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I am trying to get 64 bit output from an LFSR. I found a code in the net and edited it for 64 bit. But I am not getting the output.

    module LFSR8_8E(reset_, clock, q, lfsr_to);
    input clock, reset_;
    output [63:0] q, lfsr_to;

    reg [63:0] LFSR;
    wire  lfsr_to;
    assign lfsr_to = (LFSR == 64'h9C69832196724182);

    always @(posedge clock or negedge reset_)
    begin
    if (!reset_) LFSR[63:0] <= 64'h0000000000000000;
      else
    begin
    if (lfsr_to) LFSR[63:0] <= 64'h0000000000000000;
    else
    begin
    LFSR[63:63] <= LFSR[62:62]^LFSR[61:61]; 
    LFSR[62:62] <= LFSR[61:61]^LFSR[60:60];
    LFSR[61:61] <= LFSR[60:60]^LFSR[59:59];
    LFSR[60:60] <= LFSR[59:59]^LFSR[58:58];
    LFSR[59:59] <= LFSR[58:58];   
    LFSR[58:58] <= LFSR[57:57]; 
    LFSR[57:57] <= LFSR[56:56]; 
    LFSR[56:56] <= LFSR[55:55]; 
    LFSR[54:54] <= LFSR[53:53]; 
    LFSR[53:53] <= LFSR[52:52]; 
    LFSR[52:52] <= LFSR[51:51]; 
    LFSR[51:51] <= LFSR[50:50];
    LFSR[49:49] <= LFSR[48:48]; 
    LFSR[48:48] <= LFSR[47:47]; 
    LFSR[47:47] <= LFSR[46:46]; 
    LFSR[46:46] <= LFSR[45:45]; 
    LFSR[45:45] <= LFSR[44:44]; 
    LFSR[44:44] <= LFSR[43:43]; 
    LFSR[43:43] <= LFSR[42:42]; 
    LFSR[42:42] <= LFSR[41:41]; 
    LFSR[41:41] <= LFSR[40:40];
    LFSR[39:39] <= LFSR[38:38]; 
    LFSR[38:38] <= LFSR[37:37]; 
    LFSR[37:37] <= LFSR[36:36]; 
    LFSR[36:36] <= LFSR[35:35]; 
    LFSR[35:35] <= LFSR[34:34]; 
    LFSR[34:34] <= LFSR[33:33]; 
    LFSR[33:33] <= LFSR[32:32]; 
    LFSR[32:32] <= LFSR[31:31]; 
    LFSR[31:31] <= LFSR[30:30];
    LFSR[29:29] <= LFSR[28:28]; 
    LFSR[28:28] <= LFSR[27:27]; 
    LFSR[27:27] <= LFSR[26:26]; 
    LFSR[26:26] <= LFSR[25:25]; 
    LFSR[25:25] <= LFSR[24:24]; 
    LFSR[24:24] <= LFSR[23:23]; 
    LFSR[23:23] <= LFSR[22:22]; 
    LFSR[22:22] <= LFSR[21:21]; 
    LFSR[21:21] <= LFSR[20:20];
    LFSR[29:29] <= LFSR[28:28]; 
    LFSR[28:28] <= LFSR[27:27]; 
    LFSR[27:27] <= LFSR[26:26]; 
    LFSR[26:26] <= LFSR[25:25]; 
    LFSR[25:25] <= LFSR[24:24]; 
    LFSR[24:24] <= LFSR[23:23]; 
    LFSR[23:23] <= LFSR[22:22]; 
    LFSR[22:22] <= LFSR[21:21]; 
    LFSR[21:21] <= LFSR[20:20];
    LFSR[20:20]<= LFSR[19:19];
    LFSR[19:19] <= LFSR[18:18]; 
    LFSR[18:18] <= LFSR[17:17]; 
    LFSR[17:17] <= LFSR[16:16]; 
    LFSR[16:16] <= LFSR[15:15]; 
    LFSR[15:15] <= LFSR[14:14]; 
    LFSR[14:14] <= LFSR[13:13]; 
    LFSR[13:13] <= LFSR[12:12]; 
    LFSR[12:12] <= LFSR[11:11]; 
    LFSR[11:11] <= LFSR[10:10];
    LFSR[10:10]<= LFSR[9:9];
    LFSR[9:9] <= LFSR[8:8]; 
    LFSR[8:8] <= LFSR[7:7]; 
    LFSR[7:7] <= LFSR[6:6]; 
    LFSR[6:6] <= LFSR[5:5]; 
    LFSR[5:5] <= LFSR[4:4]; 
    LFSR[4:4] <= LFSR[3:3]; 
    LFSR[3:3] <= LFSR[2:2]; 
    LFSR[2:2] <= LFSR[1:1]; 
    LFSR[1:1] <= LFSR[0:0];
    LFSR[0:0] <= LFSR[63:63];
    end
    end
    end

    assign q = LFSR;
    endmodule

I am trying to get 64 bit output to drive a 64 input circuit in FPGA. When I synthesize the code its omitting the LFSR.

[Synth 8-3332] Sequential element (LFSR_reg[63]) is unused and will be removed from module LFSR8_8E.(63 more like this)

Any idea how to get this working.

Thanks in Advance

SIMULATION

4

1 回答 1

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LFSR 的初始化值或种子对其正常运行至关重要。在上面的设计中，您为 LFSR 寄存器分配了一个复位值或“0”。如果初始值为“0”，则在每个时钟沿，将通过 XOR 运算计算的 LFSR 的新值也将为“0”。因此，LFSR 将始终具有值“0”。

综合工具解析您的代码，并发现您的代码中的任何操作都不会导致 LFSR 寄存器的值发生任何变化。因此，它会删除整个 LFSR 寄存器，这就是为什么您会看到警告 [Synth 8-3332]

LFSR 永远不应该有一个全零的值。如果我将重置值修改为 64'hffff_ffff_ffff_ffff，则 LFSR 工作正常。

修改后的代码如下。

module LFSR8_8E(reset_, clock, q, lfsr_to);
  input clock, reset_;
  output [63:0] q;
  output lfsr_to;

  reg [63:0] LFSR;
  wire  lfsr_to;

  assign lfsr_to = (LFSR == 64'h9C69832196724182);

  always @(posedge clock or negedge reset_)
  begin
    if (!reset_) 
      LFSR[63:0] <= 64'hffff_ffff_ffff_ffff;
    else if (lfsr_to) 
      LFSR[63:0] <= 64'hffff_ffff_ffff_ffff;
    else
    begin
      LFSR[63:0] <= {(LFSR[62]^LFSR[61]),
                     (LFSR[61]^LFSR[60]),
                     (LFSR[60]^LFSR[59]),
                     (LFSR[59]^LFSR[58]),
                     LFSR[58:0],
                     LFSR[63]};
    end
  end

  assign q = LFSR;
endmodule

我想指出的另一件事是，如果 LFSR 是最大长度 LFSR，那么条件

    else if (lfsr_to) 
      LFSR[63:0] <= 64'hffff_ffff_ffff_ffff;

没有必要。LFSR 寄存器将自动翻转到初始值。查看代码，我认为 LFSR 多项式不会生成最大长度。你可能想看看维基百科上的一些标准 LFSR 多项式

在此处查看 EDA 操场上的设计仿真 https://www.edaplayground.com/x/5NpK

如果您需要根据外部信号禁用 LFSR，可在https://www.edaplayground.com/x/n7Q获得修改后的代码

于 2016-10-18T05:42:00.833 回答