Abstract:
A bus structure providing pipelined busing of data between logic circuits and special-purpose circuits of an integrated circuit, the bus structure including a network of pipelined conductors, and connectors selectively joining the pipelined conductors between the special-purpose circuits, other pipelined connectors, and the logic circuits.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to a network for efficient communication within a digital system and, in particular, to a multi-stationed grid of stations and interconnecting buses providing a high-speed pipelined and configurable communication network for a field-programmable gate array. 
         [0003]    2. History of the Prior Art 
         [0004]    Digital systems can be implemented using off-the-shelf integrated circuits. However, system designers can often reduce cost, increase performance, or add capabilities by employing in the system some integrated circuits whose logic functions can be customized. Two common kinds of customizable integrated circuits in digital systems are application-specific integrated circuits (ASICs) and field-programmable gate arrays (FPGAs). 
         [0005]    ASICs are designed and manufactured for a particular application. An ASIC includes circuits selected from a library of small logic cells. A typical ASIC also includes large special-purpose blocks that implement widely-used functions, such as a multi-kilobit random-access memory (RAM) or a microprocessor. The logic cells and special-function blocks are placed at suitable locations on the ASIC and connected by means of wiring. 
         [0006]    Application-specific integrated circuits (ASICs) have several advantages. Because an ASIC contains only the circuits required for the application, it has a small die size. An ASIC also has low power consumption and high performance. 
         [0007]    ASICs have some disadvantages. It takes a lot of time and money to design ASICs because the design process is complex. Creating prototypes for an ASIC is complex as well, so prototyping also takes a lot of time and money. 
         [0008]    Field-programmable gate arrays (FPGAs) are another kind of customizable integrated circuit that is common in digital systems. An FPGA is a general-purpose device. It is meant to be configured for a particular application by the system designer. 
         [0009]      FIG. 21  provides a schematic diagram of a portion of a conventional FPGA. The FPGA includes a plurality of general-purpose configurable logic blocks, a plurality of configurable special-purpose blocks, and a plurality of routing crossbars. In an example, each logic block, such as logic block  101 , may include a plurality of four-input lookup tables (LUTs) and a plurality of configurable one-bit sequential cells, each of which can be configured as a flip-flop or a latch. A configurable special-purpose block, such as special-purpose blocks  151  and  155 , implements a widely-used function. An FPGA may have more than one type of special-purpose block. 
         [0010]    The routing crossbars form a two-dimensional routing network that provides configurable connections among the logic blocks and the special-purpose blocks. In the illustrative FPGA, each routing crossbar is connected to the nearest-neighbor routing crossbars in four directions and to either a logic block or a special-purpose block. For example, routing crossbars  125  and  100  are connected by buses  104 . In the example FPGA, each logic block, such as logic block  101 , is connected to one routing crossbar, such as routing crossbar  100 . Special-purpose blocks are typically much larger than logic blocks and typically have more input and output signals, so a special-purpose block, such as special-purpose block  151 , may be connected by a plurality of buses to a plurality of routing crossbars, such as routing crossbars  130 - 133 . 
         [0011]    The logic blocks, special-purpose blocks, and routing crossbars contain circuitry (called configuration memory) which allows their operation to be configured. A user&#39;s design is implemented in the FPGA by setting the configuration memory appropriately. Several forms of configuration memory are used by contemporary FPGAs, the most common form being static random-access memory. Configuring an FPGA places it in a condition to perform a specific one of many possible applications. 
         [0012]    Field-programmable gate arrays (FPGAs) have advantages over application-specific integrated circuits (ASICs). Prototyping an FPGA is a relatively fast and inexpensive process. Also, it takes less time and money to implement a design in an FPGA than to design an ASIC because the FPGA design process has fewer steps. 
         [0013]    FPGAs have some disadvantages, the most important being die area. Logic blocks use more area than the equivalent ASIC logic cells, and the switches and configuration memory in routing crossbars use far more area than the equivalent wiring of an ASIC. FPGAs also have higher power consumption and lower performance than ASICs. 
         [0014]    The user of an FPGA may improve its performance by means of a technique known as pipelining. The operating frequency of a digital design is limited, in part, by the number of levels of look-up tables that data must pass through between one set of sequential cells and the next. The user can partition a set of look-up tables into a pipeline of stages by using additional sets of sequential cells. This technique may reduce the number of levels of look-up tables between sets of sequential cells and, therefore, may allow a higher operating frequency. However, pipelining does not improve the performance of FPGAs relative to that of ASICs, because the designer of an ASIC can also use the pipelining technique. 
         [0015]    It would be desirable to provide circuitry which allows the configurability, low time and cost of design, and low time and cost of prototyping typical of an FPGA while maintaining the high performance, low die area, and low power expenditure of an ASIC. Specialized special-purpose blocks might help the integrated circuit resemble an ASIC by having relatively high performance and relatively low die area. The integrated circuit might retain most of the benefits of an FPGA in being relatively configurable and in needing low time and cost for design and low time and cost for prototyping. 
         [0016]    However, a conventional FPGA routing crossbar network cannot accommodate the high data bandwidth of the special-purpose blocks in such an integrated circuit. The operating frequency of signals routed through a routing crossbar network is relatively low. A user may employ pipeline registers to increase the frequency somewhat, but doing so consumes register resources in the logic blocks. Building an FPGA with a much greater number of routing crossbars than usual would increase the data bandwidth, but it is impractical because routing crossbars use a large area. 
       SUMMARY OF THE INVENTION 
       [0017]    It is an object of the present invention to provide area-efficient routing circuitry capable of transferring data at high bandwidth to realize the high performance potential of a hybrid FPGA having special-purpose blocks thereby combining the benefits of FPGAs and ASICs. 
         [0018]    The present invention is realized by a bus structure providing pipelined busing of data between logic circuits and special-purpose circuits of an integrated circuit, the bus structure including a network of pipelined conductors, and connectors selectively joining the pipelined conductors between the special-purpose circuits, other connectors, and the logic circuits. 
         [0019]    These and other objects and features of the invention will be better understood by reference to the detailed description which follows taken together with the drawings in which like elements are referred to by like designations throughout the several views. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  illustrates the relationship of stations in the inventive network to a routing crossbar network and to special-purpose blocks; 
           [0021]      FIG. 2  illustrates a connection routed through stations in the inventive network; 
           [0022]      FIG. 3  shows a network-oriented view of a station; 
           [0023]      FIG. 4  is a block diagram of a station; 
           [0024]      FIG. 5  is a simplified schematic diagram of a connection through the inventive network that has multiple destinations; 
           [0025]      FIG. 6  shows input and output connections for one input port and one output port; 
           [0026]      FIG. 7  is a block diagram of the input port logic of a station; 
           [0027]      FIG. 8  shows data zeroing logic for one input port; 
           [0028]      FIG. 9  shows parity generation and checking logic for one input port; 
           [0029]      FIG. 10  shows byte shuffling logic for input ports of a station; 
           [0030]      FIG. 11  is a schematic diagram of the effective behavior of the latency padding logic for one input port; 
           [0031]      FIG. 12  summarizes the preferred embodiment of the latency padding logic for one input port; 
           [0032]      FIG. 13  shows serializing logic for one input port; 
           [0033]      FIG. 14  shows a station&#39;s network switch; 
           [0034]      FIG. 15  shows a routing multiplexer for an output link of network switch; 
           [0035]      FIG. 16  is a block diagram of output port logic of a station; 
           [0036]      FIG. 17  shows deserializing logic for one output port; 
           [0037]      FIG. 18  is a schematic diagram of the effective behavior of the latency padding logic for one output port; 
           [0038]      FIG. 19  shows byte shuffling logic for output ports of a station; 
           [0039]      FIG. 20  shows parity generation and checking logic for one output port; and 
           [0040]      FIG. 21  shows a schematic diagram of a portion of a conventional field-programmable gate array (FPGA). 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    This description applies to an embodiment of the present invention in a field-programmable gate array (FPGA). However, most aspects of the invention can also be embodied in other kinds of integrated circuit, such as an integrated circuit that consists of numerous digital signal processors. 
         [0042]    The preferred embodiment uses static RAM cells for the FPGA configuration memory. However, most aspects of the invention can also be embodied in an FPGA with other kinds of configuration memory, such as fuses, antifuses, or flash memory. 
         [0043]    The present invention is a cross-connection network for data (DCC network). A DCC network consists of a grid of stations that spans the entire field-programmable gate array (FPGA). A DCC network has several key advantages over traditional FPGA routing networks. The combination of features enables many applications in the context of a field-programmable integrated circuit. 
