Abstract:
A method is provided that compensates for misalignment on a synchronous data bus. The method includes: replicating propagation path lengths, loads, and buffering of a radial distribution network for a strobe; receiving a first signal, and generating a second signal by employing the replicated propagation path lengths, loads, and buffering; when an update signal is asserted, when an update signal is asserted, measuring a propagation time beginning with assertion of the first signal and ending with assertion of the second signal by selecting one of a plurality of successively delayed versions of the first signal that coincides with the assertion of the second signal, wherein said selecting comprises incrementing and decrementing bus states of select inputs on a mux, wherein the plurality of successively delayed versions of the first signal comprises inputs to the mux; gray encoding a value on a lag bus that indicates the propagation time; and receiving one of a plurality of radially distributed strobes and a data bit, and delaying registering of the data bit by the propagation time. The receiving includes generating successively delayed versions of the data bit; receiving the value on the lag bus, and selecting one of the successively delayed versions of the data bit that corresponds to the value; and registering the state of the one of the successively delayed versions of the data bit upon assertion of one of a plurality of radially distributed strobe signals.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/757,575 (Docket: CNTR.2582), filed on Feb. 1, 2013, which is herein incorporated by reference for all intents and purposes. 
         [0002]    U.S. patent application Ser. No. 13/757,575 is a continuation of U.S. Nonprovisional patent application Ser. No. 13/747,038 (Docket: CNTR.2539), filed on Jan. 22, 2013. 
         [0003]    This application is related to the following co-pending U.S. Patent Applications, each of which has a common assignee and common inventors. 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                   
                 FILING 
                   
               
               
                 Ser. No. 
                 DATE 
                 TITLE 
               
               
                   
               
             
             
               
                 — 
                 — 
                 SOURCE SYNCHRONOUS DATA STROBE 
               
               
                 (CNTR.2539-C1) 
                   
                 MISALIGNMENT COMPENSATION MECHANISM 
               
               
                 — 
                 — 
                 SOURCE SYNCHRONOUS DATA STROBE 
               
               
                 (CNTR.2539-C2) 
                   
                 MISALIGNMENT COMPENSATION MECHANISM 
               
               
                 13/747,140 
                 Jan. 22, 2013 
                 APPARATUS AND METHOD FOR DYNAMIC 
               
               
                 (CNTR.2540) 
                   
                 ALIGNMENT OF SOURCE SYNCHRONOUS BUS 
               
               
                   
                   
                 SIGNALS 
               
               
                 13/747,187 
                 Jan. 22, 2013 
                 SOURCE SYNCHRONOUS BUS SIGNAL ALIGNMENT 
               
               
                 (CNTR.2576) 
                   
                 COMPENSATION MECHANISM 
               
               
                 — 
                 Feb. 1, 2013 
                 APPARATUS AND METHOD FOR AUTOMATICALLY 
               
               
                 (CNTR.2581) 
                   
                 ALIGNING DATA SIGNALS AND STROBE SIGNALS 
               
               
                   
                   
                 ON A SOURCE SYNCRHONOUS BUS 
               
               
                 — 
                 — 
                 APPARATUS AND METHOD FOR AUTOMATICALLY 
               
               
                 (CNTR.2581-C1) 
                   
                 ALIGNING DATA SIGNALS AND STROBE SIGNALS 
               
               
                   
                   
                 ON A SOURCE SYNCRHONOUS BUS 
               
               
                 — 
                 — 
                 APPARATUS AND METHOD FOR AUTOMATICALLY 
               
               
                 (CNTR.2581-C2) 
                   
                 ALIGNING DATA SIGNALS AND STROBE SIGNALS 
               
               
                   
                   
                 ON A SOURCE SYNCRHONOUS BUS 
               
               
                 — 
                 — 
                 APPARATUS AND METHOD FOR DYNAMICALLY 
               
               
                 (CNTR.2582-C1) 
                   
                 ALIGNED SOURCE SYNCHRONOUS RECEIVER 
               
               
                   
               
             
          
         
       
     
     
    
