Abstract:
A method of manufacturing a system on a chip and a system on a chip including a set of pre-designed modules. These modules are place on a semiconductor and connecting by a set of busses formed according to a set of design rules specifying tracks having a minimum size of conductors and a minimum spacing between conductors. The busses are routed in a preferred direction. The busses include minimum size conductors at alternate tracks within a selected metal layer of the semiconductor and minimum size conductors at alternate tracks in a different metal layer. The conductors in the different metal layer are connected to corresponding connectors in the selected metal layer by vias. Shields of conductors not connected to the bus may be included in tracks not including bus conductors.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. 119(e)(1) to U.S. Provisional Application No. 61/702,035 filed Sep. 17, 2012. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The technical field of this invention is integrated circuit manufacture of connections between pre-designed parts. 
       BACKGROUND OF THE INVENTION 
       [0003]    This invention is directed to module interconnections in system on a chip (SOC) integrated circuits. The advent of increased density integrated circuit manufacture has enabled all or all crucial parts of an end user system to be constructed on a signal integrated circuit. Such integrated circuits are called system on a chip (SOC). 
         [0004]    Typically as SOC is constructed using as assortment of previously designed subassemblies. A typical SOC design begins with a system definition. This determines the desired functionality of the SOC. The SOC designer then selects a set of already designed modules to form the desired system. This set of modules may include modules previously designed by the SOC designer or available to the SOC manufacture or modules designed by outside parties available for license to use in the SOC. In some instances a needed module is newly designed or acquired. Part of the selection process includes selecting the set of modules having an aggregate construction size within the budget of the SOC. Integrated circuits are manufactured in slices of silicon of predetermined size. The manufacturing cost of a slice is relatively constant regardless of the number of integrated circuit dies included. Thus more and smaller integrated circuit dies having less electrical circuitry can be fitted on a slice than larger integrated circuit dies. Accordingly, the manufacturing cost of integrated circuits is directly related to its die size. This is in turn related to its electrical complexity. The SOC designer often trades functionality based on electrical complexity with aggregate module size and the consequent cost of manufacture. 
         [0005]    The SOC designer then assembles these modules on a single integrated circuit to be manufactured. This process includes forming connections between the modules. These connections often include busses of plural lines in parallel for data and addresses. Determination of the connections between modules in a SOC is often nontrivial. The size of such connections often adversely increases the SOC die size. This typically increases the cost of the SOC. There are often timing issues between modules that must be dealt with in the connections. Busses often involve adverse cross talk between connections or between busses. 
       SUMMARY OF THE INVENTION 
       [0006]    This invention permits a very high occupancy buffered bus channel which is crosstalk de-sensitized. This invention thus enables tighter channel implementation in SOCs thereby reducing die area. This invention enhances the predictability of channel timing closure, especially for skew sensitive buses. This improves top level timing closure cycle time. This invention enables isolation between buses thus localizing the timing/crosstalk impact to the relevant bus. This allows clean, predictable passage of timing critical buses and lesser criticality interfaces through the same channel without interference. This invention enables early prediction of timing criticality in point-to-point connected interfaces in SOCs. 
         [0007]    This invention creates a custom-implemented buffered channel using the following key techniques: non-default direction routing; three dimensional inter layer wire spacing rules and shielding; and on route buffering for predictable buffering span. 
