各种外设驱动样例：
2、其次，楼主只贴了RCC代码，无法明确问题原因，只能帮楼主分析下，如果使用完整的HAL驱动的话，串口初始化，会执行底层的HAL_StatusTypeDef UART_SetConfig(UART_HandleTypeDef *huart)函数，里面会获取UART时钟源，并根据时钟源去查找系统当前时钟，进而进行波特率等更时钟相关的寄存器设置，而程序是如何知道系统当前时钟的呢？答案是stm32l4xx_hal_conf.h中定义的，并不是神奇的软件能够自动识别硬件得到的，只是根据RCC寄存器值获取分频、倍频等参数状态+基础时钟（HSE\HSI\LSE\LSI等）计算出来的，关键的就在这些定义上，如：
/* ########################## Oscillator Values adaptation ####################*/
/**
* @brief Adjust the value of External High Speed oscillator (HSE) used in your application.
* This value is used by the RCC HAL module to compute the system frequency
* (when HSE is used as system clock source, directly or through the PLL).
*/
#if !defined(HSE_VALUE)
#define HSE_VALUE ((uint32_t)8000000) /*!< Value of the External oscillator in Hz */
#endif /* HSE_VALUE */
#if !defined(HSE_STARTUP_TIMEOUT)
#define HSE_STARTUP_TIMEOUT ((uint32_t)100) /*!< Time out for HSE start up, in ms */
#endif /* HSE_STARTUP_TIMEOUT */
/**
* @brief Internal Multiple Speed oscillator (MSI) default value.
* This value is the default MSI range value after Reset.
*/
#if !defined(MSI_VALUE)
#define MSI_VALUE ((uint32_t)4000000) /*!< Value of the Internal oscillator in Hz*/
#endif /* MSI_VALUE */
/**
* @brief Internal High Speed oscillator (HSI) value.
* This value is used by the RCC HAL module to compute the system frequency
* (when HSI is used as system clock source, directly or through the PLL).
*/
#if !defined(HSI_VALUE)
#define HSI_VALUE ((uint32_t)16000000) /*!< Value of the Internal oscillator in Hz*/
#endif /* HSI_VALUE */
/**
* @brief Internal Low Speed oscillator (LSI) value.
*/
#if !defined(LSI_VALUE)
#define LSI_VALUE((uint32_t)32000) /*!< LSI Typical Value in Hz*/
#endif /* LSI_VALUE */ /*!< Value of the Internal Low Speed oscillator in Hz
The real value may vary depending on the variations
in voltage and temperature.*/
/**
* @brief External Low Speed oscillator (LSE) value.
* This value is used by the UART, RTC HAL module to compute the system frequency
*/
#if !defined(LSE_VALUE)
#define LSE_VALUE ((uint32_t)32768) /*!< Value of the External oscillator in Hz*/
#endif /* LSE_VALUE */
#if !defined(LSE_STARTUP_TIMEOUT)
#define LSE_STARTUP_TIMEOUT ((uint32_t)5000) /*!< Time out for LSE start up, in ms */
#endif /* HSE_STARTUP_TIMEOUT */
/**
* @brief External clock source for SAI1 peripheral
* This value is used by the RCC HAL module to compute the SAI1 & SAI2 clock source
* frequency.
*/
#if !defined(EXTERNAL_SAI1_CLOCK_VALUE)
#define EXTERNAL_SAI1_CLOCK_VALUE ((uint32_t)48000) /*!< Value of the SAI1 External clock source in Hz*/
#endif /* EXTERNAL_SAI1_CLOCK_VALUE */
/**
* @brief External clock source for SAI2 peripheral
* This value is used by the RCC HAL module to compute the SAI1 & SAI2 clock source
* frequency.
*/
#if !defined(EXTERNAL_SAI2_CLOCK_VALUE)
#define EXTERNAL_SAI2_CLOCK_VALUE ((uint32_t)48000) /*!< Value of the SAI2 External clock source in Hz*/
#endif /* EXTERNAL_SAI2_CLOCK_VALUE */
比如如果你使用外部8M高速晶振，你需要修改HSE_VALUE为8000000，假设你实际使用的外部晶振为12M，而你的HSE_VALUE仍为8000000，系统也会跑起来，但是通过HAL_RCC_GetSysClockFreq()等获取时钟的函数得到的值就不对了。也许这就是问题原因的所在，希望楼主能解决此问题。
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