     BACKGROUND OF THE INVENTION 
       [0004]    Field of the Invention 
         [0005]    This invention relates in general to the field of microelectronics, and more particularly to an apparatus and method for synchronizing and clocks and data related to the transmission and reception of source synchronous signals. 
         [0006]    Description of the Related Art 
         [0007]    A present day computer system employs a source synchronous system bus to provide for exchange of data between bus agents, such as between a microprocessor and a memory hub. A “source synchronous” bus protocol allows for the transfer of data at very high bus speeds. Source synchronous protocols operate on the principle that a transmitting bus agent places data out on the bus for a fixed time period and asserts or switches a “strobe” signal corresponding to the data to indicate to a receiving bus agent that the data is valid. Both data signals and their corresponding strobe are routed over the bus along equal propagation paths (both physically and electromagnetically), thus enabling a receiver to be relatively certain that when switching of the corresponding strobe is detected, data is valid on the data signals. For purposes of the present invention, a bus agent may be any electronic element that utilizes source synchronous signaling for the transfer of data to/from another bus agent over a source synchronous bus. Exemplary bus agents may be, but are not limited to, central processing units (CPUs), microprocessors, memory controllers, memory hubs, chipsets, and graphics controllers. The source synchronous bus may also be known as a system bus, a front side bus, or a back side bus. Bus agents may be individually packaged, disposed on a motherboard, and interconnected by conductive traces on the motherboard. Additionally, a plurality of bus agents may be disposed within the same package that is mounted to a motherboard, where the plurality of bus agents may be individual dies within the package or they may be integrated into the same integrated circuit die and are interconnected via traces on the die. 
         [0008]    Yet, source synchronous data strobes and data signals are subject to error for a number of different reasons. These inaccuracies may be the result of uncontrollable design margins, fabrication tolerances, or environmental factors such as voltage or temperature. In most cases, it is desired that a strobe signal switch precisely halfway through a data validity period so that there is equal set up and hold time for the data as seen at the receiver. However, inaccuracies resulting from the above factors may result in skewing of the data signals and/or their strobes such that reception conditions are not optimum. Consequently, operating frequency of associated devices is limited. 
         [0009]    Another source of error may be caused by distribution of a strobe signal within a receiving device. While system designers go to great lengths to ensure that a strobe and its associated data signals are routed along the same propagation path on a system board (or, motherboard), it is well known that once the strobe enters the receiving device, it must be distributed to all of the internal synchronous receivers that are associated with that strobe. Some techniques for distributing a strobe signal to internal receivers simply adds propagation lengths that are required to route the strobe to the internal receivers, which may add delay over that of the data signals, thereby skewing the phase of the synchronous transmission. More recent mechanisms for strobe distribution also introduce buffering of the disturbed strobe signals, thereby skewing the phase of the synchronous transmission even more. 
         [0010]    Therefore, what is needed are apparatus and methods that compensate for misalignment of signals and strobes on a source synchronous data bus, thus allowing optimization of a device&#39;s operating frequency. 
         [0011]    What is also needed is a technique that allows the signals on a synchronous bus to be optimized for reception by modifying the phase alignment of a data strobe and its corresponding data signals. 
         [0012]    What is furthermore needed is an automatic mechanism that allows the phase alignment of a data strobe and its associated data signals to be dynamically optimized at a receiving device. 
         [0013]    What is moreover needed is an apparatus that is programmable at the motherboard level to compensate for fabrication and design inaccuracies, voltage variations, and temperature variations in an automated signal alignment mechanism. 
         [0014]    What is additionally needed is a synchronous receiver that automatically compensates for misalignment of signals on a source synchronous data bus. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention, among other applications, is directed to solving the above-noted problems and addresses other problems, disadvantages, and limitations of the prior art. In addition, the present invention provides a superior technique for automatically and dynamically optimizing the phase alignment of data signals and associated strobes that are received over a source synchronous bus. In one embodiment, an apparatus is provided that compensates for misalignment on a synchronous data bus, the apparatus includes a replica radial distribution element, a bit lag control element, and a synchronous lag receiver. The replica radial distribution element is configured to receive a first signal, and is configured to generate a second signal, where the replica radial distribution element comprises replicated propagation path lengths, loads, and buffering of a radial distribution network for a strobe. The bit lag control element is configured to measure, when an update signal is asserted, a propagation time beginning with assertion of the first signal and ending with assertion of the second signal, and is configured to generate a value on a lag bus that indicates the propagation time. The bit lag control element has delay lock control and a gray encoder. The delay lock control is configured to select one of a plurality of successively delayed versions of the first signal that coincides with the assertion of the second signal, where the delay lock control selects the one of a plurality of successively delayed versions of the first signal by incrementing and decrementing bus states of select inputs on a second mux, and where the plurality of successively delayed versions of the first signal comprises inputs to the mux. The gray encoder is configured to gray encode the propagation time to generate the value on the lag bus. The synchronous lag receiver is configured to receive one of a plurality of radially distributed strobes and a data bit, and is configured to delay registering of the data bit by the propagation time. The synchronous lag receiver includes a first plurality of matched inverters, a first mux, and a bit receiver. The first plurality of matched inverters is configured to generate successively delayed versions of the data bit. The first mux is coupled to the first plurality of matched inverters, and is configured to receive the value on the lag bus, and is configured to select one of the successively delayed versions of the data bit that corresponds to the value. The bit receiver is configured to receive the one of the successively delayed versions of the data bit and one of a plurality of radially distributed strobe signals, and is configured to register the state of the one of the successively delayed versions of the data bit upon assertion of the one of a plurality of radially distributed strobe signals. 
         [0016]    In one aspect, the present invention contemplates an apparatus that compensates for misalignment on a synchronous data bus. The apparatus includes a microprocessor. The microprocessor has a replica radial distribution element, a bit lag control element, and a synchronous lag receiver. The replica radial distribution element is configured to receive a first signal, and is configured to generate a second signal, where the replica radial distribution element comprises replicated propagation path lengths, loads, and buffering of a radial distribution network for a strobe. The bit lag control element is configured to measure, when an update signal is asserted, a propagation time beginning with assertion of the first signal and ending with assertion of the second signal, and is configured to generate a value on a lag bus that indicates the propagation time. The bit lag control element has delay lock control and a gray encoder. The delay lock control is configured to select one of a plurality of successively delayed versions of the first signal that coincides with the assertion of the second signal, where the delay lock control selects the one of a plurality of successively delayed versions of the first signal by incrementing and decrementing bus states of select inputs on a second mux, and where the plurality of successively delayed versions of the first signal comprises inputs to the mux. The gray encoder is configured to gray encode the propagation time to generate the value on the lag bus. The synchronous lag receiver is configured to receive one of a plurality of radially distributed strobes and a data bit, and is configured to delay registering of the data bit by the propagation time. The synchronous lag receiver includes a first plurality of matched inverters, a first mux, and a bit receiver. The first plurality of matched inverters is configured to generate successively delayed versions of the data bit. The first mux is coupled to the first plurality of matched inverters, and is configured to receive the value on the lag bus, and is configured to select one of the successively delayed versions of the data bit that corresponds to the value. The bit receiver is configured to receive the one of the successively delayed versions of the data bit and one of a plurality of radially distributed strobe signals, and is configured to register the state of the one of the successively delayed versions of the data bit upon assertion of the one of a plurality of radially distributed strobe signals. 
         [0017]    Another aspect of the present invention comprehends a method that compensates for misalignment on a synchronous data bus. The method includes: replicating propagation path lengths, loads, and buffering of a radial distribution network for a strobe; receiving a first signal, and generating a second signal by employing the replicated propagation path lengths, loads, and buffering; when an update signal is asserted, when an update signal is asserted, measuring a propagation time beginning with assertion of the first signal and ending with assertion of the second signal by selecting one of a plurality of successively delayed versions of the first signal that coincides with the assertion of the second signal, wherein said selecting comprises incrementing and decrementing bus states of select inputs on a mux, wherein the plurality of successively delayed versions of the first signal comprises inputs to the mux; gray encoding a value on a lag bus that indicates the propagation time; and receiving one of a plurality of radially distributed strobes and a data bit, and delaying registering of the data bit by the propagation time. The receiving includes generating successively delayed versions of the data bit; receiving the value on the lag bus, and selecting one of the successively delayed versions of the data bit that corresponds to the value; and registering the state of the one of the successively delayed versions of the data bit upon assertion of one of a plurality of radially distributed strobe signals. 
         [0018]    Regarding industrial applicability, the present invention is implemented within a MICROPROCESSOR which may be used in a general purpose or special purpose computing device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    These and other objects, features, and advantages of the present invention will become better understood with regard to the following description, and accompanying drawings where: 
