Patent Publication Number: US-7584448-B1

Title: Constructing a model of a programmable logic device

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to programmable logic devices and more particularly to a model or representation of a programmable logic device. 
   BACKGROUND 
   Programmable logic devices (PLDs) are a well-known type of integrated circuit that can be programmed to perform specified logic functions. One type of PLD, the field programmable gate array (FPGA), typically includes an array of programmable tiles. These programmable tiles can include, for example, input/output blocks (IOBs), configurable logic blocks (CLBs), dedicated random access memory blocks (BRAM), multipliers, digital signal processing blocks (DSPs), processors, clock managers, delay lock loops (DLLs), and so forth. 
   Each programmable tile typically includes both programmable interconnect and programmable logic resources. The programmable interconnect typically includes a large number of interconnect lines of varying lengths interconnected by programmable interconnect points (PIPs). The programmable logic implements the logic of a user design using programmable elements that can include, for example, function generators, registers, arithmetic logic, and so forth. 
   The programmable interconnect and programmable logic are typically programmed by loading a stream of configuration data into internal configuration memory cells that define how the programmable elements are configured. The configuration data can be read from memory (e.g., from an external PROM) or written into the FPGA by an external device. The collective states of the individual memory cells then determine the function of the FPGA. 
   Another type of PLD is the Complex Programmable Logic Device, or CPLD. A CPLD includes two or more “function blocks” connected together and to input/output (I/O) resources by an interconnect switch matrix. Each function block of the CPLD includes a two-level AND/OR structure similar to those used in Programmable Logic Arrays (PLAs) and Programmable Array Logic (PAL) devices. In some CPLDs, configuration data is stored on-chip in non-volatile memory. In other CPLDs, configuration data is stored on-chip in non-volatile memory, then downloaded to volatile memory as part of an initial configuration sequence. 
   For all of these programmable logic devices (PLDs), the functionality of the device is controlled by data bits of configuration data provided to the device for that purpose. The data bits can be stored in volatile memory (e.g., static memory cells, as in FPGAs and some CPLDs), in non-volatile memory (e.g., FLASH memory, as in some CPLDs), or in any other type of memory cell. 
   Other PLDs are programmed by applying a processing layer, such as a metal layer, that programmably interconnects the various elements on the device. These PLDs are known as mask programmable devices. PLDs can also be implemented in other ways, e.g., using fuse or antifuse technology. The terms “PLD” and “programmable logic device” include but are not limited to these exemplary devices, as well as encompassing devices that are only partially programmable. 
   To implement a user design in a PLD, various software tools may generate corresponding configuration data and program the PLD with the configuration data. The configuration data may be generated by these software tools from a specification of the user design. These software tools may utilize models of various aspects of the circuitry of the PLD, and to correctly generate the configuration data for a user design, the models must accurately reflect the circuitry of the PLD. 
   The generation of models for a PLD may be time-consuming and expensive. The extraction of a model from the circuitry of the PLD may be done manually by inspection of the schematics of the PLD and certain associated information. Frequently, manual generation of models involves tracing of signal connections between various circuits of the PLD. Because the circuits and signals connections are numerous and may have similar names, the manual extraction of a model may be confusing and prone to error. In addition, circuit changes may be made quite late in the development of the PLD, and these circuit changes may affect the models for certain aspects of the circuitry of the PLD. After a circuit change, the availability of the models may be delayed by the time-consuming manual extraction process. 
   The present invention may address one or more of the above issues. 
   SUMMARY OF THE INVENTION 
   Various embodiments of the invention provide a processor-implemented method for constructing a model of a programmable logic device (PLD) design. A netlist is input that describes the PLD design. The netlist includes instances of modules with the modules including programmable tile modules and the instances including tile instances of the programmable tile modules. An identification is input of each of the programmable tile modules. Each programmable tile module includes at least one programmable resource, with each programmable resource being either a programmable interconnect resource configurable to implement a plurality of routing arcs or a programmable logic resource configurable to implement a plurality of logic functions. A first map is generated from the netlist that links each routing arc to at least one bit of configuration data for programming the programmable interconnect resources of the programmable tile modules. A second map is generated from the netlist that links each logic function to at least one bit of configuration data for programming the programmable logic resources of the programmable tile modules. 
   It will be appreciated that various other embodiments are set forth in the Detailed Description and Claims which follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various aspects and advantages of the invention will become apparent upon review of the following detailed description and upon reference to the drawings in which: 
       FIG. 1  is a block diagram of a programmable logic device (PLD) in accordance with various embodiments of the invention; 
       FIG. 2  is a data flow diagram of a process for generating configuration data for a user design in accordance with various embodiments of the invention; 
       FIG. 3  is a flow diagram of a process for constructing models of a programmable logic device in accordance with various embodiments of the invention; 
       FIG. 4  is a block diagram of a programmable tile module of a programmable logic device illustrating repeaters in accordance with various embodiments of the invention; 
       FIG. 5  is a data flow diagram of a process for determining pins that are reachable from a starting network in accordance with various embodiments of the invention; 
       FIG. 6  is a flow diagram of a process for determining pins that are reachable from a starting network in accordance with various embodiments of the invention; 
       FIG. 7  is a flow diagram of a process for determining a queue of reachable pins in accordance with various embodiments of the invention; 
       FIG. 8  is a block diagram of a programmable switchbox of a programmable logic device in accordance with various embodiments of the invention; 
       FIG. 9  is a data flow diagram of a process for determining programmable connections through a switchbox of a programmable logic device in accordance with various embodiments of the invention; 
       FIG. 10  is a flow diagram of a process for determining programmable connections through a switchbox of a programmable logic device in accordance with various embodiments of the invention; 
       FIG. 11  is a block diagram of a programmable tile of a programmable logic device illustrating a switchbox and a logic site in accordance with various embodiments of the invention; 
       FIG. 12  is a data flow diagram of a process for determining connections of a programmable tile of a programmable logic device in accordance with various embodiments of the invention; 
       FIG. 13  is a flow diagram of a process for determining connections of a programmable tile of a programmable logic device in accordance with various embodiments of the invention; 
       FIG. 14  is a block diagram illustrating configuration memory of a programmable tile of a programmable logic device in accordance with various embodiments of the invention; 
       FIG. 15  is a data flow diagram of a process for determining control of a select pin of a programmable tile of a programmable logic device in accordance with various embodiments of the invention; 
       FIG. 16  is a flow diagram of a process for determining control of a select pin of a programmable tile of a programmable logic device in accordance with various embodiments of the invention; 
       FIG. 17  is a data flow diagram of a process for determining indices of an instance of a cell of configuration memory in accordance with various embodiments of the invention; 
       FIG. 18  is a flow diagram of a process for determining indices of an instance of a cell of configuration memory in accordance with various embodiments of the invention; 
       FIG. 19  is a block diagram of a portion of a programmable logic device illustrating connections between programmable tiles in accordance with various embodiments of the invention; 
       FIG. 20  is an example diagram of a user interface illustrating a graphical representation of a programmable logic device in accordance with various embodiments of the invention; 
       FIG. 21  is a data flow diagram of a process for comparing connections between a graphic representation of a programmable logic device and a netlist for the programmable logic device in accordance with various embodiments of the invention; 
       FIG. 22  is a flow diagram of a process for comparing connections between a graphic representation of a programmable logic device and a netlist for the programmable logic device in accordance with various embodiments of the invention; and 
       FIG. 23  is a block diagram of a system for processing a PLD netlist in accordance with various embodiments of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram of a programmable logic device (PLD) in accordance with various embodiments of the invention.  FIG. 1  illustrates an FPGA architecture  100  for a PLD that includes a large number of different programmable tiles including multi-gigabit transceivers (MGTs  101 ), configurable logic blocks (CLBs  102 ), random access memory blocks (BRAMs  103 ), input/output blocks (IOBs  104 ), configuration and clocking logic (CONFIG/CLOCKS  105 ), digital signal processing blocks (DSPs  106 ), specialized input/output blocks (I/O  107 ) (e.g., configuration ports and clock ports), and other programmable logic  108  such as digital clock managers, analog-to-digital converters, system monitoring logic, and so forth. Some FPGAs also include dedicated processor blocks (PROC  110 ). 
   In some FPGAs, each programmable tile includes a programmable interconnect element (INT  111 ) having standardized connections to and from a corresponding interconnect element in each adjacent tile. Therefore, the programmable interconnect elements taken together implement the programmable interconnect structure for the illustrated FPGA. The programmable interconnect element (INT  111 ) also includes the connections to and from the programmable logic element within the same tile, as shown by the examples included at the top of  FIG. 1 . 
   For example, a CLB  102  can include a configurable logic element (CLE  112 ) that can be programmed to implement user logic plus a single programmable interconnect element (INT  111 ). A BRAM  103  can include a BRAM logic element (BRL  113 ) in addition to one or more programmable interconnect elements. Typically, the number of interconnect elements included in a tile depends on the height of the tile. In the pictured embodiment, a BRAM tile has the same height as four CLBs, but other numbers (e.g., five) can also be used. A DSP tile  106  can include a DSP logic element (DSPL  114 ) in addition to an appropriate number of programmable interconnect elements. An IOB  104  can include, for example, two instances of an input/output logic element (IOL  115 ) in addition to one instance of the programmable interconnect element (INT  111 ). As will be clear to those of skill in the art, the actual I/O pads connected, for example, to the I/O logic element  115  are manufactured using metal layered above the various illustrated logic blocks, and typically are not confined to the area of the input/output logic element  115 . 
   In the pictured embodiment, a columnar area near the center of the die (shown shaded in  FIG. 1 ) is used for configuration, clock, and other control logic. Horizontal areas  109  extending from this column are used to distribute the clocks and configuration signals across the breadth of the FPGA. 
   Some FPGAs utilizing the architecture illustrated in  FIG. 1  include additional logic blocks that disrupt the regular columnar structure making up a large part of the FPGA. The additional logic blocks can be programmable blocks and/or dedicated logic. For example, the processor block PROC  110  shown in  FIG. 1  spans several columns of CLBs and BRAMs. 
   Note that  FIG. 1  is intended to illustrate only an exemplary FPGA architecture. The numbers of logic blocks in a column, the relative widths of the columns, the number and order of columns, the types of logic blocks included in the columns, the relative sizes of the logic blocks, and the interconnect/logic implementations included at the top of  FIG. 1  are purely exemplary. For example, in an actual FPGA more than one adjacent column of CLBs is typically included wherever the CLBs appear, to facilitate the efficient implementation of user logic. 
     FIG. 2  is a data flow diagram of a process for generating configuration data  132  for a user design  134  in accordance with various embodiments of the invention. The configuration data  132  may be generated from the user design  134  by translation tools including a mapper/placer  136 , router  138 , and serializing generator  140 . Various embodiment of the invention create models of a PLD that are used by the translation tools to create configuration data  132  from a user design  134 . The models may be generated by various analyzers  142  of a PLD netlist  144  specifying the design of the PLD. The models may include tile programmable-logic resources  146 , fixed tile routing arcs  148 , programmable tile routing arcs  150 , memory cell relationships  152 , and memory cell organization  154 . 
   The user design  134  may be a specification of the user design in a hardware description language, such as Verilog or VHDL. The mapper/placer  136  may map the logic functions of the user design into the programmable logic resources  146  that are available in the PLD. The mapper/placer  136  may also place each mapped logic function of the user design into a specific tile instance of the PLD. 
   With the logic functions of the user design placed in the tile instances, the appropriate connections between the inputs and outputs of these placed logic functions are generated by the router  138 . The router  138  may use arcs  148  and  150  of the fixed interconnect and the programmable interconnect resources to produce these connections. The programmable interconnect resources may include multiplexers that may each provide one of several programmable connections depending on which input is selected to drive the output of the multiplexer. The output of the router  138  may include the values of the selection controls  156  of the multiplexers of the programmable interconnect resources. 
   The mapper  136  may generate selection controls  158  that determine the logic functions implemented by the programmable logic resources. The combination of the selection controls  156  and  158  may determine the programming of the PLD that is required to implement the user design  134  in the PLD. The serializing generator  140  may put the values of the selection controls  156  and  158  into a particular order for the PLD to generate the configuration data  132 . The particular order for the selection controls  156  and  158  may be specified by configuration memory cell relationships  152  and configuration memory cell organization  154 . 
   The process of translating a user design  134  into configuration data  132  for a PLD may require certain models of the design of the PLD and these models of the PLD may be generated from a PLD netlist  144  according to various embodiments of the invention. In one embodiment, a netlist extractor  160  extracts the PLD netlist  144  from a schematic  162  of the design of the PLD. 
     FIG. 3  is a flow diagram of a process for constructing models of a programmable logic device in accordance with various embodiments of the invention. The models may be generated from a netlist that specifies the design of the PLD. 
