Verilog Coding Tips and Tricks: Synthesiable Verilog code for a 4 tap FIR Filter

Monday, November 16, 2015

Synthesiable Verilog code for a 4 tap FIR Filter

Few years back I wrote a VHDL code for implementing a FIR filter. In this post, I want to implement the same algorithm in Verilog.

Finite Impulse Response(FIR) filters are one of the two main type of filters available for signal processing. As the name suggests the output of a FIR filter is finite and it settles down to zero after some time. For a basic FAQ on FIR filters see this post by dspguru.

A FIR filter output, 'y' can be defined by the following equation:

Here, 'y' is the filter output, 'x' in the input signal and 'b' is the filter coefficients. 'N' is the filter order. The higher the value of N is, the more complex the filter will be.

For writing the code in Verilog I have referred to the paper, VHDL generation of optimized FIR filters. You can say I have coded the exact block diagram available in the paper, "Figure 2".

This is a 4 tap filter. That means the order of the filter is 4 and so it has 4 coefficients. The inputs are chosen to be 8 bits wide and outputs are chosen to be 16 bits wide. Both inputs and outputs can store negative numbers, in two's complement format. If you want to handle inputs with bigger range you can simply increase the size of the inputs and intermediate variables. The structure of the code will remain the same.

The design contains two files. One is the main file with all the multiplications and adders defined in it, and another one for defining the D flip flop operation.

The codes are given below:


module fir_4tap(
        input Clk,
        input signed [7:0] Xin,
        output reg signed [15:0] Yout
    //Internal variables.
    wire signed   [7:0] H0,H1,H2,H3;
    wire signed   [15:0] MCM0,MCM1,MCM2,MCM3,add_out1,add_out2,add_out3;
    wire signed     [15:0] Q1,Q2,Q3;
//filter coefficient initializations.
//H = [-2 -1 3 4].
    assign H0 = -2;
    assign H1 = -1;
    assign H2 = 3;
    assign H3 = 4;

//Multiple constant multiplications.
    assign MCM3 = H3*Xin;
    assign MCM2 = H2*Xin;
    assign MCM1 = H1*Xin;
    assign MCM0 = H0*Xin;

    assign add_out1 = Q1 + MCM2;
    assign add_out2 = Q2 + MCM1;
    assign add_out3 = Q3 + MCM0;    

//flipflop instantiations (for introducing a delay).
    DFF dff1 (.Clk(Clk),.D(MCM3),.Q(Q1));
    DFF dff2 (.Clk(Clk),.D(add_out1),.Q(Q2));
    DFF dff3 (.Clk(Clk),.D(add_out2),.Q(Q3));

//Assign the last adder output to final output.
    always@ (posedge Clk)
        Yout <= add_out3;



module DFF
        (input Clk,
        input [15:0] D,
        output reg [15:0]   Q
    always@ (posedge Clk)
        Q = D;

Testbench for the FIR filter:

module tb;

    // Inputs
    reg Clk;
    reg signed [7:0] Xin;

    // Outputs
    wire signed [15:0] Yout;

    // Instantiate the Unit Under Test (UUT)
    fir_4tap uut (
    //Generate a clock with 10 ns clock period.
    initial Clk = 0;
    always #5 Clk =~Clk;

//Initialize and apply the inputs.
    initial begin
          Xin = 0;  #40;
          Xin = -3; #10;
          Xin = 1;  #10;
          Xin = 0;  #10;
          Xin = -2; #10;
          Xin = -1; #10;
          Xin = 4;  #10;
          Xin = -5; #10;
          Xin = 6;  #10;
          Xin = 0;  #10;

Simulation waveform:

The following waveform was obtained in Xilinx ISE 14.6 after successful simulation.

Synthesis Results:

The modules were successfully synthesised for Virtex 4 device and a maximum frequency of 528 MHz was obtained.


  1. how to add DFF module code to to fir_4tap code? is there any stepts for that?

    1. create a .v file for every module you see above. create a new project and add all the three files to it. the you should be able to compile the codes. Just make sure the compile order is correct. dff first, then fir and then tb.

  2. is it ok to mention input in integers format( -3,1,-2, etc...)?
    or it should be in binary format?

    1. you can see the testbench code. As you can see, the inputs can be given in integer format. If you want, the inputs can be given in binary format too.

  3. Hi,
    Thank you very much for the code. I am trying to implement the 4-tap FIR filter. I have just copied-paste the codes (fir_4tap, DFF and tb) and run simulations on Vivado. I set the Radix to signed decimal, however I get different output for the filter.

    The Yout is [-5, -1528, 1267, 1019, 1025, 2299, 1022, 24,0].

    I would appreciate if you could advise me what the problem could be?

    1. This comment has been removed by the author.

    2. Thanks for noticing the error. The code had a bug. I have corrected it and uploaded the new code. Please check now.

  4. This comment has been removed by a blog administrator.

  5. can i get the code for direct form fir filter in verilog

  6. can u plzz use retimming in this filter and write verilog code for that

  7. Is this code is not synthesizable on VIVADO 2014?