Systick 更新

之前写的systick_config(loadvalue) 根据系统时钟为72Mhz来写的，如果system clock不是72MHz怎么办？

重新写了一下，先获取，系统时钟频率。

//参数为ms
void Systick_Init(u16 ms)
{     
    RCC_ClocksTypeDef RCC_Clocks;
     /* SystTick configuration: an interrupt every 1ms */
    RCC_GetClocksFreq(&RCC_Clocks);
    SysTick_Config(ms*RCC_Clocks.SYSCLK_Frequency / 1000);
    NVIC_SetPriority (SysTick_IRQn, 1);    
}

说下关键的RCC_ClocksTypeDef

typedef struct
{
  uint32_t SYSCLK_Frequency;  /*!< returns SYSCLK clock frequency expressed in Hz */
  uint32_t HCLK_Frequency;    /*!< returns HCLK clock frequency expressed in Hz */
  uint32_t PCLK1_Frequency;   /*!< returns PCLK1 clock frequency expressed in Hz */
  uint32_t PCLK2_Frequency;   /*!< returns PCLK2 clock frequency expressed in Hz */
  uint32_t ADCCLK_Frequency;  /*!< returns ADCCLK clock frequency expressed in Hz */
}RCC_ClocksTypeDef;