   At step  170 , a netlist is input that specifies connections between instances of a modules, with the modules including programmable tile modules and the instances including tile instances of the programmable tile modules. At step  172 , an identification of one or more of tile modules is input. At step  174 , a characterization is input of one or more sub-modules of the programmable tile modules, and the characterization may specify certain sub-module characteristics. At step  176 , routing arcs are generated for the programmable interconnect resources of the identified programmable tile modules. The routing arcs may be generated from the netlist that describes the PLD design using the characterization of step  174 . 
   At step  178 , a characterization is input for a configuration memory cell of the PLD, with each configuration memory cell being a module of the PLD. At step  180 , a characterization is input for one or more configuration control modules of the PLD. The configuration control module or modules may be used to set the values of the instances of the configuration memory cell within the PLD. At step  182 , a map is generated that links the routing arcs to values of configuration data for the programmable interconnect resources. The map also links logic functions of programmable logic resources of the programmable tile modules to other values of the configuration data for the programmable logic resources. The map may specify relationships between the instances of the configuration memory cells and the selection controls of the programmable logic and interconnect resources. The map may also specify an organization, which may be a two-dimensional organization, of the instances of the configuration memory cells. 
     FIG. 4  is a block diagram of a programmable tile module  200  of a programmable logic device (PLD) illustrating repeaters  202 ,  204 ,  206 , and  208  in accordance with various embodiments of the invention. The programmable tile module  200  may be a CLB  102  as shown in  FIG. 1  and a PLD may include multiple instances of the programmable tile module  200 . The programmable tile module  200  may itself include instances, such as repeaters  202 ,  204 ,  206 , and  208 , of corresponding modules. 
   An internal source within tile  200 , for example, look-up table  210 , may have an output pin  212  that drives the network on line  214 . Repeater  202  may be an instance of a module having the type of a buffer, and repeater  202  may have an input pin  216  receiving a value from the network on line  214  and an output pin  218  that drives this received value to the network on line  220 . Because repeater  202  repeats the value received from input pin  216  at output pin  218 , pins  216  and  218  are functionally equivalent pins that are functionally connected. For clarity, the input and output pins of repeaters  204 ,  206 , and  208  are not explicitly shown. 
   Tile module  200  may have an output pin  222  connected to network  220 , and the pins that are reachable from network  214  may include pin  222 . The pins that are reachable from network  214  may also include pin  212  of look-up table  210  and pins  216  and  218  of repeater  202 . Network  214  may be identified directly in one embodiment, and network  214  may be identified indirectly in another embodiment by identifying one of the pins, such as pin  212 , of network  214 . 
   The programmable resources of a PLD may include multiplexer  224  that is programmable by the value of memory configuration cell  226  to select either an input on line  228  or an input on line  230 . The inputs  228  and  230  of multiplexer  224  may be buses containing multiple bits and multiplexer  224  may be an array of multiplexer gates having a common select input that is controlled by the value of memory configuration cell  226 . An example usage for multiplexer  224  is for selecting either an asynchronous input on line  228  or a synchronous input on line  230  for the data input of a block RAM, such as block RAM  103  of  FIG. 1 . Because an example block RAM may have a data input with up to 36-bits, the multiplexer  224  may be an array of 36 multiplexer gates. An instance of the module for memory configuration cell  226  may have insufficient drive strength to directly drive the select input of 36 multiplexer gates, and repeater  204  may be included to increase the drive strength of memory configuration cell  226 . Repeater  204  may be an inverting buffer that receives a logical value at the input using positive logic and repeats the logical value at the output using negative logic. Thus, while the repeater  204  may invert the electrical value, the repeater  204  might not invert the logical value. 
   A PLD may have a global configuration signal on line  232  that indicates that the PLD is initialized with configuration data that implements a user design in the programmable logic and interconnect resources of the PLD. Prior to completion of the programming of the PLD with the configuration data, the global configuration signal on line  232  may force certain signals to a specific value. For example, the global configuration signal on line  232  may cause repeater  206  to drive the value of zero from tie-down  234  to the output on line  236  prior to the completion of programming. 
   During normal operation of the user design in the PLD, the global configuration signal on line  232  may have a static value that causes repeater  206  to drive the value received on line  238  onto line  236  and causes repeater  208  to drive the value received on line  240  onto line  242 . Thus, the input pin of repeater  206  for the network on line  238  may be functionally connected to the output pin of repeater  206  for the network on line  236 , and the input pin of repeater  208  for the network on line  240  may be functionally connected to the output pin of repeater  208  for the network on line  242 . The type for the repeater  206  may be a multiplexer gate and the type for the repeater  208  may be an AND gate. 
     FIG. 5  is a data flow diagram of a process for determining pins that are reachable from a starting network in accordance with various embodiments of the invention. The design of a PLD may be specified by a list of networks, PLD “netlist”  252 , which may specify the connections between the instances of the modules of the PLD. The PLD netlist  252  may be searched by the generator  254  to determine the pins that a reachable from the starting network. 
   The identification  256  may identify a network from the netlist  252  as the starting network from which to begin the search. The starting network may be identified by a name of the starting network or by a node for the starting network in a graph representing the PLD netlist  252 . A pin of an instance of a module that is connected to the starting network may also be used to identify the starting network. 
   Each of the characterizations  258  and  260  for the repeater modules may include a type of a repeater module and a set of functionally connected pins of the repeater module. An example characterization  258  for the repeater  202  of  FIG. 4  may include the type of a buffer and the set of functionally connected pins that includes pins  216  and  218 . The type for the repeater module may be a name from a module definition of the repeater module that is found in the PLD netlist  252 , and the functionally connected pins may determine the pins that are functionally equivalent for each instance of the repeater module. 
   The reachable pins generator  254  may search the PLD netlist  252  for pins that are functionally equivalent to the pins of the starting network from identification  256 . The reachable pins generator  254  may use pin queue  262  to track the reached pins. The pins in the pin queue  262  after finishing the search of the PLD netlist  252  may be the reachable pins that are functionally connected to the starting network. 
     FIG. 6  is a flow diagram of a process  270  for determining pins that are reachable from a starting network in accordance with various embodiments of the invention. The PLD may be specified by a designer using a hardware description language and a PLD netlist may be generated from this specification of the PLD. 
   At step  272 , the PLD netlist is input, with each network listed in the PLD netlist including a network name and a list of the pins of instances of the modules of the PLD that are connected by the network. The PLD netlist may be a flattened netlist having networks that interconnect pins of instances of elementary modules that do not contain any instances, or the PLD netlist may be a hierarchical netlist having networks that interconnect pins of a hierarchy of instances of modules. 
   At step  274 , an identification of a starting network is input and the starting network may be identified directly or the starting network may be identified indirectly by identifying a pin that is included in the starting network. At step  276 , a characterization of one or more repeater modules is input including a type of the repeater module and a set of functionally connected pins of the repeater module. In one embodiment, the PLD netlist is generated from a specification of the PLD in a hardware description language (HDL), and the characterization for a repeater module is an abbreviation or annotation of the specification of the repeater module from the HDL specification of the PLD. For example, the characterization for the repeater module may include the name of the repeater module and an abbreviated port list of the repeater module that only includes the ports of the repeater module that are functionally connected by the repeater module. 
   At step  278 , the starting network may be found in the PLD netlist and the corresponding list of pins from the PLD netlist may be used to initialize a queue. At step  280 , a pin in the queue is selected. Decision  282  checks whether the pin is one of the functionally connected pins of a repeater module. When the pin is not one of the functionally connected pins of a repeater module, process  270  returns to step  280  and the next pin from the queue is selected; otherwise, process  270  proceeds to step  284 . At step  284 , the corresponding network is found in the PLD netlist for each of the functionally connected pins of the pin selected at step  280 , and each pin of each of these corresponding networks is appended to the queue, excluding those pins that are already in the queue. In one embodiment, a “visited” flag is maintained for each pin such that the queue does not need to be searched to determine whether a particular pin is already in the queue. 
   Decision  286  checks whether every pin in the queue has been considered. When the queue includes as yet unprocessed pins, process  270  returns to step  280  and another pin is selected from the queue; otherwise, process  270  proceeds to step  288 . At step  288  the pins from the queue are output as the reachable pins that are functionally connected to the starting network. 
     FIG. 7  is a flow diagram of a process  300  for determining a queue of reachable pins in accordance with various embodiments of the invention. The reachable pins from the queue at the end of process  300  are a set of pins that are functionally connected to a starting network. 
   A netlist is input that specifies the networks of a PLD at step  302  and an identification is input that identifies the starting network at step  304 . At step  306 , a characterization of a repeater module is input that specifies the type of the repeater module and a set of functionally connected pins of the repeater module. Frequently, a repeater module has the set of functionally connected pins that includes one input pin and one output pin that repeats the value of the input pin. It will be appreciated that process  300  may be modified to handle a repeater module that has an alternative set of functionally connected pins, such as one input pin and multiple output pins. 
   A queue is initialized at step  308  with the pins of the starting network and a path for each pin. The pins of the starting network may be the pins that are specified in the netlist for the starting network. The queue may be organized as a list of pairs with each pair including a pin and a path from the starting network to the pin. Because the queue is initialized with the pins of the starting network, the path from the starting network to a pin in the queue at step  308  is a path including the starting network and the pin. 
   At step  310 , a pin and the associated path are selected from the queue. The queue may be a FIFO of pairs and the pin and associated path may be obtained from the next pair in the FIFO. In one embodiment, a pair from the FIFO may be selected at step  310  without removing the pair from the FIFO. For example, the FIFO may be a list of pairs having a pointer to the pair that is selected at step  310  and following the selection of a pair at step  310  the pointer may move to the next pair in the queue. In another embodiment, the selection of a pair at step  310  may remove the pair from the FIFO and the removed pair may be saved in separate data structure. It will be appreciated that the queue may include the FIFO and this separate data structure. 
   Decision  312  checks whether the pin selected at step  310  is a pin of the set of functionally connected pins of a repeater module. Process  300  may proceed to optional decision  314  when the pin is such a pin of a repeater; otherwise, process  300  may return to step  310  for selection of the next pin in the queue. For a netlist having more than one path between the starting network and a reachable pin, optional decision  314  may ensure that the queue does not include more than one path between the starting network and any pin, as may be required in certain applications. For other applications, either the netlist does not include multiple paths or the queue is permitted or required include all paths between the starting network and the reachable pins. If optional decision  314  is included in process  300 , process  300  returns to step  310  when the appropriate set of functionally connected pins of the repeater module includes the pin selected at step  310  and a remote pin that is already included in the queue; otherwise, process  300  proceeds to step  316 . 
   At step  316 , the queue is updated to append a pair of the remote pin of the repeater module and a corresponding repeater path. When the pin selected at step  310  is an input pin from the set of functionally connected pins of a repeater module, the remote pin may be an output pin of the repeater module. Similarly, when the pin selected at step  310  is an output pin from the set of functionally connected pins of a repeater module, the remote pin may be an input pin of the repeater module. In certain embodiments, a path between the starting network and a pin may be formatted as a sequence that alternates networks and pins, for example, starting network, first pin, second network, second pin, third network, third pin, and et cetera. To retain the alternating format of networks and pins, the corresponding repeater path for the remote pin of the repeater module may add a null network and the remote pin to the path for the pin selected at step  310 . 
   The netlist may specify that the remote pin of the repeater module is connected to a network that may be denoted the remote network. The remote network generally includes the remote pin and one or more additional pins of the remote network. At step  318 , one of these additional pins of the remote network is selected and decision  320  checks whether this additional pin is already included in the queue. Step  322  is skipped for an additional pin that is already in the queue. At step  322 , the queue is updated to append a pair of the additional pin and a corresponding path that adds the remote network and the additional pin to the repeater path generated at step  316 . After all of the additional pins of the remote network have been sequentially selected at step  318 , process  300  proceeds from decision  324  to decision  326 , and otherwise process  300  returns to step  318  for the selection of the next one of these additional pins of the remote network. 
   Decision  326  checks whether all of the pins of the pairs of the queue are processed. If the queue includes as yet unprocessed pairs, process  300  returns to step  310  for the selection of the next pin and associated path from the queue; otherwise, process  300  proceeds to step  328 . Upon returning to step  310  for the selection of the next pin and associated path, the pin and associated path that are selected may be a pin and associated path previously appended to the queue at step  322 . After the search of the netlist is completed, the pairs are output with each pair including a reachable pin and a path from the starting network to the reachable pin. 
   Example 1 shows pseudo-code for determining a queue of reachable pins according to one embodiment of the invention. 
   
