  illustrates a simplified timing diagram illustrating a portion of the timing signals used in the operation of the  FIG. 2  circuit; 
         FIG. 4  illustrates a graphic example of the synchronization technique used to synchronize the memory circuit of  FIG. 1 ; 
         FIG. 5  illustrates a pattern of acceptable delay values for synchronization used in the invention; 
         FIG. 6  is a representative circuit for generating a calibration pattern used in one embodiment of the invention; 
         FIG. 7  illustrates exemplary clock divider circuitry in accordance with an embodiment of the invention; 
         FIG. 8  illustrates a processor system using a DRAM memory which employs calibration structures and process methodologies in accordance with the invention; 
         FIG. 9(A)  illustrates a digital circuit including two logic devices employing calibration structures and process methodologies in accordance with an embodiment of the invention; 
         FIG. 9(B)  illustrates a portion of the digital circuit shown in  FIG. 9(A) ; 
         FIG. 10(A)  illustrates a graphic example of using a prefetch memory scheme with an embodiment of the invention; 
         FIG. 10(B)  illustrates a portion of a memory circuit using the prefetch memory scheme illustrated in  FIG. 10(A) ; 
         FIG. 11  illustrates a series of simplified timing diagrams showing exemplary timing settings for data capture timing calibration in accordance with an embodiment of the invention; 
         FIG. 12  illustrates a graphic example of data rate expansion in accordance with an embodiment of the invention; 
         FIG. 13  illustrates a modified linear address counter used in an embodiment of the invention; 
         FIG. 14(A)  illustrates a JEDEC test mode circuit that may be used in conjunction with an embodiment of the invention; 
         FIG. 14(B)  illustrates a modified JEDEC test mode circuit which may be used in an embodiment of the invention; and 
         FIG. 15  illustrates exemplary patterns detected during timing calibration of a circuit in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described with respect to the aspect of the invention wherein any pattern of bits may be used as a calibration pattern. In the following, a novel method and associated apparatus is described for transmitting and receiving a calibration bit pattern at a rate slower than a normal operating rate of a receiving logic circuit to ensure correct capture of the calibration bit pattern. However, other methods of ensuring the correct transmission and capture of a calibration pattern at a logic device in a digital circuit are possible, and the invention is not to be limited to any particular method of transmission or capture of digital bits. 
       FIGS. 9(A) and 9(B)  show an embodiment of the invention used in an exemplary digital circuit, such as a memory circuit. Referring to  FIG. 9(A) , a digital circuit topology is shown including two logic devices  101 ,  103  connected by a bus  107 . Each of the logic devices  101 ,  103  includes a control logic circuit  21 , such as that illustrated in  FIG. 9(B) . Each control logic circuit  21  includes a compare circuit  123 , a pattern register  127  for storing a calibration bit pattern, a repeater circuit  125 , and a command recognition circuit  113 . The output of the compare circuit  123  is used to control a variable delay circuit  27  in the data path of the signal under calibration. At least one of the logic devices  101 ,  103  is connected to receive the calibration bit pattern from an external calibration pattern storage device  105 , shown in  FIG. 9(A) . 
     The logic devices  101 ,  103  may be, for example, a memory controller and a memory device, but this is not required. Other examples of logic devices  101 ,  103  that may utilize the invention include, respectively, a bus master and bus slave, a DRAM controller and a DRAM memory device, a microprocessor and a control logic chip, and a network hub and/or switch and an adapter card. 
     The external calibration pattern storage device  105  may be a circuit or a combination of circuits capable of storing and/or transmitting a pattern of digital bits, including a shift register or other pattern generation circuit, static pins or other hardwired circuit for storing a pattern, a cache memory, a network connection to an external bit pattern source, a read only memory (ROM) or a basic input/output system (BIOS). 
