//======================================================================
//
// external_avalanche_entropy.v
// ----------------------------
// Entropy provider for an external entropy source based on
// avalanche noise. (or any other source that ca toggle a single
// bit input).
//
// Currently the design consists of a free running counter. At every
// positive flank detected the LSB of the counter is pushed into
// a 32bit shift register.
//
//
// Author: Joachim Strombergson
// Copyright (c) 2011, NORDUnet A/S All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
//   * Redistributions of source code must retain the above copyright
//     notice, this list of conditions and the following disclaimer.
//
//   * Redistributions in binary form must reproduce the above
//     copyright notice, this list of conditions and the following
//     disclaimer in the documentation and/or other materials
//     provided with the distribution.
//
//   * Neither the name of the NORDUnet nor the names of its
//    contributors may be used to endorse or promote products
//    derived from this software without specific prior written
//    permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
// FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
// COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
// INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
// BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
// LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
// CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
// ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
// POSSIBILITY OF SUCH DAMAGE.
//
//======================================================================

module external_avalanche_entropy(
                                  input wire           clk,
                                  input wire           reset_n,

                                  input wire           cs,
                                  input wire           we,
                                  input wire  [7 : 0]  address,
                                  input wire  [31 : 0] write_data,
                                  output wire [31 : 0] read_data,
                                  output wire          error,

                                  input wire           noise,

                                  input wire           entropy_read,
                                  output wire          entropy_ready,
                                  output wire [31 : 0] entropy_data,
                                  output wire [7 : 0]  debug
                                 );


  //----------------------------------------------------------------
  // Internal constant and parameter definitions.
  //----------------------------------------------------------------
  parameter ADDR_STATUS      = 8'h00;
  parameter ADDR_ENTROPY     = 8'h10;
  parameter ADDR_POS_FLANKS  = 8'h20;
  parameter ADDR_NEG_FLANKS  = 8'h21;
  parameter ADDR_TOT_FLANKS  = 8'h22;

  parameter DEBUG_RATE   = 32'h00300000;
  parameter SECONDS_RATE = 32'h02faf080;


  //----------------------------------------------------------------
  // Registers including update variables and write enable.
  //----------------------------------------------------------------
  reg          noise_sample0_reg;
  reg          noise_sample_reg;

  reg          flank0_reg;
  reg          flank1_reg;

  reg [31 : 0] cycle_ctr_reg;

  reg [31 : 0] seconds_ctr_reg;
  reg [31 : 0] seconds_ctr_new;

  reg [5 :  0] bit_ctr_reg;
  reg [5 :  0] bit_ctr_new;
  reg          bit_ctr_inc;
  reg          bit_ctr_rst;
  reg          bit_ctr_we;

  reg [31 : 0] entropy_reg;
  reg [31 : 0] entropy_new;
  reg          entropy_we;

  reg          entropy_ready_reg;
  reg          entropy_ready_new;

  reg [31 : 0] debug_ctr_reg;
  reg [31 : 0] debug_ctr_new;
  reg          debug_ctr_we;

  reg [7 : 0]  debug_reg;
  reg [7 : 0]  debug_new;
  reg          debug_we;

  reg [31 : 0] posflank_ctr_reg;
  reg [31 : 0] posflank_ctr_new;
  reg          posflank_ctr_rst;
  reg          posflank_ctr_we;

  reg [31 : 0] negflank_ctr_reg;
  reg [31 : 0] negflank_ctr_new;
  reg          negflank_ctr_rst;
  reg          negflank_ctr_we;

  reg [31 : 0] posflank_sample_reg;
  reg [31 : 0] posflank_sample_new;
  reg          posflank_sample_we;

  reg [31 : 0] negflank_sample_reg;
  reg [31 : 0] negflank_sample_new;
  reg          negflank_sample_we;

  reg [31 : 0] totflank_sample_reg;
  reg [31 : 0] totflank_sample_new;
  reg          totflank_sample_we;


  //----------------------------------------------------------------
  // Wires.
  //----------------------------------------------------------------
  reg [31 : 0]   tmp_read_data;
  reg            tmp_error;