     
       
         
             
           
             
                 
             
             
               Example 1: 
             
             
                 
             
           
          
             
               Initialize a queue to empty. 
             
             
               Initialize a set of reachable pins to empty. 
             
             
               If the starting point is specified using a starting pin instead of a starting 
             
             
               network { 
             
          
         
         
             
             
          
             
                 
               Set the starting network to the network of the starting pin. 
             
          
         
         
             
          
             
               } 
             
             
               For each pin of the starting network { 
             
          
         
         
             
             
          
             
                 
               Set a path to the starting network and the pin. 
             
             
                 
               Add the pin and the path to the queue and the set of reachable pins. 
             
          
         
         
             
          
             
               } 
             
             
               While the queue is not empty { 
             
          
         
         
             
             
          
             
                 
               Pop the current pin and corresponding current path from the queue. 
             
             
                 
               If the current pin is in the set of functionally connected pins of a 
             
             
                 
               repeater module { 
             
          
         
         
             
             
          
             
                 
               Set a remote pin to the other pin of the set of functionally 
             
             
                 
               connected pins. 
             
             
                 
               Set a remote path to the current path plus the null network and the 
             
             
                 
               remote pin. 
             
             
                 
               For each pin of the remote network of the remote pin { 
             
          
         
         
             
             
          
             
                 
               If the pin is not already in the set of reachable pins { 
             
          
         
         
             
             
          
             
                 
               Set a path to the remote path plus the remote network and the 
             
             
                 
               pin. 
             