     For example,  FIG. 6  illustrates an exemplary circuit which may be used as a calibration pattern storage device  105 . The exemplary circuit of  FIG. 6  generates the calibration pattern, such as a 2 N −1 bit pattern, where N=4, to produce a repeating 15 bit pattern. It includes a four stage shift register  151  having bit positions &lt; 0 &gt;&lt; 1 &gt;&lt; 2 &gt;&lt; 3 &gt; and an exclusive OR gate  157  having a pair of inputs respectively connected to the output of the first stage output &lt; 0 &gt; and the last stage output &lt; 3 &gt; of shift register  151 . The output of exclusive OR gate  157  is applied as an input to stage &lt; 0 &gt; of shift register  151 . The clock signal CLK is applied to shift register  151 . The shift register  151  can initially be seeded with all ones “1” at stages &lt; 0 &gt;&lt; 1 &gt;&lt; 2 &gt;&lt; 3 &gt; and it will generate the repeating 15 bit pattern “111101011001000.” 
     In conventional memory systems, the circuit shown in  FIG. 6  was formerly included at each memory device as well as at the memory controller. In the present invention, only one such circuit may be required to generate the calibration pattern which is transmitted and stored at each memory device. Thus, the present invention conserves resources at each device by eliminating the need to have the four-bit shift register circuit of  FIG. 6 . Instead, only a minimal storage capability (pattern register  127 ) and a repeater circuit  125  may be used. 
     Alternatively, instead of generating the repeating bit pattern with a shift register circuit as shown in  FIG. 6 , the calibration pattern storage device  105  may be a ROM, BIOS or other hardwired circuitry from which the calibration pattern is repeatedly read out during calibration. 
     During preparation for timing calibration of the data paths of the logic devices  101 ,  103 , the calibration pattern, originally generated at and/or stored at the pattern storage device  105 , is received at each logic device  101 ,  103  shown in  FIG. 9(A) . For example, a first logic device  101  receives the calibration pattern from the pattern storage device  105  and transmits the calibration pattern to a second logic device  103 . When the calibration pattern is received at the second logic device  103 , it is stored in the pattern register  127  at the second logic device. As noted, transmission of the calibration pattern from a single pattern storage device  105  to each of the logic devices  101 ,  103  permits one pattern generation circuit to serve the pattern-generation needs of an entire system. For example, only one pattern generation circuit may be needed for a memory system containing a memory controller and a plurality of memory devices. After logic devices  101 ,  103  have received the calibration pattern, preparation is complete and calibration of signals paths of each logic device  101 ,  103  may begin. 
     In order to ensure correct transmission and capture of the calibration pattern at each logic device  101 ,  103 , the calibration pattern is transmitted to and between the logic devices  101 ,  103  at a first data rate slower than a normal operating rate of the logic devices. For example, the calibration pattern may be transmitted to the first logic device  101  and between the first logic device  101  and the second logic device  103  at a rate which is 25% of the rate at which the logic devices  101 ,  103  normally transmit and receive data (i.e. the normal rate is four times as fast as the pattern transmission rate). The slow rate ensures that each bit of the calibration pattern is correctly received at the logic device  101 ,  103 . 
     Before slow transfer of the calibration bit pattern may take place, the system may be commanded to slow the rate of data transfer during transmission of the pattern. Because no previous timing calibration may have occurred prior to transmission of the calibration pattern, the target logic device  103 ,  101  may not be able to properly recognize commands transmitted at normal operating speed on a high speed bus. Therefore the target logic device  103 ,  101  should be commanded into the slow pattern mode via a low speed bus or via a lower effective data rate code on a higher speed bus. This will ensure that the command for entering the slow pattern input mode will be recognized. 
     Referring to  FIG. 9(B) , a pattern input mode command from an external source (i.e., from a memory controller) may be received at the command detector  113  in the control logic circuit  21  of a logic device  101 ,  103 . A combination of slow speed signal paths or an unusual state not used in normal operating modes may be used to recognize the pattern input mode command at the command detector  113 , as is well known in the art. For example, a slower speed pin, such as a CKE pin in an SDRAM, may be toggled a number of times to signal the slow pattern input mode. Another example is a combination of four consecutive burst terminate commands in a DDR SDRAM. Since each burst terminate command is identical, the command pins would not transition for at least four clock cycles, and thus the command input is easily captured. To be reliable, the transmitting device may send more than the minimum number of repeated commands, for example six repeated commands. Alternately, a separate pattern input mode pin could be provided, but would require an additional signal path. 
