Merge pull request #837 from mwalker33/master

[proxmark3-svn] / fpga / hi_iso14443a.v

diff --git a/fpga/hi_iso14443a.v b/fpga/hi_iso14443a.v
index ec5aa75771023b75fe5b7ee0426b7aac4d5390db..e460a2ccfd57e4ad54af6158ae9af43bec33b7ef 100644 (file)
--- a/fpga/hi_iso14443a.v
+++ b/fpga/hi_iso14443a.v
@@ -3,54 +3,81 @@
 // Gerhard de Koning Gans, April 2008
 //-----------------------------------------------------------------------------
 
 // 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(
 module hi_iso14443a(

-    pck0, ck_1356meg, ck_1356megb,
+    ck_1356meg,

     pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4,
     adc_d, adc_clk,
     ssp_frame, ssp_din, ssp_dout, ssp_clk,
     pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4,
     adc_d, adc_clk,
     ssp_frame, ssp_din, ssp_dout, ssp_clk,

-    cross_hi, cross_lo,

     dbg,
     mod_type
 );
     dbg,
     mod_type
 );

-    input pck0, ck_1356meg, ck_1356megb;
+    input ck_1356meg;

     output pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4;
     input [7:0] adc_d;
     output adc_clk;
     input ssp_dout;
     output ssp_frame, ssp_din, ssp_clk;
     output pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4;
     input [7:0] adc_d;
     output adc_clk;
     input ssp_dout;
     output ssp_frame, ssp_din, ssp_clk;

-    input cross_hi, cross_lo;

     output dbg;
     input [2:0] mod_type;
 
     output dbg;
     input [2:0] mod_type;
 

-reg ssp_clk;
-reg ssp_frame;

 
 

-wire adc_clk;
-assign adc_clk = ck_1356meg;
+wire adc_clk = ck_1356meg;
+
+

 
 

-reg after_hysteresis, pre_after_hysteresis, after_hysteresis_prev1, after_hysteresis_prev2, after_hysteresis_prev3, after_hysteresis_prev4;
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// 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 [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
 
 always @(negedge adc_clk)
 begin

-       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 >= 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.
+       

        
        

+       // 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 <= 12'd0;
+    end
+    else
+    begin
+        if(has_been_low_for == 12'd4095)
+        begin
+            has_been_low_for <= 12'd0;
+            after_hysteresis <= 1'b1;
+        end
+        else
+               begin
+            has_been_low_for <= has_been_low_for + 1;
+               end     
+    end
+       
+end
+
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// 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(~(| adc_d[7:0]))                                                                     // if adc_d == 0 (U <= 0,94V)
        begin

-               if(deep_counter == 3'd7)                                                // adc_d == 0 for 7 adc_clk ticks -> deep_modulation (by reader)
+               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;
                begin
                        deep_modulation <= 1'b1;
                        saw_deep_modulation <= 8'd0;


@@ -61,136 +88,143 @@ begin
        else                                                    
        begin
                deep_counter <= 3'd0;
        else                                                    
        begin
                deep_counter <= 3'd0;

-               if(saw_deep_modulation == 8'd255)                               // adc_d != 0 for 255 adc_clk ticks -> deep_modulation is over, now waiting for tag's response
+               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
                        deep_modulation <= 1'b0;
                else
                        saw_deep_modulation <= saw_deep_modulation + 1;
        end

-       
-       if(after_hysteresis)
-    begin
-        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;                                  // 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
 
 
 
 end
 
 
 

-// Report every 4 subcarrier cycles
-// 128 periods of carrier frequency => 7-bit counter [negedge_cnt]
-reg [6:0] negedge_cnt;
-reg bit1, bit2, bit3, bit4;
-reg curbit;
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// 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;

 
 

-// 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;
+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    

 
 

-// 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);
+// 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;

 
 

-// Registers to store steepest edges detected:
-reg [7:0] rx_mod_falling_edge_max;
-reg [7:0] rx_mod_rising_edge_max;
+wire [9:0] tmp1 = input_prev_4_times_2 + input_prev_3;
+wire [9:0] tmp2 = adc_d_times_2 + input_prev_1;

 
 

-// A register to send 8 Bit results to the arm
-reg [7:0] to_arm;
+// convert intermediate signals to signed and calculate the filter output
+wire signed [10:0] adc_d_filtered = {1'b0, tmp1} - {1'b0, tmp2};

