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//   ____  ____

//  /   /\/   /

// /___/  \  /    Vendor                : Xilinx

// \   \   \/     Version               : %version

//  \   \         Application           : MIG

//  /   /         Filename              : ecc_gen.v

// /___/   /\     Date Last Modified    : $date$

// \   \  /  \    Date Created          : Tue Jun 30 2009

//  \___\/\___\

//

//Device            : 7-Series

//Design Name       : DDR3 SDRAM

//Purpose           :

//Reference         :

//Revision History  :

//*****************************************************************************

`timescale 1ps/1ps

// Generate the ecc code.  Note that the synthesizer should

// generate this as a static logic.  Code in this block should

// never run during simulation phase, or directly impact timing.

//

// The code generated is a single correct, double detect code.

// It is the classic Hamming code.  Instead, the code is

// optimized for minimal/balanced tree depth and size.  See

// Hsiao IBM Technial Journal 1970.

//

// The code is returned as a single bit vector, h_rows.  This was

// the only way to "subroutinize" this with the restrictions of

// disallowed include files and that matrices cannot be passed

// in ports.

//

// Factorial and the combos functions are defined.  Combos

// simply computes the number of combinations from the set

// size and elements at a time.

//

// The function next_combo computes the next combination in

// lexicographical order given the "current" combination.  Its

// output is undefined if given the last combination in the 

// lexicographical order. 

// 

// next_combo is insensitive to the number of elements in the

// combinations.

//

// An H transpose matrix is generated because that's the easiest

// way to do it. The H transpose matrix is generated by taking

// the one at a time combinations, then the 3 at a time, then

// the 5 at a time.  The number combinations used is equal to

// the width of the code (CODE_WIDTH).  The boundaries between

// the 1, 3 and 5 groups are hardcoded in the for loop.

//

// At the same time the h_rows vector is generated from the

// H transpose matrix.

module mig_7series_v2_3_ecc_gen

  #(

    parameter CODE_WIDTH        = 72,

    parameter ECC_WIDTH         = 8,

    parameter DATA_WIDTH        = 64

   )

   (

     /*AUTOARG*/

  // Outputs

  h_rows

  );

  function integer factorial (input integer i);

    integer index;

    if (i == 1) factorial = 1;

    else begin

      factorial = 1;

      for (index=2; index<=i; index=index+1)

        factorial = factorial * index;

    end

  endfunction // factorial

  function integer combos (input integer n, k);

    combos = factorial(n)/(factorial(k)*factorial(n-k));

  endfunction // combinations

  // function next_combo

  // Given a combination, return the next combo in lexicographical

  // order.  Scans from right to left.  Assumes the first combination

  // is k ones all of the way to the left.

  //

  // Upon entry, initialize seen0, trig1, and ones.  "seen0" means

  // that a zero has been observed while scanning from right to left.

  // "trig1" means that a one have been observed _after_ seen0 is set.

  // "ones" counts the number of ones observed while scanning the input.

  //

  // If trig1 is one, just copy the input bit to the output and increment

  // to the next bit.  Otherwise  set the the output bit to zero, if the 

  // input is a one, increment ones.  If the input bit is a one and seen0

  // is true, dump out the accumulated ones.  Set seen0 to the complement

  // of the input bit.  Note that seen0 is not used subsequent to trig1 

  // getting set.

  function [ECC_WIDTH-1:0] next_combo (input [ECC_WIDTH-1:0] i);

    integer index;

    integer dump_index;

    reg seen0;

    reg trig1;

//    integer ones;

    reg [ECC_WIDTH-1:0] ones;

    begin

      seen0 = 1'b0;

      trig1 = 1'b0;

      ones = 0;

      for (index=0; index<ECC_WIDTH; index=index+1)

        begin

          // The "== 1'bx" is so this will converge at time zero.

          // XST assumes false, which should be OK.

          if ((&i == 1'bx) || trig1) next_combo[index] = i[index];

          else begin

            next_combo[index] = 1'b0;

            ones = ones + i[index];

            if (i[index] && seen0) begin

              trig1 = 1'b1;

              for (dump_index=index-1; dump_index>=0;dump_index=dump_index-1)

                if (dump_index>=index-ones) next_combo[dump_index] = 1'b1;

            end

            seen0 = ~i[index];

          end // else: !if(trig1)

        end

    end // function

  endfunction // next_combo

  wire [ECC_WIDTH-1:0] ht_matrix [CODE_WIDTH-1:0];

  output wire [CODE_WIDTH*ECC_WIDTH-1:0] h_rows;

  localparam COMBOS_3 = combos(ECC_WIDTH, 3);

  localparam COMBOS_5 = combos(ECC_WIDTH, 5);

  genvar n;

  genvar s;

  generate

    for (n=0; n<CODE_WIDTH; n=n+1) begin : ht

      if (n == 0)

         assign ht_matrix[n] = {{3{1'b1}}, {ECC_WIDTH-3{1'b0}}};

      else if (n == COMBOS_3 && n < DATA_WIDTH)

         assign ht_matrix[n] = {{5{1'b1}}, {ECC_WIDTH-5{1'b0}}};

      else if ((n == COMBOS_3+COMBOS_5) && n < DATA_WIDTH)

         assign ht_matrix[n] = {{7{1'b1}}, {ECC_WIDTH-7{1'b0}}};

      else if (n == DATA_WIDTH)

         assign ht_matrix[n] = {{1{1'b1}}, {ECC_WIDTH-1{1'b0}}};

      else assign ht_matrix[n] = next_combo(ht_matrix[n-1]);

      for (s=0; s<ECC_WIDTH; s=s+1) begin : h_row

        assign h_rows[s*CODE_WIDTH+n] = ht_matrix[n][s];

      end

    end

  endgenerate

endmodule // ecc_gen