source: PlatformSupport/Deprecated/pcores/SPI_Controller/hdl/verilog/spi_clgen.v

//////////////////////////////////////////////////////////////////////
////                                                              ////
////  spi_clgen.v                                                 ////
////                                                              ////
////  This file is part of the SPI IP core project                ////
////  http://www.opencores.org/projects/spi/                      ////
////                                                              ////
////  Author(s):                                                  ////
////      - Simon Srot (simons@opencores.org)                     ////
////                                                              ////
////  All additional information is avaliable in the Readme.txt   ////
////  file.                                                       ////
////                                                              ////
//////////////////////////////////////////////////////////////////////
////                                                              ////
//// Copyright (C) 2002 Authors                                   ////
////                                                              ////
//// This source file may be used and distributed without         ////
//// restriction provided that this copyright statement is not    ////
//// removed from the file and that any derivative work contains  ////
//// the original copyright notice and the associated disclaimer. ////
////                                                              ////
//// This source file 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 source 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 source; if not, download it   ////
//// from http://www.opencores.org/lgpl.shtml                     ////
////                                                              ////
//////////////////////////////////////////////////////////////////////

`include "spi_defines.v"
`include "timescale.v"

module spi_clgen (clk_in, rst, go, enable, last_clk, divider, clk_out, pos_edge, neg_edge); 

  parameter Tp = 1;
  
  input                            clk_in;   // input clock (system clock)
  input                            rst;      // reset
  input                            enable;   // clock enable
  input                            go;       // start transfer
  input                            last_clk; // last clock
  input     [`SPI_DIVIDER_LEN-1:0] divider;  // clock divider (output clock is divided by this value)
  output                           clk_out;  // output clock
  output                           pos_edge; // pulse marking positive edge of clk_out
  output                           neg_edge; // pulse marking negative edge of clk_out
                            
  reg                              clk_out;
  reg                              pos_edge;
  reg                              neg_edge;
                            
  reg       [`SPI_DIVIDER_LEN-1:0] cnt;      // clock counter 
  wire                             cnt_zero; // conter is equal to zero
  wire                             cnt_one;  // conter is equal to one
  
  
  assign cnt_zero = cnt == {`SPI_DIVIDER_LEN{1'b0}};
  assign cnt_one  = cnt == {{`SPI_DIVIDER_LEN-1{1'b0}}, 1'b1};
  
  // Counter counts half period
  always @(posedge clk_in or posedge rst)
  begin
    if(rst)
      cnt <= #Tp {`SPI_DIVIDER_LEN{1'b1}};
    else
      begin
        if(!enable || cnt_zero)
          cnt <= #Tp divider;
        else
          cnt <= #Tp cnt - {{`SPI_DIVIDER_LEN-1{1'b0}}, 1'b1};
      end
  end
  
  // clk_out is asserted every other half period
  always @(posedge clk_in or posedge rst)
  begin
    if(rst)
      clk_out <= #Tp 1'b0;
    else
      clk_out <= #Tp (enable && cnt_zero && (!last_clk || clk_out)) ? ~clk_out : clk_out;
  end
   
  // Pos and neg edge signals
  always @(posedge clk_in or posedge rst)
  begin
    if(rst)
      begin
        pos_edge  <= #Tp 1'b0;
        neg_edge  <= #Tp 1'b0;
      end
    else
      begin
        pos_edge  <= #Tp (enable && !clk_out && cnt_one) || (!(|divider) && clk_out) || (!(|divider) && go && !enable);
        neg_edge  <= #Tp (enable && clk_out && cnt_one) || (!(|divider) && !clk_out && enable);
      end
  end
endmodule