注意看上面的CTRL寄存器的CLKSOURCE位（时钟源位）,它有两个选择,一个是内核时钟源FLCK（72MHz），一个是外部时钟源HCLK,这里应该是
我们知道,我们设定一个计数,那么每次计数器减到0，时间经过了：系统时钟周期 *计数器初值.我们使用72M作为系统时钟，那么每次计数器减1所用的时间是1/72M，计数器的初值如果是72000，那么每次计数器减到0，时间经过(1/72M)*72000= 0.001，（简单理解：用72M的时钟频率，即1s计数72M=72000000次，那1ms计数72000次，所以计数值为72000）,
让我们来看看这些寄存器的配置：
系统库core_cm3.h中
STATIC_INLINE void LL_InitTick(uint32_t HCLKFrequency, uint32_t Ticks)
{
/* Configure the SysTick to have interrupt in 1ms time base */
SysTick->LOAD = (uint32_t)((HCLKFrequency / Ticks) - 1UL); /* set reload register */
SysTick->VAL = 0UL; /* Load the SysTick Counter Value */
SysTick->CTRL = SysTick_CTRL_CLKSOURCE_Msk |
SysTick_CTRL_ENABLE_Msk; /* Enable the Systick Timer */
}再看看
/* SysTick Control / Status Register Definitions */
#define SysTick_CTRL_COUNTFLAG_Pos 16 /*!< SysTick CTRL: COUNTFLAG Position */
#define SysTick_CTRL_COUNTFLAG_Msk (1ul << SysTick_CTRL_COUNTFLAG_Pos) /*!< SysTick CTRL: COUNTFLAG Mask */
#define SysTick_CTRL_CLKSOURCE_Pos 2 /*!< SysTick CTRL: CLKSOURCE Position */
#define SysTick_CTRL_CLKSOURCE_Msk (1ul << SysTick_CTRL_CLKSOURCE_Pos) /*!< SysTick CTRL: CLKSOURCE Mask */
#define SysTick_CTRL_TICKINT_Pos 1 /*!< SysTick CTRL: TICKINT Position */
#define SysTick_CTRL_TICKINT_Msk (1ul << SysTick_CTRL_TICKINT_Pos) /*!< SysTick CTRL: TICKINT Mask */
#define SysTick_CTRL_ENABLE_Pos 0 /*!< SysTick CTRL: ENABLE Position */
#define SysTick_CTRL_ENABLE_Msk (1ul << SysTick_CTRL_ENABLE_Pos) /*!< SysTick CTRL: ENABLE Mask */
/* SysTick Reload Register Definitions */
#define SysTick_LOAD_RELOAD_Pos 0 /*!< SysTick LOAD: RELOAD Position */
#define SysTick_LOAD_RELOAD_Msk (0xFFFFFFul << SysTick_LOAD_RELOAD_Pos) /*!< SysTick LOAD: RELOAD Mask */
/* SysTick Current Register Definitions */
#define SysTick_VAL_CURRENT_Pos 0 /*!< SysTick VAL: CURRENT Position */
#define SysTick_VAL_CURRENT_Msk (0xFFFFFFul << SysTick_VAL_CURRENT_Pos) /*!< SysTick VAL: CURRENT Mask */
#define SysTick_CTRL_CLKSOURCE_Pos 2 /*!< SysTick CTRL: CLKSOURCE Position */
#define SysTick_CTRL_CLKSOURCE_Msk (1ul << SysTick_CTRL_CLKSOURCE_Pos) /*!< SysTick CTRL: CLKSOURCE Mask */ul是unsigned long 型, 这两行代码的意思是1左移2位,也就是将2这位置1,这里就是选择了内核时钟源HCLK(72MHz)，所以我们要把计数初值设置为72000,这样才能产生1ms的基准时钟
1、 LL_Init1msTick(72000000);
2、 LL_InitTick(HCLKFrequency, 1000U);/初始化systick定时器
3、void LL_mDelay(uint32_t Delay)
void LL_Init1msTick(uint32_t HCLKFrequency)
{
/* Use frequency provided in argument */
LL_InitTick(HCLKFrequency, 1000U);
}初始化systick定时器跳转；
STATIC_INLINE void LL_InitTick(uint32_t HCLKFrequency, uint32_t Ticks)
{
/* Configure the SysTick to have interrupt in 1ms time base */
SysTick->LOAD = (uint32_t)((HCLKFrequency / Ticks) - 1UL); /* set reload register */
SysTick->VAL = 0UL; /* Load the SysTick Counter Value */
SysTick->CTRL = SysTick_CTRL_CLKSOURCE_Msk |
SysTick_CTRL_ENABLE_Msk; /* Enable the Systick Timer */
}初始化systick定时器，并重装载为1MS（72Mhz对应1MS为72M/1000）的时间;
void LL_mDelay(uint32_t Delay)
{
__IO uint32_t tmp = SysTick->CTRL; /* Clear the COUNTFLAG first */
/* Add this code to indicate that local variable is not used */
((void)tmp);
/* Add a period to guaranty minimum wait */
if (Delay < LL_MAX_DELAY)
{
Delay++;
}while (Delay)
{
if ((SysTick->CTRL & SysTick_CTRL_COUNTFLAG_Msk) != 0U)
{
Delay--;
}
}
}延时函数，判断SysTick寄存器是否数到0若数到0（SysTick_CTRL_COUNTFLAG_Msk数到0为1读取后自动置0）则Delay减1