Abstract:
An improved binary decoder incorporating a selection circuit that activates a selected output corresponding to a input binary value, and a deselecting circuit coupled to each output that deactivates all other outputs when the selected output is activated. The deselecting circuit arrangement has a single input connected to the selected output and a plurality of outputs each of which is connected to one of the remaining outputs and forces them to the inactive state whenever the selected output is activated.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to electronic integrated circuits, and in particular to improved binary decoders in integrated circuits. 
   2. Description of the Related Art 
   A binary decoder is a circuit that enables a single selected digital output from a defined set of digital outputs, based on an input binary value. The decoder produces an output signal on the single output that matches the input binary value. Binary decoders are essentially combinatorial logic circuits formed by an arrangement of logic gates. A binary decoder having an input value “n” bits is required to have “2 n ” outputs one of which corresponds with the n-bit value applied at the input. Binary decoders are widely used for performing selection functions where any single device or any single set of devices are required to be selected or enabled from a defined collection of devices or defined collection of device sets. A typical application is in selecting a row of memory cells in a memory device. Another typical application is in selecting a single IO device from a collection of IO devices in an electronic circuit. 
   A 2-to-4 decoder implementation according to the known art requires 16 transistors covering four different circuits, one for each combination of the input. At the same time each input signal is required to drive four GATE loads. Similarly, a 3-to-8 decoder requires as many as 32 transistors covering 8 different circuits, one for each combination of inputs, with each input having to drive 4 gate loads. This manner of implementation results in a bulky and expensive circuit requiring an undesirably large chip area. The capacitive loading on the input signals reduces the speed of the operation of the device. 
   BRIEF SUMMARY OF THE INVENTION 
   The disclosed embodiments of the invention provide binary decoders requiring fewer transistors, decoders with reduced loading on input signals, reduced size, reduced power consumption, and reduced delay. 
   To achieve the foregoing, one embodiment of the invention provides an improved binary decoder incorporating a selection circuit that activates a selected output corresponding to the input binary value, and a deselecting circuit coupled to each output that deactivates all other outputs when the selected output is activated. The deselecting circuit arrangement has a single input connected to the selected output and a plurality of outputs, each of which is connected to one of the remaining outputs and forces them to the inactive state whenever the selected output is activated. 
   More particularly, a binary decoder is provided that, in one embodiment, includes a first pair of parallel-coupled transistors and a second pair of parallel-coupled transistors, the first and second pair of parallel-coupled transistors coupled between a voltage source and first and second outputs, respectively; a third transistor coupled between the first output and a reference potential: a fourth transistor coupled between the second output and the reference potential; and the first output coupled to a first input of the second pair of parallel-coupled transistors, the second output coupled to a first input of the second pair of parallel-coupled transistors, the first and second pair of parallel-coupled transistors each having an input terminal, and the third and fourth transistors each having an input terminal. 
   In accordance with another aspect of the invention, an decoder for integrated circuits is provided that includes a first set of parallel-coupled transistors coupled between a voltage source and a first output and comprising first, second, and third transistors each having respective inputs; a second set of parallel-coupled transistors coupled between the voltage source and a second output and comprising fourth, fifth, and sixth transistors each having respective inputs; a seventh transistor coupled between the first output and a common node and having an input: an eighth transistor coupled between the second output and the common node and having an input: and a ninth transistor coupled between the common node and a voltage reference and having an input. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention will now be described in accordance with the accompanying drawings, wherein: 
       FIG. 1  shows the schematic diagram of a conventional 2-to-4 decoder, which provides an active low output for an input=A′.B′. 
       FIG. 2  shows the schematic diagram of a conventional 2-to-4 decoder, which provides an active low output for an input=A′.B. 
       FIG. 3  shows the schematic diagram of a conventional 2-to-4 decoder, which provides an active low output for an input=A.B′. 
       FIG. 4  shows the schematic diagram of a conventional 2-to-4 decoder, which provides an active low output for an input=A.B. 
       FIG. 5  shows the schematic circuit diagram of a 2-to-4 decoder according to this invention, which provides an active low output for an input=A′.B′ and A.B′. 
       FIG. 6  shows the schematic circuit diagram of a 2-to-4 decoder according to this invention, which provides an active low output for an input=A′.B and A.B. 
       FIG. 7  shows the schematic circuit diagram of a 3-to-8 decoder according to this invention, which provides an active low output for an input=A′.B′.C′ and A.B′.C′. 
       FIG. 8  shows the schematic circuit diagram of a 3-to-8 decoder according to this invention, which provides an active low output for an input=A′.B′.C and A.B′.C. 
       FIG. 9  shows the schematic circuit diagram of a 3-to-8 decoder according to this invention, which provides an active low output for an input=A′.B.C′ and A.BC′. 
       FIG. 10  shows the schematic circuit diagram of a 3-to-8 decoder according to this invention, which provides an active low output for an input=A′.B.C and A.B.C. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows one circuit of a conventional 2-to-4 decoder, corresponding to a single combination of the inputs=  AB  or  AB . The output Y 0  is set to active low whenever this combination is true, i.e., when A=0 and B=0, transistors  1 . 3  and  1 . 4  conduct and pull output Y 0  to 0. For any other condition Y 0  is set to HIGH. 
