Abstract:
Techniques are provided to allow users of Electronic Design Automation (EDA) tools to define constraint template types for efficiently associating constraints with design objects to specify design intent. Processes for creating and validating the constraints are provided by user-defined plug-ins, thereby allowing users to annotate circuit designs with constraints for proprietary layout and fabrication processes without exposing the constraints to tool vendors or competitors. The constraints are re-created from the template types whenever the circuit design is loaded into the EDA tool, so that the constraints are based upon the latest version of the template type. The constraints are also re-created when validation fails. Constraint groups are reusable in different designs, technology nodes, and processes because they automatically expand to provide the specific constraints to achieve a goal. The specific constraints in a template type can be modified as technology changes, and the modification will automatically be applied to the design objects.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 11/527,199 filed Sep. 25, 2006, currently pending, which is incorporated in its entirety herein by this reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to the design of electronic circuits and more particularly to association of constraints with design objects in electronic circuit designs. 
         [0004]    2. Description of the Related Art 
         [0005]    Electronic Design Automation (EDA) tools are software programs that are used to design electronic circuits. A suite or combination of EDA tools may be used to convert a circuit design idea to a physical design including logic gates, circuit components, and their interconnections. A circuit designer typically uses an EDA tool to create a schematic design of a circuit. The designer may use another EDA tool from the suite to convert the schematic to a physical circuit layout, which includes the shapes, positions, and dimensions of semiconductor and conductor materials such as silicon and metal. The EDA design tool in essence translates the schematic design to a physical layout using layout rules, which specify, for example, that a particular type of transistor is to have particular dimensions and be a particular distance from other transistors in the physical layout. The physical layout can be fabricated to produce a physical implementation, which is typically a semiconductor chip. 
         [0006]    The integrated circuit may be, for example, an analog circuit such as an amplifier, a digital circuit such as a microprocessor, or a combination analog and digital circuit, such as a cellular telephone radio. In one approach to circuit design, a designer creates the schematic diagram by adding design objects such as transistors, resistors, digital logic gates, and other devices to a design canvas. The designer connects the design objects using connections that are represented as lines in the schematic. 
         [0007]    A design canvas is typically a two-dimensional area displayed by the EDA tool. The EDA tool may provide a user interface to allow the designer to modify the schematic design by modifying existing design objects on the canvas, by adding new design objects to the canvas, and by modifying or adding connections between the objects on the canvas. Some prior custom design tools allow the designer to produce “custom designs” by providing a selection of design objects that can be placed at locations on the canvas specified by the designer. Locations on the canvas may correspond to locations on the physical material on which the circuit is to be fabricated, thereby allowing the designer to control the placement of physical devices on the physical circuit. 
         [0008]    Often, design annotations are used to communicate design intent among designers, and to archive design intent. A design annotation may include one or more data values such as parameter values for layout rules. The data values may be, for example, text in a format that is readable by humans or machines. A design annotation may also include one or more names associated with the data values, e.g., to identify specific parameters to which the data values correspond. Design annotations are typically created by a user via an EDA tool user interface or by a programmatic script, and are typically associated with particular design objects. 
         [0009]    Annotations that can be recognized and used by the design tool or fabrication process are called “constraints”. Constraint-driven design is a state-of-the-art technique for productivity enhancement in modern design automation software. A constraint is typically identified by a name, which is associated with an optional value, which may specify details about the constraint. A constraint can be attached to one or more design objects, typically by a designer who wishes to specify information about a design object for the purpose of, for example, overriding default layout parameters provided by the base EDA tool, or for providing additional layout or process parameters not provided by the base EDA tool. For example, a symmetry constraint may specify that two devices are to be positioned symmetrically in the physical layout. An orientation constraint may specify that a device is to be positioned with a particular orientation, where the orientation is specified by the value of the constraint. The design tool may recognize and act on the constraints when generating the layout. The constraint may also be used in the fabrication process when fabricating the physical circuit. 
         [0010]    Designers typically enter constraints for a circuit into one or more EDA tools as part of the process of designing the circuit. However, as more constraint-driven algorithms become available, there is a corresponding increase in the number, complexity, and specialization of constraints that are available to the designer. Unfortunately, the designer&#39;s productivity can be impeded by time spent selecting and entering the appropriate constraints. Automated constraint entry techniques such as scripts and hard-coded entry of constraint groups have been developed to address this productivity impediment. Scripts have been used to generate the constraints automatically, e.g., by generating multiple constraints in response to one user action. However, the scripts often fail to accommodate continuous incremental changes to the constraints. Changes ordinarily must be made to the scripts over time in response to changes in the underlying technology that implements the circuit. Unfortunately, using existing change methodologies to change scripts can be error-prone, because, for example, an operator may enter incorrect constraints, or a script that generates constraints may contain an error that may introduce incorrect or inconsistent constraints. 
         [0011]    Hard-coded entry of groups of constraints is another technique meant to address the productivity impediment. With this group-based approach, multiple constraints can be entered at once, e.g., in response to one user action, so the constraint entry process is somewhat simplified, but such techniques still require restarting the constraint entry task from scratch for new technology or new constraint driven tools and are, therefore, subject to technological obsolescence. 
         [0012]    Users of the EDA tool may define constraints and associated instructions for creating the constraints, such as scripts or constraint groups. Such user-defined constraints are typically used by user-defined tools that interact with the EDA tool, such as tools for generating physical circuit layouts and tools for controlling the chip fabrication process. Different designs and different users may employ different specialized constraints to achieve design goals. The goals and the constraints used to achieve them may be proprietary. Therefore, there is a benefit to permitting users to define customized, specialized constraints without revealing such constraints to other parties, such as the EDA tool vendor, for example. Therefore, the constraints, associated instructions, and circuit designs that use the constraints should not ordinarily be accessible by other users, or by the EDA tool vendor, for example. 
         [0013]    There has been a need for techniques of automatically creating constraints for a circuit design in an EDA tool while maintaining correctness of the constraints over time, e.g., as technology, the circuit design, and the EDA tool change. There also has been a need to enable users to extend the EDA tool to allow the tool to create and maintain constraints according to user-defined instructions. There has further been a need to allow users to protect their extensions, and circuit designs that use their extensions, from unauthorized use by other users. Therefore, it would be desirable to have a technique for automatic management of constraints that accommodates changes to the constraints and changes to the tools, and permits easy development of customized constraints while minimizing the risk of revealing proprietary information. 
       SUMMARY OF THE INVENTION 
       [0014]    In general, in a first aspect, the invention features a method of automating the application of constraints to design objects in a circuit design created using an electronic design automation tool. The method comprises receiving selection of a circuit design object that represents a physical circuit object in a circuit being designed in the electronic design automation tool, receiving selection of a template type that produces a constraint that represents a physical limitation upon implementation of the circuit design object in the circuit, wherein the template type is represented in the electronic design automation tool by a selectable user interface feature, and producing a template instance based upon the template type, wherein the template instance associates the constraint with the circuit design object to represent in the electronic design automation tool that the physical limitation of the constraint applies to the circuit design object. 
         [0015]    Embodiments of the invention may include one or more of the following features. Producing the template instance may include identifying a template instance generation process associated with the template type, invoking the generation process to create the template instance, and associating the circuit design object with the template instance. The template instance generation process may be provided by a user of the design automation tool. The method of automating the application of constraints may further comprise providing the design object and a template instance parameter to the template instance generation process. The template instance generation process may comprise creating the constraint; associating the parameter with the constraint, and associating the design object with the constraint. 
         [0016]    The circuit design object may be stored in a circuit design file, wherein a representation of the template instance is stored to the circuit design file, and wherein the representation includes an instance name, a template type identifier that specifies the template instance&#39;s type, and a design object identifier that specifies the design object. The design automation tool may be stored in at least one tool file provided by a design automation tool vendor and the template instance&#39;s type may be stored in a template type plug-in file, wherein the at least one tool file, the template type plug-in file, and the circuit design file are separate files. The template instance may include a template instance parameter, and the method of automating the application of constraints may further comprise storing in the circuit design file the template instance parameter, wherein the instance parameter is associated on the storage medium with the template instance. 
         [0017]    The template instance may include at least one constraint, and the at least one constraint may include a constraint parameter, and the method of automating the application of constraints may further comprise storing in the circuit design file the at least one constraint, wherein the at least one constraint is associated in the circuit design file with the template instance. The circuit design may include a plurality of design objects, the at least one constraint may include a reference to at least one of the design objects, and the method of automating the application of constraints may further comprise storing in the circuit design file the reference, wherein the reference is associated in the circuit design file with the individual constraint. 
         [0018]    In general, in a second aspect, the invention features a method of automating the validation of constraints associated with circuit design objects in a circuit design created using an electronic design automation tool. The method of automating validation comprises receiving notification of a change action that affects the design, identifying a template instance validation process associated with the template type, invoking the validation process to validate the constraints, receiving a validity indicator from the validation process, and if the validity indicator indicates that the constraints are not valid, invoking a recovery process. 