 
 
 
 

-reg bit_to_arm;
-reg fdt_indicator, fdt_elapsed;
-reg [10:0] fdt_counter;
-//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;
-reg [3:0] sub_carrier_cnt;
-reg[3:0] reader_falling_edge_time;
+       
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// 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;

 
 

-// ADC data appears on the rising edge, so sample it on the falling edge

 always @(negedge adc_clk)
 begin
 always @(negedge adc_clk)
 begin

-       // ------------------------------------------------------------------------------------------------------------------------------------------------------------------
-       // 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 ((mod_type == `TAGSIM_MOD) || (mod_type == `TAGSIM_LISTEN))
+       // detect a reader signal's falling edge and remember its timing:
+       pre_after_hysteresis <= after_hysteresis;
+       if (pre_after_hysteresis && ~after_hysteresis)

        begin
        begin

-               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.
+               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 == `FPGA_HF_ISO14443A_SNIFFER || mod_type == `FPGA_HF_ISO14443A_TAGSIM_LISTEN) && deep_modulation)
+       begin
+               if (reader_falling_edge_time == 4'd1)                   // reader signal changes right after sampling. Better sample earlier next time. 

                begin
                begin

-                       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
+                       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
                end
                else
                begin

-                       fdt_counter <= fdt_counter + 1; // Count until 1155
+                       negedge_cnt <= negedge_cnt + 1;                         // Continue as usual

                end
                end

+               reader_falling_edge_time[3:0] <= 4'd8;                  // adjust only once per detected edge

        end
        end

-       else // other modes: don't use the delay line.
+       else if (negedge_cnt == 7'd127)                                         // normal operation: count from 0 to 127

        begin
        begin

-               mod_sig_coil <= ssp_dout;
+               negedge_cnt <= 0;

        end     
        end     

-       
-       
-       //-------------------------------------------------------------------------------------------------------------------------------------------
-       // Relevant for READER_LISTEN only
-       // look for steepest falling and rising edges:
+       else
+       begin
+               negedge_cnt <= negedge_cnt + 1;
+       end
+end    
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// Tag -> PM3:
+// determine best possible time for starting/resetting the modulation detector.
+reg [3:0] mod_detect_reset_time;

 
 

-       if(negedge_cnt[3:0] == 4'd1)                                    // reset modulation detector. Save current edge.
+always @(negedge adc_clk)
+begin
+       if (mod_type == `FPGA_HF_ISO14443A_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
        begin

-               if (adc_d_filtered > 0)
-               begin
-                       rx_mod_falling_edge_max <= adc_d_filtered;
-                       rx_mod_rising_edge_max <= 0;
-               end     
-               else
+               mod_detect_reset_time <= 4'd4;
+       end
+       else
+       if (mod_type == `FPGA_HF_ISO14443A_SNIFFER)
+       begin
+               // 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
                begin

-                       rx_mod_falling_edge_max <= 0;
-                       rx_mod_rising_edge_max <= -adc_d_filtered;
+                       mod_detect_reset_time <= negedge_cnt[3:0] - 4'd3;

                end
        end
                end
        end

-       else                                                                                    // detect modulation
+end
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// 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
        begin
                if (adc_d_filtered > 0)
                begin


@@ -199,255 +233,334 @@ begin
                end
                else
                begin
                end
                else
                begin

-                       if (-adc_d_filtered > rx_mod_rising_edge_max)
-                               rx_mod_rising_edge_max <= -adc_d_filtered;
+                       if (adc_d_filtered < rx_mod_rising_edge_max)
+                               rx_mod_rising_edge_max <= adc_d_filtered;

                end
        end
 
                end
        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;
+end

 
 
 
 

-       // 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)
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// 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
        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     
+        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;

 
 

-       if(negedge_cnt[3:0] == 4'd1)
+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
        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;
+               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
                else

-                       sendbit = 1'b0;
+                       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

+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];
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// 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 is assigned to sendbit after at least 1 tick, the transfer to ARM needs minimum 8 ticks. Response from
+// ARM could appear at ssp_dout 8 ticks later.
+// 1128 - 464 - 1 - 8 - 8 = 647
+`define FDT_INDICATOR_COUNT 11'd647
+// Note: worst case, assignment to sendbit takes 15 ticks more, and transfer to ARM needs 7*16 = 112 ticks more.
+//       When the ARM's response then appears, the fdt_count is already 647 + 15 + 112 = 774, which still allows the ARM a possible
+//       response window of 1128 - 774 = 354 ticks. 

