SysTick-系统定时器

Systick系统定时器是内核的24位定时器,只能够递减,嵌套在NVIC中,所有的Cortex-M内核的单片机都具有这个定时器,系统嘀嗒校准值固定为9000,当系统嘀嗒时钟设定为9MHz(HCLK/8的大值),产生1ms时间基准
功能框图如下:

下面是SysTick 控制和状态寄存器,只有0,1,2,16这四个位有效

Systick的时钟源可以选择AHB/8或者AHB

Systick寄存器操作如下

SysTick定时时间计算,最好设置成毫秒级别,防止中断的频繁切换

在core_cm3.h里面查看SysTick_Type的定义,因为SysTick属于内核的外设,

/** @addtogroup CMSIS_CM3_SysTick CMSIS CM3 SysTick
  memory mapped structure for SysTick
  @{
 */
typedef struct
{
  __IO uint32_t CTRL;                         /*!< Offset: 0x00  SysTick Control and Status Register */
  __IO uint32_t LOAD;                         /*!< Offset: 0x04  SysTick Reload Value Register       */
  __IO uint32_t VAL;                          /*!< Offset: 0x08  SysTick Current Value Register      */
  __I  uint32_t CALIB;                        /*!< Offset: 0x0C  SysTick Calibration Register        */
} SysTick_Type;

下面是SysTick的配置库函数,对于systick中断优先级配置的是scb->shprx寄存器,而外设的中断优先级配置的是nvic->iprx,有优先级分组,有抢占优先级和子优先级的说法

/**
 * @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(uint32_t ticks)
{ 
  if (ticks > SysTick_LOAD_RELOAD_Msk)  return (1);            /* 如果ticks大于2的24次方则返回1 */
                                                               
  SysTick->LOAD  = (ticks & SysTick_LOAD_RELOAD_Msk) - 1;      /* 初始化reload重装载寄存器的值 */
  NVIC_SetPriority (SysTick_IRQn, (1<<__NVIC_PRIO_BITS) - 1);  /* 配置中断优先级为15,默认的最大优先级*/
  SysTick->VAL   = 0;                                          /* Sys当前计数器的值赋值为0 */
  SysTick->CTRL  = SysTick_CTRL_CLKSOURCE_Msk | 
                   SysTick_CTRL_TICKINT_Msk   | 
                   SysTick_CTRL_ENABLE_Msk;                    /* 使能systick中断和systick时间值 */
  return (0);                                                  /* Function successful */
}

补充:

对systick进行配置

#include "bsp_systick.h"
void systick_config_us(uint32_t us)
{
   SysTick_Config(72);
	 uint32_t i;
	 for(i=0;i<us;i++)
   {
	   while(!((SysTick->CTRL)&(1<<16)));
	 }
	 SysTick->CTRL&=~SysTick_CTRL_ENABLE_Msk;
}
void systick_config_ms(uint32_t ms)
{
   SysTick_Config(72000);
	 uint32_t i;
	 for(i=0;i<ms;i++)
   {
	   while(!((SysTick->CTRL)&(1<<16)));
	 }
	 SysTick->CTRL&=~SysTick_CTRL_ENABLE_Msk;
}

int main(void)
{
	HseConfig(9);
	lcd_init();
	while(1)
	{
		

    //systick_config_ms(5000);	  
		GPIO_ResetBits(LED_PORT,GPIO_Pin_2);
		systick_config_ms(500);	
		GPIO_SetBits(LED_PORT,GPIO_Pin_2);
		systick_config_ms(500);	
	}
	return 0;
}

实现灯500毫秒亮灭