Abstract:
A hierarchial bus structure having at least three dimensions provides improved interconnect flexibility between nodes located on one or more levels of the structure. Nodes are defined on at least first and second &#34;horizontal&#34; (or &#34;H&#34;) rings, the rings being coupled by at least one &#34;vertical&#34; (or &#34;V&#34;) ring. Each node is identified in terms of its (H,V) coordinates in the hierarchial interconnect structure, and an M-dimensional structure will provide an M-way multiplex unit at each node. For an M=3, e.g., three-dimensional structure, each multiplex unit has three-inputs, a Localout, a Vin, and an Hin input, and couples one of these inputs to an output port in response to a Local select arbitration signal. The output signal is coupled to Hout and Vout, and to Localin. Nodes on the same horizontal level will drive their Hin signal to Vout and Hout, whereas all other nodes receive the Vin signal. The arbitration select signals may reconfigure the overall bus structure dynamically or statically, preferably according to demand of the nodes required interconnection. Providing additional vertical rings provides redundancy and can reduce latency time. Because the multi-dimensional hierarchial structure is point-to-point, low module current may be used, the width of the metallized bus traces may be reduced, and contention-type overlap damage is minimized.

Description:
RELATION TO OTHER APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 08/390,052, filed Feb. 17, 1995 now U.S. Pat. No. 5,555,540, entitled ASIC BUS STRUCTURE, assigned to the assignee of this application. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to bus structures that couple digital signals within an integrated circuit, and more specifically to multidimensional bus structures that avoid contention damage, large current handling metal bus traces, and testing difficulties associated with prior art tristate buffer modules. 
     BACKGROUND OF THE INVENTION 
     In fabricating large application specific integrated circuits (&#34;ASICs&#34;), it is common practice to design the ASIC as a series of sub-sections whose nodes are interconnected with wide buses. If interconnecting requires, say 16 bits, the integrated circuit (&#34;IC&#34;) chip may contain sixteen 1-bit wide buses, with enabling or arbitration signals determining the source of the bit coupled to the one-bit bus. The buses are low resistivity metal traces, sized to accommodate the current flow associated with the signals carried across the bus. 
     FIG. 1 depicts a prior art configuration wherein data are coupled to or from a one-bit bus 2 on an integrated circuit chip 4 using tristate buffer modules, 6-0, 6-1, 6-2, 6-3 (although more or less than four buffer modules may instead be used). Each buffer module defines an input/output node (&#34;I/O&#34;) that may be coupled to one or more sub-sections or circuits on IC chip 4 by assertion of an appropriate enabling arbitration select signal. 
     Each buffer module has an input port, an enabling or arbitration port and an output port. For example, buffer 6-0 has an input port coupled to receive an I/O signal Dout0 from elsewhere on IC chip 4. Buffer 6-0 has an enabling port coupled to receive an output enabling or arbitration signal ARB-0, and also has an output port coupled to deliver a signal Din0. In common tristate buffer fashion, the signal Din0 will replicate the signal Dout0 only when ARB-0 is in an enabling state. In this fashion the sub-section circuit generating Dout0 can communicate one-bit of this signal across bus 2 to some other sub-section circuit also coupleable to bus 2. 
     If I/O signal Dout0 is to be coupled to bus 2, ARB-0 will be in an enabling state permitting buffer 6-0 to output a replica of signal Dout0. By contrast, enabling signals ARB-1, ARB-2, ARB-3 would each be in a disabling state that precluded respective buffers 6-1, 6-2, 6-3 from outputting a signal to bus 2. Only one ARB signal is to be in the enabling state at a time, which means that only one tristate buffer module is selected at any given time for coupling Dout signals to bus 2. 
     In the manner described, the buffer modules serve as mechanisms for coupling signals between the buffer I/O node and bus 2, the coupling being determined by the enabling ARB signals. 
     It is understood that if the bus is N-bits wide, there will be N buses 2, and N groups of buffers 6, each buffer having an input port, an arbitration port, and an output port. Using FIG. 1 as an example, an IC requiring a 16-bit wide bus would replicate the structure of FIG. 1 sixteen times. There would be sixteen bus 2 structures and 64 (e.g., 16×4) buffer modules 6, one such structure being present for each bit-position in the bus. However, each group of sixteen buffer modules would be coupled to the same ARB signal, with the four ARB signals thus each being coupled to blocks of sixteen buffer modules. 
