Abstract:
A method for minimizing coupling capacitance between wires in a bus comprising shifting by rearranging the order of said wires in said bus so that, aside from said first and last wires in said arrangement, the coupling capacitance across said bus is uniform and minimized relative to the original arrangement. Alternatively, a method for minimizing coupling capacitance between wires in a bus comprising shifting by rearranging the order of said wires in said bus so that, aside from said first and last wires in said arrangement, one of said wires incurs the smallest possible amount of coupling capacitance and then the coupling capacitance across the rest of said wires in said bus gets progressively worse relative to the original arrangement.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods for resolving problems caused by coupling effects between neighboring wires, and particularly to adjusting the effects of inductive coupling and/or capacitive coupling in bus lines. 
     2. Description of Background 
     Heretofore, in many cases busses in integrated circuits have been shifted a certain number of bits in the horizontal or vertical direction. This is commonly done in shifters, multipliers, and unit wiring.  FIGS. 1A and 1B  illustrate a common, i.e. normal, way people and tools wire provide the shifting of wiring  FIG. 1A  is a table illustrating the shifting of six wires in an array of busses between two stages of a prior art integrated circuit device.  FIG. 1B  illustrates a prior art example of the physical configuration of the busses of the table in  FIG. 1A . The order for N wires in the vertical directions is 0,1,2,3, . . . , N−1, For N wires in the bus and a shift or throw of length L bits there are (L−1)*2 of bits of neighboring wire for each wire aside from the periphery of the lines in the bus. 
     From the point of view of the problems associated with coupling capacitance and noise, this is undesirable. Any given wire is completely surrounded by neighbors that are usually hostile, i.e. cross talk is created by induction, which degrades the signal to noise ratio and slows down signal propagation especially when common timing signals are employed thereon. For N wires in a bus and a shift or throw of length L bits there is (L−1)*2 bits of neighboring wire for each wire. This provides the maximum amount of coupling capacitance due to proximity of wires to each other. For example, consider a system with ten wires, shifting fourteen bits. This assumes that there is no freedom to space wires out and that the designer or tool has the exact amount of wiring tracks as wires. 
     U.S. Patent Application Publication No. 2006/0143586 of Suaya entitled “Synthesis Strategies Based on the Appropriate use of Inductance Effects” describes optimizing the signal propagation speed on a wiring layout. 
     U.S. Pat. No. 7,139,993 of Proebsting entitled “Method and Apparatus for Routing Differential Signals Across a Semiconductor Chip” provides an arrangement of pairs of wires carrying differential signals across a semiconductor chip with those pairs of wires organized within a set of parallel tracks on the chip. 
     U.S. Pat. No. 6,999,375 of Jung entitled “Synchronous Semiconductor Device And Method Of Preventing Coupling Between Data Buses” describes a synchronous semiconductor device and a method for preventing coupling between data buses. 
     U.S. Pat. No. 6,772,406 of Trimberger entitled “Method For Making Large-Scale ASIC Using Pre-Engineered Long Distance Routing Structure” describes optimal routing line segments and associated buffers. 
     U.S. Pat. No. 6,388,277 of Kobayashi entitled “Auto Placement and Routing Device and Semiconductor Integrated Circuit” provides an auto placement and routing device that lays out wiring with consideration for influences of an increase in an effective coupling capacitance. 
     U.S. Pat. No. 6,189,133 of Durham, which is assigned to IBM, entitled “Coupling Noise Reduction Technique Using Reset Timing” describes reducing false transitions resulting from capacitive coupling between parallel interconnects driven by dynamic circuits by classifying interconnects based on the timing of expected data transitions in the signals they carry. 
     SUMMARY OF THE INVENTION 
     The shortcomings of the prior art are overcome and additional advantages are provided through the provision of an improved method of shifting wires in a bus to minimize problems caused by coupling effects between neighboring wires, including adjusting the effects of inductive coupling and/or capacitive coupling in bus lines. Only wires running horizontally are considered to contribute to coupling capacitance because they are spaced closely. The vertical components of the wires are not considered as they are relatively distant from each other thus the coupling is negligible. 