         [0044]    One advantage of the inventive network is that user data is serialized and then pipelined across the chip. In the preferred embodiment the pipeline frequency can be as high as two GHz, which is difficult to achieve in an ASIC and impossible to achieve in an FPGA. The high frequency provides a performance advantage. 
         [0045]    Another advantage is that the pipeline registers are built into the stations. They do not consume register resources in the logic blocks, which provides an area advantage over FPGAs. 
         [0046]    A third advantage is that the routing multiplexers in the network switches of the inventive network are configured on a granularity coarser than a single bit. This greatly reduces the number of configuration memory bits and multiplexer ports compared to an FPGA routing network, so it saves a great deal of die area. 
         [0047]    These three advantages provide enough on-chip bandwidth for high-speed special-purpose blocks to communicate with each other, while using much less die area than an FPGA to provide equivalent bandwidth. 
         [0048]    Organization of the Inventive Network: The inventive network consists of a grid of stations that spans the entire field-programmable gate array (FPGA). The two-dimensional network formed by the stations is like a plane that is parallel to the two-dimensional routing crossbar network. These two parallel planes are analogous to the roadways in a city, where the network of freeways is parallel to the network of surface streets. 
         [0049]      FIG. 1  shows the relationship of stations to the routing crossbar network and to special-purpose blocks in one embodiment of the invention. The repeating unit in the routing crossbar network is a four-by-four array of routing crossbars, each with a logic block attached, plus an extra vertical set of four routing crossbars (such as routing crossbars  130 - 133 ). The four extra routing crossbars connect the four-by-four segment of the routing crossbar network to the next group of four-by-four routing crossbars. The repeating unit in the inventive network is the station. Each station has direct connections to the nearest station above it, below it, and to the left and right of it. For example, station  152  is connected to the neighboring station  150  above it by buses  153 . (Note that there are horizontal connections between stations, but  FIG. 1  does not show them.) Typically, each station is connected to one repeating unit of the routing crossbar network. The station is connected to the four extra routing crossbars at the routing crossbar ports which could otherwise be connected to logic blocks. For example, station  150  is connected to routing crossbar  133  by buses  154 . Typically, each station is also connected to a special-purpose block. For example, station  150  is connected to special-purpose block  151  by buses. Multiplexers in the station give the special-purpose block access to the routing crossbar network as well as to the inventive network. 
         [0050]    Computer-aided design (CAD) software routes a path through the inventive network by configuring switches in the stations. This is similar to the process of routing a signal through an FPGA routing network, such as the routing crossbar network. Unlike an FPGA network, the inventive network provides one stage of pipeline register at each station, which allows the data to flow at a very high rate. 
         [0051]      FIG. 2  illustrates a connection routed through a series of stations  210 - 215  in the inventive network. User module  200  is implemented with logic blocks. User module  200  sends data into the inventive network through routing crossbar-to-station bus  201 . In this example, the user module sends eighty-bit-wide data at two hundred MHz. Input-port logic in station  210  serializes the data to be ten bits wide at one thousand, six hundred MHz. Data travels from station to station over ten-bit buses  230 - 234  at one thousand, six hundred MHz, with one pipeline register at each station. At the destination station  215 , output-port logic deserializes the data to be forty bits wide and presents it to special-purpose block  221  on bus  220  at four hundred MHz. 
         [0052]    Overview of a Station in the Inventive Network:  FIG. 3  shows a network-oriented view of a station in the inventive network. It contains four twenty-bit input ports  300 , input port logic  301  for processing input data, network switch  302  for passing data from station to station, output port logic  303  for processing output data, and four twenty-bit output ports  304 . The station&#39;s external connections consist of sixteen five-bit output links  310 - 313  to neighboring stations, and sixteen five-bit input links  320 - 323  from neighboring stations, many input connections  330  from and output connections  331  to routing crossbars and a special-purpose block, and a small number of clock inputs  332 . Some of the clocks operate at the frequencies of user logic and some operate at the faster internal frequencies of the inventive network. 
         [0053]      FIG. 4  is a block diagram of a station. A station includes input and output multiplexers  400 , five layers of input port logic  410 - 414 , a network switch  420 , and four layers of output port logic  431 - 434 . 
         [0054]    The input and output multiplexers  400  give a special-purpose block  401  access to the routing crossbar network through four routing crossbars  402 . The input and output multiplexers  400  connect both the special-purpose block  401  and the routing crossbars  402  to the input ports  415  and output ports  435  of the station. Each station has four twenty-bit input ports  415  and four twenty-bit output ports  435 . 
         [0055]    The input port logic  410 - 414  performs a series of functions: data zeroing, parity generation and checking, byte shuffling, latency padding, and serialization. 
         [0056]    The data-zeroing logic  410  can dynamically or statically zero out five-bit portions of the twenty-bit user bus. This feature helps implement multiplexers in the inventive network and also allows the use of five, ten, or fifteen bits of the input port instead of all twenty bits. 
         [0057]    The parity logic  411  can generate parity over nineteen bits or over two groups of nine bits, and it can check parity over all twenty bits or over two groups of ten bits. Output ports have similar parity logic  431 , so parity can be generated or checked at both input ports and output ports. By default, each twenty-bit input port will be serialized onto one five-bit bundle in the inventive network. This implies a default frequency ratio of 4:1 between the internal clock of the inventive network and the user port clock. When the user requires a 2:1 ratio, the byte-shuffling logic  412  can steer twenty bits of data from one user port toward two internal bundles. 
         [0058]    The latency padding logic  413  can add up to fourteen user clock cycles of latency to an input port, and output ports have similar latency padding logic  433 . CAD software uses this logic to pad the end-to-end latency through the inventive network to equal the value specified by the user, largely independent of the number of stations that the data has to pass through. 
         [0059]    The last layer in the input port logic is the serializers  414 , which serialize each twenty-bit input port at the user clock rate onto a five-bit internal bundle. In the preferred embodiment, internal bundles can be clocked at up to two GHz. 
         [0060]    In  FIG. 4 , the network switch  420  is a partially populated crossbar switch. It routes five-bit bundles  421  from the four input ports to the sixteen station-to-station output links  422 , from the sixteen station-to-station input links  423  to the sixteen station-to-station output links  422 , and from the sixteen station-to-station input links  423  to the five-bit bundles  424  that feed the four output ports. (The sixteen station-to-station output links  422  correspond to elements  310 - 313  in  FIG. 3 , and the sixteen station-to-station input links  423  correspond to elements  320 - 323  in  FIG. 3 .) There is a multi-port OR gate at the root of each routing multiplexer in the switch. If a multiplexer is configured to allow more than one bundle into the OR gate, then the data-zeroing logic at the input ports determines which input bus is allowed through the OR gate. This lets the inventive network perform cycle-by-cycle selection for applications such as high-bandwidth multiplexers, user crossbar switches, and time-slicing a connection through the inventive network. In  FIG. 4 , the output port logic  431 - 434  performs a series of functions that reverse the functions of the input port. The deserializer  434  distributes a five-bit internal bundle onto a twenty-bit output port at the user clock rate. The latency padding logic  433  can add up to fourteen user clock cycles of latency. Byte-shuffling logic  432  can steer data from one internal bundle toward two user output ports, which is often used with a 2:1 clock ratio. The parity logic  431  can generate parity over nineteen bits or two groups of nine bits, and it can check parity over twenty bits or two groups of ten bits. There is no data-zeroing logic in an output port. 
         [0061]    Creating a Connection through the Inventive Network: To create a connection through the inventive network between two pieces of logic, the user selects logic models from a library provided by the manufacturer of the integrated circuit. CAD software converts the models to physical stations in the inventive network and routes a path through the inventive network. Beginpoint and endpoint models can be provided that have user bus widths in every multiple of five bits from five to eighty. 
         [0062]      FIG. 5  is a simplified schematic diagram of a connection through the inventive network that has more than one destination. In this example, user module  520  is implemented with logic blocks. The user sends the output of module  520  to two destinations, parser ring  522  for header parsing and dual-port random-access memory (RAM)  524  for packet buffering. User module  520  in this example produces eighty-bit data  521  at two hundred MHz, and parser ring  522  and dual-port RAM  524  consume forty-bit data  505  and  507 , respectively, at four hundred MHz. The data travels over the inventive network as two five-bit bundles at one thousand, six hundred MHz. The frequency ratio of internal clock  512  to user clock is 8:1 at the input to the network (signal  514 ) and 4:1 at the output from the network (signal  513 ). 
