请问这个超声波模块为什么测试不准

//定时器2通道2输入捕获配置

TIM_ICInitTypeDef TIM2_ICInitStructure;

void TIM2_Cap_Init(u16 arr,u16 psc)
{
GPIO_InitTypeDef GPIO_InitStructure;
TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
NVIC_InitTypeDef NVIC_InitStructure;

RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2, ENABLE);    //使能TIM5时钟
 RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA, ENABLE);  //使能GPIOA时钟

GPIO_InitStructure.GPIO_Pin  = GPIO_Pin_1;  //PA0 清除之前设置  
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IPD; //PA0 输入  
GPIO_Init(GPIOA, &GPIO_InitStructure);
GPIO_ResetBits(GPIOA,GPIO_Pin_1);                         //PA0 下拉

//初始化定时器5 TIM5     
TIM_TimeBaseStructure.TIM_Period = arr; //设定计数器自动重装值 
TIM_TimeBaseStructure.TIM_Prescaler =psc;     //预分频器   
TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1; //设置时钟分割:TDTS = Tck_tim
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;  //TIM向上计数模式
TIM_TimeBaseInit(TIM2, &TIM_TimeBaseStructure); //根据TIM_TimeBaseInitStruct中指定的参数初始化TIMx的时间基数单位

//初始化TIM5输入捕获参数
TIM2_ICInitStructure.TIM_Channel = TIM_Channel_2; //CC1S=01     选择输入端 IC1映射到TI1上
  TIM2_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_Rising;    //上升沿捕获
  TIM2_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI; //映射到TI1上
  TIM2_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_DIV1;     //配置输入分频,不分频 
  TIM2_ICInitStructure.TIM_ICFilter = 0x00;//IC1F=0000 配置输入滤波器 不滤波
  TIM_ICInit(TIM2, &TIM2_ICInitStructure);

//中断分组初始化
NVIC_InitStructure.NVIC_IRQChannel = TIM2_IRQn;  //TIM3中断
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 2;  //先占优先级2级
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;  //从优先级0级
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE; //IRQ通道被使能
NVIC_Init(&NVIC_InitStructure);  //根据NVIC_InitStruct中指定的参数初始化外设NVIC寄存器 

TIM_ITConfig(TIM2,TIM_IT_Update|TIM_IT_CC2,ENABLE);//允许更新中断 ,允许CC1IE捕获中断    

   TIM_Cmd(TIM2,ENABLE );     //使能定时器5

}

u8 TIM2CH2_CAPTURE_STA=0; //输入捕获状态
u16 TIM2CH2_CAPTURE_VAL; //输入捕获值

//定时器5中断服务程序
void TIM2_IRQHandler(void)
{

 if((TIM2CH2_CAPTURE_STA&0X80)==0)//还未成功捕获    
{      
    if (TIM_GetITStatus(TIM2, TIM_IT_Update) != RESET)
     
    {        
        if(TIM2CH2_CAPTURE_STA&0X40)//已经捕获到高电平了
        {
            if((TIM2CH2_CAPTURE_STA&0X3F)==0X3F)//高电平太长了
            {
                TIM2CH2_CAPTURE_STA|=0X80;//标记成功捕获了一次
                TIM2CH2_CAPTURE_VAL=0XFFFF;
            }else TIM2CH2_CAPTURE_STA++;
        }     
    }
if (TIM_GetITStatus(TIM2, TIM_IT_CC2) != RESET)//捕获1发生捕获事件
    {    
        if(TIM2CH2_CAPTURE_STA&0X40)        //捕获到一个下降沿         
        {                  
            TIM2CH2_CAPTURE_STA|=0X80;        //标记成功捕获到一次上升沿
            TIM2CH2_CAPTURE_VAL=TIM_GetCapture2(TIM2);
               TIM_OC2PolarityConfig(TIM2,TIM_ICPolarity_Rising); //CC1P=0 设置为上升沿捕获
        }else                                  //还未开始,第一次捕获上升沿
        {
            TIM2CH2_CAPTURE_STA=0;            //清空
            TIM2CH2_CAPTURE_VAL=0;
             TIM_SetCounter(TIM2,0);
            TIM2CH2_CAPTURE_STA|=0X40;        //标记捕获到了上升沿
               TIM_OC2PolarityConfig(TIM2,TIM_ICPolarity_Falling);        //CC1P=1 设置为下降沿捕获
        }            
    }                                                
 }

TIM_ClearITPendingBit(TIM2, TIM_IT_CC2|TIM_IT_Update); //清除中断标志位

}

/超声波距离反馈封装/
int SR04_Distance(void)
{
SR04=1;
delay_us(13);
SR04=0;//超声波模块的GPIO信号
if(TIM2CH2_CAPTURE_STA&0X80)//成功捕获到了一次上升沿
{
time=TIM2CH2_CAPTURE_STA&0X3F;
time*=65536;//溢出时间总和
time+=TIM2CH2_CAPTURE_VAL;//得到总的高电平时间
Distance=(time*0.033/2);//长度单位：cm

// sprintf((char*)string,"vol:%.2f",Distance);
// OLED_ShowString(0,44,string,16);
// OLED_Refresh_Gram();
TIM2CH2_CAPTURE_STA=0;//开启下一次捕获
}
return Distance;
｝

测不准有很多原因，1.超出测量范围，2.没有对数据进行滤波处理，3.算法设计