     System and computer program products corresponding to the above-summarized methods are also described and claimed herein. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 
     In accordance with this invention a method and a program product are provided for minimizing coupling capacitance between wires in a bus. The method and program product assume the prior art arrangement of wires in a bus. The method and program product then achieve desired minimization by rearranging positions of the wires in said bus so that, aside from the first and last wires in the resulting arrangement, a subset of the wires receive a drastic minimization relative to the remaining wires. More specifically, one wire has a minimum amount of coupling capacitance, and each other wire has progressively more coupling capacitance. Preferably, there are N wires in a bus shifting a length L, the method and program product rearranges the order of the wires resulting in a pattern of 0, (N−1), 1, (N−2), 2, . . . , (N/2)−1, N/2. The above pattern is for even values of N. For odd values of N, the order is determined to be 0, (N−1), 1, (N−2), . . . , Ceiling (N/2), Floor(N/2). 
     TECHNICAL EFFECTS 
     The technical effect of software used in the invention is to provide improved arrangements of wires shifted in a bus thereby minimizing problems caused by coupling effects between neighboring wires, including adjusting the effects of inductive coupling and/or capacitive coupling in bus lines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIGS. 1A and 1B  illustrate a common, i.e. normal, way people and tools arrange the wires for a bus that is shifting.  FIG. 1A  is a table illustrating the shifting of six wires in a bus between two stages of a prior art integrated circuit device.  FIG. 1B  illustrates a prior art example of the physical configuration of the bus in the table in  FIG. 1A . 
         FIGS. 2A and 2B  illustrate a simple three wire example of a structure provided in accordance with this invention formed by employing a method in accordance with this invention of rearranging the paths of an array of three wires WIRE 0 , WIRE 1 , and WIRE 2  in a bus interconnecting an array of terminals LL 1 , LL 2  and LL 3  to an array of terminals LL 4 ′, LL 5 ′ and LL 6 ′ between two successive stages of an integrated circuit device.  FIG. 2A  is a table illustrating the arrangement of the three wires WIRE 0 , WIRE 1 , and WIRE 2  in the bus that is shifting from the output terminals LL 1 , LL 2 , and LL 3  to the input terminals LL 4 ′, LL 5 ′ and LL 6 ′ between the two successive stages of the integrated circuit device.  FIG. 2B  illustrates an example of the physical configuration of the paths of the wires WIRE 0 , WIRE 1 , and WIRE 2  in the bus with the intermediate shift in the path of WIRE 1  which crosses WIRE 2  and then re-crosses WIRE  2  so that there is a gap of  2 X between the adjacent wires WIRE 0  and WIRE 1  in the horizontal runs thereof as shown in the table in  FIG. 2A  with a gap of 1× between WIRE 2  and each of WIRE 0  and WIRE 1  in the horizontal runs thereof. The three wires WIRE 0 , WIRE 1 , and WIRE 2  extend, initially, in parallel in an output array (upper vertical lines in  FIG. 2B ) from the output terminals LL 1 , LL 2 , and LL 3 . The wires WIRE 0 , WIRE 1 , and WIRE 2  extend in that order from the output terminals LL 1 , LL 2 , and LL 3  until they reach a set of points at which they extend at right angles in a transverse parallel array WIRE 0 , WIRE 2 , and WIRE 1 , i.e. with WIRE 2  sandwiched between WIRE 0  and WIRE 1  (horizontal lines in  FIG. 2B ). Then the three wires WIRE 0 , WIRE 1 , and WIRE 2  reach a second set of points at which they extend in a parallel input array (lower vertical lines in  FIG. 2B ). The input array is parallel to the direction of the output array but shifted to the right in  FIG. 2B . The three wires WIRE 0 , WIRE 1 , and WIRE 2  extend in that direction reaching contact with the input terminals LL 4 ′, LL 5 ′ and LL 6 ′ in that order. The three wires WIRE 0 , WIRE 1 , and WIRE 2  reach the three input terminals LL 4 ′, LL 5 ′ and LL 6  in the original order WIRE 0 , WIRE 1 , WIRE 2  but with a greater gap between WIRE 0  and WIRE 1  in the region of the transverse parallel array. That greater gap exists because WIRE 1  has crossed WIRE  2  twice since the output parallel array WIRE 2  is 1× shorter than WIRE 1  and 1× longer than WIRE 0 , but in the input parallel array WIRE 2  is 1× shorter than WIRE 0  and 1× longer than WIRE 1 . 
