模拟交通信号灯控制，一般情况下正常显示，有急救车到达时，两个方向交通 信号灯全红，以便让急救车通过。设急救车通过路口时间为10 秒，急救车通过后，交通恢 复正常，用单次脉冲申请外部中断，表示有急救车通过。-simulated traffic signal control, under normal circumstances normal show that emergency vehicles arrived, the two directions of traffic lights all red, in order to allow emergency vehicles through. Based emergency vehicles through the intersection for 10 seconds, after the passage of emergency vehicles, traffic returned to normal, with a single pulse for external interrupts, saying that a first aid trucks.