code_gen2.v

GPS中C/A码生成电路

//                              -*- Mode: Verilog -*-
// Filename        : code_gen.v
// Description     : Generates early prompt and late C/A code chips.

// Author          : Peter Mumford, 2005, UNSW

// Function        : Generate the C/A code early, prompt and late chipping sequence.

/*
	Copyright (C) 2007  Peter Mumford

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2.1 of the License, or (at your option) any later version.

    This library is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
*/
  
module code_gen (clk, rstn, tic_enable, hc_enable, prn_key_enable, prn_key, code_slew, slew_enable,
                 dump_enable, code_phase, early, prompt, late);

   input clk, rstn;
   input tic_enable;       // the TIC
   input hc_enable;        // the half-chip enable pulse from the code_nco
   input prn_key_enable;   // pulse to latch in the prn_key and reset the logic (write & chip_select)
   input slew_enable;      // pulse to set the slew_flag (write & chip select)
   input [9:0] prn_key;    // 10 bit number used to select satellite PRN code
   input [10:0] code_slew; // number of half chips to delay the C/A code after the next dump_enable 

   output dump_enable;          // pulse at the begining/end of prompt C/A code cycle
   output reg [10:0] code_phase;// the phase of the C/A code at the TIC
   output early, prompt, late;  // half-chip spaced C/A code sequences

   reg [9:0] g1;          // the g1 shift register
   reg g1_q;              // output of the g1 shift register
   reg [9:0] g2;          // the g2 shift register
   reg g2_q;              // output of the g2 shift register
   wire ca_code;          // the C/A code chip sequence from g1 and g2 shifters
   wire [2:0] srq;        // the output of the chip spreader
   
   reg fc_enable;         // full-chip enable that drives the g1 and g2 shifters
   reg dump_enable;       // pulse generated at the begining/end of the prompt C/A code cycle
   reg [10:0] hc_count1;  // counter used for generating the fc_enable and slew logic (max slew 2045)

   reg [10:0] slew;       // the code_slew latched if the slew_flag is set  
   reg [11:0] hc_count2;  // counter for keeping track of the begining/end of C/A code cycle (max count 4091)
   reg [11:0] max_count2; // limit of hc_count2, normally = 2045, but increased when slew delays the C/A code
   
   //reg [1:0] dump = 3;    // dump_enable is generated when hc_count2 = 3
   reg slew_flag;         // slew_flag is set on the slew_enable pulse and cleared on the dump_enable
   reg slew_trigger;      // triggers the slew event

   reg [10:0] hc_count3;  // this counter is reset at the dump_enable, latched into the code_phase on the TIC
   
   
   
   // chip spreader: shift register for generating early, prompt and late chips
   //--------------------------------------------------------------------------
   // Half a chip separates the early and prompt, and prompt and late codes.
   lpm_shiftreg sr(
     .clock(clk),
     .sclr(prn_key_enable),
     .enable(hc_enable),
     .shiftin(ca_code),
     .q(srq)
     );

   defparam   sr.lpm_width= 3;
    
   // The G1 shift register
   //----------------------
   always @ (posedge clk)
   begin
      if (prn_key_enable) // set up shift register
         begin
         g1_q <= 0;
         g1 <= 10'b1111111111;
         end
      else if (fc_enable) // run
         begin
         g1_q <= g1[0];
         g1 <= {(g1[7] ^ g1[0]), g1[9:1]};
         end
   end

   // The G2 shift register
   //----------------------
   always @ (posedge clk)
   begin
      if (prn_key_enable) // set up shift register
         begin
         g2_q <= 0;
         g2 <= prn_key;
         end
      else if (fc_enable) // run
         begin
         g2_q <= g2[0];
         g2 <= {(g2[8] ^ g2[7] ^ g2[4] ^ g2[2] ^ g2[1] ^ g2[0]), g2[9:1]};
         end
   end

   assign ca_code = g1_q ^ g2_q;

   // assign the early, prompt and late chips, one half chip apart
   assign early = srq[0];
   assign prompt = srq[1];
   assign late = srq[2];

   // hc_count3 process
   //------------------
   // Counter 3 counts hc_enables, reset on dump_enable.
   // If there is slew delay this counter will roll over
   // before the next dump. However, code_phase measurements
   // are not valid during slewing.
   always @ (posedge clk)
   begin
      if (prn_key_enable || dump_enable)
         hc_count3 <= 0;
      else if (hc_enable)
         hc_count3 <= hc_count3 + 1;
   end
   
   // capture the code phase at TIC
   //------------------------------
   // The code_phase is the half-chip count
   // at the TIC. Half-chips are numbered 0 to 2045.
   // The code_nco_phase (from the code_nco) provides
   // the fine (sub half-chip) code phase.
   always @ (posedge clk)
   begin
      if (tic_enable)
         code_phase <= hc_count3;
   end

   // The full-chip enable generator
   //--------------------------------
   // Without the code_slew being set
   // this process just creates the full-chip enable
   // at half the rate of the half-chip enable.
   // When the code_slew is set, the fc_enable
   // is delayed for a number of half-chips.
   always @ (posedge clk)
   begin
      if (prn_key_enable)
         begin
         hc_count1 <= 0;
         fc_enable <= 0;
         slew <= 0; // reset slew   
         end
      else
         begin 
         if (slew_trigger)
            slew <= code_slew;
         if (hc_enable)
            begin
            if (slew == 0) // no delay on code
               begin
               if (hc_count1 == 1)
                  begin
                  hc_count1 <= 0;
                  fc_enable <= 1; // create fc_enable pulse
                  end
               else
                  hc_count1 <= hc_count1 + 1; // increment count
               end
            else
            slew <= slew - 1; // decrement slew
            end
         else
            fc_enable <= 0;
      end
   end

   // The dump_enable generator
   //--------------------------
   // create the dump_enable
   //
   // When a slew value (= x) is written to the code_slew register,
   // the C/A code is delayed x half-chips at the next dump.
   always @ (posedge clk)
   begin
   if (prn_key_enable)
      begin
      dump_enable <= 0;
      hc_count2 <= 0;
      slew_trigger <= 0;
      max_count2 <= 2045; // normal half-chip count in one C/A cycle
      end
     
   else if (hc_enable)
      begin
      hc_count2 <= hc_count2 + 1;
      if (hc_count2 == 3)//dump)
         dump_enable <= 1;
      else if (hc_count2 == max_count2)
         hc_count2 <= 0;
      else if (hc_count2 == 1)  // signals the arrival of the first hc_enable
         begin
         if (slew_flag) // slew delay
            begin
            slew_trigger <= 1;
            max_count2 <= 2045 + code_slew;
            end
         else
            max_count2 <= 2045;
         end
      end
   else
      begin
      dump_enable <= 0;
      slew_trigger <= 0;
      end

   end     


   
   // slew_flag process
   //------------------
   // The slew_flag is set on slew_enable and cleared on the dump_enable.
   always @ (posedge clk)
   begin
   if (prn_key_enable)
      slew_flag <= 0;
   else if (slew_enable)
      slew_flag <= 1;
   else if (dump_enable)
      slew_flag <= 0;
   end
    
endmodule // code_gen