     FIG. 2A shows a typical implementation of a tristate buffer, e.g., buffer 6-0, which typically operate from an upper power supply Vdd and a lower power supply Vss that is often ground. At its input port, buffer 6-0 receives Dout0, and at its output port outputs Din0, providing the ARB-0 enabling signal is present (e.g., is a digital &#34;1&#34;). Depending upon the circuit design, Dino may replicate or be an inverted version of Dout0, and buffer 6-0 may enable when ARB-0 is a digital &#34;0&#34;. 
     The output of buffer 6-0 is shown coupled to a load impedance Z L  that may be represented generally by a resistance R L  shunted by an effective capacitive load C L . Load impedance Z L  represents the load seen by the buffer output. As will be described later, Z L  includes load contributions from the bus, from the three other buffer modules, and from the Din0 port of buffer 6-0 itself. 
     As shown in FIG. 2A, buffer 6 may be implemented with bipolar transistors, complementary metal-on-semiconductor (&#34;CMOS&#34;) transistors, or a combination of each (&#34;BiCMOS&#34;). Buffer 6 typically will include two inverters I1 (here a NAND gate) and I2 coupled in series, or I3 (here a NOR gate) and I2 coupled in series. The output of the first inverter is presented as input to the second inverter, and the output of the second inverter is the buffer output, which has the same phase as the input to the first inverter. 
     In the CMOS implementation of FIG. 2A, each inverter comprises a P-type pull-up metal-on-semiconductor (&#34;PMOS&#34;) transistor and an N-type MOS (&#34;NMOS&#34;) transistor coupled in series between Vdd and Vss. For example, I1 may comprise a PMOS transistor P1 (not shown) and an NMOS transistor N1 (not shown), I2 comprises PMOS transistor P2 and NMOS transistor N2, and I3 comprises transistors P3, N3 (not shown). Because I2 drives a relatively large load, output transistors P2 and N2 will generally be larger sized devices than the transistors comprising I1 or I3. 
     The arbitration or enabling function may be implemented using the NAND gate (I1), INVERTER and NOR gate (I3) logic shown, or using other techniques well known to those skilled in the relevant art. 
     When Dout0 is a digital &#34;1&#34;, within I1 transistor P1 turns off and N1 turns on, and the first inverter output is a digital &#34;0&#34;. Upon receipt of this &#34;0&#34;, in the second inverter I2, P2 turns on, N2 turns off, and the signal Din0 will be a digital &#34;1&#34;, and buffer 6-0 sources current into bus 2. When Dout0 is a digital &#34;0&#34;, P1 turns on, N1 turns off, and the output from the first inverter is a &#34;1&#34;. Upon receipt of this &#34;1&#34;, P2 in the second inverter turns off, N2 turns on, signal Din0 is a &#34;0&#34;, and buffer 6-0 sinks current from bus 2. 
     FIGS. 2B-1 through 2B-4 depict voltage and current waveforms associated with output buffer 6-0. For example, although Douto is &#34;1&#34; before time t 0 , it is only after the enabling ARB-0 signal goes high that buffer 6-0 is enabled to provide the Din0 output signal. At time t 1 , Dout0 goes low and, since ARB-0 is still enabling buffer 6-0, the Din0 signal also goes low. In the Din0 waveform, the voltage waveform drawn in phantom represents the case of a relatively large load capacitance C L . When C L  is not especially large, the output voltage waveform slews more rapidly, but can overshoot and undershoot as shown. 
