Abstract:
A method and apparatus for calibrating a data path of a digital circuit uses an even bit pseudo-random calibration pattern. A portion of the pattern is captured in a capture period and used to predict a next arriving portion of the calibration pattern. The next arriving portion of the calibration pattern is captured and then compared to the predicted pattern in a compare period, and the result of the comparison is used to relatively time data arriving in the data path to a clocking signal which clocks in the data. The time duration of the compare period may be varied to ensure that all possible bits of the calibration pattern are used in the calibration procedure.

Description:
This application is a divisional of application Ser. No. 09/637,088 filed on Aug. 14, 2000, which issued as U.S. Pat. No. 6,587,804 on Jul. 1, 2003, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for improving, calibration of data paths in memory devices. 
     DISCUSSION OF THE RELATED ART 
     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 extended data output (EDO) to static random access memory (SRAM) to double data rate static random access memory (DDR SRAM) to synchronous link dynamic random access memory (SLDRAM), the latter of which is the subject of much current industry interest. SLDRAM has a high sustainable bandwidth, low latency, low power, user upgradeability and support for large hierarchical memory applications. It also provides multiple independent banks, fast read/write bus turn-around, and the capability for small fully pipelined burst. 
     One characteristic of modern high speed memory devices is that they use both the positive- and negative-going edges of a clock cycle to READ and WRITE data from/to the memory cells and to receive command and FLAG data from a memory controller. 
     An overview of one type of high speed memory device, an SLDRAM, can be found in the specification entitled “SLDRAM Architectural and Functional Overview,” by Gillingham, 1997 SLDRAM Consortium (Aug. 29, 1997), the disclosure of which is incorporated by reference herein. 
     Because of the required high speed operation of SLDRAM, and other 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. 
     One of the several calibration procedures which is performed in current SLDRAM devices is a timing synchronization of clock signals CCLK (command clock signal) and DCLK (data clock signal) with data provided on an incoming command data path CA and FLAG data path (for the CCLK signal) and on the READ/WRITE data paths DQ (for the DCLK signal) so that incoming data is correctly sampled. Currently, a memory controller achieves this timing calibration at system initialization by sending continuous CCLK and DCLK signals on those clock paths and transmitting inverted and non-inverted versions of a fifteen-bit repeating pseudo-random SYNC sequence “1111010110010000” on each of the data paths DQ, the command data path CA, and the FLAG data path. The SLDRAM recognizes the beginning of the transmission of this pseudo-random sequence from a memory controller by the appearance of a predetermined bit pattern appearing on the FLAG data path and determines an optimal relative internal delay for CCLK and DCLK to optimally sample the known bit pattern. This optimal delay is achieved by adjusting the position of the received data bits to achieve a desired bit alignment relative to the clock. This is accomplished by adjusting a delay in the receiving data path until the received data is properly sampled by the clock and recognized internally. Once synchronization has been achieved, that is, the proper delays on the data receiving paths have been set, the memory controller stops sending the SYNC sequence and the SLDRAM, after all calibrations are completed, can be used for normal memory READ and WRITE access. 
     In order to improve reliability of calibration of incoming data to a correct edge of the clock (CCLK or DCLK), an even bit (e.g., sixteen-bit) repeating pseudo-random SYNC sequence, e.g., 1111010110010000, has been proposed to be used in place of the fifteen-bit pseudo-random sequence in the pending U.S. application Ser. No. 09/568,155 filed on May 10,2000, and entitled IMPROVED CALIBRATION PATTERN FOR SLDRAM, the disclosure of which is incorporated herein by reference. 
     The even bit pattern works well in attaining desired calibration, but the randomness of the pattern cannot be fully exploited in some predictive calibration techniques when the receiving memory device has only one serial register in the control logic for registering incoming data bits. This limitation is illustrated by the circuit diagram of FIG.  1  and the timing diagram of FIG.  2 . 
     FIG. 1 illustrates a simplified proposed predictive calibration circuit for positioning a clocking edge of clock signal CCLK at the center of a data eye to latch data arriving on one command data path CA of a command and address bus CA . . . CA 9  of a memory storage device. The data on the data path CA passes through a buffer amplifier  11 , an adjustable ring delay circuit  13  and is latched in by latch  15 . During calibration, the data appearing on the CA data path is a known repeating even-bit calibration pattern, e.g. 1111010110010000, which is sequentially latched bit-by-bit by data latch  15  on a clocking edge of the clock signal CCLK. The data on the data path CA arrives in four-bit data bursts and are latched into the memory device by rising and falling clocking edges of CCLK. The period defined by a rising or falling edge of a clock signal which extends to the next rising or falling edge is referred to herein as a clock “tick.” Calibration techniques are used to center a rising or falling clocking edge in the data eye, which is the period of time where the data is valid on the data path, at start-up and reset of the memory device so that the memory device can properly latch in data during subsequent READ/WRITE operations. 
