SystemInit()函数的结构分析

具体的分析见：http://wenku.baidu.com/view/3b995d708e9951e79b8927f5.html

http://blog.sina.com.cn/s/blog_60e5783601017vbt.html

系统函数调用顺序startup_stm32f10x_cl.s→ SystemInit() → SetSysClock()→ SetSysClockTo72()

总结:开始是在keil4里看代码，很混乱，后来把他粘到Notepad编辑器，一点击就可以看出这个子函数到哪结束

#if defined (STM32F10X_LD_VL) || (defined STM32F10X_MD_VL) || (defined STM32F10X_HD_VL)
/* #define SYSCLK_FREQ_HSE    HSE_VALUE */
#define SYSCLK_FREQ_24MHz  24000000
#else
/* #define SYSCLK_FREQ_HSE    HSE_VALUE */
/* #define SYSCLK_FREQ_24MHz  24000000 */
/* #define SYSCLK_FREQ_36MHz  36000000 */
/* #define SYSCLK_FREQ_48MHz  48000000 */
/* #define SYSCLK_FREQ_56MHz  56000000 */
#define SYSCLK_FREQ_72MHz  72000000
#endif

下面定义的SystemCoreClock 变量是为了方便取出几种情况下系统时钟的实际值，例SYSCLK_FREQ_24MHz 也就是24000000。

#ifdef SYSCLK_FREQ_HSE
uint32_t SystemCoreClock         = SYSCLK_FREQ_HSE;        /*!< System Clock Frequency (Core Clock) */
#elif defined SYSCLK_FREQ_24MHz
uint32_t SystemCoreClock         = SYSCLK_FREQ_24MHz;        /*!< System Clock Frequency (Core Clock) */
#elif defined SYSCLK_FREQ_36MHz
uint32_t SystemCoreClock         = SYSCLK_FREQ_36MHz;        /*!< System Clock Frequency (Core Clock) */
#elif defined SYSCLK_FREQ_48MHz
uint32_t SystemCoreClock         = SYSCLK_FREQ_48MHz;        /*!< System Clock Frequency (Core Clock) */
#elif defined SYSCLK_FREQ_56MHz
uint32_t SystemCoreClock         = SYSCLK_FREQ_56MHz;        /*!< System Clock Frequency (Core Clock) */
#elif defined SYSCLK_FREQ_72MHz
uint32_t SystemCoreClock         = SYSCLK_FREQ_72MHz;        /*!< System Clock Frequency (Core Clock) */
#else /*!< HSI Selected as System Clock source */
uint32_t SystemCoreClock         = HSI_VALUE;        /*!< System Clock Frequency (Core Clock) */
#endif

static void SetSysClock(void)
{
#ifdef SYSCLK_FREQ_HSE
SetSysClockToHSE();
#elif defined SYSCLK_FREQ_24MHz
SetSysClockTo24();
#elif defined SYSCLK_FREQ_36MHz
SetSysClockTo36();
#elif defined SYSCLK_FREQ_48MHz
SetSysClockTo48();
#elif defined SYSCLK_FREQ_56MHz
SetSysClockTo56();
#elif defined SYSCLK_FREQ_72MHz
SetSysClockTo72();
#endif

/* If none of the define above is enabled, the HSI is used as System clock
source (default after reset) */
}

#ifdef SYSCLK_FREQ_HSE
/**
* @brief  Selects HSE as System clock source and configure HCLK, PCLK2
*         and PCLK1 prescalers.
* @note   This function should be used only after reset.
* @param  None
* @retval None
*/
static void SetSysClockToHSE(void)
{
......
}

#elif defined SYSCLK_FREQ_24MHz
/**
* @brief  Sets System clock frequency to 24MHz and configure HCLK, PCLK2
*         and PCLK1 prescalers.
* @note   This function should be used only after reset.
* @param  None
* @retval None
*/
static void SetSysClockTo24(void)
{
........
}

#elif defined SYSCLK_FREQ_36MHz
/**
* @brief  Sets System clock frequency to 36MHz and configure HCLK, PCLK2
*         and PCLK1 prescalers.
* @note   This function should be used only after reset.
* @param  None
* @retval None
*/
static void SetSysClockTo36(void)
{
......
}

省略几个......

#elif defined SYSCLK_FREQ_72MHz
/**
* @brief  Sets System clock frequency to 72MHz and configure HCLK, PCLK2
*         and PCLK1 prescalers.
* @note   This function should be used only after reset.
* @param  None
* @retval None
*/
static void SetSysClockTo72(void)
{
.......
}

这样结构就很明晰了。