         [0019]    Embodiments of the invention may include one or more of the following features. The method of automating validation may further comprise retrieving the template instance validation process from a plug-in file associated with the template type. The template instance validation process may be provided by a user of the design automation tool. The method of automating validation may further comprise providing the at least one design object and a template instance parameter to the template instance validation process. The template instance validation process may comprise determining if the value of at least one constraint parameter associated with at least one of the constraints is valid; if the constraint parameter is valid, generating a success indicator; and if the constraint parameter is not valid, generating a failure indicator. The recovery process may comprise deleting the constraints, identifying a template instance generation process associated with the template type, and invoking the template instance generation process to re-create the constraints. 
         [0020]    The change action may comprise reading in of a template instance from a storage medium, modification of a template instance parameter, or a combination thereof. The change action may comprise modification of the circuit design. The change action may comprise a change to the at least one constraint, and the recovery process may comprise rejecting the change action. The method of automating the application of constraints may include displaying a template instance representation corresponding to the template instance. The method of automating the application of constraints may further include displaying a template instance tree comprising a template instance node, wherein the template instance node represents the template instance. The method of automating the application of constraints may further include displaying a constraint node as a descendant of the template instance node in the template instance tree, wherein the constraint node represents a constraint associated with the template instance, and displaying a design object node in the tree as a descendant of the constraint node, wherein the design object node represents the design object. 
         [0021]    In general, in a third aspect, the invention features a system for automating the application of constraints to design objects in a circuit design created using an electronic design automation tool. The system comprises a design object selector to receive selection of a circuit design object that represents a physical circuit object in a circuit being designed in the electronic design automation tool, a template type selector to receive selection of a template type that produces a constraint that represents a physical limitation upon implementation of the circuit design object in the circuit, wherein the template type is represented in the electronic design automation tool by a selectable user interface feature; and a template instance producer to produce a template instance based upon the template type, wherein the template instance associates the constraint with the circuit design object to represent in the electronic design automation tool that the physical limitation of the constraint applies to the circuit design object. 
         [0022]    Embodiments of the invention may include one or more of the following features. The system may comprise a process selector to identify a template instance generation process associated with the template type, a process invoker to invoke the generation process to create the template instance, and an associator to associate the circuit design object with the template instance. The system may comprise a design object writer to store a design object of the circuit design to the circuit design file, and a template instance writer to store a representation of the template instance to the circuit design file, wherein the representation includes an instance name, a template type identifier that specifies the template instance&#39;s type, and a design object identifier that specifies the design object. 
         [0023]    In general, in a fourth aspect, the invention features a system for retrieving a circuit design annotated with a constraint from a circuit design file on a persistent storage medium. The system comprises a template instance reader to retrieve a representation of a template instance from the circuit design file, wherein the representation includes an instance name, a template type identifier, and at least one design object identifier; a template type selector to identify a template type specified by the template type identifier, a generator plug-in locator to identify a template instance generator associated with the template type if the template type is accessible, and a generator plug-in invoker to invoke the template instance generator to create a restored template instance if the instance generator is accessible, wherein the restored template instance includes a constraint that references at least one design object, wherein the at least one design object is specified by the at least one design object identifier. 
         [0024]    In general, in a fifth aspect, the invention features a system for providing a constraint manager user interface in an electronic design automation tool. The system comprises a template instance generator to create a template instance in response to selection of a template type button, wherein the template instance is an instance of a template type associated with the button, a design object selector to receive selection of a design object, a design object mapping component to associate the design object with the template instance, and a template instance display component to display a template instance tree comprising a template instance node, wherein template instance node represents the template instance. 
         [0025]    In general, in a sixth aspect, the invention features a computer program product comprising program code for automating the application of constraints to design objects in a circuit design created using an electronic design automation tool. The computer program product comprises program code operable to receive selection of a circuit design object that represents a physical circuit object in a circuit being designed in the electronic design automation tool, program code operable to receive selection of a template type that produces a constraint that represents a physical limitation upon implementation of the circuit design object in the circuit, wherein the template type is represented in the electronic design automation tool by a selectable user interface feature, and program code operable to produce a template instance based upon the template type, wherein the template instance associates the constraint with the circuit design object to represent in the electronic design automation tool that the physical limitation of the constraint applies to the circuit design object. 
         [0026]    In general, in a seventh aspect, the invention features a computer program product comprising program code for automating the validation of constraints associated with design objects in a design created using an electronic design automation tool. The computer program product comprises program code operable to receive notification of an change action that affects the design, program code operable to receive identify a template instance validation process associated with the template type, program code operable to receive invoke the validation process to validate the constraints, program code operable to receiving a validity indicator from the validation process, and program code operable to invoke a recovery process if the validity indicator indicates that the constraints are not valid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1A  is an illustrative drawing of a constraint manager and related components in accordance with some embodiments of the invention. 
           [0028]      FIG. 1B  is a schematic drawing of an illustrative computer system  10  that can run novel processes for generating and validating constraints in accordance with embodiments of the invention. 
           [0029]      FIG. 2  is an illustrative drawing of prior art constraints in an Electronic Design Automation tool. 
           [0030]      FIG. 3A  is an illustrative drawing of a constraint manager user interface in an Electronic Design Automation tool in accordance with some embodiments of the invention. 
           [0031]      FIG. 3B  is an illustrative drawing of a constraint manager user interface displaying specific constraints in an Electronic Design Automation tool in accordance with some embodiments of the invention. 
           [0032]      FIG. 4  is an illustrative drawing of a process for generating a template instance in accordance with some embodiments of the invention. 
           [0033]      FIG. 5  is an illustrative drawing of a process for validating a template instance in accordance with some embodiments of the invention. 
           [0034]      FIG. 6  is an illustrative drawing of a process for validating a template instance in accordance with some embodiments of the invention. 
           [0035]      FIG. 7  is an illustrative drawing of a process for defining a template type in accordance with some embodiments of the invention. 
           [0036]      FIG. 8  is an illustrative drawing of a process for creating a template instance in an Electronic Design Automation tool user interface in accordance with some embodiments of the invention. 
           [0037]      FIG. 9  is an illustrative drawing of a template instance generation sub-process in accordance with some embodiments of the invention. 
           [0038]      FIG. 10  is an illustrative drawing of a template instance validation sub-process in accordance with some embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0039]    The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of particular applications and their requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Moreover, in the following description, numerous details are set forth for the purpose of explanation. However, one of ordinary skill in the art will realize that the invention might be practiced without the use of these specific details. In other instances, well-known structures and devices are shown in block diagram form in order not to obscure the description of the invention with unnecessary detail. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0040]      FIG. 1A  is an illustrative drawing of a constraint manager and related components in accordance with some embodiments of the invention. An EDA tool  100  allows a user  101  to create a circuit design  102  that represents an electronic circuit. The circuit design  102  is typically a data structure in a computer-readable memory. The user  101  may be, for example, a human user of the EDA tool. The user  101  is typically associated with an organization such as a chip design company. The EDA tool  101  is typically provided by an organization such as an EDA tool vendor. The circuit design  102  includes a set of design objects  103  which represent devices, connections, and other features in the electronic circuit. The design objects  103  include at least one design object, shown as Design Object A  104 , also referred to herein as a design object  104 . The user  101  interacts with the EDA tool  100  via a user interface  105 . The EDA tool  100  may be, for example, the Cadence Virtuoso™ Custom Design Platform from Cadence Design Systems of San Jose, Calif., or another tool for creating circuit designs. 
         [0041]    Each design object, such as the design object  104  may be associated with one or more constraints  133 , as illustrated by the arrows from the constraints  133  to the design objects  103 . The arrows are exemplary and represent one possible set of associations; other associations, i.e., relations or arrows, are possible between the design objects  103  and the constraints  133 . The particular set of associations is established by the user  101  for a particular circuit design  102 . In general, each of the deign objects  103  may be associated with zero or more of the constraints  133 , and each of the constraints  133  may be associated with zero or more of the design objects  103 . 
         [0042]    The design object  104  may be, for example, a data structure stored in a computer-readable memory. As described above, the constraints  133  represent design intent, i.e., goals, for the design object  104 , e.g., desired properties of the design object. The constraints  133  may be used by other portions of the EDA tool or by other EDA tools, such as a layout tool or a placement and routing tool (not shown), which may determine a position or orientation of the device based upon the constraints, or in fabrication of a physical device or feature represented by the design object  104 , where parameters of the fabrication process may be based upon the constraints  133 . A constraint may be, for example, a minimum or maximum distance between two design objects, or an orientation between two design objects. Examples of typical constraint types are shown in the table below. 
         [0000]    
       