         FIG. 3A  is a flow chart of an algorithm for rearranging wires in accordance with this invention.  FIGS. 3B-3D  show how the algorithm of  FIG. 3A  applies to the example of  FIGS. 2A and 2B . 
         FIGS. 4A and 4B  illustrate a six wire example of method of rearranging of wires in a bus that is shifting interconnecting stages of an integrated circuit in accordance with this invention.  FIG. 4A  is a table illustrating the rearrangement of six wires in a bus that is shifting between two stages of an integrated circuit device.  FIG. 4B  illustrates an example of the physical configuration of the busses in the table in  FIG. 4A . 
         FIGS. 5A and 5B  illustrate a six wire example of method of rearranging of wires in a bus that is shifting interconnecting stages of an integrated circuit in accordance with this invention.  FIG. 5A  is a table illustrating the rearrangement of six wires in a bus that is shifting between two stages of an integrated circuit device.  FIG. 5B  illustrates an example of the physical configuration of the busses in the table in  FIG. 5A . 
     
    
    
     The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings in greater detail, it will be seen that  FIGS. 2A and 2B  illustrate a structure in accordance with this invention which was provided by employing a method in accordance with this invention of rearranging the paths of three exemplary wires connected between including the wire of bit  0  (WIRE 0 ), the wire of bit  1  (WIRE 1 ) and the wire of bit  2  (WIRE 2 ) in a bus connected in the same sequence or order respectively from output terminals LL 0 , LL 1  and LL 2  to input terminals LL 4 ′, LL 5 ′ and LL 6 ′. The method of this invention as illustrated by  FIGS. 2A and 2B  provides for the intermediate shifting of the paths of the wires WIRE 0 , WIRE 1  and WIRE 2  between those output terminals LL 0 , LL 1  and LL 2  and those input terminals LL 4 ′, LL 5 ′ and LL 6 ′ without changing the sequence of connections provided by the wires WIRE 0 , WIRE 1  and WIRE 2  even though WIRE  2  shifts its path and returns its path into the original sequence of wires as it crosses over WIRE  1  twice, which in fact is in accordance with the method of this invention and which provides a structure in accordance with this invention. The invention employs an algorithm that can be manifested in software that minimizes the adversity of the coupling effect by neighboring wires WIRE 0  WIRE 1  and WIRE 2 .  FIG. 2A  is a table illustrating the rearrangement of three wires WIRE 0  WIRE 1  and WIRE 2  in the bus that is the intermediate shifting of the path of WIRE 2  between two stages of an integrated circuit device.  FIG. 2B  illustrates an example of the physical configuration of the busses in the table in  FIG. 2A . 
     Block A
     middle =Floor((N+1)/2);   end =N-1;   Proceed to Block B   

     Block B
     Then the program tests as to whether the value of “i” is less than one less than the number N of wires, i.e. i&lt;(N−1)?   If YES proceed to Block C. If NO proceed to Block D.   

     Block C
     tmp =Floor(i/2);   ArrayOut(i) =Arrayln(tmp);   ArrayOut(i+1) =Arrayln(tmp+middle);   Set i =i+2   Proceed to Block B   

     Block D
     i=end? If YES, proceed to Block E. If NO, Proceed to Block F.   