 
 

-       // 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))
+// reset on a pause in listen mode. I.e. the counter starts when the pause is over:
+assign fdt_reset = ~after_hysteresis && mod_type == `FPGA_HF_ISO14443A_TAGSIM_LISTEN;
+
+always @(negedge adc_clk)
+begin
+       if (fdt_reset)

        begin
        begin

-               // adjust clock if necessary:
-               if (reader_falling_edge_time < 4'd8 && reader_falling_edge_time > 4'd1)
-               begin
-                       negedge_cnt <= negedge_cnt;                             // freeze time
+               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
+                               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
+                               sub_carrier_cnt <= sub_carrier_cnt + 1;
+                       end     

                end     
                end     

-               else if (reader_falling_edge_time == 4'd8)
-               begin
-                       negedge_cnt <= negedge_cnt + 1;                 // the desired state. Advance as usual;
-               end

                else
                begin
                else
                begin

-                       negedge_cnt[3:0] <= 4'd15;                              // time warp
+                       fdt_counter <= fdt_counter + 1;

                end
                end

-               reader_falling_edge_time <= 4'd8;                       // only once per detected rising edge

        end
        
        end
        

+       if(fdt_counter == `FDT_INDICATOR_COUNT) fdt_indicator <= 1'b1;
+end

 
 
 
 

-       //------------------------------------------------------------------------------------------------------------------------------------------
-       // Prepare 8 Bits to communicate to ARM
-       if (negedge_cnt == 7'd63)
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// 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 == `FPGA_HF_ISO14443A_TAGSIM_MOD)                   // need to take care of proper fdt timing

        begin
        begin

-               if (mod_type == `SNIFFER)
+               if(fdt_counter == `FDT_COUNT)

                begin
                begin

-                       if(deep_modulation) // a reader is sending (or there's no field at all)
+                       if(fdt_elapsed)

                        begin
                        begin

-                               to_arm <= {after_hysteresis_prev1,after_hysteresis_prev2,after_hysteresis_prev3,after_hysteresis_prev4,1'b0,1'b0,1'b0,1'b0};
+                               if(negedge_cnt[3:0] == mod_sig_flip) mod_sig_coil <= mod_sig;

                        end
                        else
                        begin
                        end
                        else
                        begin

-                               to_arm <= {after_hysteresis_prev1,after_hysteresis_prev2,after_hysteresis_prev3,after_hysteresis_prev4,bit1,bit2,bit3,bit4};
-                       end                     
-                       negedge_cnt <= 0;
+                               mod_sig_coil <= mod_sig;        // just reached fdt. Immediately assign signal to coil
+                       end

                end
                end

-               else
+       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
                begin

-                       negedge_cnt <= negedge_cnt + 1;
+                       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
                end

-       end     
-       else if(negedge_cnt == 7'd127)
+       end
+end
+
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// 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
        begin

-               if (mod_type == `TAGSIM_MOD)
+               if (mod_type == `FPGA_HF_ISO14443A_SNIFFER)

                begin
                begin

-                       to_arm[7:0] <= {mod_sig_ptr[4:0], mod_sig_flip[3:1]};
-                       negedge_cnt <= 0;
+                       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
                end
                else
                begin

-                       to_arm[7:0] <= 8'd0;
-                       negedge_cnt <= negedge_cnt + 1;
+                       to_arm[7:0] <= {mod_sig_ptr[4:0], mod_sig_flip[3:1]}; // feedback timing information

                end
                end

-       end
-       else
-       begin
-               negedge_cnt <= negedge_cnt + 1;
-       end
+       end     

 
 