         
               
             
               
               
             
           
               
                   
               
               
                 Constraint Types 
               
             
          
           
               
                 Constraint Type 
                 Description 
               
               
                   
               
               
                 Symmetry 
                 Specifies a desired symmetry relation between design objects. 
               
               
                 Correlation 
                 Specifies a desired correlation between design objects. 
               
               
                 matchedParameters 
                 Specifies a desired parameter matching between design objects. 
               
               
                 Orientation 
                 Specifies a desired orientation relationship between design objects. 
               
               
                 Alignment 
                 Specifies a desired alignment relationship between design objects. 
               
               
                 layoutStructure 
                 Specifies a desired layout structure for design objects. 
               
               
                 Cluster 
                 Specifies a desired cluster relationship between design objects. 
               
               
                 parasiticEstimate 
                 Specifies an estimated parasitic value for design objects. 
               
               
                 parasiticFilter 
                 Specifies a desired parasitic filter for design objects. 
               
               
                 areaUtilization 
                 Specifies a desired area utilization for design objects. 
               
               
                 boundaryArea 
                 Specifies a desired boundary area for design objects. 
               
               
                 Distance 
                 Specifies a desired distance between design objects. 
               
               
                 powerStructure 
                 Specifies a desired power structure for design objects. 
               
               
                 IRDrop 
                 Specifies a desired voltage drop between design objects. 
               
               
                 relativeOrientation 
                 Specifies a relative orientation between design objects. 
               
               
                   
               
             
          
         
       
     
