说明：  当接收到一个信号(D_start)时，开始计时，再收到另一个信号(D_stop)时，计时结束，得到计时时间A，然后将时间A与给定时间B进行比较，如果小于时间B,程序结束，进行下一环节(LED)，否则返回重新等待计时(cnt:=0)
(When receiving a signal (D_start), the start time, and then received another signal (D_stop), the time the end of time by time A, then the time A and B compare a given time, if less than the time B, end of the process, proceed to the next link (LED), or return to re-wait time (cnt: = 0))