STM32F4时钟配置分析

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STM32F4时钟设置分析

原文博客链接:http://blog.csdn.net/jdh99,作者:jdh,转载请注明.

环境：

主机:WIN7

开发环境:MDK4.72

MCU:STM32F407VGT6

STM32F4启动与STM32F10X不同,时钟已经默认配置好.

1.启动代码:

文件:startup_stm32f4xx.s

<span style="font-family:KaiTi_GB2312;font-size:18px;">; Reset handler
Reset_Handler    PROC
                 EXPORT  Reset_Handler             [WEAK]
        IMPORT  SystemInit
        IMPORT  __main

                 LDR     R0, =SystemInit
                 BLX     R0
                 LDR     R0, =__main
                 BX      R0
                 ENDP</span>

可以看出,在进入main函数之前,系统调用了SystemInit函数.

2.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:

<span style="font-family:KaiTi_GB2312;font-size:18px;">/************************* 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

/******************************************************************************/</span>

而晶振频率则是在文件stm32f4xx.h中进行设置:

外部晶振:

<span style="font-family:KaiTi_GB2312;font-size:18px;">#if !defined  (HSE_VALUE)
  #define HSE_VALUE    ((uint32_t)25000000) /*!< Value of the External oscillator in Hz */
#endif /* HSE_VALUE */</span>

内部晶振:

<span style="font-family:KaiTi_GB2312;font-size:18px;">#if !defined  (HSI_VALUE)
  #define HSI_VALUE    ((uint32_t)16000000) /*!< Value of the Internal oscillator in Hz*/
#endif /* HSI_VALUE */ </span>

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

时钟图:

SystemInit函数代码:

<span style="font-family:KaiTi_GB2312;font-size:18px;">/**
  * @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
}</span>

3.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:

<span style="font-family:KaiTi_GB2312;font-size:18px;">/**
  * @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 start-up, the application will have wrong clock
         configuration. User can add here some code to deal with this error */
  }

}</span>

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