Abstract:
A method ( 400 ) of determining widths (W) and/or routes of I/O power routes ( 112 ) between one or more power distribution networks ( 108 ) and a plurality of I/O circuits ( 104 ) based on IR drop, electromigration, and electrostatic discharge electrical requirements. The method includes initially routing the I/O power routes and then iteratively analyzing the I/O power routes and iteratively incrementing the width of each power route that fails one or more of the electrical requirements until all power routes meet all electrical requirements. Once all power routes meet the electrical requirements, power routing is performed again to re-route any power routes as necessary to accommodate their wider widths. The method may be implemented in system ( 300 ) that includes a power routing tool ( 304 ), an electrical analysis tool ( 308 ), and a tool integrator ( 312 ) that implements an integrated power routing algorithm ( 316 ).

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to the field of integrated circuits. In particular, the present invention is directed to an I/O circuit power routing system and method.  
         [0003]     2. Background of the Invention  
         [0004]     Thus far, the semiconductor industry has succeeded in pushing forward the famous Moore&#39;s law on technology scaling. This continuous push results in future very large scale integration (VLSI) designs characterized by higher integration densities, higher operating frequencies, and reduced feature size. Reduced feature size leads to higher sheet resistivity for the metal wires that connect electrical devices to their corresponding electrical networks. Higher operating frequencies result in an increase in the dynamic power dissipated by the chips. Higher integration densities increase the number of transistors on the chip and accordingly increase the chip power dissipation.  
         [0005]     Furthermore, leakage power, also referred to as static power, is increasing significantly from one technology to the next. In an attempt to address the increased power dissipation, as well as address other reliability requirements such as oxide breakdown voltage, the supply voltage is reduced in newer technologies. This reduces the noise margins and makes the designs more sensitive to voltage drops, also known as IR drops. While excessive voltage drops may cause functional failures, less severe voltage drops increase gate delays, which affect chip timing and make it harder to meet a chip&#39;s timing requirements.  
         [0006]     The trend of increased power dissipation, lower supply voltage, and smaller feature size leads to higher current densities flowing in the power distribution networks of modem VLSI chips. Higher current densities and reduced sheet resistivity raise the chip susceptibility to reliability concerns, such as electromigration (EM) and electrostatic discharge (ESD), both of which can cause physical damage and chip failure.  
         [0007]     Presently, a large number of chips are made using designs in which the input/output (I/O) circuits can be placed essentially anywhere on the chip and are not limited to the periphery of the chip. This type of chip is often referred to as a “flip chip.” An important aspect of the physical design of flip chips relative to I/O circuits is the sizing and routing of the wiring that connects the I/O circuits to the appropriate on-chip power distribution networks. “Power routing” of I/O circuits is the process of connecting the power service terminals (PSTs) of every I/O circuit (i.e., I/O pins where power is supplied to the I/O circuit) to the power distribution network. The metal wires connecting the I/O PSTs to the power distribution network are referred to as “power routes.” By controlling the widths of the I/O power routes, the effective resistance of the power routes, as well as the current densities in those routes, can be controlled to satisfy the electrical requirements of the design. The process of modifying the widths of the power distribution wires, also referred to as “wire sizing,” has been discussed in the literature to satisfy EM reliability requirements of generic power mesh structures.  
         [0008]     For I/O circuits to function properly and meet their specifications, a set of electrical constraints, defined by either the technology developers or the chip designers, needs to be satisfied. A subset of these constraints related to the power routes of the I/O circuits are checked by the IR, EM, and ESD constraints.  
         [0009]     IR checks: Supply currents flowing through metal conductors cause voltage drops across the conductors. Consequently, the voltage at the circuit pins is less than the voltage applied at the module pins. The IR drop checks are defined to guarantee that the voltage drops at the PSTs of the I/O circuits are less than a specified percentage of the supply voltage. This guarantees that the I/O circuits meet their performance specifications, which strongly depend on the value of the voltage at the PSTs of the I/O circuits.  
