使用STM32定時器輸入捕獲子產品控制3路超音波傳感器
本次使用的超音波傳感器是常見HC-SR04,該傳感器常常使用在小型機器人和智能小車的避障系統中。
在上圖中，5v和GND為子產品提供電能，Trig用于觸發子產品測距，Echo用于接受傳回電平信号。
其操作時序圖如下：
如上圖所示，STM32給Trig引腳一個超過10us的高電平，就可以使能子產品内部的測距電路，子產品會循環發出8個40kHz脈沖，發射出超音波，然後通過檢測Echo引腳的高電平時間就可以測量出子產品與障礙物之間的距離。其計算公式可表示如下：
distance=340∗Echo高電平時間2 d i s t a n c e = 340 ∗ E c h o 高 電 平 時 間 2
子產品驅動程式如下：
void HC_SR04_Init(void)
{
/*初始化GPIO*/
GPIO_InitTypeDef GPIO_InitStructure;
TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
NVIC_InitTypeDef NVIC_InitStructure;
TIM_ICInitTypeDef TIM_ICInitStructure;
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);//使能GPIOA時鐘
RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2,ENABLE);//使能定時器2
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_4|GPIO_Pin_5|GPIO_Pin_6; //Trig:PA4,PA5,PA6
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_OUT;
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;//100MHz
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_DOWN;
GPIO_Init(GPIOA, &GPIO_InitStructure);
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_2|GPIO_Pin_3; //Echo: PA2,PA3對應TIM2的通道3,4
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;//打開引腳複用
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;//100MHz
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
GPIO_PinAFConfig(GPIOA,GPIO_PinSource2,GPIO_AF_TIM2);
GPIO_PinAFConfig(GPIOA,GPIO_PinSource3,GPIO_AF_TIM2);
GPIO_Init(GPIOA, &GPIO_InitStructure);//初始化GPIOA
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOB, ENABLE);//Enable GPIOB's Clock
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_3;//PB3 refers to TIM2's channel 2
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;//100MHz
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
GPIO_PinAFConfig(GPIOB,GPIO_PinSource3,GPIO_AF_TIM2);
GPIO_Init(GPIOB, &GPIO_InitStructure);//initialize GPIOB
/初始化TIM2*/
TIM_TimeBaseStructure.TIM_Period = ; //傳感器最大探測距離4000mm,一般不會溢出
TIM_TimeBaseStructure.TIM_Prescaler =-; //設定TIM2時鐘頻率為1MHz
TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1;
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
TIM_TimeBaseInit(TIM2, &TIM_TimeBaseStructure);
//初始化TIM2輸入捕獲
TIM_ICInitStructure.TIM_Channel = TIM_Channel_2;
TIM_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_BothEdge; //上升和下降都觸發輸入捕獲
TIM_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI;
TIM_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_DIV1;
TIM_ICInitStructure.TIM_ICFilter = ;
TIM_ICInit(TIM2, &TIM_ICInitStructure);
TIM_ICInitStructure.TIM_Channel = TIM_Channel_3;
TIM_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_BothEdge;
TIM_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI;
TIM_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_DIV1;
TIM_ICInitStructure.TIM_ICFilter = ;
TIM_ICInit(TIM2, &TIM_ICInitStructure);
TIM_ICInitStructure.TIM_Channel = TIM_Channel_4;
TIM_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_BothEdge;
TIM_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI;
TIM_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_DIV1;
TIM_ICInitStructure.TIM_ICFilter = ;
TIM_ICInit(TIM2, &TIM_ICInitStructure);
//設定輸入捕獲中斷
NVIC_InitStructure.NVIC_IRQChannel = TIM2_IRQn;
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = ; NVIC_InitStructure.NVIC_IRQChannelSubPriority = 3;
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
NVIC_Init(&NVIC_InitStructure);
TIM_ITConfig(TIM2,TIM_IT_Update|TIM_IT_CC2|TIM_IT_CC3|TIM_IT_CC4,ENABLE);//使能中斷
TIM_Cmd(TIM2,ENABLE);
dist.overflow=;
}
值得注意的是，這裡的輸入捕獲的邊緣極性設定為上升和下降沿都捕獲，效率比較高，也有大神開始使用上升沿捕獲，然後在中斷中将捕獲記性設定為下降沿捕獲，我試過這種辦法，程式容易卡死，是以沒有采用。
中斷程式可以這樣編寫：
void TIM2_IRQHandler(void)
{
static uint16_t cnt_f,cnt_l,cnt_r;
if(TIM_GetITStatus(TIM2,TIM_IT_Update)!=RESET)
{
dist.overflow++;
TIM_ClearITPendingBit(TIM2, TIM_IT_Update);
// printf("overflow\n");
}
if(TIM_GetITStatus(TIM2, TIM_IT_CC2)!=RESET)
{
cnt_f++;
if(cnt_f%==)//檢測到了上升電平
{
TIM2->CNT=;//計數器清零
}
else//檢測到了下降沿電平，讀取距離值
{
dist.cnt=TIM2->CNT;
TIM2->CNT=;
dist.f_distance=(dist.overflow*+dist.cnt)*f;
printf("F_dis=%.2fmm\n",dist.f_distance);
dist.overflow=;
}
if(cnt_f>)cnt_f=;
TIM_ClearITPendingBit(TIM2, TIM_IT_CC2);
}
if(TIM_GetITStatus(TIM2, TIM_IT_CC3)!=RESET)
{
cnt_l++;
if(cnt_l%==)//檢測到了上升電平
{
TIM2->CNT=;
}
else
{
dist.cnt=TIM2->CNT;
TIM2->CNT=;
dist.l_distance=(dist.overflow*+dist.cnt)*f;
printf("L_dis=%.2fmm\n",dist.l_distance);
dist.overflow=;
}
if(cnt_l>)cnt_l=;
TIM_ClearITPendingBit(TIM2, TIM_IT_CC3);
}
if(TIM_GetITStatus(TIM2, TIM_IT_CC4)!=RESET)
{
cnt_r++;
if(cnt_r%==)
{
TIM2->CNT=;
}
else
{
dist.cnt=TIM2->CNT;
TIM2->CNT=;
dist.r_distance=(dist.overflow*+dist.cnt)*f;
printf("R_dis=%.2fmm\n",dist.r_distance);
dist.overflow=;
}
if(cnt_r>)cnt_r=;
TIM_ClearITPendingBit(TIM2, TIM_IT_CC4);
}
}
這樣設計的好處是中斷程式設計簡單，可以保證明時性。
觸發子產品進行測量，通過給子產品的Trig引腳一個超過10us高電平信号就可以觸發子產品進行距離測量，程式代碼如下：
/*
function:啟動距離測量
choice: :左邊超音波子產品
：中間超音波子產品
：右邊超音波子產品
*/
void Get_Distance(uint8_t choice)
{
switch(choice)
{
case :
GPIO_ResetBits(GPIOA,GPIO_Pin_4);
GPIO_SetBits(GPIOA,GPIO_Pin_4);
Delay_us();
GPIO_ResetBits(GPIOA,GPIO_Pin_4);
break;
case :
GPIO_ResetBits(GPIOA,GPIO_Pin_5);
GPIO_SetBits(GPIOA,GPIO_Pin_5);
Delay_us();
GPIO_ResetBits(GPIOA,GPIO_Pin_5);
break;
case :
GPIO_ResetBits(GPIOA,GPIO_Pin_6);
GPIO_SetBits(GPIOA,GPIO_Pin_6);
Delay_us();
GPIO_ResetBits(GPIOA,GPIO_Pin_6);
break;
}
}