         [0063]    The output bus  521  of user module  520  is connected to beginpoint module  500 , which is chosen from a library of logic models for the cross-connection network for data (DCC network). A beginpoint module is a logic model for input ports of a station. The user input port is eighty bits wide and the clock division ratio is 8:1, so a beginpoint module is used that has an eighty-bit user input port and that serializes data at an 8:1 ratio. CAD software will route the user&#39;s eighty-bit bus through routing crossbars to all four input ports of a station and configure the station to steer the user&#39;s data onto two five-bit internal bundles. 
         [0064]    The output  501  of beginpoint module  500  is connected to latency module  502 . A latency module is a logic model for the end-to-end latency of a connection through the inventive network. This example uses a latency module whose input and output ports are both ten bits wide. The user sets a parameter on latency module  502  to tell software the desired end-to-end latency of the connection. After the design is placed and routed, software can pad out the latency at the input and output ports if the routed delay through the sequence of physical stations is less than the user-specified latency. 
         [0065]    Output  503  of latency module  502  is connected to endpoint modules  504  and  506 , one for each of the two destinations. An endpoint module is a logic model for output ports of a station. This example uses endpoint modules that have a forty-bit user output port and that deserialize data at a 4:1 ratio, because the user output ports  505  and  507  are forty bits wide and the clock division ratio is 4:1. At each destination station, software will steer the data from two five-bit internal bundles to two of the four output ports of the station, and from there directly to the special-purpose block ( 522  or  524 ). 
         [0066]    The field-programmable gate array (FPGA) containing the inventive network has a clock distribution network with built-in clock dividers. In the proposed embodiment, the dividers can create any integer clock ratio from 1:1 to 16:1. For a connection through the inventive network, the internal clock is typically at a 1:1 ratio to the root of a clock tree. The user clocks are divided down from the same root. The clock distribution network ensures that any clocks divided down from the same root are aligned and have low skew. This guarantees synchronous interfacing between the user clock domain and the internal clock domain. In the example of  FIG. 5 , the root  511  of the clock tree operates at one thousand, six hundred MHz. The clock tree divides down root  511  by a 1:1 ratio to produce internal clock  512  at one thousand, six hundred MHz. The clock tree divides down root  511  by 4:1 and 8:1 ratios to produce user clocks  513  and  514 , respectively, at four hundred MHz and two hundred MHz, respectively. 
         [0067]    Different connections in the inventive network can use different clock trees. For example, a design can use a one thousand, six hundred MHz root clock for some connections and a one thousand, two hundred fifty MHz root clock for others. 
         [0068]    After placement and routing the user&#39;s data will travel through a sequence of stations, but those stations do not appear in the user&#39;s netlist. The actual latency through the inventive network is simulated by the begin, latency, and end modules that the user selects, such as modules  500 ,  502 ,  504 , and  506  in  FIG. 5 . This is similar to the routing of a signal through the routing crossbar network; back-annotation represents the delay of the routed signal, but the routing switches do not appear in the user&#39;s netlist. 
         [0069]    Uses of the Inventive Network: The hardware characteristics of the inventive network make various uses possible. 
         [0070]    The simplest use of the inventive network is a point-to-point connection between two pieces of user logic having the same bus width and clock frequency. For example, suppose that the integrated circuit includes a special-purpose block that performs the media access control (MAC) function for a ten Gbps Ethernet connection, and a ring of special-purpose blocks that can be programmed to perform simple parsing of Ethernet frames. Suppose further that the output bus from the MAC block for received frames is forty bits wide (including data and tag bits) and has a clock frequency of three hundred fifty MHz. Suppose further that the input bus to the parser ring also is forty bits wide and also clocks at three hundred fifty MHz. In this example, the user can send data from the media access control (MAC) block to the parser ring over the inventive network by using an internal clock frequency in the network of one thousand, four hundred MHz. MAC data enters the inventive network through two twenty-bit input ports near the MAC block. The input data is serialized at a 4:1 ratio onto two five-bit internal bundles. The ten-bit-wide internal data travels a configured path through a series of stations in the inventive network at one thousand, four hundred MHz. At two output ports of a station near the parser ring, the data is deserialized at a 4:1 ratio onto two twenty-bit buses and presented to the parser ring at three hundred fifty MHz. 
         [0071]    Another use of the inventive network is a point-to-point connection between two pieces of user logic that have the same data rate but different bus widths and clock frequencies. This bandwidth-matching is made possible by the independently configurable serializer and deserializer ratios in the input port and output port, respectively. For example, consider the schematic diagram in  FIG. 5 . User module  520  sends eighty-bit data at two hundred MHz into beginpoint module  500 , which is a logical representation of four twenty-bit input ports. The input data is serialized at an 8:1 ratio onto two five-bit internal bundles. The ten-bit-wide internal data travels a configured path through a series of stations at one thousand, six hundred MHz. At endpoint module  506 , which is a logical representation of two twenty-bit output ports, the output data is deserialized at a 4:1 ratio onto two twenty-bit buses and presented to dual-port RAM  524  at four hundred MHz. The data rate is sixteen thousand Mbps throughout the path: eighty bits times two hundred MHz leaving the user module, ten bits times one thousand, six hundred MHz inside the inventive network, and forty bits times four hundred MHz entering the dual-port RAM. 
         [0072]    The inventive network can fan out data from one source to multiple destinations. Network switch  420 , shown in  FIG. 4 , makes this possible. A data bundle can enter the switch through one of the input links  423  or one of the input ports  421 . The network switch can send the bundle to more than one output bundle among output links  422  and output ports  424 .  FIG. 5  illustrates a connection with multiple destinations. In this example, the user sends data from user module  520  to two destinations, parser ring  522  and dual-port RAM  524 . 
         [0073]    As well as transporting data at a high bandwidth, a connection through the inventive network can implement a high-bandwidth user multiplexer. This function relies on two features of the hardware. The first feature is the data zeroing logic  410  in an input port of a station (see  FIG. 4 ). An input port can be configured to allow a user input signal to zero out the port&#39;s twenty-bit bus on a cycle-by-cycle basis. The second feature is that the routing multiplexers in a network switch can OR together two or more five-bit bundles of data. As shown in  FIG. 15 , a routing multiplexer consists of multiple AND gates that feed into an OR gate. Configuration memory bits can enable two or more of the AND gates in the multiplexer, which causes two or more input bundles to be ORed together onto an output bundle. To implement a high-bandwidth user multiplexer, computer-aided design (CAD) software routes bundles corresponding to two or more user multiplexer input buses to a routing multiplexer in the network switch of some station. Within that network switch, CAD software enables the AND gates that correspond to all of those bundles, thereby ORing the bundles together. The user connects their multiplexer input buses to separate input ports and provides a control signal to each port to function as the select signals for the user multiplexer. 
         [0074]    A user can combine fanout and high-bandwidth multiplexing in one connection through the inventive network. That is, a connection can have multiple user input buses, with each bus enabled cycle-by-cycle by a separate control signal. The connection can OR the user data together, thereby forming a high-bandwidth user multiplexer. The output data of the user multiplexer can be fanned out to multiple user output destination buses. Multiple such connections can be used to implement a non-blocking user crossbar, in which multiple user output buses can independently receive data from a cycle-by-cycle choice of multiple input buses. 
         [0075]    A connection through the inventive network can time-slice data from two or more input ports onto one internal bundle. This function can be used to time-division-multiplex two or more user buses, each of which does not need the full bandwidth of a bundle, onto one bundle. This function can also be used to concatenate two or more user buses that originate at widely separated locations on the integrated circuit. This function relies on the data zeroing logic, the serializer and deserializer, and the ORing function of the network switch. For example, suppose that the user wishes to time-slice two ten-bit user buses A and B onto one five-bit internal bundle. The user connects ten-bit buses A and B to separate input ports of the inventive network and connects an output port to twenty-bit user bus C. The user connects bus A[9:0] to bits [9:0] of its input port, and bits [19:10] of the port are forced to 0 by configuration memory. ( FIG. 8  shows the configuration memory bits in the data zeroing logic that perform this function.) The user connects bus B[9:0] to bits [19:10] of its input port, and bits [9:0] of the port are forced to 0 by configuration bits. The serializers in both input ports are configured to serialize at a frequency ratio of 4:1. For each user clock cycle, the sequence of five-bit nybbles on the output of bus A&#39;s serializer is A[4:0], A[9:5], 0, 0, and the sequence of nybbles on the output of bus B&#39;s serializer is 0, 0, B[4:0], B[9:5]. CAD software routes the output bundles of the two serializers to a network switch in some station of the inventive network, where it ORs them together. The sequence of nybbles on the ORed-together bundle is therefore A[4:0], A[9:5], B[4:0], B[9:5]. The combined bundle is routed to an output port and deserialized at 4:1. Twenty-bit output bus C displays B[9:0] concatenated with A[9:0] on every cycle. 