           [0020]      FIG. 1  is a block diagram illustrating a present day system wherein source synchronous data is transmitted and received; 
           [0021]      FIG. 2  is a timing diagram depicting two source synchronous signaling scenarios that may occur in the present day system of  FIG. 1 : one scenario in which a data strobe in a receiving device is in synchronization with associated data, and a second scenario in which the data strobe and the associated data are unsynchronized. 
           [0022]      FIG. 3  is a block diagram featuring an apparatus for automated local synchronous signals alignment according to the present invention; 
           [0023]      FIG. 4  is a block diagram showing an apparatus for automated dynamic synchronous signals alignment according to the present invention; 
           [0024]      FIG. 5  is a block diagram one embodiment of a bit lag control element according to the present invention; 
           [0025]      FIG. 6  is a block diagram showing a fuse-adjustable bit lag control element according to the present invention; 
           [0026]      FIG. 7  is a block diagram illustrating a JTAG-adjustable bit lag control element according to the present invention; 
           [0027]      FIG. 8  is a block diagram depicting a synchronous lag receiver according to the present invention; and 
           [0028]      FIG. 9  is a block diagram detailing a precision delay element according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Exemplary and illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification, for those skilled in the art will appreciate that in the development of any such actual embodiment, numerous implementation-specific decisions are made to achieve specific goals, such as compliance with system-related and business related constraints, which vary from one implementation to another. Furthermore, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. Various modifications to the preferred embodiment will be apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described herein, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed. 
         [0030]    The present invention will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present invention with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present invention. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
         [0031]    In view of the above background discussion on source synchronous signaling and associated techniques employed within present day devices for the transmission and reception of data, a discussion of the disadvantages and limitations of the present day techniques be discussed with reference to  FIGS. 1-2 . Following this, a discussion of the present invention will be presented with reference to  FIGS. 3-9 . The present invention overcomes these limitations and disadvantages by providing mechanisms that allow for the detection of the precise lag of a data strobe from associated data group bits in a receiving device and also techniques for delaying those associated data group bits in corresponding receivers thereby providing for correction of strobe and data misalignment caused by any of a number of reasons, thus enabling throughput to be optimized between the a transmitting device and a receiving device. 
         [0032]    Turning to  FIG. 1 , a block diagram is presented illustrating a present day computer system  100  where two or more bus agents  101  exchange data over a source synchronous system bus  102 . The bus agents  101  may be any element or elements of the computer system  100  that are employed to transmit or receive data via the bus  102 , as is alluded to above. The source synchronous bus  102  may be known by other names as well including, but not limited to, a system bus, a front side bus, and a back side bus. 
         [0033]    As one skilled in the art will appreciate, a typical present day bus agent  101  may be embodied as, but not limited to, a microprocessor or central processing unit (CPU), a memory hub or memory controller, a chipset, a master or slave peripheral device, a direct memory access unit, a graphics controller, or another type of bus interface unit. In broad terms, to transfer data, one of the bus agents  101  will drive a subset of the signals on the bus  102  while another of the bus agents  101  detects and receives the driven signals, thus capturing the data that is represented by the states of one or more of the subset of the signals on the bus  102 . One or more of the bus agents  101  may be devices each disposed on an individual integrated circuit die and encapsulated in a device package, where the device package is disposed on a motherboard (or “system board”) by conventional means, and where the system bus  102  is disposed as metal traces (or “lands”) on the motherboard. Alternatively, two or more of the bus agents  101  may be devices each disposed on an individual integrated circuit die, where two or more of the integrated circuit die are disposed on a substrate and encapsulated in a single device package, and where the bus  102  is disposed as metal traces on the substrate, and where the single device package is disposed on a motherboard and is coupled to other device packages on the motherboard via interconnecting metal traces on the motherboard, where those interconnecting metal traces include the bus  102 . Furthermore, two or more of the bus agents  101  may be disposed on a single integrated circuit die that is encapsulated in a device package that is disposed on a motherboard, where the system bus  102  comprises metal traces on the single integrated circuit die to interconnect the two or more bus agents  101 , and also as metal traces on the motherboard to interconnect the device package housing the single integrated circuit die to other device packages disposed on the motherboard. 
         [0034]    There are a number of different bus protocols provided for in the present day art for transferring data between two bus agents  101 , and it is beyond the scope of this application to include a detailed description of these various techniques. It is sufficient for purposes of the present application to appreciate that the “data” which is communicated between two or more bus agents  101  during a bus transaction may include, but is not limited to, address information, data that is associated with one or more addresses, control information, or status information. Regardless of the type of data that is communicated over the bus  102 , it is germane to this application that more and more present day computer systems  100  are employing a particular type of bus protocols commonly known as “source synchronous” protocols, to affect the transfer of data at very high bus speeds. In contrast to prior art sampled data bus protocols, source synchronous protocols operate on the principle that a transmitting bus agent  101  places data out on the bus  102  for a fixed time period (i.e., “setup time”) and asserts a “strobe” signal corresponding to the data to indicate to a receiving bus agent  101  that the data is valid. The transmitting bus agent  101  holds the data on the bus  102  for an amount of time (i.e., “hold time”) approximately equal to the setup time so that a receiving bus agent  101  can detect the state of the date prior to assertion of the strobe signal and to latch the data subsequent to assertion of the strobe signal. One skilled in the art will appreciate that the propagation path, to include physical and electromagnetic parameters, of one set of data and corresponding strobe signals, at very high transfer speeds, may very well be quite different from the propagation path that is associated with another set of signals on the bus, whether that propagation path is from the transmitting device to another receiving device or whether the propagation path is from the transmitting bus agent  101  to the same receiving bus agent  101 , but corresponds to another data group and the group&#39;s associated strobe signal. In particular, propagation delay, bus impedance, and electromagnetic characteristics of a propagation path affect the times (i.e., the setup and hold times) at which the data signals are stable, (i.e., “valid”) for reception by the receiving bus agent  101 . It is for this reason that source synchronous bus protocols are now prominent in the market of fielded devices. In a typical configuration, a data strobe that is associated with a corresponding set (or “group”) of data signals is purposely routed along the same propagation path as the set of data signals, and thus the strobe sees the same propagation characteristics as the data signals themselves. If the strobe is asserted during the period in which the data is valid (preferably such that setup and hold times are approximately equal), when the receiving bus agent  101  detects a valid transition of the strobe, it is relatively certain that the data signals will be valid as well. 
         [0035]    To more particularly describe the interaction of signals on a source synchronous bus, attention is now directed to  FIG. 2 , where is a timing diagram  200  is presented depicting two source synchronous signaling scenarios that may occur in the present day system of  FIG. 1 : one scenario in which a data strobe in a receiving device is in synchronization with associated data, and a second scenario in which the data strobe and the associated data are unsynchronized. The diagram  200  shows interaction of signals within an exemplary data signal group for performing the data phase of an 8-byte burst bus transaction. For clarity, assertion of signals is shown in the diagram  200  as a logic low level, although one skilled in the art will appreciate that assertion can as well be indicated by a logic high level, or by toggling between a high and low levels. Cycles of a differential bus clock BCLK[1:0] are shown across the top of the timing diagram  200 . For an x86-compatible microprocessor, the bus clock BCLK[1:0] is distributed to all bus agents in order to facilitate synchronization of transactions between the bus agents. 
         [0036]    A source synchronous protocol provides for a 16-bit data bus D[15:0] that supports transfer during the data phase of an 8-byte cache line over two cycles of the bus clock BCLK[1:0] through the employment of source synchronous data strobe signals DSTBPB 0 , DSTBNB 0 . The transfer of one byte over the 16-bit data bus D[15:0] is known as a beat, and 4 beats  1 - 4 ,  5 - 8  are transferred during each cycle of the bus clock BCLK[1:0]. The data bus signals D[15:0] and their corresponding strobe signals DSTBPB 0 , DSTBNB 0  are routed along the same propagation path to individual bit receivers for each of the bits in D[15:0]. The falling edges of data strobe DSTBPB 0  are used to indicate validity of words  1 ,  3 ,  5 , and  7  on D[15:0]. The falling edges of data strobe DSTBNB 0  are used to indicate validity of words  2 ,  4 ,  6 , and  8  on D[15:0]. Note that the frequency of the data strobe signals DSTBPB 0 , DSTBNB 0  is twice that of the bus clock BCLK[1:0] and that the two strobes DSTBPB 0 , DSTBNB 0  exhibit a relative ½-cycle lag in phase. Consequently, the exemplary bus protocol supports transfer of four sets (i.e., beats) of data during a single bus clock cycle. The signals noted above are presented to teach aspects of the present invention, and for clarity sake bus interactions are simplified, however, as one skilled in the art will appreciate, the bus could be expanded to support any number of bits. 
