Compact multiplexer circuit
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to multiplexers and, in particular, to a compact multiplexer design that reduces the number of transistors required for a tree structure multiplexer. 
     2. Discussion of Prior Art 
     A multiplexer performs a binary decode of input signals to select a single output from a number of potential outputs. For purposes of discussion in this document, multiplexer input signals will be referred to as &#34;addresses&#34; and multiplexer output signals will be referred to as &#34;selects&#34;. 
     For purposes of simplicity, two prior art multiplexers having only two addresses will be described. Those skilled in the art will appreciate that multiplexers having more than two addresses are common. Both of the described multiplexers can be designed for either a select logic low or a select logic high. 
     A first conventional multiplexer design, and the most simple, uses NOR-gates or NAND-gates. FIGS. 1A-1D show a NOR-gate design that generates four selects from two addresses. In this case, a select is enabled when it is high. FIGS. 1A-1D combine to show both the logic-level and transistor-level schematics for each of the four respective selects. 
     FIGS. 2A-2D combine to show the corresponding NAND-gate design. In this case, a select is enabled when it is low. 
     A second multiplexer design is commonly referred to as a tree structure. In this design, logically identical transistors are combined. For example, transistor M1 in FIG. 1A is combined with transistor M5 in FIG. 1B and transistor M9 in FIG. 1C is combined with transistor M13 in FIG. 1D to arrive at the tree structures shown in FIGS. 3 and 3B, respectively. Similarly, transistor M4 in FIG. 2A is combined with transistor M8 in FIG. 2B and transistor M12 in FIG. 2C is combined with transistor M16 in FIG. 2D to arrive at the tree structures shown in FIGS. 4A and B, respectively. 
     SUMMARY OF THE INVENTION 
     The present invention provides a design approach for the compact layout of mutliplexer circuits. The multiplexer is used to perform a binary decode of the input signal to generate a single selection of one of a number potential outputs. Since the inputs undergo a binary decode, the number of outputs is equal to 2 x  where x is the number of inputs. The design approach of the present invention allows a reduction in the number of transistors necessary for implementation of a tree structure multiplexer. 
     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings which set forth an illustrative embodiment in which the principals of the invention are utilized. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1D are schematic diagrams that combine to illustrate a NOR-gate multiplexer in which SELECT1-SELECT4 are respectively enabled high. 
     FIGS. 2A-2D are schematic diagrams that combine to illustrate a NAND-gate multiplexer in which SELECT1-SELECT4 are respectively enabled low. 
     FIGS. 3A and 3B are transistor-level schematic diagrams illustrating the respective combination of the FIGS. 1A-1B and FIGS. 1C-1D NOR-gates into corresponding tree multiplexer structures. 
     FIGS. 4A and 4B are transistor-level schematic diagrams illustrating the respective combination of the FIGS. 2A-2B and FIGS. 2C-2D NAND-gates into corresponding tree multiplexer structures. 
     FIG. 5 is a transistor-level schematic diagram illustrating transistor count reduction in the FIGS. 3A and 3B tree structures to arrive at a multiplexer design in accordance with the present invention. 
     FIG. 6 is a transistor-level schematic diagram illustrating transistor count reduction in the FIGS. 4A and 4B tree structure to arrive at a multiplexer design in accordance with the present invention. 
     FIGS. 7 provides a truth table and a typical waveform, respectively, illustrating the characteristics of the multiplexer shown in FIG. 5. 
     FIGS. 8 provides a truth table and a typical waveform, respectively, illustrating the characteristics of the multiplexer shown in FIG. 6. 
     FIG. 9 is a transistor-level schematic diagram illustrating the FIG. 5 2-address multiplexer design extended to a corresponding 3-address multiplexer design in accordance with the present invention. 
     FIG. 10 is a transistor-level schematic diagram illustrating the extension of the FIG. 6 2-address multiplexer design to a corresponding 3-address multiplexer design in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention allows reduction of the number of transistors necessary in a tree multiplexer structure. 
