NVIC_InitStructure.NVIC_IRQChannel = TIM3_IRQn;
 NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0; //设置抢占优先级为0
 NVIC_InitStructure.NVIC_IRQChannelSubPriority = 3;   //设置次要优先级为3
 NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;     //IRQ通道使能
 NVIC_Init(&NVIC_InitStructure);         //函数调用,按照结构体变量的值设置相关的寄存器
TIM_Cmd(TIM3,ENABLE);                                    //启动定时器TIM3
   为什编译器老是说我的TIM3_IRQn;未定义啊 ！什么原因啊！