X-Git-Url: https://git.zerfleddert.de/cgi-bin/gitweb.cgi/proxmark3-svn/blobdiff_plain/6658905f18a1eebc148836f26c731dea9c1377dc..b5cefff12c4b8c415926a1ccef3f3f528629e2d0:/fpga/hi_iso14443a.v

diff --git a/fpga/hi_iso14443a.v b/fpga/hi_iso14443a.v
index eb03fa23..ccb51d8f 100644
--- a/fpga/hi_iso14443a.v
+++ b/fpga/hi_iso14443a.v
@@ -3,6 +3,13 @@
 // Gerhard de Koning Gans, April 2008
 //-----------------------------------------------------------------------------
 
+// constants for the different modes:
+`define SNIFFER			3'b000
+`define TAGSIM_LISTEN	3'b001
+`define TAGSIM_MOD		3'b010
+`define READER_LISTEN	3'b011
+`define READER_MOD		3'b100
+
 module hi_iso14443a(
     pck0, ck_1356meg, ck_1356megb,
     pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4,
@@ -22,48 +29,33 @@ module hi_iso14443a(
     output dbg;
     input [2:0] mod_type;
 
-reg ssp_clk;
-reg ssp_frame;
 
-reg fc_div_2;
-always @(posedge ck_1356meg)
-    fc_div_2 = ~fc_div_2;
+wire adc_clk = ck_1356meg;
+
 
-wire adc_clk;
-assign adc_clk = ck_1356meg;
 
-reg after_hysteresis, after_hysteresis_prev1, after_hysteresis_prev2, after_hysteresis_prev3;
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// Reader -> PM3:
+// detecting and shaping the reader's signal. Reader will modulate the carrier by 100% (signal is either on or off). Use a 
+// hysteresis (Schmitt Trigger) to avoid false triggers during slowly increasing or decreasing carrier amplitudes
+reg after_hysteresis;
 reg [11:0] has_been_low_for;
-reg [8:0] saw_deep_modulation;
-reg [2:0] deep_counter;
-reg deep_modulation;
+
 always @(negedge adc_clk)
 begin
-	if(& adc_d[7:6]) after_hysteresis <= 1'b1;
-    else if(~(| adc_d[7:4])) after_hysteresis <= 1'b0;
+	if(adc_d >= 16) after_hysteresis <= 1'b1;			// U >= 1,14V 	-> after_hysteresis = 1
+    else if(adc_d < 8) after_hysteresis <= 1'b0;  		// U < 	1,04V 	-> after_hysteresis = 0
+	// Note: was >= 3,53V and <= 1,19V. The new trigger values allow more reliable detection of the first bit 
+	// (it might not reach 3,53V due to the high time constant of the high pass filter in the analogue RF part).
+	// In addition, the new values are more in line with ISO14443-2: "The PICC shall detect the ”End of Pause” after the field exceeds 
+	// 5% of H_INITIAL and before it exceeds 60% of H_INITIAL." Depending on the signal strength, 60% might well be less than 3,53V.
 	
-	if(~(| adc_d[7:0]))
-	begin
-		if(deep_counter == 3'd7)
-		begin
-			deep_modulation <= 1'b1;
-			saw_deep_modulation <= 8'd0;
-		end
-		else
-			deep_counter <= deep_counter + 1;
-	end
-	else
-	begin
-		deep_counter <= 3'd0;
-		if(saw_deep_modulation == 8'd255)
-			deep_modulation <= 1'b0;
-		else
-			saw_deep_modulation <= saw_deep_modulation + 1;
-	end
 	
-	if(after_hysteresis)
+	// detecting a loss of reader's field (adc_d < 192 for 4096 clock cycles). If this is the case, 
+	// set the detected reader signal (after_hysteresis) to '1' (unmodulated)
+	if(adc_d >= 192)
     begin
-        has_been_low_for <= 7'b0;
+        has_been_low_for <= 12'd0;
     end
     else
     begin
@@ -73,288 +65,513 @@ begin
             after_hysteresis <= 1'b1;
         end
         else
+		begin
             has_been_low_for <= has_been_low_for + 1;
+		end	
     end
+	
 end
 