         [0037]    As one skilled in the art will acknowledge, a transmitting bus agent (e.g., microprocessor, chipset, or other bus agent) places its data D[15:0] on the bus and then asserts a corresponding data strobe DSTBPB 0 , DSTBNB 0  to indicate validity of the data, preferably halfway through the validity period of the data so that setup and hold times are approximately equal. Hence, in contrast to older, sampled data/address buses, where data was placed on the bus and held for a sampling period, the present synchronous bus mechanisms strobe data out over bus subgroups in a plurality of bursts, where the validity of each burst is indicated by the state of the corresponding strobe DSTBPB 0 , DSTBNB 0 , and since the corresponding strobe DSTBPB 0 , DSTBNB 0  is routed along the same propagation path as its associated data signals D[15:0], it is virtually certain that when a receiver detects assertion of the data strobe DSTBPB 0 , DSTBNB 0 , the associated data D[15:0] will be valid. 
         [0038]    From the perspective of a receiving bus agent, assertions of the data/address strobes DSTBPB 0 , DSTBNB 0  appear to be indeterminate with respect to assertions of the bus clock BCLK#, but as alluded to above, the period for each of the data strobes DSTBPB 0 , DSTBNB 0  is equal to approximately one-half of the period of the bus clock BCLK#. As previously noted, the timing of data and strobe transitions is indeed a function of the bus clock frequency, but at a receiving bus agent the switching of any given data strobe seems, for all intents and purposes, to be asynchronous to the bus clock BCLK[1:0]. This is because there is a fixed, but unknown, phase difference between the bus clock BCLK[1:0] and transitions of the data subgroup signals and corresponding data strobes as the bus clock may BCLK[1:0] have traversed a different propagation path between a clock generator and the receiving bus agent. 
         [0039]    Note that the transitions of D[15:0] and associated strobes DSTBPB 0 , DSTBNB 0  in a first scenario  201  appear to be in phase with the transitions of BCLK[1:0] while the transitions of D[15:0] and associated strobes DSTBPB 0 , DSTBNB 0  in a second scenario  202  appear to have no phase relationship with BCLK[1:0] whatsoever. These differences may be due to that manner in which a transmitting bus agent transfers data over the bus, or it may be due to a different propagation path length for the data bus D[15:0] relative to BCLK[1:0], or it may be due to both transmitter characteristics and propagation path lengths. 
         [0040]    As long as the data signals within the bus D[15:0] are received approximately in proper phase with their corresponding strobe signals DSTBPB 0 , DSTBNB 0 , because setup and hold times are approximately equal, effective data transfer can be accomplished at very high bus speeds. This is the case illustrated the first scenario  201 . Note that at time T 1 , from the perspective of the receiving bus agent, DSTBPB 0  is asserted halfway through the period when burst  1  is valid on the bus, thus enabling optimum conditions for reception of the burst  1 . Likewise, at time T 2 , from the perspective of the receiving bus agent DSTBNB 0  is asserted halfway through the period when burst  4  is valid on the bus, thus enabling optimum conditions for reception of the burst  4 . 
         [0041]    The conditions in the first scenario  201 , although desirable, are not realistic. This is because at the high speeds corresponding to a present day synchronous data bus, even the propagation paths and corresponding loads within a receiving device affect the relative skew of each of the data bits D[15:0] and their corresponding strobe signals DSTBPB 0 , DSTBNB 0 . In prior art designs, data bit signals and strobe signals were routed using brute force techniques such that the signals and strobes incurred the least amount of propagation path delay and loading that was possible on a die. And because each bit was individually routed to its receiver, the phase difference between data bit and strobe signal varied from receiver to receiver. 
         [0042]    Because these individual propagation paths differ internal to a receiving device, designers often utilize a radial distribution scheme for the strobe where an equivalent propagation path (including loads and buffering) is applied to every distributed strobe signal. The result is that the phase lag between every data bit within the subgroup and their respective distributed strobe signal, as seen at a bit receiver, is approximately equal. Thus, radial distribution introduces phase lags into distributed strobe signals so that each of the receivers within a data group see the same amount of lag in their respective strobe signal relative to their corresponding data bit. Radial distribution schemes are very useful from a design standpoint because every data bit in a group sees the same phase lag for its corresponding strobe. However, the present inventors have observed that radial distribution limits the operating frequency of a device as a result of the lag that is introduced into the strobe signals. That is, setup times are much longer than hold times, which limits overall operating frequency. 
         [0043]    This case is what is depicted for in the second scenario  202  for D[15:0], which for purposes of illustrating an extreme case, renders its associated data bit receivers inoperable. That is, because DSTBPB 0  and DSTBNB 0  are distributed to data bit receivers for bits D[15:0] within the receiving bus agent according to a radial distribution scheme, the amount of lag introduced into the distributed strobes causes the distributed strobes to be asserted when the data bits D[15:0] are no longer valid. Clearly, this is undesirable. Consider that at time T 3 , from the perspective of the bit receivers, DSTBPB 0  is asserted when burst  5  is no longer valid on the bus, thus precluding any chance for reception of the burst  5 . Likewise note that at time T 4 , DSTBNB 0  is asserted when burst  8  is no longer valid on the bus, thus precluding any chance for reception of the burst  8 . 
         [0044]    In order to compensate for misalignment of a data bit and its corresponding data strobe, as noted above, various techniques are provided for in the art to introduce phase lag into data bits within a subgroup, or to accelerate assertion of data strobe signals, so that the signals (in the presence of radial strobe distribution) are optimally aligned. Yet, all of these mechanisms require experimentation, testing, circuitry external to a device, and/or programming of devices comprising a system on a motherboard. And the present inventors have noted that such experimentation, testing, circuitry, and/or programming is limiting in that each design must be uniquely configured to compensate for differences in the phase of a data strobe signal and its associated data bits, when the phase difference is chiefly due to radial distribution of the data strobe signal within a given receiving device. 
         [0045]    In addition, the present inventors note that although the length of any particular propagation path for a strobe signal may be known, even in the presence of a radial distribution scheme, the timing of this path (and the resultant phase lag) will dynamically change as a result of voltage, temperature, and fabrication process variations. Consequently, to introduce a specified amount of phase delay into data bits within a subgroup, as is presently provided for by the prior art, is a suboptimal compensation technique at best. 
         [0046]    The present invention overcomes the above noted limitations and disadvantages, and others, by providing a mechanism that automatically and dynamically aligns the phase of a data strobe and its associated data bit signals within a receiving device. The present invention dynamically adjusts the alignment of these signals as environmental factors (e.g., voltage, temperature, and process) change within a host device. The present invention will now be discussed with reference to  FIGS. 3-9 . 
         [0047]    Referring now to  FIG. 3 , a block diagram is presented featuring an apparatus  300  for automated local synchronous signals alignment according to the present invention. The apparatus  300  is preferably disposed within a receiving device (e.g., “bus agent”) that is coupled to a source synchronous bus, such as has been discussed above. In one embodiment, the receiving device comprises an x86-compatible microprocessor disposed as a die within an integrated circuit package that is physically coupled to a motherboard or system board. In another embodiment, the receiving device comprises an x86-compatible microprocessor configured as one or a plurality of x86-compatible microprocessors disposed on a single die within an integrated circuit package. One or more of the apparatuses  300  may be included within the receiving device to synchronize one or more data groups and their corresponding strobe signals, regardless of the type of data involved (e.g., data, address, or control). The apparatus  300  includes a radial distribution element  303  for a synchronous data strobe DSTROBE, as will be described below in further detail. The radial distribution element  303  equalizes all of the propagation paths (including loads and buffering) for DSTROBE as it is distributed. DSTROBE is received from a transmitting device (e.g., “bus agent”) (not shown) as is described above. 
         [0048]    The apparatus  300  may have a plurality of synchronous lag receivers  304  configured to receive one or more data bit signals DATA 1 -DATAN along with phase-aligned and load-matched strobe signals DSTROBE 1 -DSTROBN, which are derived from DSTROBE. A first one of the plurality of data signals DATA 1  enters the receiving device at a first point  311  and a first signal  312  is routed to a first synchronous receiver  304 . A last one of the plurality of data signals DATAN enters the device at a last point  3 N 1  and a last signal  3 N 2  is routed to its associated synchronous receiver  304 . The receivers  304  output respective received data signals OUT 1 -OUTN. 
         [0049]    The data strobe DSTROBE enters the device at point  301  where an internal strobe signal  302  is routed to a strobe receiver  313 , which receives the strobe signal  302 . The output of the strobe receiver  313  is coupled the radial distribution element  303 . The radial distribution element  303  includes a plurality of delay elements  303 . 1 - 303 .N, each associated with a corresponding one of the plurality of synchronous receivers  304 . Each of the plurality of delay elements  303 . 1 - 303 .N is configured to introduce a portion of a radial propagation path into the propagation path of DSTROBE as it is routed from the radial distribution element  303  to a corresponding receiver  304 . In one embodiment, the radial propagation path may comprise a worst-case path in terms of load, trace length, and buffering that is associated with one of a plurality of distributed strobe signals DSTROBE 1 -DSTROBEN. The portion of the radial propagation path corresponding to a particular receiver  304  introduces additional propagation length, load, and buffering beyond the length, load, and buffering associated with the corresponding strobe signal DSTROBE 1 -DSTROBEN such that the cumulative length, load, and buffering for that corresponding strobe signal DSTROBE 1 -DSTROBEN is equal to the radial propagation path described above. Thus, from the perspective of a particular receiver  304 , its corresponding data strobe signal DSTROBE 1 -DSTROBEN lags its corresponding data signal  321 - 3 N 2  in phase by the same amount as is seen by all other receivers  304  within a given data subgroup. 
         [0050]    The apparatus  300  also includes bit lag control  305  that receives the data strobe signal  302 , an update signal UPDATE, and one of the plurality of distributed data strobe signals DSTROBEN. In one embodiment, the bit lag control generates a 4-bit lag bus LAG[3:0] that indicates an amount of phase that the distributed strobe signals DSTROBE 1 -DSTROBEN lag behind the received data strobe signal DSTROBE. The lag bus LAG[3:0] is routed to each of the lag receivers  304  in the data subgroup. 
