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//-----------------------------------------------------------------------------

// Copyright (C) 2009 OutputLogic.com 

// This source file may be used and distributed without restriction 

// provided that this copyright statement is not removed from the file 

// and that any derivative work contains the original copyright notice 

// and the associated disclaimer. 

// 

// THIS SOURCE FILE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS 

// OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED 

// WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE. 

//-----------------------------------------------------------------------------

// scrambler module for data[7:0],   lfsr[22:0]=1+x^18+x^23;

//-----------------------------------------------------------------------------

module prbs_23_to_8(

  input [7:0] data_in,

  input scram_en,

  input scram_rst,

  output reg [7:0] data_out,

  input rst,

  input clk);

  reg [22:0] lfsr_q,lfsr_c;

  reg [7:0] data_c;

  always @( * ) begin

    lfsr_c[0] = lfsr_q[15] ^ lfsr_q[20];

    lfsr_c[1] = lfsr_q[16] ^ lfsr_q[21];

    lfsr_c[2] = lfsr_q[17] ^ lfsr_q[22];

    lfsr_c[3] = lfsr_q[18];

    lfsr_c[4] = lfsr_q[19];

    lfsr_c[5] = lfsr_q[20];

    lfsr_c[6] = lfsr_q[21];

    lfsr_c[7] = lfsr_q[22];

    lfsr_c[8] = lfsr_q[0];

    lfsr_c[9] = lfsr_q[1];

    lfsr_c[10] = lfsr_q[2];

    lfsr_c[11] = lfsr_q[3];

    lfsr_c[12] = lfsr_q[4];

    lfsr_c[13] = lfsr_q[5];

    lfsr_c[14] = lfsr_q[6];

    lfsr_c[15] = lfsr_q[7];

    lfsr_c[16] = lfsr_q[8];

    lfsr_c[17] = lfsr_q[9];

    lfsr_c[18] = lfsr_q[10] ^ lfsr_q[15] ^ lfsr_q[20];

    lfsr_c[19] = lfsr_q[11] ^ lfsr_q[16] ^ lfsr_q[21];

    lfsr_c[20] = lfsr_q[12] ^ lfsr_q[17] ^ lfsr_q[22];

    lfsr_c[21] = lfsr_q[13] ^ lfsr_q[18];

    lfsr_c[22] = lfsr_q[14] ^ lfsr_q[19];

    data_c[0] = data_in[0] ^ lfsr_q[22];

    data_c[1] = data_in[1] ^ lfsr_q[21];

    data_c[2] = data_in[2] ^ lfsr_q[20];

    data_c[3] = data_in[3] ^ lfsr_q[19];

    data_c[4] = data_in[4] ^ lfsr_q[18];

    data_c[5] = data_in[5] ^ lfsr_q[17] ^ lfsr_q[22];

    data_c[6] = data_in[6] ^ lfsr_q[16] ^ lfsr_q[21];

    data_c[7] = data_in[7] ^ lfsr_q[15] ^ lfsr_q[20];

  end // always

  always @(posedge clk, posedge rst) begin

    if(rst) begin

      lfsr_q <= {23{1'b1}};

      data_out <= {8{1'b0}};

    end

    else begin

      lfsr_q <= scram_rst ? {23{1'b1}} : scram_en ? lfsr_c : lfsr_q;

      data_out <= scram_en ? data_c : data_out;

    end

  end // always

endmodule // scrambler