Verilog code for fixed-point 16-bit 4-tap FIR filter module (state machine version) | Codetter（こーどったー）

// Fixed-point 16 bit 4-tap FIR filter module [state machine version] -----------------------
module FIRfilterS (
  input wire[15:0] indata,
  input wire update,
  input wire clk,
  output wire [15:0] outdata
  );
  parameter NTAPS = 4;
  wire signed [15:0] coef[NTAPS-1:0];
  reg  signed [15:0] buff[NTAPS-1:0];
  reg  signed [15:0] outreg,mul_in1,mul_in2;
  wire signed [31:0] mul_out;
  reg  signed [31:0] addreg;
  integer statecnt,rptr,wptr;
  reg inprocess;
  assign outdata = outreg;
  
  // coefficients --------------- change here
  assign coef[0] = 16'h2000;
  assign coef[1] = 16'h2000;
  assign coef[2] = 16'h2000;
  assign coef[3] = 16'h2000;  
  // ------------------------------
  
  // initital register settings
  initial begin
    rptr <= 0;
    wptr <= 0;
    inprocess <= 0;
    statecnt <= NTAPS*4+1;
  end
  // mulitplication module
  multiplier mul(.clk(clk),.a(mul_in1),.b(mul_in2),.p(mul_out)); 
    // Optimized 'black-box' module : p = a * b
    //    input clk
    //    input [15:0] a
    //    input [15:0] b
    //    output [31:0] p
  
  // internal calculations
  always @(posedge clk) begin
    // inpute detection
    if(update==1 && inprocess==0) begin 
      buff[wptr] <= indata;
      statecnt <= 0;
      rptr <= wptr; 
      addreg <= 0;
      inprocess <= 1;
    end
    // processing state machine
    if(statecnt<NTAPS*4) begin 
      if(statecnt%4==0) begin // input data to multiplier
        mul_in1 <= buff[rptr];
        mul_in2 <= coef[statecnt>>4];
      end
      if(statecnt%4==3) begin // get data from multiplier and accumulate
        addreg <= addreg + mul_out;
        rptr <= (rptr+NTAPS-1)%NTAPS;
      end
      statecnt <= statecnt+1;
    end
    if(statecnt==NTAPS*4) begin // output data
      outreg <= addreg>>>15;
      wptr <= (wptr+1)%NTAPS;
      statecnt <= statecnt+1;
    end
    if(statecnt==NTAPS*4+1 && inprocess==1 && update==0) begin 
      inprocess <= 0;
    end
  end
endmodule