         [0051]    Operationally, when UPDATE is asserted, the bit lag control  305  measures the lag between assertion of DSTROBE and assertion of DSTROBEN when DSTROBE is received by the receiving device, and the lag is indicated by the value of LAG[3:0]. The receivers  304  may register the value of LAG[3:0] and introduce an equal amount of lag into their corresponding data signals  312 - 3 N 2  during a following data cycle when DSTROBE is asserted. Thus, the amount of phase lag in the distributed data strobe signals DSTROBE 1 -DSTROBEN is updated at each data cycle and this lag is employed for a following data cycle, where each of the receivers  304  will introduce this same amount of delay into reception of their corresponding data signal  312 - 3 N 2 , consequently centering assertion of the distributed data strobe signals DSTROBE 1 -DSTROBEN in a period when the data signals  312 - 3 N 2  are valid. Accordingly, the present invention delays each of the data signals  312 - 3 N 2  by an amount indicated by LAG[3:0] to provide for equal setup and hold times for each of the receivers  304 , thus allowing higher frequency bus transactions than have heretofore been provided for. 
         [0052]    A 4-bit lag bus LAG[3:0] is employed to provide an acceptable amount of resolution in the amount of lag delay, however higher or lower resolution may be achieved by increasing or decreasing the complexity of the bit lag control  305 , the number of bits on the lag bus LAG[3:0], and the complexity of the receivers  304  to introduce lag. 
         [0053]    Signal UPDATE may be deasserted for any number of well known reasons to include reset states, sleep states, power control, and the like. In one embodiment, when UPDATE is not asserted, the bit lag control  305  may not update the value of the lag bus LAG[3:0], and the former value is employed by the receivers  304  during all subsequent data cycles until UPDATE is again asserted. 
         [0054]    As one skilled in the art will appreciate, the worst-case propagation path (and the resulting lag) dynamically changes as a function of temperature, voltage, operating frequency, and fabrication process variation (die-to-die variation and also point-to-point location variation on a die). Advantageously, since the amount of lag measured by the bit lag control  305  is replicated by each of the receivers  304 , the value indicated by LAG[3:0] also dynamically adjusts as a function of the above noted attribute variations. 
         [0055]    The apparatus  300  according to the present invention is configured to perform the functions and operations as discussed above. The apparatus  300  comprises logic, circuits, devices, or microcode, or a combination of logic, circuits, devices, or microcode, or equivalent elements that are employed to execute the functions and operations according to the present invention as noted. The elements employed to accomplish these operations and functions within the apparatus  300  may be shared with other circuits, microcode, etc., that are employed to perform other functions and/or operations within the receiving device. 
         [0056]    The apparatus  300  provides a mechanism that directly measures the lag between a received strobe DSTROBE and its distributed strobe signals DSTROBE 1 -DSTROBEN, and thus provides a simple technique for compensating for radial strobe lag within a particular data subgroup. However, the present inventors have noted that alternative embodiments of the present invention may provide for a more timely dynamic adjustment of the lag by employing a replica radial distribution mechanism where the lag is measured offline. That is, according to the alternative embodiments, the lag may be measured and distributed to lag receivers asynchronous to when the synchronous bus is active. Accordingly, attention is now directed to  FIG. 4 , where a block diagram is presented showing an apparatus  400  for automated dynamic synchronous signals alignment according to the present invention. 
         [0057]    The apparatus  400  is preferably disposed within a receiving device that is coupled to a source synchronous bus, such as has been discussed above. In one embodiment, the receiving device comprises an x86-compatible microprocessor disposed as a die within an integrated circuit package that is physically coupled to a motherboard or system board. In another embodiment, the receiving device comprises an x86-compatible microprocessor configured as one or a plurality of x86-compatible microprocessors disposed on a single die within an integrated circuit package. One or more of the apparatuses  400  may be included within the receiving device to synchronize one or more data groups and their corresponding strobe signals, regardless of the type of data involved (e.g., data, address, or control). Like the apparatus  300  discussed with reference to  FIG. 3 , the apparatus  400  of  FIG. 4  includes a radial distribution element  403  for a synchronous data strobe DSTROBE, as will be described below in further detail. The radial distribution element  403  equalizes all of the propagation paths (including loads and buffering) for DSTROBE. DSTROBE is received from a transmitting bus agent (not shown) as described above. 
         [0058]    The apparatus  400  has a plurality of synchronous lag receivers  404  configured to receive one or more data bit signals DATA 1 -DATAN along with phase-aligned and load-matched strobe signals DSTROBE 1 -DSTROBN, which are derived from DSTROBE. A first one of the plurality of data signals DATA 1  enters the receiving device at a first point  411  and a first signal  412  is routed to a first synchronous receiver  404 . A last one of the plurality of data signals DATAN enters the device at a last point  4 N 1  and a last signal  4 N 2  is routed to its associated synchronous receiver  404 . The receivers  404  output respective received data signals OUT 1 -OUTN. 
         [0059]    The data strobe DSTROBE enters the device at point  401  where an internal strobe signal  402  is routed to a strobe receiver  413 , which receives the strobe signal  402 . The output of the strobe receiver  413  is coupled the radial distribution element  403 . The radial distribution element  403  includes a plurality of delay elements  403 . 1 - 403 .N, each associated with a corresponding one of the plurality of synchronous receivers  404 . Each of the plurality of delay elements  403 . 1 - 403 .N is configured to introduce a portion of a radial propagation path into the propagation path of DSTROBE as it is routed from the radial distribution element  403  to a corresponding receiver  404 . In one embodiment, the radial propagation path comprises a worst-case path in terms of load, trace length, and buffering that is associated with one of a plurality of distributed strobe signals DSTROBE 1 -DSTROBEN. The portion of the radial propagation path corresponding to a particular receiver  404  introduces additional propagation length, load, and buffering beyond the length, load, and buffering associated with the corresponding strobe signal DSTROBE 1 -DSTROBEN such that the cumulative length, load, and buffering for that corresponding strobe signal DSTROBE 1 -DSTROBEN is equal to the radial propagation path described above. Thus, from the perspective of a particular receiver  404 , its corresponding data strobe signal DSTROBE 1 -DSTROBEN lags its corresponding data signal  412 - 4 N 2  in phase by the same amount as all other is seen by all other receivers  404  within a given data subgroup. 
         [0060]    The apparatus  400  also includes a replica strobe receiver element (REPRCVR)  415 , that receives a lag pulse signal LAGPLS. In one embodiment, LAGPLS may be an internal clock signal. The replica strobe receiver element  415  is a matched replica of the strobe receiver  413 . The output of the replica receiver  415  is coupled to a replica radial distribution element  406  that is a replica of the radial distribution element  403 , including a matched circuit configuration, propagation path lengths, loads, and buffering. The replica radial distribution element  406  includes a plurality of delay elements  406 . 1 - 406 .N, each associated with a corresponding one of the plurality of synchronous receivers  404 . Each of the plurality of delay elements  406 . 1 - 406 .N is configured to introduce a portion of a radial propagation path into the propagation path of DSTROBE as it is routed from the radial distribution element  403  to a corresponding receiver  404 . In one embodiment, the radial propagation path comprises a worst-case path in terms of load, trace length, and buffering that is associated with one of a plurality of distributed strobe signals DSTROBE 1 -DSTROBEN. In another embodiment, the replica radial distribution element  406  may comprise only one delay element  406 .X, which replicates the worst-case path. One of the outputs REPS 1  of the replica radial distribution element  406  is coupled to a bit lag control element  405 , which generates an output lag bus LAG[3:0], and which is coupled to each of the receivers  404 . An update signal UPDATE and LAGPLS are coupled as well to the bit lag control  405 . In one embodiment, the bit lag control  405  generates a 4-bit lag bus LAG[3:0] that indicates an amount of phase that the output REPS 1  lags behind LAGPLS. Since the combination of elements  415  and  406  completely replicates the propagation path exhibited by the strobe receiver  413  and radial distribution element  403 , it is noted that the amount of phase lag indicated by LAG[3:0] represents the same phase lag that is exhibited by the strobe receiver  413  and the radial distribution element  403 , and thus is substantially equivalent to the amount of phase that the distributed strobes DSTROBE 1 -DSTROBEN lag behind DSTROBE. 
         [0061]    Operationally, when UPDATE is asserted, the bit lag control  405  measures the lag between assertion of LAGPLS and assertion of RESP 1 , and the lag is indicated by the value of LAG[3:0]. In one embodiment, LAGPLS is a continuous signal derived from a core processor clock signal (not shown). In one embodiment, UPDATE is asserted every 64 cycles of the core processor clock signal. Other embodiments are contemplated as well, with the express purpose of ensuring a timely update of LAG[3:0] without exhibiting a processing or power burden on remaining elements of a bus agent. The receivers  404  register the value of LAG[3:0] and introduce an equal amount of lag into their corresponding data signals  412 - 4 N 2  during a next data cycle when DSTROBE is asserted. Thus, the amount of phase lag in the distributed data strobe signals DSTROBE 1 -DSTROBEN is updated at each data cycle, as replicated by pulsing LAGPLS through the replica receiver  415  and distribution element  406 , and this lag is employed for a next data cycle and all data cycles occurring until the next periodic update of LAG[3:0], where each of the receivers  404  will introduce this same amount of delay into reception of their corresponding data signal  412 - 4 N 2 , consequently centering assertion of the distributed data strobe signals DSTROBE 1 -DSTROBEN in a period when the data signals  412 - 4 N 2  are valid. Accordingly, the present invention delays each of the data signals  412 - 4 N 2  by an amount indicated by LAG[3:0] to provide for equal setup and hold times for each of the receivers  404 , thus allowing higher frequency bus transactions than have heretofore been provided for. 
         [0062]    In contrast to the local alignment apparatus  300  of  FIG. 3 , the dynamic alignment apparatus  400  of  FIG. 4  does not depend upon assertion of DSTROBE in order to measure and indicate how much a distributed strobe DSTROBE 1 -DSTROBEN will lag behind the data strobe DSTROBE. 