-// Report every 4 subcarrier cycles
-// 64 periods of carrier frequency => 6-bit counter [negedge_cnt]
-reg [5:0] negedge_cnt;
-reg bit1, bit2, bit3;
-reg [3:0] count_ones;
-reg [3:0] count_zeros;
-wire [7:0] avg;
-reg [7:0] lavg;
-reg signed [12:0] step1;
-reg signed [12:0] step2;
-reg [7:0] stepsize;
-reg curbit;
-reg [12:0] average;
-wire signed [9:0] dif;
 
-// A register to send the results to the arm
-reg signed [7:0] to_arm;
 
-assign avg[7:0] = average[11:4];
-assign dif = lavg - avg;
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// Reader -> PM3
+// detect when a reader is active (modulating). We assume that the reader is active, if we see the carrier off for at least 8 
+// carrier cycles. We assume that the reader is inactive, if the carrier stayed high for at least 256 carrier cycles. 
+reg deep_modulation;
+reg [2:0] deep_counter;
+reg [8:0] saw_deep_modulation;
+
+always @(negedge adc_clk)
+begin
+	if(~(| adc_d[7:0]))									// if adc_d == 0 (U <= 0,94V)
+	begin
+		if(deep_counter == 3'd7)						// adc_d == 0 for 8 adc_clk ticks -> deep_modulation (by reader)
+		begin
+			deep_modulation <= 1'b1;
+			saw_deep_modulation <= 8'd0;
+		end
+		else
+			deep_counter <= deep_counter + 1;
+	end
+	else							
+	begin
+		deep_counter <= 3'd0;
+		if(saw_deep_modulation == 8'd255)				// adc_d != 0 for 256 adc_clk ticks -> deep_modulation is over, probably waiting for tag's response
+			deep_modulation <= 1'b0;
+		else
+			saw_deep_modulation <= saw_deep_modulation + 1;
+	end
+end
 
-reg bit_to_arm;
-reg fdt_indicator, fdt_elapsed;
-reg [10:0] fdt_counter;
-reg [47:0] mod_sig_buf;
-wire mod_sig_buf_empty;
-reg [5:0] mod_sig_ptr;
-reg [3:0] mod_sig_flip;
-reg mod_sig, mod_sig_coil;
-reg temp_buffer_reset;
-reg sendbit;
 
-assign mod_sig_buf_empty = ~(|mod_sig_buf[47:0]);
-reg [2:0] ssp_frame_counter;
 
-// ADC data appears on the rising edge, so sample it on the falling edge
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// Tag -> PM3
+// filter the input for a tag's signal. The filter box needs the 4 previous input values and is a gaussian derivative filter
+// for noise reduction and edge detection.
+// store 4 previous samples:
+reg [7:0] input_prev_4, input_prev_3, input_prev_2, input_prev_1;
+
 always @(negedge adc_clk)
 begin
+	input_prev_4 <= input_prev_3;
+	input_prev_3 <= input_prev_2;
+	input_prev_2 <= input_prev_1;
+	input_prev_1 <= adc_d;
+end	
+
+// adc_d_filtered = 2*input_prev4 + 1*input_prev3 + 0*input_prev2 - 1*input_prev1 - 2*input
+//					= (2*input_prev4 + input_prev3) - (2*input + input_prev1) 
+wire [8:0] input_prev_4_times_2 = input_prev_4 << 1;
+wire [8:0] adc_d_times_2 		= adc_d << 1;
+
+wire [9:0] tmp1 = input_prev_4_times_2 + input_prev_3;
+wire [9:0] tmp2 = adc_d_times_2 + input_prev_1;
+
+// convert intermediate signals to signed and calculate the filter output
+wire signed [10:0] adc_d_filtered = {1'b0, tmp1} - {1'b0, tmp2};
+
 
-	// last bit = 0 then fdt = 1172, in case of 0x26 (7-bit command, LSB first!)
-	// last bit = 1 then fdt = 1236, in case of 0x52 (7-bit command, LSB first!)
-	if(fdt_counter == 11'd740) fdt_indicator = 1'b1;
 	
