Patent Publication Number: US-7587693-B2

Title: Apparatus and method of delay calculation for structured ASIC

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an apparatus and method of delay calculation for structured ASICs (application-specific integrated circuits). More specifically, the present invention is directed to delay calculation for ASICs incorporating clock circuits that used to distribute clock signals. 
     2. Description of the Related Art 
     Delay calculation is one of the most important techniques in integrated circuit design. Operating an integrated circuit at desired timings requires accurately estimating delay times of respective circuit components within the integrated circuit by delay calculation, and verifying operation timings of the integrated circuit on the basis of the estimated delay times. For instance, a delay time of a clock circuit integrated within the integrated circuit must be accurately estimated, and then clock skews must be verified as being appropriate on the basis of the estimated delay times. In addition, delay times of desired paths within the integrated circuit must be verified as being appropriate. Under such technical background, various sorts of delay calculation methods have been proposed. 
     One of the most typical delay calculation methods is a cell-library-based method, which uses a cell delay library describing delay times of respective kinds of cells (refer to, for example, JP-A-Heisei 11-259555). The cell-library-based method is directed to delay calculation of integrated circuits that adopt a cell-based design technique. The cell-library-based method uses a cell library that contains tables describing associations of input slews and/or load capacitances with delay times for respective kinds of cells. Delay times of respective cells integrated within the target integrated circuit are acquired by table lookup from the cell library, referring a netlist of the target integrated circuit. This is followed by delay calculation of desired paths based on the acquired delay times of the respective cells. 
     Delay calculation of integrated circuits incorporating macro circuits is also known in the art, for instance, in JP-A-Heisei 10-162040 and JP-A 2001-273338. In the known techniques, delay calculation of an integrated circuit incorporating macro circuits is achieved using a macro delay library containing: tables describing associations of delay times of boundary cells connected to input terminals of macro circuits with input slews of the boundary cells; tables describing associations of delay times of boundary cells connected to output terminals of macro circuits with input load capacitances of the boundary cells; and delay times among the boundary cells. Delay calculation for desired paths within the integrated circuit is performed using the delay times acquired from the macro delay library. 
     Another typical delay calculation method is an analog simulation method. SPICE is one of the most typical simulators for analog simulation. In the analog simulation, a target integrated circuit is expressed by a circuit model described by resistors, capacitors, coils, and other primitive elements, and delay times are calculated by solving the circuit equation corresponding to the circuit model. For instance, a document entitled “A Clock Distribution Circuit Layout Design Tool for Large-scale and High-Speed ASICs” written by Masayuki Terai et al., Institute of Japanese Information Processing, Volume 43, No. 5, Pages 1294 to 1303, May in 2002, discloses that SPICE-compatible circuit simulation is applied to analyze clock skew of a clock circuit within an ASIC. 
     The inventor of the present invention has discover the fact that delay calculations of clock circuits are wastefully repeated when the conventional delay calculation methods are applied to delay calculations of structured ASICs. Conventionally, every time an integrated circuit is designed, delay times of a clock circuit integrated within the integrated circuit are estimated. For instance, when two structured ASICs are designed, delay times of clock circuits within the respective ASICs are separately calculated. In accordance with investigations by the inventor, performing delay calculations of the clock circuits within the respective structured ASICs wastefully uses design resources, which may undesirably increase the TAT in designing structure ASICs. 
     SUMMARY OF THE INVENTION 
     The inventor has discovered that delay calculations of structured ASICs are effectively facilitated by preparing a clock circuit delay library describing a result of delay calculation of a clock circuit integrated within structured ASICs. A clock circuit can be commonly integrated within various structured ASICs by incorporating the clock circuit within master slices of the structured ASICs. Once a clock circuit delay library that describes delay times of the commonly integrated clock circuit is prepared, commonly using the clock circuit delay library for the delay calculations of structured ASICs incorporating the clock circuit effectively reduces the time necessary for the delay calculations. 
     As long as the inventor knows, preparing a delay library for a clock circuit is not known in the art. This may be because usual integrated circuits have different layout patterns of clock circuits, and there is no technical sense in preparing a delay library of a clock circuit. Interconnect resistances and capacitances within clock circuits are different depending on layout patterns of the clock circuits. In the case that individual integrated circuits have different layout patterns of clock circuits, preparing a delay library for a clock circuit is not effective; however, the structured ASIC technique allows clock circuits having the same layout pattern to be integrated within various integrated circuits. As for structured ASICs, preparing a delay library for a clock circuit effectively reduces the time necessary for delay calculation. 
