X-Git-Url: http://git.zerfleddert.de/cgi-bin/gitweb.cgi/proxmark3-svn/blobdiff_plain/6658905f18a1eebc148836f26c731dea9c1377dc..347dea669a04d1cc692600d06324eb9df7438de4:/fpga/hi_iso14443a.v

diff --git a/fpga/hi_iso14443a.v b/fpga/hi_iso14443a.v
index eb03fa23..ec5aa757 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,
@@ -25,26 +32,25 @@ module hi_iso14443a(
 reg ssp_clk;
 reg ssp_frame;
 
-reg fc_div_2;
-always @(posedge ck_1356meg)
-    fc_div_2 = ~fc_div_2;
-
 wire adc_clk;
 assign adc_clk = ck_1356meg;
 
-reg after_hysteresis, after_hysteresis_prev1, after_hysteresis_prev2, after_hysteresis_prev3;
+reg after_hysteresis, pre_after_hysteresis, after_hysteresis_prev1, after_hysteresis_prev2, after_hysteresis_prev3, after_hysteresis_prev4;
 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[7:6]) after_hysteresis <= 1'b1;			// adc_d >= 196 (U >= 3,28V) -> after_hysteris = 1
+    else if(~(| adc_d[7:4])) after_hysteresis <= 1'b0;  // if adc_d <= 15 (U <= 1,13V) -> after_hysteresis = 0
+
+	pre_after_hysteresis <= after_hysteresis;
 	
-	if(~(| adc_d[7:0]))
+	if(~(| adc_d[7:0]))									// if adc_d == 0 (U <= 0,94V)
 	begin
-		if(deep_counter == 3'd7)
+		if(deep_counter == 3'd7)						// adc_d == 0 for 7 adc_clk ticks -> deep_modulation (by reader)
 		begin
 			deep_modulation <= 1'b1;
 			saw_deep_modulation <= 8'd0;
@@ -52,10 +58,10 @@ begin
 		else
 			deep_counter <= deep_counter + 1;
 	end
-	else
+	else							
 	begin
 		deep_counter <= 3'd0;
-		if(saw_deep_modulation == 8'd255)
+		if(saw_deep_modulation == 8'd255)				// adc_d != 0 for 255 adc_clk ticks -> deep_modulation is over, now waiting for tag's response
 			deep_modulation <= 1'b0;
 		else
 			saw_deep_modulation <= saw_deep_modulation + 1;
@@ -63,229 +69,309 @@ begin
 	
 	if(after_hysteresis)
     begin
-        has_been_low_for <= 7'b0;
+        has_been_low_for <= 12'd0;
     end
     else
     begin
         if(has_been_low_for == 12'd4095)
         begin
             has_been_low_for <= 12'd0;
-            after_hysteresis <= 1'b1;
+            after_hysteresis <= 1'b1;					// reset after_hysteresis to 1 if it had been 0 for 4096 cycles (no field)
         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;
+// 128 periods of carrier frequency => 7-bit counter [negedge_cnt]
+reg [6:0] negedge_cnt;
+reg bit1, bit2, bit3, bit4;
 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;
+// storage for four previous samples:
+reg [7:0] adc_d_1;
+reg [7:0] adc_d_2;
+reg [7:0] adc_d_3;
+reg [7:0] adc_d_4;
+
+// the filtered signal (filter performs noise reduction and edge detection)
+// (gaussian derivative)
+wire signed [10:0] adc_d_filtered;
+assign adc_d_filtered = (adc_d_4 << 1) + adc_d_3 - adc_d_1 - (adc_d << 1);
+
+// Registers to store steepest edges detected:
+reg [7:0] rx_mod_falling_edge_max;
+reg [7:0] rx_mod_rising_edge_max;
+
+// A register to send 8 Bit results to the arm
+reg [7:0] to_arm;
 
-assign avg[7:0] = average[11:4];
-assign dif = lavg - avg;
 