         [0010]     EM checks: Electromigration is an important reliability failure mechanism that is becoming a more serious concern in shrinking technologies. Electromigration is defined as the mass transport of metal ions due to the momentum exchange between the metal ions and the moving electrons that represent the electric current flowing through the metal wires. A direct current in a metal wire running for a substantial period of time eventually causes the formation of voids or hillocks. In circuit terms, a void formation means an open circuit in the wire and a hillock formation means that the wire gets shorted to other wires. Either scenario may cause chip failure. For each technology, the technology developers define maximum EM current density limits as a function of the chip lifetime and temperature. It is then the designer&#39;s responsibility to make sure that current densities flowing through the metal wires on the chip are less than the specified technology limits. This is basically what defines the EM checks.  
         [0011]     ESD checks: Electrostatic discharge is another important reliability failure mechanism that chip designers need to take into consideration. An ESD event is defined as the transfer of charge between bodies of different electrostatic potential in proximity or through direct contact. There are three different ESD models recognized in the semiconductor industry: (1) human body model; (2) machine model; and (3) charged device model. The difference between these models is the definition of their criteria in terms of how much charge can be injected into the system without damaging chip circuitry. To protect the chip circuitry against an ESD event, ESD clamps are utilized to help conduct a discharge path to the ground network. An ESD clamp is effectively a huge transistor (or diode) that is turned off except in the presence of an ESD event. In the case of an ESD event, the clamp turns on, creating a path for the charge to be drained into the ground network, thus, allowing the safe discharge of the ESD event while avoiding damage to chip circuitry. The ESD check is usually defined in terms of a maximum limit on the effective resistance of the power distribution network (including the power routes) from every I/O circuit to the ESD clamps.  
         [0012]     The continual push for high performance and low power designs in current and future technologies makes it more difficult to meet the different electrical requirements of the designs, such as satisfying the IR drop, EM, and ESD electrical requirements. As mentioned, the widths of the power routes of the I/O circuits can be controlled to guarantee the satisfaction of all the electrical constraints. However, the processes of power routing and electrical analysis are typically independent. Most existing techniques rely on generic guidelines for power routing the I/O circuits. These guidelines are usually manually developed by experienced engineers relying on their knowledge of typical operation of I/O circuits and the design of the on-chip power distribution. Such guidelines are usually not I/O instance-specific and they do not necessarily guarantee the satisfaction of the electrical constraints for all I/O circuits. On the other hand, analysis tools have been developed to check for and capture the electrical violations in a design. Such tools utilize techniques that extract and simulate the power distribution networks excited by the different I/O circuits.  
         [0013]     Consequently, a major drawback of existing design techniques is that the power routing design step is invoked independently of the electrical analysis design step. Thus, any violations reported by the electrical analysis step are then fixed manually by the designers. This is usually a tedious process that requires a number of iterations that may result in schedule delays. With newer technologies, the electrical constraints are becoming more stringent and consequently, the process of manual fix-up of electrical violations is becoming even more tedious.  
       SUMMARY OF INVENTION  
       [0014]     In one aspect, the present invention is directed to a method of floorplanning an integrated circuit chip. The method includes: a) routing a plurality of power routes corresponding to a plurality of integrated circuits using an initial width for each of the plurality of power routes; b) performing at least one electrical check of the plurality of integrated circuits; c) determining whether any one or more of the plurality of power routes has at least one electrical violation; d) for each one of said plurality of power routes having at least one electrical violation, assigning that one of the plurality of power routes a new width greater than the initial width and e) repeating steps b) and c) at least once using the one or more new widths assigned in step d) and any of the initial widths remaining after step d).  