     It is thus appreciated from the Din0 waveform that as C L  increases, the output voltage slew rate (Dv/dt) decreases. To compensate for this, it is necessary to implement buffer 6-0 with larger output inverter transistors that can source or sink more current (i). (Of course, this assumes that the IC containing buffer 6 has sufficient area whereon to fabricate larger transistors.) The ability to compensate for a large C L  by increasing output buffer current follows from the equation: 
     
         i=C.sub.L ΔV/Δt 
    
     Although large current handling transistors can improve output voltage slewrate, a large current capability can be detrimental. In practice, buffer 6-0 will not function perfectly because the various pull-up and pull-down transistors do not change states in perfect synchronism. The output buffer current waveform depicts the total current i o  flowing through buffer 6-0. The i o  current waveform drawn in phantom represents total current drawn by the buffer when the various buffer transistors are themselves large devices, e.g., devices with a relatively large drain current. 
     Note from this waveform that current spikes occur when the buffer transistors change states, for example at times t 0  and t 1 . These spikes are created because for a brief moment, the PMOS and NMOS transistors in each inverter are simultaneously on, thus presenting a low impedance current path between the Vdd and Vss power supplies. In addition, current spiking occurs because the load capacitance C L  component of Z L  is being charged toward Vdd or discharged toward Vss (depending upon the direction of the output state change). 
     Thus, the i o  waveform in FIG. 2B-4 suggests that compensating for a large load capacitance C L  by implementing buffer 6-0 with large current transistors will aggravate current spiking. Those skilled in the art will appreciate that the current spiking waveforms can contain many high frequency components that represent electromagnetic (&#34;EM&#34;) and radio frequency (&#34;RF&#34;) noise that can interfere with other signals implemented on the IC containing buffer 6, and with signals elsewhere in a system contain this IC. 
     It will be appreciated from the foregoing that the use of tristate buffers 6 presents many problems. Although the configuration of FIG. 1 is commonly used in fully customized integrated circuit chips, this configuration aggravates current spiking and the need for fabricating relatively wide bus 2 metal traces. In practice, the width of the bus 2 metal will be in the range of about 3 μm. In some applications, having to provide a sufficiently wide metal bus trace may compromise the layout of other portions of the IC due to space considerations. 
     The configuration of FIG. 1 is not point-to-point in that each buffer module is always coupled to more than one other buffer module, e.g., to three other buffer modules. As will now be described, this causes each buffer to see a substantial load impedance Z L , with resultant degradation of signal voltage slewrate. 
     Assume for example that ARB-0 enables buffer 6-0, and that ARB-1, ARB-2, ARB-3 disable buffers 6-1, 6-2, 6-3. The load Z L  seen by the enabled (e.g., turned-on) buffer 6-0 includes (a) the metal trace bus 2, (b) whatever is coupled to Din0, (c) the output impedance capacitance of each of the other three disabled (e.g., turned-off) buffers, and (d) the Din1, Din2, Din3 loads contributed by each of the other three input buffers. The resultant load is the metal trace load and seven buffer loads. Since the metal trace typically is equivalent to about twelve buffer loads, the turned-on buffer must drive approximately sixteen equivalent loads. One standard equivalent load is about 0.032 Pf, which is to say that 31.3 standard equivalent loads represent approximately 1.0 pF. 
     Unfortunately, if the tristate buffers are to drive sixteen equivalent loads and still provide output Din signals having a sufficiently rapid voltage slewrate, the buffer current i o  must be increased. This in turn requires larger-sized buffer transistors, and can increase current spiking and noise generation. 
     It is also apparent from FIG. 1 and FIG. 2A that no more than one output enabling ARB signal may be on (e.g., &#34;1&#34;) at any time. Any overlap in time between enabling signals, or &#34;arbitration contention&#34;, can cause one turned-on buffer to attempt to drive a very low impedance load that includes another turned-on buffer, and vice verse. The resultant high current flow will usually damage if not destroy IC 4. 
     Testing prior art tristate buffer configurations such as shown in FIG. 1 is extremely challenging, and generally cannot be accomplished using conventional automatic testing routines and equipment. It is very difficult for conventional testing routines to determine which of a group of tristate buffers is actually driving the bus at a given time. Further, conventional test routines cannot detect the occurrence of contention with any great certainty. Stated differently, to successfully test the configuration of FIG. 1, it is necessary to demonstrate that contention can never occur. The testing procedures and equipment necessary to demonstrate this are difficult to implement. 