 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 [47:0] mod_sig_buf;
+reg [31:0] mod_sig_buf;
+//reg [5:0] mod_sig_ptr;
+reg [4:0] 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;
+reg [3:0] sub_carrier_cnt;
+reg[3:0] reader_falling_edge_time;
 
 // ADC data appears on the rising edge, so sample it on the falling edge
 always @(negedge adc_clk)
 begin
-
-	// 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;
+	// ------------------------------------------------------------------------------------------------------------------------------------------------------------------
+	// relevant for TAGSIM_MOD only. Timing of Tag's answer relative to a command received from a reader
+	// ISO14443-3 specifies:
+	// fdt = 1172, if last bit was 0.
+	// fdt = 1236, if last bit was 1.
+	// the FPGA takes care for the 1172 delay. To achieve the additional 1236-1172=64 ticks delay, the ARM must send an additional 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 delay.
+	if(fdt_counter == 11'd547) fdt_indicator <= 1'b1; 	// The ARM must not send earlier to prevent mod_sig_buf overflow.
+														// 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, 16 ticks for the transfer, 128 ticks until response is sended.
+														// 1148 - 464 - 1 - 128 - 8 = 547
 	
-	if(fdt_counter == 11'd1148)
+	if ((mod_type == `TAGSIM_MOD) || (mod_type == `TAGSIM_LISTEN))
 	begin
-		if(fdt_elapsed)
+		if(fdt_counter == 11'd1148) // the RF part delays the rising edge by approx 5 adc_clk_ticks, the ADC needs 3 clk_ticks for A/D conversion,
+									// 16 ticks delay by mod_sig_buf
+									// 1172 - 5 - 3 - 16 = 1148.
 		begin
-			if(negedge_cnt[3:0] == mod_sig_flip[3:0]) mod_sig_coil <= mod_sig;
+			if(fdt_elapsed)
+			begin
+				if(negedge_cnt[3:0] == mod_sig_flip) mod_sig_coil <= mod_sig; // start modulating (if mod_sig is already set)
+				sub_carrier_cnt[3:0] <= sub_carrier_cnt[3:0] + 1;
+			end
+			else
+			begin
+				mod_sig_flip <= negedge_cnt[3:0];			// start modulation at this time
+				sub_carrier_cnt[3:0] <= 0;					// subcarrier phase in sync with start of modulation
+				mod_sig_coil <= mod_sig;					// assign signal to coil
+				fdt_elapsed = 1'b1;
+				if(~(| mod_sig_ptr[4:0])) mod_sig_ptr <= 5'd9;  // if mod_sig_ptr == 0 -> didn't receive a 1 yet. Delay next 1 by n*128 ticks.
+				else temp_buffer_reset = 1'b1; 					// else fix the buffer size at current position
+			end
 		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
+			fdt_counter <= fdt_counter + 1; // Count until 1155
 		end
 	end
-	else
+	else // other modes: don't use the delay line.
 	begin
-		fdt_counter <= fdt_counter + 1;
-	end
+		mod_sig_coil <= ssp_dout;
+	end	
 	