     Upon recognizing the pattern input mode command, the command detector  113  transmits a signal to both the compare circuit  123  and the repeater circuit  125  to signal them of the slow pattern input mode operation (e.g., both circuits are shut off during transmission and storage of the calibration bit pattern). After the calibration bit pattern is received and stored in the pattern register  127  of the control circuit  21  of a logic device  101 , the command detector  113  activates the repeater circuit  125  for transmission of the calibration bit pattern at the normal operating rate for calibration of another logic device  103 . The repeater circuit may allow the stored pattern to be driven back out onto the external signal path or bus. Subsequently, or at the same time, the command detector  113  may activate the compare circuit  123  for calibration of the logic device  101 . The command detector  113  also may subsequently receive commands terminating calibration operations and may transmit appropriate shut-down signals to the compare circuit  123  and the repeater circuit  125 . 
     For example, after logic device  101  receives the calibration bit pattern from pattern storage device  105  (see  FIG. 9(A) ) and stores it in the pattern register  127  of its control circuit  21  (see FIG.  9 (B)), the command detector  113  activates its repeater circuit  125  for transmission of the calibration bit pattern to logic device  103 . Although shown in  FIG. 9(A)  as supplying the calibration bit pattern to logic device  101 , it is apparent that the pattern storage device  105  may instead first supply the pattern to another logic device in the system, such as logic device  103 , that may subsequently transmit the calibration bit pattern to the logic device  101 . 
     After the system is commanded to enter the pattern input mode, transfer of the calibration bit pattern at the slow rate may commence. In one embodiment of the invention, the slower transfer rate may be accomplished by reducing the frequency of the clock signal with which the calibration pattern is synchronized. This embodiment may require an additional clock signal path and/or clock divider circuitry.  FIG. 7  shows an example of a clock divider circuit interposed in the clock signal path prior to its use in  FIG. 2 . The clock divider circuit would cause the clock signal to transition fewer times per unit time and hence slow operation of the logic devices  101 ,  103  during transmission of the calibration bit pattern to the logic devices  101 ,  103 . 
     In devices that use a delay locked loop (DLL)  41  (see  FIG. 2 ) or a phase locked loop (PLL), it may be necessary to turn the DLL/PLL off, since the slower clock rate may be too low for the DLL/PLL to operate correctly. For example, when the control logic circuit  21  receives a command indicating the bit pattern is to be transmitted at the slower rate, it may command the DLL  41  to shut down. 
     In other embodiments of the invention, alternate methods may be used to slow the data rate of transmission of the calibration pattern. One method is illustrated in  FIG. 12  and includes repeating each bit to be transmitted N times before transmitting the next bit in the calibration pattern. For example,  FIG. 12  illustrates repeating each bit four consecutive times before starting the next bit. This method effectively slows the rate of transmission of the calibration pattern to one-fourth of the normal transmission rate. When the calibration pattern is received by the logic device  101 ,  103 , only one bit in N (e.g., four) may be captured and still correctly capture the calibration pattern. To avoid timing problems resulting in capture errors, a bit in the middle of each repeated sequence may be chosen to be captured. By avoiding the data bits at the beginning or the end of the 4 bit consecutive pattern, adequate timing margin is assured, since there is substantial setup and hold time of the data bit to the capture clock (over one bit time). The remaining three bits in the four bit consecutive pattern may be ignored. 
     For example, even if the data eye is misaligned such that capture of a given bit at high speed may not be possible, capture of the same bit repeated four consecutive times is likely, if a middle bit is attempted to be captured, because the correct bit is captured whether the capture is early or late.  FIG. 15  shows several exemplary received data patterns for the cases of early and late clocking of the capture of data on an incoming signal path. 
