系统时钟

一、系统定时器

SysTick叫做系统滴答时钟、系统定时器，属于Cortex-M4内核中的一个外设，它24bit向下递减的计数器

二、系统定时器的中断使用方法

1.代码的初始化

//初始化系统定时器，1S内核触发1000次中断，说白了定时1ms
    SysTick_Config(SystemCoreClock/1000);

2. 中断服务函数的编写

void SysTick_Handler(void)
{
    static uint32_t cnt=0;

    cnt++;
    
    //到达500ms的定时
    if(cnt >= 500)
    {
        cnt=0;
        
        PFout(9)^=1;
    
    }
}

3.  定时时间的计算

SysTick_Config(SystemCoreClock/频率);

让系统定时器触发1秒中断是否可以？如果不可以，最大的定时时间又是什么？

不能触发1秒中断。

在额定频率情况下，最大定时时间 = 2^24 /168000000 ≈ 99.86ms

在超频的频率（216MHz）下，最大定时时间 = 2^24 /216000000≈77.67ms

测试结果：

         //初始化系统定时器，1S内核触发1000次中断，说白了定时1ms，能够成功
         //SysTick_Config(SystemCoreClock/1000);
         
         //初始化系统定时器，1S内核触发10次中断，说白了定时100ms,现象失败
         SysTick_Config(SystemCoreClock/10);     
         
         //初始化系统定时器，1S内核触发11次中断，说白了定时90.90ms,能够成功
         SysTick_Config(SystemCoreClock/11);     

 

三、系统定时器的用途

两个方面：

没有操作系统：只用于延时 

有操作系统（ucos2 ucos3 freertos....）:为操作系统提供精准的定时中断（1ms~50ms）

四、使用系统定时器用于延时的用途

If you want to use the SysTick timer in polling mode, you can use the count flag in the SysTick Control and

Status Register (SysTick->CTRL) to determine when the timer reaches zero.

For example, you can create a timed delay by setting the SysTick timer to a certain value and waiting until it reaches zero:

 

SysTick->CTRL = 0; // Disable SysTick
SysTick->LOAD = 0xFF; // Count from 255 to 0 (256 cycles)
SysTick->VAL = 0; // Clear current value as well as count flag
SysTick->CTRL = 5; // Enable SysTick timer with processor clock
while ((SysTick->CTRL & 0x00010000)==0);// Wait until count flag is set
SysTick->CTRL = 0; // Disable SysTick

1.  配置系统定时器的时钟源

 

/**
  * @brief  Configures the SysTick clock source.
  * @param  SysTick_CLKSource: specifies the SysTick clock source.
  *   This parameter can be one of the following values:
  *     @arg SysTick_CLKSource_HCLK_Div8: AHB clock divided by 8 selected as SysTick clock source.
  *     @arg SysTick_CLKSource_HCLK: AHB clock selected as SysTick clock source.
  * @retval None
  */
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;
  }
  else
  {
    SysTick->CTRL &= SysTick_CLKSource_HCLK_Div8;
  }
}

2，系统定时器寄存器

1.  当SysTick使用168MHz系统时钟频率时，代码编写如下：

void delay_us(uint32_t nus)
{
 
    SysTick->CTRL = 0;                      // Disable SysTick
    SysTick->LOAD = (SystemCoreClock/1000000)*nus; // 计数值
    SysTick->VAL = 0;                       // Clear current value as well as count flag
    SysTick->CTRL = 5;                      // Enable SysTick timer with processor clock
    while ((SysTick->CTRL & 0x00010000)==0);// Wait until count flag is set
    SysTick->CTRL = 0;                      // Disable SysTick
}
 
void delay_ms(uint32_t nms)
{
    SysTick->CTRL = 0;                      // Disable SysTick
    SysTick->LOAD = (SystemCoreClock/1000)*nms; // 计数值
    SysTick->VAL = 0;                       // Clear current value as well as count flag
    SysTick->CTRL = 5;                      // Enable SysTick timer with processor clock
    while ((SysTick->CTRL & 0x00010000)==0);// Wait until count flag is set
    SysTick->CTRL = 0;                      // Disable SysTick
}
 
最大的延时为99.86ms

2.  当SysTick使用168MHz系统时钟频率并进行8分频时，代码编写如下：

 

void delay_us(uint32_t nus)
{
    SysTick->CTRL = 0;                      // Disable SysTick
    SysTick->LOAD = (SystemCoreClock/8/1000000)*nus; // 计数值
    SysTick->VAL = 0;                       // Clear current value as well as count flag
    SysTick->CTRL = 1;                   // Enable SysTick timer with processor clock
    while ((SysTick->CTRL & 0x00010000)==0);// Wait until count flag is set
    SysTick->CTRL = 0;                      // Disable SysTick
}
 
 
void delay_ms(uint32_t nms)
{
    SysTick->CTRL = 0;                      // Disable SysTick
    SysTick->LOAD = (SystemCoreClock/8/1000)*nms; // 计数值
    SysTick->VAL = 0;                       // Clear current value as well as count flag
    SysTick->CTRL = 1;                   // Enable SysTick timer with processor clock
    while ((SysTick->CTRL & 0x00010000)==0);// Wait until count flag is set
    SysTick->CTRL = 0;                      // Disable SysTick
}
 
思考题，当前最大的延时时间是多少？如何优化代码，支持秒级别或更长时间的延时？
 
最大的延时时间 = 2^24 / 21000000 ≈ 798.91ms
 
 
void delay_ms(uint32_t nms)
{
    uint32_t m,n;
    
    m = nms/500;
    
    n = nms %500;
    
    //m个500ms的延时
    while(m--)
    {
        SysTick->CTRL = 0;                      // Disable SysTick
        SysTick->LOAD = (SystemCoreClock/8/1000)*500; // 计数值
        SysTick->VAL = 0;                       // Clear current value as well as count flag
        SysTick->CTRL = 1;                      // Enable SysTick timer with processor clock，当使用21MHz的时候，1；当使用168MHz的时候，5；
        while ((SysTick->CTRL & 0x00010000)==0);// Wait until count flag is set
        SysTick->CTRL = 0;                      // Disable SysTick  
    }
 
    //不足500ms的延时
    if(n)
    {
        SysTick->CTRL = 0;                      // Disable SysTick
        SysTick->LOAD = (SystemCoreClock/8/1000)*n; // 计数值
        SysTick->VAL = 0;                       // Clear current value as well as count flag
        SysTick->CTRL = 1;                      // Enable SysTick timer with processor clock，当使用21MHz的时候，1；当使用168MHz的时候，5；
        while ((SysTick->CTRL & 0x00010000)==0);// Wait until count flag is set
        SysTick->CTRL = 0;                      // Disable SysTick      
    
    }
}