void SysTick_CLKSourceConfig(uint32_t SysTick_CLKSource)
{
  /* Check the parameters */
  assert_param(IS_SYSTICK_CLK_SOURCE(SysTick_CLKSource));
  if (SysTick_CLKSource == SysTick_CLKSource_HCLK)
  {
    SysTick->CTRL |= SysTick_CLKSource_HCLK;//内核时钟的HCLK
  }
  else
  {
    SysTick->CTRL &= SysTick_CLKSource_HCLK_Div8;//外部时钟HCLK的1/8
  }
}

/**
 * @brief  Initialize and start the SysTick counter and its interrupt.
 *
 * @param   ticks   number of ticks between two interrupts
 * @return  1 = failed, 0 = successful
 *
 * Initialise the system tick timer and its interrupt and start the
 * system tick timer / counter in free running mode to generate 
 * periodical interrupts.
 */
static __INLINE uint32_t SysTick_Config(u**int32_t ticks**)//两次中断，有多少次ticks
{ 
  if (ticks > SysTick_LOAD_RELOAD_Msk)  return (1);            /* Reload value impossible 判断有效性*/
                                                               
  SysTick->LOAD  = (ticks & SysTick_LOAD_RELOAD_Msk) - 1;      /* set reload register 设定装载值*/
  NVIC_SetPriority (SysTick_IRQn, (1<<__NVIC_PRIO_BITS) - 1);  /* set Priority for Cortex-M0 System Interrupts */
  SysTick->VAL   = 0;                                          /* Load the SysTick Counter Value 设定初值*/
  SysTick->CTRL  = SysTick_CTRL_CLKSOURCE_Msk |                //开启中断
                   SysTick_CTRL_TICKINT_Msk   |                //使能定时器
                   SysTick_CTRL_ENABLE_Msk;                    /* Enable SysTick IRQ and SysTick Timer 选择时钟源*/
  return (0);                                                  /* Function successful */
}

void delay_init()
{
#if SYSTEM_SUPPORT_OS  							//如果需要支持OS.
	u32 reload;
#endif
	SysTick_CLKSourceConfig(SysTick_CLKSource_HCLK_Div8);	//选择外部时钟  HCLK/8
	fac_us=SystemCoreClock/8000000;				//为系统时钟的1/8  
#if SYSTEM_SUPPORT_OS  							//如果需要支持OS.
	reload=SystemCoreClock/8000000;				//每秒钟的计数次数 单位为M  
	reload*=1000000/delay_ostickspersec;		//根据delay_ostickspersec设定溢出时间
												//reload为24位寄存器,最大值:16777216,在72M下,约合1.86s左右	
	fac_ms=1000/delay_ostickspersec;			//代表OS可以延时的最少单位	   

	SysTick->CTRL|=SysTick_CTRL_TICKINT_Msk;   	//开启SYSTICK中断
	SysTick->LOAD=reload; 						//每1/delay_ostickspersec秒中断一次	
	SysTick->CTRL|=SysTick_CTRL_ENABLE_Msk;   	//开启SYSTICK    

#else
	fac_ms=(u16)fac_us*1000;					//非OS下,代表每个ms需要的systick时钟数   
#endif
}	
void delay_us(u32 nus)
{		
	u32 temp;	    	 
	SysTick->LOAD=nus*fac_us; 					//时间加载，数不能超过2^（24-1）  		 
	SysTick->VAL=0x00;        					//清空计数器
	SysTick->CTRL|=SysTick_CTRL_ENABLE_Msk ;	//开始倒数	  
	do
	{
		temp=SysTick->CTRL;
	}while((temp&0x01)&&!(temp&(1<<16)));		//等待时间到达   
	SysTick->CTRL&=~SysTick_CTRL_ENABLE_Msk;	//关闭计数器
	SysTick->VAL =0X00;      					 //清空计数器	 
}
//延时nms
//注意nms的范围
//SysTick->LOAD为24位寄存器,所以,最大延时为:
//nms<=0xffffff*8*1000/SYSCLK
//SYSCLK单位为Hz,nms单位为ms
//对72M条件下,nms<=1864 
void delay_ms(u16 nms)
{	 		  	  
	u32 temp;		   
	SysTick->LOAD=(u32)nms*fac_ms;				//时间加载(SysTick->LOAD为24bit)
	SysTick->VAL =0x00;							//清空计数器
	SysTick->CTRL|=SysTick_CTRL_ENABLE_Msk ;	//开始倒数  
	do
	{
		temp=SysTick->CTRL;
	}while((temp&0x01)&&!(temp&(1<<16)));		//等待时间到达   
	SysTick->CTRL&=~SysTick_CTRL_ENABLE_Msk;	//关闭计数器
	SysTick->VAL =0X00;       					//清空计数器	  	    
}