A Decoder is a combinational circuit that converts binary information from input lines to unique output lines. Apart from the Input lines, a decoder may also have an Enable input line.
Decoder as a De-Multiplexer –
A Decoder with Enable input can function as a demultiplexer. A demultiplexer is a circuit that receives information from a single line and directs it to one of possible output lines.
A demultiplexor receives as input, selection lines and one Input line. These selection lines are used to select one output line out of possible lines. To implement a demultiplexor, we use a decoder with Enable input. The selection lines of the demultiplexer are the input lines that the decoder gets and the one input line of demulitplexer is the Enable input of the Decoder.
Making 1:4 demultiplexer using 2:4 Decoder with Enable input. Let A, B be the selection lines and EN be the input line for the demultiplexer.
The decoder shown below functions as a 1:4 demultiplexer when EN is taken as a data input line and A and B are taken as the selection inputs. The single input variable E has a path to all four outputs, but the input information is directed to only one of the output lines, as specified by the binary combination of the two selection lines A and B. This can be verified from the truth table of the circuit.
Combinational Logic Implementation using Decoder –
A decoder takes input lines and has output lines. These output lines can provide the minterms of input variables.
Since any boolean function can be expressed as a sum of minterms, a decoder that can generate these minterms along with external OR gates that form their logical sums, can be used to form a circuit of any boolean function.
For example, if we need to implement the logic of a full adder, we need a 3:8 decoder and OR gates. The input to the full adder, first and second bits and carry bit, are used as input to the decoder. Let x, y and z represent these three bits. Sum and Carry outputs of a full adder have the following truth tables-
Therefore we have-
The following circuit diagram shows the implementation of Full adder using a 3:8 Decoder and OR gates.
Digital Design, 5th edition by Morris Mano and Michael Ciletti
Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.
- Construction of Combinational Circuits
- Combinational and Sequential Circuits
- Classifications of Combinational and Sequential circuits
- Analysis and Design of Combinational and Sequential circuits
- Difference between Characteristics of Combinational and Sequential circuits
- BCD to 7 Segment Decoder
- Difference between Decoder and Demultiplexer
- Difference between Multiplexer and Decoder
- Binary Decoder in Digital Logic
- Difference between combinational and sequential circuit
- Introduction of Sequential Circuits
- Asynchronous Sequential Circuits
- Switch Debounce in Digital Circuits
- Synchronous Sequential Circuits in Digital Logic
- Difference between Synchronous and Asynchronous Sequential Circuits
- Mathematics | Walks, Trails, Paths, Cycles and Circuits in Graph
If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to email@example.com. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.
Improved By : cmkmanwani