module pn_encode(data_source,init,clk_pn,reset_pn,flag_tra,pn_out);
    input data_source;
    input init;	   //init為1時，賦予x[3:0]初值0001，下拉
    input clk_pn;	  //pn碼的碼元速率
    input reset_pn;	 //pn碼重新回零,上拉
    output flag_tra; //當reset_pn重啟后，flag_tra開始計數，計夠兩個來一個0，計夠四個來一個1，并回零重來
    output pn_out;	 //pn碼與輸入全一信號相乘后的輸出信號
			   
    reg flag_tra; //4b5b的轉換信號，上升沿有效
    reg[3:0] flag_tra_counter;
    reg[3:0] x; //寄存器

    assign pn_out=data_source^x[3]^x[0];

    always @(posedge clk_pn)
      begin
	   if(!reset_pn)		  //零的時候回零
	     x[3:0]<='b0000;
	   else
	     begin
		  if(init==1)
		    begin 
		      x[3:0]<='b0001;
		      flag_tra_counter<='b000;
		    end
		  else
		    begin
		      x[0]<=x[3]^x[0]^data_source;
			 x[1]<=x[0];
			 x[2]<=x[1];
			 x[3]<=x[2];
			 if(flag_tra_counter==3)
			   flag_tra_counter<='b000;
			 else
			   begin
			     flag_tra_counter<=flag_tra_counter+1;
                    if(flag_tra_counter<=1)
			       flag_tra<=1;
			     else
			       flag_tra<=0;  
			   end	    
		    end   
		end
	     
	 end
    
     


endmodule