#if !defined  (HSE_VALUE) 
  #define HSE_VALUE    ((uint32_t)24000000U) /*!< Value of the External oscillator in Hz */
#endif /* HSE_VALUE */

HSE_VALUE   值的大小决定了，时钟滴答的周期。  这个值的设置不会影响主频的时钟周期以及相关timer的变化。

__weak uint32_t HAL_RCC_GetSysClockFreq(void)
{
  uint32_t pllm = 0U, pllvco = 0U, pllp = 0U;
  uint32_t sysclockfreq = 0U;

  /* Get SYSCLK source -------------------------------------------------------*/
  switch (RCC->CFGR & RCC_CFGR_SWS)
  {
    case RCC_CFGR_SWS_HSI:  /* HSI used as system clock source */
    {
      sysclockfreq = HSI_VALUE;
       break;
    }
    case RCC_CFGR_SWS_HSE:  /* HSE used as system clock  source */
    {
      sysclockfreq = HSE_VALUE;
      break;
    }
    case RCC_CFGR_SWS_PLL:  /* PLL used as system clock  source */
    {
      /* PLL_VCO = (HSE_VALUE or HSI_VALUE / PLLM) * PLLN
      SYSCLK = PLL_VCO / PLLP */
      pllm = RCC->PLLCFGR & RCC_PLLCFGR_PLLM;
      if(__HAL_RCC_GET_PLL_OSCSOURCE() != RCC_PLLSOURCE_HSI)
      {
        /* HSE used as PLL clock source */
        pllvco = ((HSE_VALUE / pllm) * ((RCC->PLLCFGR & RCC_PLLCFGR_PLLN) >> POSITION_VAL(RCC_PLLCFGR_PLLN)));
      }
      else
      {
        /* HSI used as PLL clock source */
        pllvco = ((HSI_VALUE / pllm) * ((RCC->PLLCFGR & RCC_PLLCFGR_PLLN) >> POSITION_VAL(RCC_PLLCFGR_PLLN)));    
      }
      pllp = ((((RCC->PLLCFGR & RCC_PLLCFGR_PLLP) >> POSITION_VAL(RCC_PLLCFGR_PLLP)) + 1U) *2U);
      
      sysclockfreq = pllvco/pllp;
      break;
    }
    default:
    {
      sysclockfreq = HSI_VALUE;
      break;
    }
  }
  return sysclockfreq;
}