分享一个非8M晶振系统配置串口时遇到的问题

d1z1y2

我用的是库函数，配置串口波特率时，USART_Init()里会调用 RCC_GetClocksFreq(&RCC_Clocks)取时钟频率值， 来配置波特率，RCC_GetClocksFreq() 会用到宏定义 HSE_VALUE，此为外部晶振

值，默认为8M,此宏定义位于 stm32f10x.h ，当用非8M晶振时，要更改 HSE_VALUE的值,否则RCC_GetClocksFreq(&RCC_Clocks)得到的值是不对的，会导致波特率异常

//USART_Init()函数部分:
/*---------------------------- USART BRR Configuration -----------------------*/
  /* Configure the USART Baud Rate -------------------------------------------*/
  RCC_GetClocksFreq(&RCC_ClocksStatus);
  if (usartxbase == USART1_BASE)
  {
    apbclock = RCC_ClocksStatus.PCLK2_Frequency;
  }
  else
  {
    apbclock = RCC_ClocksStatus.PCLK1_Frequency;
  }


//void RCC_GetClocksFreq(RCC_ClocksTypeDef* RCC_Clocks)函数部分:
   /* Get SYSCLK source -------------------------------------------------------*/
  tmp = RCC->CFGR & CFGR_SWS_Mask;

  switch (tmp)
  {
    case 0x00:  /* HSI used as system clock */
      RCC_Clocks->SYSCLK_Frequency = HSI_VALUE;
      break;
    case 0x04:  /* HSE used as system clock */
      RCC_Clocks->SYSCLK_Frequency = HSE_VALUE;
      break;
    case 0x08:  /* PLL used as system clock */

      /* Get PLL clock source and multiplication factor ----------------------*/
      pllmull = RCC->CFGR & CFGR_PLLMull_Mask;
      pllsource = RCC->CFGR & CFGR_PLLSRC_Mask;

#ifndef STM32F10X_CL      
      pllmull = ( pllmull >> 18) + 2;

      if (pllsource == 0x00)
      {/* HSI oscillator clock divided by 2 selected as PLL clock entry */
        RCC_Clocks->SYSCLK_Frequency = (HSI_VALUE >> 1) * pllmull;
      }
      else
      {
#if defined (STM32F10X_LD_VL) || defined (STM32F10X_MD_VL) || defined (STM32F10X_HD_VL)
       prediv1factor = (RCC->CFGR2 & CFGR2_PREDIV1) + 1;
       /* HSE oscillator clock selected as PREDIV1 clock entry */
       RCC_Clocks->SYSCLK_Frequency = (HSE_VALUE / prediv1factor) * pllmull;
#else
        /* HSE selected as PLL clock entry */
        if ((RCC->CFGR & CFGR_PLLXTPRE_Mask) != (uint32_t)RESET)
        {/* HSE oscillator clock divided by 2 */
          RCC_Clocks->SYSCLK_Frequency = (HSE_VALUE >> 1) * pllmull;
        }
        else
        {
          RCC_Clocks->SYSCLK_Frequency = HSE_VALUE * pllmull;
        }
#endif


//stm32f10x.h 里有:
#if !defined  HSE_VALUE
#ifdef STM32F10X_CL   
  #define HSE_VALUE    ((uint32_t)25000000) /*!< Value of the External oscillator in Hz */
#else
  #define HSE_VALUE    ((uint32_t)8000000) /*!< Value of the External oscillator in Hz */
#endif /* STM32F10X_CL */
#endif /* HSE_VALUE */

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