     Such a procedure of repeating each bit of the calibration pattern N consecutive times may be desired in order to avoid any timing errors of the system prior to calibration. Because the calibration pattern may be transmitted on a signal path of a logic device  101 ,  103  that has not yet been calibrated, repetition of the bits, combined with capture of bits in the middle of the received sequence, may advantageously avoid errors and ensure correct capture of the calibration pattern. The logic devices  101 ,  103  may then proceed with calibration operations at the normal operating speed. 
     Storage of the calibration pattern in the pattern register  127  once it is captured may be carried out in different ways. Where each bit of the calibration pattern is repeated N consecutive times, one embodiment stores each and every bit transmitted in the pattern register  127 . However, when the calibration pattern is retrieved from the pattern register  127  for production of the calibration signal consisting of the repeated calibration pattern, only every Nth bit is retrieved (i.e. a middle bit, such as the third bit, is retrieved, for N=4). This embodiment may be implemented using the modified linear address counter shown in  FIG. 13  or using the modified JEDEC test mode circuit shown in  FIG. 14(   b ). Referring to  FIG. 13 , an 8-bit linear address counter is shown with A 7  as the most significant bit (MSB) and A 0  as the least significant bit (LSB). The Carry-In bit (CIN) is always set to logic one “1” for the LSB. When the calibration pattern is retrieved from the pattern register  127 , the PATMODE signal is set to a logic one “1”, which forces A 0  to a logic zero “0” and A 1  to a logic one “1”. This also forces the CIN of A 2  to a logic one “1”, so that A 2  toggles on every clock cycle, as A 0  would toggle in the unmodified linear address counter. If A 2 -A 7  are all logic zero “0”, the first state has A 0 =logic zero “0” and A 1 =logic “1” (i.e., the binary number equivalent of address location  3 ). At the next clock cycle, A 2  transitions to logic one “1”, and all other bits remain the same (i.e., the binary number equivalent of address location  7 ). Thus, every Nth address is counted where N=4. Referring to  FIGS. 14(   a ) and  14 ( b ), a conventional JEDEC test mode circuit ( FIG. 14(   a )) is modified ( FIG. 14(   b )) so that the output data (DOUT) is always the contents of the Nth address read out of memory, where N=4. 
     In another embodiment the pattern register  127  stores only every Nth bit transmitted to the logic device  101 ,  103 . This embodiment may also use the modified linear address counter shown in  FIG. 13  or the modified JEDEC test mode circuit shown in  FIG. 14(   b ), connected to a read-in buffer of the incoming data path. For example, for a burst mode device which reads four bits at time, the pattern register may store only every fourth bit. Then the calibration pattern can be read out of the pattern register  127  like any other data. 
     In yet another embodiment, a prefetch scheme memory architecture may be used. The prefetch architecture is often used when the data rate of the information entering or leaving a memory device is faster than the cycle times of a memory array of the memory device. Referring to  FIG. 10(B) , data entering a memory device (e.g., incoming WRITE data) is first demultiplexed in a demultiplexer  201  to a slower rate, then written to the memory array multiple bits at a time (with a wider data path) after a minimum amount of data has been received. When the same data is read out of the memory array (e.g., for a READ operation), the same minimum amount of data is read simultaneously from the memory array and multiplexed to the higher data rate (e.g., the normal operating rate) at the output. The prefetch memory technique can be used with a slow data pattern calibration scheme in which each bit of the calibration pattern is repeated M times, for example four times, by writing the incoming data to the memory array in multiple array access times, so that the bits of the calibration pattern are not all written simultaneously. This may require the use of memory data write masking modes. For example, for a 16-bit calibration pattern, each bit of the calibration pattern incoming to the prefetch circuitry shown in  FIG. 10(B)  may be repeated 4 times and thus 64 bits may be received before the 16 bits of the calibration pattern may be written to the memory array. To write the correct 16 bits to the array, a write masking mode may be used to select only every third bit received to be written to the array. 
