Abstract:
A logic cluster for a field programmable gate array integrated circuit device is disclosed. The cluster comprises a plurality of functional blocks and three levels of routing multiplexers. External signals enter the logic cluster primarily at the third level multiplexers with a few signals entering at the second level. Combinational outputs are fed back into the first and second level multiplexers while sequential outputs are fed back into the third level multiplexers. The logic function generators have permutable inputs with varying propagation delays. Routing signals between the first and second level multiplexers are grouped into speed classes and coupled to first level multiplexers associated with different logic function generators according to their speed class. Second and third level multiplexers are organized into groups such that routing signals between the second and third level multiplexers can be localized within the area occupied by the group. Groups are pitch matched to logic function generators to optimize and modularize area. Provision is made for global and local control of the sequential elements.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims an invention that was disclosed in Provisional Application No. 61/024,867 filed Jan. 30, 2008, entitled “Clustered Field Programmable Gate Array Architecture.” The benefit under 35 USC §119(e) of the United States provisional application is hereby claimed, and the aforementioned provisional application is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention pertains to the field of integrated circuits. More particularly, the invention pertains to field programmable gate array integrated circuit devices. 
     2. The Prior Art 
     Field Programmable Gate Array (FPGA) integrated circuit devices are known in the art. An FPGA comprises any number of initially uncommitted functional blocks arranged in an array along with an appropriate amount of initially uncommitted routing resources. Functional blocks are circuits which can be configured to perform a variety of logic functions like, for example, AND-gates, OR-gates, NAND-gates, NOR-gates, XOR-gates, XNOR-gates, inverters, multiplexers, adders, latches, and flip/flops. Routing resources can include a mix of components such as wires, switches, multiplexers, and buffers. Logic modules, routing resources, and other features like, for example, I/O buffers and memory blocks, are the programmable elements of the FPGA. 
     The programmable elements have associated control elements (sometimes known as programming bits or configuration bits) which determine their functionality. The control elements may be thought of as binary bits having values such as on/off, conductive/non-conductive, true/false, or logic-1/logic-0 depending on the context. The control elements vary according to the technology employed and their mode of data storage may be either volatile or non-volatile. Volatile control elements, such as SRAM bits, lose their programming data when the PLD power supply is disconnected, disabled or turned off. Non-volatile control elements, such as antifuses and floating gate transistors, do not lose their programming data when the PLD power supply is removed. Some control elements, such as antifuses, can be programmed only one time and cannot be erased. Other control elements, such as SRAM bits and floating gate transistors, can have their programming data erased and may be reprogrammed many times. The detailed circuit implementation of the functional blocks and routing resources can vary greatly and is appropriate for the type of control element used. 
     Typically a user creates a logic design inside manufacturer-supplied design software. The design software then takes the completed design and converts it into the appropriate mix of configured logic modules and other programmable elements, maps them into physical locations inside the FPGA, configures the interconnect to route the signals from one logic module to another, and generates the data structure necessary to assign values to the various control elements inside the FPGA. 
     Many FPGA architectures employing various different functional blocks and interconnect arrangements are known in the art. Some architectures are flat while others are clustered. In a flat architecture, the logic modules may or may not be grouped together with other logic modules, but all of the logic modules have similar or nearly equivalent access to the larger routing architecture. In a clustered architecture, the logic modules are grouped together into clusters, meaning that all of the logic modules in the cluster have some degree of exclusive routing interrelationship between themselves relative to logic modules in other clusters. 
       FIG. 1  illustrates a block diagram of a prior art logic cluster  100  which illustrates some basic principles of a clustered architecture. The illustrative logic cluster contains four functional blocks  102 , though any number can be present. Typically a functional block  102  comprises a logic function generator circuit (or LFG)  104  and an associated sequential element  106  (designated SE in the diagram) which is typically a flip/flop that can also be configured to be a latch. The ones in the diagram have four logic input interconnects since in practice a look-up table (LUT) with four inputs (LUT4) is the most common function generator in this sort of architecture. The output of each LFG is coupled to the data input of the associated sequential element. The output of each logic function generator and each sequential element is coupled to a functional block output. The output coupled to the function generator is a combinational output while the output coupled to the sequential element is a sequential output. 
     Functional block  102  is a very simple exemplary functional block. Many others are known in the art. Typically there are additional routing multiplexers present in the functional block  102  to allow one of the LFG  104  inputs to bypass the LFG  104  and enter the data input of the sequential element  104 . Sometimes there is a fifth input that can bypass LFG  104  without stealing one of its inputs. Often the output of the sequential element  106  can be fed back into one of the LFG  104  inputs through another routing multiplexer. Support circuits for other functions like binary arithmetic or cascading LFGs into wide functions are present. While the place and route tools in the design software will utilize these features when available, these are generally special cases separate from the general cluster routing functionality. 
     Typically the sequential element is more complicated than the simple D-flip/flop shown in  FIG. 1 . Often there will be, for example, a set signal, a reset signal, an enable signal, a load signal, and a clock signal (shown but without its source) or some combination thereof present. Collectively these signals are called sequential control inputs or global inputs (since they are often driven by signals distributed globally around the FPGA) and they typically have their own associated routing resources that may or may not be coupled to the cluster routing resources shown in  FIG. 1 . 
     The box designated Cluster Internal Routing (CIR)  110  contains the cluster internal interconnects and the box designated External Horizontal &amp; Vertical Routing (EHVR)  114  contains the external interconnects and other routing resources of the larger FPGA (not shown). Thus in  FIG. 1 , CIR  100  contains only routing wires, while EHVR  114  contains a variety of different elements like switches, multiplexers, and buffers in addition to routing wires. Functional block output interconnects are cluster output interconnects if they connect to EHVR  114 . If a functional block output interconnect connects to the CIR  110 , it is called a feedback interconnect since it allows cluster outputs to feed back to inputs of LFGs in the same cluster without leaving and reentering the cluster by means of cluster outputs and external interconnect and routing resources. A functional block output interconnect can be both a cluster output interconnect and a feedback interconnect if it connects to both the CIR  110  and EHVR  114 . 
     In  FIG. 1 , the Logic Function Generator Input Multiplexers (some exemplary instances being labeled  108 ) are coupled between the Cluster Internal Routing  110  and the various logic inputs coupled to the functional blocks  102 . Since there are four functional blocks each with four input interconnects, there are a total of sixteen Logic Function Generator Input Multiplexers in the exemplary cluster  100 . Typically, the number of data channels (or data inputs) on each Logic Function Generator Input Multiplexer is less than the total number of lines in CIR  110 , so each Logic Function Generator Input Multiplexer can only route a subset of the signals inside CIR  110  to its associated LFG input. 
     In the architecture  100  of  FIG. 1 , signals are transmitted from the EHVR  114  to the CIR  110  by ten Cluster Input Multiplexers, two exemplary ones being labeled  112 . Various interconnects and resources from other parts of the FPGA are connected to the inputs of the Cluster Input Multiplexers by means of the External Horizontal &amp; Vertical Routing  114 . The lines internal to the Cluster Internal Routing box  110  come from a variety of sources: the outputs of the Cluster Input Multiplexers, the outputs of the cluster&#39;s LFGs and SEs, and possibly other sources such as clock networks and other special functions not shown in  FIG. 1  to avoid overcomplicating the diagram. 
     As FPGAs get larger, clustered architectures get favored over completely flat ones, based on the ease of place and route, and how fast this task can be accomplished by the design software. There are many examples of clustered architectures in both the academic literature as well as in commercial products. 