-	if(fdt_counter == 11'd1148)
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// internal FPGA timing. Maximum required period is 128 carrier clock cycles for a full 8 Bit transfer to ARM. (i.e. we need a 
+// 7 bit counter). Adjust its frequency to external reader's clock when simulating a tag or sniffing.
+reg pre_after_hysteresis; 
+reg [3:0] reader_falling_edge_time;
+reg [6:0] negedge_cnt;
+
+always @(negedge adc_clk)
+begin
+	// detect a reader signal's falling edge and remember its timing:
+	pre_after_hysteresis <= after_hysteresis;
+	if (pre_after_hysteresis && ~after_hysteresis)
 	begin
-		if(fdt_elapsed)
+		reader_falling_edge_time[3:0] <= negedge_cnt[3:0];
+	end
+
+	// adjust internal timer counter if necessary:
+	if (negedge_cnt[3:0] == 4'd13 && (mod_type == `SNIFFER || mod_type == `TAGSIM_LISTEN) && deep_modulation)
+	begin
+		if (reader_falling_edge_time == 4'd1) 			// reader signal changes right after sampling. Better sample earlier next time. 
 		begin
-			if(negedge_cnt[3:0] == mod_sig_flip[3:0]) mod_sig_coil <= mod_sig;
+			negedge_cnt <= negedge_cnt + 2;				// time warp
+		end	
+		else if (reader_falling_edge_time == 4'd0)		// reader signal changes right before sampling. Better sample later next time.
+		begin
+			negedge_cnt <= negedge_cnt;					// freeze time
 		end
 		else
 		begin
-			mod_sig_flip[3:0] <= negedge_cnt[3:0];
-			mod_sig_coil <= mod_sig;
-			fdt_elapsed = 1'b1;
-			fdt_indicator = 1'b0;
-
-			if(~(| mod_sig_ptr[5:0])) mod_sig_ptr <= 6'b001001;
-			else temp_buffer_reset = 1'b1; // fix position of the buffer pointer
+			negedge_cnt <= negedge_cnt + 1;				// Continue as usual
 		end
+		reader_falling_edge_time[3:0] <= 4'd8;			// adjust only once per detected edge
 	end
+	else if (negedge_cnt == 7'd127)						// normal operation: count from 0 to 127
+	begin
+		negedge_cnt <= 0;
+	end	
 	else
 	begin
-		fdt_counter <= fdt_counter + 1;
+		negedge_cnt <= negedge_cnt + 1;
 	end
-	
-	if(& negedge_cnt[3:0])
+end	
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// Tag -> PM3:
+// determine best possible time for starting/resetting the modulation detector.
+reg [3:0] mod_detect_reset_time;
+
+always @(negedge adc_clk)
+begin
+	if (mod_type == `READER_LISTEN) 
+	// (our) reader signal changes at negedge_cnt[3:0]=9, tag response expected to start n*16+4 ticks later, further delayed by
+	// 3 ticks ADC conversion. The maximum filter output (edge detected) will be detected after subcarrier zero crossing (+7 ticks).
+	// To allow some timing variances, we want to have the maximum filter outputs well within the detection window, i.e.
+	// at mod_detect_reset_time+4 and mod_detect_reset_time+12  (-4 ticks).
+	// 9 + 4 + 3 + 7 - 4  = 19.    19 mod 16 = 3
+	begin
+		mod_detect_reset_time <= 4'd4;
+	end
+	else
+	if (mod_type == `SNIFFER)
 	begin
-		// When there is a dip in the signal and not in reader mode
-		if(~after_hysteresis && mod_sig_buf_empty && ~((mod_type == 3'b100) || (mod_type == 3'b011) || (mod_type == 3'b010))) // last condition to prevent reset
+		// detect a rising edge of reader's signal and sync modulation detector to the tag's answer:
+		if (~pre_after_hysteresis && after_hysteresis && deep_modulation)
+		// reader signal rising edge detected at negedge_cnt[3:0]. This signal had been delayed 
+		// 9 ticks by the RF part + 3 ticks by the A/D converter + 1 tick to assign to after_hysteresis.
+		// Then the same as above.
+		// - 9 - 3 - 1 + 4 + 3 + 7 - 4 = -3
 		begin
-			fdt_counter <= 11'd0;
-			fdt_elapsed = 1'b0;
-			fdt_indicator = 1'b0;
-			temp_buffer_reset = 1'b0;
-			mod_sig_ptr <= 6'b000000;
+			mod_detect_reset_time <= negedge_cnt[3:0] - 4'd3;
 		end
-		
-		lavg <= avg;
-		
-		if(stepsize<16) stepsize = 8'd16;
+	end
+end
 
