Subversion Repositories scalable_arbiter

/*

 * Copyright (c) 2009, Kendall Correll

 *

 * Permission to use, copy, modify, and distribute this software for any

 * purpose with or without fee is hereby granted, provided that the above

 * copyright notice and this permission notice appear in all copies.

 *

 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES

 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF

 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR

 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES

 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN

 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF

 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

 */

`timescale 1ns / 1ps

module lfsr #(

        parameter width = 0,

        parameter reset_value = {width{1'b1}}

)(

        input enable,

        input load,

        input in,

        output reg [width-1:0] out,

        input clock,

        input reset

);

// it's assumed that the msb is a tap so it need not be in this table,

// indexes are 0-based, you can add cases for any other widths

function is_tap (

        input integer width, index

);

begin

        case(width)

        3:   is_tap = index == 1;

        4:   is_tap = index == 2;

        5:   is_tap = index == 2;

        6:   is_tap = index == 4;

        7:   is_tap = index == 5;

        8:   is_tap = index == 5   || index == 4   || index == 3;

        15:  is_tap = index == 13;

        16:  is_tap = index == 14  || index == 12  || index == 3;

        31:  is_tap = index == 27;

        32:  is_tap = index == 21  || index == 1   || index == 0;

        63:  is_tap = index == 61;

        64:  is_tap = index == 62  || index == 60  || index == 59;

        127: is_tap = index == 125;

        128: is_tap = index == 125 || index == 100 || index == 98;

        default: is_tap = 0;

        endcase

end

endfunction

// combine the taps to compute the next lsb

function [0:0] feedback (

        input [width-1:0] value

);

integer i;

begin

        // always include the msb

        feedback = value[width-1];

        // include the other taps specified by the table

        for(i = 0; i < width - 1; i = i + 1)

        begin

                if(is_tap(width, i))

                        feedback = feedback ^ value[i];

        end

end

endfunction

// the shift register

always @(posedge clock, posedge reset)

begin

        if(reset)

                out <= reset_value;

        else

        begin

                if(enable)

                        out <= {out[width-2:0], load ? in : feedback(out)};

        end

end

endmodule