超声波测距死在while（！aa）

void GPIO_Configuration(void)
{
GPIO_InitTypeDef GPIO_InitStructure;
  
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB|RCC_APB2Periph_AFIO , ENABLE);


GPIO_InitStructure.GPIO_Pin = GPIO_Pin_5;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_Init(GPIOB, &GPIO_InitStructure);

GPIO_InitStructure.GPIO_Pin =  GPIO_Pin_6 ;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AIN;
//GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_Init(GPIOB, &GPIO_InitStructure);
}
void chaoshengbo()
{     
aa=GPIO_ReadInputDataBit(GPIOB, GPIO_Pin_6);
while(!aa)
TIM_Cmd(TIM2, ENABLE);  
aa=GPIO_ReadInputDataBit(GPIOB, GPIO_Pin_6);
while(aa)
TIM_Cmd(TIM2, DISABLE);     //RE回响信号
}
void csPZ()
{
GPIO_SetBits(GPIOB, GPIO_Pin_5);
delay_ms(10);
GPIO_ResetBits(GPIOB, GPIO_Pin_5); //TX  触发信号
}
void NVIC_Configuration(void)
{
  NVIC_InitTypeDef NVIC_InitStructure;
  
#ifdef  VECT_TAB_RAM  
  /* Set the Vector Table base location at 0x20000000 */
  NVIC_SetVectorTable(NVIC_VectTab_RAM, 0x0);
#else  /* VECT_TAB_FLASH  */
  /* Set the Vector Table base location at 0x08000000 */
  NVIC_SetVectorTable(NVIC_VectTab_FLASH, 0x0);   
#endif
  /* Configure one bit for preemption priority */
  NVIC_PriorityGroupConfig(NVIC_PriorityGroup_0);
  
  /* Enable the EXTI9_5 Interrupt */
  NVIC_InitStructure.NVIC_IRQChannel =TIM2_IRQChannel;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);
}
void Timer_Configuration(void)
{
TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2,ENABLE);
TIM_DeInit(TIM2);
TIM_TimeBaseStructure.TIM_Period = 9000;
TIM_TimeBaseStructure.TIM_Prescaler = 8000;
TIM_TimeBaseStructure.TIM_ClockDivision =TIM_CKD_DIV1;
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
TIM_TimeBaseInit(TIM2, & TIM_TimeBaseStructure);
TIM_ClearITPendingBit(TIM2,TIM_FLAG_Update);
TIM_ITConfig(TIM2,TIM_IT_Update,ENABLE);
}