鉴相器，pf_up是local相比reference朝前，pf_down是滞后， 经仿真可以实现功能。请各位评判下这样实现好不好，还有更好或可靠的方法没有，谢谢！

module top_jianxiang(reference_frq, local_frq, pf_up, pf_down );
    input reference_frq;
    input local_frq;
    output pf_up;
    output pf_down ;
  
  reg pf_up,pf_down;

  always @(posedge reference_frq or posedge local_frq)
    begin
    if (local_frq == 0)
      pf_down <= 1;
 
      else
      if (local_frq == 1)
        pf_down <= 0;
    end 
   
  always @(posedge reference_frq or posedge local_frq)
    begin
    if (reference_frq == 0)
      pf_up <= 1;
 
      else
      if (reference_frq == 1)
        pf_up <= 0;
    end 
   

endmodule