       FIG. 2A  shows an exemplary cluster  200  of a type known in the art. Present in  FIG. 2A  are functional blocks  202 , Level 1 multiplexers  204   a  through  204   j , EHVR  206 , cluster input interconnect busses  208   a  through  208   j , functional block output bus  210 , and feedback interconnects  212 . There can be any number of functional blocks of various types inside of box  202  as a matter of design choice. This is an abstraction intended to focus attention on the relationships between classes of interconnects inside cluster  200  rather than on the detailed connections of a specific circuit topology. 
     The inputs of the functional blocks  202  are coupled to the outputs of the first level (or level 1) multiplexers  204   a  through  204   j . While ten level 1 multiplexers  204   a  through  204   j  are shown in  FIG. 2A , the exact number present is equal to the total of all of the inputs of all of the functional blocks  202 . The number of input channels of the multiplexers  204   a  through  204   h  need not be equal. Similarly, the portion of the input channels into each multiplexer from each of the sources shown need not be equal. 
     The external horizontal and vertical routing (EHVR)  206  contains routing interconnects and other routing resources such as, for example, multiplexers, buffers, and control elements for programming and enabling them. Placing the balance of the FPGA routing in box  206  is a deliberate abstraction to allow focusing on the relationships of classes of interconnects inside cluster  200 . 
     The level 1 multiplexers  204   a  through  204   j  are coupled to EHVR  206  by cluster input interconnect busses  208   a  through  208   j . While each cluster input interconnect bus  208   a  through  208   j  is shown with exactly five lines in  FIG. 2A , this is an abstraction and any number of interconnects may be present as a matter of design choice. 
     The outputs of functional blocks  202  form functional block output bus  210 , which is shown coupling functional blocks  202  to EHVR  206  and feedback interconnects  212 . The portion of the interconnects in functional block output bus  210  that couple to either EHVR  206  or feedback interconnects  212  is a matter of design choice. 
     Feedback interconnects  212  are coupled to inputs on level 1 multiplexers  204   a  through  204   j  by interconnect busses  214   a  through  214   j  (though only  214   j  is labeled in  FIG. 2A  to avoid cluttering and obscuring the drawing figure). The feedback interconnects are functional block outputs that can be routed back to the functional block inputs without exiting and reentering cluster  200  through EHVR  206 , giving the feedback interconnects a significantly faster return path. Level 1 multiplexer  204   a  is coupled to interconnect bus  214   a , level 1 multiplexer  204   b  is coupled to interconnect bus  214   b , and so on through Level 1 multiplexer  204   j  and interconnect bus  214   j . Although five wires are shown in each of interconnect busses  214   a  through  214   j  the exact number of wires present can vary from one bus to another and is a matter of design choice. 
     While interconnect busses  208   a  through  208   j  couple EHVR  206  to the level 1 multiplexers  204   a  through  204   j , they do not connect to the feedback interconnects  212 . In  FIG. 2A , they can be thought of as “passing under” them instead. One example is indicated by an oval designated “Bus Passes Under, Does Not Connect.” This convention will be used with respect to various interconnect representations throughout this application, since drawing such busses in the more conventional “passing over” style makes the drawing figures harder to read and obscures the concepts being illustrated. 
     In  FIG. 2A , EHVR  206  contains the external interconnects and other routing resources of the larger FPGA (not shown) while feedback interconnects  212  contains only routing wires. 
     Cluster  200  can be described as an I1F1 type cluster. The “I1” signifies that inputs to the cluster enter the cluster internal routing resources at the first level of multiplexers while the “F1” signifies that the feedback interconnects also enter the cluster internal routing resources at the level 1 multiplexers. This type of shorthand description is useful for characterizing many types of clustered architectures. Examples of I1F1 clusters are found in a number of different commercial FPGA families offered by Xilinx, Inc., of San Jose, Calif. 
     An I2F1 cluster  220  of the prior art is shown in  FIG. 2B . Present in the drawing figure are functional blocks  222 , level 1 multiplexers  224   a  through  224   j , local interconnects  226 , level 2 multiplexers  228   a  through  228   j , EHVR  230 , interconnect busses  232   a  through  232   j , interconnect busses  234   a  through  234   j  (though only  234   j  is labeled in the figure), functional block output bus  236 , and feedback interconnects  238 . 
     Any number of functional blocks may be present inside box  222  as a matter of design choice. Although ten level 1 multiplexers are shown, the number of level 1 multiplexers is equal to the total number of functional block inputs and the number of data channels on the various level 1 multiplexers can vary from one to another as a matter of design choice. While each level 1 multiplexer is shown coupled to two busses of five interconnects each, the number of interconnects in each bus can vary as a matter of design choice. Similarly, while ten level 2 multiplexers  228   a  through  228   j  are shown, the exact number will vary from architecture to architecture as a matter of design choice. 
     The data flow for external signals is interconnects originating in EHVR  230  are coupled to the inputs of the second level multiplexers  228   a  through  228   j . While seven interconnects are shown for each level 2 multiplexer in the drawing figure, the exact number can vary from multiplexer to multiplexer as a matter of design choice. The outputs of the level 2 multiplexers are coupled to the level 1 interconnections  226 , which in turn are coupled to some of the data channels on the level 1 multiplexers, which in turn have their outputs coupled to the inputs of the functional blocks  222 . Thus the cluster inputs enter the internal cluster routing resources (meaning multiplexers and interconnects) at the level 2 multiplexers (I2 in I2F1). The box labeled level 1 interconnects  226  in  FIG. 2B  contains only routing wires and all of the routing decisions are implemented by control elements (not shown) governing the level 1 multiplexers  224   a  through  224   j  and level 2 multiplexers  228   a  through  228   j.    
     The data flow for feedback signals starts from the outputs of the functional blocks  222  that form functional block output bus  236 . Some of the interconnects in bus  236  are coupled to the feedback interconnects  238  which in turn are coupled to some of the data channels on the level 1 multiplexers. Thus the feedback interconnects enter the internal cluster routing resources at the level 1 multiplexers (the F1 in I2F1). The box labeled feedback interconnects  238  in the drawing figure contain only routing wires and routing decisions internal to cluster  220  with respect to these wires are made by the control elements governing the level 1 multiplexers  224   a  through  224   j . Interconnects in functional block output bus  336  can be coupled to feedback interconnects  238 , EHVR  230 , or both. The number and destinations of the interconnects in functional block output bus  236  is a matter of design choice. 
     I2F1 clusters are well known in both academia and in commercial FPGAs. Such a cluster is described in the textbook Guy Lemieux and David Lewis,  Design of Interconnection Networks for Programmable Logic , Kluwer Academic Publishers, 2004 (henceforth “Lemieux”), page 28, FIG. 3.4. Commercial products using this architecture can be found in a number of FPGA families offered by Altera Corporation of San Jose, Calif. 
     Other FPGA cluster types with two levels of routing multiplexers are possible. In U.S. Pat. No. 6,975,139 to Pani, et al, (henceforth “Pani”) another architecture is shown in FIG. 7. In that figure, one set of input wires 601-604 is shown coupled to first level multiplexers while another set of input wires 605-612 is shown coupled to second level multiplexers. Since no feedback interconnects are explicitly shown in conjunction with the figure, it is reasonable to assume they are included in the input wires 601-612. The input interconnects could reasonably be coupled exclusively to the second level multiplexers (I2) or to a combination of first and second level multiplexers (I2—pronounced “i-one-two,” not “i-twelve”). Similarly, the feedback interconnects could reasonably be coupled exclusively to first level multiplexers (F1), exclusively to second level multiplexers (F2), or to a combination of first and second level multiplexers (F12—pronounced “ef-one-two,” not “ef-twelve”). Thus the cluster combinations taught in Pani FIG. 7 could reasonably be inferred to be I12F1, I12F2, I12F12, I2F1 or I2F12 depending on the particular design choices made in assigning input and feedback connections in any given embodiment. 
