stm32 uses systick to accurately delay

To achieve precise delay using the system tick timer, the following five steps are required:
1. Set the tick timer clock by setting its control register and selecting the external clock, which is one eighth of the system clock. If the system clock is 72M, the tick timer clock is 9M. That is, it takes 9M clock cycles to time one second.
2. Set the value of the tick timer reload register, which is the number of clock cycles required for delay. For example, if a delay of 20 microseconds is required, the reload value is 20*9. If a delay of 20 milliseconds is required, the reload value is set to 20*9000.
3. Clear the value of the tick timer current value register to zero, so that it can count down from the set value when counting is enabled.
4. Set the tick timer control register to start countdown.
5. Query the status bit of the tick timer. When the timing time is up, turn off the timer and clear the current value of the timer.

void delay_init(u8 SYSCLK)
{
   SysTick->CTRL&=0xfffffffb;//Control register, select external clock, which is one eighth of the system clock (HCLK/8; 72M/8=9M)
   fac_us=SYSCLK/8; //Define global variables, that is, the number of clock cycles required to delay one microsecond (72/8=9, in microseconds)
   fac_ms=(u16)fac_us*1000; //The number of clock cycles required for one millisecond (9000)
}
void Delay_us(u32 nus)
{
  u32 temp;   
  SysTick->LOAD=nus*fac_us; //Time loading (that is, how many clock cycles to delay)  
  SysTick->VAL=0x00; //Clear counter
  SysTick->CTRL=0x01; //Start countdown
do
{
  temp=SysTick->CTRL;
}
  while(temp&0x01&&!(temp&(1<<16)));//Wait for time to arrive  
  SysTick->CTRL=0x00; //Turn off counter
  SysTick->VAL =0X00; //Clear counter
}
orvoid
Delay_us(u32 Nus)   
{   
SysTick->LOAD=Nus*9; //Time loading 72M main frequency     
SysTick->CTRL|=0x01; //Start countdownwhile      
(!(SysTick->CTRL&(1<<16))); //Wait for time to arrive   
SysTick->CTRL=0X00000000; //Turn off counter   
SysTick->VAL=0X00000000; //Clear counter        
}