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//======================================================================
//
// rosc.v
// ------
// Digital ring oscillator used as entropy source. Based on the
// idea of using carry chain in adders as inverter by Bernd Paysan.
//
//
//
// Author: Joachim Strombergson
// Copyright (c) 2014, 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 rosc #(parameter WIDTH = 2)
(
input wire clk,
input wire reset_n,
input wire we,
input wire [(WIDTH - 1) : 0] opa,
input wire [(WIDTH - 1) : 0] opb,
output wire dout
);
//----------------------------------------------------------------
// Registers.
//----------------------------------------------------------------
reg dout_reg;
reg dout_new;
//----------------------------------------------------------------
// Concurrent assignment.
//----------------------------------------------------------------
assign dout = dout_reg;
//----------------------------------------------------------------
// reg_update
//----------------------------------------------------------------
always @ (posedge clk or negedge reset_n)
begin
if (!reset_n)
begin
dout_reg <= 1'b0;
end
else
begin
if (we)
begin
dout_reg <= dout_new;
end
end
end
//----------------------------------------------------------------
// adder_osc
//
// Adder logic that generates the oscillator.
//
// NOTE: This logic contains a combinational loop and does
// not play well with an event driven simulator.
//----------------------------------------------------------------
always @*
begin: adder_osc
reg [WIDTH : 0] sum;
reg cin;
cin = ~sum[WIDTH];
sum = opa + opb + cin;
dout_new = sum[WIDTH];
end
endmodule // rosc
//======================================================================
// EOF rosc.v
//======================================================================
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