     FPGA cluster types of three or more stages are also known in the art. In Pani, FIGS. 9-12 an I5F5 cluster is disclosed. In Lemieux, Chapter 2, Section 2.1, pages 9-17, highly routable switching networks are discussed in general, including references to a number of well known switching networks such as Clos networks and Benes networks. These networks typically have at least three stages of switches and can often be optimized for decreased numbers of switches and improved routablility by increasing the number of levels of switches that signals must pass through. Unfortunately, when such an approach is used in an FPGA cluster, the resulting performance degradation is undesirable to the point of being unacceptable in commercial FPGAs. 
     Since switches are typically constructed out of pass transistors or analogous floating gate structures in FPGAs, a series of switches and interconnects creates an RC network. As the number of series switches increases, both the resistance and capacitance also increase which greatly increases the propagation delay through the network. Adding buffers can help, but often the propagation delay through the buffers offsets the decrease in RC delays. At present, no three or more level clusters are used in commercially available products. 
       FIG. 2C  shows an I3F3 type FPGA cluster  250  known in the prior art. Present in the drawing figure are functional blocks  252 , level 1 multiplexers  254   a  through  254   j , level 1 interconnects  256 , level 2 multiplexers  258   a  through  258   j , level 2 interconnects  260 , level 3 multiplexers  262   a  through  262   j , input busses  264   a  through  264   j , EHVR  266 , functional block output bus  268 , and feedback interconnects  270 . Similar to the clusters shown in  FIG. 2A  and  FIG. 2B , the numbers of functional blocks, the numbers of first, second and third level multiplexers, the numbers of interconnects in the various interconnect busses, and the number of input channels on the various multiplexers are all a matter of design choice. 
     The data flow for external signals is through interconnects originating in EHVR  266  that are coupled to some of the data channels of the third level multiplexers  262   a  through  262   j . The outputs of the level 3 multiplexers are coupled to the level 2 interconnections  260  which in turn are coupled to the data channels on the level 2 multiplexers  258   a  through  258   j . The outputs of the level 2 multiplexers  258   a  through  258   j  are coupled to the level 1 interconnects  256  which are coupled to the data channels of the level 1 multiplexers  254   a  through  254   j , which in turn have their outputs coupled to the inputs of the functional blocks  252 . Thus the cluster inputs enter the internal cluster routing resources at the level 3 multiplexers (the I3 in I3F3). 
     The data flow for feedback signals starts from the outputs of the functional blocks  252  that form functional block output bus  268 . Some or all of the interconnects in bus  268  (the number and destinations of the interconnects in functional block output bus  268  is a matter of design choice) are coupled to the feedback interconnects  270  which in turn are coupled to some of the data channels on the level 3 multiplexers. The outputs of the level 3 multiplexers are coupled to the level 2 interconnections  260  which in turn are coupled to the data channels on the level 2 multiplexers  258   a  through  258   j . The outputs of the level 2 multiplexers  258   a  through  258   j  are coupled to the level 1 interconnects  256  which are coupled to the data channels of the level 1 multiplexers  254   a  through  254   j , which in turn have their outputs coupled to the inputs of the functional blocks  252 . Thus the feedback interconnects reenter the internal cluster routing resources at the level 3 multiplexers (the F3 in I3F3). 
     Level 1 interconnects  256 , level 2 interconnects  260  and feedback interconnects  270  contain only routing wires. All of the routing decisions are implemented by control elements (not shown) governing the level 1 multiplexers  254   a  through  254   j , level 2 multiplexers  258   a  through  258   j  and level 3 multiplexers  262   a  through  262   j.    
     Turning to  FIG. 3 , a typical lookup table (or LUT) functional block  300  of the prior art is shown. LUT  300  has four inputs A, B, C and D and an output OUT and when programmed is capable of generating any four variable Boolean function where OUT=f(A,B,C,D) where “f” is the programmed Boolean function. Such a four input lookup table has been used as the LFG in many academic studies and commercial FPGAs. Typically a buffer or a sense amplifier (not shown) is present at the output to provide output drive. 
     LUT  300  comprises one first stage 2:1 multiplexer  302 , two second stage 2:1 multiplexers  304   a  through  304   b , four third stage 2:1 multiplexers  306   a  through  306   d , eight fourth stage 2:1 multiplexers  308   a  through  308   h , and sixteen control elements  310   a  through  310   p . The select inputs of the first, second, third and fourth stage 2:1 multiplexers are coupled to inputs A, B, C, and D respectively. The output of first stage 2:1 multiplexer  302  is coupled to OUT while its two data channels are each coupled to one of the outputs of the two second stage 2:1 multiplexers  304   a  through  304   b . The four data channels of the two second stage 2:1 multiplexers  304   a  through  304   b  are each coupled to one of the four outputs of third stage 2:1 multiplexers  306   a  through  306   d . The eight data channels of the four third stage 2:1 multiplexers  306   a  through  304   d  are each coupled to one of the eight outputs of fourth stage 2:1 multiplexers  308   a  through  308   h . The sixteen data channels of the eight fourth stage 2:1 multiplexers  308   a  through  308   h  are each coupled to one of the sixteen outputs of the control elements  310   a  through  310   p . Collectively, this arrangement of 2:1 multiplexers forms a 16:1 multiplexer that presents the output of one of the sixteen control elements  310   a  through  310   p  to the output OUT. 
     Control elements  310   a  through  310   p  produce either a logic-0 or a logic-1 depending on how they are programmed. Boolean function f(A,B,C,D) of four variables will have sixteen unique input states, each with a corresponding output state. The value of that output state is placed in the control elements  310   a  through  310   p  such that the correct value is gated to the output OUT for the logic values presented at the inputs A, B, C and D. Thus the circuit  300  is said to “look up” the logic function rather than calculate it. 
     The nature of the control elements  310   a  through  310   p  will vary with the technology employed in constructing the FPGA. For example, in an SRAM-based FPGA each of the control elements  310   a  through  310   p  will typically be one bit of SRAM, with or without a buffer. Or in an antifuse based FPGA, each of the control elements would typically comprise a first antifuse coupled between the control element output and VCC and a second antifuse coupled between the control element output and ground. Or in a flash based FPGA, each of the control elements would typically comprise a first floating gate transistor coupled between the control element output and VCC and a second floating gate transistor coupled between the control element output and ground. In addition, some programming signals and circuitry may also be present in each control element. 
     It is worth noting that the propagation delay through LUT  300  of  FIG. 3  will be different for each of the four inputs A, B, C and D. Input A will be the fastest, having the least propagation delay, since a logic transition on input A will only need to propagate through first stage multiplexer  302  to reach the output OUT. Input B will be the second fastest, having the second least propagation delay, since a logic transition on input B will need to propagate through one of the second stage multiplexers  304   a  through  304   b  as well as through first stage multiplexer  302  to reach the output OUT. In a similar manner, input C will be the third fastest, having the third least propagation delay, since a logic transition on input C will need to propagate through one of the third stage multiplexers  306   a  through  306   d , through one of the second stage multiplexers  304   a  through  304   b , as well as through first stage multiplexer  302  to reach the output OUT. Similarly, input D will be the slowest input, since an input transition on D will need to propagate through a multiplexer in each of the four stages. 