         [0008]    This invention uses three dimensional wire spacing and custom shielding. According to this invention criticality dependent lane re-assignment is made with minor re-routing. This invention uses non-default direction routing for bus wires. This invention adapts based on bus timing constraints. Because it is structured and predictable, this invention enables timing or functionality aware neighbor signal wire or shield selection for critical signals. 
         [0009]    This invention has the following advantages over the prior art. This invention provides bus throughput improvement using non-default direction routing. This invention enables much higher efficiency than conventional place and route tools. This invention enables predictable interface timing closure. This invention is a robust bus routing implementation which reduces the number of vias and jogs. This invention provides less variation. This invention reduces cross talk impact on bus routing. This invention eases source synchronous bus timing closure. This invention enables optimized routing resource utilization. This invention enables early I/O interface timing closure. I/O timing feasibility is known at the floorplan stage according to this invention. This invention reduces project turn around time by identifying interface closure issues at the floorplan stage. This invention permits the alternate of closing SOC level inter-block communication timings very early in the flow. This invention provides uniform bus buffering reducing bus variation with reduced AOCV and reduced SKEW. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    These and other aspects of this invention are illustrated in the drawings, in which: 
           [0011]      FIG. 1  illustrates the process of this invention; 
           [0012]      FIG. 2  illustrates an example system-on-chip (SOC)  200  including modules connected by respective buses; 
           [0013]      FIGS. 3A and 3B  illustrate the results of a prior art first pass bus routing routine,  FIG. 3A  illustrates a top view of an example bus and  FIG. 3B  illustrates a corresponding cross section view; 
           [0014]      FIGS. 4A and 4B  illustrate a prior art design response to excessive cross-talk between conductors,  FIG. 4A  illustrates a top view of an example bus and  FIG. 4B  illustrates a corresponding cross section view; 
           [0015]      FIGS. 5A and 5B  illustrate another prior art design response to excessive cross-talk between conductors,  FIG. 5A  illustrates a top view of an example bus and  FIG. 5B  illustrates a corresponding cross section view; 
           [0016]      FIGS. 6A and 6B  illustrate the inventive response to excessive cross-talk between conductors,  FIG. 6A  illustrates a top view of an example bus and  FIG. 6B  illustrates a corresponding cross section view; 
           [0017]      FIG. 7  illustrates the key showing the track types illustrated in  FIGS. 8 to 16  including wire carrying tracks, empty tracks and shield carrying tracks; 
           [0018]      FIG. 8  illustrates a cross section view of a prior art disposition of conductors within a bus on a SOC; 
           [0019]      FIG. 9  illustrates a cross section view of application of this invention to the problem of the prior art of  FIG. 8 ; 
           [0020]      FIG. 10  illustrates a cross section view of another example of a prior art disposition of conductors within a bus on a SOC; 
           [0021]      FIG. 11  illustrates a cross section view of a first embodiment of this invention modifying the prior art disposition of conductors of  FIG. 10 ; 
           [0022]      FIG. 12  illustrates a cross section view of a second embodiment of this invention modifying the prior art disposition of conductors of  FIG. 10 ; 
           [0023]      FIG. 13  illustrates a cross sectional view of another example prior art disposition of conductors; 
           [0024]      FIG. 14  illustrates a cross section view of an embodiment of this invention modifying the prior art disposition of conductors of  FIG. 13 ; 
           [0025]      FIG. 15  illustrates a cross sectional view of another example prior art disposition of conductors; 
           [0026]      FIG. 16  illustrates a cross section view of an embodiment of this invention modifying the prior art disposition of conductors of  FIG. 15 ; 
           [0027]      FIG. 17  illustrates the key showing the track types illustrated in  FIGS. 18 and 19  including wire carrying tracks, staggered tracks, empty tracks and shield carrying tracks; 
           [0028]      FIG. 18  illustrates a cross sectional view of another example prior art disposition of conductors; 
           [0029]      FIG. 19  illustrates a cross section view of an embodiment of this invention modifying the prior art disposition of conductors of  FIG. 18 ; 
