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//======================================================================
//
// 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 noise,
output wire [7 : 0] debug
);
//----------------------------------------------------------------
// Internal constant and parameter definitions.
//----------------------------------------------------------------
parameter DEBUG_RATE = 32'h00300000;
//----------------------------------------------------------------
// 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] entropy_reg;
reg [31 : 0] entropy_new;
reg entropy_we;
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;
//----------------------------------------------------------------
// Concurrent connectivity for ports etc.
//----------------------------------------------------------------
assign debug = debug_reg;
//----------------------------------------------------------------
// reg_update
//----------------------------------------------------------------
always @ (posedge clk or negedge reset_n)
begin
if (!reset_n)
begin
noise_sample_reg <= 1'b0;
flank0_reg <= 1'b0;
flank1_reg <= 1'b0;
cycle_ctr_reg <= 32'h00000000;
debug_ctr_reg <= 32'h00000000;
entropy_reg <= 32'h00000000;
debug_reg <= 8'h00;
end
else
begin
noise_sample0_reg <= noise;
noise_sample_reg <= noise_sample0_reg;
flank0_reg <= noise_sample_reg;
flank1_reg <= flank0_reg;
cycle_ctr_reg <= cycle_ctr_reg + 1'b1;
debug_ctr_reg <= debug_ctr_new;
if (entropy_we)
begin
entropy_reg <= entropy_new;
end
if (debug_we)
begin
debug_reg <= debug_new;
end
end
end // reg_update
//----------------------------------------------------------------
// entropy_collect
//
// This is where we collect entropy by adding the LSB of the
// cycle counter to the entropy shift register every time
// we detect a positive flank at the noise source.
//----------------------------------------------------------------
always @*
begin : entropy_collect
entropy_new = 32'h00000000;
entropy_we = 1'b0;
// Update the entropy shift register every positive flank.
if ((flank0_reg) && (!flank1_reg))
begin
entropy_new = {entropy_reg[30 : 0], cycle_ctr_reg[0]};
entropy_we = 1'b1;
end
end // entropy_collect
//----------------------------------------------------------------
// 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
endmodule // external_avalanche_entropy
//======================================================================
// EOF external_avalanche_entropy.v
//======================================================================
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