STM32F4系统时钟树
STM32F4的系统时钟非常重要，涉及到整个系统的运行结果，无论是什么操作，都需要时钟信号，不同型号的微控制器的默认系统时钟配置是不同的，这里，给出两种配置STM32F407系统时钟的方法。方法一，采用官方库提供的配置(这里外部晶振8MHz，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 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:
我使用的是STM32F407，筛选可用信息如下：
/************************* PLL Parameters *************************************/
#if defined (STM32F40_41xxx) || defined (STM32F427_437xx) || defined (STM32F429_439xx) || defined (STM32F401xx)
/* PLL_VCO = (HSE_VALUE or HSI_VALUE / PLL_M) * PLL_N */
#define PLL_M 8
#else /* STM32F411xE */
#if defined (USE_HSE_BYPASS)
#define PLL_M 8
#else /* STM32F411xE */
#define PLL_M 16
#endif /* USE_HSE_BYPASS */
#endif /* STM32F40_41xxx || STM32F427_437xx || STM32F429_439xx || STM32F401xx */
/* USB OTG FS, SDIO and RNG Clock = PLL_VCO / PLLQ */
#define PLL_Q 7
#if defined (STM32F40_41xxx)
#define PLL_N 336
/* SYSCLK = PLL_VCO / PLL_P */
#define PLL_P 2 //2 //2---168M 4---84M
#endif /* STM32F40_41xxx */
#if defined (STM32F427_437xx) || defined (STM32F429_439xx)
#define PLL_N 360
/* SYSCLK = PLL_VCO / PLL_P */
#define PLL_P 2
#endif /* STM32F427_437x || STM32F429_439xx */
#if defined (STM32F401xx)
#define PLL_N 336
/* SYSCLK = PLL_VCO / PLL_P */
#define PLL_P 4
#endif /* STM32F401xx */
#if defined (STM32F411xE)
#define PLL_N 400
/* SYSCLK = PLL_VCO / PLL_P */
#define PLL_P 4
#endif /* STM32F411xx */
/******************************************************************************/
/************************* PLL Parameters *************************************/
/* PLL_VCO = (HSE_VALUE or HSI_VALUE / PLL_M) * PLL_N */
#define PLL_M 8
#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)8000000) /*!< 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,如果使用外部晶振8MHz，则可以得出默认配置中:
锁相环压腔振荡器时钟PLL_VCO =（HSE_VALUE/PLL_M）* PLL_N=8/ 8* 336 = 336MHz
系统时钟SYSCLK = PLL_VCO / PLL_P=336 / 2 = 168MHz
USB,SD卡时钟 = PLL_VCO / PLLQ=336 / 7 = 48MHz
- SystemInit函数代码:
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;
#if defined (DATA_IN_ExtSRAM) || defined (DATA_IN_ExtSDRAM)
SystemInit_ExtMemCtl();
#endif /* DATA_IN_ExtSRAM || DATA_IN_ExtSDRAM */
/* 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:
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);
点击打开链接
/* 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 ,AHB总线时钟
PCLK2 = HCLK / 2 = 84MHz
PCLK1 = HCLK / 4 = 42MHz
定时器初始化设置时计算定时时间需要用到该定时器时钟频率，具体原因详细看我整理的一篇博客文章，链接如下：
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
/*******************************************************************************
* Function Name : RCC_Configuration
* Description : Configures the different system clocks.
* Input : None
* Output : None
* Return : None
*******************************************************************************/
void RCC_Configuration(void)
{
ErrorStatus HSEStartUpStatus;
uint32_t PLL_M_Temp = 0;
uint32_t PLL_N_Temp = 0;
uint32_t PLL_P_Temp = 0;
uint32_t PLL_Q_Temp = 0;
RCC_DeInit(); //½«ËùÓÐRCCÖØÖÃΪ³õʼֵ
RCC_HSEConfig(RCC_HSE_ON);
HSEStartUpStatus = RCC_WaitForHSEStartUp(); //Ñ¡ÔñÍⲿ¾§Õñ(HSE)×÷ΪʱÖÓÔ´ µÈ´ýÍⲿʱÖÓ×¼±¸ºÃ
if (HSEStartUpStatus == SUCCESS) //ÉèÖÃʱÖÓΪ168M
{
/* Enable Prefetch Buffer */
//FLASH_PrefetchBufferCmd(ENABLE);
/* Flash 2 wait state */
//FLASH_SetLatency(FLASH_Latency_5);
//HSE_VALUE = 8MHz,PLL_VCO input clock = (HSE_VALUE or HSI_VALUE)/PLL_M,½¨Òé´ËֵΪ1~2MHz,Òò´ËÈ¡PLL_M=8£¬
//PLL_VCO input clock = 1MHz;
PLL_M_Temp = 8;
//PLL_VCO output clock = (PLL_VCO input clock)*PLL_N
//PLL_VCO output clock = 336;
PLL_N_Temp = 336;
//System Clock = (PLL_VCO output clock)/PLL_P ,
//System Clock = 84MHz
PLL_P_Temp = 4;
//´ËϵÊýÓÃÓÚÉèÖÃSD¿¨¶Áд£¬USBµÈ¹¦ÄÜ£¬ÔÝʱ²»ÓÃ
PLL_Q_Temp = 7;
/* PLL configuration */
RCC_PLLConfig(RCC_PLLSource_HSE, PLL_M_Temp, PLL_N_Temp, PLL_P_Temp, PLL_Q_Temp);
/* Enable PLL */
RCC_PLLCmd(ENABLE);
/* Wait till PLL is ready */
while (RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET)
{
}
/* Select PLL as system clock source */
RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);
/* Wait till PLL is used as system clock source */
while(RCC_GetSYSCLKSource() != 0x08)
{
}
}
}
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