     In an aspect of the present invention, a delay calculation apparatus is provided for delay calculation of a structured ASIC in which a clock circuit is integrated within a master slice. The delay calculation apparatus is composed of a storage unit storing a clock circuit delay library containing clock circuit delay data obtained by delay calculation of a clock circuit, and a delay calculation section performing delay calculation of a structured ASIC incorporating the clock circuit to thereby generate delay calculation result data of the structured ASIC. The delay calculation section is designed to obtain at least a portion of the clock circuit delay data from the clock circuit delay library, and to merge the obtained portion of the clock circuit delay data into the delay calculation result data. 
     The delay calculation apparatus thus constructed effectively eliminates the need for repeating the delay calculation of the clock circuit by through incorporating the clock circuit delay data archived in the clock circuit delay library into the delay calculation result data of various structured ASICs within which the clock circuit is commonly integrated. This effectively reduces time duration necessary for delay calculation of the structured ASICs, and thereby reduces design TATs of the structured ASICs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanied drawings, in which: 
         FIG. 1  is a sectional view illustrating an exemplary structure of a structured ASIC in one embodiment of the present invention; 
         FIG. 2  is a circuit diagram illustrating circuit topology of the structured ASIC in one embodiment of the present invention; 
         FIG. 3  is a block diagram illustrating a structure of a delay calculation system in one embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating an exemplary procedure of generating a clock circuit delay library in one embodiment of the present invention; 
         FIG. 5  is a diagram illustrating contents of a netlists in one embodiment of the present invention; 
         FIG. 6A  is a conceptional diagram illustrating contents of a clock circuit delay library in one embodiment of the present invention; 
         FIG. 6B  is a conceptional diagram illustrating contents of a clock circuit delay library in another embodiment of the present invention; 
         FIG. 7  is a conceptional diagram illustrating a method of calculating a timing margin in one embodiment of the present invention; 
         FIG. 8  is a flowchart illustrating an exemplary procedure of generating a clock circuit delay library in an alternative embodiment of the present invention; 
         FIG. 9  is a conceptional diagram for explaining signal integrity verification in one embodiment of the present invention; and 
         FIG. 10  is a flowchart illustrating an exemplary procedure of calculating entire delays in the structured ASIC in one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art would recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposed. 
     Structures of Structured ASIC, and Delay Calculating System 
     In one embodiment, as shown in  FIG. 1 , a technical idea for calculating a delay of a structured ASIC (Application Specification Integrated Circuit)  1  is provided, while the structured ASIC  1  is composed of a semiconductor substrate  2  within which MOS transistors are integrated; common layers  3 , and customizing layers  4 . The common layers  3  are interconnection layers incorporating interconnections which are commonly integrated within various sorts of structured ASICs  1  to provide electrical connections among circuits (for example, I/O buffers and power supply circuits). The customizing layers  4  are interconnection layers incorporating interconnections designed in correspondence with various user specifications. The semiconductor substrate  2  and the common layers  3  are portions which have been preliminarily prepared by a vendor, and may be referred to as a “master slice.” In this embodiment, a clock circuit used to distribute clocks is integrated within the master slice. This implies that the clock circuit is commonly integrated within various structured ASICs  1 . 
       FIG. 2  is a conceptional diagram illustrating an exemplary circuit topology of the structured ASIC  1  in this embodiment. The structured ASIC  1  is provided with a clock circuit  5  which is commonly used, and a logic circuit  6  designed in accordance with user&#39;s requests. Interconnections of the clock circuit  5  are integrated within the common layers  3 , and interconnections of the logic circuit  6  are integrated within the customizing layers  4 . 
     The clock circuit  5  is composed of cells  7  and interconnections  8  that provide electrical connections among the cells  7 . The cells  7  and interconnections  8  are connected to function as a clock tree. One of the cells  7  which is located at a root of the clock tree is referred to as a “root cell  7   a ”, whereas a cell  7  which is located at a leaf of the clock tree is referred to as a “leaf cell  7   b .” The root cell  7   a  receives a clock signal to be distributed. The root cell  7   a  may be a macro circuit, such as a PLL (Phase Locked-loop) circuit and a DLL (Digital lock loop) circuit, a cell which is directly or indirectly connected to an output of such macro circuit, an I/O cell which externally receives a clock signal, and a cell which is directly or indirectly connected to an output of such I/O cell. On the other hand, the leaf cells  7   b  are used to output clock signals from the clock circuit  5 . The leaf cells  7   b  may be flip-flops or macro circuits to which clock signals are distributed, cells connected to inputs of the flip-flops or the macro circuits, I/O cells that output clock signals, or cells which is directly or indirectly connected to inputs of the I/O cells. 