-       
-    if(negedge_cnt == 7'd1)
-       begin
-        after_hysteresis_prev1 <= after_hysteresis;
-               bit1 <= curbit;
-       end
-    if(negedge_cnt == 7'd17)
-       begin
-        after_hysteresis_prev2 <= after_hysteresis;
-               bit2 <= curbit;
-       end
-    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
-       
-       //--------------------------------------------------------------------------------------------------------------------------------------------------------------
-       // 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.
+       if(negedge_cnt[2:0] == 3'b000 && mod_type == `FPGA_HF_ISO14443A_SNIFFER)        // shift at double speed

        begin
        begin

-               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.
+               // Don't shift if we just loaded new data, obviously.
+               if(negedge_cnt[5:0] != 6'd0)

                begin
                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 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.
+                       to_arm[7:1] <= to_arm[6:0];

                end
                end

-               else
+       end
+
+       if(negedge_cnt[3:0] == 4'b0000 && mod_type != `FPGA_HF_ISO14443A_SNIFFER)
+       begin
+               // Don't shift if we just loaded new data, obviously.
+               if(negedge_cnt[6:0] != 7'd0)

                begin
                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];
+                       to_arm[7:1] <= to_arm[6:0];

                end
        end
        
                end
        end
        

-       //-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
-       // Communication to ARM (SSP Clock and data)
-       // SNIFFER mode (ssp_clk = adc_clk / 8, ssp_frame clock = adc_clk / 64)):
-       if(mod_type == `SNIFFER)
+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 == `FPGA_HF_ISO14443A_SNIFFER)
+       // FPGA_HF_ISO14443A_SNIFFER mode (ssp_clk = adc_clk / 8, ssp_frame clock = adc_clk / 64)):

        begin
        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;
                        ssp_clk <= 1'b1;

-                       // 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(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
        end
        else

-       //-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
-       // Communication to ARM (SSP Clock and data)

        // all other modes (ssp_clk = adc_clk / 16, ssp_frame clock = adc_clk / 128):
        begin
        // 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;
-                       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[6:0] == 7'd7)    // ssp_frame rising edge indicates start of frame, sampled on falling edge of ssp_clk
+                       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;
-                       // 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
-               
-               if (mod_type == `TAGSIM_MOD && fdt_elapsed && temp_buffer_reset)
-                       // transmit timing information
-                       bit_to_arm = to_arm[7];
+
+
+////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
+// 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 == `FPGA_HF_ISO14443A_TAGSIM_LISTEN) 
+                       sendbit = after_hysteresis;
+               else if(mod_type == `FPGA_HF_ISO14443A_TAGSIM_MOD)
+                       /* if(fdt_counter > 11'd772) sendbit = mod_sig_coil; // huh?
+                       else */ 
+                       sendbit = fdt_indicator;
+               else if (mod_type == `FPGA_HF_ISO14443A_READER_LISTEN)
+                       sendbit = curbit;

                else
                else

-                       // transmit data or fdt_indicator
-                       bit_to_arm = sendbit;
-               end
-       
-end    //always @(negedge adc_clk)
+                       sendbit = 1'b0;
+       end

 
 

-assign ssp_din = bit_to_arm;

 
 

+       if(mod_type == `FPGA_HF_ISO14443A_SNIFFER)
+               // send sampled reader and tag data:
+               bit_to_arm = to_arm[7];
+       else if (mod_type == `FPGA_HF_ISO14443A_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;

 
 

-// Subcarrier (adc_clk/16, for TAGSIM_MOD only).
+// Subcarrier (adc_clk/16, for FPGA_HF_ISO14443A_TAGSIM_MOD only).

 wire sub_carrier;
 assign sub_carrier = ~sub_carrier_cnt[3];
 
 wire sub_carrier;
 assign sub_carrier = ~sub_carrier_cnt[3];
 

-// 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))); 
+// in FPGA_HF_ISO14443A_READER_MOD: drop carrier for mod_sig_coil==1 (pause); in FPGA_HF_ISO14443A_READER_LISTEN: carrier always on; in other modes: carrier always off
+assign pwr_hi = (ck_1356meg & (((mod_type == `FPGA_HF_ISO14443A_READER_MOD) & ~mod_sig_coil) || (mod_type == `FPGA_HF_ISO14443A_READER_LISTEN)));      

 
 
 // Enable HF antenna drivers:
 assign pwr_oe1 = 1'b0;
 assign pwr_oe3 = 1'b0;
 
 
 
 // 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)
+// FPGA_HF_ISO14443A_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
 // 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 pwr_oe4 = mod_sig_coil & sub_carrier & (mod_type == `FPGA_HF_ISO14443A_TAGSIM_MOD);

 
 // This is all LF, so doesn't matter.
 assign pwr_oe2 = 1'b0;
 
 // This is all LF, so doesn't matter.
 assign pwr_oe2 = 1'b0;