基于STM32外部中断捕获低频频率的系统配置和代码书写

STM32外部中断捕获低频频率的系统配置和代码书写步骤如下：

1. 配置GPIO口为输入模式，对应的外部中断线路与GPIO口建立映射关系；
2. 配置外部中断线路触发方式为上升沿、下降沿或者双边沿触发；
3. 配置TIM计数器为定时器模式，设置定时器时钟频率和计数器最大值，使得定时器的计数时间足够长；
4. 配置TIM通道为输入捕获模式，选择对应的GPIO口和外部中断线路映射，设置输入捕获的触发方式；
5. 在外部中断的回调函数中，读取TIM计数器的当前值，计算捕获到的脉冲宽度和频率。

下面是代码实现的示例：

#include "stm32f10x.h"

#define TIMx            TIM2
#define TIMx_IRQn       TIM2_IRQn
#define GPIO_Pin        GPIO_Pin_0
#define GPIO_Port       GPIOA
#define EXTI_Line       EXTI_Line0
#define EXTI_PortSource EXTI_PortSourceGPIOA
#define EXTI_PinSource  EXTI_PinSource0

volatile uint32_t g_pulseWidth = 0;
volatile uint32_t g_frequency = 0;

void TIM_Configuration(void)
{
  TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
  TIM_ICInitTypeDef TIM_ICInitStructure;
  NVIC_InitTypeDef NVIC_InitStructure;

  RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2, ENABLE);

  TIM_TimeBaseStructure.TIM_Period = 0xFFFF;
  TIM_TimeBaseStructure.TIM_Prescaler = 71;
  TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1;
  TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
  TIM_TimeBaseInit(TIMx, &TIM_TimeBaseStructure);

  TIM_ICInitStructure.TIM_Channel = TIM_Channel_1;
  TIM_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_Rising;
  TIM_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI;
  TIM_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_DIV1;
  TIM_ICInitStructure.TIM_ICFilter = 0x0;
  TIM_ICInit(TIMx, &TIM_ICInitStructure);

  NVIC_InitStructure.NVIC_IRQChannel = TIMx_IRQn;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);

  TIM_Cmd(TIMx, ENABLE);
  TIM_ITConfig(TIMx, TIM_IT_CC1, ENABLE);
}

void EXTI_Configuration(void)
{
  GPIO_InitTypeDef GPIO_InitStructure;
  EXTI_InitTypeDef EXTI_InitStructure;
  NVIC_InitTypeDef NVIC_InitStructure;

  RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA, ENABLE);

  GPIO_InitStructure.GPIO_Pin = GPIO_Pin;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
  GPIO_Init(GPIO_Port, &GPIO_InitStructure);

  EXTI_InitStructure.EXTI_Line = EXTI_Line;
  EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt;
  EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Rising_Falling;
  EXTI_InitStructure.EXTI_LineCmd = ENABLE;
  EXTI_Init(&EXTI_InitStructure);

  NVIC_InitStructure.NVIC_IRQChannel = EXTI0_IRQn;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);
}

void EXTI0_IRQHandler(void)
{
  if (EXTI_GetITStatus(EXTI_Line) != RESET)
  {
    g_pulseWidth = TIM_GetCapture1(TIMx);
    g_frequency = SystemCoreClock / (TIMx->PSC + 1) / g_pulseWidth;

    EXTI_ClearITPendingBit(EXTI_Line);
  }
}

int main(void)
{
  SystemInit();

  TIM_Configuration();
  EXTI_Configuration();

  while (1)
  {
    // do something
  }
}

在这个例子中，我们使用了TIM2的通道1作为输入捕获通道，对应的GPIO口为PA0。TIM2被配置为定时器模式，时钟频率为72MHz，计数器最大值为0xFFFF。在外部中断的回调函数中，计算捕获到的脉冲宽度和频率，然后将它们保存在全局变量中。注意，在计算频率时，需要除以脉冲宽度，再除以2，因为每个脉冲包含了两个边沿。