倒计时秒表设计:使秒表在上电/复位后，具有：开始时显示 9.9 ；并开始倒计时， 当9.9S计时到，数码管显示0.0并闪烁，同时蜂鸣器发声报警3S，倒计时误差小于0.2秒。 晶振频率：11.0592MHz-Countdown stopwatch design: stopwatch after power-on/reset, comprising: a display at the beginning of 9.9 and begins the countdown, When t