  //----------------------------------------------------------------
  // Concurrent connectivity for ports etc.
  //----------------------------------------------------------------
  assign entropy_ready = entropy_ready_reg;
  assign entropy_data  = entropy_reg;
  assign debug         = debug_reg;

  assign read_data     = tmp_read_data;
  assign error         = tmp_error;


  //----------------------------------------------------------------
  // reg_update
  //----------------------------------------------------------------
  always @ (posedge clk or negedge reset_n)
    begin
      if (!reset_n)
        begin
          noise_sample0_reg   <= 1'b0;
          noise_sample_reg    <= 1'b0;
          flank0_reg          <= 1'b0;
          flank1_reg          <= 1'b0;
          debug_reg           <= 8'h00;
          entropy_ready_reg   <= 1'b0;
          entropy_reg         <= 32'h00000000;
          cycle_ctr_reg       <= 32'h00000000;
          seconds_ctr_reg     <= 32'h00000000;
          debug_ctr_reg       <= 32'h00000000;
          bit_ctr_reg         <= 6'h00;
          posflank_ctr_reg    <= 32'h00000000;
          negflank_ctr_reg    <= 32'h00000000;
          posflank_sample_reg <= 32'h00000000;
          negflank_sample_reg <= 32'h00000000;
          totflank_sample_reg <= 32'h00000000;
        end
      else
        begin
          noise_sample0_reg <= noise;
          noise_sample_reg  <= noise_sample0_reg;

          flank0_reg        <= noise_sample_reg;
          flank1_reg        <= flank0_reg;

          entropy_ready_reg <= entropy_ready_new;

          cycle_ctr_reg     <= cycle_ctr_reg + 1'b1;
          seconds_ctr_reg   <= cycle_ctr_reg + 1'b1;
          debug_ctr_reg     <= debug_ctr_new;

          if (bit_ctr_we)
            begin
              bit_ctr_reg <= bit_ctr_new;
            end

          if (entropy_we)
            begin
              entropy_reg <= entropy_new;
            end

          if (debug_we)
            begin
              debug_reg <= debug_new;
            end

          if (posflank_ctr_we)
            begin
              posflank_ctr_reg <= posflank_ctr_new;
            end

          if (negflank_ctr_we)
            begin
              posflank_ctr_reg <= posflank_ctr_new;
            end

          if (posflank_sample_we)
            begin
              posflank_sample_reg <= posflank_sample_new;
            end

          if (negflank_sample_we)
            begin
              negflank_sample_reg <= negflank_sample_new;
            end

          if (totflank_sample_we)
            begin
              totflank_sample_reg <= totflank_sample_new;
            end
        end
    end // reg_update


  //----------------------------------------------------------------
  // entropy_collect
  //
  // We collect entropy by adding the LSB of the cycle counter to
  // the entropy shift register every time we detect a positive
  // flank in the noise source.
  //----------------------------------------------------------------
  always @*
    begin : entropy_collect
      entropy_new = 32'h00000000;
      entropy_we  = 1'b0;
      bit_ctr_inc = 1'b0;

      if ((flank0_reg) && (!flank1_reg))
        begin
          entropy_new = {entropy_reg[30 : 0], cycle_ctr_reg[0]};
          entropy_we  = 1'b1;
          bit_ctr_inc = 1'b1;
        end
    end // entropy_collect


  //----------------------------------------------------------------
  // entropy_read_logic
  //
  // The logic needed to handle detection that entropy has been
  // read and ensure that we collect more than 32 entropy
  // bits beforeproviding more entropy.
  //----------------------------------------------------------------
  always @*
    begin : entropy_read_logic
      bit_ctr_new       = 6'h00;
      bit_ctr_we        = 1'b0;
      entropy_ready_new = 1'b0;

      if (bit_ctr_reg == 6'h20)
        begin
          entropy_ready_new = 1'b1;
        end

      if ((bit_ctr_inc) && (bit_ctr_reg < 6'h20))
        begin
          bit_ctr_new = bit_ctr_reg + 1'b1;
          bit_ctr_we  = 1'b1;
        end
      else if (entropy_read || bit_ctr_rst)
        begin
          bit_ctr_new = 6'h00;
          bit_ctr_we  = 1'b1;
        end
      end // entropy_read_logic