         [0076]    The output of a connection through the inventive network can be used in a time-sliced fashion as well. In the example described in the preceding paragraph, the combined bundle can be routed to two output ports of the network. At one output port, the user can ignore bits [19:10] of the port and receive bus A from bits [9:0]. At the other output port, the user can ignore bits [9:0] of the port and receive bus B from bits [19:10]. 
         [0077]    CAD software can implement fixed, user-specified end-to-end latency in a connection through the inventive network, largely independent of the number of stations that the data passes through. For example, when the user sends a data bus through the inventive network while sending control signals through the routing crossbar network, it may be important to have the same number of cycles of latency along both paths. This function uses the latency padding logic in input ports and output ports of the inventive network. When defining a connection through the inventive network, the user sets a parameter on the latency module (such as latency module  502  in  FIG. 5 ), to tell CAD software the desired end-to-end latency. After the design is placed and routed, CAD software can pad out the latency at the input and output ports if the routed delay through the sequence of physical stations is less than the user-specified latency. 
         [0078]    The inventive network can detect single-bit errors in user logic or in a connection through the inventive network, thanks to the parity generation and checking logic found in both input ports and output ports. To detect parity errors in user logic, such as a RAM special-purpose block, the user can provide input data to the RAM from an output port of the inventive network that has parity generation enabled. If the output data from the RAM goes to an input port that has parity checking enabled, then the input port detects any single-bit errors that occurred on the data while it was stored in the RAM. To detect single-bit errors that occur while data is traveling through the inventive network, the user can enable parity generation in the connection&#39;s input port and parity checking in the connection&#39;s output port. 
         [0079]    Further Details of the Input and Output Connections: Stations in the inventive network connect the routing crossbar network to the inventive network and connect both of them to special-purpose blocks. As  FIG. 1  shows, each station, such as station  150 , is attached to four routing crossbars, such as routing crossbars  130 - 133 , which are part of the routing crossbar network. A special-purpose block, such as special-purpose block  151 , gets access to those routing crossbars through the input and output connections of the station. 
         [0080]    A station has four twenty-bit input ports and four twenty-bit output ports. Each pair of ports, consisting of one input port and one output port, has its own set of input and output connections. The connections for one pair of ports are completely independent of the other pairs.  FIG. 6  shows the input and output connections for one pair of ports. There are three types of connections: input multiplexers that drive the input port, output multiplexers that drive the routing crossbar and the special-purpose block, and feedthrough connections between the routing crossbar and the special-purpose block. All of the multiplexers are controlled by configuration memory. 
         [0081]    Input multiplexers  610  and  615  drive the first layer of the station&#39;s input port, which is the data zeroing logic  600 . The twenty-bit, two-port multiplexer  610  and the one-bit, two-port multiplexer  615  select the User Data Input (UDI) bus  620  and the Valid Input (VI) control signal  625 , respectively, from either routing crossbar  602  or special-purpose block  603 . Both multiplexers are controlled by the same configuration memory bit  630 , so either UDI and VI both come from the routing crossbar or both come from the special-purpose block. Not all special-purpose blocks have a dedicated output signal  663  to indicate that the twenty-bit data word is valid. For information on the Valid Input (VI) signal, see the description under subsection “Further Details of the Input Port Logic.” 
         [0082]    The twenty-bit, two-port output multiplexer  612  drives routing crossbar  602 , and the twenty-bit, two-port output multiplexer  613  drives special-purpose block  603 . These multiplexers are controlled by independent configuration memory bits  632  and  633 , respectively. The last layer of the station&#39;s output port, which is the parity generation and checking logic  601 , drives the User Data Output (UDO) bus  621 . UDO fans out to both output multiplexers. The output multiplexer  612  that drives routing crossbar  602  selects between UDO  621  and the same twenty-bit bus  643  from the special-purpose block that drives input multiplexer  610 . Similarly, the output multiplexer  613  that drives special-purpose block  603  selects between User Data Output (UDO)  621  and the same twenty-bit bus  642  from the routing crossbar that drives input multiplexer  610 . 
         [0083]    In addition to the multiplexers, there are feedthrough signals  652  from the routing crossbar  602  to the special-purpose block  603  and feedthrough signals  653  from the special-purpose block to the routing crossbar. None of the feedthrough signals has a connection to the input or output port of the station. Therefore, although all bits of the routing crossbar&#39;s outputs (except for signal  662  to the Valid Input (VI) input multiplexer  615 ) have some path to the special-purpose block, only twenty bits have a path to the input port. Similarly, all bits of the special-purpose block&#39;s outputs (except for Valid Output (VO) signal  663  to the VI input multiplexer  615 ) have some path to the routing crossbar, but only twenty bits have a path to the input port. 
         [0084]    Note that the input and output multiplexers operate on twenty bits as a unit. For example, there is no way to select the high ten bits of the input port from the routing crossbar and the low ten bits from the special-purpose block. 
         [0085]    A station is connected to four routing crossbars and therefore has four copies of the input and output connections that are shown in  FIG. 6 . A typical special-purpose block, such as a dual-port RAM, is connected to one station, which in turn connects it to four routing crossbars. 
         [0086]    Further Details of the Input Port Logic: The input port logic of each station is depicted by elements  410 - 414  in  FIG. 4 . More detail is provided by  FIG. 7 , which is a block diagram of the input port logic. Each group of buses  415  and  720 - 723  consists of four buses. Each of the buses is twenty bits wide and clocked by a user clock. Buses  724  consist of four buses; each of the buses, also referred to herein as bundles, is five bits wide and clocked by an internal clock of the inventive network. 
         [0087]    Input multiplexers  700  drive the four twenty-bit input buses  415 . Buses  415  drive data zeroing logic  410 , which consists of four data zeroing units  710   a - 710   d , one for each port. Data zeroing units  710   a - 710   d  drive the four twenty-bit buses  720 . Buses  720  drive parity generation and checking logic  411 , which consists of four parity generation and checking units  711   a - 711   d , one for each port. Parity units  711   a - 711   d  drive the four twenty-bit buses  721 . Buses  721  drive byte shuffling logic  412 , which can steer data from one port to another port. Byte shuffling logic  412  drives the four twenty-bit buses  722 . Buses  722  drive latency padding logic  413 , which consists of four latency padding units  713   a - 713   d , one for each port. Latency padding units  713   a - 713   d  drive the four twenty-bit buses  723 . Buses  723  drive serializers  414 , which consist of four serializers  714   a - 714   d , one for each port. Serializers  714   a - 714   d  drive the four five-bit bundles  724 . Bundles  724  drive network switch  420 . 
         [0088]      FIG. 8  shows the data zeroing logic for one input port, such as data zeroing unit  710   a . The data zeroing logic for a port has three functions: to register the user&#39;s input data; to statically set the width of the port; and to allow the user&#39;s logic to zero out the entire port on a cycle-by-cycle basis. 
         [0089]    The user&#39;s input data for the port is twenty-bit bus  802 , which is one of the four buses  415  driven by input multiplexers  700 . Bus  802  is captured by register  803 , which is clocked by user clock  805 . The output of register  803  is treated as four independent five-bit nybbles. Element  820  is the logic for a representative nybble. The output nybbles are concatenated to form twenty-bit bus  830 , which drives the port&#39;s parity generation and checking logic. 
         [0090]    The port also has one-bit Valid Input (VI) signal  800 . Signal  800  is captured by register  801 , which is clocked by user clock  805 . 
         [0091]    An input port can be configured to be five, ten, fifteen, or twenty bits wide. Each of the port&#39;s four nybbles has a configuration memory bit that forces the entire nybble to 0 if the nybble is unused. In representative nybble  820 , AND gates  824  consist of five two-input AND gates, where the first input of each gate is driven by signal  823  and the second input is driven by one of the bits of the nybble. If the nybble is unused, configuration bit  821  is programmed to 0. This forces output  823  of AND gate  822  to 0, which in turn forces the outputs of all five AND gates  824  to 0. 