     Referring to FIG. 5, in accordance with the present invention, the transistor count of the FIG. 3A multiplexer structure is reduced by replacing transistors M4 and M7 with transistor M17. Similarly, as shown in FIG. 5, transistors M12 and M15 in FIG. 3A are replaced by transistor M18. 
     Referring to FIG. 6, the transistor count in the FIG. 4 multiplexer structure is reduced by replacing transistors M2 and M5 with transistor M17; also in FIG. 6, transistors M10 and M13 in FIG. 4 are replaced by transistor M18. 
     Although the resulting circuits are not logically identical, they are functionally equivalent, as will be described in greater detail below. In addition, since the SELECTS in the FIG. 5 circuit are enabled with a high signal, and the SELECTS in the FIG. 6 circuit are enabled with a low signal, these circuits are no slower in enabling a select than are the prior art circuits shown in FIGS. 1-4. 
     FIG. 5 shows a 2-input multiplexer with a select enabled when it is high. Consider the case when addresses A0 and A1 are both low. Since A0 and A1 are logically opposite signals, they are both high. In this state, SELECT 0 is high and the SELECTS 1-3 are all low. 
     An aspect of this invention lies in the way selects are enabled and disabled when an address switches. Consider the case when address A1 switches from a low to a high and, thus, A1 switches from a high to a low. A1 switches from to high to low, turning off transistor M18. This isolates SELECT 2 from SELECT 3. Simultaneously, transistor M9 is turned on. Since address A0 is low, transistor M10 is also on, and SELECT 2 goes high. Since transistors M9 and M10 are the same size and configuration in the FIG. 5 circuit as in the FIG. 1 and FIG. 3 circuits, select speed is the same. 
     When address A1 switches from low to high, transistor M17 is turned on. Simultaneously, transistor M1 is turned off. Since A0 is high, SELECT 0 is discharged from high to low through transistors M17 and M8. 
     A further aspect of this invention is that, since address A0 and its complement A0 are guaranteed opposite signals, there will always be a discharge path to disable the SELECT signal. Although this deselect time is slower by virtue of having to discharge through two transistors instead of one as in the FIGS. 1 and 3 circuits, deselect time is typically not critical for multiplexer applications. 
     FIG. 7 provides a truth table and typical waveform characteristics, respectively, for the multiplexer circuit shown in FIG. 5. 
     FIG. 6 shows a 2-input multiplexer with a select enabled when it is low. Consider the case when addresses A0 and A1 are both low. Since A0 and A1 are logically opposite to A0 and A1, respectively, they are both high. In this state, SELECT 0 is low, and the SELECTS 1-3 are all high. 
     Consider what happens when address A1 switches from a low to a high and, thus, its complement A1 switches from a high to a low. Address A1 switches form low to high, turning off transistor M17. This isolates SELECT 2 from SELECT 3. Simultaneously, transistor M4 is turned on. Since A0 is high, transistor M7 is also on, and Select 2 goes low. Since transistor M7 and transistor M4 are the same size and configuration in FIG. 6 as in the FIG. 2 and FIG. 4 embodiments, select speed is the same. 
     When A1 switches from high to low, transistor M18 is turned on. Simultaneously, transistor M12 is turned off. Since address A0 is low, SELECT 0 is charged from low to high through transistors M9 and M18. 
     Again, since address A0 and A0 are guaranteed opposite signals, there will always be a charge path to disable the SELECT signal. Although this deselect time is slower by virtue of having the charge flow through two transistors instead of the one transistor as shown in FIGS. 2 and 4, deselect time is typically not critical for multiplexer applications. 
     FIG. 8 provides a truth table and typical waveforms, respectively, for the multiplexer circuit shown in FIG. 6. 
     FIGS. 9 and 10 show examples of how the 2-input multiplexer circuits shown in FIGS. 5 and 6, respectively, can be extended to multiplexers with more inputs. The examples provided are for 3-address multiplexers, but demonstrate the generic process by which the concepts of this invention can be utilized to extend to multiplexers to any number of addresses. 
     It should be understood that various alternatives to the embodiment of the invention described herein may be employed in practicing invention. It is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.