Abstract:
N stage tree-type mutliplexers having multiple selects and associated processes for configuring the same are disclosed. The basic multiplexer has control signals which are disbursed throughout the tree for high performance multiplexing. Control signals are distributed such that different signals control at least one stage of the N stage tree and such that the signals controlling the selectors in each of the plurality of selector paths from the input stage to the output stage of the tree are unique. As an enhancement, circuitry for buffering the control signals provided to the input stage of the tree can be used to further reduce the capacitive load thereon.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates in general to multiplexer circuits and, more particularly, to tree-type multiplexers having multiple selects wherein selector control signals are dispersed for high performance multiplexing, and to associated processes for configuring such multiplexers. 
     2. Description of the Prior Art 
     Multiplexers of different type are known in the art. One specific type of multiplexer is a tree arrangement such as disclosed in several earlier U.S. patents, e.g., U.S. Pat. No. 3,614,327, entitled &#34;Data Multiplexer Using Tree Switching Configuration,&#34; and U.S. Pat. No. 3,654,394, entitled &#34;Field Effect Transistor Switch, Particularly for Multiplexing.&#34; Another tree approach to multiplexing is to combine multiple data selects (such as the one of two select depicted in FIG. 1, having the logical function set forth in FIG. 2) in a layered configuration such as that depicted in FIG. 3. The multiplexer of FIG. 3 comprises a decoder wherein a particular data input from the thirty-two inputs X 0  14 X 31  is selected for output on line R by the signals appearing on control lines A 0  -A 4 . This type of multiplexer has a significant advantage over other multiplex implementations in that the control lines A 0  -A 4  are much closer (in terms of logic depth) to the output and, therefore, provide a faster control path than other types of decode devices, such as a conventional AND function with decode control logic to drive the inputs. The AND function with decode control logic approach places loads on the control lines at a rate proportional to the number of data being selected. 
     The inherent drawback to tree-type multiplexing is that the approach also suffers from heavy loading conditions on the control lines. Loading within such structures typically increases at a rate proportional to the number of input data being selected. For example, in FIG. 3, control line A 0  is loaded with one data selector(s), line A 1  with two data selectors, line A 2  with four data selectors, line A 3  with eight data selectors, and line A 4  with sixteen data selectors. This significantly higher load on control line A 4  limits performance of the circuit. The disadvantage becomes more pronounced as the number of stages of multiplexed data selects increases, such as typically encountered with RAMs and ROMs. The present invention is designed to address this loading problem inherent in existing tree-type multiplexers. 
     SUMMARY OF THE INVENTION 
     Briefly summarized, a method and structure for reducing peak loading on the control signals of an N stage tree-type multiplexer are provided. The tree-type multiplexer has an input stage, an output stage and a plurality of data flow selector paths therebetween. In a basic embodiment, the method includes the steps of: distributing the control signals such that different signals control at least one stage of the N stage tree and such that the signals controlling the selectors in each of the plurality of selector paths from the input stage to the output stage are unique; and identifying data inputs to the first input stage of the N stage tree-type multiplexer using the distributed control signals. 
     In another basic embodiment, the invention comprises a method for fabricating a multiplexer from a plurality of selects. Each select has at least two data inputs, a control signal input and an output. The fabrication method includes connecting the multiple selects in a tree-type configuration of N layered stages. The selects are interconnected such that the output of each select in stage i is fed to an input of a select in stage i+1, wherein i=1 . . . N-1. Further, stage i=1 of the configuration comprises an input stage and stage i=N of the configuration comprises an output stage. A plurality of selector paths are defined by the interconnected selects between the input stage and the output stage. The method further includes distributing control signals to the tree-type configuration such that different signals control at least one stage of the N stages and such that the signals controlling the selectors in each path for each of the plurality of selector paths from the input stage to the output stage are unique. 