           [0030]      FIG. 20  illustrates the delay versus channel number for the two topologies of bus construction; 
           [0031]      FIG. 21  illustrates in schematic form the prior art aligned bus buffering; and 
           [0032]      FIG. 22  illustrates in schematic form the inventive staggered buffering. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    This invention includes the concept of buffered conduits as pre-implemented buffered bus blocks in the SOC. This invention eliminates all through-fare channels. 
         [0034]    In SOCs long, buffered channels for functional buses are fairly common. Such channels normally run long distances in one orientation: North-South; or East-West. Typical prior art place and route solutions honor a preferred direction routing in a strict way while not using available orthogonal routing resources in an efficient manner. This invention includes using high packing density for channel routes by exploiting all available routing resources 
         [0035]    This invention produces improved channel efficiency. In the conventional flows, channels are sized based on preferred direction routing resources. Conventional place and route rules use alternate orthogonal preferred routing layers, such as: Metal 1 Vertical; Metal 2 Horizontal; and Metal 3 Vertical. Thus a vertical channel implementation using conventional place and route flows may be only using 50% of the available routing tracks. Channel sizing is usually done as follows: the number of Signals to route is R; the Router Efficiency Loss is E, thus if the route was utilizing 90% of the resources, E would be 0.9; the Xtalk based extra spacing is X, where extra spacing per signal will make X=2.0 and no extra spacing will make X=1.0; the number of preferred direction usable routing layers is N. Thus the channel size is given by: 
         [0000]      Channel size=( R*X )/( E*N ) 
         [0000]    The invention is advantageous as follows. The invention increases the number of available routing layers to 2*N. 
       Case #1: 
       [0036]    Let the Original Channel Size be: 
         [0000]      ( R*X )/( E*N ) 
         [0000]    The Channel size using this invention is: 
         [0000]      ( R*X )/( E 1 *N* 2) 
         [0000]    where: E1 is the efficiency of the custom implementation, especting that E1&gt;E. 
       Case #2 
       [0037]    Let the Original Channel Size be: 
         [0000]      ( R*X )/( E*N ) 
         [0000]    where: E&lt;1. Thus X&gt;1.0, we assume X=2.0. Conventional flows don&#39;t work with design understanding. Crosstalk impact is actually a function of crosstalk coupling as well as the simultaneous activity of the aggressor and victim nets. If there is a scenario where multiple groups of signals passing through a channel are not simultaneously active, the structured implementation proposed by this invention would enable neighbor selection to avoid simultaneous activity. This would enable avoiding the additional crosstalk derived spacing X. For X=2.0 and E=0.9, the channel size is: 
         [0000]      ( R* 2.0)/(0.9 *N ). 
         [0000]    Using the invention and assuming no improvement in the router efficiency, the channel size is: 
         [0000]      ( R* 1.0)/(0.9*2 *N ). 
         [0000]    This is a very large improvement. 
         [0038]    The prior art trades significant silicon area for crosstalk and wide bus routing inefficiencies. The prior art automated wire-spacing implementations are local in their approach and do not equalize white space over larger windows. This invention enables criticality dependent crosstalk desensitization optimization at potentially zero extra cost versus the default highest density routing. 
         [0039]    This invention produces improved bus isolation. Channels have through-fare of multiple types of buses. Prior art solutions do not ensure isolation. Source synchronous buses are skew sensitive. Regular synchronous buses have a pure delay sensitivity. This invention enables bus isolation by controlling channel parameters. According to this invention skew sensitive buses get delay-matched lanes. According to this invention slow/pseudo-static buses are power optimized. According to this invention span-delay sensitive buses get fast lanes. None of these influences timing closure on each other because they are isolated. 
         [0040]    This invention provides predictability in timing closure cycle time. In the prior art timing closure remains open until the very last moment when all mode/corner static timing analysis (STA) is performed. This invention enables a pre-fabricated bus. Thus the timing closure is performed at the floorplanning stage. This is very early in the design flow. Late timing fixes can be handled in this invention by trivial lane re-assignments. 
