系统设置一个两位BCD码倒计时计数器（计数脉冲1HZ），用于记录各状态持续时间；
因为各状态持续时间不一致，所以上述计数器应置入不同的预置数；
倒计时计数值输出至二个数码管显示；
程序共设置4个进程：
① 进程P1、P2和P3构成两个带有预置数功能的十进制计数器，其中P1和P3分别为个位和十位计数器，P2产生个位向十位的进位信号；
② P4是状态寄存器，控制状态的转换，并输出6盏交通灯的控制信号。
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