-	if(& negedge_cnt[3:0])
+	
+	//-------------------------------------------------------------------------------------------------------------------------------------------
+	// Relevant for READER_LISTEN only
+	// look for steepest falling and rising edges:
+
+	if(negedge_cnt[3:0] == 4'd1)					// reset modulation detector. Save current edge.
 	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
+		if (adc_d_filtered > 0)
 		begin
-			fdt_counter <= 11'd0;
-			fdt_elapsed = 1'b0;
-			fdt_indicator = 1'b0;
-			temp_buffer_reset = 1'b0;
-			mod_sig_ptr <= 6'b000000;
+			rx_mod_falling_edge_max <= adc_d_filtered;
+			rx_mod_rising_edge_max <= 0;
+		end	
+		else
+		begin
+			rx_mod_falling_edge_max <= 0;
+			rx_mod_rising_edge_max <= -adc_d_filtered;
 		end
-		
-		lavg <= avg;
-		
-		if(stepsize<16) stepsize = 8'd16;
-
-		if(dif>0)
+	end
+	else											// detect modulation
+	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)
+	end
+
+	// detect modulation signal: if modulating, there must be a falling and a rising edge
+	if (rx_mod_falling_edge_max > 6 && rx_mod_rising_edge_max > 6)
+			curbit <= 1'b1;	// modulation
+		else
+			curbit <= 1'b0;	// no modulation
+			
+	
+	// store previous samples for filtering and edge detection:
+	adc_d_4 <= adc_d_3;
+	adc_d_3 <= adc_d_2;
+	adc_d_2 <= adc_d_1;
+	adc_d_1 <= adc_d;
+
+
+	// Relevant for TAGSIM_MOD only (timing the Tag's answer. See above)
+	// When we see end of a modulation and we are emulating a Tag, start fdt_counter.
+	// Reset fdt_counter when modulation is detected.
+	if(~after_hysteresis /* && mod_sig_buf_empty */ && mod_type == `TAGSIM_LISTEN)
+	begin
+		fdt_counter <= 11'd0;
+		fdt_elapsed = 1'b0;
+		fdt_indicator <= 1'b0;
+		temp_buffer_reset = 1'b0;
+		mod_sig_ptr <= 5'b00000;
+		mod_sig = 1'b0;
+	end	
+
+
+	if(negedge_cnt[3:0] == 4'd1)
+	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
+
+
+
+	// check timing of a falling edge in reader signal
+	if (pre_after_hysteresis && ~after_hysteresis)
+		reader_falling_edge_time[3:0] <= negedge_cnt[3:0];
+
+
+
+	// sync clock to external reader's clock:
+	if (negedge_cnt[3:0] == 4'd13 && (mod_type == `SNIFFER || mod_type == `TAGSIM_MOD || mod_type == `TAGSIM_LISTEN))
+	begin
+		// adjust clock if necessary:
+		if (reader_falling_edge_time < 4'd8 && reader_falling_edge_time > 4'd1)
 		begin
-			count_zeros <= 4'd0;
-			if(& count_ones[3:2])
-			begin
-				curbit = 1'b0; // suppressed signal
-				stepsize = 8'd24; // just a fine number
-			end
-			else
-			begin
-				count_ones <= count_ones + 1;
-			end
+			negedge_cnt <= negedge_cnt;				// freeze time
+		end	
+		else if (reader_falling_edge_time == 4'd8)
+		begin
+			negedge_cnt <= negedge_cnt + 1;			// the desired state. Advance as usual;
 		end
 		else
 		begin
-			count_ones <= 4'd0;
-			if(& count_zeros[3:0])
+			negedge_cnt[3:0] <= 4'd15;				// time warp
+		end
+		reader_falling_edge_time <= 4'd8;			// only once per detected rising edge
+	end
+	
+
+
+	//------------------------------------------------------------------------------------------------------------------------------------------
+	// Prepare 8 Bits to communicate to ARM
+	if (negedge_cnt == 7'd63)
+	begin
+		if (mod_type == `SNIFFER)
+		begin
+			if(deep_modulation) // a reader is sending (or there's no field at all)
 			begin
-				stepsize = 8'd24;
+				to_arm <= {after_hysteresis_prev1,after_hysteresis_prev2,after_hysteresis_prev3,after_hysteresis_prev4,1'b0,1'b0,1'b0,1'b0};
 			end
 			else
 			begin
-				count_zeros <= count_zeros + 1;
-			end
+				to_arm <= {after_hysteresis_prev1,after_hysteresis_prev2,after_hysteresis_prev3,after_hysteresis_prev4,bit1,bit2,bit3,bit4};
+			end			
+			negedge_cnt <= 0;
 		end
-		
-		// What do we communicate to the ARM
-		if(mod_type == 3'b001) sendbit = after_hysteresis;
-		else if(mod_type == 3'b010)
+		else
 		begin
-			if(fdt_counter > 11'd772) sendbit = mod_sig_coil;
-			else sendbit = fdt_indicator;
+			negedge_cnt <= negedge_cnt + 1;
 		end
-		else if(mod_type == 3'b011) sendbit = curbit;
-		else sendbit = 1'b0;
-
-	end
-
-	if(~(| negedge_cnt[3:0])) average <= adc_d;
-	else average <= average + adc_d;
-
-	if(negedge_cnt == 7'd63)
-    begin
-		if(deep_modulation)
+	end	
+	else if(negedge_cnt == 7'd127)
+	begin
+		if (mod_type == `TAGSIM_MOD)
 		begin
-			to_arm <= {after_hysteresis_prev1,after_hysteresis_prev2,after_hysteresis_prev3,after_hysteresis,1'b0,1'b0,1'b0,1'b0};
+			to_arm[7:0] <= {mod_sig_ptr[4:0], mod_sig_flip[3:1]};
+			negedge_cnt <= 0;
 		end
 		else
 		begin
-			to_arm <= {after_hysteresis_prev1,after_hysteresis_prev2,after_hysteresis_prev3,after_hysteresis,bit1,bit2,bit3,curbit};
+			to_arm[7:0] <= 8'd0;
+			negedge_cnt <= negedge_cnt + 1;
 		end
+	end
+	else
+	begin
+		negedge_cnt <= negedge_cnt + 1;
+	end
 