         [0041]    The prior art used control of EDA tools space, jog, swizzle routes to reduce cross talk impact. Generally this adds excessive vias and detours. The datapath RC-spreads are quite uncontrolled in the prior art and the timing response across corners can be very poor especially for skew-sensitive buses. This invention strictly controls RC spread, cross corner timing responses and the like. Thus this invention is more robust than the prior art. 
         [0042]      FIG. 1  illustrates process  100  of this invention. Process  100  begins with start block  101 . Process  100  next executes preferred direction routing step  102 . Preferred direction routing step  102  routes required busses among plural modules of a SoC in a preferred direction. This could be either horizontal or vertical. The preferred direction routing depends upon the relative placement of the two connected modules on the SOC. This preferred direction routing includes minimal required spacing between wires according to a custom script with minimal jogging. 
         [0043]    Process  100  next executes wire pushdown step  103 . Wire pushdown step  103  pushes down nets in alternative track to a lower metal. This routes in the non-preferred direction. 
         [0044]    Process  100  next executes shield insertion step  104 . Shield insertion step  104  adds shield wires to reduce cross talk. 
         [0045]    Process  100  next executes on route buffering step  105 . On route buffering step  105  inserts staggered buffers in-line the routing of the bus using custom scripts. This process includes selection of inverting or non-inverting buffers. This invention may include staggered buffers (described below). 
         [0046]    Process  100  next executes wire widening step  106 . Wire widening step  106  optimizes wire width for additional performance of the bus. 
         [0047]    Process  100  next executes buffer sizing step  107 . Buffer sizing step  107  selects the buffer size of the various buffers to account for loading mismatches. These loading mismatches could occur from top/bottom layers, difference the number of vias in the routes and other factors. Process  100  ends with end block  108 . 
         [0048]    This invention has the following advantages. This invention provides logical decoupling. According to this invention different interfaces use different buffered channels alleviating cross talk impact. This invention provides physical decoupling of multiple interface uses. This invention provides high throughtput through the use of non-default routing with different configurations of 3D-wire space. This invention provides floorplan level interface timing budgeting and closure. According to this invention the conduits are pre-implemented as modules. Thus the propagation delay and skew across lanes are guaranteed by the module design. This invention provides improved dynamic current-resistance (IR) voltage drop performance. Because the buffering is custom implemented, appropriate decap insertion is possible to reduce the local dynamic IR effects. Each conduit is dynamic IR drop de-sensitized therefore the SOC doesn&#39;t see a problem. This invention provides timing window correlation. This invention enables logical correlation based crosstalk minimization in addition to the shielding/spacing options. This invention maximizes utilization of resources. This invention maximizes the available routing resources enabling die area optimization. 
         [0049]    This invention includes pre-built buffered channels. This invention implements channels as pre-built buffered, timing optimized hard blocks that are instantiated at the SOC level.  FIG. 2  illustrates an example of this technique. 
         [0050]      FIG. 2  illustrates SOC  200  including modules  201 ,  202 ,  203 ,  204 ,  205  and  206 . As illustrated in  FIG. 2  bus  211  connects modules  201  and  202 , bus  212  connects modules  203  and  204  and bus  213  connects modules  205  and  206 . 
         [0051]    This invention replaces the original channel buffers  211 ,  212  and  213  with higher efficiency in timing and area, pre-fabricated units. These pre-fabricated units optimize area, timing closure cycle time and place and route efficiency. Guidance may be needed to ensure routes follow intended paths. Table 1 shows a comparison between the width, height and net count of the original channel buffers and the inventive channel buffers. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Net 
               
