unstable_counters.v

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// baeckler - 11-16-2005

module unstable_counters (clk,rst,dat);

input clk,rst;
output [7:0] dat;
wire [7:0] dat;

function [7:0] rev_byte;
	input [7:0] b;
	begin
		rev_byte = {b[0],b[1],b[2],b[3],b[4],b[5],b[6],b[7]};
	end
endfunction

function [7:0] mash_word;
	input [15:0] w;
	begin
		mash_word = w[7:0] ^ rev_byte(w[15:8]);
	end
endfunction

wire [15:0] nd0_out;
wire [15:0] nd1_out;
wire [15:0] nd2_out;
wire [15:0] nd3_out;

// unstable counters with delay loops in the 75-100 cell neighborhood
ring_counter nd0 (.clk(clk),.rst(rst),.out(nd0_out));
defparam nd0 .DELAY = 79;
ring_counter nd1 (.clk(clk),.rst(rst),.out(nd1_out));
defparam nd1 .DELAY = 83;
ring_counter nd2 (.clk(clk),.rst(rst),.out(nd2_out));
defparam nd2 .DELAY = 89;
ring_counter nd3 (.clk(clk),.rst(rst),.out(nd3_out));
defparam nd3 .DELAY = 97;

assign dat = (mash_word(nd0_out) ^ mash_word(nd3_out) ^ 
		rev_byte(mash_word(nd1_out)) ^ rev_byte(mash_word(nd2_out)));

endmodule