How to implement a (pseudo) hardware random number generator

Question

How do you implement a hardware random number generator in an HDL (verilog)?

What options need to be considered?

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This question is following the self-an

Answer 1

As noted in Morgan's answer this will only produce a single random bit. The number of bits in the LFSR only set how many values you get before the sequence repeats. If you want an N bit random number you have to run the LFSR for N cycles. However, if you want a new number every clock cycle the other option is to unroll the loop and predict what the number will be in N cycles. Repeating Morgan's example below, but to get a 5 bit number each cycle:

module fibonacci_lfsr_5bit(
  input clk,
  input rst_n,

  output reg [4:0] data
);

reg [4:0] data_next;

always @* begin
  data_next[4] = data[4]^data[1];
  data_next[3] = data[3]^data[0];
  data_next[2] = data[2]^data_next[4];
  data_next[1] = data[1]^data_next[3];
  data_next[0] = data[0]^data_next[2];
end

always @(posedge clk or negedge rst_n)
  if(!rst_n)
    data <= 5'h1f;
  else
    data <= data_next;

endmodule

Edit: Added a new version below which doesn't require you to do the math. Just put it in a loop and let the synthesis tool figure out the logic:

module fibonacci_lfsr_nbit
   #(parameter BITS = 5)
   (
    input             clk,
    input             rst_n,

    output reg [4:0] data
    );

   reg [4:0] data_next;
   always_comb begin
      data_next = data;
      repeat(BITS) begin
         data_next = {(data_next[4]^data_next[1]), data_next[4:1]};
      end
   end

   always_ff @(posedge clk or negedge reset) begin
      if(!rst_n)
         data <= 5'h1f;
      else
         data <= data_next;
      end
   end

endmodule

I would like to make the LFSR length parameterizable as well, but that is much more difficult since the feedback taps don't follow a simple pattern.