STM32F4时钟设置分析

只看该作者 · 2015-3-22 23:28

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

[cpp] view plaincopy

; 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.

只看该作者 · 2015-3-22 23:28

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:

[cpp] view plaincopy

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

只看该作者 · 2015-3-22 23:29

而晶振频率则是在文件stm32f4xx.h中进行设置:

外部晶振:

[cpp] view plaincopy

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

内部晶振:

[cpp] view plaincopy

#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

只看该作者 · 2015-3-22 23:29

时钟图:

只看该作者 · 2015-3-22 23:30

SystemInit函数代码:

[cpp] view plaincopy

/**
* @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
}

只看该作者 · 2015-3-22 23:30

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:
[cpp] view plaincopy在CODE上查看代码片派生到我的代码片
/**
 * @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