关于 System Tick设置，给出3种方法，学习并确认OK：
使用 M051BSPv3.01.001版本
一、使用函数CLK_EnableSysTick（）
1 //Enable System Tick counter, Select HXT/2 as the clock source of SysTick,reload value. It could be 0~0xFFFFFF. 2 //HXT=12mHZ=（1/12）us
3 //12000*（1/12）=1000us=1ms 即1ms中断一次 4 CLK_EnableSysTick(CLK_CLKSEL0_STCLK_S_HXT, 12000);
函数CLK_EnableSysTick（）在CLK.C
注意：此函数有保护寄存器操作，所在需在解保时执行。
1 /**
2 * @brief Enable System Tick counter
3 * @param[in] u32ClkSrc is System Tick clock source. Including:
4 * - \ref CLK_CLKSEL0_STCLK_S_HXT
5 * - \ref CLK_CLKSEL0_STCLK_S_HXT_DIV2
6 * - \ref CLK_CLKSEL0_STCLK_S_HCLK_DIV2
7 * - \ref CLK_CLKSEL0_STCLK_S_HIRC_DIV2
8 * - \ref CLK_CLKSEL0_STCLK_S_HCLK
9 * @param[in] u32Count is System Tick reload value. It could be 0~0xFFFFFF.
10 * @return None
11 * @details This function set System Tick clock source, reload value, enable System Tick counter and interrupt.
12 * The register write-protection function should be disabled before using this function.
13 */
14 void CLK_EnableSysTick(uint32_t u32ClkSrc, uint32_t u32Count)
15 {
16 /* Set System Tick counter disabled */
17 SysTick->CTRL = 0;
18
19 /* Set System Tick clock source */
20 if( u32ClkSrc == CLK_CLKSEL0_STCLK_S_HCLK )
21 SysTick->CTRL |= SysTick_CTRL_CLKSOURCE_Msk;
22 else
23 CLK->CLKSEL0 = (CLK->CLKSEL0 & ~CLK_CLKSEL0_STCLK_S_Msk) | u32ClkSrc;
24
25 /* Set System Tick reload value */
26 SysTick->LOAD = u32Count;
27
28 /* Clear System Tick current value and counter flag */
29 SysTick->VAL = 0;
30
31 /* Set System Tick interrupt enabled and counter enabled */
32 SysTick->CTRL |= SysTick_CTRL_TICKINT_Msk | SysTick_CTRL_ENABLE_Msk;
33 }
二、使用函数SysTick_Config（）
1 if (SysTick_Config(SysTick_int_us*50))//此函数SysTick时钟源默认为系统时钟源。
2 {
3 //Capture error
4 do
5 printf("SysTick_Init_失败 \n\n\n");
6 while (1);
7 }
8 else
9 printf("SysTick_Init_成功 \n");
此函数在core_cm0.h
1 /** \brief System Tick Configuration
2
3 The function initializes the System Timer and its interrupt, and starts the System Tick Timer.
4 Counter is in free running mode to generate periodic interrupts.
5
6 \param [in] ticks Number of ticks between two interrupts.
7
8 \return 0 Function succeeded.
9 \return 1 Function failed.
10
11 \note When the variable <b>__Vendor_SysTickConfig</b> is set to 1, then the
12 function <b>SysTick_Config</b> is not included. In this case, the file <b><i>device</i>.h</b>
13 must contain a vendor-specific implementation of this function.
14
15 */
16 __STATIC_INLINE uint32_t SysTick_Config(uint32_t ticks)
17 {
18 if (ticks > SysTick_LOAD_RELOAD_Msk) return (1); /* Reload value impossible */
19
20 SysTick->LOAD = (ticks & SysTick_LOAD_RELOAD_Msk) - 1; /* set reload register */
21 NVIC_SetPriority (SysTick_IRQn, (1<<__NVIC_PRIO_BITS) - 1); /* set Priority for Systick Interrupt */
22 SysTick->VAL = 0; /* Load the SysTick Counter Value */
23 SysTick->CTRL = SysTick_CTRL_CLKSOURCE_Msk |
24 SysTick_CTRL_TICKINT_Msk |
25 SysTick_CTRL_ENABLE_Msk; /* Enable SysTick IRQ and SysTick Timer */
26 return (0); /* Function successful */
27 }
三、直接寄存器操作
1 //主时钟为：50Mhz 2 //HXT=50mHZ=1/50us 50000*1/50=1000us=1ms//OK 3 //重装载值=ticks 时钟频率(Hz)X想要的定时时间（S）=50x1000=50000 4 //SysTick控制与状态（SYST_CSR） 5 SysTick->CTRL = SysTick_CTRL_CLKSOURCE_Msk | // 1= 内核时钟用于SysTick. 6 SysTick_CTRL_TICKINT_Msk | // 1:向下计数到0将引起SysTick中断。清SysTick当前值寄存器SYST_CVR的值将清除SysTick中断. 7 SysTick_CTRL_ENABLE_Msk; // ENABLE:1:计数器运行于多脉冲方式. 0:禁用计数器 8 // COUNTFLAG:计数由1到0时，COUNTFLAG 置位.在读该位或向系统定时器当前值寄存器（SYST_CVR）写时，COUNTFLAG 被清零。 9 //SysTick重装载值寄存器（SYST_RVR） 10 SysTick->LOAD = ((SysTick_int_us*50) & SysTick_LOAD_RELOAD_Msk) - 1; //RELOAD:当计数器达到0时，值加载到当前值寄存器. 11 12 //SysTick当前值寄存器（SYST_CVR） 13 SysTick->VAL = 0; //CURRENT:该寄存器为写清零软件写入任何值将清寄存器为0。 14 15 // set Priority for Systick Interrupt 16 //NVIC_SetPriority (SysTick_IRQn, (1<<__NVIC_PRIO_BITS) - 1); 17 NVIC_EnableIRQ(SysTick_IRQn);
最后中断入口程序：
1 /*******************************************************************
2 功能 ：SysTick中断入口
3 说明 ：嘀嗒心跳
4 入口参数 ：无
5 返回值 ：无
6 应用子程序 ：
7 全局变量 ：
8 *******************************************************************/
9 void SysTick_Handler(void) //非SysTick_IRQHandler(void)
10 {
11 SysTick_CountValue_Add(); //计数加1
12 /*SysTick当前值寄存器（SYST_CVR）*/
13 SysTick->VAL = 0; //CURRENT:该寄存器为写清零,软件写入任何值将清寄存器为0，写值清中断
14 GPIO_TOGGLE(P00); //Test 1ms P00切换一次电平
15 }