Verilog Coding Tips and Tricks: Verilog Code for 3:8 Decoder using Case statement

Sunday, October 18, 2015

Verilog Code for 3:8 Decoder using Case statement

Decoders are combinational circuits used for breaking down any combination of inputs to a set of output bits that are all set to '0' apart from one output bit. Therefore when one input changes, two output bits will change.

Lets say we have N input bits to a decoder, the number of output bits will be equal to 2^N.
In this blog post, we implement a 3:8 decoder using behavioral modelling. We use case statements for this purpose. The number of input bits are 3 and number of output bits are 8.

3:8 Decoder:

//declare the Verilog module - The inputs and output port names.
module decoder3to8(

    //what are the input ports and their sizes.
    input [2:0] Data_in;
    //what are the output ports and their sizes.
    output [7:0] Data_out;
    //Internal variables
    reg [7:0] Data_out;

    //Whenever there is a change in the Data_in, execute the always block.
    always @(Data_in)
    case (Data_in)   //case statement. Check all the 8 combinations.
        3'b000 : Data_out = 8'b00000001;
        3'b001 : Data_out = 8'b00000010;
        3'b010 : Data_out = 8'b00000100;
        3'b011 : Data_out = 8'b00001000;
        3'b100 : Data_out = 8'b00010000;
        3'b101 : Data_out = 8'b00100000;
        3'b110 : Data_out = 8'b01000000;
        3'b111 : Data_out = 8'b10000000;
        //To make sure that latches are not created create a default value for output.
        default : Data_out = 8'b00000000; 

Testbench for testing 3:8 Decoder:

//This is a testbench code used for testing the 3:8 decoder module.
//Since its a testbench code we dont need to define any inputs or outputs for the block.
module tb_decoder;

    // Declaring Inputs
    reg [2:0] Data_in;

    // Declaring Outputs
    wire [7:0] Data_out;

    // Instantiate the Unit Under Test (UUT)
    decoder3to8 uut (

    initial begin
        //Apply Input and wait for 100 ns
        Data_in = 3'b000;     #100;
        Data_in = 3'b001;     #100;
        Data_in = 3'b010;     #100;
        Data_in = 3'b011;     #100;
        Data_in = 3'b100;     #100;
        Data_in = 3'b101;     #100;
        Data_in = 3'b110;     #100;
        Data_in = 3'b111;     #100;

Simulated Waveform:

When the codes are correctly simulated in Xilinx ISE, you will get the following waveform.

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