99秒马表设计。主要实现以下功能：（1)开始时，显示“00”，第1次按下SP1后就开始计时；（2)第2次按SP1后，计时停止；（3)第3次按SP1后，计时归零。实验原理图：-99 seconds stopwatch design. Achieve the following functions: (1) beginning to show " 00" , 1st time by pressing start after SP1　(2) 2 nd by SP1, the time to stop　(3) 3 times by SP1, time zero. Experimental schematic diagram: