STM32F407学习记录2:SysTick嘀嗒定时器学习

Systick嘀嗒定时器是一个24位的递减计数器。该定时器的时钟源可以是内部时钟，也可以是外部时钟。 M4的Systick有四个寄存器，分别是

1. Systick control and  status register(STK_CTRL) 控制和状态寄存器；

2. Systick reload value register（STK_LOAD) 重装数值寄存器；

3. Systick current value register（STK_VAL) 当前计数值寄存器；

4. Systick calibration value register（STK_CALIB) 校准数值寄存器。

STM32F4中Systick相关函数主要在misc.c和core.cm4.h中。在misc.c中 void Systic_CLKSourceConfig(uint32_t SysTick_CLKSource) 主要实现时钟源的选择。

/**
  * @brief  Configures the SysTick clock source.
  * @param  SysTick_CLKSource: specifies the SysTick clock source.
  *   This parameter can be one of the following values:
  *     @arg SysTick_CLKSource_HCLK_Div8: AHB clock divided by 8 selected as SysTick clock source.
  *     @arg SysTick_CLKSource_HCLK: AHB clock selected as SysTick clock source.
  * @retval None
  */
void SysTick_CLKSourceConfig(uint32_t SysTick_CLKSource)
{
  /* Check the parameters */
  assert_param(IS_SYSTICK_CLK_SOURCE(SysTick_CLKSource));
  if (SysTick_CLKSource == SysTick_CLKSource_HCLK)
  {
    SysTick->CTRL |= SysTick_CLKSource_HCLK;
  }
  else
  {
    SysTick->CTRL &= SysTick_CLKSource_HCLK_Div8;
  }
}

在core_cm4.h中函数SysTick_Config(uint32_t ticks)实现了嘀嗒定时器的配置。

/* ##################################    SysTick function  ############################################ */
/** \ingroup  CMSIS_Core_FunctionInterface
    \defgroup CMSIS_Core_SysTickFunctions SysTick Functions
    \brief      Functions that configure the System.
  @{
 */


#if (__Vendor_SysTickConfig == 0)


/** \brief  System Tick Configuration


    The function initializes the System Timer and its interrupt, and starts the System Tick Timer.
    Counter is in free running mode to generate periodic interrupts.


    \param [in]  ticks  Number of ticks between two interrupts.


    \return          0  Function succeeded.
    \return          1  Function failed.


    \note     When the variable <b>__Vendor_SysTickConfig</b> is set to 1, then the
    function <b>SysTick_Config</b> is not included. In this case, the file <b><i>device</i>.h</b>
    must contain a vendor-specific implementation of this function.


 */
__STATIC_INLINE uint32_t SysTick_Config(uint32_t ticks)
{
  if ((ticks - 1) > SysTick_LOAD_RELOAD_Msk)  return (1);      /* Reload value impossible */


  SysTick->LOAD  = ticks - 1;                                  /* set reload register */
  NVIC_SetPriority (SysTick_IRQn, (1<<__NVIC_PRIO_BITS) - 1);  /* set Priority for Systick Interrupt */
  SysTick->VAL   = 0;                                          /* Load the SysTick Counter Value */
  SysTick->CTRL  = SysTick_CTRL_CLKSOURCE_Msk |
                   SysTick_CTRL_TICKINT_Msk   |
                   SysTick_CTRL_ENABLE_Msk;                    /* Enable SysTick IRQ and SysTick Timer */
  return (0);                                                  /* Function successful */
}


#endif


/*@} end of CMSIS_Core_SysTickFunctions */

嘀嗒定时器使用方法：

1. 函数SysTick_Config()是属于CMSIS里面的一个函数，实现配置如下
  - 函数的参数是重载寄存器reload要赋予的初值
  - 配置嘀嗒定时器的优先级是最低优先级15
  - 配置HCLK作为系统的时钟源
  - 使能嘀嗒定时器中断
  - 启动嘀嗒定时器计数

2. 用户可以在调用函数SysTick_Config()后通过函数 SysTick_CLKSourceConfig()
  更改嘀嗒定时器的时钟源为HCLK/8。SysTick_CLKSourceConfig()在文件misc.c里面

3. 用户可以在调用函数SysTick_Config()后通过函数 NVIC_SetPriority()修改优先级，
  函数NVIC_SetPriority()在文件core_cm4.h文件里面

4. 通过下面的公式调整时基
  Reload Value = SysTick Counter Clock (Hz) x  Desired Time base (s)
  Reload Value 是函数SysTick_Config()的参数，但是不能超过0xFFFFFF