-		if(dif>0)
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// Tag -> PM3:
+// modulation detector. Looks for the steepest falling and rising edges within a 16 clock period. If there is both a significant
+// falling and rising edge (in any order), a modulation is detected.
+reg signed [10:0] rx_mod_falling_edge_max;
+reg signed [10:0] rx_mod_rising_edge_max;
+reg curbit;
+
+`define EDGE_DETECT_THRESHOLD	5
+
+always @(negedge adc_clk)
+begin
+	if(negedge_cnt[3:0] == mod_detect_reset_time)
+	begin
+		// detect modulation signal: if modulating, there must have been a falling AND a rising edge
+		if ((rx_mod_falling_edge_max > `EDGE_DETECT_THRESHOLD) && (rx_mod_rising_edge_max < -`EDGE_DETECT_THRESHOLD))
+				curbit <= 1'b1;	// modulation
+			else
+				curbit <= 1'b0;	// no modulation
+		// reset modulation detector
+		rx_mod_rising_edge_max <= 0;
+		rx_mod_falling_edge_max <= 0;
+	end
+	else											// look for steepest edges (slopes)
+	begin
+		if (adc_d_filtered > 0)
 		begin
-			step1 = dif*3;
-			step2 = stepsize*2; // 3:2
-			if(step1>step2)
-			begin
-				curbit = 1'b0;
-				stepsize = dif;
-			end
+			if (adc_d_filtered > rx_mod_falling_edge_max)
+				rx_mod_falling_edge_max <= adc_d_filtered;
 		end
 		else
 		begin
-			step1 = dif*3;
-			step1 = -step1;
-			step2 = stepsize*2;
-			if(step1>step2)
-			begin
-				curbit = 1'b1;
-				stepsize = -dif;
-			end
+			if (adc_d_filtered < rx_mod_rising_edge_max)
+				rx_mod_rising_edge_max <= adc_d_filtered;
 		end
-		
-		if(curbit)
-		begin
-			count_zeros <= 4'd0;
-			if(& count_ones[3:2])
+	end
+
+end
+
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// Tag+Reader -> PM3
+// sample 4 bits reader data and 4 bits tag data for sniffing
+reg [3:0] reader_data;
+reg [3:0] tag_data;
+
+always @(negedge adc_clk)
+begin
+    if(negedge_cnt[3:0] == 4'd0)
+	begin
+        reader_data[3:0] <= {reader_data[2:0], after_hysteresis};
+		tag_data[3:0] <= {tag_data[2:0], curbit};
+	end
+end	
+
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// PM3 -> Reader:
+// a delay line to ensure that we send the (emulated) tag's answer at the correct time according to ISO14443-3
+reg [31:0] mod_sig_buf;
+reg [4:0] mod_sig_ptr;
+reg mod_sig;
+
+always @(negedge adc_clk)
+begin
+	if(negedge_cnt[3:0] == 4'd0) 	// sample data at rising edge of ssp_clk - ssp_dout changes at the falling edge.
+	begin
+		mod_sig_buf[31:2] <= mod_sig_buf[30:1];  			// shift
+		if (~ssp_dout && ~mod_sig_buf[1])
+			mod_sig_buf[1] <= 1'b0;							// delete the correction bit (a single 1 preceded and succeeded by 0)
+		else
+			mod_sig_buf[1] <= mod_sig_buf[0];
+		mod_sig_buf[0] <= ssp_dout;							// add new data to the delay line
+
+		mod_sig = mod_sig_buf[mod_sig_ptr];					// the delayed signal.
+	end
+end
+
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// PM3 -> Reader, internal timing:
+// a timer for the 1172 cycles fdt (Frame Delay Time). Start the timer with a rising edge of the reader's signal.
+// set fdt_elapsed when we no longer need to delay data. Set fdt_indicator when we can start sending data.
+// Note: the FPGA only takes care for the 1172 delay. To achieve an additional 1236-1172=64 ticks delay, the ARM must send
+// a correction bit (before the start bit). The correction bit will be coded as 00010000, i.e. it adds 4 bits to the 
+// transmission stream, causing the required additional delay.
+reg [10:0] fdt_counter;
+reg fdt_indicator, fdt_elapsed;
+reg [3:0] mod_sig_flip;
+reg [3:0] sub_carrier_cnt;
+
+// we want to achieve a delay of 1172. The RF part already has delayed the reader signals's rising edge
+// by 9 ticks, the ADC took 3 ticks and there is always a delay of 32 ticks by the mod_sig_buf. Therefore need to
+// count to 1172 - 9 - 3 - 32 = 1128
+`define FDT_COUNT 11'd1128
+
+// The ARM must not send too early, otherwise the mod_sig_buf will overflow, therefore signal that we are ready
+// with fdt_indicator. The mod_sig_buf can buffer 29 excess data bits, i.e. a maximum delay of 29 * 16 = 464 adc_clk ticks.