     In a more specific embodiment, the method includes the steps of: partitioning the tree-type multiplexer into multiple sections; assigning from a plurality of multiplexer control signals an arbitrary control signal to control the output stage of the tree; assigning a unique control signal to each partitioned section in the input stage of the tree such that each of the unique control signals is different from the signal arbitrarily assigned to the output stage; and assigning a control signal to each section in the remaining stage of the N stage tree-type multiplexer such that the control signals assigned in each selector path of the tree are unique from input to output. As an enhanced processing step, the method can include identifying specific data inputs at the input stage of the multiplexer using the assigned control signals. In addition, control signal loading can be further reduced by buffering the unique control signals supplied to the input stage of the tree for subsequent application to the stages between the input stage and the output stage. The buffering delay of input stage control signals is commensurate with the delay through the selectors of the first stage in order that the control signals for the second stage through stage N-1 arrive at the appropriate selectors while the input data is being passed therethrough. 
     In yet another aspect, the invention comprises a novel tree-type multiplexer structure. The structure has a plurality of selects distributed in N interconnected stages. The interconnected stages form a pyramid structure which has an input stage, an output stage and a plurality of data flow selector paths therebetween. The input stage has a plurality of data inputs. A plurality of control lines are connected to the selectors such that at least one stage of the N stages is controlled by different control lines and such that each control line controlling the selectors in each of the plurality of selector paths between the input stage and the output stage are unique. As an enhancement, buffers can be provided to further reduce the loading on the plurality of control lines by duplicating each of the unique control signals applied to the input stage of the tree which can then be used to control subsequent selector stages. 
     The tree-type multiplexer and methods of fabrication disclosed herein define a structure which significantly improves upon the performance of conventional tree-type multiplexers, without changing the logical characteristics thereof. Improved performance is obtained by reducing the capacitive loads on the control logic through various signal distribution techniques. The multiplexer and fabrication methods can be used in any circuitry where high performance multiplexing is required, including data flow elements, RAMs, ROMs and/or control logic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, advantages and features of the present invention will be more readily understood from the following detailed description of certain preferred embodiments thereof, when considered in conjunction with the accompanying drawings in which: 
     FIG. 1 depicts a one of two select used in a tree-type multiplexer configuration in accordance with the present invention; 
     FIG. 2 is a table of the logic function of the one of two select of FIG. 1; 
     FIG. 3 is a schematic of a prior art example of a tree-type multiplexer using the select of FIG. 1; 
     FIG. 4 is a flowchart of one processing embodiment of the present invention; 
     FIG. 5 is a schematic of a tree-type multiplexer containing one of two selects partitioned into sections in accordance with one embodiment of the present invention; 
     FIG. 6 is a schematic of a tree-type multiplexer containing one of two selects wherein control signals are distributed through the tree and input data is assigned based thereon, all in accordance with the present invention; and 
     FIG. 7 is a schematic of an alternate embodiment of a tree-type multiplexer containing one of two selects having control signals and data inputs distributed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Broadly stated, the present invention comprises a tree-type multiplexer and associated methods of implementation in which peak loading on the control signals to the multiplexer are reduced such that no one signal experiences a significantly greater capacitive load than other signals. Along with distributing the load, the data inputs to the tree-type multiplexer can be reassigned using the distributed control signals. One specific technique for implementing the tree-type multiplexer of the present invention is set forth in the flowchart of FIG. 4. 
     Pursuant to this detailed example, the first step is to partition the tree into P sections, 20 &#34;Partition the Tree.&#34; The variable P is defined as an integer which is the maximum power of two which is less than or equal to N-1 (wherein N=the number of control signals or address lines). Using this criteria, a table such as Table 1 can be created to identify the number of partition sections required for various numbers of control signals. 
     
                       TABLE 1______________________________________  N  Control         P  lines  Sections______________________________________  2      1  3      2  4      2  5      4  6      4  7      4  8      4  9      8  10     8  11     8  12     8  13     8  14     8  15     8  16     8  17     16  .      .  .      .  .      .______________________________________ 
    
     By way of example, since the multiplexer of FIG. 3 has five control lines A 0  -A 4 , the tree is partitioned into four sections by the criteria of step 20. An appropriately partitioned multiplexer is depicted in FIG. 5 wherein phantom lines represent partition lines. In the example provided, partitioning to arrive at the desired number of sections is accomplished by repeatedly dividing from input to output subsequent sections of the tree in half (as shown in FIG. 5). In an alternate embodiment, partitioning can be accomplished separately for each stage of the tree, for example, when control signals are to be assigned to the selectors thereof. 