             
                 
               Add the pin and the path to the queue and the set of reachable 
             
             
                 
               pins. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
          
             
               } 
             
             
                 
             
          
         
       
     
   
     FIG. 8  is a block diagram of a programmable switchbox  330  of a programmable logic device in accordance with various embodiments of the invention. The programmable switchbox  330  may include programmable multiplexers  332  that are programmed to implement a portion of the programmable interconnect of the PLD. The programmable selection of the multiplexers  332  may be controlled by the value of configuration memory cells  334 . Programmable switchbox  330  may correspond, for example, to programmable interconnect element (INT  111 ) of  FIG. 1 . 
   Various embodiments of the invention automatically determine the various programmable connections that might be made by the switchbox  330 . The programmable connections may be described using arcs from certain input pins  336  and tie-down zero and one values to certain output pins including pin  338 . For the appropriate programming of configuration memory cells  334 , any one of the input pins  336  may be coupled to the output pin  338 . In addition, the zero value from tie-down  339  may be coupled to the output pin  338 . Thus, the programmable connections that are provided by switchbox  330  may be specified by a respective arc from each of the input pins  336  and from a zero value for tie-down  339  to the output pin  338 . The specification of each arc from one of the input pins  336  or from a zero or one tie-down to the output pin  338  may include a specification of the series of multiplexers that may be programmed to provide the programmable connection. The specification of each arc may also include a specification of the pins of each multiplexer that are used to provide the programmable connection. 
   It will be appreciated that a switchbox  330  may include pins  340  that are not used for making programmable connections. For example, a switchbox  330  may be specified by a schematic that includes circuitry for power distribution, testing, and configuration, including programming of configuration memory cells  334 . Generally, pins  340  are not included in the model of the switchbox that is generated by various embodiments of the invention. 
   While a PLD may include many multiplexers that are not used to make programmable connections, frequently all of the multiplexers that are used to make programmable connections are collected together into switchboxes. Thus, all of multiplexers  334  or certain specified multiplexers within switchbox  330  may be used to make programmable connections. A switchbox  330  may include hierarchy, such as instance  342  of a sub-module and other levels of hierarchy, and multiplexers  332  may be considered internal to switchbox  330  regardless of whether each multiplexer is included directly within the switchbox  330  or within the hierarchy of the switchbox  330 . 
     FIG. 9  is a data flow diagram of a process for determining programmable connections through a switchbox of a programmable logic device in accordance with various embodiments of the invention. The design of a PLD may be specified by a PLD netlist  352  that is searched by programmable connection generator  354  to determine the programmable connections of a switchbox that is identified by identification  356 . The identification  356  of the switchbox module may be a name from a module definition for the switchbox from a hardware description language specification of the PLD. Because the PLD netlist  352  may be generated from the hardware description language specification of the PLD, the netlist  352  may include the name for the switchbox as the type for the switchbox. 
   The switchbox may include instances of one or more types of multiplexer modules and these instances may be used to provide programmable connections between the inputs and outputs of the switchbox. For each multiplexer, values from configuration memory cells may steer the selection of one of the multiplexer inputs that is coupled to the multiplexer output. During programming of the PLD, the configuration values provided for these configuration memory cells may determine the selected programmable connections. Generally, each possible programmable connection passes through at least one of the instances of the multiplexer modules of the switchbox from an input of the multiplexer module to an output of the multiplexer module. 
   Respective characterizations  358  may be created for the multiplexer module or modules used to provide the programmable connections between the input and outputs of the switchbox. The generator  354  may use the characterizations  358  to determine the programmable connections specified in the PLD netlist  352  for the switchbox of identification  356 . The generator  354  may first determine the switchbox pins  360  that are used to provide the programmable connections. Each programmable connection may couple an input pin of the switchbox to an output pin of the switchbox via one or more instances of a multiplexer module, and the switchbox pins  360  may include all pins of the switchbox module that may be used to provide the programmable connections. The generator  354  may trace connections in the PLD netlist  352  starting from the switchbox pins  360  and passing through one or more instances of a multiplexer module or modules having characterizations  358 . 
     FIG. 10  is a flow diagram of a process  380  for determining programmable connections through a switchbox of a programmable logic device in accordance with various embodiments of the invention. A netlist that describes the PLD design is input at step  382 , an identification of a switchbox module is input at step  384 , and a characterization of one or more multiplexer modules is input at step  386 . The characterization of a multiplexer module may include the input pins and one or more output pins that provide the programmable connections. 
   At step  388 , the switchbox pins are determined that provide programmable connections. A search of the PLD netlist may be performed beginning at the input and output pins of the instances within the switchbox of the multiplexer modules having the characterization of step  386 . The switchbox pins for the programmable connections are the reachable pins from the search that are also pins of the switchbox. Example 2 shows pseudo-code for determining the switchbox pins providing programmable connections according to one embodiment of the invention. 
   
     
       
         
             
           
             
                 
             
             
               Example 2: 
             
             
                 
             
           
          
             
               For each instance within the hierarchy of the switchbox { 
             
          
         
         
             
             
          
             
                 
               If the instance has a corresponding characterization of a multiplexer { 
             
          
         
         
             
             
          
             
                 
               For each pin on the instance of the multiplexer { 
             
          
         
         
             
             
          
             
                 
               If that pin is included in the corresponding characterization of the 
             
             
                 
               multiplexer { 
             
          
         
         
             
             
          
             
                 
               Trace from that pin of the instance of the multiplexer to all 
             
             
                 
               reachable pins { 
             
          
         
         
             
             
          
             
                 
               For each reachable pin { 
             
          
         
         
             
             
          
             
                 
               If the reachable pin is a pin of the switchbox module { 
             
          
         
         
             
             
          
             
                 
               Determine the direction for the reachable pin. 
             
             
                 
               Add the reachable pin to a list of switchbox pins. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
          
             
               } 
             
             
                 
             
          
         
       
     
   
   At step  390 , the programmable connections are determined between switchbox pins via one or more instances of a multiplexer module. A search of the PLD netlist may be performed beginning at the switchbox pins from step  388 . The search may track each pin of each instance of a multiplexer module that is used to make the programmable connection. Example 3 shows pseudo-code for determining the programmable connections according to one embodiment of the invention. 
   
     
       
         
             
           
             
                 
             
             
               Example 3: 
             
             
                 
             
           
          
             
               For each pin in the list of switchbox pins providing programmable 
             
             
               connections { 
             
          
         
         
             
             
          
             
                 
               Set the starting pin to the pin from the list. 
             
             
                 
               Trace from the starting pin to all reachable pins. 
             
             
                 
               Execute ProcessReachedPins ( reachable pins ). 
             
          
         
         
             
          
             
               } 
             
             
               ProcessReachedPins ( pins ) : 
             
          
         
         
             
             
          
             
                 
               For each of the pins { 
             
          
         
         
             
             
          
             
                 
               If the pin is an input pin on an instance of a multiplexer module { 
             
          
         
         
             
             
          
             
                 
               Push the input pin of the multiplexer onto a stack. 
             
             
                 
               For each output pin of the instance of the multiplexer module { 
             
          
         
         
             
             
          
             
                 
               If a programmable path exists from the input pin to the output 
             
             
                 
               pin { 
             
          
         
         
             
             
          
             
                 
               Push the output pin of the multiplexer onto a stack. 
             
             
                 
               Trace from the output pin to all reachable pins. 
             
             
                 
               Execute ProcessReachedPins ( reachable pins ). 
             
             
                 
               Pop the output pin of the multiplexer from the stack. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
             
                 
               Pop the input pin of the multiplexer from the stack. 
             
          
         
         
             
             
          
             
                 
               } else if the pin is a pin of the switchbox { 
             
          
         
         
             
             
          
             
                 
               Set the ending pin. 
             
             
                 
               Create arc using starting and ending pins and the multiplexer pins 
             
             
                 
               on the stack. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
          
             
               } 
             
             
                 
             
          
         
       
     
   
