STM32F407系统时钟配置

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STM32F407系统时钟配置

时钟树

方法一，采用官方库提供的配置(这里外部晶振25MHz，系统配置为168MHz)

STM32F4启动与STM32F10X不同,时钟已经默认配置好
启动代码，文件:startup_stm32f4xx.s

Reset handler  
Reset_Handler    PROC  EXPORT  Reset_Handler             [WEAK]  IMPORT  SystemInit  IMPORT  __main  LDR     R0, =SystemInit  BLX     R0  LDR     R0, =__main  BX      R0  ENDP

可以看出,在进入main函数之前,系统调用了SystemInit函数.

SystemInit函数分析:SystemInit函数位于system_stm32f4xx.c文件中.此文件提供几个宏定义可以设置各个时钟:

/************************* PLL Parameters *************************************/  
/* PLL_VCO = (HSE_VALUE or HSI_VALUE / PLL_M) * PLL_N */  
#define PLL_M      25  
#define PLL_N      336  /* SYSCLK = PLL_VCO / PLL_P */  
#define PLL_P      2  /* USB OTG FS, SDIO and RNG Clock =  PLL_VCO / PLLQ */  
#define PLL_Q      7  /******************************************************************************/

而晶振频率则是在文件stm32f4xx.h中进行设置:
外部晶振:

#if !defined  (HSE_VALUE)   #define HSE_VALUE    ((uint32_t)25000000) /*!< Value of the External oscillator in Hz */  
#endif /* HSE_VALUE */

内部晶振:

#if !defined  (HSI_VALUE)     #define HSI_VALUE    ((uint32_t)16000000) /*!< Value of the Internal oscillator in Hz*/  
#endif /* HSI_VALUE */

综上,可以得出默认配置中:
锁相环压腔振荡器时钟PLL_VCO = 25 / 25 * 336 = 336MHz
系统时钟SYSCLK = 336 / 2 = 168MHz
USB,SD卡时钟 = 336 / 7 = 48MHz

SystemInit函数代码:

/** * @brief  Setup the microcontroller system *         Initialize the Embedded Flash Interface, the PLL and update the  *         SystemFrequency variable. * @param  None * @retval None */  
void SystemInit(void)  
{  /* FPU settings ------------------------------------------------------------*/  #if (__FPU_PRESENT == 1) && (__FPU_USED == 1)  SCB->CPACR |= ((3UL << 10*2)|(3UL << 11*2));  /* set CP10 and CP11 Full Access */  #endif  /* Reset the RCC clock configuration to the default reset state ------------*/  /* Set HSION bit */  RCC->CR |= (uint32_t)0x00000001;  /* Reset CFGR register */  RCC->CFGR = 0x00000000;  /* Reset HSEON, CSSON and PLLON bits */  RCC->CR &= (uint32_t)0xFEF6FFFF;  /* Reset PLLCFGR register */  RCC->PLLCFGR = 0x24003010;  /* Reset HSEBYP bit */  RCC->CR &= (uint32_t)0xFFFBFFFF;  /* Disable all interrupts */  RCC->CIR = 0x00000000;  #ifdef DATA_IN_ExtSRAM  SystemInit_ExtMemCtl();   
#endif /* DATA_IN_ExtSRAM */  /* Configure the System clock source, PLL Multiplier and Divider factors,  AHB/APBx prescalers and Flash settings ----------------------------------*/  SetSysClock();  /* Configure the Vector Table location add offset address ------------------*/  
#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  
}

SetSysClock函数分析,在SetSysClock函数中,配置了系统时钟,PLL倍频以及分频系数:

/** * @brief  Configures the System clock source, PLL Multiplier and Divider factors,  *         AHB/APBx prescalers and Flash settings * @Note   This function should be called only once the RCC clock configuration   *         is reset to the default reset state (done in SystemInit() function).    * @param  None * @retval None */  
static void SetSysClock(void)  
{  
/******************************************************************************/  
/*            PLL (clocked by HSE) used as System clock source                */  
/******************************************************************************/  __IO uint32_t StartUpCounter = 0, HSEStatus = 0;  /* 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)  {  /* Select regulator voltage output Scale 1 mode, System frequency up to 168 MHz */  RCC->APB1ENR |= RCC_APB1ENR_PWREN;  PWR->CR |= PWR_CR_VOS;  /* HCLK = SYSCLK / 1*/  RCC->CFGR |= RCC_CFGR_HPRE_DIV1;  /* PCLK2 = HCLK / 2*/  RCC->CFGR |= RCC_CFGR_PPRE2_DIV2;  /* PCLK1 = HCLK / 4*/  RCC->CFGR |= RCC_CFGR_PPRE1_DIV4;  /* Configure the main PLL */  RCC->PLLCFGR = PLL_M | (PLL_N << 6) | (((PLL_P >> 1) -1) << 16) |   (RCC_PLLCFGR_PLLSRC_HSE) | (PLL_Q << 24);  /* Enable the main PLL */  RCC->CR |= RCC_CR_PLLON;  /* Wait till the main PLL is ready */  while((RCC->CR & RCC_CR_PLLRDY) == 0)  {  }  /* Configure Flash prefetch, Instruction cache, Data cache and wait state */  FLASH->ACR = FLASH_ACR_ICEN |FLASH_ACR_DCEN |FLASH_ACR_LATENCY_5WS;  /* Select the main PLL as system clock source */  RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));  RCC->CFGR |= RCC_CFGR_SW_PLL;  /* Wait till the main PLL is used as system clock source */  while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS ) != RCC_CFGR_SWS_PLL);  {  }  }  else  {/* If HSE fails to startup, the application will have wrong clock configuration. User can add here some code to deal with this error */  }  }

如果外部时钟启动失败,系统会使用内部时钟
默认配置:
HCLK = SYSCLK / 1 = 168MHz
PCLK2 = HCLK / 2 = 84MHz
PCLK1 = HCLK / 4 = 42MHz

方法二，根据需要重新进行配置(这里外部晶振25MHz，系统配置为168MHz)

自己根据自己外部晶振大小和需要进行配置


void RCC_Config(void)
{RCC_DeInit();    //RCC寄存器初始化RCC_HSEConfig(RCC_HSE_ON);    //使用外部时钟if(RCC_WaitForHseStartUp() == SUCCESS)     //等待外部时钟启动{RCC_PLLCmd(DISABLE);    //配置PLL前应先关闭主PLLRCC_SYSCLKConfig(RCC_SYSCLKSOURCE_PLLCLK);    //选择PLL时钟为系统时钟RCC_HCLKConfig(RCC_SYSCLK_Div1);    //HCLK（AHB）时钟为系统时钟1分频RCC_PCLK1Config(RCC_HCLK_Div4);    //PCLK（APB1）时钟为HCLK时钟8分频RCC_PCLK2Config(RCC_HCLK_Div2);    //PCLK（APB2）时钟为HCLK时钟2分频RCC_PLLConfig(RCC_PLLSource_HSE,25,336,2,7);    //PLL时钟配置，外部晶振为25MHz，系统配置为168MHzRCC_PLLCmd(ENABLE);    //PLL时钟开启while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET);    //等待PLL时钟准备好}}