     On the other hand, the logic circuit  6  is composed of circuits  9  operating in response to signals received from the leaf cells  7   b . The circuits  9  may be composed of combination circuits. 
     A detailed description is made of the structure and function of a delay calculation system used for delay calculation of the structured ASIC  1  arranged as shown in  FIGS. 1 and 2 . 
       FIG. 3  is a block diagram illustrating an exemplary structure of a delay calculation system  10  in one embodiment of the present invention. In this embodiment, commonly-used computer architecture is employed in the delay calculation system  10 . Specifically, the delay calculation system  10  is composed of a CPU (Central Processing Unit)  11 , an input device  12 , such as a keyboard and a mouse, an output device  13 , such as a display device and a printer, and a storage device  14  such as an HDD (Hard Disk Drive). 
     Stored in the storage device  14  are a clock circuit delay library generation tool  15  and a delay calculation tool  16 . The clock circuit delay library generation tool  15  is a computer program used to generate a clock circuit delay library  17 . The CPU  11  executes code modules contained in the clock circuit delay library generation tool  15  to generate the clock circuit delay library  17 . The clock circuit delay library generation tool  15  performs analog simulation as to the clock circuit  5 , and stores delay calculation results obtained by the analog simulation in the clock circuit delay library  17 . The contents of the clock circuit delay library  17  will be later explained in detail. 
     The delay calculation tool  16  is a computer program used for delay calculation of an entire circuitry of the structured ASIC  1 . The CPU  11  executes code modules contained in the delay calculation tool  16  to perform delay calculation of the entire structured ASIC  1 . The delay calculation tool  16  performs the delay calculation of the entire structured ASIC  1  on the basis of the clock circuit delay library  17  and a cell delay library  18 . The cell delay library  18  is a library describing delay times of various sorts of cells. Specifically, the cell delay library  18  contains two-dimensional tables describing associations of input slews (typically defined as slew rates of input signals) and load capacitances with delay times for respective cells, and two-dimensional tables describing associations of input slews and load capacitances with output slews (typically defined as slew rates of output signals). 
     One feature of the delay calculation system  10  in this embodiment is that the clock circuit delay library  17  is preliminarily prepared to store the results of the delay calculations of the clock circuit  5 , and used for delay calculations of various structured ASICs  1 . This avoids wastefully repeating delay calculations of the same clock circuit  5  integrated within various structured ASICs  1 . A detailed explanation will be made of exemplary methods of generating the clock circuit delay library  17  and of performing delay calculation using the clock circuit delay library  17 . 
     Clock Circuit Delay Library 
       FIG. 4  is a flowchart illustrating an exemplary procedure of generating the clock circuit delay library  17  in one embodiment. As previously explained, the clock circuit delay library  17  is generated by the clock circuit delay library generation tool  15 . 
     The procedure of generating the clock circuit delay library  17  begins with clock circuit extraction at Step S 01 . In the clock circuit extraction, a netlist  19  of a certain structured ASIC  1  is given to the clock circuit delay library generation tool  15 . The netlist  19  describes connections of cells integrated within the structured ASIC  1 , and capacitances of interconnections connecting the cells. In one embodiment, a netlist of a structured ASIC  1  which incorporates no customizing layer  4  (namely, a netlist of the master slice of the structured ASIC  1 ) is given as the target netlist  19  to the clock circuit delay library generation tool  15 . 
     The clock circuit delay library generation tool  15  extracts a clock circuit netlist  20  that is a netlist of the clock circuit  5  from the netlist  19 . 
       FIG. 5  is a conceptional view illustrating contents of the clock circuit netlist  20 . The clock circuit netlist  20  describes connections of the cells  7  integrated within the clock circuit  5 , and capacitances of the interconnections integrated within the clock circuit  5 . It should be noted that the logic circuit  6  is excluded from the clock circuit netlist  20 . 