               
                   
                 Buffer Number 
                 Width 
                 Height 
                 Count 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Original 
                 183μ 
                  950μ 
                 1350 
               
               
                   
                 Invention 
                 105μ 
                  950μ 
                 1350 
               
               
                 2 
                 Original 
                 204μ 
                 1950μ 
                 2040 
               
               
                   
                 Invention 
                 160μ 
                 1950μ 
                 2040 
               
               
                 3 
                 Original 
                 395μ 
                 2030μ 
                 4200 
               
               
                   
                 Invention 
                 325μ 
                 2030μ 
                 4200 
               
               
                   
               
             
          
         
       
     
         [0052]    The following observations are made regarding this solution. The channel utilization is not generally balanced. The utilization efficiency is low. The inventors believe that pumping it up can yield die area improvements. A buffer width reduction of about 60μ seems feasible. If the Channels are closed pre-placement, significant improvement in the place and route (PnR) convergence can be achieved. Dependency on the routing engine to take certain routes, for example for channel balancing, can be avoided. This permits a determinists solution to the problem. 
         [0053]    This is a significant challenge in prior art SOC place and route methodologies, especially as constraints mature late and quite a few nets are deemed non-critical at the early stages and detoured significantly. Buffers  211 ,  212  and  213  can be pre-constructed as a set of buffered conduits. This involves inserting black box models of these conduits and blocking off the area for PnR. The logical insertion of these into the netlist is much like a buffer insertion. These modules split the net and connect the two pieces to the two end of the conduits. 
         [0054]    Another key concept of this invention is three dimensional (3D) wire spacing. This is called 3D because regular “push” happens in the same horizontal or vertical layer. This invention pushes to a layer below or above. This invention works best on low occupancy orthogonal layers which enable higher packing density with sidewall capacitance reduction. 
         [0055]      FIGS. 3A and 3B  illustrate the results of a prior art first pass bus routing routine.  FIG. 3A  illustrates a top view of an example bus including conductors  301 ,  302  and  303 .  FIG. 3B  illustrates a corresponding cross section view. The first pass bus routing routine places a distance  310  between each conductor pair  301 - 302  and  302 - 303 . Wires are routed as straight as possible generally with minimum spacing between conductors. Custom routing is potentially employed. The routing routine avoids jogs/detours as much as possible. Generally there is no reason to block the layers orthogonal to the signal flow direction. The conductor spacing is typically the minimum distance possible in the target processing node but this is not required.  FIG. 3B  illustrates that conductors  301 ,  302  and  303  are disposed between semiconductor layers  321  and  322 . As noted in  FIG. 3B  the plane of the conductors  301 ,  302  and  303  of this example lie in a region  311 , which in this example is metal level  5 . 
         [0056]      FIGS. 4A and 4B  illustrate a prior art design response to excessive cross-talk between conductors  301 ,  302  and  302 .  FIG. 4A  illustrates a top view of an example bus including conductors  401 ,  402  and  403 .  FIG. 4B  illustrates a corresponding cross section view. The revised routing places a distance  410  between each conductor pair  301 - 302  and  302 - 303 . As illustrated in  FIG. 4A  this distance  410  is greater than the distance  310  illustrated in  FIG. 3A . This greater distance tends to reduce the crosstalk between conductors. This reduced crosstalk may be sufficient to meet the bus crosstalk tolerance.  FIG. 4B  illustrates that conductors  401 ,  402  and  403  are disposed between semiconductor layers  321  and  322  in metal level  5  in region  411 . 
         [0057]      FIGS. 5A and 5B  illustrates another example of the results of a prior art first pass bus routing routine.  FIG. 5A  illustrates a top view of an example bus including conductors  501 ,  502  and  503 .  FIG. 5B  illustrates a corresponding cross section view. The first pass bus routing routine places a distance  510  between each conductor pair  501 - 502 ,  502 - 503  and  503 - 504 .  FIG. 5B  illustrates that conductors  501 ,  502 ,  503  and  504  are disposed between semiconductor layers  321  and  322 . As noted in  FIG. 5B  the plane of the conductors  501 ,  502 ,  503  and  505  of this example lie in metal level  5  region  511 . 