+// fdt_indicator could appear at ssp_din after 1 tick, the transfer needs 16 ticks, the ARM can send 128 ticks later.
+// 1128 - 464 - 1 - 128 - 8 = 535
+`define FDT_INDICATOR_COUNT 11'd535
+
+// reset on a pause in listen mode. I.e. the counter starts when the pause is over:
+assign fdt_reset = ~after_hysteresis && mod_type == `TAGSIM_LISTEN;
+
+always @(negedge adc_clk)
+begin
+	if (fdt_reset)
+	begin
+		fdt_counter <= 11'd0;
+		fdt_elapsed <= 1'b0;
+		fdt_indicator <= 1'b0;
+	end	
+	else
+	begin
+		if(fdt_counter == `FDT_COUNT)
+		begin						
+			if(~fdt_elapsed)							// just reached fdt.
 			begin
-				curbit = 1'b0; // suppressed signal
-				stepsize = 8'd24; // just a fine number
+				mod_sig_flip <= negedge_cnt[3:0];		// start modulation at this time
+				sub_carrier_cnt <= 4'd0;				// subcarrier phase in sync with start of modulation
+				fdt_elapsed <= 1'b1;
 			end
 			else
 			begin
-				count_ones <= count_ones + 1;
-			end
-		end
+				sub_carrier_cnt <= sub_carrier_cnt + 1;
+			end	
+		end	
 		else
 		begin
-			count_ones <= 4'd0;
-			if(& count_zeros[3:0])
+			fdt_counter <= fdt_counter + 1;
+		end
+	end
+	
+	if(fdt_counter == `FDT_INDICATOR_COUNT) fdt_indicator <= 1'b1;
+end
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// PM3 -> Reader or Tag
+// assign a modulation signal to the antenna. This signal is either a delayed signal (to achieve fdt when sending to a reader)
+// or undelayed when sending to a tag
+reg mod_sig_coil;
+
+always @(negedge adc_clk)
+begin
+	if (mod_type == `TAGSIM_MOD)			 // need to take care of proper fdt timing
+	begin
+		if(fdt_counter == `FDT_COUNT)
+		begin
+			if(fdt_elapsed)
 			begin
-				stepsize = 8'd24;
+				if(negedge_cnt[3:0] == mod_sig_flip) mod_sig_coil <= mod_sig;
 			end
 			else
 			begin
-				count_zeros <= count_zeros + 1;
+				mod_sig_coil <= mod_sig;	// just reached fdt. Immediately assign signal to coil
 			end
 		end
-		
-		// What do we communicate to the ARM
-		if(mod_type == 3'b001) sendbit = after_hysteresis;
-		else if(mod_type == 3'b010)
+	end
+	else 									// other modes: don't delay
+	begin
+		mod_sig_coil <= ssp_dout;
+	end	
+end
+
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// PM3 -> Reader
+// determine the required delay in the mod_sig_buf (set mod_sig_ptr).
+reg temp_buffer_reset;
+
+always @(negedge adc_clk)
+begin
+	if(fdt_reset)
+	begin
+		mod_sig_ptr <= 5'd0;
+		temp_buffer_reset = 1'b0;
+	end	
+	else
+	begin
+		if(fdt_counter == `FDT_COUNT && ~fdt_elapsed)							// if we just reached fdt
+			if(~(| mod_sig_ptr[4:0])) 
+				mod_sig_ptr <= 5'd8;  											// ... but didn't buffer a 1 yet, delay next 1 by n*128 ticks.
+			else 
+				temp_buffer_reset = 1'b1; 										// else no need for further delays.
+
+		if(negedge_cnt[3:0] == 4'd0) 											// at rising edge of ssp_clk - ssp_dout changes at the falling edge.
 		begin
-			if(fdt_counter > 11'd772) sendbit = mod_sig_coil;
-			else sendbit = fdt_indicator;
+			if((ssp_dout || (| mod_sig_ptr[4:0])) && ~fdt_elapsed)				// buffer a 1 (and all subsequent data) until fdt is reached.
+				if (mod_sig_ptr == 5'd31) 
+					mod_sig_ptr <= 5'd0;										// buffer overflow - data loss.
+				else 
+					mod_sig_ptr <= mod_sig_ptr + 1;								// increase buffer (= increase delay by 16 adc_clk ticks). mod_sig_ptr always points ahead of first 1.
+			else if(fdt_elapsed && ~temp_buffer_reset)							
+			begin
+				// wait for the next 1 after fdt_elapsed before fixing the delay and starting modulation. This ensures that the response can only happen
+				// at intervals of 8 * 16 = 128 adc_clk ticks (as defined in ISO14443-3)
+				if(ssp_dout) 
+					temp_buffer_reset = 1'b1;							
+				if(mod_sig_ptr == 5'd1) 
+					mod_sig_ptr <= 5'd8;										// still nothing received, need to go for the next interval
+				else 
+					mod_sig_ptr <= mod_sig_ptr - 1;								// decrease buffer.
+			end
 		end
-		else if(mod_type == 3'b011) sendbit = curbit;
-		else sendbit = 1'b0;
-
 	end
+end
 
