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 */