     One such proposed calibration technique, illustrated in FIGS. 1 and 2, involves receiving a four-bit data burst of the calibration pattern in a register  17 , using the registered four bits to predict a subsequently arriving four bits with a predictor circuit  19 , and then comparing in compare circuit  21  the predicted four bits with the subsequently arriving four bits loaded in register  17 . Register  17  is loaded with four bits during periods of time known as capture periods and the comparison of a predicted bit patterns with an arriving bit pattern occurs during a compare period which occurs in between successive capture periods. The capture and compare periods are produced by a symmetric clock generator  27  from the received CCLK clock signal and are shown in FIG.  2 . 
     Referring again to FIG. 1, when the compared data is not coincident, this indicates a lack of calibration to a delay adjust circuit  23  which adjusts ring delay circuit  13  to increase or decrease the amount of delay applied to the incoming data signal on the data path CA. This repositions the data eye of the incoming data on path CA relative to the clocking edges of clock signal CCLK. The process of loading a four-bit data pattern into register  17 , predicting the next four bits, comparing the predicted four bits with a subsequently arriving four bits, and adjusting ring delays  13  when no coincidence is indicated by compare circuit  21  repeats until coincidence is detected at compare circuit  21 , indicating that the clock transition is within the data eye of arriving data. Even with a detected coincidence, the process will typically continue until ring delay  13  is adjusted to the point where coincidence is no longer obtained. Thus, in actual practice, the FIG. 1 circuit operates by stepping through all possible delay stages of ring delay  13  and noting and storing in store and logic circuit  25  those delay values where compare circuit  21  finds coincidence of a predicted data sequence to an arriving data sequence. This establishes a range of delay values for delay  13  where data is correctly received and establishes the boundaries of the data eye. The delay adjust circuit  23  may then be operated by the store and logic circuit  25  to set a final delay for ring delay  13  which positions the clocking edge at or near the center of the data eye. 
     One predictive calibration technique similar to that described above in simplified form is also described in greater detail in U.S. application Ser. No. 09/568,016 filed May 10, 2000, which issued as U.S. Pat. No. 6,606,041 on Aug. 12, 2003 and entitled PREDICTIVE TIMING CALIBRATION FOR MEMORY DEVICES, the disclosure of which is incorporated herein by reference. 
     When the proposed predictive calibration circuit of FIG. 1 is used with only a single four-bit register  17  to capture incoming data and with an even bit number calibration pattern, only portions of the calibration pattern are fully utilized during calibration. This is illustrated in FIG. 2 which shows use of the clock signal CCLK to bit-wise latch the data pattern into latch  15  which proceeds from there into register  17 . For purposes of illustration, FIG. 2 shows the data pattern “1111010110010000” as the arriving calibration data pattern. It should be remembered that in actuality, for an SLDRAM, data arrives in four bit data bursts clocked in latch  15  by two cycles of clock signal CCLK When a single four-bit register  17  is used in the control logic  35 , the control logic  35  must first clock in a four-bit data burst and use that registered data burst to predict a subsequently arriving and registered four-bit data burst. Assuming the first registered four bits is “1111,” then the predictor circuit will predict that a subsequent four-bit pattern as “1001.” This predicted value will then be compared by compare circuit  21  to the next four bits captured and registered in register  17 . It should be noted that because the compare periods are interleaved with the capture periods, the predictor circuit is set to predict a four-bit pattern which will be the next four-bit pattern captured, e.g. “1001,” rather than the next four-bit pattern which actually exits exists in the calibration pattern, e.g. “0101.” 
     Accordingly, the control logic circuit establishes a pattern of registering four bits and generating the predictive pattern over two cycles of clock CCLK and using the next two clock cycles to compare the first captured four-bit pattern with a four-bit pattern predicted from a previously captured four-bit pattern. This process can be thought of as having a data “capture” period of two clock cycles followed by a “compare” period of two clock cycles as illustrated in FIG.  2 . The arrows in FIG. 2 show how a just captured four-bit pattern, e.g. “1111,” is used both in the comparison operation (arrow A) and to predict a subsequent four-bit pattern for the comparison operation which occurs after the next ensuing comparison (arrow B). Because a single four-bit register  17  is used, the capture and compare cycles must be performed sequentially and cannot overlap. 
     When an even number of bits are used in the calibration pattern, the capture and compare periods will repeatedly sequentially capture and compare only a portion of the entire calibration pattern which is available. For example, assuming the “1111” pattern is first captured, this is followed by a comparison of the predicted pattern “1001” with the next captured four-bit pattern. If this process repeats, then only the “11111” and subsequent “1001” patterns are used in the evaluation, while patterns “0101” and “0000” and other possible four-bit patterns of the sixteen-bit calibration pattern are never used. Thus, only a small repeating portion of the sixteen-bit calibration pattern is evaluated during calibration. 