         [0058]      FIGS. 6A and 6B  illustrate the inventive response to excessive cross-talk between conductors  501 ,  502 ,  503  and  504 . This corresponds to step  103  illustrated in  FIG. 1 .  FIG. 6A  illustrates a top view of an example bus including conductors  601 ,  602 ,  603  and  604 .  FIG. 6B  illustrates a corresponding cross section view. The revised of  FIGS. 6A and 6B  maintains the same horizontal distance  510  between each conductor pair  601 - 602 ,  602 - 603  and  603 - 604  as shown in  FIG. 5A . As illustrated in  FIG. 6B  alternate conductors  602  and  604  are pushed down to metal level  4  region  612 .  FIG. 6B  illustrates that conductors  601  and  603  are disposed between semiconductor layers  321  and  322  in metal level  5  in region  611  and that conductors  602  and  604  are disposed between semiconductor layers  322  and  622  in metal level  6  in region  612 . This increases the distance between conductor pairs  601 - 602 ,  602 - 603  and  603 - 604 . This greater distance tends to reduce the crosstalk between conductors. This reduced crosstalk may be sufficient to meet the bus crosstalk tolerance. 
         [0059]    In accordance with this aspect of the invention alternate tracks are pushed down one metal level after initially routing. These run in long segments in the non-preferred direction. Thus metal level  5  alternate tracks are pushed down to metal level  4  while metal level  4  alternate tracks are pushed down to metal level  3 . Open-patch up is done to resolve discontinuities. Spacing/Short cleanup resolves power-grid collisions, pin access collisions and the like. Alternately, the pushdown solution can include a smart handling of this by modeling obstructions and avoiding collisions. Empty tracks can either be left open or metal-filled (shielded) to further reduce conductor cross coupling. Each routing segment not metal-filled can now be widened as well. 
         [0060]    Crosstalk de-sensitization can be enhanced in a number of ways. Shield insertion (step  104 ) employs buffered conduits using the following techniques. The structural uniformity of wire lengths and shield is the important concept. Conventional tool flows do opportunistic shield insertion. This invention guarantees-by-design shield lanes. 
         [0061]      FIG. 7  illustrates the key showing the track types illustrated in  FIGS. 8 to 16 . These track types include wire carrying tracks  701 , empty tracks  702  and shield carrying tracks  703 . 
         [0062]      FIG. 8  illustrates a cross section view of a prior art disposition of conductors within a bus on a SOC.  FIG. 8  illustrates ten adjacent tracks  801 ,  802 ,  803 ,  804 ,  805 ,  806 ,  807 ,  808 ,  809  and  810 . Not shown in  FIG. 8  is the necessary minimal spacing for insolation between adjacent tracks.  FIG. 8  illustrates metal level  5 , metal level  4 , metal level  3  and metal level  2 .  FIG. 8  illustrates that all ten tracks  801  to  810  of metal level  5  include wire carrying tracks and ten tracks  801  to  810  of metal level  3  include wire carrying tracks. This prior art arrangement may lead excessive crosstalk between conductors at the same metal layer. 
         [0063]      FIG. 9  illustrates a cross section view of application of this invention to the problem of the prior art of  FIG. 8 . Every other conductor in metal lever  5  (tracks  902 ,  904 ,  906 ,  908  and  910 ) are pushed down to metal level  4 . Every other conductor in metal lever  3  (tracks  902 ,  904 ,  906 ,  908  and  910 ) are pushed down to metal level  3 .  FIG. 9  illustrates that all other channel/metal level combinations are shield carrying tracks. This disbursal of the conductors (wire carrying tracks) together with the shield carrying tracks reduces the crosstalk between conductors. This arrangement permits diagonal coupling between wire carrying tracks on plural edges. 
         [0064]      FIG. 10  illustrates a cross section view of another example of a prior art disposition of conductors within a bus on a SOC. In  FIG. 10  both metal layer  5  and metal layer  3  include alternating two wire carrying tracks and two shield carrying tracks. Every track in metal layer  4  and metal layer  2  is an empty track in  FIG. 10 . 
         [0065]      FIG. 11  illustrates a cross section view of a first embodiment of this invention modifying the prior art disposition of conductors of  FIG. 10 . Alternate tracks of metal level  5  are pushed down to metal level  4  and alternate tracks of metal level  3  are pushed down to metal level  2 . For example: the shield carrying track at track  1102  on metal level  5  is pushed down to metal level  4 ; the wire carrying track at track  1104  on metal level  5  is pushed down to metal level  4 ; the shield carrying track at track  1106  on metal level  5  is pushed down to metal level  4 ; the wire carrying track at track  1108  on metal level  5  is pushed down to metal level  4 ; and the shield carrying track at track  1110  on metal level  5  is pushed down to metal level  4 . The shield carrying and wire carrying tracks on metal level  3  are similarly pushed down to metal level  2 . The additional distance between wire carrying tracks and the change distribution of shield carrying tracks reduces the crosstalk between conductors. This arrangement illustrated in  FIG. 11  does include diagonal coupling between conductors. 