         [0043]    A template manager  120  interacts with a set of user-defined template types  150  to manage, i.e., create and verify, using user-defined instructions, sets of constraints associated with the design objects  103 . Each set of constraints  133  may, for example, represent one or more design goals or designer-specified parameters for the design objects  103 . For example, the constraints  133  may specify that placement of a specified set of design objects in a particular orientation or within a particular distance is desired. 
         [0044]    The constraint manager produces a technical effect by assigning constraints to circuit design objects and enforcing the validity of the constraints in an electronic circuit design representation in an Electronic Design Automation tool. The constraints produce a technical effect by, for example, affecting the process of manufacturing a physical semiconductor chip by providing parameter values for steps in the chip layout or fabrication processes. In one aspect, the constraint manager improves the efficiency of the process of assigning constraints to circuit designs by providing features that a circuit designer can use to group constraints. The constraint manager thereby simplifies the task of assigning multiple constraints to a design object. 
         [0045]    A template type  151  includes a template instance generation process  152  and a template instance validation process  154  to, respectively, create and verify groups of constraints. Note that the processes described herein may be implemented as a subroutine that includes instructions executable by a computer. 
         [0046]    The processes  152 , 154  and other portions of the constraint template type  151  may be packaged, i.e., stored as a plug-in. A plug-in may be stored in a file, and may include files and associated instructions for registering the processes  152 , 154  with the template manager  120  so that the template manager  120  can invoke the processes. For example, the processes  152 , 154  may be registered with the template manager  120  through commands in an application command interface of the EDA tool  100 . These commands can be typed interactively by the user  101  or invoked automatically. e.g., by a startup script that is invoked when the EDA tool  100  is initialized. The plug-in may include implementations of the processes  152 , 154  for each template type in a set of user-defined template types. Template types may also be provided by the EDA tool vendor. Such vendor-provided template types may be used as-is by the user  101 , or may be customized or extended by the user to reflect specific aspects of users&#39; circuit design technology. 
         [0047]    The constraints  133  are associated with or contained in a template instance  132 . The template instance  132  may be represented as, for example, a set of data values or an object instance stored in a computer memory. Each template instance  132  may include a reference to the template type  151  that was used to create the template instance, as shown by the arrow from the template instance  132  to the template type  151 . Multiple different template instances may be created for each template type  151 , and each template instance  132  may include a corresponding set of constraints  133 . 
         [0048]    Each template instance  132  includes an instance name  134  which may be, for example, a text string that describes the template instance. Each template instance  132  may also include one or more template instance parameters  135 . The template instance parameters  135  allow the user  101  to specify per-template-instance values that may be accessed by the template instance validation process  154 . The user may specify the values for the template instance parameters via, for example, the parameter list user interface feature  348  of  FIG. 3A . 
         [0049]    In one sense, the template instance  132  can be understood as a container that allows the EDA tool, and therefore the user  101 , to treat the set of constraints as a single entity with a name and a type. The template instance can be manipulated, e.g., created, modified, and deleted, by user interface actions that apply to the constraints in the template instance, so the user  101  can more easily manage multi-constraint expressions of design intent. Furthermore, because the template instance has an associated type, changes to the set of constraints associated with a particular type of design intent, or changes to creation or validation behavior associated with the type of design intent, can be made in a single place, i.e., the template type, and may be automatically propagated to existing constraints contained in template instances, as well as to constraints in subsequently-created template instances. The template type&#39;s validation process  154  ensures that changes made to the constraints do not result in an invalid constraint set  133 . The validation process  154  ensures that the constraint set  133  remains valid by re-creating the constraint set  133  if validation fails, and, if the constraint set  133  cannot be re-created in a valid state, by rejecting the changes. 
         [0050]    A constraint  136  in the set of constraints  133  may refer to one or more design objects  103  to represent design goals for the design objects  103 . The individual constraints in the set of constraints  133  defined by a template type may be related or unrelated to each other, but typically are related to a particular design goal represented by the template type, such as placement of a specified set of design objects in a particular orientation or within a particular distance. Each of the constraints  133  may be associated with one or more constraint parameters  137 , which can be name-value pairs which represent the actual design annotations that the constraint  133  applies to the design objects  103 . 
         [0051]    Each of the constraints  133 , e.g., Constraint A  136 , may be applied to one or more of the design objects  103  by establishing an association between the constraint  136  and the design objects  103 . In one embodiment, each of the constraints  133  includes a reference, i.e., a link, to one or more of the design objects  103  with which the constraint is associated.  FIG. 1A  shows examples of possible associations between the constraints  133  and the design objects  103 . Constraint A  136  is associated with Design Object A  104  and Design Object B  106 , as shown by the arrow  161  from Constraint A  136  to Design Object A  104  and the arrow  162  from Constraint A  136  to Design Object B  106 , respectively. In one aspect, Constraint A  136  is a constraint that relates the two associated design objects  104 , 106 , such as a distance constraint that specifies a certain distance, specified by constraint parameters  137 , to be maintained between the two design objects  104 , 106 . 
         [0052]    Similarly, Constraint B  138  is associated with Design Object B  108 , Design Object C  110 , and Design Object X  114 , as shown by the arrows from constraint B  138  to the respective design objects. Constraint B  138  may be, for example, an alignment constraint that specifies how the associated design objects are to be aligned relative to each other, in which case Constraint parameters  139  of Constraint B  138  would specify a type of alignment, e.g., edge, to indicate that the edges of the associated design objects should be aligned. 
         [0053]    Constraint X  140  may be, for example, an IR Drop constraint, which specifies a desired voltage drop for the associated Design Object C  110 , in which case constraint parameters  141  of Constraint X  140  specify the value of the desired voltage drop. 
         [0054]    In  FIG. 1 , the association between design objects and constraints is represented as arrows from constraints  133  to design objects  103 . For example, the arrows  161 , 162  represent the association of the Design Object A  104  and the Design Object B  108  with the Constraint A  136 , as described above. These arrows may be implemented in a computer program as, for example, pointers or object references stored in the constraints  133 , where the pointers or object references point to or refer to the design objects  103  associated with the constraints  133 . 
         [0055]    The association between design objects and constraints may be represented in other ways, e.g., as a list of design objects and associated constraints, or as reference links in the design objects, where the reference links refer to constraints. 
         [0056]    The template manager  120  creates the template instances  130 , which may include template instance objects that are instances of template types in the list of template types  150 . The template instances  130  may be, for example, data structures or objects stored in a computer readable memory. For illustrative purposes, three template instances are shown, including a Template Instance A  132 , a Template Instance B  132 , and a Template Instance X  144 . Each of the template instances  130  is created by and associated with the design object  104  by the template manager  120 . For example, in response to a user-initiated command to create a template instance, a template instance generator  122  of the template manager  120  invokes a user-defined template instance generation process  152  to create a template instance  132  and create a set of constraints  133  of the template instance  132 . The set of constraints  133  is created by the template instance generation process  152 . 
         [0057]    The template manager  120  also causes the template instances  130  to be validated when appropriate. For example, in response to modification of at least one of the design object references  161  associated with a constraint  136 , or in response to modification of name-value pair parameters associated with the template instance  132  or with the constraint  136 , or in response to reading in of a template instance from a file, a template instance validator  124  of the template manager  120  invokes a user-defined template instance validation process  154  to validate the template instance  132 . 
         [0058]    The template manager  120 , including the template instance generator  122 , which may be a process, and the template instance validator  124 , which may also be a process, may be implemented in a computer program encoded in computer readable media. For example, the template manager  120  may be implemented in a programming language such as Cadence™ SKILL, Lisp, C++, Java™ or the like. 
         [0059]    The template instances  130  can be created when a user  101  interacts with the user interface  105  by selecting a particular template type  151  to be associated with one or more user-selected design objects  103 . The template instance  132  maintains a link, i.e., a reference, to the template type  151  from which the template instance  132  was created. This link provides for updating of the template instance  132  over time as technology changes. For example, if the technology underlying the constraints for a goal associated with the template type  152  changes, then the template instance  132  may be re-created with the new constraints. The template instance validation process  154  may initiate such a re-creation by examining the template instance  132  to determine if the template instance includes appropriate, e.g., up-to-date, constraints  133 . The template instance generation process  142  and the template instance validation process  154  may be modified over time to cause re-creation of template instances and constraints to reflect changes in technology. Automatic creation of constraints while maintaining constraints in a correct-by-construction manner is provided by the constraint manager and user-defined template instance generation and validation sub-processes, which the user can define to provide technology-specific constraints and validation behavior. The user can extend the features of the EDA tool with template types that provide those automatic constraint creation and correctness preservation features. 
