用中小规模集成电路设计一个公用电话计时系统。通过本设计使其完成一下几项基本功能：每3分钟计时一次；显示通话次数，最多为99次；每次定时误差小于1秒；具有手动复位功能；具有声响提醒功能。-Small-scale integrated circuit design with a public telephone timing system. Through this design it is completed at several basic functions: timer once every