js 倒计时 button不可用

　　

  function showtime() {
             fun_timedown(5);
         }
         function fun_timedown(time) {
             $("#timedown").val(time + "秒");
             time = time - 1;
             if (time >= 0) {
                 $("#timedown").attr("disabled", "disabled");
                 setTimeout("fun_timedown(" + time + ")", 1000);

             } else {

                 alert('倒计时结束啦');

             }
         }

 <input type="button" id="timedown" value="确定"/>
  <a href="javascript:showtime()">点击</a>