     Using circuit design techniques well known in the art such as adding buffers and inverters, using different styles of multiplexer, sizing transistors, decoding the select inputs, using level restorers, etc., LUT  300  can be optimized in many different ways. For example, LUT  300  can be designed for minimum area or minimum power or minimum propagation delay as a matter of design choice. Optimizing for minimum propagation delay can be done in a number of different ways. For example, LUT  300  can be optimized so that the average of the propagation delays from each of the four inputs A, B, C and D is minimized. Alternatively, the propagation delay of the fastest input can be minimized, even if this causes the average propagation delay of the four inputs A, B, C and D to be greater than the minimum for any given area and power budget. 
     In U.S. Pat. No. 7,443,198 to McCollum, et al, (henceforth “McCollum”) a second look up table functional block is disclosed in FIG. 8A, FIG. 8B and FIG. 9. Since multiplexer  312  shown in FIG. 9 of McCollum is constructed in a manner similar to the first three multiplexer stages of LUT  300  in FIG. 3, the propagation delay through the McCollum look up table will be different for each of the four inputs A, B, C and D. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1  is a diagram showing a logic cluster of the prior art. 
         FIG. 2A  is a diagram showing a logic cluster of the prior art. 
         FIG. 2B  is a diagram showing a logic cluster of the prior art. 
         FIG. 2C  is a diagram showing a logic cluster of the prior art. 
         FIG. 3  is a diagram showing a look up table of the prior art. 
         FIG. 4A  is a diagram showing a logic cluster of the present invention. 
         FIG. 4B  is a diagram showing a logic cluster of the present invention. 
         FIG. 4C  is a diagram showing a logic cluster of the present invention. 
         FIG. 4D  is a diagram showing a logic cluster of the present invention. 
         FIG. 4E  is a diagram showing a logic cluster of the present invention. 
         FIG. 4F  is a diagram showing a logic cluster of the present invention. 
         FIG. 5  is a diagram showing details of a CCRU of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Persons of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons. 
     The present invention is a three level logic cluster which is organized in a manner that overcomes the performance limitations of the prior art. In comparison to the unacceptably slow I3F3 architecture shown in  FIG. 2C  the present invention has commercially acceptable performance while maintaining substantially the same area and routeability. 
     In the descriptions in this specification for the purpose of consistency and clarity of explanation only, the functional blocks in the logic cluster are assumed to include a mixture of LFGs (implemented as four input lookup tables or LUT4s) and sequential elements. In a preferred embodiment, an equal number of LUT4S and sequential elements are present, but this is not true in all embodiments. In the preferred embodiment, the LUT4 employed is of the type disclosed in McCollum FIG. 8A, FIG. 8B and FIG. 9, although other LUT4 designs like the one in  FIG. 3  of this application may be used. The number of LUT inputs is not limited to four and LUTs of other widths such as LUT2, LUT3, LUT5, LUT6 or even wider LUTs fall within the scope of the present invention. 
     The present invention exploits the property of unequal propagation delays through the LUT4 from its various inputs. In the preferred embodiment, the propagation delay from the fastest input (by convention always the A input in this specification) to the output is minimized as much as area and power constraints will allow as a matter of design choice using circuit design techniques known in the art. Then the propagation delay from the second fastest input (by convention always the B input in this specification) to the output is minimized as much as area and power constraints will allow. In a similar manner, the third fastest and fourth fastest inputs (by convention the C and D inputs in this specification) to output delays are each minimized in turn. It is important to optimize the LUT4 to make the A input propagation delay as fast as possible, even if this results in the average delay of all four inputs being slower than if the optimization were done to make the average of the propagation delays as small as possible (which is done in some prior art architectures like Pani). 
     Lookup table LFGs have logically permutable inputs, meaning that the place and route software can route any signal to any input and then change the lookup function to compensate. FPGA design software can exploit this to increase routeability (like in Pani where all LUT inputs are assumed equal and the emphasis is on minimizing the number of routing switches), or the design software can exploit this to increase speed by routing critical signal paths through the fastest inputs—but it cannot do both because both directives are continually in conflict by competing for the fastest LUT input and only by coincidence can both objectives be achieved for the same LUT in a user design. Since the starting point I3F3 architecture is already extremely routable, the approach of using the design software to increase speed is used. Although only LUTs are discussed in this specification, persons of ordinary skill in the art will recognize that any other LFG with permutable inputs having different propagation delays would be suitable for use with the present invention. 
       FIG. 4A  shows an I3F123 (pronounced “i-three-ef-one-two-three”) logic cluster  400 A of the present invention. Present in  FIG. 4A  are functional blocks  402 , level 1 multiplexers  404   a  through  404   h , level 1 interconnects  406 , level 2 multiplexers  408   a  through  408   h , level 2 interconnects  410 , level 3 multiplexers  412   a  through  412   j , EHVR  414 , functional block output bus  416 , feedback interconnects  418 , and feedback interconnects  420 . 
     The functional blocks  402  have inputs coupled to level 1 multiplexers  404   a  through  404   h . The inputs are labeled A 0  through A n , B 0  through B n , C 0  through C n , and D 0  through D n . The number of functional blocks can vary as a matter of design choice, and in any given design n+1 are present (n is a positive integer). The A i , B i , C i , and D i  inputs are coupled to the A, B, C, and D inputs respectively of the LUT4s inside functional blocks  402 . The handling of sequential control inputs will be discussed later in conjunction with  FIG. 5 . 
     Level 1 internal interconnects  406  comprise a number of routing wires. Each wire is coupled to the output of one of the level 2 multiplexers  408   a  through  408   h  as well as to one or more data channels of the level 1 multiplexers. The exact number of inputs and outputs and pattern of their connections will vary from embodiment to embodiment and is a matter of design choice. 
     Level 2 internal interconnects  410  comprise a number of routing wires. Each wire is coupled to the output of one of the level 3 multiplexers  412   a  through  412   j  as well as to one or more data channels of the level 2 multiplexers. The exact number of inputs and outputs and pattern of their connections will vary from embodiment to embodiment and is a matter of design choice. 
     EHVR  414  comprises a number of routing wires and other routing resources not shown such as multiplexers, switches and buffers. External interconnects originating in EHVR  414  are coupled to one or more of the data channels of the level 3 multiplexers. The exact number of external interconnects and the pattern of their connections will vary from embodiment to embodiment as a matter of design choice. 
     Functional blocks  402  have outputs coupled to the functional block output bus  416 . The outputs of the LFGs are coupled to combinational interconnects and the outputs of the sequential inputs are coupled to sequential interconnects. The sequential interconnects can be coupled to either feedback interconnects  418 , EHVR  414 , or both, while the combinational interconnects can be coupled to either feedback interconnects  420 , EHVR  414 , or both. 
     In a preferred embodiment, the sequential feedback interconnects  418  only reenter the logic cluster internal routing at the level 3 multiplexers (the F3 in I3F123), while the combinational feedback interconnects  420  only reenter the logic cluster internal routing at the level 1 and level 2 multiplexers (the F12 in I3F123). Place and route experiments indicate that LUTs feed back into the same cluster far more frequently than sequential elements do. Also, F2 connections are more expensive than F3 connections (since an F2 connection must connect to multiple level 2 multiplexers while an F3 connection needs to connect to only one level 3 multiplexer for equivalent routeability), so only the feedback connections that improve speed significantly (the combinational ones) reenter as F2 connections. 