     A very important attribute of the calibration pattern, which is the pseudo-random nature of the sequence, is not exploited. This pseudo-random property of the calibration pattern is what exposes inter-symbol interference (ISI) and dynamic skew of the data path. Accordingly, this limits the utility of the even-bit calibration pattern in detecting ISI and bus skew. 
     A method and apparatus which more fully exploits the pseudo-random nature of an even-bit pseudo-random calibration pattern when a single register is used in a predictive calibration technique would be desirable. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus which more fully utilizes the pseudo-random properties of a repeating even-bit pseudo-random calibration pattern in a predictive calibration system. 
     In the invention, repeating sequential capture and compare periods are utilized to capture an N-bit pattern, e.g., N=4, of the even numbered repeating M-bit calibration pattern and e.g. M=16, and to subsequently compare a predicted N-bit sequence generated from the captured pattern with a subsequently captured N-bit pattern. A constant even number of clock cycles are always used to capture the data (e.g., two clock cycles for four data bits), but the number of clock cycles used for the compare period changes from time to time. 
     As a result, the capture periods will not continually capture the same data pattern, and during the calibration period a larger number of N-bit sequences which are contained in the even M-bit calibration pattern, sequences which utilize all of the bits of the calibration path, will be captured and used in the calibration process, thereby more fully exploiting the pseudo-randomness of the calibration pattern. The compare period can vary between two preset values, e.g., two clock cycles, and three clock cycles for alternating compare periods, or other arrangements for varying the duration of the compare period from time to time, can be used. 
     In an alternative implementation of the invention, the capture and compare periods are arranged to capture and compare a larger portion of the calibration pattern, e.g. eight bits of a sixteen-bit calibration pattern, so that a larger portion of the sixteen-bit calibration pattern is used during calibration. Here, too, the captured sequences will utilize all of the bits of the calibration pattern. With this arrangement, the compare periods can be of equal duration, or can be varied to even more fully exploit the randomness of the calibration pattern. 
    
    
     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 is a block diagram of one proposed predictive calibration circuit for a data path of a memory device; 
     FIG. 2 illustrates signal patterns associated with the operation of the FIG. 1 circuit; 
     FIG. 3 illustrates signal patterns associated with two exemplary embodiments of a predictive calibration technique performed in accordance with the invention; 
     FIG. 4 is a block diagram of an exemplary embodiment of a predictive calibration system constructed in accordance with the invention; 
     FIG. 5 illustrates signal patterns associated with a third exemplary embodiment of the invention; and 
     FIG. 6 illustrates a processor system which may employ the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a first exemplary embodiment, the invention provides an asymmetric clock generator  28 , FIG. 4, in place of the symmetric clock generator  27  employed in the FIG. 1 circuit. The clock generator  28  functions to vary the time duration of the compare periods so that more of the possible N-bit sequences of an even M-bit calibration pattern can be used during the calibration process described above with reference to FIG.  1 . For purposes of simplification it will be assumed that M=16 and N=4, but it should be understood that these values are merely exemplary of an embodiment of the invention and that other values can be used as well. 
     In the invention, instead of the compare period always being two clock cycles long, as in the FIG. 1 circuit and shown in FIG. 2, it now varies. For example, in the first exemplary embodiment described in more detail below, successive capture periods alternate between being two clock cycles long and three clock cycles long. Other capture period durations are also possible, the important point being that the capture periods are not all of the same duration. This causes more of the possible N-bit patterns of the M-bit calibration pattern to be used in the calibration process. 
     The first exemplary embodiment of the invention is now described in greater detail with reference to FIGS. 3 and 4. FIG. 4 is similar to FIG. 1, except that the symmetric clock generator of FIG. 1 is replaced by an asymmetric clock generator  28 . In this embodiment, the FIG. 4 asymmetric clock generator  28  produces, from clock signal CCLK, an asymmetric clock signal ACLK which has alternating capture  31  and compare  33   a ,  33   b ,  33   a  periods (FIG.  3 ). The capture periods  31  are all of equal time duration which corresponds to two cycles (four ticks) of clock signal CCLK, but the compare periods  33   a ,  33   b  have different durations. In the illustrated embodiment, compare period  33   a  has a time duration corresponding to three cycles (six ticks) of clock signal CCLK, while compare period  33   b  has a time duration corresponding to two cycles (four ticks) of clock signal CCLK. In the illustrated embodiment, successive compare periods  33   a ,  33   b ,  33   a  . . . have time periods which alternate between a first time duration corresponding to two cycles of the CCLK clock signal and a second time duration corresponding to three cycles of the CCLK clock signal. 