         [0015]     In another aspect, the present invention is directed to a system for floorplanning an integrated circuit chip that includes a plurality of integrated circuits electrically connected to one or more power distribution networks via a corresponding plurality of power routes. The system comprises a power routing tool operatively configured to route the plurality of power routes. An electrical analysis tool operatively configured to perform at least one electrical check on the plurality of integrated circuits and the plurality of power routes. A tool integrator implements an integrated power routing algorithm that performs the steps of: i) routing, using the power routing tool, the plurality of power routes using an initial width for each of the plurality of power routes; ii) performing, using the electrical analysis tool, at least one electrical check of the plurality of integrated circuits; iii) determining whether any one or more of the plurality of power routes has at least one electrical violation; iv) for each one of the plurality of power routes having at least one electrical violation, assigning that one of the plurality of power routes a new width greater than the initial width; and v) repeating steps ii) and iii) at least once using the one or more new widths assigned in step iv) and any of the initial widths remaining after step iv). 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0016]     For the purpose of illustrating the invention, the drawings show a form of the invention that is presently preferred. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein:  
         [0017]      FIG. 1  is a high-level schematic diagram of an integrated circuit chip made using an integrated power routing system and method of the present invention;  
         [0018]      FIG. 2  is a schematic diagram illustrating the modeling of the power distribution network of  FIG. 1 ;  
         [0019]      FIG. 3  is a high-level schematic diagram of an integrated power routing system of the present invention;  
         [0020]      FIG. 4  is a flow diagram illustrating an integrated power routing method of the present invention that may be implemented in the integrated power routing system of  FIG. 3 ;  
         [0021]      FIG. 5  is a histogram of the number of I/O power routes of each width after a first iteration of a power routing method of the present invention for each of four test cases TC 1 , TC 2 , TC 3 , and TC 4 ; and  
         [0022]      FIG. 6  is a graph of number of failed power routes versus an iteration index for test cases TC 1 , TC 2 , TC 3 , and TC 4 . 
     
    
     DETAILED DESCRIPTION  
       [0023]     Referring now to the drawings,  FIG. 1  illustrates an integrated circuit chip  100  that contains a plurality of I/O circuits  104  each electrically coupled to a power distribution network  108  via a corresponding power route  112 . Power routes  112  have been sized and/or routed using an integrated power routing system and method of the present invention, e.g., system  300  of  FIG. 3  and method  400  of  FIG. 4 . Integrated power routing system  300  and integrated power routing method  400  are each described below in detail. However, in order to provide a context for a macro placement system and method of the present invention, chip  100  is described first.  
         [0024]     Chip  100  may be any type that utilizes one or more power distribution networks  108  and I/O circuits  104  that need to be power routed to one of the power distribution networks. Chip  100  may be of any type, e.g., an application specific integrated circuit (ASIC) chip, processor, memory, system on chip or controller, among others. Those skilled in the art will readily appreciate that chip  100  may be designed to perform any functions typical of integrated circuits and that the type of chip is generally not relevant to the broad scope of the present invention. Power distribution network  108  may comprise a plurality of wires  120  located on one or more metal levels, e.g., M 1  and M 2 , of chip  100  and a plurality of power pads  124  for connecting the chip to an external power supply (not shown). Wires  120  may be arranged in any manner suitable for a particular design, such as the rectangular grid arrangement shown.  
         [0025]     It is clear from the discussion in the background section that power distribution networks, such as power distribution network  108 , are becoming performance limiting factors in modern chip designs. In addition to IR, EM, and ESD concerns, transient power supply collapse is a serious concern that may cause chip timing violations and potentially functional failure. In order to capture the transient response of the system, power distribution network  108  may be modeled as a linear RLC network. Correspondingly, power pads  124  may be modeled as ideal voltage sources, and power distribution network  108  may be excited by time-varying current sources that capture the switching behavior of the active circuits. These current sources are applied at the locations of the circuits they represent. Such a model  200  is illustrated in  FIG. 2 .  
         [0026]     Referring again to  FIG. 1 , although each power distribution network may be modeled as an RLC network, for simplicity, power distribution network  108  may, if desired, be modeled as a resistive model, since DC simulation is sufficient for checking IR, EM, and ESD requirements. This significantly simplifies, and speeds up, the modeling, analysis, and checking for any electrical violations. Correspondingly, the power pads  124  may still be modeled as ideal voltage sources and the active circuits may be modeled as DC current sources.  