     The extraction of the clock circuit netlist  20  is followed by delay calculation of the clock circuit  5  at Step S 02 . The delay calculation of the clock circuit  5  is achieved by analog simulation based on the clock circuit netlist  20 . The use of analog simulation for delay calculation of the clock circuit  5  is advantageous because analog simulation provides highly precise delay calculation for the clock circuit  5 , which may be commonly integrated within various structured ASICs  1 . The clock circuit  5  is desirably designed so as to reduce clock skews as far as possible, since logic circuits  6  within structured ASICs  1  may be arranged in various manners. In order to assure that the clock skews are sufficiently reduced, analog simulation is used to perform highly precise delay calculation of the clock circuit  5 . 
     Specifically, the following data is calculated in the delay calculation at Step S 02 : 
     (1) delay times of the cells  7  and the interconnections  8  integrated within the clock circuit  5 . 
     (2) input slews (typically, slew rates of input signals of the cells  7 ) of the respective cells  7  except for the root cell  7   a ; and 
     (3) a load capacitance of an output terminal of the root cell  7   a.    
     The delay calculation of the clock circuit  5  is followed by clock circuit library archiving at Step S 03 . In the clock circuit library archiving, the data obtained by the delay calculating operation of the clock circuit  5  is archived as clock circuit delay data into the clock circuit delay library  17 . 
     Specifically, the clock circuit delay data archived in the clock circuit delay library  17  is composed of the following data: 
     (1) delay time data of the circuit portion between the output terminal of the root cell  7   a  and the input terminals of the respective leaf cells  7   b;    
     (2) a load capacitance of the output terminal of the root cell  7   a  (namely, the sum of the capacitances of the interconnections  8  connected to the root cell  7   a , and input capacitances of the cells  7  connected to the relevant interconnections  8 ); and 
     (3) input slews of the respective leaf cells  7   b  within the clock circuit  5 . The following is a description of the respective data archived in the clock circuit delay library  17 . 
       FIGS. 6A and 6B  are conceptional diagrams illustrating contents of delay time data stored in the clock circuit delay library  17 . In one embodiment, as shown in  FIG. 6A , the delay time data archived in the clock circuit delay library  17  include delay times of the respective cells  7  and interconnections  8  located between the output terminal of the root cell  7   a  and the input terminals of the leaf cells  7   b . In another embodiment, as shown in  FIG. 6B , the delay time data archived in the clock circuit delay library  17  include delay times of respective paths from the output terminal of the root cell  7   a  to the input terminals of the respective leaf cells  7   b , which may be referred to as path delay times. In an alternative embodiment, the delay time data archived in the clock circuit delay library  17  may include both of the delay times of the respective cells  7  and interconnections  8 , and the path delay times from the output terminal of the root cell  7   a  up to the input terminals of the respective leaf cells  7   b . Archiving the delay times of the respective cells  7  and interconnections  8  in the clock circuit delay library  17  is advantageous for performing verification depending upon the paths of the clock circuit  5 . This allows, for example, calculating a delay time between two desired points within the clock circuit  5 . On the other hand, archiving the path delay times in the clock circuit delay library  17  is advantageous for executing verification that does not depend upon the paths of the clock circuit  5 . In this case, each of the paths from the output terminal of the root cell  7   a  of the clock circuit  5  to the input terminals of the respective leaf cells  7   b  is entirely handled as one delay element. 
     It should be noted that the circuit portion of the clock circuit  5  for which the delay times are archived in the clock circuit delay library  17  is defined as the portion from the output terminal of the root cell  7   a  to the input terminals of the respective leaf cells  7   b ; the root cell  7   a  and the leaf cells  7   b  themselves are not involved in this range. In other words, the delay times of the root cell  7   a  and the leaf cells  7   b  are not described in the clock circuit delay library  17 . 
     The reason why the delay time of the root cell  7   a  is excluded from the clock circuit delay library  17  is that the delay time of the root cell  7   a  varies depending on the input slew of the root cell  7   a . In order to improve the preciseness of the delay calculations of the entire structured ASIC  1  on the basis of the association of the delay time of the root cell  7   a  with the input slew thereof, it is preferable to calculate the input slew of the root cell  7   a  from the arrangement of the circuit connected to the input of the root cell  7   a , and then to calculate the delay time of the root cell  7   a  from the input slew calculated. 
     On the other hand, the reason why the delay times of the leaf cells  7   b  are excluded from the clock circuit delay library  17  is that the data amount of the clock circuit delay library  17  and the calculation amount required for generating the clock circuit delay library  17  are reduced. In general, a delay library of a macro circuit contains delay tables describing associations of delay times of respective output cells within the macro circuit (namely, cells directly connected to the respective output terminals of the macro circuit) with load capacitances of the respective output terminals. However, such approach is not employed for the clock circuit delay library  17 . This is because the number of the output terminals of the clock circuit  5  (namely, the number of the leaf cells  7   b ) is extremely large, due to the fact that clock signals are distributed to a large number of circuits. The number of the leaf cells  7   b  of the clock circuit  5  may be increased up to several thousands. Providing the delay tables for the respective leaf cells  7   b  undesirably increases the data amount of the clock circuit delay library  17 . 