     Next, a control signal is assigned to the Nth control stage, 22 &#34;Assign Control Signal to Control Stage N.&#34; The tree has a pyramid structure such that control stages narrow from an input stage (stage 1) to an output stage (stage N), which in the example depicted is stage 5. The control signal assigned to control stage N is arbitrary, and can comprise any one of signals A 0  -A 4 . 
     The third step is to arbitrarily assign from the remaining group of control signals a unique control signal to each partitioned section of stage 1, 24 &#34;Assign Control Signals to Control Stage 1.&#34; In other words, the control signals are assigned to the various stage 1 sections such that no data flow path from the input stage to the output stage of the tree uses the same control signal to control more than one selector(s). 
     After instruction 24, an index value &#34;i&#34; is assigned the number 2, which represents the second control stage, 26 &#34;i=2.&#34; Thereafter, control signals are arbitrarily assigned to each partitioned section of control stage &#34;i&#34;, again such that no path in the tree from the input stage to the output stage uses the same address line as a control more than once, 28 &#34;Assign Control Signals to Control Stage i.&#34; Inquiry is then made whether control stage &#34;i&#34; comprises the stage N-1 in the tree such that all stages have been assigned control signals, 30 &#34;i=N-1? (All Stages Assigned?).&#34; If &#34;no&#34;, value &#34;i&#34; is incremented, 32 &#34;i=i+1,&#34; and return is made to junction 27 and hence instruction 28 wherein control signals are assigned to the new, incremented stage. 
     If all control stages have been assigned a control signal, then from inquiry 30 the method requires that an index value &#34;j&#34;, representative of the data input, be indexed to zero (i.e., X 0 ), 34 &#34;j=0,&#34; after which the location of data input &#34;j&#34; is assigned based on the control signals which have been distributed throughout the multiplexer, 36 &#34;Assign Data Input j.&#34; Thereafter, inquiry is made whether all data inputs have been assigned, 38 &#34;j=Q-1? (All Inputs Assigned?)&#34;. Assuming that all data inputs have not been assigned, index &#34;j&#34; is incremented, 40 &#34;j=j+1,&#34; and return is made to junction 35 and hence instruction 36 where the next data input is assigned. 
     Again, assigning a data input involves ascertaining an input location using the value of the control signals for the selected data input and the distributed tree. For example, FIG. 6 depicts a distributed tree structure wherein input X 17  is conventionally selected in binary with control signals A 0  -A 4  equal to 10001. The input is identified by tracing a sensitized path from the output stage to the unique input of the input stage associated with this setting of the control signals. The same process continues until all data inputs have been assigned. Once the inputs are assigned, the distribution method has been completed and processing is terminated, 42 &#34;End.&#34; 
     By way of more detailed process explanation, step 1 of the flowchart of FIG. 4 requires that the tree be partitioned into P sections. Using the tree of FIGS. 5 &amp; 6 as an example, since there are five control signals (A 0  -A 4 ) the partition formula dictates that four partition sections be created in the tree (FIG. 5). Next, the Nth control stage, stage 5, is arbitrarily assigned control signal A 0 . Thereafter, signals for the uppermost control stage, stage 1, are arbitrarily assigned from the remaining signals such that each partitioned section is assigned a unique control signal (FIG. 6). The control signals are arbitrarily chosen as A 4 , A 2 , A 3 , and A 1  for the left to right partition sections, respectively. According to the process, signal A 0  may not be used as a stage 1 section control since it has already been assigned to the Nth stage of the tree and, therefore, is already associated with every data flow path of the tree from the input stage to the output stage. 
     Control signals for the remaining control stages of the tree are then assigned, again such that no control signal influences more than one circuit in any input to output path of the tree. This allows signal A 1  or A 3  to be chosen for the leftmost selector and signal A 2  or A 4  to be chosen for the rightmost selector in stage 4. Signals A 1  and A 4  are arbitrarily chosen in the example of FIG. 6. The process similarly repeats for stage 3 and stage 2, again such that no control signal influences more than one selector in a path of the tree. Note that this criterion leaves no choice with respect to the assignment of control signals to the last stage in the logic tree. Also, note that the order of assigning control signals to the stages subsequent assignment of stage N and stage 1 is immaterial. 