-        negedge_cnt <= 0;
 	
-		end
-    else
-    begin
-        negedge_cnt <= negedge_cnt + 1;
-    end
-
-    if(negedge_cnt == 6'd15)
+    if(negedge_cnt == 7'd1)
 	begin
         after_hysteresis_prev1 <= after_hysteresis;
 		bit1 <= curbit;
 	end
-    if(negedge_cnt == 6'd31)
+    if(negedge_cnt == 7'd17)
 	begin
         after_hysteresis_prev2 <= after_hysteresis;
 		bit2 <= curbit;
 	end
-    if(negedge_cnt == 6'd47)
+    if(negedge_cnt == 7'd33)
 	begin
         after_hysteresis_prev3 <= after_hysteresis;
 		bit3 <= curbit;
 	end
+    if(negedge_cnt == 7'd49)
+	begin
+        after_hysteresis_prev4 <= after_hysteresis;
+		bit4 <= curbit;
+	end
 	
-
-	if(mod_type != 3'b000)
+	//--------------------------------------------------------------------------------------------------------------------------------------------------------------
+	// Relevant in TAGSIM_MOD only. Delay-Line to buffer data and send it at the correct time
+	if(negedge_cnt[3:0] == 4'd0) 	// at rising edge of ssp_clk - ssp_dout changes at the falling edge.
 	begin
-		if(negedge_cnt[3:0] == 4'b1000)
+		mod_sig_buf[31:0] <= {mod_sig_buf[30:1], ssp_dout, 1'b0};  			// shift in new data starting at mod_sig_buf[1]. mod_sig_buf[0] = 0 always.
+		// asign the delayed signal to mod_sig, but don't modulate with the correction bit (which is sent as 00010000, all other bits will come with at least 2 consecutive 1s)
+		// side effect: when ptr = 1 it will cancel the first 1 of every block of ones. Note: this would only be the case if we received a 1 just before fdt_elapsed.
+		if((ssp_dout || (| mod_sig_ptr[4:0])) && ~fdt_elapsed)				// buffer a 1 (and all subsequent data) until fdt_counter = 1148 adc_clk ticks.
+			//if(mod_sig_ptr == 6'b101110)									// buffer overflow at 46 - this would mean data loss
+			//begin
+			//	mod_sig_ptr <= 6'b000000;
+			//end
+			if (mod_sig_ptr == 5'd30) mod_sig_ptr <= 5'd0;
+			else mod_sig_ptr <= mod_sig_ptr + 1;							// increase buffer (= increase delay by 16 adc_clk ticks). ptr always points to first 1.
+		else if(fdt_elapsed && ~temp_buffer_reset)							
+		// fdt_elapsed. If we didn't receive a 1 yet, ptr will be at 9 and not yet fixed. Otherwise temp_buffer_reset will be 1 already.
 		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
+			// 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 intervals (as defined in ISO14443-3)
+			if(ssp_dout) temp_buffer_reset = 1'b1;							
+			if(mod_sig_ptr == 5'd2) mod_sig_ptr <= 5'd9;					// still nothing received, need to go for the next interval
+			else mod_sig_ptr <= mod_sig_ptr - 1;							// decrease buffer.
+		end
+		else
+		begin
+			if(~mod_sig_buf[mod_sig_ptr-1] && ~mod_sig_buf[mod_sig_ptr+1]) mod_sig = 1'b0;
+			// finally, assign the delayed signal:
+			else mod_sig = mod_sig_buf[mod_sig_ptr];
 		end
 	end
 	