     In this embodiment, the clock circuit delay library  17  describes the input slews of the respective leaf cells  7   b , instead of incorporating tables describing the delay times of the leaf cells  7   b . It should also be noted that the input slews of the respective leaf cells  7   b  are fixed values. As will be explained later, the delay times of the respective leaf cells  7   b  are separately calculated on the basis of the input slews of the leaf cells  7   b  described in the clock circuit delay library  17 . This effectively reduces the data amount of the clock circuit delay library  17 , and thereby reduces the calculation amount required for generating the clock circuit delay library  17 . 
     Archiving the input slews of the respective leaf cells  7   b  in the clock circuit delay library  17  is also advantageous in that this eliminates the need for archiving the tables describing associations of output slews of the leaf cells  7   b  (that is, the slew rates of the output signals of the leaf cells  7   b ) with the load capacitances thereof in the clock circuit delay library  17 . In general, a delay library of a macro circuit contains output slew tables describing associations of output slews of respective output cells within the macro circuit with load capacitances of the respective output terminals. The output slew tables are used for delay calculations of the output cells of the macro circuit. However, preparing the output slew tables for the respective output cells undesirable increases the data amount of the clock circuit delay library  17 . In this embodiment, the input slews are archived in the clock circuit delay library  17  instead of output slew tables, to thereby reduce the data amount of the clock circuit delay library  17 . 
     Strictly speaking, the input slew of the root cell  7   a , which influences the delay time of the root cell  7   a , also slightly influences delay times of the cells  7  connected to the output terminal of the root cell  7   a  (which will be referred to as the next-staged cells  7   c , hereinafter). This is because the input slew of the root cell  7   a  influences input slews of the next-staged cells  7   c , which influences the delay times of the next-stage cells  7 . The operation timings of the structured ASIC  1  are desirably verified on the basis of the influences of the input slew of the root cell  7   a  on the delay times of the next-staged cells  7   c.    
     In order to allow for the influences of the input slew of the root cell  7   a  on the delay times of the next-staged cells  7   c , it is desirable that timing margins are determined on the basis of the influences of the input slew of the root cell  7   a  on the delay times of the respective next-staged cells  7   c , and then archived into the clock circuit delay library  17 . The timing margin for a target next-staged cell  7   c  is desirably calculated as follows: 
     Referring now to  FIG. 7 , a delay time “delay 1 ” of the target next-staged cell  7   c  is firstly calculated for the case when the input slew rate of the root cell  7   a  is a predetermined standard value. Specifically, time duration between the pull-up of the input signal of the target next-staged cell  7   c  up to a threshold value “Vth” (or the pull-down of the input signal to the threshold value “Vth”) and the pull-up of the output signal of the target next-staged cell  7   c  up to the threshold value Vth (or the pull-down of the output signal to the threshold value “Vth”) is calculated for the case when the input slew of the root cell  7   a  is the standard value, and the calculated time duration is defined as the delay time “delay 1 .” It should be understood that, in  FIG. 7 , the solid lines  31  and  32  indicates the waveforms of the input and output signals of the target next-staged cell  7   c  with the input slew of the root cell  7   a  being the standard value, respectively. 
     Furthermore, a delay time “delay 2 ” of the next-staged cell  7   c  is calculated for the case when the input slew rate of the root cell  7   a  is an allowable minimum value. Specifically, time duration between the pull-up of the input signal of the target next-staged cell  7   c  up to a threshold value “Vth” (or the pull-down of the input signal to the threshold value “Vth”) and the pull-up of the output signal of the target next-staged cell  7   c  up to the threshold value Vth (or the pull-down of the output signal to the threshold value “Vth”) is calculated for the case when the input slew rate of the root cell  7   a  is the allowable minimum value, and the calculated time duration is defined as the delay time “delay 2 .” 
     The timing margin Δdelay for the target next-staged cell  7   c  is defined as the difference between the delay time “delay 2 ” and the delay time “delay 1 ”, and is archived in the clock circuit delay library  17 . In timing verification of a certain path within the clock circuit  5 , the operation timing is judged to be proper, when the sum of the delay time of the certain path and the associated timing margin “Δdelay” is in a predetermined range. 