   At step  392 , a specification is output for an arc between the input pin and the output pin of the switchbox module for each programmable connection. In one embodiment, an arc specifies the input pin and the output pin of the switchbox module. In another embodiment, an arc specifies the input and output pins of the switchbox module and the input and output pins of each instance of a multiplexer module along the programmable connection. 
   The specification of the arcs for the programmable connections may be used by a router of tools that translate a user design into configuration data that implements the used design in a PLD. After mapping and placement of the logic of the user design, the router may select arcs that connect the outputs of the placed logic with the appropriate inputs of the placed logic. It will be appreciated that a single connection may require the usage of multiple switchboxes to complete the connection, corresponding to the selection of multiple arcs for the connection by the router. To route each user design without mistakes, the specified arcs should accurately model the programmable connections. Because the specification of the arcs is automatically generated, the specification of the arcs can be quickly generated following, for example, a design change of the PLD that is reflected in an updated netlist for the PLD. In addition, for a newly designed PLD that may potentially use different switchbox and multiplexer designs, the software code that implements process  380  should not need time-consuming modifications. Instead the modifications for the newly designed PLD may be limited to simply creating new identifications of the switchbox modules and new characterizations of the multiplexer module. 
   At optional step  394 , a specification is output of the switchbox pins that are used to provide the programmable connections. These connectivity pins of a switchbox may be used to determine the non-programmable connections of a programmable tile that includes the switchbox as discussed below in connection with  FIGS. 12 and 13 . These non-programmable connections may be used to extend the programmable connections throughout the programmable tile. 
     FIG. 11  is a block diagram of a programmable tile  400  of a programmable logic device illustrating a switchbox  402  and a logic site  404  in accordance with various embodiments of the invention. A PLD may include many instances of a particular tile module  400 . For example, tile module  400  may correspond to CLB  102  of  FIG. 1 , and switchbox  402  and logic site  404  may respectively correspond to INT  111  and CLE  112  of  FIG. 1 . Alternatively, tile module  400  may correspond to BRAM  103  of  FIG. 1 , and switchbox  402  and logic site  404  may respectively correspond to INT  111  and BRL  113  of  FIG. 1 . A tile module may contain one or more switchboxes  402  and/or one or more logic sites  404 . Generally, a tile module contains at least one sub-module that is either a switchbox  402  or a logic site  404 , and this sub-module may be positioned within the hierarchy of the tile module. 
   Tile module  400  may have pins  406  and  408  that are connected within tile module  400  to pins  410  of switchbox  402  and pins  412  of logic site  404 . The sub-modules of switchbox  402  and logic site  404  may also have pins  414  that are connected amongst themselves within tile module  400 . The tile module  400  may also have certain pins  416  and  418  that are directly connected amongst themselves and are not connected to pins  410 ,  412 , and  414  of switchbox  402  and logic site  404 . 
   Pins  406 ,  408 ,  416 , and  418  of the tile module and pins  410 ,  412 , and  414  of switchbox  402  and logic site  404  are pins that may be used to implement signals of a user design. As described in connection with  FIG. 2 , a specification of the user design may be translated into configuration data, and a PLD may be configured to implement the user design by programming the PLD with the configuration data. Certain signals of the user design may be implemented in an instance of the tile module  400  in the PLD using the pins  406 ,  408 ,  416 , and  418  of the tile module and pins  410 ,  412 , and  414  of switchbox  402  and logic site  404 . Tile module  400 , switchbox  402 , and logic site  404  may also include pins  420  that might not directly implement a signal of the user design because these pins  420  are used for other purposes, such as power distribution, configuration, and testing of the PLD. 
   While pins  416  and  418  are not connected to switchbox  402  and logic site  404  of tile module  400 , a PLD may contain many instances of tile module  400  and pins  416  and  418  of one instance of tile module  400  may be connected to a switchbox  402  or a logic site  404  of another instance of tile module  400  in the PLD. For example, a PLD may include two instances of tile module  400 , with pin  416  of one instance connected in the PLD to pin  406  of the other instance. Thus, pin  416  of one instance of tile module  400  may be connected to a switchbox  402  of another instance of the tile module  400 . 
     FIG. 12  is a data flow diagram of a process for determining connections of a programmable tile of a programmable logic device in accordance with various embodiments of the invention. A tile connection generator  432  searches a PLD netlist  434  for certain connections within and between the instances in the PLD of the tile module with identification  436 . These certain connections may connect the pins of the instances of sub-modules having characterizations  438  and  440  within and between the instances of the tile module in the PLD. 
   The identification  436  of the tile module may specify the type of the tile module. In one embodiment, identification  436  may also specify each instance within the tile module of the sub-modules having characterizations  438  and  440 . In another embodiment, the characterizations  438  and  440  include a type for the corresponding sub-module and the instances of these sub-modules are determined by examining the PLD netlist for instances that match the types of these sub-modules. 
   Generally, the characterizations  438  and  440  include modeled pins of the corresponding sub-modules that may be used to implement signals of a user design. For the characterization  438  of a switchbox module, these modeled pins may be automatically generated by a process for determining programmable connections through a switchbox, as discussed in connections with  FIGS. 9 and 10 . For example, these modeled pins may be the switchbox pins  360  for programmable connections of  FIG. 9 . For characterization  440  of a logic site module, the modeled pins may be manually modeled for a processor-implemented generator  432  according to one embodiment of the invention. It will be appreciated that a particular tile module may have characterizations for multiple switchbox modules and/or multiple logic site modules, or that a particular tile module may have a characterization or characterizations only for switchbox modules or only for logic site modules. 
   The tile connectivity generator  432  may determine pins  442  of the tile module that may be used to implement signals of a user design. The pins  442  of the tile module that may be used to implement signals of a user design may be pins of the tile module that are functionally connected to a pin of a switchbox module or a logic site module of one or more instances of the tile module. Each of the pins  442  of the tile module may be functionally connected to a modeled pin of a sub-module with characterization  438  or  440  within the tile module or between two instances in the PLD of the tile module. The PLD netlist  434  may be searched for all reachable pins within the PLD that are functionally connected to the modeled pins of the sub-modules with characterizations  438  and  440  for all instances in the PLD of the tile module. The reachable pins that are also pins of an instance of the tile module may determine the pins  442  of the tile module that may be used to implement signals of a user design. It will be appreciated that certain of the reachable pins may be functionally connected via one or more repeaters that each functionally connect specific input and output pins of the repeater. 
   After searching the entire PLD netlist  434  to generate certain tile pins  442 , the generator  432  may determine the connectivity pins of list  444  with a search that may examine the connectivity within the tile module instead of examining the connectivity of every instance of the tile module in the PLD. The generator  432  may search for pins within the tile module that are functionally connected to starting pins that are either the tile pins  442  or the modeled pins of every instance within the tile module of a switchbox or logic site sub-module. It will be appreciated that certain of the reachable pins may be functionally connected via one or more repeaters. The generator  432  may create a network for each starting pin that is determined to not already be included in any previously created network. Each network may specify a set of pins that are functionally connected either directly or indirectly via one or more repeaters. The generator  422  may output the networks, including the set of functionally connected pins, in the list  444  of networks. 
     FIG. 13  is a flow diagram of a process for determining connections of a programmable tile of a programmable logic device in accordance with various embodiments of the invention. A specification of the connections of a programmable tile may be used by a router to determine interconnections for implementing a user design in a PLD. 
   At step  460 , a netlist that describes the PLD design is input that specifies the interconnections between the instances of various modules of the design of the PLD. An identification of the programmable tile is input at step  462 , and a characterization is input for one or more sub-modules that are each either a switchbox module or a logic site module at step  464 . A characterization for a sub-module includes modeled pins that may be used to implement the user design in the PLD. 
   The connectivity pins are determined at step  466 , and the connectivity pins are a subset of the pins of the tile module that are used to provide connections for implementing a user design in a PLD. The connectivity pins of the tile module may be determined by finding all pins of the instances of the tile module that are functionally connected to an instance of either a switchbox or a logic site within an instance of a tile module. 
   Example 4 shows pseudo-code for determining the connectivity pins of a tile module according to one embodiment of the invention. 
   
     
       
         
             
           
             
                 
             
             
               Example 4: 
             
             
                 
             
           
          
             
               For each sub-module of the PLD { 
             
          
         
         
             
             
          
             
                 
               If the sub-module has a corresponding characterization of a switchbox 
             
             
                 
               or logic site { 
             
          
         
         
             
             
          
             
                 
               For each sub-module pin of the sub-module { 
             
          
         
         
             
             
          
             
                 
               If the sub-module pin is a modeled pin of the corresponding 
             
             
                 
               characterization { 
             
          
         
         
             
             
          
             
                 
               Determine all of the reachable pins from the sub-module pin. 
             
             
                 
               For each reachable pin { 
             
          
         
         
             
             
          
             
                 
               If the reachable pin is a pin of the tile module { 
             
          
         
         
             
             
          
             
                 
               Determine the direction of the pin of the tile module. 
             
             
                 
               Add the pin and the direction to a list of tile pins for 
             
             
                 
               connections. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
          
             
               } 
             
             
                 
             
          
         
       
     
   
   At step  468 , a list of networks may be determined that interconnects the connectivity pins and the modeled pins of the tile module. The networks may be determined by finding all pins that are functionally connected to the connectivity pins and the modeled pins in the PLD netlist. Example 5 shows pseudo-code for determining the networks according to one embodiment of the invention. 
   
     
       
         
             
           
             
                 
             
             
               Example 5: 
             
             
                 
             
           
          
             
               For each tile pin that is a connectivity pin { 
             
          
         
         
             
             
          
             
                 
               Add the tile pin to a list of starting pins. 
             
          
         
         
             
          
             
               } 
             
             
               For each sub-module of the tile { 
             
          
         
         
             
             
          
             
                 
               If the sub-module has a corresponding characterization of a switchbox 
             
             
                 
               or logic site { 
             
          
         
         
             
             
          
             
                 
               For each sub-module pin of the sub-module { 
             
          
         
         
             
             
          
             
                 
               If the sub-module pin is a modeled pin of the corresponding 
             
             
                 
               characterization { 
             
          
         
         
             
             
          
             
                 
               Add the sub-module pin to the list of starting pins. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
          
             
               } 
             
             
               For each starting pin of the list { 
             
          
         
         
             
             
          
             
                 
               If the starting pin is not already visited { 
             
          
         
         
             
             
          
             
                 
               Create a network that includes the starting pin. 
             
             
                 
               Mark the starting pin as visited. 
             
             
                 
               Determine all of the reachable pins from the starting pin. 
             
             
                 
               For each reachable pin { 
             
          
         
         
             
             
          
             
                 
               Add the reachable pin to the network. 
             
