stm32 SysTick system timer

It is a 24-bit downward-counting timer. Each count takes 1/SYSTICK. SYSTICK is the system timer clock. It can be directly obtained from the system clock or obtained by dividing the system clock by 8. 

When the timer counts to 0, the initial value of the timer will be automatically reloaded from the LOAD register and count down again. 
If the SysTick interrupt is enabled, an interrupt signal will be generated when the timer counts to 0.

void SysTick_Init(u8 SYSCLK)

{

    SysTick_CLKSourceConfig(SysTick_CLKSource_HCLK_Div8); 

    fac_us=SYSCLK/8; //1us times             

    fac_ms=(u16)fac_us*1000; //1ms times                

}

void delay_us(u32 nus)

{       

    u32 temp;            

    SysTick->LOAD=nus*fac_us; //Time initial value           

    SysTick->VAL=0x00; //Clear the counter

    SysTick->CTRL|=SysTick_CTRL_ENABLE_Msk; //Turn on the timer   

    do

    {

        temp=SysTick->CTRL;

    }while((temp&0x01)&&!(temp&(1<<16))); //Wait for time to arrive   

    SysTick->CTRL&=~SysTick_CTRL_ENABLE_Msk; //Turn off the counter

    SysTick->VAL=0x00; //Clear the counter  

}

void delay_ms(u16 nms) //Maximum 1864ms

{                 

    u32 temp;          

    SysTick->LOAD=(u32)nms*fac_ms;

    SysTick->VAL=0x00;

    SysTick->CTRL|=SysTick_CTRL_ENABLE_Msk; 

    do

    {

        temp=SysTick->CTRL;

    }while((temp&0x01)&&!(temp&(1<<16)));

    SysTick->CTRL&=~SysTick_CTRL_ENABLE_Msk;

    SysTick->VAL=0x00;

}