         [0063]    The 4-bit lag bus LAG[3:0] is employed to provide an acceptable amount of resolution in the amount of lag delay, however higher or lower resolution may be achieved by increasing or decreasing the complexity of the bit lag control  405 , the number of bits on the lag bus LAG[3:0], and the complexity of the receivers  404 . 
         [0064]    Signal UPDATE may be deasserted for any number of well known reasons to include reset states, sleep states, power control, and the like. When UPDATE is not asserted, the bit lag control  405  does not update the value of the lag bus LAG[3:0], and the former value is employed by the receivers  404  during subsequent data cycles. 
         [0065]    The apparatus  400  according to the present invention is configured to perform the functions and operations as discussed above. The apparatus  400  comprises logic, circuits, devices, or microcode, or a combination of logic, circuits, devices, or microcode, or equivalent elements that are employed to execute the functions and operations according to the present invention as noted. The elements employed to accomplish these operations and functions within the apparatus  400  may be shared with other circuits, microcode, etc., that are employed to perform other functions and/or operations within the receiving device. 
         [0066]    Turning to  FIG. 5 , a block diagram is presented detailing one embodiment of a bit lag control element  500  according to the present invention. The bit lag control  500  may be employed in the embodiments of  FIGS. 3 and 4 . The bit lag control  500  includes a delay element  501  that is coupled to a mux  502 . The mux  502  is coupled to delay lock control  503  via signal SLAG. The delay lock control  503  generates a 4-bit lag select signal LAGSELECT[3:0] that is coupled to the mux  502  and to a gray encoder  504 . An update signal UPDATE is coupled to the gray encoder  504 , which generates a gray-encoded 4-bit lag signal LAG[3:0] indicating the number of matched inverter pairs U 1 A/B-U 15 A/B that a radially distributed pulse RESP 1  lags behind a lag clock pulse LAGCLK. 
         [0067]    The delay element  501  and the delay lock control  503  receive the lag clock LAGCLK. The delay lock control  503  also receives the distributed lag clock REPS 1 . In the embodiment of  FIG. 3 , LAGCLK is represented by signal DSTROBE and REPS 1  is represented by DSTROBEN. In the apparatus  400  of  FIG. 4 , LAGCLK is represented by LAGPLS and REPS 1  is represented by the like-named signal. The delay element  501  includes a plurality of inverter pairs U 1 A/B-U 15 A/B. A tap LC 0 -LC 15  is coupled to each of the pairs U 1 A/B-U 15 A/B, and the taps LC 0 -LC 15  are coupled to the register mux  502 . In the embodiment of  FIG. 5 , 15 inverter pairs U 1 A/B-U 15 A/B are depicted having matched inverters U 1 A/B-U 15 A/B each exhibiting a delay of 20 picoseconds per inverter U 1 A/B-U 15 A/B (40 picoseconds per inverter pair U 1 A/B-U 15 A/B, which is acceptable resolution for measuring phase lag in a receiving device operating at but speeds from approximately 500 Megahertz to 1.5 Gigahertz. Other embodiments are contemplated comprising different numbers of inverter pairs U 1 A/B-U 15 A/B as is appropriate with the application. An inverter pair U 1 A/B-U 15 A/B exhibiting a 40 picosecond delay is commensurate with receiving devices fabricated according to a 28-nanometer CMOS fabrication process and operating within the aforementioned frequency range. It is noted that the configuration shown in  FIG. 5  is presented to teach the present invention and that modifications can be made to provide accuracy and resolution under different fabrication processes and different operating frequencies. 
         [0068]    As noted above, the gray encoder  504  generates a gray-encoded bus LAG[3:0] that indicates the amount of time that RESP 1  lags in phase behind LAGCLK, which is the amount of time that it takes for a data strobe to propagate through a radial distribution network up to a data bit receiver according to the present invention. 
         [0069]    In operation, UPDATE enables or disables operation of the bit lag control  500 , as has been described above. When UPDATE is asserted, upon assertion of LAGCLK, successively delayed versions of LAGCLK are generated by the delay element  501  and are provided on taps LC 0 -LC 15  to the mux  502 . The delay lock control increments or decrements the value of LAGSELECT[3:0] in order to select one of the taps LC 0 -LC 15  on signal SLAG such that the value of SLAG is equal to RESP 1  subsequent to assertion of LAGCLK. Thus, the delay lock control  503  operates substantially similar to a delay lock loop in order to converge on a phase delay that is one inverter pair U 1 A/B-U 15 A/B less than the delay corresponding to one of the inverter pairs U 1 A/B-U 15 A/B. In one embodiment, to provide for stability of the bit lag control  500 , once a phase lag is locked in place, the delay lock control increments/decrements LAGSELECT[3:0] about the selected value such that changes of measured delay vary only by one bit. 
         [0070]    In one embodiment, measurement of the phase lag operates independently and asynchronously from assertion of the update signal UPDATE. When UPDATE is asserted, the gray-encoded value of LAGSELECT[3:0] is placed on bus LAG[3:0]. Accordingly, a 4-bit value of 0011 on LAGSELECT[3:0] may indicate that RESP 1  lags behind LAGCLK by 120 picoseconds under certain temperature, voltage, and frequency conditions. But since the present invention is configured to provide for automatic and dynamic measurement of phase lag and adjustment of the same timing in a data bit receiver, it is more precise to state that the above noted value of LAGSELECT[3:0] indicates that RESP 1  lags behind LAGCLK by three inverter pairs U 1 A/B-U 15 A/B. Since matched replicas of these inverter pairs U 1 A/B-U 15 A/B are present in every data bit receiver according to the present invention, this phase “delay” can be replicated at each of the data bit receivers to provide for optimum reception of data. 
         [0071]    The gray-encoded 4-bit lag bus LAG[3:0] is distributed to each of the data bit receivers that are associated with the radial distribution network being measured. Typically, these will comprise all of the data bit receivers in a particular data subgroup that each are activated by the same synchronous data strobe signal. In one embodiment, a different bit lag control  500  is employed for each different radial distribution network. In alternative embodiments, the gray encoder  504  may be deleted and the lag select bus LAGSELECT[3:0] is sent directly to the receivers. In such alternative embodiments, provisions must be made to accommodate glitches in LAGSELECT[3:0]. 
         [0072]    The apparatus  500  according to the present invention is configured to perform the functions and operations as discussed above. The apparatus  500  comprises logic, circuits, devices, or microcode, or a combination of logic, circuits, devices, or microcode, or equivalent elements that are employed to execute the functions and operations according to the present invention as noted. The elements employed to accomplish these operations and functions within the apparatus  500  may be shared with other circuits, microcode, etc., that are employed to perform other functions and/or operations within the receiving device. 
         [0073]    Now turning to  FIG. 6 , a block diagram is presented showing a fuse-adjustable bit lag control element  600  according to the present invention. The bit lag control element  600  is provided to enable the amount of delay indicated by a delay lock control element  603  via LAGSELECT[3:0] in such a manner as to provide compensation for lot variations, process variations, and other factors that may come to light during or following manufacture of a host device. The bit lag control  600  may be employed in the embodiments of  FIGS. 3 and 4 . The bit lag control  600  includes a delay element  601  that is coupled to a mux  602 . The mux  602  is coupled to delay lock control  603  via signal SLAG. The delay lock control  603  generates a 4-bit lag select signal LAGSELECT[3:0] that is coupled to the mux  602  and to adjust logic  606 . The adjust logic  606  is coupled to a gray encoder  604 . The adjust logic  606  is also coupled to an adjust value ADJVAL  605  via bus SUB[1:0]. An update signal UPDATE is coupled to the gray encoder  604 , which generates a gray-encoded 4-bit lag signal LAG[3:0] indicating the number of matched inverter pairs U 1 A/B-U 15 A/B that a radially distributed pulse RESP 1  lags behind a lag clock pulse LAGCLK, as adjusted by the value indicated on SUB[1:0]. 
         [0074]    The delay element  601  and the delay lock control  603  receive the lag clock LAGCLK. The delay lock control  603  also receives the distributed lag clock REPS 1 . In the embodiment of  FIG. 3 , LAGCLK is represented by signal DSTROBE and REPS 1  is represented by DSTROBEN. In the apparatus  400  of  FIG. 4 , LAGCLK is represented by LAGPLS and REPS 1  is represented by the like-named signal. The delay element  601  includes a plurality of inverter pairs U 1 A/B-U 15 A/B. A tap LC 0 -LC 15  is coupled to each of the pairs U 1 A/B-U 15 A/B, and the taps LC 0 -LC 15  are coupled to the register mux  602 . In the embodiment of  FIG. 6 , 15 inverter pairs U 1 A/B-U 15 A/B are depicted having matched inverters U 1 A/B-U 15 A/B each exhibiting a delay of 20 picoseconds per inverter U 1 A/B-U 15 A/B (40 picoseconds per inverter pair U 1 A/B-U 15 A/B, which is acceptable resolution for measuring phase lag in a receiving device operating at but speeds from approximately 500 Megahertz to 1.5 Gigahertz. Other embodiments are contemplated comprising different numbers of inverter pairs U 1 A/B-U 15 A/B as is appropriate with the application. 
         [0075]    The gray encoder  604  generates a gray-encoded bus LAG[3:0] that indicates the amount of time that RESP 1  lags in phase behind LAGCLK, as adjusted by the value of bus ALAG[3:0], which is an adjusted amount of time that it takes for a data strobe to propagate through a radial distribution network up to a data bit receiver according to the present invention. 
         [0076]    In operation, UPDATE enables or disables operation of the bit lag control  600 , as has been described above. When UPDATE is asserted, upon assertion of LAGCLK, successively delayed versions of LAGCLK are generated by the delay element  601  and are provided on taps LC 0 -LC 15  to the mux  602 . The delay lock control increments or decrements the value of LAGSELECT[3:0] in order to select one of the taps LC 0 -LC 15  on signal SLAG such that the value of SLAG is equal to RESP 1  subsequent to assertion of LAGCLK. Thus, the delay lock control  603  operates substantially similar to a delay lock loop in order to converge on a phase delay that is one inverter pair U 1 A/B-U 15 A/B less than the delay corresponding to one of the inverter pairs U 1 A/B-U 15 A/B. In one embodiment, to provide for stability of the bit lag control  600 , once a phase lag is locked in place, the delay lock control increments/decrements LAGSELECT[3:0] about the selected value such that changes of measured delay vary only by one bit. 
         [0077]    In operation, the adjust logic  606  that receives a compensation value over bus SUB[1:0] and performs a subtraction function, in one embodiment, from LAGSELECT[3:0]. The amount to be subtracted from LAGSELECT[3:0] is indicated by the value of signal SUB[1:0], which is received from the ADJVAL logic  605 . In one embodiment, SUB[1:0] indicates a number of bits to right shift the valued of LAGSELECT[3:0]. Then the right-shifted version of LAGSELECT[3:0] is subtracted from LAGSELECT[3:0] by the adjust logic  606  to produce an adjusted 4-bit vector ALAG[3:0]. In one embodiment, the number of bits to right shift LAGSELECT[3:0] is as shown below in Table 1. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Adjustment Values for 4-Bit Select Vector 
               
