STM32时钟初始化函数SystemInit()详解

花了一天的时间，总算是了解了SystemInit()函数实现了哪些功能，初学STM32，，现记录如下(有理解错误的地方还请大侠指出)：

使用的是3.5的库，用的是STM32F107VC，开发环境RVMDK4.23

我已经定义了STM32F10X_CL，SYSCLK_FREQ_72MHz

函数调用顺序：

startup_stm32f10x_cl.s(启动文件) → SystemInit() → SetSysClock () → SetSysClockTo72()

初始化时钟用到的RCC寄存器复位值：

RCC_CR = 0x0000 xx83; RCC_CFGR = 0x0000 0000;RCC_CIR = 0x0000 0000; RCC_CFGR2 = 0x0000 0000;

SystemInit()

在调用 SetSysClock()之前RCC寄存器的值如下(都是一些与运算，或运算，在此就不赘述了)：

RCC->CR = 0x0000 0083; RCC->CIR = 0x00FF0000; RCC->CFGR2 = 0x00000000;至于这些寄存器都代表着什么意思，详见芯片资料RCC寄存器，该文重点不在此处;

SetSysClock()函数如下：

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 //我的定义的是SYSCLK_FREQ_72MHz，所以调用SetSysClockTo72()

SetSysClockTo72();

#endif

}

SetSysClockTo72()函数如下：

static void SetSysClockTo72(void)

{

__IO uint32_t StartUpCounter = 0, HSEStatus = 0;

/* SYSCLK, HCLK, PCLK2 and PCLK1 configuration ---------------------------*/

/* Enable HSE */

RCC->CR |= ((uint32_t)RCC_CR_HSEON);

/* Wait till HSE is ready and if Time out is reached exit */

do

{

HSEStatus = RCC->CR & RCC_CR_HSERDY;

StartUpCounter++;

} while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT));

if ((RCC->CR & RCC_CR_HSERDY) != RESET)

{

HSEStatus = (uint32_t)0x01;

}

else

{

HSEStatus = (uint32_t)0x00;

}

if (HSEStatus == (uint32_t)0x01)

{

/* Enable Prefetch Buffer */

FLASH->ACR |= FLASH_ACR_PRFTBE;

/* Flash 2 wait state */

FLASH->ACR &= (uint32_t)((uint32_t)~FLASH_ACR_LATENCY);

FLASH->ACR |= (uint32_t)FLASH_ACR_LATENCY_2;

/* HCLK = SYSCLK */

RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_div1;

/* PCLK2 = HCLK */

RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE2_div1;

/* PCLK1 = HCLK */

RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE1_div2;

#ifdef STM32F10X_CL

/* Configure PLLs ------------------------------------------------------*/

/* PLL2 configuration: PLL2CLK = (HSE / 5) * 8 = 40 MHz */

/* PREdiv1 configuration: PREdiv1CLK = PLL2 / 5 = 8 MHz */

RCC->CFGR2 &= (uint32_t)~(RCC_CFGR2_PREdiv2 | RCC_CFGR2_PLL2MUL |

RCC_CFGR2_PREdiv1 | RCC_CFGR2_PREdiv1SRC);

RCC->CFGR2 |= (uint32_t)(RCC_CFGR2_PREdiv2_div5 | RCC_CFGR2_PLL2MUL8 |

RCC_CFGR2_PREdiv1SRC_PLL2 | RCC_CFGR2_PREdiv1_div5);

/* Enable PLL2 */

RCC->CR |= RCC_CR_PLL2ON;

/* Wait till PLL2 is ready */

while((RCC->CR & RCC_CR_PLL2RDY) == 0)

{

}

/* PLL configuration: PLLCLK = PREdiv1 * 9 = 72 MHz */

RCC->CFGR &= (uint32_t)~(RCC_CFGR_PLLXTPRE | RCC_CFGR_PLLSRC | RCC_CFGR_PLLMULL);

RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLXTPRE_PREdiv1 | RCC_CFGR_PLLSRC_PREdiv1 |

RCC_CFGR_PLLMULL9);

#else

/* PLL configuration: PLLCLK = HSE * 9 = 72 MHz */

RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLXTPRE |

RCC_CFGR_PLLMULL));

RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_HSE | RCC_CFGR_PLLMULL9);

#endif /* STM32F10X_CL */

/* Enable PLL */

RCC->CR |= RCC_CR_PLLON;

/* Wait till PLL is ready */

while((RCC->CR & RCC_CR_PLLRDY) == 0)

{

}

/* Select PLL as system clock source */

RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));

RCC->CFGR |= (uint32_t)RCC_CFGR_SW_PLL;

/* Wait till PLL is used as system clock source */

while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS) != (uint32_t)0x08