     Since the combinational F2 connections to the level 2 multiplexers are costly in terms of silicon area, in a preferred embodiment some selective depopulation of the level 2 multiplexers is done to recoup the area required. Depopulation means removing data channels from the group of level 2 multiplexers eliminating some routing possibilities. In the I3F3 cluster  250  of  FIG. 2C , each level 3 multiplexer input can reach 100% of the LUT4 inputs. By reducing size of the level 2 multiplexers so that each level 3 multiplexer can reach 85% of the LUT4 inputs on average and no less than 75% in any given instance, the area penalty incurred by the additional F2 connections is removed with no loss of performance and almost no loss of routing flexibility. 
     A few of the combinational feedback interconnects  420  are also fed back as F1 connections, but only to the level 1 multiplexers associated with the fastest A inputs of the LUT4s. An exemplary of one of these connections is indicated by reference number  422  in the drawing figure. From a routeability perspective, these are redundant with respect to the F2 connections (i.e., there is more than one way to reach the involved level 1 multiplexers) but the connections provide an even faster option for the place and route software to improve the speed of critical paths. No depopulation of the level 1 multiplexers is attempted to recoup the additional area cost of these connections since that would greatly harm routeability. 
       FIG. 4A  uses the convention from earlier drawing figures that busses drawn as wires “pass under” interconnects drawn as polygons (here the feedback interconnects  418  and  420 ) without connecting. An exemplary instance of each of the three cases is labeled “Does Not Connect” in the diagram. In general, this style of drawing figure is intended to emphasize the interrelationships between classes of components and interconnects without specific reference to any particular numbers or detailed circuit connections. 
     Persons of ordinary skill in the art will appreciate that the numbers of functional blocks, the number of level 1, level 2 and level 3 multiplexers, the number of data channels on each level 1, level 2 and level 3 multiplexer, the interconnections to the various level 1, level 2 and level 3 multiplexers are a matter of design choice. Such skilled persons will realize that other connections not shown in  FIG. 4A  or described in the specification will be present in many practical embodiments of the invention. Circuits such as carry chains, sequential element control signals and their routing multiplexers and interconnects, other functional block inputs (like bypass inputs allowing connection to the data input of the sequential element while avoiding the LUT4 and its level 1 multiplexers), etc., may be present and their presence or absence is in no way limiting of the present invention. 
       FIG. 4B  shows an I23F123 (pronounced “i-two-three-ef-one-two-three”) logic cluster  400 B of the present invention. Present in  FIG. 4B  are functional blocks  402 , level 1 multiplexers  404   a  through  404   h , level 2 multiplexers  408   a  through  408   h , level 2 interconnects  410 , level 3 multiplexers  412   a  through  412   j , EHVR  414 , functional block output bus  416 , and feedback interconnects  418 ,  420  and  422  previously discussed.  FIG. 4B  is identical to  FIG. 4A  except that level 1 interconnects  406  have been replaced by fast level 1 interconnects  424  and level 1 interconnects  426  and interconnections  428   a  and  428   b  have been added. 
     In order to further improve performance over the embodiment  400 A of  FIG. 4A , a group of level 2 multiplexers represented by multiplexers  408   g  and  408   h  (and labeled with an “F” for “fast”) are selected as fast level 2 multiplexers. The outputs of the fast level 2 multiplexers  408   g  and  408   h  are coupled to fast level 1 internal interconnects  424 , while the outputs of the remaining level 2 multiplexers  408   a  through  408   f  are coupled to level 1 interconnects  426 . Fast level 1 internal interconnects  424  are coupled only to data channels on level 1 multiplexers (represented by  404   g  and  404   h ) associated with the fastest (A) inputs to the LUT4s, while the remaining level 1 interconnects  426  are coupled only to data channels on the level 1 multiplexers associated with the B, C and D inputs. 
     Several factors contribute to the speed improvement of the fast level 1 internal interconnects  424 . First, since they are only coupled to level 1 multiplexers associated with the fastest (A) inputs to the LUT4s, they have an intrinsic speed advantage relative to signals entering the LUT4s through the B, C or D inputs. Second, since they only connect to one quarter of the total number of level 1 multiplexers (multiplexers  404   g  and  404   h  in  FIG. 4B ), there are substantially fewer data channels adding capacitance to the routing wires in the fast level 1 internal interconnects  424 . By comparison, the level 1 internal interconnects  426  couple to the level 1 multiplexers coupled to the B inputs, the C inputs and the D inputs to maintain routeability. Third, since the level 1 multiplexers associated with the fastest (A) inputs to the LUT4s have fewer data channels, their output capacitance on the routing wire to the LUT4 A inputs is reduced as well. Fourth, a LUT4 typically has at most one speed critical path routed through it, so creating a fast class of connection paths to the A inputs of the LUT4s greatly increases the overall performance of the architecture by giving the timing driven place and route tool (or tools) a fast option for the one signal that really needs to be fast in the large majority of instances. Fifth, in some embodiments the size of the transistors in the fast level 2 multiplexers can be increased, further improving the speed. Thus, the fast level 1 internal interconnects  424  are a fast class of level 1 internal interconnects due to their architectural context rather than the routing wires themselves being intrinsically faster than their counterparts the routing wires in the level 1 internal interconnects  426 . 
     All of the paths starting from the data channel inputs of the fast level 2 multiplexers  408   g  and  408   h  through the fast level 1 internal interconnects  424  and level 1 multiplexers  404   g  and  404   h  through the A inputs to the LUT4 outputs form a first group of select signal paths of a first speed class in  FIG. 4B . Similarly, all of the paths starting from the data channel inputs of the level 2 multiplexers  408   a  through  408   f  through the level 1 internal interconnects  426  and level 1 multiplexers  404   a  through  404   f  through the B, C and D inputs to the LUT4 outputs form a second group of select signal paths of a second speed class in  FIG. 4B . 
     A few external interconnects  428   a  and  428   b  that are coupled to data channels on level 3 multiplexers  412   a  through  412   j  are now additionally coupled to data channels on the fast level 2 multiplexers  408   g  and  408   h . These are not new external connections, rather they are additional points of connection for a few of the existing I3 external interconnects, effectively bypassing the level 3 multiplexers and creating an I23 topology. This allows the timing driven place and route tool (or tools) a faster way to get to the fastest speed class of select signal paths. 
     As shown in the embodiment  400 B of  FIG. 4B , there is no interaction between the fast level 1 internal interconnects  424  and the level 1 interconnects  426 . In some embodiments a degree of interconnection between fast level 1 internal interconnects  424  and the level 1 interconnects  426  is allowed as a matter of design choice. 
       FIG. 4B  uses the convention from earlier drawing figures that busses drawn as wires “pass under” interconnects drawn as polygons (here the feedback interconnects  418  and  420 ) without connecting. An exemplary instance of each of the three cases is labeled “Does Not Connect” in the diagram. In general, this style of drawing figure is intended to emphasize the interrelationships between classes of components and interconnects without specific reference to any particular numbers or detailed circuit connections. 
     Persons of ordinary skill in the art will appreciate that the numbers of functional blocks, the number of level 1, level 2 and level 3 multiplexers, the number of data channels on each level 1, level 2 and level 3 multiplexer, and the interconnections to the various level 1, level 2 and level 3 multiplexers are a matter of design choice. Such skilled persons will realize that other connections not shown in  FIG. 4B  or described in the specification will be present in many practical embodiments of the invention. Circuits such as carry chains, sequential element control signals and their routing multiplexers and interconnects, other functional block inputs (like bypass inputs allowing connection to the data input of the sequential element while avoiding the LUT4 and its level 1 multiplexers), etc., may be present and their presence or absence is in no way limiting of the present invention. 