     The last step in the process is to identify data inputs with specific input terminals using the distributed tree. For example, data input X 28  is represented in binary as 11100 such that the control signals are A 0  =1, A 1  =1, A 2  =1, A 3  =0 &amp; A 4  =0. Following this control sequence, the particular input for X 28  can be identified as the input location depicted in FIG. 6. Specifically, since A 0  =1, the right input to the select of stage 5 is sensitized; for A 4  =0, the left input of the corresponding select in stage 4 is sensitized; for A 2  =1, the right input to the corresponding select in stage 3 is sensitized; for A 1  =1, the right input to the corresponding select in stage 2 is sensitized; and, finally, A 3  =0, such that the left input to the corresponding select of stage 1 is sensitized. This input is thus identified as circuit input X 28 . The remaining data inputs are assigned in a similar manner. 
     A comparison of the capacitive loading on each control signal in the prior art tree of FIG. 3 and the distributed tree of FIG. 6 (configured pursuant to the present invention) is provided in Table 2. 
     
                       TABLE 2______________________________________Address         FIG. 3  FIG. 6Line            Tree    Tree______________________________________A.sub.0         1       1A.sub.1         2       7A.sub.2         4       8A.sub.3         8       8A.sub.4         16      7______________________________________ 
    
     As indicated, the high loading on line A 4  in the prior art tree configuration is eliminated by the distribution of control signals such as in the tree of FIG. 6. Although the FIG. 6 and FIG. 3 tree embodiments have the same logic characteristics, performance of the FIG. 6 tree is significantly better owing to the distribution of control signals (which are in the critical path for improving performance of the multiplexer). As shown in FIG. 3, the maximum load on the control lines is the load on A 4 , which has a fan-out load of 16 drops. This heavy load on the control line farthest from the output of the circuit is the primary delay limiting the performance of the multiplexer. In comparison, the distributed tree of FIG. 6 has a load on any address line equal to a maximum of 8 such that the technique described herein gains on the order of 30 percent performance improvement in thirty-two bit multiplexers. Even greater improvement is anticipated with more complex structures. Again, the technique can be used on any tree-type logic design. (Also, those skilled in the art will recognize that the process concepts outlined herein are readily implementable in software.) 
     As a further enhancement, FIG. 7 depicts a tree-type multiplexer having distributed control signals which uses buffers 10 to produce delay control signals for control the selectors of the stages between stage 1 and stage 5 (i.e., stage N). Since control signals A 4 , A 2 , A 3  &amp; A 1  control the selects of stage 1, each of these signals is buffered to produce duplicate signals A 4  &#39;, A 2  &#39;, A 3  &#39; &amp; A 1  &#39;, respectively. The buffering is balanced such that each buffer 10 output is timed to coincide with the output from the stage 1 selects. As shown, in stages 2, 3 &amp; 4, control signals A 3  &#39;, A 4  &#39;, A 1  &#39; &amp; A 2  &#39; replace control signals A 3 , A 4 , A 1  &amp; A 2 , respectively, of the FIG. 6 tree embodiment. In this case, buffering guarantees that the maximum load on an address line of the multiplexer in any depth for the thirty-two bit example is five. The loading for each control signal is set forth in Table  3. 
     
                       TABLE 3______________________________________  Control         Capacitive  Signal Load______________________________________  A.sub.0         1  A.sub.1         5  A.sub.2         5  A.sub.3         5  A.sub.4         5  A.sub.1 &#39;         3  A.sub.2 &#39;         4  A.sub.3 &#39;         4  A.sub.4 &#39;         3______________________________________ 
    
     In will noted from the above description that certain novel tree-type multiplexers and methods of construction are provided herein. Specifically, the multiplexers and methods of distribution described provide significant improvement in performance over conventional tree-type multiplexers, without changing the logical characteristics thereof. Improved performance is obtained by reducing the capacitive loads on the control logic through various control signal distribution techniques. Further, the multiplexers and fabrication methods can be used for any circuitry where high performance multiplexing is required, including data flow elements, RAMs, ROMs and/or control logic circuitry. 
     While the invention has been described in detail herein in accordance with certain preferred embodiments thereof, many modifications and changes therein may be effected by those skilled in the art. Accordingly, it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.