第一個函數SystemInit()裡面有些啥

SystemInit()這個函數出現在main()函數的第一行，可以看出它的重要性。以前關於SystemInit()這個函數從來沒有關心過，只知道這是進行STM32系統初始化的一個函數。今天決定仔細看看，重新開始STM32的學習。這個函數在system_stm32f10x.c中，此C檔案主要就是幹具體硬體設定相關的工作。

/** @addtogroup STM32F10x_System_Private_Functions  * @{  *//**  * @brief  Setup the microcontroller system  *         Initialize the Embedded Flash Interface, the PLL and update the   *         SystemCoreClock variable.  * @note   This function should be used only after reset.  * @param  None  * @retval None  */void SystemInit (void){  /* Reset the RCC clock configuration to the default reset state(for debug purpose) */  /* Set HSION bit */  RCC->CR |= (uint32_t)0x00000001;  /* Reset SW, HPRE, PPRE1, PPRE2, ADCPRE and MCO bits */#ifndef STM32F10X_CL  RCC->CFGR &= (uint32_t)0xF8FF0000;#else  RCC->CFGR &= (uint32_t)0xF0FF0000;#endif /* STM32F10X_CL */       /* Reset HSEON, CSSON and PLLON bits */  RCC->CR &= (uint32_t)0xFEF6FFFF;  /* Reset HSEBYP bit */  RCC->CR &= (uint32_t)0xFFFBFFFF;  /* Reset PLLSRC, PLLXTPRE, PLLMUL and USBPRE/OTGFSPRE bits */  RCC->CFGR &= (uint32_t)0xFF80FFFF;#ifdef STM32F10X_CL  /* Reset PLL2ON and PLL3ON bits */  RCC->CR &= (uint32_t)0xEBFFFFFF;  /* Disable all interrupts and clear pending bits  */  RCC->CIR = 0x00FF0000;  /* Reset CFGR2 register */  RCC->CFGR2 = 0x00000000;#elif defined (STM32F10X_LD_VL) || defined (STM32F10X_MD_VL) || (defined STM32F10X_HD_VL)  /* Disable all interrupts and clear pending bits  */  RCC->CIR = 0x009F0000;  /* Reset CFGR2 register */  RCC->CFGR2 = 0x00000000;      #else  /* Disable all interrupts and clear pending bits  */  RCC->CIR = 0x009F0000;#endif /* STM32F10X_CL */    #if defined (STM32F10X_HD) || (defined STM32F10X_XL) || (defined STM32F10X_HD_VL)  #ifdef DATA_IN_ExtSRAM    SystemInit_ExtMemCtl();   #endif /* DATA_IN_ExtSRAM */#endif   /* Configure the System clock frequency, HCLK, PCLK2 and PCLK1 prescalers */  /* Configure the Flash Latency cycles and enable prefetch buffer */  SetSysClock();#ifdef VECT_TAB_SRAM  SCB->VTOR = SRAM_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal SRAM. */#else  SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal FLASH. */#endif }

從函數說明來看，此函數功能就是初始化內部FALSH，PLL並且更新系統時鐘。此函數需在複位啟動後調用。

RCC->CR |= (uint32_t)0x00000001;

第一行代碼操作時鐘控制寄存器，將內部8M高速時鐘使能，從這裡可以看出系統啟動後是首先依靠內部時鐘源而工作的。

#ifndef STM32F10X_CL  RCC->CFGR &= (uint32_t)0xF8FF0000;#else  RCC->CFGR &= (uint32_t)0xF0FF0000;

這兩行代碼則是操作時鐘配置寄存器。其主要設定了MCO（微控制器時鐘輸出）PLL相關（PLL倍頻係數，PLL輸入時鐘源），ADCPRE（ADC時鐘），PPRE2(高速APB分頻係數)，PPRE1（低速APB分頻係數），HPRE(AHB預分頻係數)，SW（系統時鐘切換），開始時，系統時鐘切換到HSI，由它作為系統初始時鐘。宏STM32F10X_CL是跟具體STM32晶片相關的一個宏。

  /* Reset HSEON, CSSON and PLLON bits */  RCC->CR &= (uint32_t)0xFEF6FFFF;  /* Reset HSEBYP bit */  RCC->CR &= (uint32_t)0xFFFBFFFF;  /* Reset PLLSRC, PLLXTPRE, PLLMUL and USBPRE/OTGFSPRE bits */  RCC->CFGR &= (uint32_t)0xFF80FFFF;

這幾句話則是先在關閉HSE，CSS,,PLL等的情況下配置好與之相關參數然後開啟，達到生效的目的。

#ifdef STM32F10X_CL  /* Reset PLL2ON and PLL3ON bits */  RCC->CR &= (uint32_t)0xEBFFFFFF;  /* Disable all interrupts and clear pending bits  */  RCC->CIR = 0x00FF0000;  /* Reset CFGR2 register */  RCC->CFGR2 = 0x00000000;#elif defined (STM32F10X_LD_VL) || defined (STM32F10X_MD_VL) || (defined STM32F10X_HD_VL)  /* Disable all interrupts and clear pending bits  */  RCC->CIR = 0x009F0000;  /* Reset CFGR2 register */  RCC->CFGR2 = 0x00000000;      #else  /* Disable all interrupts and clear pending bits  */  RCC->CIR = 0x009F0000;#endif /* STM32F10X_CL */

這一段主要是跟中斷設定有關。開始時，我們需要禁止所有中斷並且清除所有中斷標誌位。不同硬體有不同之處。

#if defined (STM32F10X_HD) || (defined STM32F10X_XL) || (defined STM32F10X_HD_VL)  #ifdef DATA_IN_ExtSRAM    SystemInit_ExtMemCtl();   #endif /* DATA_IN_ExtSRAM */#endif

這段跟設定外部RAM有關吧，我用到的STM32F103RBT與此無關。

SetSysClock();

此又是一個函數，主要是配置系統時鐘頻率。HCLK,PCLK2,PCLK1的分頻值，分別代表AHB,APB2,和APB1。當然還幹了其它的事情，配置FLASH延時周期和使能預取緩衝期。後面的這個配置具體還不瞭解。

#ifdef VECT_TAB_SRAM  SCB->VTOR = SRAM_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal SRAM. */#else  SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal FLASH. */#endif 

這段代碼主要是實現向量表的重定位。依據你想要將向量表定位在內部SRAM中還是內部FLASH中。這個SCB開始沒在STM32參考手冊中發現，原來它是跟Cortex-M3核心相關的東西。所以ST公司就沒有把它包含進來吧。核心的東西後面再瞭解，這裡給自己提個醒。

然後再看看SystemInit()中的那個函數SetClock()又做了什麼吧。

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) */ }

從中可以看出就是根據不同的宏來設定不同的系統時鐘，這些宏就在跟此函數在同一個源檔案裡。官方很是考慮周到，我們只需要選擇相應宏就能達到快速配置系統時鐘的目的。

#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

比如這裡我需要配置系統時鐘為72MHZ，則只需要將#define SYSCLK_FREQ_72MHz  72000000兩邊的注釋符去掉。

這個函數裡面又有SetSysClockTo72()函數，這個函數就是具體操作寄存器進行配置了。

#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){  __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)    {    }  }  else  { /* If HSE fails to start-up, the application will have wrong clock          configuration. User can add here some code to deal with this error */  }}#endif

上面的代碼需要細細看。SystemInit()差不多就是這樣了。