     For example, although scanning test protocols are commonly used to rapidly test ICs, such routines cannot be used with tristate buffer configurations such as shown in FIG. 1. In such testing, the various flipflops within an IC are temporarily coupled together in a ring and known data patterns are passed through the ring. Unfortunately, when tristate buffers are present, random output drive signals become propagated through the ring, introducing uncertainty and, what is worse, contention into the test procedure. 
     To summarize, there is a need for an on-IC bus structure that avoids the contention and testing problems associated with prior art tristate buffer configurations. It should be possible to fabricate such a structure using IC and metal trace areas not exceeding what would be required to implement a tristate buffer bus configuration. Most preferably, such structure should be extendable to hierarchies of at least three-dimensions to better accommodate a larger number of nodes without using tri-state buses. 
     The present invention provides such a multidimensional hierarchial bus structure. 
     SUMMARY OF THE PRESENT INVENTION 
     In applicant&#39;s above-referenced patent, a two-dimensional point-to-point ring bus structure was formed without using tri-state buses. M (an integer≧2) X:1 multiplexer modules were used in which each module was associated with an input/output port that could communicate with the bus. Each module had an output port (Dout), and arbitration (&#34;ARB&#34;) port, and X input ports (&#34;LOCALout&#34;, &#34;Din1&#34;, &#34;Din2&#34;, . . . &#34;Din X-1!&#34;). The Dout output port of an M i  module was coupled, via a portion of conductive bus, to  X-1! input ports on an adjacent D i+1  module. Thus, module M 0  &#39;s Dout 0  output port was coupled to  X-1! input ports on module M 1 , module M 1  &#39;s Dout 1  port was coupled to  X-1! input ports of module M 2 , and so forth. 
     The modules were X:1 in that the output port of each module was coupled to a chosen one of that module&#39;s X INPUT ports, as determined by the state of an arbitration select signal (ARB) coupled to the module&#39;s arbitration port. The state of the arbitration select signals defined a bus signal path between the LOCAL out  input port of a module coupled to the bus, and the D in  input ports of other modules. 
     The described point-to-point configuration presented smaller equivalent loads to module outputs, permitting low module current operation and narrower width bus metallization traces. Because large current handling output transistors were unnecessary, multiplexer modules did not incur thermal damage if contention-type overlap occurred between ARB signals, and automatic testing including scan generation testing could be used. 
     However for relatively numbers of nodes, greater interconnect flexibility may be attained by providing a three-dimensional (or greater) hierarchial interconnect configuration. Nodes are defined on at least first and second &#34;horizontal&#34; (or &#34;H&#34;) rings, the rings being coupled by at least one &#34;vertical&#34; (or &#34;V&#34;) ring. Each node is identified in terms of its (H,V) coordinates in the hierarchial interconnect structure, and an M-dimensional structure will provide an M-way multiplex unit at each node. For an M=3, e.g., three-dimensional structure, each multiplex unit has three-inputs, a Localout, a Vin, and an Hin input, and couples one of these inputs to an output port in response to a Local select arbitration signal. The output signal is coupled to Hout and Vout, and to Localin. Nodes on the same horizontal level will drive their Hin signal to Vout and Hout, whereas all other nodes receive the Vin signal. The arbitration select signals may reconfigure the overall structure dynamically or statically, preferably according to demand of the nodes required interconnection. Providing additional vertical rings provides redundancy and can reduce latency time. 
     Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail, in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts an on-chip bus configuration using tristate buffer modules, according to the prior art; 
     FIG. 2A depicts a generic tristate buffer module, according to the prior art; 
     FIGS. 2B-1 through 2B-4 depict voltage and current waveforms for the tristate buffer module of FIG. 2A; 
     FIG. 3A depicts an on-chip two-dimensional ring bus structure using multiplexer modules; 
     FIGS. 3B-1 through 3B-4 depict voltage waveforms for a multiplexer module as shown in FIG. 3A; 
     FIG. 4 depicts a generic multiplexer module, such as used in FIG. 3A; 
     FIG. 5A is a schematic depiction of a bus interface multiplexer module and node, such as used in the two-dimensional ring bus structure of FIG. 3A; 
     FIG. 5B is a generic depiction of the two-dimensional ring bus structure of FIG. 3A; 
     FIG. 6A is a schematic depiction of a nodal bus interface multiplexer module and node used in a three-dimensional ring bus structure; 
     FIG. 6B is a generic depiction of a three-dimensional ring bus structure; 
     FIG. 7A is a schematic depiction of a nodal bus interface for each node in a full three-dimensional bus structure configured as four rings; 
     FIG. 7B is a generic depiction of a three-dimensional bus structure configured as four rings; 
     FIG. 8A depicts the nodal bus interface for each connection node N(x,0) in a partial three-dimensional bus structure; 
     FIG. 8B is a generic depiction of a partial three-dimensional bus structure, used with the nodal bus interface of FIG. 8A; 
     FIG. 9A depicts a multi-node bus configuration in which functional a subset of nodes communicates between a small number of groups; 
     FIG. 9B depicts the addition of a second vertical ring to the configuration shown in FIG. 9B to reduce latency. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3A depicts a single-bit ring bus 102 formed on an integrated circuit chip 104 that may be an ASIC that includes circuitry formed on sub-sections. In the embodiment of FIG. 3A, ring bus 102 includes M=4 two-way (e.g., X=2, or 2:1) multiplexer modules 106-0, 106-1, 106-2 and 106-3. Each multiplexer module defines an I/O node that may be coupled through the bus to one or more sub-sections or circuits on IC chip 104. In an IC with an N-bit wide bus, what is shown in FIG. 3A would be replicated N times, with each ARB signal coupled to N multiplexer modules. 
     In the embodiment shown, each multiplexer module has two input ports, and output port, and an arbitration port. The multiplexer modules are two-way in that the multiplexer output port is coupled to a chosen one of the two input ports, the coupling being determined by the state of the signal coupled to the arbitration port. 
     For example, multiplexer 106-0 has a first input port coupled to receive an I/O signal Din0 that is provided as an output (Dout1) by multiplexer 106-1. Multiplexer 106-0 has a second input port coupled to receive an I/O signal LOCALout0 that may be coupled to one or more subsections or circuits on the IC chip 104. Multiplexer 106-0 also has an arbitration port coupled to receive an arbitration signal ARB-0, and an output port that couples an output signal Dout0 to an input port on an adjacent multiplexer, here module 106-3. 
     As shown by the voltage waveforms in FIGS. 3B-1 through 3B-4, in the preferred embodiment, when ARB-0 is a digital &#34;1&#34;, Dout0 is Din0, and when ARB-0 is a digital &#34;0&#34;, Dout0 is LOCALout0. FIG. 4 depicts a generic implementation of multiplexer module 106-0, as comprising two NMOS field effect transistors N4, N5 and an inverter I4. Of course other implementations could be used, including implementations that invert the polarity of the arbitration signal result in Dout being Din0 when ARB-0 is a &#34;0&#34;, and being LOCALout0 when ARB-0 is &#34;1&#34;. 
     While the embodiment shown in FIG. 4 represents a unidirectional multiplexer unit, those skilled in the art will recognize that a bidirectional multiplexer unit could instead be provided. The use of bidirectional multiplexer units would permit a ring bus structure according to the present invention to operate bidirectionally. Thus, with respect to FIG. 3A, a signal coupled to D IN  0 might be passed across the ring to LOCAL OUT  0, or a signal coupled to LOCAL OUT  0 might be passed across the ring to D IN  0. 
     At this juncture, similarities and differences between a multiplexer module ring bus 102 configuration according to FIG. 3A, and a tristate buffer module configuration according to FIG. 1A will be pointed out. 
     The configurations are similar in that a chosen I/O node associated with one module may be coupled to another node associated with another module, according to the state of the arbitration select signals. In FIG. 3A, for example, if ARB-0 is &#34;1&#34;, and ARB-1, ARB-2, and ARB-3 are &#34;0&#34;, a Din0 signal will pass through MUX 106-1, MUX 106-2 and MUX 106-3, and will appear at the LOCALout0 node of MUX 106-0. As shown in FIG. 3A, the same Din0 signal will also be present at the nodes Din1, Din2 and Din3. As such, bus 102 is a ring, as indicated in FIG. 3A. 