         [0060]    The constraint manager can improve design and manufacturing efficiency by detecting and correcting invalid, i.e., inconsistent, constraints or constraint groups. Invalid constraints may result in fabrication of defective chips, or, if the invalid constraints are detected at a later stage, in wasted effort, because errors detected later in the chip manufacturing process are more difficult to correct than errors detected earlier, such as during the design phase. 
         [0061]    When the template instance  132  is associated with a design object, such as the design object  104 , the constraints  133  in the template instance are automatically applied to (i.e., associated with) that design object for subsequent use by other portions of the EDA tool  100 . Multiple template instances  130  may be created from a single template type  151 . The constraints  133  in a template instance  132  are generated by a user-defined instance generation process  152  of the template type  151 . The instance generation process  152  may be packaged as a plug-in, and is invoked by a template manager  120  when a user requests creation of a new template instance for that type. The template manager  120  includes a template instance generator  122 , which calls the instance generation process  152  to create the template instance  132 . 
         [0062]    The user  101  may associate template instances  130  with design objects  103  by interacting with the user interface  105 . The template instance  132  may include multiple constraints, as illustrated by a Constraint A  136 , a Constraint B  138 , and a Constraint X  140 . Each constraint includes a set of one or more constraint parameters  137 , which are, for example, name-value pairs. In one aspect, the name of a constraint parameter  137  represents a required or desired property of the circuit design, and the associated value represents the value of that property. 
         [0063]    After the template instance  132  has been associated with one or more of the design objects  103  by the user  101 , the integrity, i.e., correctness, of the constraints  133  in the template instance, may be validated in response to certain events. The constraints  133  are validated because the constraints can be modified or deleted by, and such modification or deletion can lead to an incorrect or incomplete set of constraints  133 . This validation of the constraints  133  is performed by a user-defined instance validation process  154  associated with the template type. The instance validation process may be packaged as a plug-in, and is called by the template manager in response to certain events. These events include reading a template instance from a storage medium such as a disk file, modification of a template instance parameter, or modification of a constraint. A template instance parameter allows a user  101  to specify a value to be associated with a template instance. These modifications may be performed, for example, by the user. When any of those events occur, a template instance validator  124  of the template manager  120  invokes the instance validation process  154 . The instance validation process  154  checks the constraints  133  and any other content of the template instance  132  using conditions defined by the user. If the template instance validation process  154  finds that the template instance  132  is valid, then the template instance validator  124  takes no further action. However, if the template instance is found to be invalid, then the template instance validator  124  may replace the existing constraints  133  with a newly-created set of constraints, or delete the template instance, or reject the change, depending on the type of event. 
         [0064]    A process or sub-process is a set of computer program code instructions that can be invoked, i.e., called, by other computer program code instructions, which are referred to herein as invokers. The instructions in a process are executed when the process is invoked. Parameter passing may occur, i.e., input objects may be passed from the invoker to the process when the process is invoked, and output objects may be passed from the process to the invoker when the process returns, i.e. finishes executing or executes a return statement. The process may have process arguments, which are names and types of objects accepted by the process. The arguments receive particular objects provided by the invoker when the process is invoked. The objects provided by the invoker are referred to as parameters. The process may use the parameter objects passed by the invoker to perform computations, and may compute one or more output values, which are returned to the invoker when the process returns. 
         [0065]    For example, a user-defined process may be invoked by an EDA tool at a particular point in the EDA tool&#39;s execution. Such a user-defined process, which is referred to as a plug-in, allows the EDA tool to be extended with user-defined behavior so that users such as the user  101  can customize the EDA tool for use with their design, layout, and fabrication processes. 
         [0066]    In one aspect, the processes  152 , 154  are associated with each instance of the template type  151 . Invoking the template instance generation process  152  twice with the same input, i.e., with the same parameters and design objects, will produce two with the same constraints  133 . Invoking the template instance validation process  154  on two template instances of the same template type  151  with the same parameters and design objects will produce the same result, i.e., valid or invalid. 
         [0067]      FIG. 1B  is a schematic drawing of an illustrative computer system  10  that can run novel processes for generating and validating constraints in accordance with embodiments of the invention. The computer system  10  includes one or more central processing units (CPU&#39;s)  12 , a user interface  14 , computer readable storage media  16 , a system bus  18 , and one or more bus interfaces for connecting the CPU, user interface, memory and system bus together. The computer system also includes a network interface  20  for communicating with other devices  22  on a computer network. 
         [0068]    Computer-readable code such as code for implementing the template instance generation process of  FIG. 4 , the template instance validation process of  FIG. 5  or  FIG. 6 , the template instance generation sub-process of  FIG. 9 , or the template instance validation sub-process of  FIG. 10  can be stored in the storage memory  16  and used to control the CPU  12 . Moreover, a data structure representing constraint template instances such as the template instance data structures  130  can be stored in storage memory  16 . The computer program instructions can cause the CPU  12  to produce the template instance data structures  130  and to generate and validate constraint templates that associate constraints with the design objects  103  of the circuit design  102 . 
         [0069]      FIG. 2  is an illustrative drawing of prior art constraints in an Electronic Design Automation tool. An EDA tool user interface  200  is provided by an EDA tool (not shown). The user interface  200  is displayed on, for example, a workstation computer, and interacts with a user  201 , who is typically a circuit designer. The user interface  200  provides a design canvas  202 . The design canvas  202  is an area upon which design objects can be placed, positioned, and connected. The user  201  interacts with the user interface  200  to place representations of design objects, such as a transistor representation P 1   204  and a transistor representation N 2   206 , on the design canvas  202  to create a circuit diagram  203 , which is a displayed representation of a circuit design corresponding to the circuit design  102  of  FIG. 1A . 
         [0070]    The EDA tool typically stores the circuit diagram  203 , including the design objects on the design canvas  202 , on a storage medium such as a disk file. The EDA tool can subsequently read in the circuit diagram  203  from the file, so that the circuit diagram can persist after the EDA tool program has stopped running, or after the workstation has lost power. The file also allows the circuit diagram  203  to be shared among different users in different locations. The file may be stored on a storage medium such as a magnetic disk or tape. Users&#39; intellectual property is typically encoded in constraint management and validation sub-processes. Such intellectual property is protected from unauthorized distribution because users need not make their template types or sub-processes public. In one aspect, users can control the distribution of circuit designs that use their constraint types because the template instances, i.e., constraints, associated with circuit designs can only be loaded by the EDA tool if the template types and sub-processes are available. That is, loading of a template instance is prevented if the template instance&#39;s corresponding template type definition is not available to the EDA tool. 
         [0071]    The user  201  may add design objects such as the transistor P 1   204  to the circuit design  203 , e.g., by selecting a transistor device from a palette of device types (not shown) and placing the transistor device at an appropriate position on the canvas  202 . The user  201  may connect the design objects using connectors, which are another type of design object. The user may associate constraints with the design objects of the circuit design  203  by, for example, selecting a design object from the palette, selecting a menu option to add a constraint, and entering the name and value of the constraint. In this example, a constraint browser  208  shows two constraints, an orientation constraint  210  and a distance constraint  218 . The orientation constraint  210  indicates, e.g., to a circuit designer or to another component of the EDA tool, that associated design objects  216  should be oriented in a certain way, as specified by parameters  214 . The orientation constraint  210  is associated with the transistor P 1   204  and the transistor N 1   206 , as shown by the associated design objects  216 . The distance constraint  218  is similarly associated with the transistor P 1   204  and the transistor N 1   206 , as shown by associated design objects  224 . The distance constraint  218  indicates that the associated design objects  224  should be separated by a distance specified by a distance parameter  222 . 
         [0072]      FIG. 3A  is an illustrative drawing of a constraint manager user interface in an Electronic Design Automation (EDA) tool in accordance with some embodiments of the invention. An EDA tool user interface  300  includes a design canvas component  302  similar to the design canvas  202  described above. The EDA tool user interface is an example of the user interface  105  of  FIG. 1A . The EDA tool user interface  300  also includes a constraint manager component  303 , which is a user interface that displays a set of template types  310  and allows a user to create a template instance  326 . 