     Block E
     ArrayOut(i)=ArrayIn(middle−1)   Proceed to Block F   

     Block F Return ArrayOut Proceed to Block G 
     Block G DONE 
       FIGS. 3B-3D  show how the algorithm described above is performed for the simple three wire illustrative example of  FIGS. 2A and 2B . 
       FIGS. 4A and 4B  show a six wire example of a structure in accordance with this invention which was provided by employing a method in accordance with this invention of rearranging the paths of wires in a bus interconnecting stages of an integrated circuit which is shifting bit lines from left to right in accordance with this invention. As in  FIGS. 2A and 2B , the wires from the output terminals to the input terminals have paths which are altered to minimize the proximity of parallel wires between two stages of an integrated circuit device by crossing wires to increase the minimum spacing between horizontal portions of the adjacent wires without changing the sequence of connection of the wires from the input terminals to the output terminals.  FIG. 4A  is a table illustrating the rearrangement of the paths of the six wires in the bus that is shifting between two stages of an integrated circuit device in accordance with this invention.  FIG. 4B  illustrates an example of the physical configuration of the busses in the table in  FIG. 4A  with the sequence of wires WIRE 0  to WIRE 5  connected respectively from output terminals LL 0  to LL 5  in the same sequence, i.e. in the same order, to a sequence of input terminals LL 4 ′ to LL 9 ′, but with the routing of the wires WIRE 0 , WIRE 1 , WIRE 2 , WIRE 3 , WIRE 4 , and WIRE 5  arranged with paths which provide minimum capacitance therebetween from the input terminals LL 0  to LL 5  and the set input terminals LL 4 ′ to LL 9 ′ terminals in accordance with this invention. 
     In  FIG. 4A , there are the six wires WIRE 0 , WIRE 1 , WIRE 2 , WIRE 3 , WIRE 4 , and WIRE 5  in the bus that are shifting the routing to minimize interwire capacitance. Wires WIRE 5  and WIRE 0  have only two bit-lengths of coupling, but wires WIRE 1 -WIRE 4  have five bit-lengths of coupling as can be seen by inspection. The process as described in  FIG. 3A  yields the results shown in  FIGS. 4A and 4B . Inspection of  FIG. 4B  shows the coupling of the six wires in a bus that is shifting from left to right. The horizontal component of WIRE 0 , which extends from output terminal LL 0  to input terminal LL 4 ′, is for a very short span a space of 1× away from WIRE  3 . The horizontal component of WIRE 1 , which extends from output terminal LL 1  to input terminal LL 5 ′, is for two different and longer spans a space of 1× away from both WIRE 3  and WIRE 2 , and WIRE 1  crosses WIRE 3  at right angles both horizontally and vertically . The horizontal component of WIRE 2 , which extends from output terminal LL 2  to input terminal LL 6 ′, is a space of 1× away from both WIRE 4  and WIRE 5  and WIRE 2  crosses WIRE 3  and WIRE 4  at right angles both horizontally and vertically. The horizontal component of WIRE 3 , which extends from output terminal LL 3  to input terminal LL 7 ′, is a space of 1× away from both WIRE 1  and WIRE 0 , and WIRE 3  crosses WIRE 4  and WIRE 5  at right angles horizontally. The horizontal component of WIRE  4 , which extends from output terminal LL 4  to input terminal LL 8 ′, is a space of 1× away from both WIRE 2  and WIRE 1  and WIRE 4  crosses WIRE 2  at right angles both horizontally and vertically. The horizontal component of WIRE 5 , which extends from output terminal LL 5  to input terminal LL 9 ′, is only a space of 1× away from WIRE  2 . The remainder of the wires from terminals LL 6  to LL 9  have not been shown or described. 