         [0092]    If the user wants to be able to zero out the entire port on a cycle-by-cycle basis, then configuration memory bit  811  is programmed to pass the output of register  801  through multiplexer  810  to signal  812 . If Valid Input (VI) signal  800  is 0, then signal  812  is 0 during the following cycle. That forces a 0 onto output  823  of AND gate  822  and onto the outputs of the other three like AND gates. That in turn forces 0 onto the output of AND gates  824  and the other three like sets of AND gates, regardless of the value of configuration bit  821  and the other three like configuration bits. On the other hand, if VI signal  800  is 1, then signal  812  is 1 during the following cycle, and the five-bit nybbles pass through the data zeroing logic unchanged unless the nybble&#39;s individual configuration bit, such as configuration bit  821 , is 0. 
         [0093]    If the user wants Valid Input (VI) signal  800  to be ignored and wants the port to be enabled on every cycle, then configuration memory bit  811  can be programmed to pass a constant 1 through multiplexer  810  to signal  812 . 
         [0094]      FIG. 9  is a schematic diagram of the parity generation and checking logic for one input port, such as parity unit  711   a . It can be configured for bypass (leaving all twenty bits unchanged), parity generation, or parity checking. The parity logic can be configured to operate on all twenty bits as a group or on the two ten-bit bytes as independent groups. The twenty-bit input to the parity unit is one of the four buses  720  driven by one of the four data zeroing units  710   a - 710   d  (see  FIG. 7 ). The low-order input byte consists of bit  0   900  and bits  9 : 1901 , and the high-order input byte consists of bit  10   910  and bits  19 : 11   911 . The high nine bits of both bytes (bits  9 : 1   901  and bits  19 : 11   911 ) always pass through the parity logic unchanged. The twenty-bit output of the parity unit (bit  0   950 , bits  9 : 1   901 , bit  10   960 , and bits  19 : 11   911 ) drive the station&#39;s byte shuffling logic. 
         [0095]    To generate parity, the logic computes the exclusive-OR (XOR) of the high nineteen bits or nine bits of the parity group and injects the computed parity on the low-order bit of the group (bit  0   950  in twenty-bit mode or bit  10   960  and bit  0950  in ten-bit mode). To check parity, the logic computes the XOR of all twenty bits or ten bits of the parity group and injects the error result on the low-order bit of the group; the result is 1 if and only if a parity error has occurred. 
         [0096]    The multiplexers in  FIG. 9  are controlled by configuration memory. The multiplexers determine whether the parity logic operates in bypass, generate, or check mode. The multiplexers also determine whether the parity logic operates in twenty-bit mode or ten-bit mode. 
         [0097]    The byte shuffling logic is the only layer of the input logic where the four ports can exchange data with each other. Its main function is to support a 2:1 frequency ratio between an internal clock of the inventive network and a user clock. For all other frequency ratios, computer-aided design (CAD) software configures this logic to pass the twenty bits of each port straight through on the same port. 
         [0098]      FIG. 10  shows the byte shuffling logic for all four input ports; the multiplexers in the figure are controlled by configuration memory. The byte shuffling unit has one twenty-bit input bus  1000 - 1003  for each of ports  0 - 3 , respectively. These input buses are the four buses  721  in  FIG. 7 , which are driven by the four parity units  711   a - 711   d . The byte shuffling unit has one twenty-bit output bus  1060 - 1063  for each of ports  0 - 3 , respectively. These output buses drive the four latency padding units  713   a - 713   d  (see  FIG. 7 ). 
         [0099]    The byte shuffling logic treats each port as two ten-bit bytes. For example, port  1 &#39;s input bus  1001  consists of low-order byte  1051   l  and high-order byte  1051   h . Configurable multiplexers either keep the low-order byte of port i on port i, or steer it to the high-order byte position of port i−1 (mod 4). For example, multiplexers either direct port  1 &#39;s low-order input byte  1051   l  to port  1 &#39;s output bus  1061 , or steer it to the high-order byte of port  0 &#39;s output bus  1060 . Similarly, the multiplexers either keep the high-order byte of port i on port i, or steer it to the low-order byte position of port i+1 (mod 4). For example, multiplexers either direct port  1 &#39;s high-order input byte  1051   h  to port  1 &#39;s output bus  1061 , or steer it to the low-order byte of port  2 &#39;s output bus  1062 . 
         [0100]    The 2:1 frequency ratio works with byte shuffling as follows. Each twenty-bit input port, clocked at a user clock frequency, is associated with a five-bit internal bundle, clocked at the faster frequency of the internal clock of the inventive network. When the ratio of internal clock to user clock is 2:1, only ten bits of the twenty-bit port can be serialized onto the five-bit bundle. If all twenty bits of the port are in use, the byte shuffling multiplexers keep ten bits within the given port and steer the other ten bits to an adjacent port. Therefore, the twenty bits that originally came into the port will be serialized onto two five-bit internal bundles. 
         [0101]    Each input port has latency padding logic, such as latency padding unit  713   a  in  FIG. 7 . CAD software can use this logic to pad the end-to-end latency through the inventive network to equal the value specified by the user. 
         [0102]      FIG. 11  is a schematic diagram of the effective behavior of the latency padding logic for one input port, such as latency padding unit  713   a . It behaves as a shift register that is clocked by user clock  805 . The effective shift register depth is determined by the configuration memory bits that control multiplexer  1101 . The twenty-bit input  1102  to the latency padding unit is one of the four buses  722  driven by the byte shuffling logic (see  FIG. 7 ). The twenty-bit output  1103  drives the port&#39;s serializer. 
         [0103]    The logic can be configured to behave like a twenty-bit-wide shift register with zero to seven stages or like a ten-bit-wide shift register with zero to fourteen stages. When the logic is configured as a zero-stage shift register, it passes data through from input bus  1102  to output bus  1103  without any register delays. The deeper-and-narrower fourteen-by-ten configuration is useful when only ten bits or five bits of the port are meaningful, which is the case when the frequency ratio between the internal clock of the inventive network and the user clock is 2:1 or 1:1. 
         [0104]      FIG. 12  summarizes the preferred embodiment of the latency padding logic. Twenty-bit input data  1102  from the byte shuffling logic is written into a seven-word by twenty-bit RAM  1204  on every cycle of user clock  805 , and twenty-bit output data  1103  for the serializer is read from RAM  1204  on every cycle. 
         [0105]    Random-access memory (RAM)  1204  has separate write bit lines and read bit lines. During the first half of the cycle, the write bit lines are driven with write data, the read bit lines get precharged, and the output latches are held closed so they retain the results of the previous read. During the second half of the cycle, RAM bit cells can pull down the read bit lines, and the output latches are held open so they can capture the values from the sense amplifiers. 
         [0106]    The RAM addresses are furnished by read pointer  1205  and write pointer  1206 . The pointers are implemented by identical state machines that have a set of states that form a graph cycle. The state machines can be configured with different initial states, and they advance to the next state at every cycle of user clock  805 . As pointers  1205  and  1206  “chase” each other around RAM  1204 , the effect is that RAM  1204  delays its input data by a fixed number of cycles. In the preferred embodiment, the state machines are three-bit linear feedback shift registers (LFSRs) that have a maximal-length sequence of seven states. Other possible embodiments include binary counters, which are slower, and one-hot state machines, which use more area. 
         [0107]    To emulate a zero-stage shift register, RAM  1204  has several features to pass data through from its input bus  1102  to its output bus  1103 . The linear feedback shift registers (LFSRs) in read and write pointers  1205  and  1206  can be initialized to the one state that does not belong to the seven-state graph cycle, and the LFSR remains in that state at every clock cycle; in this state, no word lines are enabled. The precharge circuits have additional circuitry that can steadily short the write bit lines to the read bit lines and never precharge the read bit lines. The clock for the output latches can be configured to hold the latches steadily open. 
         [0108]    RAM  1204  can also operate as fourteen words by ten bits. It has separate write word lines for the high and low bytes of each word, and there is a ten-bit-wide two-to-one multiplexer preceding the low byte of the output latches. In addition to the three-bit state of the linear feedback shift register, read pointer  1205  and write pointer  1206  both include an additional state bit to select the high or low byte of RAM  1204 . 
         [0109]    Read and write pointers  1205  and  1206  are initialized at some rising edge of user clock (UCLK)  805 . A synchronization (sync) pulse causes this initialization. The integrated circuit&#39;s clock system distributes sync alongside clock throughout each clock tree. The period of sync is a multiple of seven cycles of the internal clock of the inventive network because the read and write pointers cycle back to their initial values every seven (or fourteen) UCLK cycles, and because the clock tree issues sync pulses repeatedly. For more information about the sync pulse, see subsection “Providing Clocks and Synchronization Pulses for the Inventive Network”. 