         [0027]     Power routes  112  of I/O circuits  104  typically do not follow a grid structure, as power distribution network  108  may. Often, I/O power routes  112  look more like signal routes (not shown). Thus, there is no regular power distribution grid that I/O circuits  104  simply tap into. Instead, a connection, i.e., a corresponding one of power routes  112 , has to be routed from the corresponding PST  128  to on-chip power distribution network  108  of the voltage domain to which the respective I/O circuit  104  belongs.  FIG. 1  also shows various blockages  132 , each of which is a physical area on chip  100  where I/O circuits  104  cannot be placed or where power routes  112  cannot pass through because some other circuit(s) is/are placed in that area.  
         [0028]     Given a model, e.g., model  200  ( FIG. 2 ), of power distribution network  108 , modified nodal analysis can be applied to extract the system of equations represented by Equation {2}. 
 
Gx=I   {2}
 
         [0029]     where G is a conductance matrix, x is a vector of node voltages, and I is a set of current stimulus exciting the system. The solution of the system of Equation {2} provides the voltages at all the nodes and the currents flowing in all the branches. A node is defined as the intersection of two adjacent (or same) metal layers of like polarity (e.g., VDD, GND, or VDDx). A branch is the metal segment between two nodes. Node voltages are required for IR and ESD checking. Branch currents, on the other hand, are required for EM checking.  
         [0030]     Referring to  FIG. 3 , and also to  FIG. 1 ,  FIG. 3  illustrates an integrated power routing system  300  of the present invention that may be used to automatically size and/or route I/O power routes, e.g., power routes  112  of  FIG. 1 . Integrated power routing system  300  may include, among other things, a power routing tool  304 , an electrical analysis tool  308 , and a tool integrator  312  that controls the power routing and electrical analysis tools in an iterative manner so as to automatically size and/or route the I/O power routes so as to meet IR, EM, and ESD and other requirements. Power routing tool  304  may be any suitable tool known in the art for routing I/O power routes  112 .  
         [0031]     Similarly, electrical analysis tool  308  may be any suitable tool for electrical analysis of I/O circuits, power network(s), power routes and other elements. An example of an electrical analysis tool that may be adapted for use as electrical analysis tool  308  in integrated power routing system  300  of the present invention is VOLTAGE STORM®, available from Cadence Design Systems, Inc. of San Jose, Calif. Of course, each of power routing tool  304  and electrical analysis tool  308  may be custom made and/or integrated with one another. Those skilled in the art readily understand the design and use of power routing and electrical analysis tools  304 ,  308 , such that they need not be described herein in any further detail in order for those skilled in the art to make and use the present invention to its fullest scope.  
         [0032]     Tool integrator  312  may be operatively configured to perform an integrated power routing algorithm  316  that utilizes the functionality of power routing tool  304  and electrical analysis tool  308  in an iterative manner to arrive at suitable widths (W) and/or routings for I/O power routings  112  being routed using integrated power routing system  300 . The functions of tool integrator  312  are described below in connection with method  400  of  FIG. 4 . It is noted that tool integrator  312  need not be separate and distinct from power routing tool  304  and/or electrical analysis tool  308  as shown. Rather, tool integrator  312  may be integrated into one, the other, or both of power routing and electrical analysis tools  304 ,  308 . Tool integrator  312  is shown as separate from power routing and electrical analysis tools  304 ,  308  merely to illustrate its separateness in terms of function.  
         [0033]     Referring to  FIG. 4 , and also to  FIGS. 1 and 3 ,  FIG. 4  illustrates an integrated power routing method  400  of the present invention that may be implemented by tool integrator  312  ( FIG. 3 ) to automatically size and route I/O power routes, e.g., power routes  112  of  FIG. 1 . As those skilled in the art will appreciate, method  400 , and other methods in accordance with the present invention, may be executed in any suitable software/hardware context.  