-	if(~(| negedge_cnt[3:0])) average <= adc_d;
-	else average <= average + adc_d;
 
-	if(negedge_cnt == 7'd63)
-    begin
-		if(deep_modulation)
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// FPGA -> ARM communication:
+// buffer 8 bits data to be sent to ARM. Shift them out bit by bit.
+reg [7:0] to_arm;
+
+always @(negedge adc_clk)
+begin
+	if (negedge_cnt[5:0] == 6'd63)							// fill the buffer
+	begin
+		if (mod_type == `SNIFFER)
 		begin
-			to_arm <= {after_hysteresis_prev1,after_hysteresis_prev2,after_hysteresis_prev3,after_hysteresis,1'b0,1'b0,1'b0,1'b0};
+			if(deep_modulation) 							// a reader is sending (or there's no field at all)
+			begin
+				to_arm <= {reader_data[3:0], 4'b0000};		// don't send tag data
+			end
+			else
+			begin
+				to_arm <= {reader_data[3:0], tag_data[3:0]};
+			end			
 		end
 		else
 		begin
-			to_arm <= {after_hysteresis_prev1,after_hysteresis_prev2,after_hysteresis_prev3,after_hysteresis,bit1,bit2,bit3,curbit};
+			to_arm[7:0] <= {mod_sig_ptr[4:0], mod_sig_flip[3:1]}; // feedback timing information
 		end
+	end	
 
-        negedge_cnt <= 0;
-	
-		end
-    else
-    begin
-        negedge_cnt <= negedge_cnt + 1;
-    end
-
-    if(negedge_cnt == 6'd15)
-	begin
-        after_hysteresis_prev1 <= after_hysteresis;
-		bit1 <= curbit;
-	end
-    if(negedge_cnt == 6'd31)
+	if(negedge_cnt[2:0] == 3'b000 && mod_type == `SNIFFER)	// shift at double speed
 	begin
-        after_hysteresis_prev2 <= after_hysteresis;
-		bit2 <= curbit;
-	end
-    if(negedge_cnt == 6'd47)
-	begin
-        after_hysteresis_prev3 <= after_hysteresis;
-		bit3 <= curbit;
+		// Don't shift if we just loaded new data, obviously.
+		if(negedge_cnt[5:0] != 6'd0)
+		begin
+			to_arm[7:1] <= to_arm[6:0];
+		end
 	end
-	
 