       FIG. 8  is a flowchart illustrating an exemplary procedure for generating the clock circuit delay library  17  in an alternative embodiment of the present invention. The procedure shown in  FIG. 8  addresses allowing for influences of the logic circuit  6  designed in accordance with the user&#39;s request on the delay times of the interconnections  8  within the clock circuit  5  in timing verification of the structured ASIC  1 . Strictly speaking, the capacitances of the interconnections  8  within the clock circuit  5  changes depending on the layout of the logic circuit  6  due to the capacitive coupling between the interconnections  8  within the clock circuit  5  and the interconnections within logic circuit  6 . The capacitive coupling may influence the generation of the clock skews. In this embodiment, clock skew margins are determined on the basis of influences of the changes in the capacitances of the interconnections  8 , and archived in the clock circuit delay library  17 . In the timing verification of the structured ASIC  1 , a clock skew of the clock circuit  5  is judged to be proper, when the sum of the clock skew of the clock circuit  5  calculated by delay calculation and the associated skew margin is smaller than a predetermined reference value. 
     More specifically, the generation of the clock circuit delay library  17  in this embodiment begins with clock circuit extractions at Steps S 01 A and S 01 B for a user-circuit-excluded netlist  19 A and a use-circuit-including netlist  19 B, respectively. The user-circuit-excluded netlist  19 A is a netlist of a structured ASIC  1  incorporating no logic circuit  6  (namely, a netlist of a structured ASIC  1  in which no customized layer  4  is integrated), while the use-circuit-including netlist  19 B is a netlist of a structured ASIC  1  incorporating a logic circuit  6  (namely, a netlist of a structured ASIC  1  in which one or more customized layers  4  are integrated. The user-circuit-excluded netlist  19 A and the use-circuit-including netlist  19 B describe connections of the cells, and capacitances of interconnections providing electrical connections among the cells. As will be understood from the following description, it is important that the user-circuit-excluded netlist  19 A and the use-circuit-including netlist  19 B describe the capacitances of the interconnections. 
     In a preferred embodiment, the user-circuit-including netlist  19 B is a netlist of the structured ASIC  1  that is designed to cause most severe capacitive coupling, namely, the structured ASIC  1  incorporating a logic circuit  6  having such a structure that the coupling capacitances between the interconnections within the clock circuit  5  and the logic circuit  6  are increased as far as possible. 
     Upon receiving the user-circuit-excluded netlist  19 A and the user-circuit-including netlist  19 B, the clock circuit delay library generation tool  15  extracts clock circuit netlists  20 A and  20 B from the user-circuit-excluded netlist  19 A and the use-circuit-including netlist  19 B, respectively. The clock circuit netlist  20 A is a netlist corresponding to the clock circuit  5  extracted from user-circuit-excluded netlist  19 A, and the clock circuit netlist  20 B is a netlist corresponding to the clock circuit  5  extracted from user-circuit-including netlist  19 B. Although describing the structure of the clock circuit  5  (namely, the connections of the cells within the clock circuit  5 ) in the same manner, the clock circuit netlists  20 A and  20 B differently describes the capacitances of the interconnections within the clock circuit  5 . The differences in the interconnection capacitances give influences on the delay times of the clock circuit  5 . 
     After the extraction of the clock circuit netlists  20 A and  20 B, clock circuit delay calculations are carried out for the clock circuit netlists  20 A and  20 B at Steps S 02 A and S 02 B. Specifically, the following data are calculated in the clock circuit delay calculations at Steps S 02 A and S 02 B for the respective clock circuit netlists  20 A and  20 B: 
     (1) delay times of the respective cells  7  and interconnections  8  within in the clock circuit  5 ; 
     (2) input slews of the cells  7  (typically, slew rates of input signals thereof) within the clock circuit  5  except for the root cell  7   a ; and 
     (3) a load capacitance of the output terminal of the root cell  7   a.    
     It should be noted that the clock circuit netlist  20 A is associated with the case where no logic circuit  6  is integrated within the structured ASIC  1 , while the clock circuit netlist  20 B is associated with the case where a specific logic circuit  6  is integrated within the structured ASIC  1 . The delay calculations are carried out by using analog simulation. 
