STM32 HAL库微秒延时函数的实现

天下苦STM32 HAL库微秒延时久已。不占用其他定时器资源又不使用循环的方式就不能实现微秒延时函数了吗？答案是否定的，我们还有方式实现，且还不止一种方法。详情且看下文分解：

以下两种延时方式来源：Arduino_Core_STM32源码delayMicroseconds(uint32_t us)函数的实现。

利用SysTick再实现微秒延时函数

虽然SysTick已经被配置为1ms中断一次的模式，但每个1ms之间SysTick的当前值寄存器是一直在计数的（每计一个数的时间是1/SytemCoreClock）我们便可以利用该机制实现微秒延时函数。

void delayMicroseconds(uint32_t us)
{
    __IO uint32_t currentTicks = SysTick->VAL;
  /* Number of ticks per millisecond */
  const uint32_t tickPerMs = SysTick->LOAD + 1;
  /* Number of ticks to count */
  const uint32_t nbTicks = ((us - ((us > 0) ? 1 : 0)) * tickPerMs) / 1000;
  /* Number of elapsed ticks */
  uint32_t elapsedTicks = 0;
  __IO uint32_t oldTicks = currentTicks;
  do {
    currentTicks = SysTick->VAL;
    elapsedTicks += (oldTicks < currentTicks) ? tickPerMs + oldTicks - currentTicks :
                    oldTicks - currentTicks;
    oldTicks = currentTicks;
  } while (nbTicks > elapsedTicks);
}

以上函数可以直接复制到工程中使用，不需要额外的任何配置。

---

Note

虽然函数参数us为uint32_t类型，但是延时数不能过大，原因自己分析。建议超过1ms的延时时间使用HAL_Delay()。

---

利用DWT（数据观测点）实现微秒延时函数

对于DWT大家可以搜索具体了解，这里我也不是很了解，就直说实现方法好了。

dwt.h文件

/**
  ******************************************************************************
  * @file    dwt.h
  * @author  Frederic Pillon
  * @brief   Header for dwt.c module
  ******************************************************************************
  * @attention
  *
  * Copyright (c) 2019, STMicroelectronics
  * All rights reserved.
  *
  * This software component is licensed by ST under BSD 3-Clause license,
  * the "License"; You may not use this file except in compliance with the
  * License. You may obtain a copy of the License at:
  *                        opensource.org/licenses/BSD-3-Clause
  *
  ******************************************************************************
  */

/* Define to prevent recursive inclusion -------------------------------------*/
#ifndef _DWT_H_
#define _DWT_H_

#include "stm32f4xx.h"
#include <stdbool.h>

#ifndef UNUSED
#define UNUSED(x) (void)x
#endif

#ifdef DWT_BASE
uint32_t dwt_init(void);

#ifdef __cplusplus
extern "C" {
#endif

//uint32_t dwt_init(void);
void dwt_access(bool ena);

static inline uint32_t dwt_max_sec(void)
{
  return (UINT32_MAX / SystemCoreClock);
};

static inline uint32_t dwt_max_msec(void)
{
  return (UINT32_MAX / (SystemCoreClock / 1000));
};

static inline uint32_t dwt_max_usec(void)
{
  return (UINT32_MAX / (SystemCoreClock / 1000000));
};

static inline uint32_t dwt_getCycles(void)
{
  return (DWT->CYCCNT);
};

#ifdef __cplusplus
}
#endif

#endif /* DWT_BASE */
#endif /* _DWT_H_ */

dwt.c文件

/**
  ******************************************************************************
  * @file    dwt.c
  * @author  Frederic Pillon
  * @brief   Provide Data Watchpoint and Trace services
  ******************************************************************************
  * @attention
  *
  * Copyright (c) 2019, STMicroelectronics
  * All rights reserved.
  *
  * This software component is licensed by ST under BSD 3-Clause license,
  * the "License"; You may not use this file except in compliance with the
  * License. You may obtain a copy of the License at:
  *                        opensource.org/licenses/BSD-3-Clause
  *
  ******************************************************************************
  */

#include "dwt.h"

#ifdef DWT_BASE
#ifdef __cplusplus
extern "C" {
#endif


uint32_t dwt_init(void)
{

  /* Enable use of DWT */
  if (!(CoreDebug->DEMCR & CoreDebug_DEMCR_TRCENA_Msk)) {
    CoreDebug->DEMCR |= CoreDebug_DEMCR_TRCENA_Msk;
  }

  /* Unlock */
  dwt_access(true);

  /* Reset the clock cycle counter value */
  DWT->CYCCNT = 0;

  /* Enable  clock cycle counter */
  DWT->CTRL |=  DWT_CTRL_CYCCNTENA_Msk;

  /* 3 NO OPERATION instructions */
  __asm volatile(" nop      \n\t"
                 " nop      \n\t"
                 " nop      \n\t");

  /* Check if clock cycle counter has started */
  return (DWT->CYCCNT) ? 0 : 1;
}

void dwt_access(bool ena)
{
#if (__CORTEX_M == 0x07U)
  /*
   * Define DWT LSR mask which is (currentuly) not defined by the CMSIS.
   * Same as ITM LSR one.
   */
#if !defined DWT_LSR_Present_Msk
#define DWT_LSR_Present_Msk ITM_LSR_Present_Msk
#endif
#if !defined DWT_LSR_Access_Msk
#define DWT_LSR_Access_Msk ITM_LSR_Access_Msk
#endif
  uint32_t lsr = DWT->LSR;

  if ((lsr & DWT_LSR_Present_Msk) != 0) {
    if (ena) {
      if ((lsr & DWT_LSR_Access_Msk) != 0) { //locked
        DWT->LAR = 0xC5ACCE55;
      }
    } else {
      if ((lsr & DWT_LSR_Access_Msk) == 0) { //unlocked
        DWT->LAR = 0;
      }
    }
  }
#else /* __CORTEX_M */
  UNUSED(ena);
#endif /* __CORTEX_M */
}

#ifdef __cplusplus
}
#endif

#endif

delayMicroseconds()函数

void delayMicroseconds(uint32_t us)
{
#if defined(DWT_BASE) && !defined(DWT_DELAY_DISABLED)
  int32_t start  = dwt_getCycles();
  int32_t cycles = us * (SystemCoreClock / 1000000);

  while ((int32_t)dwt_getCycles() - start < cycles);
#endif
}

---

Note:

在使用DWT实现的延时函数时，程序下载到单片机中不能直接运行，需要按一下复位键才能正常运行（使用keil-MDK环境，ST-Link下载）。

---

结论

对比上面两种方式的延时效果：DWT实现的延时精度更高，SysTick的使用更加简单，大家可针对自己的需求选择。