More topics in VLSI and VHDL course

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

Nest up in this VHDL course, we are 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 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.

Contents

## Explanation of the VHDL source code for a demux 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. This is a general Demultiplexer with its single input, multiple outputs, and corresponding select lines.

We will proceed just like we did with the select statement in the VHDL coding for multiplexers.

#### Logic equation for a 1:2 Demux

Y0 = I0S’

Y1 = I0S

Here, I0 is the input. S is the select line input. Y0 and Y1 are the outputs. To see how we got this equation, you can check out the working of the 1:2 demux.

## 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 is 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 demux code is a perfect example of doing that. We’ll be using the truth table to write the demultiplexer’s VHDL source code.

```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 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 write the VHDL code for your demux? 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?