         [0034]     At step  404 , method  400  may be started. A typical starting point for method  400  occurs once chip  100  has been floorplanned and all I/O circuits  104  have been assigned and placed. However, it is noted that, depending upon the type of chip  100 , the starting point may be at another stage of design. For example, if chip  100  is of a system on chip design, starting point may occur at a time when a particular region of the chip has been floorplanned and corresponding I/O circuits  104  have been assigned and placed therein.  
         [0035]     At step  408 , tool integrator  312 , e.g., via power routing tool  304 , may assign to each power route  112  a minimum width recommended by the technology used to make chip  100 , or portions of the chip, at issue. Those skilled in the art will readily understand how to arrive at the value of minimum power route width applicable for the technology they will use.  
         [0036]     At step  412 , tool integrator  312  may cause power routing tool  304  to perform an initial power routing using the minimum widths assigned at step  408 . The result of this initial power routing is a set of routes for power routes  112  based on these power routes being the minimum width possible. At step  416 , tool integrator  312  extracts the physical design data, e.g., lengths of power routes  112 , connection locations to power distribution network  108  and placement of I/O circuits  104 , needed for electrical analysis tool  308  to perform an electrical analysis of the power routes. At step  420 , tool integrator  312  may cause electrical analysis tool  308  to perform an electrical analysis of I/O circuits  104  and power routes  112  to determine, perhaps among other things, whether any one or more IR, EM, and ESD violations exist.  
         [0037]     At step  424 , electrical analysis tool  308  or tool integrator  312  may determine whether any power route  112  has any IR, EM, and ESD violations. If not, at step  428 , the routing of I/O power routes  112  is done, and routes and widths of the power routes just analyzed at step  420  may be used in the final floorplan. In this scenario, tool integrator  312  may terminate integrated power routing algorithm  316 . If, on the other hand, electrical analysis tool  308  reports one or more IR, EM, and ESD violations, then the electrical analysis tool or tool integrator  312  may create a list of all I/O circuits  104  that fail any of the IR, EM, and ESD checks.  
         [0038]     If it is determined at step  424  that one or more IR, EM, and ESD violations exist, electrical analysis tool  308  or tool integrator  312  may, at step  432 , assign an increased width to power route(s)  112  corresponding to the one(s) of I/O circuits  104  having one or more violations. Each existing width that failed may be increased by any incremental amount, such as an incremental amount dictated by the technology used to fabricate chip  100 . For example, in one technology in which the minimum width is 6 μm, the incremental step may be 2 μm, such that the next width is 8 μm. At this point, the routes of power routes  112  may be assumed to be the same routes as just determined in step  412 .  
         [0039]     At step  436 , tool integrator  312  may cause electrical analysis tool  308  to re-run using the new widths assigned to the failing I/O circuits  104  in step  432  and all of the remaining original minimum widths. At step  440 , similar to step  424 , electrical analysis tool  308  or tool integrator  312  may determine whether any power route  112  has any IR, EM, and ESD violations. If not, method  300  may loop back to step  412  to re-run power routing tool  304  so that power routes  112 , if any, may be re-routed in the event that any of the width increases made in step  432  result in a new interference with one or more of blockages  132  or other power route(s). Once any power routes  112  have been re-routed at step  412 , method  400  may continue with steps  416 ,  420 ,  424 ,  432 ,  436  and  440  as necessary until the process ends at step  428  with one or more of the power routes being resized and/or re-routed until no IR, EM, and ESD violations occur. Once step  428  has been reached, the routes and widths of power routes  112  determined in the most recent power routing of step  412  may be used in the final floorplan. At this point, tool integrator  312  may terminate integrated power routing algorithm  316 .  