     The prefetch memory architecture may also be implemented to write only every Nth bit to the memory and read out the calibration pattern at the normal higher speed data rate. The received data may be demultiplexed as required to meet the memory prefetch width. Instead of shifting every received data bit in the demultiplexer  201 , only every Nth bit is shifted in. The shifted bit is chosen so it is not near a signal transition. X shifts occur before the contents of the demultiplexer  201  are parallel transferred into the memory array, X bits wide, which corresponds to an X bit prefetch. An example of this technique is illustrated in  FIG. 10(A) . 
       FIG. 10(A)  shows data being received on both rising and falling edges of the dock, but at the reduced data rate. In the example shown in  FIG. 10(A) , the third bit is shifted in, then the seventh bit, etc. Once the shift register is full with X bits, the entire contents are transferred to memory as in normal prefetch operations. Note that the cycle time is reduced to the memory by a factor of N×X, as opposed to a factor of X with a normal data rate prefetch scheme. Upon read out, X bits are fetched from the memory array, and when they are multiplexed out of memory, the original target calibration pattern is restored at the normal operating data rate. 
     After transmission and storage of the calibration pattern in the pattern register  127  of each logic device  101 ,  103 , calibration proceeds in a sequential manner. Referring to  FIG. 9(A) , a first logic device  101  is calibrated using a first calibration signal transmitted from the second logic device  103 . Once the first logic device  101  is correctly calibrated, the second logic device  103  is calibrated using a second calibration signal transmitted from the first logic device  101 . Referring to  FIG. 9(B) , the first and second calibration signals are produced using a repeater circuit  125  that repeats the bits comprising the calibration bit pattern stored in the pattern register  127 . Thereafter those repeated bits are transmitted and applied to a signal path of the first and second logic devices  101 ,  103 , respectively. The calibration signal may be applied to a signal path of a logic device  101 ,  103  by transmitting the calibration signal over the bus  107 . 
     During calibration of each logic device  101 ,  103 , the timing of the signal path on which the calibration signal is transmitted is synchronized with a clock signal. In order to properly synchronize the signal path under calibration with the clock signal, the control logic circuit  21  compares the pattern of bits received on the signal path under calibration with the stored calibration bit pattern using the compare circuit  123  (see  FIG. 9(B) ). If a match is reliably achieved, then the signal path is correctly calibrated. If not, the timing of one of the data or clock signal paths is adjusted, for example a delay value of the variable delay circuit  27  is adjusted. This process is repeated until a match is reliably achieved. 
     Several timing possibilities are illustrated in  FIG. 11 . Correct capture of data on the signal path under calibration may be achieved when a rising edge of the clock signal, such as the data capture clock signal DCLK, is aligned with the center of the “data eye” of the data received on the signal path.  FIG. 11  shows the data eye of the received data on the signal path under calibration as it compares to several different timing possibilities of the clock signal which may be explored by varying the timing, for example varying the delay value of the variable delay circuit  27 . These different timing possibilities include arbitrary initial placement of the rising edge of the clock signal, a retarded placement where the delay value is too high, an advanced placement where the delay value is too low, and a calibrated setting where the rising edge of the clock signal is in the center of the data eye. 
     A semiconductor memory system which may employ the invention is illustrated in  FIG. 1 . It includes a plurality of DRAM modules  11   a  . . .  11   n  which are accessed and controlled by a memory controller  13 . Memory controller  13  provides a command link to each of the DRAM modules  11   a  . . .  11   n  which includes a clock signal path and a command/address path. In addition, a bi-directional data bus is provided between memory controller  13  and each of the DRAM modules  11   a  . . .  11   n . The clock is used to strobe input/output data into and out of the DRAM modules. The memory controller  13  may contain a logic device  101  of  FIG. 9(A)  while each of the DRAM memory modules may contain a logic device  103 . 
