New LF edge detection algorithm + lowpass filter

This is a new LF edge detection algorithm for the FPGA.

- It uses a low-pass IIR filter to clean the signal
(see https://fail0verflow.com/blog/2014/proxmark3-fpga-iir-filter.html)
- The algorithm is able to detect consecutive peaks in the same
  direction
- It uses an envelope follower to dynamically adjust the peak thresholds
- The main threshold used in the envelope follower can be set from the ARM side

fpga/lf_edge_detect.v,
fpga/lp20khz_1MSa_iir_filter.v,
fpga/min_max_tracker.v: New file.

fpga/lo_edge_detect.v, fpga/fpga_lf.v: Modify accordingly.

armsrc/apps.h (FPGA_CMD_SET_USER_BYTE1,
FPGA_CMD_SET_EDGE_DETECT_THRESHOLD): New FPGA command.
fpga/fpga_lf.v: Modify accordingly/Add a 8bit user register.

fpga/fpga_lf.bit: Update accordingly.

fpga/tests: New directory for testbenches

fpga/tests/Makefile: New file. It compiles the testbenches
and runs all the tests by default (comparing with the golden output)

fpga/tests/tb_lp20khz_1MSa_iir_filter.v,
fpga/tests/tb_min_max_tracker.v,
fpga/tests/tb_lf_edge_detect.v: New testbenches

fpga/tests/plot_edgedetect.py: New script to plot the results from
the edge detection tests.

fpga/tests/tb_data: New directory for data and golden outputs

fpga/tests/tb_min_max_tracker.v

@@ -0,0 +1,74 @@
+//-----------------------------------------------------------------------------
+// Copyright (C) 2014 iZsh <izsh at fail0verflow.com>
+//
+// This code is licensed to you under the terms of the GNU GPL, version 2 or,
+// at your option, any later version. See the LICENSE.txt file for the text of
+// the license.
+//-----------------------------------------------------------------------------
+// testbench for min_max_tracker
+`include "min_max_tracker.v"
+
+`define FIN "tb_tmp/data.filtered.gold"
+`define FOUT_MIN "tb_tmp/data.min"
+`define FOUT_MAX "tb_tmp/data.max"
+
+module min_max_tracker_tb;
+
+	integer fin;
+	integer fout_min, fout_max;
+	integer r;
+
+	reg clk;
+	reg [7:0] adc_d;
+	wire [7:0] min;
+	wire [7:0] max;
+
+	initial
+	begin
+		clk = 0;
+		fin = $fopen(`FIN, "r");
+		if (!fin) begin
+			$display("ERROR: can't open the data file");
+			$finish;
+		end
+		fout_min = $fopen(`FOUT_MIN, "w+");		
+		fout_max = $fopen(`FOUT_MAX, "w+");		
+		if (!$feof(fin))
+			adc_d = $fgetc(fin); // read the first value
+	end
+
+	always
+		# 1 clk = !clk;
+
+	// input
+	initial
+	begin
+		while (!$feof(fin)) begin
+			@(negedge clk) adc_d <= $fgetc(fin);
+		end
+
+		if ($feof(fin))
+		begin
+			# 3 $fclose(fin);
+			$fclose(fout_min);
+			$fclose(fout_max);
+			$finish;
+		end
+	end
+
+	initial
+	begin
+		// $monitor("%d\t min: %x, max: %x", $time, min, max);		
+	end
+
+	// output
+	always @(negedge clk)
+	if ($time > 2) begin
+		r = $fputc(min, fout_min);
+		r = $fputc(max, fout_max);
+	end
+
+	// module to test
+	min_max_tracker tracker(clk, adc_d, 8'd127, min, max);
+
+endmodule