void TIM3_GPIO_Config(void) {   GPIO_InitTypeDef GPIO_InitStructure;

  RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3, ENABLE);

  RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB, ENABLE);

  GPIO_InitStructure.GPIO_Pin =  GPIO_Pin_0;

  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP; 

 GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;

  GPIO_Init(GPIOB, &GPIO_InitStructure); }

void TIM3_Mode_Config(void) {  

TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;  

TIM_OCInitTypeDef  TIM_OCInitStructure;    

u16 CCR1_Val = 500;       

  /* Time base configuration */    

TIM_TimeBaseStructure.TIM_Period = 999;       //当定时器从0计数到999，即为1000次，为一个定时周期  

TIM_TimeBaseStructure.TIM_Prescaler = 0;     //设置预分频：不预分频，即为36MHz   TIM_TimeBaseStructure.TIM_ClockDivision = 0; //设置时钟分频系数：不分频   TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;  //向上计数模式

  TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure);

  /* PWM1 Mode configuration: Channel1 */  

TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1;     //配置为PWM模式1   TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;    TIM_OCInitStructure.TIM_Pulse = CCR1_Val;    //设置跳变值，当计数器计数到这个值时，电平发生跳变  

TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;  //当定时器计数值小于CCR1_Val时为高电平    

TIM_OC1Init(TIM3, &TIM_OCInitStructure);  //使能通道1

  TIM_OC1PreloadConfig(TIM3, TIM_OCPreload_Enable);

TIM_ARRPreloadConfig(TIM3, ENABLE);    // 使能TIM3重载寄存器ARR

  /* TIM3 enable counter */   TIM_Cmd(TIM3, ENABLE);                   //使能定时器3  }

我是这样设置的 为何PB0不能产生波形？？