hi_flite.v

/*
  This code demodulates and modulates signal as described in ISO/IEC 18092.
  That includes packets used for Felica, NFC Tag 3, etc. (which do overlap)
  simple envelope following algorithm is used (modification of fail0verflow LF one)
  is used to combat some nasty aliasing effect with testing phone (envelope looked like sine wave)

  Speeds supported:  only 212 kbps (fc/64) for now.  Todo: 414 kbps
  though for reader, the selection has to come from ARM. modulation waits for market sprocket -doesn't really mean anything

   mod_type: bits 210:
      bit 2 : reader drive/power on/off
      bit 1 : speed bit, 0 : 212, 1 :424
      bit 0 : listen or modulate
*/

module hi_flite(
    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,
    dbg,
    mod_type
);
    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 dbg;
    input [3:0] mod_type;

assign dbg = 0;

wire power = mod_type[2];
wire speed = mod_type[1];
wire disabl = mod_type[0];

// Most off, oe4 for modulation;
// Trying reader emulation (would presumably just require switching power on, but I am not sure)
assign pwr_lo = 1'b0;

// 512x64/fc  -wait before ts0, 32768 ticks
// tslot: 256*64/fc
assign adc_clk = ck_1356meg;

///heuristic values for initial thresholds. seem to work OK
`define imin 70     // (13'd256)
`define imax 180    // (-13'd256)
`define ithrmin 91  // -13'd8
`define ithrmax 160 //  13'd8

`define min_bitdelay_212 8
//minimum values and corresponding thresholds
reg  [8:0] curmin=`imin;
reg [8:0] curminthres=`ithrmin;
reg [8:0] curmaxthres=`ithrmax;
reg [8:0] curmax=`imax;

//signal state, 1-not modulated, 0 -modulated
reg after_hysteresis = 1'b1;

//state machine for envelope tracking
reg [1:0] state=1'd0;

//lower edge detected, trying to  detect first bit of SYNC (b24d, 1011001001001101)
reg try_sync=1'b0;

//detected first sync bit, phase frozen
reg did_sync=0;

`define bithalf_212 32 // half-bit length for 212 kbit
`define bitmlen_212 63 // bit transition edge

`define bithalf_424 16 // half-bit length for 212 kbit
`define bitmlen_424 31 // bit transition edge

wire [7:0] bithalf = speed ? `bithalf_424 : `bithalf_212;
wire [7:0] bitmlen = speed ? `bitmlen_424 : `bitmlen_212;


//ssp clock and current values
reg ssp_clk;
reg ssp_frame;
reg curbit = 1'b0;

reg [7:0] fccount = 8'd0; // in-bit tick counter. Counts carrier cycles from the first lower edge detected, reset on every manchester bit detected

reg [7:0] tsinceedge = 8'd0;// ticks from last edge,  desync if the valye is too large

reg zero = 1'b0; // Manchester first halfbit low second high corresponds to this value. It has been known to change. SYNC is used to set it

//ssp counter for transfer and framing
reg [8:0] ssp_cnt = 9'd0;

always @(posedge adc_clk)
     ssp_cnt <= (ssp_cnt + 1);

//maybe change it so that ARM sends preamble as well.
//then: ready bits sent to ARM, 8 bits sent from ARM (all ones), then preamble (all zeros, presumably) - which starts modulation

always @(negedge adc_clk)
begin
     //count fc/64 - transfer bits to ARM at the rate they are received
     if( ((~speed) && (ssp_cnt[5:0] == 6'b000000)) || (speed && (ssp_cnt[4:0] == 5'b00000)))
        begin
            ssp_clk <= 1'b1;
            ssp_din <= curbit;
        end
        if( ( (~speed) && (ssp_cnt[5:0] == 6'b100000)) ||(speed && ssp_cnt[4:0] == 5'b10000))
            ssp_clk <= 1'b0;
    //create frame pulses. TBH, I still don't know what they do exactly, but they are crucial for ARM->FPGA transfer. If the frame is in the beginning of the byte, transfer slows to a crawl for some reason
    // took me a day to figure THAT out.
       if(( (~speed) && (ssp_cnt[8:0] == 9'd31)) || (speed && ssp_cnt[7:0] == 8'd15))
        begin
            ssp_frame <= 1'b1;
        end
         if(( (~speed) && (ssp_cnt[8:0] == 9'b1011111)) || (speed &&ssp_cnt[7:0] == 8'b101111) )
        begin
            ssp_frame <= 1'b0;
        end
end