         [0110]    Each of the four input ports has a serializer, such as serializer  714   a  in  FIG. 7 , that follows the latency padding logic. The serializer splits a twenty-bit input port into four five-bit nybbles and serializes them onto a five-bit internal bundle. The serializer is the only input port layer that uses an internal clock (DCLK) of the inventive cross-connection network for data. 
         [0111]      FIG. 13  shows the serializer logic for one input port. The twenty-bit input  1103  to the serializer is one of the four buses  723  driven by one of the latency padding units  713   a - d  (see  FIG. 7 ). The five-bit output  1303  of the serializer goes to the station&#39;s network switch. 
         [0112]    Each nybble has a two-to-one multiplexer and a register clocked by DCLK  512 . The multiplexers and registers are connected to form a four-stage, five-bit-wide shift register that can also load twenty bits in parallel. When control logic  1300  tells the multiplexers to shift, five-bit data  1303  for the network switch emerges from the low-order nybble  1302  of the shift register. An unused nybble is designated by a configuration memory bit, such as configuration bit  1304 , that forces the nybble to shift every cycle; this behavior is important for time-slicing, for allowing low-order nybbles to be unused, and for other functions. 
         [0113]    The inventive cross-connection network for data (DCC network) can serialize data from more than one input port onto a single five-bit bundle. For example, the library of logic models has a beginpoint model that serializes thirty bits (six nybbles) onto one five-bit bundle. The hardware of the inventive network has three features that work together to implement this function. 
         [0114]    The first feature is that the station&#39;s network switch has a multi-port OR gate at the root of each routing multiplexer. When a multiplexer is configured to allow more than one bundle into the OR gate, nybbles from all the corresponding input ports can be streamed onto the output of the multiplexer. 
         [0115]    The second feature is that in the input port serializer, a shift operation puts 0 into the high-order nybble register  1301 , and from there into the rest of the nybble registers. Except during the four cycles of the internal clock (DCLK) that immediately follow a parallel load, the serializer outputs 0 every cycle. At the OR gate in the routing multiplexer, the 0 value from the given port allows data from the other port or ports to pass through the OR gate without corruption. 
         [0116]    The third feature is that the serializer control logic  1300  has a configurable divider offset. A divider offset of zero, which is the most common case, causes the serializer to perform a parallel load one DCLK cycle after every rising edge of the user clock. A divider offset greater than zero delays the parallel load by the same number of cycles. For example, in the beginpoint model that serializes thirty bits (six nybbles) onto one five-bit bundle, the low-order port (User Data Input (UDI) bits  19 : 0 ) has a divider offset of zero and the high-order port (UDI[29:20]) has a divider offset of four. Therefore, the high-order port always performs a parallel load operation four DCLK cycles after the low-order port does. During the four DCLK cycles when the low-order serializer outputs its data to the network switch, the high-order serializer outputs 0. 
         [0117]    The serializer control logic  1300  is initialized at some rising edge of user clock (UCLK). The synchronization (sync) pulse causes this initialization. For more information about the sync pulse, see subsection “Providing Clocks and Synchronization Pulses for the Inventive Network”. 
         [0118]    Further Details of the Network Switch:  FIG. 14  illustrates the network switch in a station. The network switch routes five-bit bundles of data from sixteen input links  423  and four input ports  421  to sixteen output links  422  and four output ports  424 . As shown in  FIG. 3 , the network switch has four input links from each of the adjacent stations in four directions (sets of four input links  320 - 323  from the North, East, South, and West directions, respectively). The network switch has four output links to each of the adjacent stations in the same four directions (sets of four output links  310 - 313  to the North, East, South, and West directions, respectively). The network switch has one input bundle from each of ports  0 - 3 , respectively. These input port bundles  421  are the four buses  724  in  FIG. 7 , which are driven by the four serializers  414 . The network switch has one output bundle to each of ports  0 - 3 , respectively. These output port bundles  424  drive the four deserializer units  434  in  FIG. 16 . 
         [0119]    The network switch has twenty five-bit-wide routing multiplexers, each driven by a subset of the twenty input bundles. Thus, it implements a partially populated crossbar switch. The horizontal lines in  FIG. 14 , such as horizontal line  1410 , represent input bundles. The vertical lines, such as vertical line  1411 , represent routing multiplexers. The X symbols, such as X symbol  1412 , represent populated crosspoints from an input bundle to a routing multiplexer. 
         [0120]    The network switch has a pipeline register on every input link from another station. These registers, such as register  1413 , are clocked by internal clocks of the inventive network, and they add one cycle of latency for every station that a connection through the inventive network passes through. The pipeline registers make it practical for links in the network to transfer data at very high frequencies (up to two GHz, in the preferred embodiment). The network switch does not have pipeline registers for input ports, output ports, or output links to other stations. Note that input ports have been registered at the serializer, and output ports and output links will be registered at the deserializer or the next station, respectively. 
         [0121]    In an alternate embodiment, the pipeline register on every input link could be replaced by latches on every input link and latches clocked by the opposite phase on every output link. If the internal clock frequency of a routed connection through the network is relatively slow, it is possible to reduce the number of pipeline stages in the connection by making some of the latches along the path transparent. 
         [0122]    Every routing multiplexer is hardwired to a subset of the twenty input bundles. Compared to twenty-input multiplexers, narrower multiplexers use less die area and cause less circuit delay. The multiplexer for each of the sixteen output links  422  has six inputs, four of which come from input links and two from input ports. The multiplexer for each of the four output ports  424  has ten inputs, eight of which come from input links and two from input ports. 
         [0123]    The network switch is not a full crossbar, but the populated inputs of the routing multiplexers were chosen to make it easier for computer-aided design (CAD) software to find Manhattan-distance routes through congested regions of the inventive network. In the preferred embodiment, the inventive network can be thought of as having four routing planes, numbered  0 - 3 . Every input or output bundle belongs to one of the planes. A station&#39;s four input ports  0 - 3  belong to planes  0 - 3 , respectively. Similarly, a station&#39;s four output ports  0 - 3  belong to planes  0 - 3 , respectively. In each plane a station has four output links, one to each of the four directions (North, East, South, and West, respectively). Similarly, in each plane a station has four input links, one from each of the four directions. For an output link that belongs to a given plane, the link&#39;s routing multiplexer has more inputs from the same plane than inputs from the other planes. 
         [0124]    The routing multiplexer for an output link has inputs from four of the station&#39;s sixteen input links. Three of these inputs come from input links in the same routing plane and from different stations than the destination of the given output link. The fourth input comes from an input link in a different plane and from the station on the opposite side of the given station from the given output link, thus providing extra routing flexibility for routes that go straight through the station without turning. For example, the routing multiplexer for the South output link in plane  2  has inputs from the West, North, and East input links in plane  2 . It has a fourth input from the North input link in plane  3 , which provides extra routing flexibility for routes that go straight through the station from North to South. 
         [0125]    The routing multiplexer for an output link has inputs from two of the station&#39;s four input ports. One of these inputs comes from the input port in the same routing plane. The other input comes from the input port in the plane numbered  2  greater, modulo 4. For example, the routing multiplexer for the South output link in plane  2  has inputs from the input ports in planes  2  and  0 . This feature gives CAD software the ability to launch a connection into a different plane in the network than the plane that the input port belongs to. 
         [0126]    The routing multiplexer for an output port has inputs from eight of the station&#39;s sixteen input links. Four of these inputs come from input links in an even routing plane, specifically, one from the station in each of the four directions. The other four inputs come from input links in an odd plane, specifically, one from the station in each of the four directions. For example, the routing multiplexer for the output port in plane  1  has inputs from the North, East, South, and West input links in plane  2  and from the North, East, South, and West input links in plane  3 . 
         [0127]    The routing multiplexer for an output port has inputs from two of the station&#39;s four input ports. One of these inputs comes from the input port in the same routing plane. The other input comes from the input port in the plane numbered  2  higher, modulo 4. For example, the routing multiplexer for the output port in plane  1  has inputs from the input ports in planes  1  and  3 . The input-port-to-output-port path provides a loopback capability within a station. 