         [0066]      FIG. 12  illustrates a cross section view of a second embodiment of this invention modifying the prior art disposition of conductors of  FIG. 10 . Alternate tracks of metal level  5  are pushed down to metal level  4  and shifted one track. As an example, the shield in metal level  5  in track  1202  is pushed down to metal level  5  and shifted to track  1203 . Similarly the conductor in metal level  5  in track  1204  is pushed down to metal level  4  and shifter to track  1205 . Similarly alternate tracks of metal level  3  are pushed down to metal level  2  and shifted one track. This arrangement is believed to provide better crosstalk immunity than the arrangement of  FIG. 11 . Note that each pair of vertically aligned conductors is separated by a shield track. In addition, no conductors are diagonally adjacent as in  FIG. 11 . This provides near zero crosstalk at the expense of somewhat greater loading caused by shield capacitance. 
         [0067]      FIG. 13  illustrates a cross sectional view of another example prior art disposition of conductors. Metal layer  5  includes shields in tracks  1301 ,  1302 ,  1307  and  1308  and conductors in tracts  1303 ,  1304 ,  1305 ,  1306 ,  1309  and  1310 . Metal layer  3  includes a similar disposition of shields and conductors. Every track in metal layer  4  and metal layer  2  is an empty track in  FIG. 13 . 
         [0068]      FIG. 14  illustrates a cross section view of an embodiment of this invention modifying the prior art disposition of conductors of  FIG. 13 . Alternate tracks of metal level  5  are pushed down to metal level  4  and alternate tracks of metal level  3  are pushed down to metal level  2 . For example, the shield in metal layer  5  in track  1402  is pushed down to layer  4 , the conductor in metal layer  5  in track  1404  is pushed down to layer  4 , the conductor in metal layer  5  in track  1406  is pushed down to layer  4 , the shield in metal layer  5  in track  1408  is pushed down to layer  4  and the conductor in metal layer  5  in track  1410  is pushed down to layer  4 . Wire carrying tracks are no longer adjacent except diagonally. This provides good crosstalk immunity. 
         [0069]      FIG. 15  illustrates a cross sectional view of another example prior art disposition of conductors. Metal layer  5  includes shields in tracks  1501 ,  1504 ,  1507  and  1510  and conductors in tracts  1502 ,  1503 ,  1504 ,  1506 ,  1508  and  1509 . Metal layer  3  includes a similar disposition of shields and conductors. Every track in metal layer  4  and metal layer  2  is an empty track in  FIG. 15 . 
         [0070]      FIG. 16  illustrates a cross section view of an embodiment of this invention modifying the prior art disposition of conductors of  FIG. 15 . Alternate tracks of metal level  5  are pushed down to metal level  4  and alternate tracks of metal level  3  are pushed down to metal level  2 . For example, the conductor in metal layer  5  in track  1502  is pushed down to layer  4 , the shield in metal layer  5  in track  1504  is pushed down to layer  4 , the conductor in metal layer  5  in track  1506  is pushed down to layer  4 , the conductor in metal layer  5  in track  1508  is pushed down to layer  4  and the shield in metal layer  5  in track  1510  is pushed down to layer  4 . Wire carrying tracks are no longer adjacent except diagonally. This provides good crosstalk immunity. 
         [0071]      FIG. 17  illustrates the key showing the track types illustrated in  FIGS. 18 and 19 . These track types include wire carrying tracks  1701 , staggered tracks  1702 , empty tracks  1703  and shield carrying tracks  1704 . 
         [0072]      FIG. 18  illustrates a cross sectional view of another example prior art disposition of conductors. In  FIG. 18  metal layer  3  has conductors 0 to 42 in alternate tracks. In  FIG. 18  metal layer  5  has conductors 43 to 127 in every track. All tracks in metal layer  4  and  2  at empty. Tracks other than conductors 0 to 42 in metal layer  3  are empty tracks. 
         [0073]      FIG. 19  illustrates a cross section view of an embodiment of this invention modifying the prior art disposition of conductors of  FIG. 18 . Alternate tracks of metal level  5  are pushed down to metal level  4 . As shown in  FIG. 19  conductors 44, 46, 48 . . . 124 to 126 are now in metal layer  4 . Alternate tracks in metal layer  5 , metal layer  4  and metal layer  3 , those that are not conductors, are replaced with shields. Alternate non-conductor tracks in layer  3  are replaced with shields. Metal layer  2  is unchanged from  FIG. 18  to  FIG. 19 . The resulting changes in  FIG. 19  make no conductor horizontally adjacent to another conductor track to track within the same metal layer. In each of metal layers  3 ,  4  and  5  a shield is in the track between each pair of conductors. In  FIG. 19  no conductor is vertically adjacent to another conductor metal layer to metal layer within the same track. The disposition of  FIG. 19  provides substantial cross talk isolation between conductor pairs without requiring additional silicon area. 
