STM32F4时钟设置分析

TM32F4时钟设置分析

本文博客链接: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

 "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

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:

 "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

 /******************************************************************************/

而晶振频率则是在文件stm32f4xx.h中进行设置:

外部晶振:

 "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 */

内部晶振:

 "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 */

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:

 "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
 }

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:

 "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 */
   }

 }

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