             
          
           
               
                   
                 NUMBER OF BITS  
               
               
                 SUB[1:0] VALUE 
                 TO RIGHT SHIFT 
               
               
                   
               
               
                 00 
                 0 BITS 
               
               
                 01 
                 1 BIT 
               
               
                 10 
                 2 BITS 
               
               
                 11 
                 3 BITS 
               
               
                   
               
             
          
         
       
     
         [0078]    In one embodiment, the ADJVAL logic  605  comprises one or more metal or poly fuses which are blown during fabrication of the device or IC. An alternative embodiment contemplates the ADJVAL logic circuit  606  as programmable, read-only memory located on the device or IC. A further alternative embodiment comprehends ADJVAL logic  605  that is located off the device or IC and that provides SUB[1:0] as signals to I/O pins (not shown) on the device or IC. Other embodiments of the ADJVAL logic  605  are contemplated as well, to include, but not limited to, a number of signals of bus SUB which are more or less than two signals. By providing the ADJVAL logic circuit  605  and the adjust logic circuit  606 , a designer is allowed to tweak the amount of delay indicated by the delay lock control  603  via LAGSELECT[3:0] in such a manner as to provide compensation for lot variations, process variations, and other factors that may come to light during or following manufacture of the IC. The adjust logic  606  thus generates an adjusted 4-bit select vector ALAG[3:0] by subtracting a right-shifted value of LAGSELECT[3:0] from LAGSELECT[3:0] as indicated by SUB[1:0]. 
         [0079]    In one embodiment, measurement of the phase lag operates independently and asynchronously from assertion of the update signal UPDATE. When UPDATE is asserted, the gray-encoded value of ALAG[3:0] is placed on bus LAG[3:0]. Accordingly, a 4-bit value of 0011 on LAGSELECT[3:0] may indicate that RESP 1  lags behind LAGCLK by 120 picoseconds under certain temperature, voltage, and frequency conditions. But since the present invention is configured to provide for automatic and dynamic measurement of phase lag and adjustment of the same timing in a data bit receiver, it is more precise to state that the above noted value of LAGSELECT[3:0] indicates that RESP 1  lags behind LAGCLK by three inverter pairs U 1 A/B-U 15 A/B. Since matched replicas of these inverter pairs U 1 A/B-U 15 A/B are present in every data bit receiver according to the present invention, this phase “delay” can be replicated at each of the data bit receivers to provide for optimum reception of data. A value of 01 on SUB[1:0] indicates to the adjust logic  606  to right shift the value of LAGSELECT[3:0] by one bit and subtract this right shifted value (i.e., 0001) from the true value of LAGSELECT[3:0] (i.e., 0011), yielding a value of LAG[3:0] of 0010, which indicates that RESP 1  lags behind LAGCLK by only 80 picoseconds, as opposed to the 120-picosecond lag indicated by LAGSELECT[3:0]. 
         [0080]    The gray-encoded 4-bit lag bus LAG[3:0] is distributed to each of the data bit receivers that are associated with the radial distribution network being measured. Typically, these will comprise all of the data bit receivers in a particular data subgroup that each are activated by the same synchronous data strobe signal. In one embodiment, a different bit lag control  600  is employed for each different radial distribution network. In alternative embodiments, the gray encoder  604  may be deleted and the adjusted lag select bus ALAG[3:0] is sent directly to the receivers. In such alternative embodiments, provisions must be made to accommodate glitches in LAGSELECT[3:0]. 
         [0081]    The apparatus  600  according to the present invention is configured to perform the functions and operations as discussed above. The apparatus  600  comprises logic, circuits, devices, or microcode, or a combination of logic, circuits, devices, or microcode, or equivalent elements that are employed to execute the functions and operations according to the present invention as noted. The elements employed to accomplish these operations and functions within the apparatus  600  may be shared with other circuits, microcode, etc., that are employed to perform other functions and/or operations within the receiving device. 
         [0082]    Now turning to  FIG. 7 , a block diagram is presented showing a JTAG-adjustable bit lag control element  700  according to the present invention. The bit lag control element  700  is provided to enable the amount of delay indicated by a delay lock control element  703  via LAGSELECT[3:0] in such a manner as to provide compensation for lot variations, process variations, and other factors that may come to light during or following manufacture of a host device. The bit lag control  700  may be employed in the embodiments of  FIGS. 3 and 4 . The bit lag control  700  includes a delay element  701  that is coupled to a mux  702 . The mux  702  is coupled to delay lock control  703  via signal SLAG. The delay lock control  703  generates a 4-bit lag select signal LAGSELECT[3:0] that is coupled to the mux  702  and to adjust logic  706 . The adjust logic  706  is coupled to a gray encoder  704 . The adjust logic  706  is also coupled to a Joint Test Action Group (JTAG) interface  705  via bus SUB[1:0]. The JTAG interface  705  receives control information over a standard JTAG bus JTAG[N:0] that provides information applicable for the adjustment of the delay determined by the delay lock control  703 . An update signal UPDATE is coupled to the gray encoder  704 , which generates a gray-encoded 4-bit lag signal LAG[3:0] indicating the number of matched inverter pairs U 1 A/B-U 15 A/B that a radially distributed pulse RESP 1  lags behind a lag clock pulse LAGCLK, as adjusted by the value indicated on SUB[1:0]. 
         [0083]    The delay element  701  and the delay lock control  703  receive the lag clock LAGCLK. The delay lock control  703  also receives the distributed lag clock REPS 1 . In the embodiment of  FIG. 3 , LAGCLK is represented by signal DSTROBE and REPS 1  is represented by DSTROBEN. In the apparatus  400  of  FIG. 4 , LAGCLK is represented by LAGPLS and REPS 1  is represented by the like-named signal. The delay element  701  includes a plurality of inverter pairs U 1 A/B-U 15 A/B. A tap LC 0 -LC 15  is coupled to each of the pairs U 1 A/B-U 15 A/B, and the taps LC 0 -LC 15  are coupled to the register mux  702 . In the embodiment of  FIG. 7 , 15 inverter pairs U 1 A/B-U 15 A/B are depicted having matched inverters U 1 A/B-U 15 A/B each exhibiting a delay of 20 picoseconds per inverter U 1 A/B-U 15 A/B (40 picoseconds per inverter pair U 1 A/B-U 15 A/B, which is acceptable resolution for measuring phase lag in a receiving device operating at but speeds from approximately 500 Megahertz to 1.5 Gigahertz. Other embodiments are contemplated comprising different numbers of inverter pairs U 1 A/B-U 15 A/B as is appropriate with the application. 
         [0084]    The gray encoder  704  generates a gray-encoded bus LAG[3:0] that indicates the amount of time that RESP 1  lags in phase behind LAGCLK, as adjusted by the value of bus ALAG[3:0], which is an adjusted amount of time that it takes for a data strobe to propagate through a radial distribution network up to a data bit receiver according to the present invention. 
         [0085]    In operation, UPDATE enables or disables operation of the bit lag control  700 , as has been described above. When UPDATE is asserted, upon assertion of LAGCLK, successively delayed versions of LAGCLK are generated by the delay element  701  and are provided on taps LC 0 -LC 15  to the mux  702 . The delay lock control increments or decrements the value of LAGSELECT[3:0] in order to select one of the taps LC 0 -LC 15  on signal SLAG such that the value of SLAG is equal to RESP 1  subsequent to assertion of LAGCLK. Thus, the delay lock control  703  operates substantially similar to a delay lock loop in order to converge on a phase delay that is one inverter pair U 1 A/B-U 15 A/B less than the delay corresponding to one of the inverter pairs U 1 A/B-U 15 A/B. In one embodiment, to provide for stability of the bit lag control  700 , once a phase lag is locked in place, the delay lock control increments/decrements LAGSELECT[3:0] about the selected value such that changes of measured delay vary only by one bit. 
         [0086]    In operation, well-known JTAG programming techniques are employed to program the precise amount of compensation that is indicated over SUB[1:0]. Such programming is performed when a host device is in a state where JTAG programming is allowed, such as a RESET state. Upon exit from the state, bus SUB[1:0] indicates a compensation value. As with the embodiment  700  of  FIG. 7 , the adjust logic  706  that receives the compensation value over bus SUB[1:0] and performs a subtraction function, in one embodiment, from LAGSELECT[3:0]. The amount to be subtracted from LAGSELECT[3:0] is indicated by the value of signal SUB[1:0]. In one embodiment, SUB[1:0] indicates a number of bits to right shift the valued of LAGSELECT[3:0]. Then the right-shifted version of LAGSELECT[3:0] is subtracted from LAGSELECT[3:0] by the adjust logic  706  to produce an adjusted 4-bit vector ALAG[3:0]. In one embodiment, the number of bits to right shift LAGSELECT[3:0] is as shown below in Table 2. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Adjustment Values for 4-Bit Select Vector 
               
             
          
           
               
                   
                 SUB[1:0] VALUE 
                 NUMBER OF BITS TO RIGHT SHIFT 
               
               
                   
                   
               
               
                   
                 00 
                 0 BITS 
               
               
                   
                 01 
                 1 BIT   
               
               
                   
                 10 
                 2 BITS 
               
               
                   
                 11 
                 3 BITS 
               
               
                   
                   
               
             
          
         
       