       FIGS. 5A and 5B  illustrate a six wire example of method of rearranging of wires in a bus that is shifting bit lines between interconnecting stages of an integrated circuit in accordance with this invention. In  FIGS. 5A and 5B  there is a minimum degree of coupling for wires WIRE 0  and WIRE 5  with higher degrees of coupling for other the other wires from WIRE 1  to WIRE  4  in the bus. As in the case of  FIG. 4B ,  FIG. 5B  illustrates an example of the physical configuration of the wires in the table in  FIG. 5A  with the sequence of wires from output terminals LL 0 -LL 5  connected in the same sequence, i.e. in the same order, to input terminals LL 4 ′-LL 9 ′, but with the routing of the wires WIRE 0 , WIRE 1 , WIRE 2 , WIRE 3 , WIRE 4 ,and WIRE 5  arranged to provide minimum capacitance therebetween in a modified arrangement in accordance with this invention. 
     For this algorithm, the largest amount of bit-length coupling capacitance is determined by: (L−1)*2−1, and the least amount is determined by: (L−N+1)*2+1, where L is the bit-lengths of the shift and N is the number of wires in the bus and always greater than 2. It should be noted that the wire with the most coupled capacitance in the final arrangement has less coupling capacitance than every wire in the prior art arrangement. 
     When wires in a bus assume the prior art arrangement, the chance of their switching windows which overlap becomes very high. For example WIRE 0  and WIRE 1  usually switch at the same time whereas WIRE 9  (not shown but connected to an output terminal later in the sequence) will switch at a different time. This difference in switching time is due to the physical location of the latches and corresponding clock buffers. That is, the latches and clock buffers associated with WIRE 0  and WIRE 1  are physically close to one another and therefore incur a similar clock skew. Whereas the latches and clock buffers associated with WIRE 0  and WIRE 9  (not shown) are physically distant from one another and therefore incur different clock skews. Since coupling capacitance can be exacerbated by neighboring wires switching at the same time, it is thus beneficial to arrange wires in such a way that neighboring wires do not have similar switching windows. The algorithm described in  FIG. 3A , and the modification exemplified in  FIG. 5  have this advantage. 
     No more wiring resources are being used in any of these alternatives versus the prior art arrangement, that is, wires are merely being reordered to reduce coupling capacitance thereby improving performance. 
     GLOSSARY 
     i is a variable that is being incremented and then compared. 
     Floor is an operator that rounds a number down to an integer; in accordance with floor and ceiling functions. 
     tmp is a temporary variable 
     middle is a variable that is the middle of the number of wires; the floor of N/2. 
     tmp=Floor(i/2) This assigns the variable ‘tmp’ the Floor(i/2) 
     ArrayOut(i) Array of N wires (indexed as 0 to N−1) ordered sequentially (i.e. 0,1,2,3,4, . . . , N). According to computer protocols, the items in the array are indexed by zero. 
     ArrayIn(tmp) This is another array called “ArrayIn”. 
     ArrayIn(tmp+middle) We are adding two numbers (tmp and middle hold numbers, like 5, 6 . . . ) and adding them to get some other number (e.g. 11), and then looking at the “tmp+middle−th” box of the array. 
     ArrayIn(middle−1) Same as above. 
     Return ArrayOut This function returns the answer to the user. 
     The capabilities of the present invention can be implemented in software, firmware, hardware or some combination thereof. 
     As one example, one or more aspects of the present invention can be included in an article of manufacture (e.g., one or more computer program products) having, for instance, computer usable media. The media has embodied therein, for instance, computer readable program code means for providing and facilitating the capabilities of the present invention. The article of manufacture can be included as a part of a computer system or sold separately. 
     Additionally, at least one program storage device readable by a machine, tangibly embodying at least one program of instructions executable by the machine to perform the capabilities of the present invention can be provided. 
     The flow diagrams depicted herein are just examples. There may be many variations to these diagrams or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
     While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.