//send current bit (detected in SNIFF mode or the one being modulated in MOD mode, 0 otherwise)
reg ssp_din;

//previous signal value, mostly to detect SYNC
reg prv = 1'b1;

// for simple error correction in mod/demod detection, use maximum of modded/demodded in given interval. Maybe 1 bit is extra? but better safe than sorry.
reg[7:0] mid = 8'd128;

// set TAGSIM__MODULATE on ARM if we want to write... (frame would get lost if done mid-frame...)
// start sending over 1s on ssp->arm when we start sending preamble
// reg sending = 1'b0;  // are we actively modulating?
reg [11:0] bit_counts = 12'd0; // for timeslots. only support ts=0 for now, at 212 speed  -512 fullbits from end of frame. One hopes.   might remove those?


//we need some way to flush bit_counts triggers on mod_type changes don't compile
reg dlay;
always @(negedge adc_clk) // every data ping?
begin
  //envelope follow code...
  ////////////
  if (fccount == bitmlen)
  begin
        if ((~try_sync) && (adc_d < curminthres) && disabl )
        begin
         fccount <= 1;
          end
         else
         begin
          fccount <= 0;
         end
           dlay <= ssp_dout;
           if (bit_counts > 768) // should be over ts0 now, without ARM interference... stop counting...
              begin
               bit_counts <= 0;
               end
           else
             if (power)
             bit_counts <= 0;
             else
               bit_counts <= bit_counts + 1;
        end
        else
        begin
        if((~try_sync) && (adc_d < curminthres) && disabl)
        begin
         fccount <= 1;
          end
         else
         begin
          fccount <= fccount + 1;
         end
        end

      // rising edge
      if (adc_d > curmaxthres)
       begin
        case (state)
         0: begin
            curmax <= adc_d > `imax? adc_d : `imax;
            state <= 2;
            end
         1: begin
            curminthres <= ((curmin >> 1) + (curmin >> 2) + (curmin >> 4) + (curmax >> 3) + (curmax >> 4)); //threshold: 0.1875 max + 0.8125 min
            curmaxthres <= ((curmax >> 1) + (curmax >> 2) + (curmax >> 4) + (curmin >> 3) + (curmin >> 4));
            curmax <= adc_d > 155 ? adc_d : 155; // to hopefully prevent overflow from spikes going up to 255
            state <= 2;
            end
         2: begin
            if (adc_d > curmax)
               curmax <= adc_d;
            end
         default:
            begin
            end
        endcase
        after_hysteresis <= 1'b1;
        if(try_sync)
          tsinceedge <= 0;
       end
      else if (adc_d<curminthres) //falling edge
        begin
          case (state)
            0: begin
               curmin <= adc_d<`imin? adc_d :`imin;
               state <= 1;
               end
            1: begin
               if (adc_d<curmin)
                 curmin <= adc_d;
               end
            2: begin
                curminthres <= ( (curmin >> 1) + (curmin >> 2) + (curmin >> 4) + (curmax >> 3) + (curmax >> 4));
                curmaxthres <= ( (curmax >> 1) + (curmax >> 2) + (curmax >> 4) + (curmin >> 3) + (curmin >> 4));
                curmin <= adc_d < `imin ? adc_d : `imin;
                state <= 1;
               end
            default:
              begin
              end
          endcase
              after_hysteresis <= 0;
          if (~try_sync ) //begin modulation, lower edge...
             begin
             try_sync <= 1;
             fccount <= 1;
             did_sync <= 0;
             curbit <= 0;
             mid <= 8'd127;
             tsinceedge <= 0;
             prv <= 1;
             end
           else
           begin
           tsinceedge <= 0;
           end
         end
        else //stable state, low or high
         begin
            curminthres <= ( (curmin >> 1) + (curmin >> 2) + (curmin >> 4) + (curmax >> 3) + (curmax >> 4));
            curmaxthres <= ( (curmax >> 1) + (curmax >> 2) + (curmax >> 4) + (curmin >> 3) + (curmin >> 4));
            state <= 0;