         [0128]    The inputs that are available on routing multiplexers make it possible for CAD software to route a connection through the inventive network from an input port in one plane to an output port in any plane, and route all the station-to-station links within a single plane. A connection that starts from an input port in a given plane can be launched into one of two planes inside the network, because every output link&#39;s routing multiplexer has inputs from input ports in two planes. The connection can continue on the same plane within the network, because every output link&#39;s routing multiplexer has inputs from three input links that allow a route within the same plane to turn left, continue straight, or turn right. The connection can leave the network at an output port in one of two planes, because every output port&#39;s routing multiplexer has inputs from input links in two planes. The product of two choices for the station-to-station link plane inside the network and two choices for the output port plane means that a connection can be routed from an input port in a given plane to an output port in any of the four planes. Because such a connection is not required to jump from plane to plane inside the network, CAD software&#39;s ability to find a good route is not restricted much by the fact that every output link&#39;s routing multiplexer has only one input from an input link in a different plane. 
         [0129]      FIG. 15  is a schematic diagram of the six-input routing multiplexer in the preferred embodiment for an output link to an adjacent station. It has four five-bit inputs  1500  from the registered input links from other stations and two five-bit inputs  1501  from the station&#39;s input ports. It uses a conventional AND-OR multiplexer design, with the enable signal for each five-bit input bundle coming from a configuration memory bit, such as configuration bit  1502 . When one of the configuration bits is set, to 1 and the others are set to 0, the multiplexer simply routes the corresponding input bundle to the output link  1505 . It is obvious that alternate embodiments of an AND-OR multiplexer are possible. For example, to reduce circuit delay, the two-input AND gates, such as AND gate  1503 , could be replaced by two-input NAND gates, and the six-input OR gate  1504  could be replaced by a six-input NAND gate. To further reduce circuit delay, every two two-input NAND gates and two inputs of the six-input NAND gate could be replaced by a 2-2 AND-OR-INVERT gate; then the six-input NAND gate could be replaced by a three-input NAND gate. 
         [0130]    Note that the routing multiplexers in the network switches are configured on a granularity coarser than a single bit. For example, in the preferred embodiment the most commonly used frequency ratio between internal clock and user clock is 4:1. In this situation, a single configuration memory bit steers a twenty-bit user bus. The coarse granularity of the network switch greatly reduces the number of configuration memory bits and multiplexer ports compared to a field-programmable gate array (FPGA) routing network, so it saves a great deal of die area. 
         [0131]    When two or more configuration memory bits are set to 1, the routing multiplexer in  FIG. 15  ORs together the corresponding input bundles. With appropriate logic upstream to zero out all of the input bundles except one during every cycle, the multiplexer performs cycle-by-cycle selection. In this configuration, the multiplexer can implement a high bandwidth multiplexer (as described under “Uses of the Inventive Network”), time-slice a connection through the inventive network (also described under “Uses of the Inventive Network”), or serialize data from more than one input port onto a single five-bit bundle (as described under “Further Details of the Input Port Logic”). 
         [0132]    Other embodiments of the multiplexer are possible that use fewer than one configuration memory bit per five-bit input bundle. In one such embodiment, the number of configuration bits equals the base-2 logarithm of the number of input bundles, rounded up to the next integer. In this embodiment, the configuration bits allow no more than one bundle to pass through the multiplexer. Such an embodiment cannot OR together two or more bundles of data and, therefore, cannot perform cycle-by-cycle selection in the network switch. 
         [0133]    The ten-input routing multiplexer for an output port in the preferred embodiment is similar to the multiplexer for an output link, but it has inputs from eight input links instead of only four. It has the same ability to perform cycle-by-cycle selection by ORing together two or more input bundles. 
         [0134]    Further Details of the Output Port Logic: The output port logic of each station is depicted by elements  431 - 434  in  FIG. 4 . More detail is provided by  FIG. 16 , which is a block diagram of the output port logic. Each group of buses  435  and  1641 - 1643  consists of four buses. Each of the buses is twenty bits wide and clocked by a user clock. Buses  1644  consist of four buses. Each of the buses, also referred to herein as bundles, is five bits wide and clocked by an internal clock of the inventive network. 
         [0135]    Network switch  420  drives the four five-bit bundles  1644 . Bundles  1644  drive deserializers  434 , which consist of four deserializers  1634   a - d , one for each port. Deserializers  1634   a - d  drive the four twenty-bit buses  1643 . Buses  1643  drive latency padding logic  433 , which consists of four latency padding units  1633   a - d , one for each port. Latency padding units  1633   a - d  drive the four twenty-bit buses  1642 . Buses  1642  drive byte shuffling logic  432 , which can steer data from one port to another port. Byte shuffling logic  432  drives the four twenty-bit buses  1641 . Buses  1641  drive parity generation and checking logic  431 , which consists of four parity generation and checking units  1631   a - d , one for each port. Parity generation and checking units  1631   a - d  drive the four twenty-bit buses  435 . Buses  435  drive output multiplexers  1600 . 
         [0136]    Each of the four output ports has a deserializer, such as deserializer  1634   a  in  FIG. 16 , that receives a five-bit bundle of data from the network switch. The deserializer first shifts the five-bit data through a five-bit-wide shift register clocked by an internal clock (DCLK) of the inventive cross-connection network for data. Then it does a parallel load into a twenty-bit output register. The deserializer is the only output port layer that uses DCLK. 
         [0137]      FIG. 17  shows the deserializer logic for one output port. The five-bit input  1700  to the deserializer is one of the four buses  1644  driven by the station&#39;s network switch  420  (see  FIG. 16 ). The twenty-bit output  1705  of the deserializer drives the port&#39;s latency padding unit. On every rising edge of DCLK  512 , a three-stage, five-bit-wide shift register  1702  shifts data from the high-order five-bit nybble toward the low-order nybble  1704  (bits  4 : 0 ). Therefore, the first nybble to arrive from the network switch will leave the deserializer in the lowest-order nybble position within the parallel output. The user port width can be set to five, ten, fifteen, or twenty bits by means of configuration memory bits (not shown) that control multiplexers to set the length of shift register  1702  to zero, one, two, or three register stages. 
         [0138]    The deserializer control logic has a configurable divider offset. An offset of zero causes the twenty-bit output register to perform a parallel load one internal clock (DCLK) cycle before every rising edge of user clock (UCLK), and an offset greater than zero makes the parallel load occur that many DCLK cycles earlier. The routing latency through a sequence of network switches can take an arbitrary number of DCLK cycles, so the divider offset allows the deserialized word to be captured at any DCLK cycle modulo the UCLK divider ratio. 
         [0139]    The inventive cross-connection network for data (DCC network) can deserialize data from a single five-bit bundle onto more than one output port. For example, the library of logic models has an endpoint model that deserializes one five-bit bundle onto thirty bits (six nybbles). The hardware of the inventive network has two features that work together to implement this function. 
         [0140]    The first feature is that a bundle can be routed within the network to fan out to two or more output ports. All the ports receive the same nybble into their shift registers at the same internal clock (DCLK) cycle. 
         [0141]    The second feature is that each output port can be configured with a different divider offset, so at any given cycle at most one port does a parallel load into its output register. For example, in the endpoint model that deserializes one five-bit bundle onto thirty bits, the low-order port (User Data Output (UDO) bits  19 : 0 ) has a divider offset of two and the high-order port (UDO[29:20]) has a divider offset of zero. Therefore, the low-order output register always performs a parallel load of its four nybbles two DCLK cycles before the high-order output register does a parallel load of its two nybbles. 
         [0142]    The deserializer control logic  1701  is initialized at some rising edge of the user clock. The synchronization (sync) pulse causes this initialization. For more information about the sync pulse, see subsection “Providing Clocks and Synchronization Pulses for the Inventive Network”. 
         [0143]    Each output port has latency padding logic, such as latency padding unit  1633   a  in  FIG. 16 . Computer-aided design (CAD) software can use this logic to pad the end-to-end latency through the inventive network to equal the value specified by the user. 
         [0144]      FIG. 18  is a schematic diagram of the effective behavior of the latency padding logic for one output port, such as latency padding unit  1633   a . It behaves as a shift register that is clocked by user clock  1800 . The effective shift register depth is determined by the configuration memory bits that control multiplexer  1801 . The twenty-bit input  1802  to the latency padding unit is one of the four buses  1643  driven by one of the four deserializer units  1634   a - 1634   d  (see  FIG. 16 ). The twenty-bit output  1803  drives the station&#39;s byte shuffling logic. 