     When the ACLK clock signal illustrated in FIGS. 3 and 4 is employed to enable (load) the register  17  and enable compare circuit  21 , the N-bit sequence captured in register  17  and used to predict a subsequent N-bit sequence will continually rotate through the sixteen-bit calibration pattern such that eight four-bit patterns are captured and used to predict a subsequent four-bit pattern. For the timing diagram illustrated in FIG. 3, these are the data patterns “0101,” “0011,” “0110,” “1111,” “1001,” “1101,” “0100” and “0000.” As a result, all sixteen bits of the calibration pattern are captured and used and more of the randomness of the even sixteen-bit calibration pattern can be exploited during calibration. 
     It should be understood that during a compare period, all bits arriving at latch  15  are skipped over, that is, they are not registered in register  17 . It is this skipping over of different numbers of bits as the duration of the compare periods vary in duration which permits more, here eight, of the sixteen possible four-bit data patterns of the sixteen-bit calibration pattern to be used for calibration. Four bits are skipped for a compare period corresponding to two cycles of the CCLK signal, while six bits are skipped for a compare period corresponding to three cycles of the CCLK signal. 
     Thus, looking at the first compare period  33   a  in FIG. 4, the four bits “0101” are received in register  17  for comparison with the four bits predicted by predictor circuit from the previously captured “0000” four bits. The bits “100100” which arrive at latch  15  during compare period  33   a  will be skipped over. As a consequence, the next four bits captured by register  17  during the next ensuing capture period  31  will be the bit sequence “0011.” This is what causes the captured pattern to eventually cycle through eight four-bit sequences of the sixteen-bit calibration pattern. 
     Although the embodiment of the invention illustrated in FIGS. 3 and 4 uses alternating compare periods which correspond respectively to two and three cycles of the CCLK clock signal, other timing patterns for the ACLK signal are also possible. For example, the compare periods may alternate between time durations corresponding to two and two-and-one-half cycles of the CCLK clock signal, i.e., between four ticks and five ticks of the CCLK signal, while the capture periods remain at two clock cycles, as shown by the ACLK′ signal in FIG.  3 . With this arrangement, all sixteen possible four-bit sequences of the sixteen-bit calibration pattern will be captured and used. 
     Another exemplary embodiment of the invention is illustrated by the timing diagrams of FIG.  5 . In this embodiment, which uses the ACLK″ signal to define the capture and compare periods, eight bits of the sixteen bit calibration pattern are captured at a time and used to predict a next captured eight-bit pattern. Thus, register  17 , predictor circuit  19  and compare circuit  21  of FIG. 4 need now handle eight bits at a time. In this embodiment, the compare periods are of equal duration, e.g. four ticks or two CCLK clock cycles long. With this embodiment, four eight-bit patterns of the sixteen-bit calibration pattern can be captured and used in the predictive calibration process described above. Using the timing illustrated in FIG. 5, the four eight-bit patterns which are captured and used are “11110101,” “00001111,” “10010000,” and “01011001.” During each of the compare periods, four incoming bits latched in latch  15  are skipped over. Although the compare period durations do not change in this embodiment, the four eight-bit patterns which are fully captured and used make full use of all sixteen bits of the calibration pattern and thus again exploit the pseudo-random nature of the calibration pattern. It is also possible to vary the duration of the compare periods, as illustrated by the ALK′″ clock signal in FIG. 5 to capture and use even more eight-bit patterns of the sixteen-bit calibration pattern in the eight-bit predictive calibration process. 
     Although exemplary embodiments of the invention have been described and illustrated with respect to use with a sixteen-bit calibration pattern, it should be apparent that the invention may be used with any even-bit calibration pattern. Also, although different exemplary embodiments have been described which capture four bits and eight bits of the calibration pattern, the invention can also be used with other numbers of captured bits. 
     The invention can be used in a predictive calibration process to calibrate any of the data paths of a memory device which have a single register for capturing incoming data. For example, such data paths include, but are not limited to, the FLAG and command CA . . . CA 9  data paths of an SLDRAM memory device. 
     FIG. 6 illustrates a processor system which employs digital circuits having data paths which can be calibrated using a method and apparatus of the invention. 
     As shown in FIG. 6, a processor based system, such as a computer system, for example, generally comprises a central processing unit (CPU)  210 , for example, a microprocessor, that communicates with one or more input/output (I/O) devices  240 ,  250  over a bus  270 . The computer system  200  also includes random access memory (RAM)  260 , a read only memory (ROM)  280  and, in the case of a computer system may include peripheral devices such as a floppy disk drive  220  and a compact disk (CD) ROM drive  230  which also communicate with CPU processor  210  over the bus  270 . The RAM  260  and/or processor  210  are preferably constructed with data paths which can be calibrated using the method and apparatus of the invention described above with reference to FIGS. 3 and 4. 
     While the invention has been described and illustrated with reference to specific exemplary embodiments, it should be understood that many modifications and substitutions can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.