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Sunday, October 21, 2012

decoder by nurul izzati farhanah

Decoders, Multiplexors and Demultiplexors


·         A decoder is a device which does the reverse operation of an encode
·         Undoing the encoding so that the original information can be retrieved the same method
·         The method used to encode is usually just reversed in order to decode
·         It is a combinational circuit that converts binary information from n input lines to a maximum of 2power n unique output lines.
http://www.youtube.com/watch?v=DgVkEVI6_Ws
·         Decoder use to build a larger component
·         Common type of decoder has an n-bit input and 2 power n output .
·         This decoder translate the n-bits input into a signal that corresponds to the binary value of the n-bits input
·         The outputs are shows a 3-bit decoder and the truth table.
·         This decoder is called a 3-to-8 decoder

  • Since  there  are 3 inputs and 8 (23) output     
  •  Ex :

               







·         There is also a logic element called an encoder  that performs the inverse function of a decoder
·         Taking n inputs and producing an n-bit output.
·         I have shown you how to make an adder using AND-OR array logic. If you look at the AND plane diagram for the adder, you will see that not every combination of inputs is accounted for.



  • The decoder takes a three-bit binary number as input and turns on the corresponding output
  • . An output is on only when its three-bit address is placed on the inputs. A decoder circuit diagram looks like this, using two-input AND gates:




  • Like other circuits, the decoder can be symbolized by a simple rectangle with the name on it.



  • Like the logic gates, decoders of several types are available in the 74LS00 series. 
  • They are used in computers to decode addresses for memory, and as the AND plane for AND-OR arrays.
  •  You can make a read-only memory (ROM) from a decoder and multiple input OR gates.
  • Memory ICs have large decoders inside them. A 2K memory needs a 10 to 2048 decoder (over 2000 outputs!).
  •  This is one reason why memory chips are slower than most other ICs in a computer system.

By using a decoder , AND gates and a multiple input OR gate, we can make data selectors, called multiplexors. Here is a four-input, one-bit multiplexor.





  • In this circuit, there are two address inputs and four data inputs.
  •  The circuit will pass the data input bit selected by the address input to the data output. 
  • The circuit can be thought of as a data selector, and the address inputs of a multiplexor are sometimes labeled as S1 and S2 for this reason.
  •  Multiplexors are used to make the data paths in a computer processor. For completeness, I will show you the de-multiplexor.
  •  While the multiplexor selects one of several inputs for one output, the demultiplexor selects one of several outputs for one input.
  • http://www.youtube.com/watch?v=x8N4XI452jM&feature=related



  • The demultiplexor is like a decoder with one extra input to determine whether the selected output will be a 1 or a 0. If the extra input is tied to 5V (made a 1 permanently), then the demultiplexor becomes a decoder. For this reason, the 7400 series of ICs has demultiplexor/decoders, and not separate demultiplexors and decoders.
  • Multiplexors and demultiplexors can be symbolized the same way as the other logic circuits. We simply draw a box around the circuit. In this case, we use a symbol with rounded edges with numbers inside, indicating the inputs or outputs to be selected by the address inputs.




  • Data is assumed to flow from left to right. The address inputs need not be shown, since the number of inputs or outputs shows how many address bits are needed. Note the rounded corners on the symbol. 
  • This will be important later as we begin to build our computer diagram. The rounded corners and numbers inside distinguish the multiplexor/demultiplexor from the data register.
  • Again, multiplexors (and demultiplexors) can be grouped together in networks. This allows us to make multiple-bit data selectors.
  •  Data inside a computer is often represented as groups of bits. Four bits makes a nybble, for example, and can represent a number from zero to 15 decimal. By connecting together four multiplexors, we can make a fourbit data selector.




  • The four multiplexors shown above each have the same address inputs. The input nybble selected by the address input will appear on the outputs.
  •  To draw a multiple-bit multiplexor or demulitplexor we use the symbol for the one-bit variety with a number above the inputs or outputs to show that it is passing groups of bits.



by Nurul Izzati Farhanah






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