     In contrast to the prior art, however, the ring bus configuration of FIG. 3A is point-to-point in that the output of one module is coupled only to one other module. For example, the output from module 106-0 sees the Din3 input of module 106-3, and nothing more. The internal circuitry associated with each multiplexer module isolates the module inputs from the load impedance coupled to the module output. 
     As such, the output transistors within each multiplexer module see a smaller Z L  load than is seen by the output transistors in a tristate buffer module in the prior art. Whereas the output impedance seen by the prior art tristate buffer configuration of FIG. 1 was about sixteen equivalent loads, the output impedance seen by a multiplexer module as described herein is only about four equivalent loads. Typical signal transition time for a 2:1 multiplexer module will be about 500 ps, e.g., about twice as fast as a prior art tristate buffer. 
     Because the multiplexer modules are less heavily loaded, they can operate with a rapid voltage slewrate using decreased output current (since C L  ΔV/Δt is decreased). As a result, the metal trace that forms the bus path between multiplexer modules may be of narrower width for a given resistivity relative to the width of a metal bus trace used in prior art FIG. 1. For example, a typical metal trace width used to implement FIG. 3A may be only 0.8 μm. The ability to use narrower metal traces advantageously provides greater flexibility when designing the IC chip layout, and can reduce the capacitive load associated with a wider metal trace. 
     Even if the sum of the operating current required by the four multiplexer modules shown in FIG. 3A should equal or even exceed the operating current required by a single enabled single prior art tristate buffer module, the present invention is still advantageous. In the present invention, the drive current is distributed among the various multiplexer modules, whereas in the prior art, all of the drive current is provided by the one enabled tristate buffer. As a result, it is easier to fabricate lower current multiplexer modules than large current handling tristate buffer modules. 
     Contention per se is not a problem with the present invention. In FIG. 3A, even if more than one ARB arbitration signal is somehow simultaneously on (e.g., &#34;1&#34;), neither the selected multiplexer modules nor IC 104 is damage. This is in contrast to what can result with the prior art configuration of FIG. 1, wherein two (or more) high output current tristate buffer modules may attempt to drive each other. 
     For the described embodiments, even if quasi-contention results in the simultaneous selection of two or more multiplexer modules, the proper &#34;0&#34; or &#34;1&#34; state of the bit coupled to bus 102 may be erroneous, but thermal damage to the buffer modules or the IC would not occur. In a multi-bit bus configuration, quasi-contention might corrupt one or more bits (e.g., a &#34;0&#34; might become a &#34;1&#34; or vice versa) but damage to the IC would not necessarily occur. 
     Because contention or quasi-contention does not present a problem that can damage or destroy IC 104, the described configurations may readily be tested using conventional test protocols and test equipment. For example, scanning test protocols may be used to rapidly test IC 104, in contrast to techniques that must be used to attempt to test prior art IC 4. 
     Although the preferred embodiment shown in FIG. 3A depicts four 2:1 multiplexer modules, as few as two 2:1 multiplexer modules may be used to implement a bus. Ring topology buses used in networks may, for example, use as many as twenty or more 2:1 multiplexer modules. Further, X:1 switching modules may be implemented other than as conventional multiplexer units. 
     As noted, the use of 2:1 (e.g., X=2) multiplexers per node results in the formation of a single data ring. However, for larger buses with many loads, it may be advantageous to use 3:1 (or arbitrarily X:1) multiplexers. Increasing X from 2 to 3 will increase the load per multiplier, as each multiplexer will then have to drive the node nearest it in two dimensions. However, increasing X will decrease the total number of elements in any one bus ring. 
     An example of such an arrangement would be a bus with 16 nodes, effectively the configuration of FIG. 3A, repeated four times, with vertically oriented ring buses passing through each node. While each node would then drive two loads (e.g., the adjacent load in the same plane, and the adjacent load &#34;above&#34;), the farthest distance between two loads decreases. The decrease is from 15 nodes for a single ring bus, to 6 nodes, namely three nodes in the horizontal plane and three nodes in the vertical plane. 