-	// SSP Clock and data
-	if(mod_type == 3'b000)
+	//-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
+	// Communication to ARM (SSP Clock and data)
+	// SNIFFER mode (ssp_clk = adc_clk / 8, ssp_frame clock = adc_clk / 64)):
+	if(mod_type == `SNIFFER)
 	begin
 		if(negedge_cnt[2:0] == 3'b100)
 			ssp_clk <= 1'b0;
@@ -294,7 +380,7 @@ begin
 		begin
 			ssp_clk <= 1'b1;
 			// Don't shift if we just loaded new data, obviously.
-			if(negedge_cnt != 7'd0)
+			if(negedge_cnt[5:0] != 6'd0)
 			begin
 				to_arm[7:1] <= to_arm[6:0];
 			end
@@ -308,53 +394,66 @@ begin
 		bit_to_arm = to_arm[7];
 	end
 	else
+	//-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
+	// Communication to ARM (SSP Clock and data)
+	// 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'b0111)
 		begin
-			if(ssp_frame_counter == 3'd7) ssp_frame_counter <= 3'd0;
-			else ssp_frame_counter <= ssp_frame_counter + 1;
+			// if(ssp_frame_counter == 3'd7) ssp_frame_counter <= 3'd0;
+			// else ssp_frame_counter <= ssp_frame_counter + 1;
+			if (negedge_cnt[6:4] == 3'b000) ssp_frame = 1'b1;
+			else ssp_frame = 1'b0;
 		end
+		// ssp_frame = (ssp_frame_counter == 3'd7);
 
 		if(negedge_cnt[3:0] == 4'b0000)
 		begin
 			ssp_clk <= 1'b1;
+			// Don't shift if we just loaded new data, obviously.
+			if(negedge_cnt[6:0] != 7'd0)
+			begin
+				to_arm[7:1] <= to_arm[6:0];
+			end
 		end
 		
-		ssp_frame = (ssp_frame_counter == 3'd7);
-	
-		bit_to_arm = sendbit;
-	end
+		if (mod_type == `TAGSIM_MOD && fdt_elapsed && temp_buffer_reset)
+			// transmit timing information
+			bit_to_arm = to_arm[7];
+		else
+			// transmit data or fdt_indicator
+			bit_to_arm = sendbit;
+		end
 	
-end
+end	//always @(negedge adc_clk)
 
 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)));
 
-// This one is all LF, so doesn't matter
-//assign pwr_oe2 = modulating_carrier;
-assign pwr_oe2 = 1'b0;
+// Subcarrier (adc_clk/16, for TAGSIM_MOD only).
+wire sub_carrier;
+assign sub_carrier = ~sub_carrier_cnt[3];
 
-// 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;
+// 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)));	
 
-// This one is always on, so that we can watch the carrier.
-//assign pwr_oe3 = modulating_carrier;
-assign pwr_oe3 = 1'b0;
 
+// Enable HF antenna drivers:
+assign pwr_oe1 = 1'b0;
+assign pwr_oe3 = 1'b0;
 
-assign dbg = negedge_cnt[3];
+// 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));
 
-// 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