80c51单片机初学者的练手程序，倒计时器：涉及按键检测、与电脑通讯、led、数码管动态显示等内容。开发环境：keil3中文版。-80c51 microcontroller beginners to practice hand program, Countdown: involving key detection, and computer communications, led, digital tube dynamic display content. Development Environment: keil3 Chinese version.