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。-System to set up a two BCD code countdown counter (count pulse 1HZ), used to record the duration of each state　because the duration of each state are inconsistent, so these counters should be placed in several different presets　countdown of numerical output to two digital display　procedures were set up four processes: ① process P1, P2 and P3 form two functions with a preset number of decimal counters, of which P1 and P3, respectively, for months, and 10-bit counters, P2 to generate a 10-bit The binary signal　② P4 is the status register, control the state of the conversion, and six output control signals of traffic lights.