这里涉及到一个很重要的寄存器，时钟配置寄存器：RCC_CFGR

 1 #if defined (STM32F10X_LD_VL) || (defined STM32F10X_MD_VL) || (defined STM32F10X_HD_VL)

 2 /* #define SYSCLK_FREQ_HSE    HSE_VALUE */

 3  #define SYSCLK_FREQ_24MHz  24000000

 4 #else

 5 /* #define SYSCLK_FREQ_HSE    HSE_VALUE */

 6 /* #define SYSCLK_FREQ_24MHz  24000000 */ 

 7 /* #define SYSCLK_FREQ_36MHz  36000000 */

 8 /* #define SYSCLK_FREQ_48MHz  48000000 */

 9 /* #define SYSCLK_FREQ_56MHz  56000000 */

10 #define SYSCLK_FREQ_72MHz  72000000

11 #endif

 1 /**

 2   * @brief  Setup the microcontroller system

 3   *         Initialize the Embedded Flash Interface, the PLL and update the 

 4   *         SystemCoreClock variable.

 5   * @note   This function should be used only after reset.

 6   * @param  None

 7   * @retval None

 8   */

 9 void SystemInit (void)

10 {

11   /* Reset the RCC clock configuration to the default reset state(for debug purpose) */

12   /* Set HSION bit */

13   RCC->CR |= (uint32_t)0x00000001;

14 

15   /* Reset SW, HPRE, PPRE1, PPRE2, ADCPRE and MCO bits */

16 #ifndef STM32F10X_CL

17   RCC->CFGR &= (uint32_t)0xF8FF0000;

18 #else

19   RCC->CFGR &= (uint32_t)0xF0FF0000;

20 #endif /* STM32F10X_CL */   

21   

22   /* Reset HSEON, CSSON and PLLON bits */

23   RCC->CR &= (uint32_t)0xFEF6FFFF;

24 

25   /* Reset HSEBYP bit */

26   RCC->CR &= (uint32_t)0xFFFBFFFF;

27 

28   /* Reset PLLSRC, PLLXTPRE, PLLMUL and USBPRE/OTGFSPRE bits */

29   RCC->CFGR &= (uint32_t)0xFF80FFFF;

30 

31 #ifdef STM32F10X_CL

32   /* Reset PLL2ON and PLL3ON bits */

33   RCC->CR &= (uint32_t)0xEBFFFFFF;

34 

35   /* Disable all interrupts and clear pending bits  */

36   RCC->CIR = 0x00FF0000;

37 

38   /* Reset CFGR2 register */

39   RCC->CFGR2 = 0x00000000;

40 #elif defined (STM32F10X_LD_VL) || defined (STM32F10X_MD_VL) || (defined STM32F10X_HD_VL)

41   /* Disable all interrupts and clear pending bits  */

42   RCC->CIR = 0x009F0000;

43 

44   /* Reset CFGR2 register */

45   RCC->CFGR2 = 0x00000000;      

46 #else

47   /* Disable all interrupts and clear pending bits  */

48   RCC->CIR = 0x009F0000;

49 #endif /* STM32F10X_CL */

50     

51 #if defined (STM32F10X_HD) || (defined STM32F10X_XL) || (defined STM32F10X_HD_VL)

52   #ifdef DATA_IN_ExtSRAM

53     SystemInit_ExtMemCtl(); 

54   #endif /* DATA_IN_ExtSRAM */

55 #endif 

56 

57   /* Configure the System clock frequency, HCLK, PCLK2 and PCLK1 prescalers */

58   /* Configure the Flash Latency cycles and enable prefetch buffer */

59   SetSysClock();

60 

61 #ifdef VECT_TAB_SRAM

62   SCB->VTOR = SRAM_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal SRAM. */

63 #else

64   SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal FLASH. */

65 #endif 

66 }

/**

  * @brief  Configures the System clock frequency, HCLK, PCLK2 and PCLK1 prescalers.

  * @param  None

  * @retval None

  */

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

 设置RCC_CFGR寄存器的位参数，使其与外部晶振匹配得到72M系统时钟。

  1 /**

  2   * @brief  Sets System clock frequency to 72MHz and configure HCLK, PCLK2 

  3   *         and PCLK1 prescalers. 

  4   * @note   This function should be used only after reset.

  5   * @param  None

  6   * @retval None

  7   */

  8 static void SetSysClockTo72(void)

  9 {

 10   __IO uint32_t StartUpCounter = 0, HSEStatus = 0;

 11   

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

 13   /* Enable HSE */    

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

 15  

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

 17   do

 18   {

 19     HSEStatus = RCC->CR & RCC_CR_HSERDY;

 20     StartUpCounter++;  

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

 22 

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

 24   {

 25     HSEStatus = (uint32_t)0x01;

 26   }

 27   else

 28   {

 29     HSEStatus = (uint32_t)0x00;

 30   }  

 31 

 32   if (HSEStatus == (uint32_t)0x01)

 33   {

 34     /* Enable Prefetch Buffer */

 35     FLASH->ACR |= FLASH_ACR_PRFTBE;

 36 

 37     /* Flash 2 wait state */

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

 39     FLASH->ACR |= (uint32_t)FLASH_ACR_LATENCY_2;    

 40 

 41  

 42     /* HCLK = SYSCLK */

 43     RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;//AHB一分频

 44       

 45     /* PCLK2 = HCLK */

 46     RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE2_DIV1;

 47     

 48     /* PCLK1 = HCLK */

 49     RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE1_DIV2;

 50 

 51 #ifdef STM32F10X_CL

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

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

 54     /* PREDIV1 configuration: PREDIV1CLK = PLL2 / 5 = 8 MHz */

 55         

 56     RCC->CFGR2 &= (uint32_t)~(RCC_CFGR2_PREDIV2 | RCC_CFGR2_PLL2MUL |

 57                               RCC_CFGR2_PREDIV1 | RCC_CFGR2_PREDIV1SRC);

 58     RCC->CFGR2 |= (uint32_t)(RCC_CFGR2_PREDIV2_DIV5 | RCC_CFGR2_PLL2MUL8 |

 59                              RCC_CFGR2_PREDIV1SRC_PLL2 | RCC_CFGR2_PREDIV1_DIV5);

 60   

 61     /* Enable PLL2 */

 62     RCC->CR |= RCC_CR_PLL2ON;

 63     /* Wait till PLL2 is ready */

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

 65     {

 66     }

 67     

 68    

 69     /* PLL configuration: PLLCLK = PREDIV1 * 9 = 72 MHz */ 

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

 71     RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLXTPRE_PREDIV1 | RCC_CFGR_PLLSRC_PREDIV1 | 

 72                             RCC_CFGR_PLLMULL9); 

 73 #else    

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

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

 76                                         RCC_CFGR_PLLMULL));

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

 78 #endif /* STM32F10X_CL */

 79 

 80     /* Enable PLL */

 81     RCC->CR |= RCC_CR_PLLON;

 82 

 83     /* Wait till PLL is ready */

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

 85     {

 86     }

 87     

 88     /* Select PLL as system clock source */

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

 90     RCC->CFGR |= (uint32_t)RCC_CFGR_SW_PLL;    

 91 

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

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

 94     {

 95     }

 96   }

 97   else

 98   { /* If HSE fails to start-up, the application will have wrong clock 

 99          configuration. User can add here some code to deal with this error */

100   }

101 }