         [0073]    A user may associate constraints with design objects by creating a template instance. To create a template instance, the user identifies the design objects that are to be constrained, e.g., by selecting one or more design objects, such as the transistors  304 , 306 , from the design canvas  302 . Next, the user selects a template type from a set of template types  310  to create the desired constraints. The set of template types  310  is shown as a set of icons, which are user interface features that have a distinct visual appearance. Two example template type icons, icon T  312  and icon U  314 , are shown. The template types  312 , 314  are displayed as icons that include illustrations of, for example, a single-letter abbreviation that identifies the template type. Alternatively, the template types could be displayed as a menu of type names, e.g., a drop-down or pop-up menu. The name or icon  312  of the template type typically indicates the goal associated with the template type, to allow the user to decide, based on the appearance of the name or icon, which template type to select to create a desired set of constraints. 
         [0074]    Next, the user may provide values for template instance parameters. The names and types of template instance parameters are provided by the template type and may be displayed as data fields for which the user can provide values. A template instance is then created by the template manager  120  of  FIG. 1A . The template instance includes the constraints from and stores references, i.e., links, to the design objects and the template instance parameters. For example, the design object  334  may represent the transistor  304 , and the design object N  336  may represent the transistor  306 . 
         [0075]    The constraint manager  303  displays previously-created independent constraints, such as an independent constraint  322 , and previously-created template instances, such as the template instance  326  and a template instance N  346 . The independent constraint  322  is a constraint that is not associated with a template instance and is ordinarily created directly, i.e., without use of a template instance, in response to a user command. The independent constraint  322  may be associated with independent constraint parameters  324 . When a new independent constraint or template instance is created, a visual representation of the constraint or template instance is displayed by the constraint manager  303  with the existing constraints and template instances, e.g., below those existing objects in a scrollable list. 
         [0076]    Each displayed template instance may be shown in an expanded state or in a collapsed state. The template instance  326  is shown in the expanded state, in which a template instance name  328 , constraints  332 , design objects  334 , 336 , and constraint parameters  333  are displayed. The template instance N  346  is shown in the collapsed state, in which only the template instance name is shown. The user can toggle between the expanded and collapsed states by selecting a tree expansion user interface feature  321 , which is displayed, for example, as a box enclosing an arrow adjacent to the object, e.g., template instance or constraint, which can be expanded or collapsed. 
         [0077]    The template instance  326  is shown in the expanded state, with an associated constraint  330  and an associated constraint N  338 . The constraint  330  is also shown in the expanded state, in which constraint parameters  333 , a design object  334 , and a design object N  336  are shown. The constraint parameters  333  have initial values, which are provided by the template type  312 . The values of the constraint parameters  333  may be changed by the user. 
         [0078]    The user may provide template instance parameter values in the parameter list user interface feature  348 , which may be, for example, a two-column list  350 , in which the first column displays a parameter name  352 , and the second column displays a parameter value  354 . The user may edit the parameter value  354  to change the value of a parameter. The parameter list feature  348  displays the predefined template type parameters. The user may select parameter values for the template instance using a drop down menu or similar user interface feature (not shown). In one example, the template instance  326  displays template instance parameters but not constraint parameters. In another example, the same parameters, e.g., both the template instance parameters and the constraint parameters, may be displayed as part of both the template instance  326  and the parameter list feature  348 . The template instance  326  appears in the EDA tool user interface  300  as a displayed representation of the template instance. 
         [0079]      FIG. 3B  is an illustrative drawing of a constraint manager user interface displaying specific constraints in an Electronic Design Automation tool in accordance with some embodiments of the invention. Specific constraints are shown in the EDA tool user interface  300 , including an independent Symmetry constraint  362  with an associated constraint parameter  364  that has the value “Mirror”. A template instance  326  is named “Match”, as shown by the name  368 . The Match template instance  326  includes a Distance constraint  370 , an Alignment constraint  380 , and an IR Drop constraint  390 . The Distance constraint  370  has a constraint parameter  373  with the value  5 , which indicates that the desired distance between the associated design objects P 11   374  and N 10   376  is 5 distance units. The Alignment constraint  380  has a constraint parameter  383  with the value side=edge, which indicates that the alignment of the associated design objects is to be determined by the edges of the objects. The IR Drop constraint  390  has a constraint parameter  393  with the value  36 , which indicates that the voltage drop between the associated design objects is 36 voltage units. A “T” icon  325  displayed adjacent to a name  368  of the Match template instance  326  indicates that the Match template instance  326  is an instance of the template type T  312 . The highlighted display state of the name  368  indicates that the Match template instance  326  is a selected template instance, i.e., has been placed in a selected state by the user. The design objects associated with the selected template instance are highlighted in the design canvas  302 . For example, since the design objects P 11   304  and N 10   306  are associated with the Match template instance  326 , and the Match template instance is selected, the design objects P 11   304  and N 10   306  are highlighted using thicker lines than other design objects on the canvas  302 . 
         [0080]      FIG. 4  is an illustrative drawing of a process for generating a template instance in accordance with some embodiments of the invention. The process of  FIG. 4  corresponds to the template instance generator  122  and is invoked by the template manager  120  of  FIG. 1A  when a new template instance is to be created, e.g., when a user selects a template type icon such as the T icon  312  in the user interface  300  of  FIG. 3A . The process creates a template instance for a given template type, a given set of design objects, and a given set of template instance parameters. The process begins at block  402  by determining if the given template type exists in a list of template types. If the template type does not exist, then the process ends by returning a failure status at block  416 . If the template type does exist, then block  404  determines if there is an instance generation sub-process associated with the template type. If there is no such sub-process, then the process ends by returning a failure status at block  416 . If there is an instance generation sub-process, then block  406  invokes, i.e., calls, the instance generation sub-process with the design objects and template instance parameters as input parameters for the sub-process. The instance generation sub-process may create a constraint set, or may return an empty set. Block  410  determines if the instance generation process created an empty constraint set, and if so, the process ends by returning failure at block  416 . If the constraint set is not empty, block  412  creates a template instance, e.g., an object in a computer programming language, and associates the constraint set, the parameters, and the design objects with the template instance, e.g., by setting corresponding data members of the object to reference the constraint set, the parameters, and the design objects. The process then ends by returning success at block  414 . 
         [0081]      FIG. 5  is an illustrative drawing of a process for validating a template instance in accordance with some embodiments of the invention. In one aspect, the validation process is invoked in response to events that could cause the template instance to become invalid. These events include changes to parameters and reading in of the template instance from a file. The validation process of  FIG. 5  corresponds to a portion of the template instance validator  124  and is invoked by the template manager  120  of  FIG. 1A  if constraint parameters or template parameters change, because changes to the parameters can cause the template instance to become invalid. The validation process of  FIG. 5  is also invoked if a template instance is read in from a file, because changes to the template instance made outside of the EDA tool can cause the template instance to become invalid. The file may be stored on persistent storage media such as a magnetic disk drive, an optical drive, random access memory, or flash memory. In other examples, the validation process may be invoked in response to other events such as changes to the circuit design, e.g., when a design object is added, deleted, or modified 
         [0082]    The validation process of  FIG. 5  invokes a template instance validation sub-process to validate a given template instance. The validation process begins at block  502  by determining if a template type for the given template instance exists in a list of template types. If the template type does not exist, then block  508  marks the template instance invalid to prevent further use of the template instance, and the process ends by returning a failure status at block  522 . If the template type exists, then block  506  invokes the instance validation sub-process associated with the template type. Block  506  passes the constraints and template instance parameters associated with the template instance to the instance validation sub-process. The instance validation sub-process is typically provided by a user, and ordinarily determines if the instance, including the constraints, is valid according to user-defined criteria. Next, block  510  determines if the instance validation sub-process returned a successful status. If the sub-process returned a successful status, then the validation process exits with a successful status at block  520 . Block  512  performs a recovery process. If the sub-process returned a failure status, then block  512  invokes the template instance generation process of  FIG. 4 , starting at block  402 , to re-create the template instance from the template type. The instance generation process then attempts to recreate a template instance, including content, e.g., constraints, and if a template instance is successfully recreated, returns a success status. Block  514  checks the return value of the instance generation process to determine if the content was successfully recreated. If the content was not successfully recreated, then block  518  deletes the template instance to prevent subsequent use of the template instance, and the process exits with a failure return status at block  522 . If the content was successfully created, then block  516  replaces the existing content (e.g., constraints) of the template instance with the newly created content, and the process exits with a successful return status at block  520 . 
