system tick

CLK_SysTickDelay 函数简介@details    Use the SysTick to generate the delay time and the unit is in us     ， The SysTick clock source is from HCLK, i.e the same as system core clock.

但CyclesPerUs是在HXT情况下，当core clock 不是hxt，一定会自动更新CyclesPerUs的吧？？？