1. Cyg_RealTimeClokc rtc
   1. Cyg_Clock
      1. Cyg_counter
         1. alarm_list
         2. tick(count)
            1. for(alarm_list) alarm->alarm();
   2. Interrupt
      1. isr
         1. (Almost do nothing, except call dsr)
      2. dsr
         1. rtc->tick(count)
2. Cyg_Thread
   1. Cyg_HardwareThread
   2. Cyg_SchedThread
   3. Cyg_ThreadTimer timer
      1. Cyg_Alarm(Cyg_Clock::real_time_clock, &alarm, CYG_ADDRWORD(this)
      2. alarm()
         1. Cyg_Scheduler::unlock
            1. unlock_inner
            2. HAL_THREAD_SWITCH_CONTEXT