/////////////////////////////////////////////////////////////////////////////////This module is used to change the 50Mhz frequency to 20Mhz.///////////////////Xiaoming,Chen,31,july,2002.//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////`timescale 1ns/100psmodule frequency5x2(in,out,rst);input in,rst;output out;   reg out;reg mid;integer counter;parameter delaytime=25;always@(posedge rst )           begin          counter=0;          out=0;          mid=0;          endalways@(posedge in)          begin             if(counter==4)                begin                mid=~mid;                counter=0;                end              else               counter=counter+1;           end             always@(negedge in)          begin             if(counter==4)                begin                mid=~mid;                counter=0;                end              else               counter=counter+1;           end       always@(posedge mid )        begin          out=~out;          #delaytime out=~out;        endalways@(negedge mid)         begin          out=~out;          #delaytime out=~out;         end                                                 endmodule //////test module/////////////////////////////////////////////////////////this module is used to test module frequency5x2.v//////////////////////// `timescale 1ns/100ps                              module test;            reg clock,reset;                                 frequency5x2 t(clock,out,reset);              initial         begin             clock=0;            reset=0;                        #10 reset=1;         end       always #10 clock=~clock;   endmodule                            //generate 20Mhz waveforme square wave               //////////////////////////////////////////////////////////////////////////////