SysTick系统滴答定时器：
core-cm4.h定义了ARM芯片自带的systick配置（使用默认时钟）：
__STATIC_INLINE uint32_t SysTick_Config(uint32_t ticks)；
The function initializes the System Timer and its interrupt, and starts the System Tick Timer.
Counter is in free running mode to generate periodic interrupts.
\param [in] ticks Number of ticks between two interrupts.
\return 0 Function succeeded.
\return 1 Function failed.
clk.c定义了systick配置：
void CLK_SetSysTickClockSrc(uint32_t u32ClkSrc)；
* @param[in] u32ClkSrc is module clock source. Including:
* - \ref CLK_CLKSEL0_STCLKSEL_HXT
* - \ref CLK_CLKSEL0_STCLKSEL_LXT
* - \ref CLK_CLKSEL0_STCLKSEL_HXT_DIV2
* - \ref CLK_CLKSEL0_STCLKSEL_HCLK_DIV2
* - \ref CLK_CLKSEL0_STCLKSEL_HIRC_DIV2
void CLK_EnableSysTick(uint32_t u32ClkSrc, uint32_t u32Count)；
* @param[in] u32ClkSrc is System Tick clock source. Including:
* - \ref CLK_CLKSEL0_STCLKSEL_HXT
* - \ref CLK_CLKSEL0_STCLKSEL_LXT
* - \ref CLK_CLKSEL0_STCLKSEL_HXT_DIV2
* - \ref CLK_CLKSEL0_STCLKSEL_HCLK_DIV2
* - \ref CLK_CLKSEL0_STCLKSEL_HIRC_DIV2
* - \ref CLK_CLKSEL0_STCLKSEL_HCLK
* @param[in] u32Count is System Tick reload value. It could be 0~0xFFFFFF.
void CLK_DisableSysTick(void) ；
clk.h定义了systick延时：
__STATIC_INLINE void CLK_SysTickDelay(uint32_t us)；
* @param us Delay time. The Max value is 2^24 / CPU Clock(MHz). Ex:
* 72MHz => 233016us, 50MHz => 335544us,
48MHz => 349525us, 28MHz => 699050us ...
__STATIC_INLINE void CLK_SysTickLongDelay(uint32_t us)；