-	if(mod_type != 3'b000)
+	if(negedge_cnt[3:0] == 4'b0000 && mod_type != `SNIFFER)
 	begin
-		if(negedge_cnt[3:0] == 4'b1000)
+		// Don't shift if we just loaded new data, obviously.
+		if(negedge_cnt[6:0] != 7'd0)
 		begin
-			// The modulation signal of the tag
-			mod_sig_buf[47:0] <= {mod_sig_buf[46:1], ssp_dout, 1'b0};
-			if((ssp_dout || (| mod_sig_ptr[5:0])) && ~fdt_elapsed)
-				if(mod_sig_ptr == 6'b101110)
-				begin
-					mod_sig_ptr <= 6'b000000;
-				end
-				else mod_sig_ptr <= mod_sig_ptr + 1;
-			else if(fdt_elapsed && ~temp_buffer_reset)
-			begin
-				if(ssp_dout) temp_buffer_reset = 1'b1;
-				if(mod_sig_ptr == 6'b000010) mod_sig_ptr <= 6'b001001;
-				else mod_sig_ptr <= mod_sig_ptr - 1;
-			end
-			else
-			begin
-				// side effect: when ptr = 1 it will cancel the first 1 of every block of ones
-				if(~mod_sig_buf[mod_sig_ptr-1] && ~mod_sig_buf[mod_sig_ptr+1]) mod_sig = 1'b0;
-				else mod_sig = mod_sig_buf[mod_sig_ptr] & fdt_elapsed; // & fdt_elapsed  was for direct relay to oe4
-			end
+			to_arm[7:1] <= to_arm[6:0];
 		end
 	end
 	
-	// SSP Clock and data
-	if(mod_type == 3'b000)
+end
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// FPGA <-> ARM communication:
+// generate a ssp clock and ssp frame signal for the synchronous transfer from/to the ARM
+reg ssp_clk;
+reg ssp_frame;
+
+always @(negedge adc_clk)
+begin
+	if(mod_type == `SNIFFER)
+	// SNIFFER mode (ssp_clk = adc_clk / 8, ssp_frame clock = adc_clk / 64)):
 	begin
-		if(negedge_cnt[2:0] == 3'b100)
-			ssp_clk <= 1'b0;
-			
-		if(negedge_cnt[2:0] == 3'b000)
-		begin
+		if(negedge_cnt[2:0] == 3'd0)
 			ssp_clk <= 1'b1;
-			// Don't shift if we just loaded new data, obviously.
-			if(negedge_cnt != 7'd0)
-			begin
-				to_arm[7:1] <= to_arm[6:0];
-			end
-		end
+		if(negedge_cnt[2:0] == 3'd4)
+			ssp_clk <= 1'b0;
 
-		if(negedge_cnt[5:4] == 2'b00)
-			ssp_frame = 1'b1;
-		else
-			ssp_frame = 1'b0;
-		
-		bit_to_arm = to_arm[7];
+		if(negedge_cnt[5:0] == 6'd0)	// ssp_frame rising edge indicates start of frame
+			ssp_frame <= 1'b1;
+		if(negedge_cnt[5:0] == 6'd8)	
+			ssp_frame <= 1'b0;
 	end
 	else
+	// all other modes (ssp_clk = adc_clk / 16, ssp_frame clock = adc_clk / 128):
 	begin
-		if(negedge_cnt[3:0] == 4'b1000) ssp_clk <= 1'b0;
+		if(negedge_cnt[3:0] == 4'd0)
+			ssp_clk <= 1'b1;
+		if(negedge_cnt[3:0] == 4'd8) 
+			ssp_clk <= 1'b0;
 
