Systick嘀嗒定時器是一個24位的遞減計數器。該定時器的時鐘源可以是內部時鐘，也可以是外部時鐘。 M4的Systick有四個暫存器，分別是

1. Systick control and  status register(STK_CTRL) 控制和狀態暫存器；

2. Systick reload value register（STK_LOAD) 重灌數值暫存器；

3. Systick current value register（STK_VAL) 當前計數值暫存器；

4. Systick calibration value register（STK_CALIB) 校準數值暫存器。

STM32F4中Systick相關函式主要在misc.c和core.cm4.h中。在misc.c中 void Systic_CLKSourceConfig(uint32_t SysTick_CLKSource) 主要實現時鐘源的選擇。

/**
  * @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中函式SysTick_Config(uint32_t ticks)實現了嘀嗒定時器的配置。

/* ##################################    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 */

嘀嗒定時器使用方法：

1. 函式SysTick_Config()是屬於CMSIS裡面的一個函式，實現配置如下
  - 函式的引數是過載暫存器reload要賦予的初值
  - 配置嘀嗒定時器的優先順序是最低優先順序15
  - 配置HCLK作為系統的時鐘源
  - 使能嘀嗒定時器中斷
  - 啟動嘀嗒定時器計數

2. 使用者可以在呼叫函式SysTick_Config()後通過函式 SysTick_CLKSourceConfig()
  更改嘀嗒定時器的時鐘源為HCLK/8。SysTick_CLKSourceConfig()在檔案misc.c裡面

3. 使用者可以在呼叫函式SysTick_Config()後通過函式 NVIC_SetPriority()修改優先順序，
  函式NVIC_SetPriority()在檔案core_cm4.h檔案裡面

4. 通過下面的公式調整時基
  Reload Value = SysTick Counter Clock (Hz) x  Desired Time base (s)
  Reload Value 是函式SysTick_Config()的引數，但是不能超過0xFFFFFF