         [0145]    The logic can be configured to behave like a twenty-bit-wide shift register with zero to seven stages or like a ten-bit-wide shift register with zero to fourteen stages. When the logic is configured as a zero-stage shift register, it passes data through from input bus  1802  to output bus  1803  without any register delays. The deeper-and-narrower fourteen-by-ten configuration is useful when only ten bits or five bits of the port are meaningful, which is the case when the frequency ratio between the internal clock of the inventive network and the user clock is 2:1 or 1:1. 
         [0146]    The hardware implementation of the latency padding logic for an output port is identical to the implementation for an input port. For more information about an input port&#39;s implementation, see the description under subsection “Further Details of the Input Port Logic.” 
         [0147]    The byte shuffling logic layer of the output logic allows the four ports to exchange data with each other. Its main function is to support a 2:1 frequency ratio between an internal clock of the inventive network and a user clock. For all other frequency ratios, CAD software configures this logic to pass the twenty bits of each port straight through on the same port. 
         [0148]    The byte shuffling logic for an output port is identical to that for an input port.  FIG. 19  shows the byte shuffling logic for all four output ports; the multiplexers in the figure are controlled by configuration memory. The byte shuffling unit has one twenty-bit input bus  1900 - 1903  for each of ports  0 - 3 , respectively. These input buses are the four buses  1642  in  FIG. 16 , which are driven by the four latency padding units  1633   a - 1633   d . The byte shuffling unit has one twenty-bit output bus  1960 - 1963  for each of ports  0 - 3 , respectively. These output buses drive the four parity units  1631   a - 1631   d  (see  FIG. 16 ). 
         [0149]    The byte shuffling logic treats each port as two ten-bit bytes. For example, port  1 &#39;s input bus  1901  consists of low-order byte  1951   l  and high-order byte  1951   h . Configurable multiplexers either keep the low-order byte of port i on port i, or steer it to the high-order byte position of port i−1 (mod 4). For example, multiplexers either direct port  1 &#39;s low-order input byte  1951   l  to port  1 &#39;s output bus  1961 , or steer it to the high-order byte of port  0 &#39;s output bus  1960 . Similarly, the multiplexers either keep the high-order byte of port i on port i, or steer it to the low-order byte position of port i+1 (mod 4). For example, multiplexers either direct port  1 &#39;s high-order input byte  1951   h  to port  1 &#39;s output bus  1961 , or steer it to the low-order byte of port  2 &#39;s output bus  1962 . 
         [0150]    The 2:1 frequency ratio works with byte shuffling as follows. Each five-bit internal bundle, clocked at the internal clock (DCLK) frequency, is associated with a twenty-bit output port, clocked at the slower user clock (UCLK) frequency. When the ratio of DCLK to UCLK is 2:1, a five-bit bundle can be deserialized onto only ten bits of the twenty-bit port. If all twenty bits of the port are in use, the port&#39;s data comes from two five-bit internal bundles. The byte shuffling multiplexers steer two ten-bit buses, which originally came from two adjacent deserializers, onto a single twenty-bit output port. 
         [0151]      FIG. 20  is a schematic diagram of the parity generation and checking logic for one output port, such as parity unit  1631   a . The parity logic can be configured for bypass (leaving all twenty bits unchanged), parity generation, or parity checking. It can be configured to operate on all twenty bits as a group or on the two ten-bit bytes as independent groups. The output of the parity logic is staged by twenty-bit register  2070  that is clocked by the output port&#39;s user clock (UCLK)  1800 . Except for having an output register, the parity logic for an output port is identical to that for an input port. The twenty-bit input to the parity unit is one of the four buses  1641  driven by the byte shuffling logic  432  (see  FIG. 16 ). The low-order input byte consists of bit  0   2000  and bits  9 : 1   2001 , and the high-order input byte consists of bit  10   2010  and bits  19 : 11   2011 . The twenty-bit output of the XOR logic (bit  0   2050 , bits  9 : 1   2001 , bit  10   2060 , and bits  19 : 11   2011 ) drives register  2070 . The output  2071  of register  2070  drives some of the station&#39;s output multiplexers. 
         [0152]    To generate parity, the logic computes the exclusive-OR (XOR) of the high nineteen bits or nine bits of the parity group and injects the computed parity on the low-order bit of the group (bit  0   2050  in twenty-bit mode or bit  10   2060  and bit  0   2050  in ten-bit mode). To check parity, the logic computes the XOR of all twenty bits or ten bits of the parity group and injects the error result on the low-order bit; the result is 1 if and only if a parity error has occurred. 
         [0153]    The multiplexers in  FIG. 20  are controlled by configuration memory. The multiplexers determine whether the parity logic operates in bypass, generate, or check mode. The multiplexers also determine whether the parity logic operates in twenty-bit mode or ten-bit mode. 
         [0154]    Providing Clocks and Synchronization Pulses for the Inventive Network: The inventive network works with the clock distribution system of the integrated circuit. A synchronization (sync) pulse initializes counters in the clock network and in the stations of the inventive network. 
         [0155]    A connection through the inventive network is completely synchronous, but it typically uses at least two clock frequencies. The user clocks have an integer frequency ratio to the internal clock of the network. This ratio is typically 2:1 or greater, but it may be 1:1. Furthermore, the user clock for different beginpoints or endpoints belonging to a connection through the network may have different frequencies. For example,  FIG. 5  illustrates a connection through the inventive network with three clock frequencies. Internal clock  512  operates at one thousand, six hundred MHz. User clock  513  operates at four hundred MHz, which has a 4:1 ratio to the internal clock. User clock  514  operates at two hundred MHz, which has an 8:1 ratio to the internal clock. 
         [0156]    These clock signals operate at different frequencies, but they have aligned edges and low skew between them to allow synchronous interfacing between the user clock domain or domains and the internal clock domain of the inventive network. The field-programmable gate array (FPGA) containing the inventive network has a clock distribution system that can produce lower-frequency clocks by dividing down a root clock by configurable integer ratios. The clock distribution system also guarantees that the root clock and the divided clocks have aligned edges and low skew among them. 
         [0157]    In the preferred embodiment, there are clock dividers at the third level of the clock distribution network, and the dividers can be configured to create any integer clock ratio from 1:1 to 16:1 relative to the root clock. In other embodiments, the dividers may be at a different level of the clock network and they may support different divider ratios. 
         [0158]    The internal clock of the inventive network and the user clock or clocks for a given connection through the network all derive from the same root clock, but different connections can use different root clocks. For example, a user can choose a one thousand, six hundred MHz root clock for some connections in their design and a one thousand, two hundred fifty MHz root clock for others. 
         [0159]    The clock distribution system and the inventive network have many counters that are initialized simultaneously. When multiple dividers in a clock tree have the same clock divider ratio, their dividers are initialized at the same rising edge of the root clock in order to cause the divided output clocks to be in phase with each other. The control logic for an input port serializer is initialized at some rising edge of the user clock; so is the control logic for an output port deserializer. In the preferred implementation, latency padding logic in input and output ports is implemented by a random-access memory (RAM); the RAM&#39;s read and write pointers are initialized at some rising edge of the user clock. 
         [0160]    To perform all of these initializations, the FPGA containing the inventive network generates a synchronization (sync) pulse and distributes it to all the clock dividers and all the stations that use those dividers. It is convenient to generate the sync pulse at the root of the clock network and distribute it alongside clock down through the levels of the network. A single synchronization pulse that occurs at the start of functional operation is enough to initialize the clock system and the stations. The counters in the clock system and the stations will remain synchronized thereafter because they are configured to cycle through a sequence of states with a fixed period. 
         [0161]    To help in ensuring that a reset pulse issued from one clock domain can be seen by clock edges in all the related domains that have different divider ratios, it is useful to issue the synchronization (sync) pulse repeatedly rather than just once. Therefore, the preferred embodiment issues periodic sync pulses. The sync pulses occur at times when the counters in the clock system and the stations would have reinitialized themselves anyway. The period of the sync pulse is configurable, and CAD software sets it to a suitable value, as measured in root clock cycles. The period is the least common multiple (LCM), or a multiple thereof, of the divider ratios of all the clock dividers that participate in connections through the inventive networks. In the preferred embodiment, the period is also a multiple of seven, because the read and write pointers in latency padding logic cycle back to their initial values every seven (or fourteen) user clock cycles. 
         [0162]    Although the present invention has been described in terms of a preferred embodiment, it will be appreciated that various modifications and alterations might be made by those skilled in the art without departing from the spirit and scope of the invention. The invention should therefore be measured in terms of the claims which follow.