VHDL code for demultiplexer using dataflow method – full code & explanation

We are now going to write the VHDL code for demultiplexer using the dataflow method. And since the dataflow architecture gives us the functionality to code circuits using their truth table, we’ll do just that. Let’s start off by understanding a demultiplexer’s external ports for the entity-architecture pair that we will define later. Then we will take a look at the truth tables.

Explanation of the VHDL code for demultiplexer using dataflow method. How does the code work?

A demultiplexer is a combinational digital logic circuit that assigns one input to one of several output lines. It selects one of these output lines depending on the value of its select inputs. So a demultiplexer has one input signal, select lines, and multiple output lines.

demultiplexer symbol
Demultiplexer with its single input, multiple outputs and corresponding select lines.

Just like we used the select statement when VHDL coding for multiplexers. I will reiterate.

VHDL code for demultiplexer using dataflow (logic equation) method – 1:2 Demux

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.STD_LOGIC_ARITH.ALL;

use IEEE.STD_LOGIC_UNSIGNED.ALL;




entity DEMUX_SOURCE is

    Port ( I,S : in  STD_LOGIC;

           O1, O2 : out  STD_LOGIC);

end DEMUX_SOURCE;


architecture dataflow of DEMUX_SOURCE is


begin


O1 <= I and (not S);

O2 <= I and S;


end dataflow;

 

Truth table of 1:4 demux

I0 S0 S1 Y0 Y1 Y2 Y3
I 0 0 I 0 0 0
I 0 1 0 I 0 0
I 1 0 0 0 I 0
I 1 1 0 0 0 I

VHDL code for demultiplexer using dataflow (truth table) method – 1:4 Demux


Usually, we see the truth table being used to code in the behavioral architecture. However, it is possible to use the truth table of a digital electronic circuit in the dataflow architecture too. This is a perfect example of doing that.

 

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.STD_LOGIC_ARITH.ALL;

use IEEE.STD_LOGIC_UNSIGNED.ALL;




entity DEMUX_USINGTRUTHTABLE_SOURCE is

    Port ( I : in  STD_LOGIC;

           S : in  STD_LOGIC_VECTOR (1 downto 0);

           Y : out  STD_LOGIC_VECTOR (3 downto 0));

end DEMUX_USINGTRUTHTABLE_SOURCE;




architecture dataflow of DEMUX_USINGTRUTHTABLE_SOURCE is




begin



--- since the output is a combination of character and binary bits, bitwise operation with bit and CHARACTER will take place (&) not logical and



with S select

Y              <=               ("000" & I) when "00",

                                ("00" & I & "0") when "01",

                                ("0" & I & "00") when "10",

                                (I & "000") when others;

                              
end dataflow;

Testbench waveform for 1:4 Demultiplexer

1 4 demultiplexer vhdl testbench waveform
f is the input signal, s0 and s1 are the select lines and a, b, c, and d are the four outputs.

Were you able to encode your own demultiplexer? Let us know what you did differently by commenting below. Also, would you like us to include the VHDL code for a 1:8 demux?


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