       FIG. 4C  shows an I23F123 logic cluster  400 C of the present invention. Present in  FIG. 4C  are functional blocks  402 , level 1 multiplexers  404   a  through  404   h , level 2 multiplexers  408   a  through  408   h , level 2 internal interconnects  410 , level 3 multiplexers  412   a  through  412   j , EHVR  414 , functional block output bus  416 , feedback interconnects  418 ,  420  and  422 , and interconnects  428   a  and  428   b  previously discussed.  FIG. 4C  is identical to  FIG. 4B  except that level 1 internal interconnects  424  and  426  have been replaced by level 1 internal interconnects  430 ,  432 ,  434  and  436 . 
     In order to further improve the area with respect to the embodiment  400 B of  FIG. 4B , the level 2 multiplexers are combined into four groups each associated with one of the LUT4 input types. Thus in  FIG. 4C  level 2 multiplexers  408   g  and  408   h  represent the group associated with the A inputs and are labeled “A” in the figure, multiplexers  408   e  and  408   f  represent the group associated with the B inputs and are labeled “B” in the figure, multiplexers  408   c  and  408   d  represent the group associated with the C inputs and are labeled “C” in the figure, and multiplexers  408   a  and  408   b  represent the group associated with the D inputs and are labeled “D” in the figure. 
     Level 1 internal interconnects (speed class A)  430  are shown coupled between the outputs of the “A” level 2 multiplexers  408   g  and  408   h  and the data channels of the representative “A” level 1 multiplexers  404   g  and  404   h . Level 1 internal interconnects (speed class B)  432  are shown coupled between the outputs of the “B” level 2 multiplexers  408   e  and  408   f  and the data channels of the representative “B” level 1 multiplexers  404   e  and  404   f . Level 1 internal interconnects (speed class C)  434  are shown coupled between the outputs of the “C” level 2 multiplexers  408   c  and  408   d  and the data channels of the representative “C” level 1 multiplexers  404   c  and  404   d . Level 1 internal interconnects (speed class D)  436  are shown coupled between the outputs of the “D” level 2 multiplexers  408   a  and  408   b  and the data channels of the representative “D” level 1 multiplexers  404   a  and  404   b.    
     Effectively this creates four different speed classes (A, B, C and D) of level 1 internal interconnect and by extension four different speed classes of select signal paths starting at the inputs of the data channels of the level 2 multiplexers and terminating at the outputs of the LUT4 function generators. The area savings in embodiment  400 C of  FIG. 4C  is due to the need for fewer input channels in the speed class A, B, C and D level 1 multiplexers. Since the speed class A, B, C and D level 2 multiplexers only have to couple to data channels on speed class A, B, C and D level 1 multiplexers respectively, all of the speed classes get the same area and speed improvement that that fast speed class got in embodiment  400 B of  FIG. 4B . Although in embodiment  400 C the primary difference in the speed classes is created by the LUT4 propagation delays, this can be enhanced by both the circuit design of the multiplexers and the layout of the routing lines. For example, larger transistors could be used for all of the class A level 1 and level 2 multiplexers relative to the other classes to further enhance the fastest connections. 
     As shown in the embodiment  400 C of  FIG. 4C , there is no interaction between the level 1 internal interconnects  424  and the level 1 interconnects  426 . In some embodiments a degree of interconnection between the class A, B, C and D level 1 internal interconnects  430 ,  432 ,  434  and  436  is allowed as a matter of design choice. 
     To maintain a high level of routeability, the number of level 2 multiplexers in any of the speed classes A, B, C and D must be at least as large as the number of level 1 multiplexers in the same speed class. The number of level 1 multiplexers is equal to the number of inputs of that class type which in turn is equal to the number of LUT4s in the logic cluster. We will define the number of LUT4s as k. Thus k=n+1 as discussed in conjunction with  FIG. 4A . 
     Place and route experiments in 90 nm and 65 nm flash based FPGA configuration technology were used to determine the optimal value of k. It was determined that the preferred embodiment of the present invention is smaller in area to a comparable I2F1 architecture in the range 7&lt;=k&lt;=18 and to a comparable I1F1 architecture for k&lt;=14. For the range 8&lt;=k&lt;18, the preferred embodiment is faster than both a comparable I2F1 architecture and a comparable I1F1 architecture. Further experiments on the architecture of the present invention utilizing determined that the optimal range for both performance and area is 11&lt;=k&lt;=14. The preferred embodiment uses k=12 (instead of k=11, k=13 or k=14) because of its larger factorization and better divisibility with small numbers, the later in particular making the design of an optimal set of multiplexers easier. In a preferred embodiment, the optimal number of second level multiplexers of each speed class was also determined to be equal to k. 
       FIG. 4C  uses the convention from earlier drawing figures that busses drawn as wires “pass under” interconnects drawn as polygons (here the feedback interconnects  418  and  420 ) without connecting. An exemplary instance of each of the three cases is labeled “Does Not Connect” in the diagram. In general, this style of drawing figure is intended to emphasize the interrelationships between classes of components and interconnects without specific reference to any particular numbers or detailed circuit connections. 
     Persons of ordinary skill in the art will appreciate that the numbers of functional blocks, the number of level 1, level 2 and level 3 multiplexers, the number of data channels on each level 1, level 2 and level 3 multiplexer, and the interconnections to the various level 1, level 2 and level 3 multiplexers are a matter of design choice. While specific values of k were cited for a particular configuration technology at particular process nodes, the value of k may differ at other process nodes and with different configuration technologies used at different process nodes and as a result of different design choices and is not limited to the range 11&lt;=k&lt;=14. Such skilled persons will realize that other connections not shown in  FIG. 4C  or described in the specification will be present in many practical embodiments of the invention. Circuits such as carry chains, sequential element control signals and their routing multiplexers and interconnects, other functional block inputs (like bypass inputs allowing connection to the data input of the sequential element while avoiding the LUT4 and its level 1 multiplexers), etc., may be present and their presence or absence is in no way limiting of the present invention. 
       FIG. 4D  shows an I23F123 logic cluster  400 D of the present invention. Present in  FIG. 4D  are functional blocks  402 , level 1 multiplexers  404   a  through  404   h , level 2 multiplexers  408   a  through  408   h , level 3 multiplexers  412   a  through  412   j , EHVR  414 , functional block output bus  416 , feedback interconnects  418 ,  420  and  422 , interconnects  428   a  and  428   b , and level 1 internal interconnects  430 ,  432 ,  434  and  436  previously discussed.  FIG. 4D  is identical to  FIG. 4C  except for three changes. 
     In the first change, the level 3 multiplexers  412   a  through  412   j  have been divided into two representative groups. Group  1  comprises level 3 multiplexers  412   b ,  412   d ,  412   f ,  412   h  and  412   j  (labeled G 1  in the drawing figure) and group  2  comprises level 3 multiplexers  412   a ,  412   c ,  412   e ,  412   g  and  412   i . While two groups are shown in  FIG. 4D , more may be present as will be discussed below. 
     In the second change, the level 2 internal interconnects  410  have been replaced by level 2 internal interconnects (group  1 )  438  that are only coupled to the outputs of the group  1  level 3 multiplexers, and level 2 internal interconnects (group  2 )  440  that are only coupled to the outputs of group  2  level 3 multiplexers. There will be as many groups of level 2 internal interconnects as there are groups of level 3 multiplexers. 
