basic_verilog/Advanced Synthesis Cookbook/ethernet_fec/fec_rot_4.v

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//
// 32 bit CRC of 4 data bits (forward - LSB first)
// polynomial : 00a00805 
//    x^23 + x^21 + x^11 + x^2 + x^0
//
//        CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC DDDD 
//        00000000001111111111222222222233 0000 
//        01234567890123456789012345678901 0123 
// C00  = ............................#... .... 
// C01  = .............................#.. .... 
// C02  = ............................#.#. .... 
// C03  = .............................#.# .... 
// C04  = #.............................#. .... 
// C05  = .#.............................# .... 
// C06  = ..#............................. .... 
// C07  = ...#............................ .... 
// C08  = ....#........................... .... 
// C09  = .....#.......................... .... 
// C10  = ......#......................... .... 
// C11  = .......#....................#... .... 
// C12  = ........#....................#.. .... 
// C13  = .........#....................#. .... 
// C14  = ..........#....................# .... 
// C15  = ...........#.................... .... 
// C16  = ............#................... .... 
// C17  = .............#.................. .... 
// C18  = ..............#................. .... 
// C19  = ...............#................ .... 
// C20  = ................#............... .... 
// C21  = .................#..........#... .... 
// C22  = ..................#..........#.. .... 
// C23  = ...................#........#.#. .... 
// C24  = ....................#........#.# .... 
// C25  = .....................#........#. .... 
// C26  = ......................#........# .... 
// C27  = .......................#........ .... 
// C28  = ........................#....... .... 
// C29  = .........................#...... .... 
// C30  = ..........................#..... .... 
// C31  = ...........................#.... .... 
//
// Number of XORs used is 32
// Maximum XOR input count is 3
//   Best possible depth in 4 LUTs = 1
//   Best possible depth in 5 LUTs = 1
//   Best possible depth in 6 LUTs = 1
// Total XOR inputs 48
//

module fec_rot_4 (
    input [31:0] c,
    output [31:0] co
);

assign co[0] =
    c[28];

assign co[1] =
    c[29];

assign co[2] =
    c[28] ^ c[30];

assign co[3] =
    c[29] ^ c[31];

assign co[4] =
    c[0] ^ c[30];

assign co[5] =
    c[1] ^ c[31];

assign co[6] =
    c[2];

assign co[7] =
    c[3];

assign co[8] =
    c[4];

assign co[9] =
    c[5];

assign co[10] =
    c[6];

assign co[11] =
    c[7] ^ c[28];

assign co[12] =
    c[8] ^ c[29];

assign co[13] =
    c[9] ^ c[30];

assign co[14] =
    c[10] ^ c[31];

assign co[15] =
    c[11];

assign co[16] =
    c[12];

assign co[17] =
    c[13];

assign co[18] =
    c[14];

assign co[19] =
    c[15];

assign co[20] =
    c[16];

assign co[21] =
    c[17] ^ c[28];

assign co[22] =
    c[18] ^ c[29];

assign co[23] =
    c[19] ^ c[28] ^ c[30];

assign co[24] =
    c[20] ^ c[29] ^ c[31];

assign co[25] =
    c[21] ^ c[30];

assign co[26] =
    c[22] ^ c[31];

assign co[27] =
    c[23];

assign co[28] =
    c[24];

assign co[29] =
    c[25];

assign co[30] =
    c[26];

assign co[31] =
    c[27];

endmodule