     Thus, in general, an X:1 multiplexer module configuration will define an X-dimensional cube. For the 2:1 multiplexer module configuration of FIG. 3A, X=2 and a two-dimensional planar configuration is defined. Where X=2, the output of each multiplexer module is coupled to  X-1! or one input on an adjacent module. If X=3 (e.g., if 3:1 multiplexer modules are used), a three-dimensional cube configuration would be realized. In a three-way configuration, each multiplexer output (Dout) would fanout to  X-1! or 2 inputs on other of the modules. 
     To recapitulate, the use of multiplexer modules enables bus structures to be easily implemented with relatively low current-handling multiplexer transistors. The IC chip area required to implement the described embodiments does not exceed the chip area required to implement a conventional tristate buffer configuration having the same number of input/output nodes. Because each multiplexer module sources or sinks relatively little current, the metal trace used to implement the ring bus shown in FIG. 3A may be thinner than would be the case for the bus shown in prior art FIG. 1. Because contention damage is not present, the described embodiments lend themselves to rapid testing using standard test techniques and equipment, including scanning. 
     Before describing three-dimensional and greater bus configurations for the present invention, it is helpful to adopt a shorthand nomenclature. FIG. 5A describes a generic node N as including, in this example, an X:2 (here X=2) multiplexer unit 106-N, whose input nodes are here denoted Local out  N, D in  N, whose output node is denoted D out  N, and whose arbitration node is denoted as Local Select N (or ARB N). Note that the D in  N input signal is shown exiting node N as a signal Local in  N+1. The similarity between node N in FIG. 5A and any of the nodes in FIG. 3A will be apparent. FIG. 5B generically depicts a four-node two-dimensional ring bus structure 102, similar, for example, to what was shown in FIG. 3A. 
     FIGS. 6A and 6B depict a generic three-dimensional node N(H,V) and a generic three-dimensional structure, respectively. The nomenclature N(H,V) denotes a node (&#34;N&#34;) coupled horizontally (&#34;H&#34;) and vertically (&#34;V&#34;) to ring structures. In FIG. 6B, for example, four &#34;horizontal&#34; ring structures 102-H0 (e.g., level-0 horizontal), 102-H1, 102-H2, 102-H3 (e.g., level-3 horizontal) are shown, as are four &#34;vertical&#34; ring structures 102-V0 (e.g., level-0 vertically), 102-V1, 102-V2, 102-V3 (e.g., level-3 vertically). In the embodiment shown, four nodes (&#34;N(H,V)&#34;) are associated with each ring bus structure, each individual node being associated with a &#34;horizontal&#34; and with a &#34;vertical&#34; ring structure. Although the &#34;4×4&#34; structure of FIG. 6B is symmetrical, the present invention does not require that the hierarchy be either symmetrical, or the same on each layer level. 
     It is understood that although the configuration of FIG. 6B could be implemented using multi-level printed circuit boards or the like, the &#34;horizontal&#34; and &#34;vertical&#34; denominated ring structures may be considered as virtual or logical ring structures. As such, &#34;horizontal&#34; ring structure 102-H0 need not physically lie above and/or be &#34;horizontally&#34; spaced-apart from &#34;horizontal&#34; ring structure 102-H1. It suffices if such ring structures be logically definable, independently of the physical implementation. 
     Thus, node N(0,1) is connected to the zero-th horizontal ring 102-H0, and to the first vertical ring 102-V1, node 3,0 is connected to the third horizontal ring 102-H3 and to the zero-th-vertical ring 102-V0, and so on. A source node drives its Localout signal to Vout and to Hout, as shown in FIG. 6A. Nodes on the same level drive Hin to Vout and Hout, whereas all other nodes receive Vin. 
     Through proper generation of arbitration or local selection logical signals to the X:1 (X=3) multiplexer units 106-N shown in FIG. 6A, any node can be coupled to any node in the structure shown. At least about ten relatively straightforward point-to-point node paths may be dynamically defined. 
     FIGS. 7A and 7B depict the nodal bus interface and structure for a full three-dimensional bus when configured as four rings. Multiplexer 106-N in FIG. 7A is a x3 MUX unit, its inputs being Dout-N, Vin, and Hin, its select signal being ARB-N, and its output being coupled to Local in, to Hout and to Vout. 
