因为眼下手里仅仅有16MHZ的2520封装的贴片晶振，8MHZ这样的封装做不到这么小，所以就先用16MHZ，这样我们就须要改动程序相关的RCC时钟：

1，stm32f4xx.h

#define HSE_VALUE    ((uint32_t)16000000) /*!< Value of the External oscillator in Hz */

2，system_stm32f4xx.c

/* PLL_VCO = (HSE_VALUE or HSI_VALUE / PLL_M) * PLL_N */

#define PLL_M      16

/* 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

#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 */