-		if(negedge_cnt[3:0] == 4'b0111)
-		begin
-			if(ssp_frame_counter == 3'd7) ssp_frame_counter <= 3'd0;
-			else ssp_frame_counter <= ssp_frame_counter + 1;
-		end
+		if(negedge_cnt[6:0] == 7'd7)	// ssp_frame rising edge indicates start of frame
+			ssp_frame <= 1'b1;
+		if(negedge_cnt[6:0] == 7'd23)
+			ssp_frame <= 1'b0;
+	end	
+end
 
-		if(negedge_cnt[3:0] == 4'b0000)
-		begin
-			ssp_clk <= 1'b1;
-		end
-		
-		ssp_frame = (ssp_frame_counter == 3'd7);
-	
-		bit_to_arm = sendbit;
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// FPGA -> ARM communication:
+// select the data to be sent to ARM
+reg bit_to_arm;
+reg sendbit;
+
+always @(negedge adc_clk)
+begin
+	if(negedge_cnt[3:0] == 4'd0)
+	begin
+		// What do we communicate to the ARM
+		if(mod_type == `TAGSIM_LISTEN) 
+			sendbit = after_hysteresis;
+		else if(mod_type == `TAGSIM_MOD)
+			/* if(fdt_counter > 11'd772) sendbit = mod_sig_coil; // huh?
+			else */ 
+			sendbit = fdt_indicator;
+		else if (mod_type == `READER_LISTEN)
+			sendbit = curbit;
+		else
+			sendbit = 1'b0;
 	end
-	
+
+
+	if(mod_type == `SNIFFER)
+		// send sampled reader and tag data:
+		bit_to_arm = to_arm[7];
+	else if (mod_type == `TAGSIM_MOD && fdt_elapsed && temp_buffer_reset)
+		// send timing information:
+		bit_to_arm = to_arm[7];
+	else
+		// send data or fdt_indicator
+		bit_to_arm = sendbit;
 end
 
+
+
+
 assign ssp_din = bit_to_arm;
 
-// Modulating carrier frequency is fc/16
-wire modulating_carrier;
-assign modulating_carrier = (mod_sig_coil & negedge_cnt[3] & (mod_type == 3'b010));
-assign pwr_hi = (ck_1356megb & (((mod_type == 3'b100) & ~mod_sig_coil) || (mod_type == 3'b011)));
+// Subcarrier (adc_clk/16, for TAGSIM_MOD only).
+wire sub_carrier;
+assign sub_carrier = ~sub_carrier_cnt[3];
 
-// This one is all LF, so doesn't matter
-//assign pwr_oe2 = modulating_carrier;
-assign pwr_oe2 = 1'b0;
+// in READER_MOD: drop carrier for mod_sig_coil==1 (pause); in READER_LISTEN: carrier always on; in other modes: carrier always off
+assign pwr_hi = (ck_1356megb & (((mod_type == `READER_MOD) & ~mod_sig_coil) || (mod_type == `READER_LISTEN)));	
 
-// Toggle only one of these, since we are already producing much deeper
-// modulation than a real tag would.
-//assign pwr_oe1 = modulating_carrier;
-assign pwr_oe1 = 1'b0;
-assign pwr_oe4 = modulating_carrier;
-//assign pwr_oe4 = 1'b0;
 
-// This one is always on, so that we can watch the carrier.
-//assign pwr_oe3 = modulating_carrier;
+// Enable HF antenna drivers:
+assign pwr_oe1 = 1'b0;
 assign pwr_oe3 = 1'b0;
 
+// TAGSIM_MOD: short circuit antenna with different resistances (modulated by sub_carrier modulated by mod_sig_coil)
+// for pwr_oe4 = 1 (tristate): antenna load = 10k || 33			= 32,9 Ohms
+// for pwr_oe4 = 0 (active):   antenna load = 10k || 33 || 33  	= 16,5 Ohms
+assign pwr_oe4 = mod_sig_coil & sub_carrier & (mod_type == `TAGSIM_MOD);
 
-assign dbg = negedge_cnt[3];
-
-// Unused.
+// This is all LF, so doesn't matter.
+assign pwr_oe2 = 1'b0;
 assign pwr_lo = 1'b0;
 
+
+assign dbg = negedge_cnt[3];
+
 endmodule