         [0074]    There are two topologies possible topologies. The first topology called EWEW uses alternate empty (E) and conductors (W) in the metal layer tracks. The second topology called SWSW uses alternate shield (S) and conductors (W) in the metal layer tracks. The EWEW topology has lower side-wall loading with a consequent lower wire capacitance and lower active power. The SWSW topology has higher side-wall loading but much lower crosstalk coupling. 
         [0075]      FIG. 20  illustrates the delay versus channel number for the two topologies. There are some lanes that spike out in delays. A skew sensitive interface should not use those channels, but they should be left unused. 
         [0076]    The per-lane delay distribution can be tightened further by sizing the drivers one notch larger on the slower lanes. For extremely high speed skew sensitive interfaces, another topology is beneficial. This third topology called SEWES employs in adjacent metal layer track: a shield (S); an empty track (E); a conductor (W); an empty track (E); and a shield (S). This shielded and spaced pipe configuration provides high crosstalk immunity. Using this third topology a channel with a slow speed bus and a skew sensitive high speed interface bus can be places in two abutted conduits running in parallel. Power consumption can be controlled via controlling the spanning distance, transistor threshold voltage (VT) choices of the repeaters and side wall loading. Note that side wall shielding causes more capacitance. 
         [0077]      FIG. 21  illustrates an example of a prior art buffered bus.  FIG. 21  illustrates what is known as an aligned buffering technique. The bus example in  FIG. 21  includes bus lines  2101 ,  2102  and  2103 . This is an illustrative example and an actual integrated circuit bus would typically include more bus lines. 
         [0078]    Bus line  2101  has buffers  2111 ,  2121  and  2131 . Bus line  2102  has buffers  2112 ,  2122  and  2132 . Bus line  2103  has buffers  2113 ,  2123  and  2133 . Each bus line includes similarly inserted buffers. Each bus line has an inter-buffer span of  2120 . The offset of the first buffer per net from the end of the bus line is the same for each bus line. The integrated circuit designer selects this offset distance based upon the expected losses in bus lines  2101 ,  2102  and  2103 . This results in a parallel set of wires running together followed by a cluster of repeating buffers in these bus lines.  FIG. 21  illustrates non-inverting buffers but those skilled in that art would realize that inverting buffers could be employed with this technique. 
         [0079]      FIG. 22  illustrates a staggered buffering technique. Each bus line  2201 ,  2202  and  2203  includes corresponding buffers  2211 ,  2221 ,  2231 ,  2241 ,  2212 ,  2222 ,  2232 ,  2213 ,  2223 ,  2233  and  2243 . The inter buffer distance on each bus line  2210  is the same as illustrated in  FIG. 21 . However, the adjacent lane buffer placement is offset for some bus lines. Thus bus lines  2201  and  2203  are the same as illustrated in  FIG. 21 . The buffers in bus line  2212  are offset from the buffers in bus lines  2201  and  2203  by an amount  2250 . In the preferred embodiment this difference  2250  is about half of the buffering span distance  2210 . This offset of buffers on alternate bus lines reduces logical correlation between buffers and results in crosstalk desensitization. 
         [0080]    Bus isolation is a key concept of this invention. Each conduit has shields on all layers on the edges. The contained bus is isolated from all surrounding influences and the design context is preserved when the conduit is installed in the SOC environment. Thus slow speed, higher density conduits can co-exist in lanes adjacent extremely skew sensitive high speed lanes. Such grouping allows optimal resource allocation for the right degree of criticality of timing and power. 
         [0081]    This invention permits design of multiple buses can be created in a short time independent of the SoC level PnR state. For each automated implementation, the delay per mm, skew per mm, crosstalk impact per mm are easily extracted. These parameters can be used at the SoC-level to enable inter-block budgeting. Based on this information, the SoC design team can validate the floorplan feasibility from a interacting block relative placement perspective. 
         [0082]    This invention enables inter-block timing closure. The ability to meet timing on interfaces in the early floor planning phase of the design is advantageous. This invention stretches the signoff tools quite significantly. This invention provides extensive fringe and inter-layer capacitance extraction accuracy when the 3-D field solver to regular extraction validation is performed. This invention provides logical correlation that is used extensively for crosstalk impact reduction.