             
                 
               Mark the reachable pin as visited. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
          
             
               } 
             
             
                 
             
          
         
       
     
   
   At step  470 , the list of networks may be output, and optionally the connectivity pins may also be output. The list of networks for the connections of the programmable tile may be combined with the programmable connections of any switchboxes included within the hierarchy of the programmable tile to yield the connections that may be created by a tile to implement a portion of a user design in a PLD. Referring to  FIG. 2 , the list of networks may correspond to fixed tile routing arcs  148  and the programmable connections of the switchbox or switchboxes may correspond to programmable tile routing arcs  150 . After the mapper and placer  136  has assigned a portion of a user design to a particular instance of a tile, the router  138  may create the appropriate connections using the routing arcs  148  and  150 . 
     FIG. 14  is a block diagram illustrating configuration memory of a programmable tile  500  of a programmable logic device in accordance with various embodiments of the invention. The configuration memory may be programmed with configuration data that is generated from a specification of a user design, and configuration memory may control the operation of the programmable interconnect and logic resources of the PLD such that the user design is implemented by the PLD. 
   The programmable tile  500  may include look-up table (LUT) logic sites  502  and  504 , and switchbox  506 . The function of LUT logic site  502  may be controlled by cells  508 ,  510 ,  512 , and  514  of the configuration memory to implement any function of two inputs. The function of LUT logic site  504  may be controlled by configuration memory cells that are external to tile module  500 . Switchbox  506  may include two multiplexers  516  and  518 . The function of multiplexer  516  may be controlled by configuration memory cells  520  and  522 , and the function of multiplexer  518  may be controlled by configuration memory cells  524  and  526 . Thus, the values of configuration memory cells  508 ,  510 ,  512 ,  514 ,  520 ,  522 ,  524 , and  526  may determine the function of tile module  500 . LUT logic sites  502  and  504  and multiplexers  516  and  518  may each have select inputs for receiving control values from the configuration memory cells  508 ,  510 ,  512 ,  514 ,  520 ,  522 ,  524 , and  526 . 
   During the programming of the PLD with configuration data generated from a specification of a user design, values from the configuration data may be written to the configuration memory cells  508 ,  510 ,  512 ,  514 ,  520 ,  522 ,  524 , and  526  using the configuration modules  530  and  532 . For example, address module  530  may generate a write enable on line  534  that writes a frame of the configuration data from data module  532  into configuration memory cells  508 ,  520 , and  524 . Generally, the configuration memory cells  508 ,  510 ,  512 ,  514 ,  520 ,  522 ,  524 , and  526  are arranged in a functional matrix determined by the addressing lines, such as write enable  534 , from the address module  530  and the data transfer lines from the data module  532 . 
   It will be appreciated that the configuration memory cells  508 ,  510 ,  512 ,  514 ,  520 ,  522 ,  524 , and  526  might not have the arrangement in a layout of a physical array of configuration memory cells. For example, during the design of the tile for tile module  500 , the circuitry for LUTs  502  and  504  and multiplexers  516  and  518  may physically placed within a tile and related interconnects routed before placing configuration memory cells in the spaces remaining available within the tile. To route the interconnect for the configuration memory cells in the routing area remaining available, the schematic for the tile module  500  may be modified to match the connections that may be made in the remaining routing area. These modifications may change the particular configuration memory cell that controls each select input of LUT logic sites  502  and  504  and multiplexers  516  and  518 , and these modifications may also change the connections between the configuration memory cells and the configuration modules  530  and  532 . Such modifications may be made during physical layout of the tile module  500  near the end of designing a PLD. Thus, the actual configuration memory cell controlling each select input and the position of each configuration memory cell in the functional matrix might not be fully determined until the design of the PLD is nearly completed. 
     FIG. 15  is a data flow diagram of a process for determining control of a select pin of a programmable tile of a programmable logic device in accordance with various embodiments of the invention. A generator  542  of control associations may determine the configuration memory cell that controls each select pin of each programmable function of the PLD. Usage of generator  542  may eliminate the manual extraction of control associations that may be time-consuming and difficult because, for example, the various configuration memory cells may have confusingly similar names. 
   A PLD netlist  544  is searched by the generator  542  to determine the functional connections between the data output pins of instances within a tile of configuration memory cells and the select pins of instances within the tile of logic site modules and multiplexer modules. The identification  546  may include a type of the tile module and the control associations may be determined for the tile module that matches the type. Characterizations  548  and  550  may specify the type and the select input pin or pins of a logic site module and a multiplexer module, respectively. It will be appreciated that, depending on the tile module, control association generator  542  may receive a respective characterization for one or more logic site modules and/or for one or more multiplexer modules. In addition, control association generator  542  may receive characterizations  552  for one or more configuration memory cells. Characterization  552  may specify the type and the data output pin of a configuration memory cell. Generator  542  may use the identification  546  of the tile module and characterizations  548 ,  550 ,  552  to search the PLD netlist  544  for the control associations. 
   The control association generator  542  may output a specification  554  of the controlling pin for each select pin of each instance of the logic site and multiplexer modules with characterizations  548  and  550 . The specification  554  may list each select pin paired with the corresponding controlling pin. The select pins may be controlled by the data output pin of a configuration memory cell that is internal or external to the tile module. For a select pin that is controlled by a configuration memory cell that is internal to the tile module, the controlling pin is the data output pin of a configuration memory cell within the tile module. For a select pin that is controlled by a configuration memory cell that is external to the tile module, the controlling pin may be specified in specification  554  as the pin of the tile module that connects the select pin to the data output pin of the external configuration memory cell. 
   In one embodiment, the characterization  548  for a multiplexer module includes a specification of paths through the multiplexer module and for each path a respective value of the select input pins that activate the path through the multiplexer. The generator  542  may output the specification  554  that includes the respective value of the select input pin for each path through each instance of the multiplexer within the tile module. In one embodiment, the characterization  550  for a logic site module includes a specification of an attribute or attributes of the logic site module and a respective value of the select input pins of the logic site module for each accepted value of each attribute. The generator  542  may output the specification  554  that includes the respective value of the select input pins of the logic site module for each accepted value of each attribute. 
   Referring back to  FIG. 2 , the memory cell relationships  152  may include the specification  554  of the select pins and the corresponding controlling pins for each type of programmable tile module in a PLD. Serializing generator  140  may use the specification  554  for a tile module to map a portion of a user design that is implemented by an instance of the tile module to specific values of the associated configuration memory cells. 
     FIG. 16  is a flow diagram of a process for determining control of a select pin of a programmable tile of a programmable logic device in accordance with various embodiments of the invention. For each select pin of a multiplexer or logic site within the tile module for the programmable tile, a corresponding controlling pin is determined. 
   A PLD netlist is input at step  562 , and an identification of a tile module within the PLD netlist is input at step  564 . At step  566 , a characterization or characterizations are input for one or more multiplexer and/or logic site sub-modules that are instantiated within the tile module. Each sub-module characterization specifies one or more select pins that determine the programmable function performed by the corresponding sub-module. For a multiplexer, the value of the select pins may determine which input of the multiplexer is selected to drive the output of the multiplexer. For a logic site, the value of the select pins may determine the logic function of the logic site. For example, the logic site may be a look-up table having a respective select input providing the output value from the look-up table for each possible combination of input values. At step  568 , a characterization or characterizations are input for one or more configuration memory cells. Each cell characterization includes a data output pin that may be connected in the PLD netlist to a select input of a multiplexer or logic site sub-module. 
   At step  570 , the controlling pin is determined for each select pin of each instance within the tile module of each multiplexer or logic site sub-module. The PLD netlist may be searched starting from the select pins of each instance of each multiplexer or logic site within the tile module to determined the reachable pin or pins that are functionally connected to each select pin. The controlling pin for a select pin is the reachable pin for the select pin that is either the data output pin of a configuration memory cell or an input pin of the tile module. The reachable pin is a pin of the tile module when the configuration memory cell controlling the select pin is external to the tile module. It will be appreciated that the data output pin of a configuration memory cell may be functionally connected to a select pin via one or more repeater modules. 
   Example 6 shows pseudo-code for determining the controlling pin for each select pin according to one embodiment of the invention. 
   
     
       
         
             
           
             
                 
             
             
               Example 6: 
             
             
                 
             
           
          
             
               For each sub-module within the tile { 
             
          
         
         
             
             
          
             
                 
               If the sub-module has a corresponding sub-module characterization { 
             
          
         
         
             
             
          
             
                 
               For each sub-module pin of the sub-module { 
             
          
         
         
             
             
          
             
                 
               If the sub-module pin is a select pin of the sub-module 
             
             
                 
               characterization { 
             
          
         
         
             
             
          
             
                 
               Determine all of the reachable pins from the sub-module pin. 
             
             
                 
               For each of the reachable pins { 
             
          
         
         
             
             
          
             
                 
               If the reachable pin is a data output pin of a configuration 
             
             
                 
               memory cell { 
             
          
         
         
             
             
          
             
                 
               Output the reachable pin as the controlling pin of the select 
             
             
                 
               pin. 
             