     After the clock circuit delay calculations, delay time differences are extracted at Step S 04 . Specifically, delay times of respective paths (that is, path delay times) from the output terminal of the root cell  7   a  to the input terminals the respective leaf cells  7   b  are calculated with respect to both of the clock circuit netlists  20 A and  20 B. The difference of the path delay times between the case where the logic circuit  6  is incorporated and the case where no logic circuit  6  is incorporated is calculated for each path within the clock circuit  5  from the clock circuit netlists  20 A and  20 B. 
     This is followed by library archiving at Step S 03 A. In the library archiving, the data obtained by the clock circuit delay calculations and the delay time difference extraction are archived in the clock circuit delay library  17 A. More specifically, the following data is archived in the clock circuit delay library  17 A: 
     (1) delay time data of the circuit portion between the output terminal of the root cell  7   a  and the input terminals of the respective leaf cells  7   b;    
     (2) a load capacitance of the output terminal of the root cell  7   a  (namely, the sum of the capacitances of the interconnections  8   a  connected to the root cell  7   a , and input capacitances of the cells  7  connected to the interconnections  8   a ); and 
     (3) input slews of the respective leaf cells  7   b  within the clock circuit  5  (typically, slew rates of input signals of the respective leaf cells  7   b ). Additionally, the differences of the path delay times obtained at Step S 04  are archived in the clock circuit delay library  17 A. 
     In the timing verification of the structured ASIC  1 , the clock skew of the clock circuit  5  is judged to be proper, when the sum of the clock skew of the clock circuit  5  calculated by the delay calculation and the clock skew margin is smaller than a predetermined reference value. 
     As previously explained, the delay time data stored in the clock circuit delay library  17 A may be the delay times of the respective cells  7  and interconnections  8  (See  FIG. 6A ), and the path delay times of the respective paths from the output terminal of the root cell  7   a  to the input terminals of the respective leaf cells  7   b  (See  FIG. 6B ). Alternatively, both of the delay times of the respective cells  7  and interconnections  8 , and the path delay times of the respective paths from the output terminal of the root cell  7   a  to the input terminals of the respective leaf cells  7   b  may be archived in the clock circuit delay library  17 A. 
     In a preferred embodiment, the generation of the clock circuit delay library  17  (or  17 A) is accompanied by performing signal integrity verification of the clock circuit  5 . This effectively eliminates the need for wastefully repeating the signal integrity verification of the clock circuit  5  in designing various structured ASICs that commonly incorporates the clock circuit  5 . 
     Referring now to  FIG. 9 , the signal integrity verification of the clock circuit  5  is achieved by using analog simulation under a condition that a clock signal having the allowable maximum frequency is fed to the root cell  7   a  within the structured ASIC  1 . The use of analog simulation for the signal integrity verification is preferable to improve the preciseness of the signal integrity verification. The signal integrity verification preferably includes full swing verification, electromigration verification, and hot carrier reliability verification. 
     The full swing verification involves calculating waveforms of the clock signals inputted to the input terminals of the respective leaf cells  7   b  by the analog simulation, and examining whether or not the respective clock signals are fully switched between the ground level “V SS ” and the power supply level “V DD .” In the case that any clock signal is not fully switched, the delay calculation system  10  outputs an error message from the output device  13  so as to notify the occurrence of the error in the full swing verification to the design engineer. This effectively promotes the design engineer to modify the specifications and/or design of the structured ASIC, so that the allowable maximum frequency is reduced, or the clock circuit  5  is redesigned. 
     The electromigration verification involves calculating current waveforms of output terminals of the respective cells  7  within the clock circuit  5  by analog simulation, and examining whether or not the calculated current waveforms satisfy a predetermined standard of the electromigration verification. Finally, the hot carrier reliability verification involves examining whether or not current waveforms of the output terminals of the respective cells  7  satisfy a predetermined standard of the hot carrier reliability verification. When the current waveform of the output terminal of any cell  7  does not satisfy the predetermined standards, the delay calculation system  10  outputs an error message from the output device  13  so as to notify the occurrence of an error in the electromigration and/or hot carrier reliability verifications to the design engineer. This effectively promotes the design engineer to modify the specifications and/or design of the structured ASIC, so that the allowable maximum frequency is reduced, or the clock circuit  5  is redesigned. 
     Delay Calculation Using Clock Circuit Delay Library 
     As previously explained, the delay calculation of the entire structured ASIC  1  is performed using the clock circuit delay library  17  (or  17 A).  FIG. 10  is a flowchart illustrating an exemplary procedure of the delay calculation of the entire structured ASIC  1 . 