     Note in FIG. 7B that the &#34;vertical&#34; rings are shown in phantom to indicate that the overall bus structure may be broken-up in different ways, dynamically or statically. For example, if Vin is ignored, the resultant structure has four rings, here denoted 102-H0, 102-H1, 102-H2 and 102-H3. On the other hand, if Hin is ignored, the resultant structure would have four vertical rings, drawn in phantom, 102-V0, 102-V1, 102-V2, and 102-V3. 
     Appropriate multiplexer signals can cause the structure of FIG. 7B (or other structures according to the present invention) to reconfigure. Such reconfiguration may be made on a timing clock cycle-by-cycle basis. For example, a 16-bit wide bus might dynamically (or statically) be reconfigured to be four horizontal 4-bit busses, and then be reconfigured to be four vertical 4-bit busses. Other reconfiguration orders could be adopted, using structures with more than 4×4 hierarchy potential, and with more nodes defined per level. Thus, in general, the present invention permits a large bus to be reconfigured to a smaller number of vertical busses or to a smaller number of horizontal busses. 
     FIGS. 8A depicts the nodal bus interface for each connection node N(x,0) in the partial three-dimensional bus structure shown in FIG. 8B. Note that the redundancy in FIG. 8B may be reduced by using a single horizontal ring between vertical rings. This is relatively easy to fabricate in hardware using fewer wires, but unfortunately latency time is increased. 
     Compare the configuration of FIG. 8B with that of FIG. 7B in traversing from node N(0,1) to node N(3,3). In FIG. 7B, four nodes were involved in the traverse:  N(0,1)→N(0,2)→N(0,3)→N(1,3)→N(2,3)→N(3,3)!. In FIG. 8B, there are fewer potential horizontal connections, and node N(0,1) must transfer its date to node N(0,0) as follows,  N(0,1)→N(0,2)→N(0,3)→N(0,0!. From there, data is transferred to N(3,0) along the path defined by  N(0,0)→N(1,0)→N(2,0)→N(3,0)!. Finally, data passes to N(3,3) along a path  N,3,0)→N(3,1)→N(3,2)→N(3,3)!, a total of eight nodes being involved in the traverse. While the above-described path can be broken into four separate vertical rings, the presence of only one horizontal ring limits flexibility of configuration. It is to be understood that for ease of fabrication and layout, a &#34;vertical&#34; ring may in fact be replaced with a tristate bus connecting the various &#34;horizontal&#34; rings. 
     Consider a bus structure in which there are many nodes, but where functionally a first group of nodes communicates primarily with a second group of nodes, but where occasionally a different communication configuration is desired. The structure of FIG. 9A lends itself to such a group-to-group application, wherein a first group of eight nodes is configured with a first horizontal ring 102-H0, and wherein a second group of eight nodes is configured with a second horizontal ring 102-HX. A single vertical ring, 102-V0, connects the two horizontal rings. To reduce latency, at least one additional vertical ring, here 102-V1 (shown in phantom), may be added. 
     FIG. 9B depicts a more complex topology to permits groups of nodes to better communicate with other groups of nodes. In FIG. 9B, nodes associated with a first horizontal ring 102-H0 can communicate readily with a second group of nodes associated with a second horizontal ring 102-H1. Communication between the first and second horizontal rings is achieved using preferably two vertical rings 102-V0 and 102-V1, to reduce latency. Communications between groups of nodes on the second and the third horizontal ring (102-H2) is facilitated by vertical rings 102-V2, 102-V3, two such rings preferably be used to reduce latency. Communication between groups of nodes on the third and fourth horizontal rings, 102-H2, 102-H3 is similarly implemented. 
     For ease of illustration, the configurations of FIGS. 6B, 7B, 8B, and 9B have been shown implemented with x3 input MUX units. As depicted, the resultant bus configurations are truly three-dimensional. If desired, x4 or even higher input MUX units could be used, to implement four-dimensional (or higher) bus structures. 
     Modifications and variations may be made to the disclosed embodiments without departing from the subject and spirit of the invention as defined by the following claims.