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

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<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:

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<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:

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<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:

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<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:

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<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:

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<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