     FIG. 2  shows another circuit of a conventional 2-to-4 decoder, corresponding to another single combination of the inputs=ĀB. The output Y 1  is set to active low whenever this combination is true, i.e., when A=0 and B=1, transistors  2 . 3  and  2 . 4  conduct and pull Y 1  to 0. In any other condition Y 1  is set to HIGH. 
     FIG. 3  shows another circuit of a conventional 2-to-4 decoder, corresponding to the combination of the inputs=A  B , The output Y 2  is set to active low whenever this combination is true, i.e., when inputs A=1 and B=0, transistors  3 . 3  and  3 . 4  conduct and pull Y 2  to 0. In any other condition Y 2  is set to HIGH. 
     FIG. 4  shows another circuit of a conventional 2-to-4 decoder, corresponding to a combination of inputs=AB, The output Y 3  is set to active low whenever this combination is true. As shown, when A=1 and B=1, transistors  4 . 3  and  4 . 4  conduct and pull Y 3  to ‘0’. In any other condition Y 3  is set to HIGH. 
     FIG. 5  shows a schematic circuit diagram for a 2-to-4 decoder in accordance with the invention. The selected output is set active low when the input=  AB  and A  B . When A=0 and B=0, transistor  5 . 6  conducts and pulls Y 0  LOW. This causes transistor  5 . 2  to conduct, pulling Y 2  HIGH. Similarly, when the inputs are A=1, B=0, transistor  5 . 3  is in conduction pulling Y 2  LOW thereby causing transistor  5 . 4  to conduct and pull Y 0  HIGH 
     FIG. 6  shows a schematic diagram of a 2-to-4 decoder in accordance with the invention. For the case when the inputs ĀB and AB. When the inputs are A=0, B=1, transistors  6 . 6  is in conducting state and pulls Y 1  to LOW. This causes transistor  6 . 2  to conduct pulling Y 3  HIGH. Similarly, when the inputs are A=1, B=1, transistor  6 . 3  is in conducting state and pulls Y 3  to LOW. This results in transistor  6 . 4  conducting and pulling Y 1  HIGH. 
     FIG. 7  shows a schematic diagram of a 3-to-8 decoder in accordance with the invention. For the case when the inputs are  ABC  and A  BC . When the inputs are A=0, B=0, C=0 transistors  7 . 8  and  7 . 9  are in conduction pulling Y 0  LOW. This sets transistor  7 . 3  conducting pulling Y 4  HIGH. Similarly, when the inputs are A=1, B=0, C=0, transistors  7 . 4  and  7 . 9  are in conduction and Y 4  is pulled LOW. This sets transistor  7 . 5  conducting, pulling Y 0  HIGH. 
     FIG. 8  shows a schematic diagram of a 3-to-8 decoder in accordance with the invention. For the case when the inputs are  AB C and A  B C. When the inputs are A=0, B=0, C=1 transistors  8 . 8  and  8 . 9  are in conduction and pull Y 1  LOW. This sets transistor  8 . 3  conducting, pulling Y 5  HIGH. When the inputs are A=1, B=0, C=1 transistors  8 . 4  and  8 . 9  are in a conducting state, pulling Y 5  LOW. This sets transistor  8 . 5  conducting, pulling Y 1  HIGH. 
     FIG. 9  shows a schematic diagram of a 3-to-8 decoder in accordance with the invention. For the case when the inputs are ĀB  C  and AB  C . When the inputs are A=0, B=1, C=0 transistors  9 . 8  and  9 . 9  are in conduction pulling Y 2  LOW. This sets transistor  9 . 3  conducting, pulling Y 6  HIGH. Similarly, when the inputs are A=1, B=1, C=0, transistors  9 . 4  and  9 . 9  are in a conducting state, pulling Y 6  LOW. This sets transistor  9 . 5  conducting, pulling Y 2  HIGH. 
     FIG. 10  shows a schematic diagram of a 3-to-8 decoder in accordance with the invention. For the case when the inputs are ĀBC and ABC. When the inputs are A=0, B=1, C=1, transistors  10 . 8  and  10 . 9  are in a conducting state, pulling Y 3  LOW. This sets transistor  10 . 3  conducting pulling Y 7  HIGH. Similarly, when the inputs are A=1, B=1, C=1, transistors  10 . 4  and  10 . 9  are in conduction, pulling Y 7  LOW. This sets transistor  10 . 5  conducting pulling Y 3  HIGH. 
   Though the invention has been described for “active low” outputs, with only two output pairs in a circuit and for a limited number of input circuits, it will be obvious to a person of ordinary skill in the art that it is possible to implement it for other requirements with minor alterations or extensions. The present invention realizes decoders with a reduced number of transistors compared to conventional implementations, e.g., for realizing a 2-to-4 decoder, the current invention requires 12 transistors as compared to 16 in the conventional scheme. The reduction in the number of transistors increases for larger decoders. For a 3×8 decoder the reduction is 12 transistors, while for a 4×16 decoder the reduction is 32 transistors and so on. 
   In order to reduce the number of transistors and to reduce the loading on the input signals and hence to increase the speed of operation of the device, the present invention utilizes the selected active output of the decoder to set the other outputs to the inactive state. This also enables the decoder to reduce the loading on the input signals to only two gate loads and two drain loads as against four gate loads in the prior art. 
   All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referenced in this specification or listed in the Application Data Sheet are incorporated herein by reference in their entirety. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.