       FIG. 2  illustrates a simplified relevant portion of one of the DRAM modules  11   a  . . .  11   n , including a control logic circuit  21  of the type illustrated in  FIG. 9(B) . The DRAM circuitry includes latches  23 ,  49 ,  59 , variable delay devices  27 ,  31 ,  55 ,  57 , buffers  35 ,  39 ,  47 ,  51 ,  53 , a delay lock loop  41 , multiplexer  43 , and respective memory banks Bank 0  and Bank 1   69 ,  71 . It should be noted that although two memory banks are illustrated in  FIG. 2 , this is just illustrative, as any number of memory banks can be used. 
     Control logic circuit  21  receives and analyzes commands on the command/address path CMD/ADDR and controls the input/output (I/O) access operations of the memory banks  69 ,  71 . The control logic circuit  21  also receives the clock signal CLK. 
     The signal on the command/address path CMD/ADDR is passed through adjustable delay circuit  27  and into latch  23  where the signal is latched by a clock signal CLK, as buffered by buffer  39  and delayed by variable delay  31 . 
     The clock signal CLK also passes into a delay lock loop circuit  41  which provides a data output timing signal into a multiplexer  43 . The multiplexer provides an output timing signal to a latch  49  which latches data output from the memory banks  69 ,  71 . The output data latched in latch  49  is provided to a buffer amplifier  47  and from there is passed back to memory controller  13  via the data bus DQ. 
     Data which is to be input to memory banks  69 ,  71  is supplied by memory controller  13  ( FIG. 1 ) on the DQ data bus, is passed through buffer  51  through variable delay  57 , into latch  59 , and into a memory bank  69 ,  71 . The data clock or data strobe signal DCLK, as buffered in buffer  53  and delayed by variable delay  55 , is used to control latch  59  to latch in incoming data on the data bus DQ. 
     In order to ensure proper timing of the various memory operations performed by the DRAM modules  11   a  . . .  11   n , the  FIG. 2  circuit may be calibrated using the apparatus and method described with reference to  FIGS. 9(A) and 9(B)  to ensure the incoming data is properly clocked in by the clock signals CCLK and DCLK. To this end, a calibration pattern is applied to the command/address input path CMD/ADDR while the data pattern is sampled in latch  23  by the delayed clock signal CLK. The control logic circuit  21  steps through all possible delay positions of delay  27  as the data sampling is performed and stores patterns representing which delay values for the variable delay  27  provide for a correct sampling and recognition of the calibration pattern. In this manner, control logic circuit  21  establishes an “eye” or “window” of acceptable delays for the variable delay  27  for the command/address data path CMD/ADDR Once a “window” of acceptable delays is found for the variable delay  27 , the control logic circuit  21  determines the “best” delay value as that value which is approximately in the middle of the window. 
     Alternatively, the control logic could vary the variable delay circuit  31  for the clock signal CLK or both variable delays circuits  27 ,  31  in order to establish the window of acceptable delays. 
     To illustrate the calibration process we will discuss calibration of the data appearing on the command/address path CMD/ADDR, it being understood that the same calibration process is also carried out on the receive path of the data bus DQ.  FIG. 3  illustrates a simplified timing diagram of the clock signal CLK, the command/address signal CMD/ADDR, and a data bus signal DQ. As shown, four bits of data on a DQ path of the data bus are clocked in on four sequential positive and negative going transitions of the clock signal CLK. The data present on the command/address signal path CMD/ADDR is also clocked in by the clock signal CLK. 