         [0040]     If, on the other hand, electrical analysis tool  308  reports one or more IR, EM, and ESD violations at step  440 , then the electrical analysis tool or tool integrator  312  may create a list of all I/O circuits  104  that fail any of the IR, EM, and ESD checks. In this case, method  400  may proceed back to step  432  so that new greater widths may be assigned to power routes  112  corresponding to the one or more IR, EM, and ESD violations. Method  400  may loop through steps  432 ,  436 ,  440  and back to step  432  until electrical analysis tool  308  or tool integrator  312  does not find any more IR, EM, and ESD violations.  
         [0041]     As explained immediately above in the flow of method  400 , integrated power routing algorithm  316  involves iterations within electrical analysis tool  308  as well as iterations between the electrical analysis tool and power routing tool  304 . Integrated power routing algorithm  316  terminates when all the electrical specifications are satisfied for all I/O circuits  104 . Using method  400 , electrical analysis tool  308  is automatically and iteratively invoked so as to arrive at a first approximation of the optimal widths for power routes  112  so as to guarantee that all electrical constraints are satisfied. This reduces the number of iterations between power routing tool  304  and electrical analysis tool  308 , thereby reducing churn in satisfying the electrical constraints.  
         [0042]     Furthermore, method  400  targets the power routing of each individual I/O circuit  104  independently. Existing techniques break up I/O circuits  104  into classes and define different power route widths for different classes. However, the electrical constraints of different I/O circuits  104  of the same class may be different. This is so because the electrical constraints required to be satisfied by an I/O circuit  104  depend on the current drawn by that I/O circuit and the location of that I/O circuit on chip  100 . The I/O current, in turn, depends on the specific loading conditions and switching activity of that specific I/O circuit  104 . Thus, defining a power route width based on an I/O class may result in some I/O circuits  104  having wider power routes  112  than necessary to satisfy the electrical constraints. This is an undesirable result since wider power routes  112  consume wiring resources that make it harder to efficiently wire chip  100 . Hence, it is important to define the minimum power route width for each I/O circuit  104  necessary to satisfy the electrical constraints of that I/O circuit.  
       EXAMPLE  
       [0043]     In this example, four test cases, referred to as TC 1 , TC 2 , TC 3 , and TC 4 , are considered in connection with implementing an integrated power routing method of the present invention, e.g., method  400 , in connection with IR requirements. However, those skilled in the art will readily understand the modifications necessary to implement this method in connection with EM and ESD requirements as well.  
         [0044]     The number of I/O circuits in each of test cases TC 1 , TC 2 , TC 3 , and TC 4  is about 150 I/O circuits, as shown in Table I.  
                                                                                                 TABLE I                                       No. of Failing I/O Circuits                    No. of I/O   vs. Power Route Width                Test Case   Circuits   6 μm   8 μm   12 μm   16 μm                    TC1   159   26   10   3   0       TC2   152   29   10   0   0       TC3   157   25   10   4   1       TC4   145   26   11   2   0                  
 
         [0045]     The supply voltage is 1.5V for each of test cases TC 1 , TC 2 , TC 3  and TC 4 . The other inputs for this example are the currents drawn by the various I/O circuits. For purposes of this example, it is assumed that each I/O circuit is drawing 30.0 mA of current from the power supply. In practice, the current demand for each I/O circuit may be obtained by running SPICE simulations under accurate loading conditions. Those skilled in the art will be familiar with SPICE, which is an acronym for “Simulation Program with Integrated Circuit Emphasis,” and the variety of SPICE implementations commercially available. Note that in such a scenario, the current demand for the various I/O circuits may be different depending on their loading conditions. However, in order to illustrate the usefulness of the present invention, it is sufficient to assume that all I/O circuits draw equal currents, each having the value of 30.0 mA. Furthermore, it is noted that the current metric suitable for IR drop analysis may be different than the current metrics suitable for EM or ESD analysis.  
