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/min_max_tracker.v

@@ -0,0 +1,65 @@
+//-----------------------------------------------------------------------------
+// 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.
+//-----------------------------------------------------------------------------
+// track min and max peak values (envelope follower)
+//
+// NB: the min value (resp. max value) is updated only when the next high peak
+// (resp. low peak) is reached/detected, since you can't know it isn't a
+// local minima (resp. maxima) until then.
+// This also means the peaks are detected with an unpredictable delay.
+// This algorithm can't therefore be used directly for realtime peak detections,
+// but it can be used as a simple envelope follower.
+module min_max_tracker(input clk, input [7:0] adc_d, input [7:0] threshold,
+	output [7:0] min, output [7:0] max);
+
+	reg [7:0] min_val = 255;
+	reg [7:0] max_val = 0;
+	reg [7:0] cur_min_val = 255;
+	reg [7:0] cur_max_val = 0;
+	reg [1:0] state = 0;
+
+	always @(posedge clk)
+	begin
+		case (state)
+		0:
+			begin
+				if (cur_max_val >= ({1'b0, adc_d} + threshold))
+					state <= 2;
+				else if (adc_d >= ({1'b0, cur_min_val} + threshold))
+					state <= 1;
+				if (cur_max_val <= adc_d)
+					cur_max_val <= adc_d;
+				else if (adc_d <= cur_min_val)
+					cur_min_val <= adc_d;					
+			end
+		1:
+			begin
+				if (cur_max_val <= adc_d)
+					cur_max_val <= adc_d;
+				else if (({1'b0, adc_d} + threshold) <= cur_max_val) begin
+					state <= 2;
+					cur_min_val <= adc_d;
+					max_val <= cur_max_val;
+				end
+			end
+		2:
+			begin
+				if (adc_d <= cur_min_val)
+					cur_min_val <= adc_d;					
+				else if (adc_d >= ({1'b0, cur_min_val} + threshold)) begin
+					state <= 1;
+					cur_max_val <= adc_d;
+					min_val <= cur_min_val;
+				end
+			end
+		endcase
+	end
+
+	assign min = min_val;
+	assign max = max_val;
+
+endmodule