         [0083]      FIG. 6  is an illustrative drawing of a process for validating a template instance in response to constraint changes in accordance with some embodiments of the invention. The validation process of  FIG. 6  corresponds to a portion of the template instance validator  124  and is called by the template manager  120  of  FIG. 1A  if there is a change in the set of constraints associated with a template instance, or if there is a change to a constraint parameter, or if there is a change in the set of design objects (i.e., a design object reference) associated with a constraint of a template instance. A template instance for which a constraint has changed is passed to the process of  FIG. 6 . The process begins at block  602  by retrieving the template type that corresponds to the template instance by, for example, retrieving a reference to the template type from the template instance. Block  604  determines if there is an instance validation sub-process associated with the template type. If no such instance validation sub-process exists, then block  606  marks the template instance as invalid to prevent it from being used subsequently, and the process ends by returning a failure status at block  614 . If an instance validation sub-process exists, then block  608  invokes the instance validation sub-process by, for example, making a procedure call. Block  608  passes the constraints and template instance parameters associated with the template instance to the instance validation sub-process. The instance validation sub-process then executes, and returns a success value to indicate whether the instance is valid. Next, block  610  determines if the instance validation sub-process returned success. If the instance validation sub-process returned success, then the validation process exits with a success value at block  614 . If the instance validator sub-process did not return success, then block  612  rejects the change to the constraints by, for example, restoring the previous value of the constraints, and the validation process ends at block  614  by returning a failure status. 
         [0084]      FIG. 7  is an illustrative drawing of a process for defining a template type in accordance with some embodiments of the invention. The process of defining a template type is typically performed by a user who interacts with a template type definition interface of the EDA tool to define the template type. The template type definition interface may be a graphical user interface, or a text-oriented command based interface, or a combination of the two. 
         [0085]    The process of defining a template type begins at block  702  by receiving a template name, template instance creation and validation processes, and template instance parameters for the new template type. Block  704  determines if a template type with the template name already exists by, for example, searching a list of template types for the template name. If a template type with the template name exists, then the process ends at block  706  by returning failure. Otherwise, block  708  creates a new object, e.g., an object in a programming language such as Cadence® SKILL, Lisp, C++, Java™ or the like, to represent the template instance, and inserts the template name into the template type object. Blocks  710  and  712  then insert references to the instance creation and validation processes, respectively, into the template type object. As described above with respect to  FIG. 1A , the template instance creation and validation processes may be registered with the template manager  120  of  FIG. 1A  through commands in an application command interface of the EDA tool using existing features of the EDA tool. Block  714  inserts the template instance parameter definitions into the template instance type, and block  716  registers the template type object with the template manager by, for example, inserting the object into the list of template types. Block  718  returns success to indicate that the template type was successfully created. In one example, only parameters that match the parameter definitions created in block  714  may be used as parameters for the template instance when the template instance is created. In that example, only parameters that match the parameter definitions will be accepted as valid template instance parameters for a template instance by the parameter list user interface feature  348  of  FIG. 3A . 
         [0086]      FIG. 8  is an illustrative drawing of a process for creating a template instance in an Electronic Design Automation tool user interface in accordance with some embodiments of the invention. This process is typically executed by the EDA tool when the tool&#39;s user interface receives a request from a user to create a template instance The process begins at block  802  by identifying, i.e., receiving selection of, a set of one or more design objects to be constrained. Block  804  identifies or receives a template type to be used to constrain the objects. Block  806  identifies or receives a parameter set for the template instance, and block  808  invokes a template instance generation process, such as the process of  FIG. 4 , to create a constraint set based on the design objects, the template type, and the parameter set. Block  810  creates the template instance as, for example, an object in a computer programming language. The template instance includes the constraints created in block  808 , a reference to the design objects and the parameters prepared in block  806 . Block  812  ads the template instance to a list of template instances. 
         [0087]      FIG. 9  is an illustrative drawing of a template instance generation sub-process in accordance with some embodiments of the invention. The template instance generation sub-process is typically a user-defined process, and is ordinarily invoked by the template instance generation process of  FIG. 4 . The instance generation sub-process shown in  FIG. 9  is an example of a user-defined sub-process and is specific to particular types of user-defined constraints, including a Matched Parameters constraint and a Relative Orientation constraint, which are related to semiconductor technology. The instance generation sub-process receives a strength parameter, which is a user-defined parameter that is meaningful to the user and is passed from the user to the instance generation sub-process when a template instance is created. In one aspect. the template instance parameters, such as the strength parameter, are treated by the template manager as opaque, uninterpreted values, The MatchedParameters and Relative Orientation constraints, and the strength parameter, are described as part of the example instance generation sub-process, and are necessarily present in other examples of the instance generation sub-process. 
         [0088]    The instance generation sub-process begins at block  902  by retrieving a strength parameter value from the template instance for which the sub-process was invoked. The template instance may be retrieved from an input parameter of the sub-process. Block  904  determines if the strength parameter is equal to 1. If the strength parameter is 1, then block  906  creates a Matched Parameters constraint, sets the parameters of the template instance to appropriate values, block  914  returns to the invoker a list that contains the Matched Parameters constraint, and the process ends. The template instance parameters may determine the constraint list and the constraint parameters, as determined by the template instance generation sub-process invoked by block  406  of  FIG. 4 . 
         [0089]    If the strength parameter is 2, block  910  creates a Matched Parameters constraint and with appropriate parameters, block  912  creates a Relative Orientation constraint with appropriate parameters, and block  914  returns to the invoker a list that contains the Matched Parameters and Relative Orientation constraints. If block  908  determines that the strength parameter is not equal to 2, then no action is taken and the sub-process ends, although different implementations of the sub-process could generate different constraints for subsequent strength values. 
         [0090]      FIG. 10  is an illustrative drawing of a template instance validation sub-process in accordance with some embodiments of the invention. The template instance validation sub-process is typically a user-defined process, and is ordinarily invoked by the template instance validation processes of  FIGS. 5 and 6  to determine if a template instance is valid, and, in one aspect, if the constraint set of the template instance valid. In this example, the constraint set is considered to be invalid if the first constraint in the set is an “alignment” type constraint, and a parameter of the first constraint has the name “side” and the value “edge”. Any other constraint set is considered to be valid in this example. 
         [0091]    The instance validation sub-process begins at block  1002  by retrieving the template instance for which the sub-process was invoked, e.g., by retrieving the template instance from an input parameter of the sub-process. Block  1004  retrieves the first constraint from the template instance. Block  1006  determines if the constraint type is equal to “alignment”. If the constraint type is not equal to “alignment”, then the constraint set is considered to be valid, block  1014  returns a success value to the invoking process to indicate that the template instance is valid, and the sub-process ends. If the constraint type is equal to “alignment”, then block  1008  attempts to get a constraint parameter with the name “side”. Block  1010  determines if the alignment constraint contains a parameter with the name “side” and the value “edge”. If such a constraint is present, then the sub-process returns failure to indicate to the invoking process that the constraint set is invalid. If no such constraint is present, then the sub-process returns success to indicate that the constraint set is valid. 
         [0092]    The template instance validation sub-process may also receive a change action parameter, which indicates the type of change action, e.g., modification of a parameter, deletion of a constraint, modification of a constraint parameter, or reading in of a template instance from storage. Other change action types may be added as well, e.g., an action type that indicates the design object has changed. The user-defined validation process may use that action parameter when validating the constraints. 
         [0093]    For example, a template instance generation process could create a template instance with three constraints, named Matched Parameters, Alignment, and Relative Orientation, as follows: 
       (Template-Instance Name=Match Parameters=(Param=Value) 
       [0094]    (constraint name=“Matched Parameters” parameters=(“l, w, m”) design-objects=(P 11  N 10 ))
 