         [0046]     Typically, the allowed power route widths are limited to a small set of discrete widths that the power routing tool, e.g., power routing tool  304  of  FIG. 3 , can use when connecting the PSTs of the I/O circuits to a corresponding power distribution network. The results presented in this section are obtained using four possible widths for the power routes, 6 μm, 8 μm, 12 μm and 16 μm. As mentioned earlier, the integrated power routing algorithm, e.g., algorithm  316  of  FIG. 3 , typically starts with the assumption that the width of the power route for every I/O circuit is the minimum possible width, which, in this example, is 6 μm. Given the initial minimum-width power routes, an electrical analysis tool, e.g., electrical analysis tool  308  of  FIG. 3 , extracts the necessary data and runs the simulations to identify the I/O circuits failing the electrical requirements. The check that is used in the present example is an IR drop of more than 5% of the supply voltage.  
         [0047]     For all the I/O circuits failing this IR check, the electrical analysis tool attempts the second larger width and reruns the simulation. The integrated power routing algorithm continues iterating the electrical analysis with one or more new power route widths until all requirements are satisfied, that is, all I/O circuits have an IR drop of less than 5% of the supply voltage. Table I shows the number of I/O circuits failing the IR drop requirement when considering the different possible widths. The first column corresponds to the different test cases T 1 , T 2 , T 3 , and T 4 . The second column shows the total number of I/O circuits in each test case. The third column reports the number of I/O circuits that fail the IR drop requirement using the initial power route width of 6 μm. Then, columns 4, 5, and 6 report the number of I/O circuits failing their drop requirement after increasing the power route width to 8 μm, 12 μm, and 16 μm respectively.  
         [0048]     Note that for TC 3 , one I/O circuit still fails the IR requirement even after the maximum possible width is considered. This usually occurs when an I/O circuit is placed in an area congested with other I/O circuits, all of which draw power from the same location of the power distribution network. In such cases, the IR drop violation may be fixed by either changing the location of that I/O circuit or re-routing its power route.  
         [0049]     Observe that the results shown in Table 1 correspond to one iteration between the power routing tool and the electrical analysis tool. Basically, the power routing tool started with the minimum width of 6 μm for all I/O power routes. The electrical analysis tool, in turn, provided a set of recommendations of increasing the widths of certain I/O circuits.  FIG. 5  shows a histogram  500  of the number of I/O circuits of each possible power route width as recommended by the electrical analysis tool after the first iteration.  
         [0050]     After the electrical analysis tool is run, the power routing tool was invoked to apply the power route widths recommended by the electrical analysis tool. Then, extraction and simulation is repeated again by the electrical analysis tool. The results for these iterations between the power routing tool and the electrical analysis tool are summarized in  FIG. 6 , which shows a graph  600  of the number of failing I/O circuits after each iteration for all four test cases TC 1 , TC 2 , TC 3 , and TC 4 . Note that iteration  0  corresponds to the initial power routes, which, at that point, all have the minimum width of 6 μm.  
         [0051]     Observe that TC 1  has no violations after iteration  1 . Test cases TC 2 , TC 3 , and TC 4 , however, still have four violations each after iteration  1  and require an additional iteration. The reason that more than one iteration may be needed is that the paths of the power routes may change for some I/O circuits. The power router will attempt to follow the minimum distance path from the I/O circuit to the power distribution network. However, as mentioned above, due to blockage and spacing requirements any one of the power routes may have to follow a different path when its width has been increased.  
         [0052]     Finally, it is noted that the run time overhead of the proposed approach is minimal. The CPU time required by the power routing tool is equivalent to any regular run. The overhead of the approach is really introduced in the electrical analysis tool as the integrated power routing algorithm tries the different possible widths before providing recommendations for another iteration of power routing tool. For all four test cases TC 1 , TC 2 , TC 3 , and TC 4 , this overhead is found to be negligible. The run time of each iteration of the electrical analysis tool is less than one second and the memory required is less than 30 MB.  
         [0053]     In view of the foregoing, it is clear that the present invention offers an advantage in reducing the number of iterations between the power routing tool and the electrical analysis tool. Furthermore, it offers an automated solution that results in power routes that satisfy all electrical requirements.  
         [0054]     Although the invention has been described and illustrated with respect to an exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.