xschem/fractional_n_divider/src/trng.v

//
// Written by StanOverflow
//   https://stackoverflow.com/questions/14497877/how-to-implement-a-pseudo-hardware-random-number-generator
//
// TRNG
//   A true random number generator.
//   
//   A Galois Ring Oscillators(GARO) which is similar to an LFSR but
//   completely asynchronous and will oscillate chaotically.
//   The output bit will be sampled randomly.
//


module trng (
        input clk, 
        input rst, 
        input stop,
        output random
    );

    (* keep *) wire [31:1] stage;
    reg meta1, meta2;

    assign random = meta2;

    always@(posedge clk or posedge rst)
    if (rst) begin
        meta1 <= 1'b0;
        meta2 <= 1'b0;
    end else begin
        meta1 <= stage[1];
        meta2 <= meta1;
    end

    assign stage[1] = ~&{stage[2] ^ stage[1],stop};
    assign stage[2] = !stage[3];
    assign stage[3] = !stage[4] ^ stage[1];
    assign stage[4] = !stage[5] ^ stage[1];
    assign stage[5] = !stage[6] ^ stage[1];
    assign stage[6] = !stage[7] ^ stage[1];
    assign stage[7] = !stage[8];
    assign stage[8] = !stage[9] ^ stage[1];
    assign stage[9] = !stage[10] ^ stage[1];
    assign stage[10] = !stage[11];
    assign stage[11] = !stage[12];
    assign stage[12] = !stage[13] ^ stage[1];
    assign stage[13] = !stage[14];
    assign stage[14] = !stage[15] ^ stage[1];
    assign stage[15] = !stage[16] ^ stage[1];
    assign stage[16] = !stage[17] ^ stage[1];
    assign stage[17] = !stage[18];
    assign stage[18] = !stage[19];
    assign stage[19] = !stage[20] ^ stage[1];
    assign stage[20] = !stage[21] ^ stage[1];
    assign stage[21] = !stage[22];
    assign stage[22] = !stage[23];
    assign stage[23] = !stage[24];
    assign stage[24] = !stage[25];
    assign stage[25] = !stage[26];
    assign stage[26] = !stage[27] ^ stage[1];
    assign stage[27] = !stage[28];
    assign stage[28] = !stage[29];
    assign stage[29] = !stage[30];
    assign stage[30] = !stage[31];
    assign stage[31] = !stage[1];

endmodule