ARM Cortex-M3 (STM32F103) Tutorial - System Timer - Delay Function

In this tutorial, I will share how to use system timer (SysTick) for making delay functions. SysTick is a basic countdown timer. SysTick can be used by an operating system to ease porting from another platform. SysTick can be polled by software or can be configured to generate an interrupt. To use SysTick we can reload a value to the reload value register. SysTick reload value register supports value between 0 - 0x00FFFFFF (24-bit).

SysTick can be configured through the 4 registers:

To configure SysTick easily, we can use this function (you can see in core_cm3.h):

__STATIC_INLINE uint32_t SysTick_Config(uint32_t ticks);

This function will initialize the SysTick timer and its interrupt, then start the SysTick. The counter is in free running mode to generate periodic interrupts. The input parameter of this function is the number of ticks between two interrupts.

In this tutorial, I will make 2 delay function (us delay and ms delay). First, to make 1us delay, I cofigure the SysTick interrupt to triggered every 1us. By using SysTick_Config() function you can initialize SysTick to trigger the interrupt every 1us.

There is a variable called usTicks that holds value of ticks in us. Every we call DelayUs() function, we reload this variable with the value of delay in us and then we poll this variable until reach 0. This variable will be decrement by 1 every 1us by SysTick_Handler() function.

To make DelayMs() function, we can reload the value of ms, then decrement it by 1 every 1000us using DelayUs() function.

This is the code for the delay library:

#ifndef __DELAY_H
#define __DELAY_H

#ifdef __cplusplus
extern "C" {
#endif

#include "stm32f10x.h"

void DelayInit(void);
void DelayUs(uint32_t us);
void DelayMs(uint32_t ms);

#ifdef __cplusplus
}
#endif

#endif

#include "delay.h"

// For store tick counts in us
static __IO uint32_t usTicks;

// SysTick_Handler function will be called every 1 us
void SysTick_Handler()
{
    if (usTicks != 0)
    {
        usTicks--;
    }
}

void DelayInit()
{
    // Update SystemCoreClock value
    SystemCoreClockUpdate();
    // Configure the SysTick timer to overflow every 1 us
    SysTick_Config(SystemCoreClock / 1000000);
}

void DelayUs(uint32_t us)
{
    // Reload us value
    usTicks = us;
    // Wait until usTick reach zero
    while (usTicks);
}

void DelayMs(uint32_t ms)
{
    // Wait until ms reach zero
    while (ms--)
    {
        // Delay 1ms
        DelayUs(1000);
    }
}

To use that delay functions don't forget to call DelayInit() function first. You can see my complete code with example in here.

For STM32 tutorial with HAL library, you can click here.