【stm32f407】SysTick to achieve delay

1. SysTick Introduction:

The processing of the CM4 core is the same as that of the CM3. It contains a SysTick timer. SysTick is a 24-bit countdown timer. When it reaches 0, it will automatically reload the initial value of the timer from the RELOAD register. As long as the enable bit in the SysTick control and status register is not cleared, it will never stop. We use the internal SysTick of STM32 to achieve delay, which does not occupy interrupts or system timers.

Usually SysTick can be implemented by interruption, which will be added later, but currently it is only implemented by polling

2. Register Introduction

SysTick has 4 registers

The corresponding code is in core_cm4.h

typedefstruct

{

  __IO uint32_t CTRL;                    /*!< Offset: 0x000(R/W)  SysTick Control and StatusRegister */

  __IO uint32_t LOAD;                    /*!< Offset: 0x004(R/W)  SysTick Reload Value Register       */

  __IO uint32_t VAL;                     /*!< Offset: 0x008(R/W)  SysTick Current ValueRegister      */

  __I uint32_t CALIB;                  /*!< Offset: 0x00C (R/ ) SysTick Calibration Register       */

} SysTick_Type; 

1) CTR register as shown in the figure:

Bit 0: ENABLE, Systick enable bit (0: turn off Systick function; 1: turn on Systick function)

Bit 1: TICKINT, Systick interrupt enable bit (0: disable Systick interrupt; 1: enable Systick interrupt)

Bit 2: CLKSOURCE, Systick clock source selection (0: use HCLK/8 as Systick clock; 1: use HCLK as Systick clock)

Bit 16: COUNTFLAG, Systick count comparison flag, if SysTick has counted to 0 after the last read of this register, this bit is 1. If this bit is read, it will be automatically cleared.

2) The LOAD register is as shown in the figure:

Systick is a decrementing timer. When the timer decrements to 0, the value in the reload register will be reloaded and continue to decrement. STK_LOAD reload register is a 24-bit register with a maximum count of 0xFFFFFF.

3) VAL register as shown in the figure:

This is also a 24-bit register. When read, it returns the current countdown value. Writing it clears it to zero and also clears the COUNTFLAG flag in the SysTick control and status register.

4) CALIB register as shown in the figure

Generally not used

3. Source code

delay.h

#ifndef _DELAY_H_H_H

#define _DELAY_H_H_H

#include "stm32f4xx.h"

void delay_init(u8 SYSCLK);

void delay_ms(u16 nms);

void delay_us(u32 nus);

#endif

delay.c

#include "delay.h"

static u8  fac_us=0;    

static u16 fac_ms=0;

void delay_init(u8 SYSCLK)

{

  SysTick->CTRL&=~(1<<2);

  fac_us=SYSCLK/8;

  fac_ms=((u32)SYSCLK*1000)/8;

}

void delay_xms(u16 nms)

{     

  u32 temp;    

  SysTick->LOAD=(u32)nms*fac_ms;

  SysTick->VAL =0x00;

  SysTick->CTRL=0x01 ;

  do

  {

    temp=SysTick->CTRL;

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

  SysTick->CTRL=0x00;

  SysTick->VAL =0X00;       

} 

void delay_ms(u16 nms)

{

  u8 repeat=nms/540;

  u16 remain=nms%540;

  while(repeat)

  {

    delay_xms(540);

    repeat--;

  }

  if(remain)delay_xms(remain);

}

void delay_us(u32 nus)

{

  u32 temp;      

  SysTick->LOAD=nus*fac_us;   

  SysTick->VAL=0x00;

  SysTick->CTRL=0x01 ;  

  do

  {

    temp=SysTick->CTRL;

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

  SysTick->CTRL=0x00;

  SysTick->VAL =0X00;  

}