[嵌入式]STM32F4系统时钟认识

关于系统时钟的配置，以stm32f429为例，确认系统时钟SYSCLK

打开stm32f4xx.h 找到如下代码

#if defined(STM32F40_41xxx) || defined(STM32F427_437xx)  || defined(STM32F429_439xx) || defined(STM32F401xx)  || defined(STM32F411xE)
 #if !defined  (HSE_VALUE) 
  #define HSE_VALUE    ((uint32_t)25000000) /*!< Value of the External oscillator in Hz */
#endif /* HSE_VALUE */
#elif defined(STM32F446xx)
 #if !defined  (HSE_VALUE) 
  #define HSE_VALUE    ((uint32_t)8000000) /*!< Value of the External oscillator in Hz */
#endif /* HSE_VALUE */
#endif /* STM32F40_41xxx || STM32F427_437xx || STM32F429_439xx || STM32F401xx || STM32F411xE */
/**
 * @brief In the following line adjust the External High Speed oscillator (HSE) Startup 
   Timeout value 
   */
#if !defined  (HSE_STARTUP_TIMEOUT) 
  #define HSE_STARTUP_TIMEOUT    ((uint16_t)0x05000)   /*!< Time out for HSE start up */
#endif /* HSE_STARTUP_TIMEOUT */   

#if !defined  (HSI_VALUE)   
  #define HSI_VALUE    ((uint32_t)16000000) /*!< Value of the Internal oscillator in Hz*/
#endif /* HSI_VALUE */   

因为我们使用的是f429 ,确认HSE_VALUE 为25000000。
之后打开system_f4xx.c找到以下代码。

/************************* 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      25
#elif defined (STM32F446xx)
#define PLL_M      8
#elif defined (STM32F411xE)

#if defined(USE_HSE_BYPASS)
#define PLL_M      8    
#else  
#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(STM32F446xx)
/* PLL division factor for I2S, SAI, SYSTEM and SPDIF: Clock =  PLL_VCO / PLLR */
#define PLL_R      7
#endif /* STM32F446xx */ 

#if defined(STM32F40_41xxx) || defined(STM32F427_437xx) || defined(STM32F429_439xx) || defined(STM32F446xx)
#define PLL_N      360
/* SYSCLK = PLL_VCO / PLL_P */
#define PLL_P      2
#endif /* STM32F40_41xxx || STM32F427_437x || STM32F429_439xx || STM32F446xx */

#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 */

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

找到PLL_M PLL_N PLL_P 分别为25 360 2。

下面进行计算

PLL_VCO =（HSE_VALUE/PLL_M）* PLL_N=25/ 25* 360= 360MHz

SYSCLK = PLL_VCO / PLL_P = 360 / 2 = 180MHz