  //----------------------------------------------------------------
  // debug_update
  //
  // Sample the entropy register as debug value at the given
  // DEBUG_RATE.
  //----------------------------------------------------------------
  always @*
    begin : debug_update
      debug_ctr_new = debug_ctr_reg + 1'b1;
      debug_new     = entropy_reg[7 : 0];
      debug_we      = 1'b0;

      if (debug_ctr_reg == DEBUG_RATE)
        begin
          debug_ctr_new = 32'h00000000;
          debug_we      = 1'b1;
        end
    end // debug_update


  //----------------------------------------------------------------
  // flank_counters
  //----------------------------------------------------------------
  always @*
    begin : flank_counters
      posflank_ctr_new = 32'h00000000;
      posflank_ctr_we  = 1'b0;
      negflank_ctr_new = 32'h00000000;
      negflank_ctr_we  = 1'b0;

      if ((flank0_reg) && (!flank1_reg))
        begin
          posflank_ctr_new = posflank_ctr_reg + 1'b1;
          posflank_ctr_we  = 1'b0;
        end

      if ((!flank0_reg) && (flank1_reg))
        begin
          negflank_ctr_new = negflank_ctr_reg + 1'b1;
          negflank_ctr_we  = 1'b0;
        end
    end // flank_counters


  //----------------------------------------------------------------
  // stats_updates
  //
  // Update the statistics counters once every second.
  //----------------------------------------------------------------
  always @*
    begin : stats_update
      seconds_ctr_new     = seconds_ctr_reg + 1'b1;
      posflank_sample_new = 32'h00000000;
      posflank_sample_we  = 1'b0;
      negflank_sample_new = 32'h00000000;
      negflank_sample_we  = 1'b0;
      totflank_sample_new = 32'h00000000;
      totflank_sample_we  = 1'b0;
      posflank_ctr_rst    = 1'b0;
      negflank_ctr_rst    = 1'b0;

      if (seconds_ctr_reg == SECONDS_RATE)
        begin
          seconds_ctr_new = 32'h00000000;
          posflank_sample_new = posflank_ctr_reg;
          negflank_sample_new = negflank_ctr_reg;
          totflank_sample_new = posflank_ctr_reg + negflank_ctr_reg;
          posflank_sample_we  = 1'b1;
          negflank_sample_we  = 1'b1;
          totflank_sample_we  = 1'b1;
          posflank_ctr_rst    = 1'b1;
          negflank_ctr_rst    = 1'b1;
        end
    end // stats_update


  //----------------------------------------------------------------
  // api_logic
  //----------------------------------------------------------------
  always @*
    begin : api_logic
      tmp_read_data = 32'h00000000;
      tmp_error     = 1'b0;
      bit_ctr_rst   = 1'b1;

      if (cs)
        begin
          if (we)
            begin
              case (address)
                // Write operations.

                default:
                  begin
                    tmp_error = 1;
                  end
              endcase // case (address)
            end // if (we)

          else
            begin
              case (address)
                // Read operations.
                ADDR_STATUS:
                  begin
                    tmp_read_data = {31'h00000000, entropy_ready_reg};
                   end

                ADDR_ENTROPY:
                  begin
                    tmp_read_data = entropy_reg;
                    bit_ctr_rst   = 1'b1;
                  end

                ADDR_POS_FLANKS:
                  begin
                    tmp_read_data = posflank_sample_reg;
                  end

                ADDR_NEG_FLANKS:
                  begin
                    tmp_read_data = negflank_sample_reg;
                  end

                ADDR_TOT_FLANKS:
                  begin
                    tmp_read_data = totflank_sample_reg;
                  end

                default:
                  begin
                    tmp_error = 1;
                  end
              endcase // case (address)
            end // else: !if(we)
        end // if (cs)
    end // api_logic

endmodule // external_avalanche_entropy

//======================================================================
// EOF external_avalanche_entropy.v
//======================================================================