     In the third change, the level 2 multiplexers are divided into groups corresponding to each of the level 3 multiplexer groups and each of the level 2 internal interconnect groups while still retaining their speed classification. Thus level 2 multiplexers  408   h ,  408   i ,  408   f ,  408   e ,  408   d ,  408   c ,  408   b  and  408   a  are now shown labeled A 1 , A 2 , B 1 , B 2 , C 1 , C 2 , D 1  and D 2  respectively. 
     The importance of the embodiment  400 D lies in the physical transformations that the logical organizing of the level 2 and level 3 multiplexers and the level 2 internal interconnects allow. These will be discussed in conjunction with  FIG. 4E  and  FIG. 4F . 
       FIG. 4D  uses the convention from earlier drawing figures that busses drawn as wires “pass under” interconnects drawn as polygons (here the feedback interconnects  418  and  420  and the level 2 internal interconnects  438  and  440 ) without connecting. An exemplary instance of each of the five cases is labeled “Does Not Connect” in the diagram. In general, this style of drawing figure is intended to emphasize the interrelationships between classes of components and interconnects without specific reference to any particular numbers or detailed circuit connections. 
     Persons of ordinary skill in the art will appreciate that the numbers of functional blocks, the number of level 1, level 2 and level 3 multiplexers, the number of data channels on each level 1, level 2 and level 3 multiplexer, and the interconnections to the various level 1, level 2 and level 3 multiplexers are a matter of design choice. Such skilled persons will realize that other connections not shown in  FIG. 4D  or described in the specification will be present in many practical embodiments of the invention. Circuits such as carry chains, sequential element control signals and their routing multiplexers and interconnects, other functional block inputs (like bypass inputs allowing connection to the data input of the sequential element while avoiding the LUT4 and its level 1 multiplexers), etc., may be present and their presence or absence is in no way limiting of the present invention. 
       FIG. 4E  shows an I23F123 logic cluster  400 E of the present invention. Present in  FIG. 4E  are functional blocks  402 , level 1 multiplexers  404   a  through  404   h , level 2 multiplexers  408   a  through  408   h , level 3 multiplexers  412   a  through  412   j , EHVR  414 , functional block output bus  416 , feedback interconnects  418 ,  420  and  422 , interconnects  428   a  and  428   b , level 1 internal interconnects  430 ,  432 ,  434  and  436 , and level 2 internal interconnects  438  and  440  previously discussed.  FIG. 4E  is functionally identical to  FIG. 4D  although the components have been reorganized. 
     In  FIG. 4E , the level 2 and level 3 multiplexers associated with each of the representative groups (group  1  and group  2 ) have been reorganized to be physically adjacent to the other components of the group. Thus level 2 multiplexers  408   a ,  408   c ,  408   e , and  408   g  (also labeled D 2 , C 2 , B 2  and A 2  respectively), level 2 internal interconnects (group  2 )  440 , level 3 multiplexers  412   a ,  412   c ,  412   e ,  412   g  and  412   i , (also labeled G 2 ) and interconnect  428   a  are all grouped together into a cross coupled routing unit (CCRU) indicated by the light dashed line labeled  442 . Similarly, level 2 multiplexers  408   b ,  408   d ,  408   f , and  408   h  (also labeled D 1 , C 1 , B 1  and A 1  respectively), level 2 internal interconnects (group  1 )  438 , level 3 multiplexers  412   b ,  412   d ,  412   f ,  412   g  and  412   j , (also labeled G 2 ) and interconnect  428   b  are all grouped together into a cross coupled routing unit (CCRU) indicated by the light dashed line labeled  444 . 
     In the drawing figure, there are no interconnections between CCRU  442  and CCRU  444  inside the logic cluster. This means that in a physical integrated circuit, each of the CCRUs  442  and  444  can have all their components laid out adjacent to one another in a small area. Unlike the organization of  FIG. 4D  where the level 2 interconnects  438  and  440  run the entire width of the logic cluster, in  FIG. 4E  the level 2 interconnects  438  and  440  are localized within CCRU  444  and  442  respectively. This greatly reduces the amount of capacitance on the level 2 internal interconnect routing wires resulting in a significant performance increase. 
     In  FIG. 4E , the level 1 internal interconnects  430 ,  432 ,  434  and  438  have also been redrawn to maintain the required connectivity. In this drawing figure they now appear to span the entire cluster. Persons of ordinary skill in the art will realize that this really was always the case. Functional blocks are constructed individually in a small area with the A, B, C and D inputs for each functional block physically close together. Representing the functional blocks  402  as a unit with inputs A 0  through A n  clustered together, inputs B 0  through B n  clustered together, inputs C 0  through C n  clustered together, and inputs D 0  through D i , grouped together was an abstraction made for clarity of understanding with respect to the grouping of level 1 internal interconnects into speed classes. The preferred physical organization will be discussed in conjunction with  FIG. 4F . 
       FIG. 4E  uses the convention from earlier drawing figures that individual wires as well as busses drawn as wires “pass under” interconnects drawn as polygons (here the feedback interconnects  418  and  420 , the level 1 internal interconnects  430 ,  432 ,  434  and  436 , and the level 2 internal interconnects  438  and  440 ) without connecting. The labeling of exemplary instances of each case has been removed in this and subsequent drawing figures to avoid cluttering them. In general, this style of drawing figure is intended to emphasize the interrelationships between classes of components and interconnects without specific reference to any particular numbers or detailed circuit connections. 
     Persons of ordinary skill in the art will appreciate that the numbers of functional blocks, the number of level 1, level 2 and level 3 multiplexers, the number of data channels on each level 1, level 2 and level 3 multiplexer, the interconnections to the various level 1, level 2 and level 3 multiplexers, and the number of CCRU blocks are a matter of design choice. Such skilled persons will realize that other connections not shown in  FIG. 4E  or described in the specification will be present in many practical embodiments of the invention. Circuits such as carry chains, sequential element control signals and their routing multiplexers and interconnects, other functional block inputs (like bypass inputs allowing connection to the data input of the sequential element while avoiding the LUT4 and its level 1 multiplexers), etc., may be present and their presence or absence is in no way limiting of the present invention. 
       FIG. 4F  shows an I23F123 logic cluster  400 F of the present invention. Present in  FIG. 4F  are level 1 multiplexers  404   a  through  404   h , level 2 multiplexers  408   a  through  408   h , level 3 multiplexers  412   a  through  412   j , EHVR  414 , functional block output bus  416 , feedback interconnects  418 ,  420  and  422 , interconnects  428   a  and  428   b , level 1 internal interconnects  430 ,  432 ,  434  and  436 , and level 2 internal interconnects  438  and  440  previously discussed.  FIG. 4F  is functionally identical to  FIG. 4D  and  FIG. 4E  although the components have been reorganized and the functional blocks  402  are now represented by two representative functional blocks  402   a  and  402   b.    
     In  FIG. 4F , two functional blocks  402   a  and  402   b  are shown, though any number may be present. Functional block 0 labeled  402   a  is shown with inputs A 0 , B 0 , C 0  and D 0  coupled to the outputs of level 1 multiplexers  404   h ,  404   f ,  404   d  and  404   b  respectively, and functional block n labeled  402   b  is shown with inputs A n , B n , C n  and D n  coupled to the outputs of level 1 multiplexers  404   g ,  404   e ,  404   c  and  404   a  respectively. 