     The delay calculation of the entire structured ASIC  1  begins with removing the clock circuit  5  from the netlist of the entire structured ASIC  1  at Step S 11 . Specifically, a target netlist  21  which is a netlist of the target structured ASIC  1  is given to the delay calculation tool  16 , and the netlist portion corresponding to the circuit portion of the clock circuit  5  for which the delay time data is archived into the clock circuit delay library  17  is removed from the target netlist  21  by the delay calculation tool  16  to thereby generate a logic circuit netlist  22 . 
     Contents of the logic circuit netlist  22  are illustrated in a lower right portion of  FIG. 5 . The logic circuit netlist  22  describes connections of the cells and capacitances of the interconnections that connect the cells for the target structured ASIC  1  except for the circuit portion between the output terminal of the root cell  7   a  and the input terminals of the leaf cells  7   b . It should also be noted that the logic circuit netlist  22  includes the descriptions as to the root cell  7   a  and the leaf cells  7   b.    
     Referring back to  FIG. 10 , the removal of the clock circuit  5  from the target netlist  21  is followed by delay calculation based on the logic circuit netlist  22  for the circuit portion other than the clock circuit  5  at Step S 12 . The delay calculation is directed to the circuit portion of the structured ASIC  1  other than the portion between the output terminal of the root cell  7   a  and the input terminals of the leaf cells  7   b . Specifically, the delay times of the respective cells and interconnections, and the input slews of the respective cells are calculated through table lookup of the cell delay library  18 , with respect to the circuit portion other than the portion from the output terminal of the root cell  7   a  to the input terminals of the leaf cells  7   b.    
     It should be noted that the delay times of the root cell  7   a  and the leaf cells  7   b  are calculated the delay calculation at Step S 12 ; the delay times of the root cell  7   a  and the leaf cells  7   b  are not described in the clock circuit delay library  17 . 
     The calculation of the delay time of the root cell  7   a  begins with calculating the input slew of the root cell  7   a  on the basis of the data described in the logic circuit netlist  22  and the cell delay library  18 . In addition, the load capacitance of the output terminal of the root cell  7   a  is obtained from the clock circuit delay library  17 . In  FIG. 10 , the broken line indicates that the load capacitance of the output terminal of the root cell  7   a  is obtained from the clock circuit delay library  17 . The delay time of the root cell  7   a  is calculated on the basis of the thus-obtained input slew and load capacitance of the root cell  7   a  through table lookup of the cell delay library  18 . 
     The calculation of the delay times of the leaf cells  7   b , on the other hand, begins with obtaining the input slews of the respective leaf cells  7   b  from the clock circuit delay library  17 . In  FIG. 10 , the broken line indicates that the input slews of the respective leaf cells  7   b  are obtained from the clock circuit delay library  17 . Additionally, the load capacitances of the output terminals of the leaf cells  7   b  are calculated from the data described in the logic circuit netlist  22  and the cell delay library  18 . The delay times of the leaf cells  7   b  are calculated from the thus-obtained input slews and load capacitances of the respective leaf cells  7   b  through table lookup of the cell delay library  18 . 
     Subsequently, this is followed by merging delay calculation result at Step S 13 . Specifically, the result of the delay calculation obtained at Step S 12  is merged with the data obtained from the clock circuit delay library  17  to generate delay calculation result data  23  which describes entire delay information of the target structured ASIC  1 . In this embodiment, the delay calculation result data  23  describes the following data: 
     (1) delay times of the respective cells and interconnections integrated within the structured ASIC  1 , including the delay times of the cells  7  and interconnections  8  within clock circuit  5 ; and 
     (2) input slews of the respective cells integrated within the structured ASIC  1 . 
     The delay calculation result data  23  may additionally or alternatively include the path delay times of the paths from the output terminal of the root cell  7   a  to the input terminals of the respective leaf cells  7   b . The timing verification of the structured ASIC  1  is carried out on the basis of the delay times and the input slews described in the delay calculation result data  23 . 
     In conclusion, the system and method of delay calculation in this embodiment involves generating the clock circuit delay library  17  storing the delay calculation results of the clock circuit  5  and performing delay calculation of the structured ASIC  1  using the clock circuit delay library  17 . This eliminates the need for repeating the delay calculation of the clock circuits  5  in the delay calculations of various structured ASICs  1 . Avoiding the wasteful repetition of the delay calculation of the clock circuit  5  effectively reduces TATs of structured ASIC design. 
     It is apparent that the present invention is not limited to the above-described embodiments, which may be modified and changed without departing from the scope of the invention.