          
         
         
             
             
          
             
                 
               } Else if the reachable pin is an input pin of the tile module { 
             
          
         
         
             
             
          
             
                 
               Output the reachable pin as the controlling pin of the select 
             
             
                 
               pin. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
          
             
               } 
             
             
                 
             
          
         
       
     
   
     FIG. 17  is a data flow diagram of a process for determining indices of an instance of a cell of configuration memory in accordance with various embodiments of the invention. The indices of an instance of a cell of configuration memory may include an address index and a data index, which may correspond to a row and a column in a functional matrix of the configuration memory cells. The functional matrix of configuration memory cells may correspond to a physical organization of the configuration memory cells in a PLD; however, the configuration memory cells might not be arranged in a physical array. In one embodiment, each configuration memory cell may be coupled to an address line and a data line. The address index may be specified by the position of the address line in an ordered set of address lines for accessing the configuration memory cells in the PLD, and the data index may specified by the position of the data line in an ordered set of data lines for accessing the configuration memory cells in the PLD. 
   A generator  582  of configuration memory indices may input a PLD netlist  584  and an identification  586  of a tile module. The generator  582  may search the PLD netlist  584  to determine the indices of the instances of the configuration memory cells within the tile module identified by identification  586 , and the identification  586  may identify the tile module using a type of the tile module. 
   Characterization  588  may specify a type of one or more cells of configuration memory along with an address input pin and a data input pin for each type of configuration memory cell. Characterizations  590  and  592  may specify a type for one or more configuration control modules, and together characterizations  590  and  592  may specify an ordered set of address output pins of the configuration control modules and an ordered set of data output pins of the configuration control modules. In one embodiment, a PLD has one configuration control module providing the address output pins and another configuration control module providing the data output pins. Thus, characterization  590  may specify the type of the address configuration-control module and the ordered set of address output pins, and characterization  592  may specify the type of the data configuration-control module and the ordered set of data output pins. 
   Generator  582  may output a specification  594  of the indices, such as a numeric address index and a numeric data index, for each configuration memory cell within the tile module of identification  586 . Each configuration memory cell may be identified in specification  594  by a hierarchical name of the configuration memory cell, which may be a hierarchical name within the tile module. A numeric address or data index may be a relative index among the configuration memory cells within the tile module. 
     FIG. 18  is a flow diagram of a process for determining indices of an instance of a cell of configuration memory in accordance with various embodiments of the invention. For each instance of a configuration memory cell within a tile module, an address index and a data index may be determined. 
   A netlist is input at step  602  and an identification of a tile module is input at step  604 . The identification  604  may include a type of the tile module. At step  606 , a characterization of one or more configuration memory cells is input. Each characterization may include the type of the configuration memory cell and an address input pin and a data input pin of the configuration memory cell. At step  608 , a characterization of one or more configuration control modules is input. Each characterization may include a type of the configuration control module and collectively the characterizations include an ordered set of address output pins of the configuration control module or modules and an ordered set of data output pins of the configuration control module or modules. 
   At step  610 , the address index and the data index is determined for each instance within the tile module of the configuration memory cells. The configuration memory cells coupled to the first address output pin in the ordered set of address output pins of a configuration control module may have an address index of one, the configuration memory cells coupled to the second address output pin in the ordered set may have an address index of two, and et cetera. The data index for each configuration memory cell may be similarly specified. The PLD netlist may be searched for pins that are reachable starting from the address input pin and the data input pin of every instance of a configuration memory cell within a tile module. The address index of a configuration memory cell may be determined by the reachable pin that is an address output pin of a configuration control module, and the data index of the configuration memory cell may be determined by the reachable pin that is a data output pin of a configuration control module. At step  612 , the address and data indices are output for each instance of the configuration memory cell within the tile module along with a name of the instance. 
   Example 7 shows pseudo-code for determining the indices of an instance of a configuration memory cell according to one embodiment of the invention. 
   
     
       
         
             
           
             
                 
             
             
               Example 7: 
             
             
                 
             
           
          
             
               For each sub-module with the tile { 
             
          
         
         
             
             
          
             
                 
               If the sub-module has a corresponding cell characterization { 
             
          
         
         
             
             
          
             
                 
               Add sub-module to a map of the cells of configuration memory. 
             
             
                 
               Call TraceToSource ( with address output pin from the cell 
             
             
                 
               characterization ). 
             
             
                 
               Call TraceToSource ( with data output pin from the cell 
             
             
                 
               characterization). 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
          
             
               } 
             
             
               TraceToSource ( cell pin ): 
             
          
         
         
             
             
          
             
                 
               Determine the reachable pins from the cell pin. 
             
             
                 
               For each of the reachable pins { 
             
          
         
         
             
             
          
             
                 
               If the module of the reachable pin has a configuration control 
             
             
                 
               characterization { 
             
          
         
         
             
             
          
             
                 
               If the reachable pin is included in an ordered set of the 
             
             
                 
               characterization { 
             
          
         
         
             
             
          
             
                 
               Associate index in the ordered set with the reachable pin and the 
             
             
                 
               cell pin. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
             
                 
                 
             
          
         
       
     
   
   Referring back to  FIG. 2 , a serializing generator  140  may receive from memory cell organization  154  the indices of the configuration memory cells within each type of tile module. The serializing generator  140  may use the indices to map the value of each configuration memory cell to a specific bit of the configuration data  132 . 
   The specification of the cell instances and the first and second indices of the cell indices that are output at step  612  may provide a mapping from each of the cell instances to the indices of the cell instance. A reverse mapping from the indices of the cell instances to the cell instance may be useful for generating the configuration data. The generation of the configuration data may loop through each combination of values for the indices for each bit of the configuration data, and the reverse mapping may be used to determine the corresponding cell instance to obtain the value to be stored in the cell instance by the bit of the configuration data. In one embodiment, a two-dimensional data structure may be output at step  614  to map from the indices to the corresponding instance of a configuration memory cell. Example 8 shows pseudo-code for generating a two-dimensional array for the reverse mapping according to one embodiment of the invention. 
   
     
       
         
             
           
             
                 
             
             
               Example 8: 
             
             
                 
             
           
          
             
               Initialize two-dimensional array to empty. 
             
             
               For each cell instance { 
             
          
         
         
             
             
          
             
                 
               Add cell instance to the two-dimensional array at the indices of the cell 
             
             
                 
               instance. 
             
          
         
         
             
          
             
               ) 
             
             
                 
             
          
         
       
     
   