(constraint name=“Alignment” parameters=(side=“right” “top/left” design-objects=(P 11  N 10 ))
 
(constraint name=“Relative Orientation” parameters=(“R 0 ”) design-objects=(P 11  N 10 )))
 
         [0095]    The validation sub-process shown in  FIG. 10  would reject any change to the Alignment constraint that would set the value of the “side” parameter to “edge”. 
         [0096]    A constraint may be changed are when (1) the template instance is read in from a file, (2) a template instance parameter in edited the EDA tool UI, or (3) a constraint is edited in the user interface, i.e., a constraint parameter or design object reference is changed in the UI. In one aspect, changes to the design objects themselves do not trigger the constraint observer. 
         [0097]    In case (1) if a change to the template instance results in the “side” parameter being set to “edge” when the template instance is read in from storage, then the template generation process shown in  FIG. 4  will be invoked to re-create the template instance&#39;s content, thereby returning the template instance to the state shown above. Any other changes would be accepted without re-creating the constraints. 
         [0098]    In case (2), changes to the template instance parameters do not affect the constraint parameters, so the “side” constraint parameter cannot be made invalid s a result of a template instance parameter change. 
         [0099]    In case (3), if a change to a constraint or design object reference results in the “side” parameter being set to “edge”, then, according to the flowchart shown in  FIG. 6 , the change would be rejected, i.e., not applied to the constraint. Furthermore, each change to a constraint or design object reference is individually verified and rejected or accepted. Any other changes would be accepted. 
         [0100]    The ability to trigger the constraint validation sub-processes upon read-in of a template instance provides for the automatic repair and re-creation of constraints. Since the instance validation and generation sub-processes may be user-defined, the user can provide instructions for repairing and re-creating the constraints in accordance with the user&#39;s technology or fabrication processes. 
         [0101]    The above description is exemplary only and it will be apparent to those of ordinary skill in the art that numerous modifications and variations are possible. For example, various exemplary methods and systems described herein may be used alone or in combination with various other computer and computer peripheral systems and methods. Additionally, particular examples have been discussed and how these examples are thought to address certain disadvantages in related art. This discussion is not meant, however, to restrict the various examples to methods and/or systems that actually address or solve the disadvantages.