     Returning to  FIG. 2 , it can be seen that the data entering on the CMD/ADDR signal path passes through variable delay circuit  27  and is latched in latch  23  by the clock signal CLK. This data is then serially applied to control logic circuit  21 . During the calibration period, a calibration signal containing the calibration pattern is applied to the CMD/ADDR path by an external logic device, such as one of the logic devices  101 ,  103  (FIG.  9 (A)), together with the free running clock signal CLK. The control logic circuit  21  knows what the calibration pattern is as it is stored in the pattern register  127 , and reads the repeating calibration signal bit-by-bit from latch  23  to detect the presence of the calibration pattern. When doing so, the control logic circuit  21  first sets variable delay  27  to one known delay setting. The control logic circuit  21  then examines the bit pattern sequentially received from latch  23  to see if it matches the known calibration bit pattern. If the timing of the calibration signal data on the CMD/ADDR path is not aligned with the transitions of the CLK signal, the correct bit pattern is not recognized at the output of latch  23  and the control logic circuit  21  will adjust variable delay  27  to the next delay setting, offset by a given amount from the prior delay setting of variable delay  27 . Control logic circuit  21  will again continue to examine the bit pattern emerging from latch  23  to see if it matches the calibration bit pattern. If not, it continues to increment the delay value of the variable delay  27  and repeat the sampling and examination process until the calibration bit pattern is recognized. In actuality, rather than stopping the calibration process when the correct calibration bit pattern is recognized at the output of latch  23 , the control logic circuit  21  will actually step through all possible delay values of variable delay  27  and keep track of which delays produced a proper recognition of the calibration bit pattern. Then the control logic circuit  21  will select as a final delay value for variable delay  27 , that value which is approximately centered between all delay values which produced a proper recognition of the calibration bit pattern. 
       FIG. 4  illustrates the command/address information envelope for consecutive bits of the calibration signal together with the clock signal CCLK which latches the command/address information in latch  23 . The relative timing of the command/address information envelope and the clock CLK is illustrated as ten possibilities CLK  1  . . .  10 , that is, ten possible delay values for variable delay  27 . The beginning and end of the command/address information envelope is where the command/address information on the CMD/ADDR path is unstable which can lead to erroneous sampling of the command/address information. As shown, reliable capture of the command/address information occurs at the relative timing location C 4  through C 7 , while unreliable capture occurs at the relative timing locations C 1  . . . C 3  and C 8  . . . C 10 . These are represented within control logic circuit  21  as delay values D 4  . . . D 7 , where the calibration bit pattern was properly recognized.  FIG. 5  illustrates how this is represented in control logic circuit  21  where delay values D 1  . . . D 3  and D 8  . . . D 10  show a “0”, logic state representing that the calibration pattern was not recognized and the logic state “1” for delay values D 4  . . . D 7 , indicating a proper recognition of the calibration pattern. It should be understood that although only 10 relative delay states of the command/address information to the command clock signal CLK are shown for simplicity, in actual practice there may be many more possible delay stages for variable delay  27  and the logic state pattern illustrated in  FIG. 5 . 
     Once the delay state pattern shown in  FIG. 5  is developed by control logic circuit  21 , it selects as a final delay for variable delay  27  a delay value which is approximately in the center of those delay values, e.g., D 4  . . . D 7 , which produced a proper recognition of the calibration pattern. In the example illustrated, the final delay would be selected as D 5  or D 6 . Once this value is set for variable delay  27 , the CMD/ADDR data path has been calibrated. The same calibration procedure is also applied to the data path of the DQ bus. 
     A logic circuit containing the calibration structure and operating as described above may also be used in a processor system of the type shown in  FIG. 8 . The processor system  90  comprises a processor  94 , a memory circuit  96 , and an I/O (input/output) device  92 . The memory circuit  96  contains the calibration structure operating as described in accordance with the present invention. In addition to, or instead of, the memory circuit  96 , the processor  94  may contain the calibration structure and operate according to the calibration methodology as described in accordance with the invention. In addition, the processor  94  may itself be an integrated processor which utilizes on chip memory devices containing the calibration structure of the present invention. 
     In the preceding discussion, the apparatus and method of the invention has been described with regard to a memory device which clocks data (i.e., reads or writes data) twice per clock cycle: on both the rising and falling edges of the clock. However, the present invention may be used in any memory device in which calibration is performed, including devices which clock data once per clock cycle, for example on one of either the rising or falling edge of the clock. 
     While the invention has been described and illustrated with reference to exemplary embodiments, many variations can be made and equivalents substituted without departing from the spirit or scope of the invention. Accordingly, the invention is not to be understood as being limited by the foregoing description, but is only limited by the scope of the appended claims.