[mw_shl_code=c,true]main
  NVIC_PriorityGroupConfig(NVIC_PriorityGroup_4); /* 设置中断嵌套模式 */
  
  SYSTICK_LEVEL = 3;
  TIMER_LEVEL = 2;
  EXIT0_LEVEL = 1;
  
  while(1) {
    int3 = 0; /* 让 systick isr 停下来 */
    while(0 == int3);
  }

systick_isr
  if(0 == int3) {
    while(0 == int2); /* 等待 timer isr 救援 */
  }
  
timer_isr
  if(0 == int2) {
    while(0 == int1); /* 等待 exti0 isr 救援 */
  }
  
exti0_isr
  int1 = 1; /* 触发 timer isr 正常响应 */[/mw_shl_code]