   It will be appreciated that the process of determining control of select pins of instances of a configuration memory cell ( FIGS. 15 and 16 ) might be integrated together with the process of determining the indices of the instances of a configuration memory cell ( FIGS. 17 and 18 ). The separation of these processes in various embodiments of the invention is useful for simplifying each of the software programs that implement these individual processes. The maintenance and clarity of these software programs is improved by the simplification resulting from using separate software programs. 
     FIG. 19  is a block diagram of a portion of a programmable logic device illustrating connections between programmable tiles in accordance with various embodiments of the invention. The PLD may include tile instances  620  and  622  of input-output modules that may correspond to IOBs  104  of  FIG. 1 , tile instances  624  and  626  of modules for configurable logic blocks that may correspond to CLBs  102  of  FIG. 1 , tile instance  628  of a block RAM module that may correspond to one of BRAMs  103  of  FIG. 1 , and tile instance  630  of a module for multi-gigabit transceivers that may correspond to one of MGTs  101  of  FIG. 1 . 
   The netlist of the PLD may specify connections between the tile instances  620  through  630 . Frequently, these connections may connect pins of modules of the tile instances  620  through  630  that are physically adjacent in a layout of the integrated circuit for the PLD. For example, the netlist and the layout of the PLD may include a network connecting the pin on line  632  of IOB tile instance  620  to the pin on line  634  of the adjacent CLB tile instance  624 . The tile instances may also include internal networks that are not connected to a network of any other tile instance, for example, network  636  may be an internal network of BRAM tile instance  628 . 
     FIG. 20  is an example diagram of a user interface illustrating a graphical representation  640  of a programmable logic device in accordance with various embodiments of the invention. The graphical representation  640  of the PLD may include occurrences  642  of a tile representation for an IOB tile module corresponding to IOB tile instances  620  and  622  of  FIG. 19 . The graphical representation  640  may also include occurrences  644  of a tile representation corresponding to CLB tile instances  624  and  626  of  FIG. 19 , occurrence  646  of a tile representation corresponding to BRAM tile instance  628  of  FIG. 19 , and occurrence  648  of a tile representation corresponding to MGT tile instance  630  of  FIG. 19 . Each of occurrences  642 ,  644 ,  646 , and  648  may include connection representations, such as connection representations  650  of occurrences  644 . In this example, the occurrence  646  of a BRAM tile representation has several errors that may be detected by certain embodiments of the invention. 
   Each connection representation  650  may represent a pin, such as the pin on line  634  of  FIG. 19 , of the corresponding tile module. Connection representations that represent a pin of the corresponding tile module generally terminate at a boundary of the occurrence. For example, each connection representation  650  of occurrences  644  represents a pin of a CLB tile module and terminates at a boundary of the respective occurrence  644 . Two tile representations that are adjacent in the graphical representation  640  may have an abutting portion of their respective boundaries, and the abutting portion may be a shared portion of their respective boundaries. The coupling of two connection representations may be indicated in the graphical representation  640  by having the two connection representations terminate at the same position of an abutting portion of the boundaries of the two corresponding occurrences. For example, the graphical representation  640  may indicate that the connection representations  650  of certain of the occurrences  644  may be coupled to connection representations  652  of adjacent occurrences  642 , while the connection representations  650  of certain other of the occurrences  644  may be coupled to connection representations  654  of adjacent occurrence  646 . 
   Generally, for a graphical representation indicating a coupling of respective connection representations of adjacent occurrences of one or more tile representations, the netlist of the PLD should include a network coupling corresponding pins of the instances of the tile modules represented by the occurrences of the tile representations. The absence of such a network in the netlist of the PLD may signify an error in one or both of the tile representations. It will be appreciated that because a tile representation may be an abstraction that omits many details of the tile module, a network coupling pins of instances does not imply that the graphical representation should indicate a coupling of corresponding connection representations. 
   In one embodiment of the invention, each connection representation that terminates on the boundary of a tile representation should have a matching connection representation at the position of the abutting portion of the boundaries between the adjacent occurrences of the tile representations. Connection representation  656 , which terminates at the boundary of one of occurrences  644 , does not have a matching connection representation at the position of the abutting portion of the boundary with occurrence  646 . This mismatch may be due to the error of a missing connection representation for occurrence  646 . Connection representation  658  does not have a matching connection representation at the position of the abutting portion of the boundaries. This mismatch may be due to the error of connection representation  660  being placed at the wrong position of the boundary of occurrence  646 . Connection representation  662  also does not have a matching connection representation, and this mismatch may be due to the error of connection representation  664  not terminating at the boundary of occurrence  646 . 
     FIG. 21  is a data flow diagram of a process for comparing connections between a graphic representation of a programmable logic device and a netlist for the programmable logic device in accordance with various embodiments of the invention. The graphic representation may be checked for matching connection representations along the abutting portion of the boundaries between each pair of adjacent occurrences of tile representations in the graphical representation. For each pair of matching connection representations between a pair of adjacent occurrences of the tile representations, the PLD netlist may be checked for a corresponding network connecting pins of the instances of the tile modules that correspond to the occurrences of the tile representations. 
   A comparer  682  of connections inputs a PLD netlist  684  and an identification  686  of each of the tile modules of the PLD. The identification  686  may include a type of each of the tile modules included in the PLD netlist  684 . 
   The comparer  682  may also input a specification  688  of the placement of the occurrences of the tile representations in the graphical representation. The placement of the occurrences and the size and shape of the tile representations may determine the occurrences that are adjacent and share an abutting portion of their boundaries. In one embodiment, the placement of the occurrences may be provided by data that determines a presentation of the occurrences of the tile representations in a user interface. Frequently, the tile representations are rectangular with various heights and/or widths, and the occurrences of the tile representations are arrayed to completely cover an area of a display of the user interface without overlapping. The arrayed placement of the occurrences of the tile representations in the graphical representation generally reflects the layout of the instances of the tile modules in the PLD. 
   The comparer  682  may input a specification  690  of the tile representations including connection representations for selected networks of the tile module for the tile representation. The specification  690  may include connection representations for selected networks, and for certain of the selected networks corresponding to pins of the tile module for the tile representation, the connection representations for these certain selected networks may be specified to terminate at a particular position on the boundary of the tile representation. The specification  690  may specify coordinates defining the boundary of the tile representation and coordinates defining a line segment for each of the connection representations of the tile representation. These boundary and connection representation coordinates may be coordinates that are used to present each occurrence of the tile representation on a display of a user interface. 
   Two connection representations from respective adjacent occurrences are coupled in the graphical representation when the connection representations terminate at the same position on the abutting portion of their shared boundaries. The comparer  682  checks for mismatched connection representations, and checks that each matched connection representation has a corresponding network connection in the PLD netlist  684 . Any detected errors may be output in connection error messages  692 . 
     FIG. 22  is a flow diagram of a process for comparing connections between a graphic representation of a programmable logic device and a netlist for the programmable logic device in accordance with various embodiments of the invention. At step  702 , a netlist that describes the PLD design is input that specifies a list of networks, with each network connecting pins of module instances, including various tile module instances. At step  704 , a respective identification is input for each of the tile modules, and these identifications may be used to identify pins of the instances of the tile modules in the PLD netlist. A specification of the arrayed placement of occurrences of the tile representations in the graphical representation is input at step  706 , and a specification of each tile representation, including connection representations, is input at step  708 . 
   At step  710 , the connection representations are determined that terminate at a shared portion of the abutting boundaries of the adjacent occurrences of the tile representations. The position of termination of each connection representation may also be determined. 
   At step  712 , mismatched connection representations are checked. In one embodiment, two connection representations of adjacent occurrences are coupled when the connection representations terminate at the same position on the abutting portion of the shared boundaries. Each connection representation may have a name in the tile representation that may be mapped to a corresponding pin of the tile module for the tile representation. For two connection representations that are coupled in the graphical representation, the PLD netlist may be checked to verify that a network forms a corresponding connection between the mapped pins of the tile instances corresponding to the adjacent occurrences. In addition, the graphical representations may be checked to verify that each connection representation that terminates at a position on the boundary of an occurrence has a matching connection representation terminating at this position for an adjacent occurrence. Example 9 shows pseudo-code for checking for mismatched connection representations according to one embodiment of the invention. 
   
     
       
         
             
           
             
                 
             
             
               Example 9: 
             
             
                 
             
           
          
             
               For each tile representation { 
             
          
         
         
             
             
          
             
                 
               For each connection representation in the tile representation { 
             
          
         
         
             
             
          
             
                 
               For each line segment of the connection representation { 
             
          
         
         
             
             
          
             
                 
               For each endpoint of the line segment { 
             
          
         
         
             
             
          
             
                 
               If the endpoint is at an abutment boundary of the tile 
             
             
                 
               representation { 
             
          
         
         
             
             
          
             
                 
               Insert connection representation into a list ordered by position 
             
             
                 
               on boundary. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
          
             
               } 
             
             
               For each pair of an adjacent first and second occurrence of tile 
             
             
               representations { 
             
          
         
         
             
             
          
             
                 
               For each coordinate position on the abutment boundary of the adjacent 
             
             
                 
               occurrences { 
             
          
         
         
             
             
          
             
                 
               Get terminating connection representation from first occurrence,  
             
             
                 
               if any. 
             
             
                 
               Get terminating connection representation from second occurrence, 
             
             
                 
               if any. 
             
             
                 
               If connection representations exist for both the first and 
             
             
                 
               second occurrence { 
             
          
         
         
             
             
          
             
                 
               Get corresponding networks from the PLD netlist. 
             
             
                 
               If the corresponding networks are not connected in the PLD 
             
             
                 
               netlist { 
             
          
         
         
             
             
          
             
                 
               Issue unexpected abutment error. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } else if only one connection representation exists for the 
             
             
                 
                   occurrences { 
             
          
         
         
             
             
          
             
                 
               Issue unmatched abutment error. 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
             
          
             
                 
               } 
             
          
         
         
             
          
             
               } 
             
             
                 
             
          
         
       
     
   
     FIG. 23  is a block diagram of a system for processing a PLD netlist in accordance with various embodiments of the invention. A processor-readable device  720  may be configured with instructions and a processor  722  or processors may perform various operations during execution of the instructions. The operations may include searching the PLD netlist to determine various properties of the PLD. 
   Software  724  may include instructions for inputting the netlist that describes the PLD design specifying connections between instances of modules. Software  726  may include instructions for inputting an identification of a network or a module of the PLD. The network that is identified may be a starting network for a search for reachable pins that are functionally connected to the starting network. The module that is identified may be a switchbox module or a tile module and the identification may include a type of the module. 
   Software  728  may include instructions for inputting a characterization a module of the PLD that may be one or more repeater modules, multiplexer modules, logic site modules, switchbox modules, configuration memory cells, or configuration control modules. The characterization may be manually or automatically generated and include selected properties of the module, such as a type of the module and one or more ordered or unordered sets of selected pins of the module. Software  730  may input a specification of one or more representations that may be a graphical representation of the PLD or respective tile representation for the tile modules of the PLD. 
   Software  732  may include instructions for managing a queue of reachable pins that are functionally connected to a starting network of the netlist for the PLD. The queue may include the paths the functionally connects each pin in the queue to the starting network. 
   Software  734  may include instructions for searching the PLD netlist to determine various properties of the PLD. In a first embodiment, the search determines pins of a switchbox module through which the programmable connections are provided and the search also determines programmable connections of the switchbox module. In a second embodiment, the search determines connectivity pins of the tile module and modeled interconnects of a tile module. In a third embodiment, the search determines a controlling pin for each select input pin of each instance of a logic site or multiplexer module within a tile module. In a fourth embodiment, the search determines the indices of a configuration memory cell within a tile module. 
   Software  736  may include instructions for outputting a specification of certain properties of the PLD, such as the pins reachable from a starting network, certain programmable connections, a list of networks for modeled interconnects, the controlling pin for each select input pin, and/or the indices of a configuration memory cell within the tile module. 
   Software  738  may include instructions for checking for a match or correspondence between connection representations terminating at the shared portion of the boundaries of adjacent occurrences of tile representations. 
   Those skilled in the art will appreciate that various alternative computing arrangements, including one or more processors and a memory arrangement configured with program code, would be suitable for hosting the processes and data structures of the different embodiments of the present invention. In addition, the processes may be provided via a variety of computer-readable media such as magnetic or optical disks or tapes, or electronic storage devices. 
   The present invention is thought to be applicable to a variety of systems for modeling a programmable logic device. Other aspects and embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and illustrated embodiments be considered as examples only, with a true scope and spirit of the invention being indicated by the following claims.