     In logic cluster  400 F, the number of CCRU blocks is made equal to the number of functional blocks. Each of the CCRU blocks is grouped together with one of the functional blocks and its associated level 1 multiplexers into a single block called a slice. Two slices  446  and  448  are indicated by heavy dotted lines. Slice  446  comprises CCRU  442 , functional block  402   b , and level 1 multiplexers  404   a ,  404   c ,  404   e  and  404   g , and slice  448  comprises CCRU  444 , functional block  402   a , and level 1 multiplexers  404   b ,  404   d ,  404   f  and  404   h.    
     In a practical FPGA integrated circuit, a cluster comprises a number of physically adjacent slices abutted together and laid out in parallel. By making the CCRU and the functional block have the same width in one physical dimension, significant modularity can be achieved. For example, if place and route experiments or some other factor indicate a different number of functional blocks per cluster is desirable, the amount of rework required to change the cluster size is reduced. As discussed in conjunction with  FIG. 4D , the number of functional blocks (and thus the number of CCRU blocks) in a preferred embodiment is 12. 
       FIG. 4F  uses the convention from earlier drawing figures that individual wires as well as busses drawn as wires “pass under” interconnects drawn as polygons (here the feedback interconnects  418  and  420 , the level 1 internal interconnects  430 ,  432 ,  434  and  436 , and the level 2 internal interconnects  438  and  440 ) without connecting. The labeling of exemplary instances of each case has been removed in this drawing figure to avoid cluttering. In general, this style of drawing figure is intended to emphasize the interrelationships between classes of components and interconnects without specific reference to any particular numbers or detailed circuit connections. 
     Persons of ordinary skill in the art will appreciate that the numbers of functional blocks, the number of level 1, level 2 and level 3 multiplexers, the number of data channels on each level 1, level 2 and level 3 multiplexer, the interconnections to the various level 1, level 2 and level 3 multiplexers, the number of CCRU blocks, and the number of slices are a matter of design choice. Such skilled persons will realize that other connections not shown in  FIG. 4F  or described in the specification will be present in many practical embodiments of the invention. Circuits such as carry chains, sequential element control signals and their routing multiplexers and interconnects, other functional block inputs (like bypass inputs allowing connection to the data input of the sequential element while avoiding the LUT4 and its level 1 multiplexers), etc., may be present and their presence or absence is in no way limiting of the present invention. 
       FIG. 5  shows a preferred embodiment CCRU  500 . CCRU  500  is a superset of CCRU  442  of  FIGS. 4E and 4F  shown in more detail. In particular, CCRU  500  comprises additional circuitry for handling special global and high fan-out signals like sequential element control signals. Present in  FIG. 5  are previously discussed level 2 multiplexers  408   a ,  408   c ,  408   e  and  408   g , level 3 multiplexers  412   a ,  412   c ,  412   e ,  412   g  and  412   i , EHVR  414 , feedback interconnect wire  418   x  (discussed as part of feedback interconnects  418 ), feedback interconnect wire  420   y  (discussed as part of feedback interconnects  420 ), and interconnect  428   b . Also shown in  FIG. 5  are interconnect  428   c , global routing  502 , level 2 multiplexer  504 , level 2 internal interconnects  506 , connect points P 1  through P 20 , and connect points Q 1  through Q 4 . In a preferred embodiment, two interconnects  428   b  and  428   c  couple to data channels on the level 3 multiplexer  412   g  and to data channels on level 2 multiplexer  408   g.    
     In  FIG. 5 , the global routing  502  (not shown in any prior drawing figure) is broken out separately from the remainder of the FPGA routing architecture external to the cluster that is still contained in EHVR  414 . The global routing resources are for signals that fan out to a large number of destinations. Their scope can be limited to a small region comprising a few logic clusters or span the entire FPGA. They are often used to carry sequential element control signals such as clocks, sets, resets, enables, and loads since often in user designs many sequential elements are controlled in parallel by the same signals. 
     Thus in  FIG. 5 , level 3 multiplexers  412   a ,  412   c ,  412   e  and  412   g  route general purpose logic signals while level 3 multiplexer  412   i  routes the sequential element control signals. Connect points P 1  through P 20  and Q 1  through Q 4  define the connectivity of the level 2 internal interconnects  506 . Connect points are an abstraction of the reasonable hard wired connections between the outputs of level 3 multiplexers  412   a ,  412   c ,  412   e ,  412   g  and  412   i  and the various data channels on level 2 multiplexers  408   a ,  408   c ,  408   e ,  408   g  and  504 . If the connect point makes a connection between the intersecting lines, the connect point is said to be populated. If the connect point does not make a connection, the connect point is said to be depopulated. Thus when the CCRU is designed, the various connect points are either populated or depopulated as a matter of design choice. Once the population decisions have been made, the actual wires on the FPGA integrated circuit are then routed in a physically efficient manner that will not resemble the pattern of wires shown in  FIG. 5  except by coincidence. 
     Populating connect points is desirable, because it increases routeability. However, each populated connect point adds a data channel to the associated level 2 multiplexer in addition to adding capacitance the output of the associated level 3 multiplexer reducing speed which is undesirable. These factors need to be balanced in any architecture and many different population patterns can be utilized as a matter of design choice. 
     Connect points P 1  through P 16  (shown inside level 2 interconnects  440 ) determine the routeability of the general logic signals originating in EHVR  414  to the functional block inputs. Connect points P 17  through P 20  allow the global routing to connect to function block logic inputs if some are populated. This can be useful in applications where a high fan out signal needs to connect to combinational logic instead of to a sequential element. 
     Level 2 multiplexer  504  is used to route the global signal selected by level 3 multiplexer  412   i  to level 1 multiplexers (not shown in any of the figures) coupled to the functional block inputs used for the sequential element control signals (not shown in any of the figures). The output of level 3 multiplexer  412   i  is always coupled to a data channel of level 2 multiplexer  504 . 
     Connect points Q 1  through Q 4  (shown inside level 2 interconnects  506 ) determine the routeability of the general logic signals originating in EHVR  414  to the sequential element control inputs of the functional block inputs if some are populated. This can be useful in applications where a subsystem uses a small number of sequential elements and it is more economical to drive the sequential element control signals using general logic signals instead of one of the resources in global routing  502 . 
     In a currently preferred embodiment, the population of the connect points varies slightly from one CCRU block to another. On average 14 of the connect points P 1  through P 16  are populated with the number ranging from 13 to 15 depending on the individual CCRU. Also, 2 of the 4 connect points P 17  through P 20  are populated, though the two connections will vary from one CCRU to another. Similarly, 2 of the 4 connect points Q 1  through Q 4  are populated, though once again the two connections will vary from one CCRU to another. In one exemplary CCRU in a currently preferred embodiment of the present invention, the connect points P 2 , P 3 , P 4 , P 5 , P 6 , P 7 , P 8 , P 9 , P 10 , P 11 , P 13 , P 14 , P 15 , P 16 , P 19 , P 20 , Q 1  and Q 2  are populated while the connect points P 1 , P 12 , P 17 , P 18 , Q 3  and Q 4  are depopulated. Persons of ordinary skill in the art will realize that many other CCRU circuit topologies with different numbers of level 2 multiplexers, level 2 internal interconnects, level 3 multiplexers, and connect points are possible. Such skilled persons will appreciate that many different population patterns of connect points are possible in any specific topology as a matter of design choice. 
     While embodiments and applications of this invention have been shown and described, it would be apparent to those of ordinary skill in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.