     
         [0087]    Other embodiments of the JTAG interface  705  are contemplated, including, but not limited to, a number of signals of bus SUB which are more or less than two signals. By providing the JTAG interface  707  and the adjust logic circuit  706 , a designer is allowed to tweak the amount of delay indicated by the delay lock control  703  via LAGSELECT[3:0] in such a manner as to provide compensation for lot variations, process variations, and other factors that may come to light during or following manufacture of the IC. The adjust logic  706  thus generates an adjusted 4-bit select vector ALAG[3:0] by subtracting a right-shifted value of LAGSELECT[3:0] from LAGSELECT[3:0] as indicated by SUB[1:0]. 
         [0088]    In one embodiment, measurement of the phase lag operates independently and asynchronously from assertion of the update signal UPDATE. When UPDATE is asserted, the gray-encoded value of ALAG[3:0] is placed on bus LAG[3:0]. Accordingly, a 4-bit value of 0011 on LAGSELECT[3:0] may indicate that RESP 1  lags behind LAGCLK by 120 picoseconds under certain temperature, voltage, and frequency conditions. But since the present invention is configured to provide for automatic and dynamic measurement of phase lag and adjustment of the same timing in a data bit receiver, it is more precise to state that the above noted value of LAGSELECT[3:0] indicates that RESP 1  lags behind LAGCLK by three inverter pairs U 1 A/B-U 15 A/B. Since matched replicas of these inverter pairs U 1 A/B-U 15 A/B are present in every data bit receiver according to the present invention, this phase “delay” can be replicated at each of the data bit receivers to provide for optimum reception of data. A value of 01 on SUB[1:0] indicates to the adjust logic  706  to right shift the value of LAGSELECT[3:0] by one bit and subtract this right shifted value (i.e., 0001) from the true value of LAGSELECT[3:0] (i.e., 0011), yielding a value of LAG[3:0] of 0010, which indicates that RESP 1  lags behind LAGCLK by only 80 picoseconds, as opposed to the 120-picosecond lag indicated by LAGSELECT[3:0]. 
         [0089]    The gray-encoded 4-bit lag bus LAG[3:0] is distributed to each of the data bit receivers that are associated with the radial distribution network being measured. Typically, these will comprise all of the data bit receivers in a particular data subgroup that each are activated by the same synchronous data strobe signal. In one embodiment, a different bit lag control  700  is employed for each different radial distribution network. In alternative embodiments, the gray encoder  704  may be deleted and the adjusted lag select bus ALAG[3:0] is sent directly to the receivers. 
         [0090]    The apparatus  700  according to the present invention is configured to perform the functions and operations as discussed above. The apparatus  700  comprises logic, circuits, devices, or microcode, or a combination of logic, circuits, devices, or microcode, or equivalent elements that are employed to execute the functions and operations according to the present invention as noted. The elements employed to accomplish these operations and functions within the apparatus  700  may be shared with other circuits, microcode, etc., that are employed to perform other functions and/or operations within the receiving device. 
         [0091]    Referring now to  FIG. 8 , a block diagram is presented depicting a synchronous lag receiver  800  according to the present invention. The receiver  800  may be employed in the embodiments of  FIGS. 3-4  and functions to introduce a delay into the propagation path of a data bit DATAX that is received from a transmitting device, where the delay is indicated by the value of a lag bus LAG[3:0] that is updated by a bit lag control element according to the present invention, such as is described above with reference to  FIGS. 3-8 . 
         [0092]    The receiver  800  includes a delay element  801  that receives the data bit DATAX. The delay element  801  is coupled to a mux  802  via a delayed data bit bus DDATAX[15:0]. The lag bus LAG[3:0] is coupled to the mux  802 . The mux  802  is coupled to a synchronous bit receiver  804  via a selected delayed data signal SDATAX. The bit receiver  804  receives SDATAX and a data strobe DSTROBEX. DSTROBEX is distributed from a radial distribution element  303 ,  403 , such as is discussed above with reference to  FIGS. 3-4 . The bit receiver  804  generates a received data bit signal RDATAX. 
         [0093]    Operationally, a bit lag controller according to the present invention updates the value of LAG[3:0] to position reception of DATAX optimally in relation to the phase of DSTROBEX. In one embodiment, this positioning is such that DSTROBEX switches approximately halfway during assertion of DATAX. Other embodiments are contemplated that enable positioning of DATAX to favor increased setup time or increased hold time for DATAX. The delay element  801  is a replica of the delay elements  501 ,  601 ,  701 ,  801  described with reference to  FIGS. 1-8 , and comprises 15 matched inverter pairs (not shown). Thus, in one embodiment, DDATAX[15:0] comprises 16 successively delayed versions of DATAX, ranging from no delay to delay through all 15 inverter pairs. 
         [0094]    The value of LAG[3:0] is employed by the mux  802  to select one of the signals on DDATAX[15:0]. The selected signal is routed to the bit receiver  804  on SDATAX. When DSTROBEX switches, the bit receiver  804  registers the value of SDATAX and outputs this value on RDATAX. RDATAX represents the received state of DATAX. 
         [0095]    Turning now to  FIG. 9 , a block diagram is presented detailing a precision delay element  900  according to the present invention. The precision delay element  900  may be substituted for any of the delay elements  501 ,  601 ,  701 ,  801  discussed above with reference to  FIGS. 5-8 , and is employed to provide both finer resolution of lag measurement and lag introduction in embodiments of the present invention. The delay element  900  includes a first mux  901  having a first input tied to a logic low level (i.e., “0”) and a second input tied to a logic high level (i.e., “1”). In one embodiment, the high level comprises a core voltage (i.e., VDD) and the low level comprises a reference voltage (i.e., ground). Other embodiments are contemplated. The first mux  901  employs a lag clock LAGCLK as a select input to select either the signal on the first input or the second input. The element  900  also includes a second mux  902  having a first input tied to a 1 and a second input tied to a 0, which is the opposite configuration from that of the first mux  901 . LAGCLK is also coupled to the select input of the second mux  902 . In the embodiments of  FIGS. 5-7 , LAGCLK represents a signal for measurement of propagation delay as the like-named signals. In the embodiment of  FIG. 8 , LAGCLK represents the data bit DATAX to be delayed. 
         [0096]    The delay element  900  includes a first group of 15 delay inverters, U 0 A-U 14 A, coupled in series cascade configuration, where the output of the first mux  901  is coupled to the input of U 0 A and the output of U 14 A is coupled to a most delayed signal LC 31 . The delay element  900  also includes a second group of 15 delay inverters, U 0 B-U 14 B, coupled in series cascade configuration, where the output of the second mux  902  is coupled to the input of U 0 B and the input of U 14 B is coupled to a next most delayed signal LC 30 . 
         [0097]    The outputs of all like numbered delay inverters (e.g., U 0 A and U 0 B, U 5 A and U 5 B) are coupled together via full keeper inverter pairs K 1 -K 15 . The outputs of even numbered inverters from the first group of 15 delay inverters (i.e., U 0 A, U 2 A, etc.) are coupled to odd numbered successively delayed signals (i.e., LC 1 , LC 3 , . . . , LC 31 ) and the inputs of even numbered inverters from the second group of 15 delay inverters (i.e., U 0 B, U 2 B, etc.) are coupled to even numbered successively delayed signals (i.e., LC 0 , LC 2 , . . . , LC 30 ). Each of the delay inverters U 0 A-U 14 A, U 0 B-U 14 B are matched. In one embodiment, the delay through each inverter is substantially 20 picoseconds and thus the most delayed signal LC 31  represents a delay in LAGCLK of approximately 300 picoseconds. 
         [0098]    In operation, either state of LAGCLK may be employed to generate the successively delayed versions that are output on LC 0 -LC 31 , although a high level will be used in this operational discussion. Accordingly, in one embodiment, when LAGCLK is 1, then the input to U 0 A is 0 and the input to U 0 B is 1. Thus, LC 0  is a 1, the output of U 0 A is 1, the output of U 0 B is a 0, and the value of LC 1  is a 1 after a delay of one inverter. And so on until the most delayed version of LAGCLK is presented on LC 31 . Keepers K 1 -K 15  function to ensure that state changes on LC 1 -LC 31  are synchronized with regard to state changes of their corresponding like numbered inverter pair U 0 [A:B]-U 14 [A:B]. 
         [0099]    The precision delay element  900  according to the present invention may be employed by any of the muxes  502 ,  602 ,  702 ,  802 ,  902  described above. However, the width of corresponding lag busses must be increased by one bit to accommodate the increased resolution provided. 
         [0100]    Portions of the present invention and corresponding detailed description are presented in terms of software, or algorithms and symbolic representations of operations on data bits within a computer memory. These descriptions and representations are the ones by which those of ordinary skill in the art effectively convey the substance of their work to others of ordinary skill in the art. An algorithm, as the term is used here, and as it is used generally, is conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of optical, electrical, or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
         [0101]    It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise, or as is apparent from the discussion, terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, a microprocessor, a central processing unit, or similar electronic computing device, that manipulates and transforms data represented as physical, electronic quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
         [0102]    Note also that the software implemented aspects of the invention are typically encoded on some form of program storage medium or implemented over some type of transmission medium. The program storage medium may be electronic (e.g., read only memory, flash read only memory, electrically programmable read only memory), random access memory magnetic (e.g., a floppy disk or a hard drive) or optical (e.g., a compact disk read only memory, or “CD ROM”), and may be read only or random access. Similarly, the transmission medium may be metal traces, twisted wire pairs, coaxial cable, optical fiber, or some other suitable transmission medium known to the art. The invention is not limited by these aspects of any given implementation. 
         [0103]    The particular embodiments disclosed above are illustrative only, and those skilled in the art will appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention, and that various changes, substitutions and alterations can be made herein without departing from the scope of the invention as set forth by the appended claims.