倒计时器

日期： 2012-05-05 23:00完成；



硬件：单片机AT89C51，8位数码管

功能要求：

（1）基本功能：起始数为9999，利用定时器定时实现每隔1秒倒计时。

（2）扩展功能1：利用按键设置起始数，可加可减。

（3）扩展功能2：利用按键控制倒计时器的启动与停止。-Countdown Timer Date: 2012-05-05 23:00 completed　hardware: the single chip microcomputer AT89C51 8-bit digital tube function requirements: (1) basic functions: the starting number for 9999, use the timer every 1 second countdown. (2) extension 1: Using the button to set the starting number can be increased or decreased. (3) extension 2: Using the button control countdown to the start and stop.