             if (try_sync )
              begin
               if (tsinceedge >= (128))
                  begin
                  //we might need to start counting... assuming ARM wants to reply to the frame.
                  bit_counts <= 1;// i think? 128 is about 2 bits passed... but 1 also works
                  try_sync <= 0;
                  did_sync <= 0;//desync
                  curmin <= `imin; //reset envelope
                  curmax <= `imax;
                  curminthres <= `ithrmin;
                  curmaxthres <= `ithrmax;
                  prv <= 1;
                  tsinceedge <= 0;
                  after_hysteresis <= 1'b1;
                  curbit <= 0;
                  mid <= 8'd128;
                  end
               else
                tsinceedge <= (tsinceedge + 1);
              end
         end


        if (try_sync && tsinceedge < 128)
            begin
            //detect bits in their middle ssp sampling is in sync, so it would sample all bits in order
            if (fccount == bithalf)
              begin
                if ((~did_sync) && ((prv == 1 && (mid > 128))||(prv == 0 && (mid <= 128))))
                  begin
                        //sync the Zero, and set curbit roperly
                        did_sync <= 1'b1;
                        zero <= ~prv;// 1-prv
                        curbit <= 1;
                  end
                else
                    curbit <= (mid > 128) ? (~zero) : zero;

                 prv <= (mid > 128) ? 1 : 0;

                 if (adc_d > curmaxthres)
                     mid <= 8'd129;
                 else if (adc_d < curminthres)
                     mid <= 8'd127;
                 else
                  begin
                   if (after_hysteresis)
                   begin
                   mid <= 8'd129;
                   end
                   else
                   begin
                   mid <= 8'd127;
                   end
                  end

               end
             else
              begin
                if (fccount==bitmlen)
                  begin
                  // fccount <= 0;
                   prv <= (mid > 128) ? 1 : 0;
                   mid <= 128;
                  end
                else
                 begin
                // minimum-maximum calc
                  if(adc_d > curmaxthres)
                     mid <= mid + 1;
                   else if (adc_d < curminthres)
                    mid <= mid - 1;
                   else
                    begin
                     if (after_hysteresis)
                       begin
                       mid <= mid + 1;
                       end
                       else
                       begin
                       mid <= mid - 1;
                       end
                    end
                 end
              end
            end
           else
           begin
            end
//       sending <= 0;
end
//put modulation here to maintain the correct clock. Seems that some readers are sensitive to that
reg pwr_hi;
reg pwr_oe1;
reg pwr_oe2;
reg pwr_oe3;
reg pwr_oe4;

wire mod = ((fccount >= bithalf) ^ dlay) & (~disabl);

always @(ck_1356meg or ssp_dout or power or disabl or mod)
begin
    if (power)
        begin
        pwr_hi <= ck_1356meg;
        pwr_oe1 <= 1'b0;//mod;
        pwr_oe2 <= 1'b0;//mod;
        pwr_oe3 <= 1'b0;//mod;
        pwr_oe4 <= mod;//1'b0;
        end
    else
        begin
        pwr_hi <= 1'b0;
        pwr_oe1 <= 1'b0;
        pwr_oe2 <= 1'b0;
        pwr_oe3 <= 1'b0;
        pwr_oe4 <= mod;
        end
end

endmodule