module period_measure(in);
 input in;
 voltage in;

 parameter real vth=0;

 integer crossings;
 real latest,previous;

 analog
 begin
   @(initial_step)
   begin
     crossings=0;previous=0;latest=0;
   end

   @(cross(V(in)-vth,+1))
   begin
     crossings=crossings+1;
     previous=latest;
     latest=$realtime;
   end

   @(final_step)
   begin
     if (crossings < 2)
       $strobe("Could not measure period.\n");
     else
       $strobe("period = %g, crossings = %d\n",
                latest-previous,crossings);
   end
 end
endmodule