SysTick timer for RTOS tick in ARM Architecture

Introduction

The SysTick timer helps the RTOS keep track of time by generating regular interrupts. This allows the RTOS to switch tasks smoothly and manage time-based events.

When you want the RTOS to switch between tasks at fixed time intervals.

When you need to measure time delays or timeouts in an embedded system.

When you want to trigger periodic actions like updating sensors or refreshing displays.

When you want to keep the system responsive by running multiple tasks cooperatively.

Core Concept

ARM Architecture

void SysTick_Config(uint32_t ticks);

This function sets up the SysTick timer to generate an interrupt every 'ticks' clock cycles.

The 'ticks' value depends on your system clock and desired tick rate.

Key Points

Configures SysTick to interrupt every 1 millisecond if SystemCoreClock is in Hz.

ARM Architecture

SysTick_Config(SystemCoreClock / 1000);

Sets SysTick to interrupt every 72000 clock cycles, which depends on your clock speed.

ARM Architecture

SysTick_Config(72000);

Detailed Explanation

This program sets up the SysTick timer to generate an interrupt every 1 millisecond. Each interrupt increases the tick_count variable. When tick_count reaches 1000, it means 1 second has passed, and you can perform actions periodically.

ARM Architecture

#include "stm32f4xx.h"

volatile uint32_t tick_count = 0;

void SysTick_Handler(void) {
    tick_count++; // Increment tick count every SysTick interrupt
}

int main(void) {
    // Configure SysTick to interrupt every 1 ms
    SysTick_Config(SystemCoreClock / 1000);

    while (1) {
        if (tick_count >= 1000) { // 1000 ms = 1 second
            // Do something every second
            tick_count = 0; // Reset counter
        }
    }
}

Important Notes

SysTick timer is a simple countdown timer built into ARM Cortex-M processors.

The interrupt handler must be named SysTick_Handler to be recognized by the system.

Make sure the system clock is correctly set before configuring SysTick.

Summary

SysTick timer generates regular interrupts to help RTOS manage time.

Use SysTick_Config() to set the interrupt frequency based on your clock.

Increment a counter in the SysTick interrupt to track elapsed time.

Practice

1. What is the main purpose of the SysTick timer in an RTOS?

easy

A. To generate regular interrupts for task switching

B. To increase CPU clock speed

C. To store data temporarily

D. To manage memory allocation

Solution

1. Step 1: Understand SysTick timer role

   The SysTick timer is designed to create periodic interrupts.

2. Step 2: Connect interrupts to RTOS function

   These interrupts allow the RTOS to switch tasks regularly.

3. Final Answer:

   To generate regular interrupts for task switching -> Option A

4. Quick Check:

   SysTick timer = periodic interrupts for RTOS [OK]

Hint: SysTick timer triggers interrupts for switching tasks [OK]

Common Mistakes:

• Confusing SysTick with CPU clock speed control
• Thinking SysTick stores data
• Assuming SysTick manages memory

2. Which of the following is the correct way to enable the SysTick timer interrupt in ARM Cortex-M?

easy

A. Write zero to the RELOAD register

B. Set the TICKINT bit in the SysTick CTRL register

C. Clear the ENABLE bit in the SysTick CTRL register

D. Disable the NVIC interrupt controller

Solution

1. Step 1: Identify the control register bits

   The SysTick CTRL register controls timer and interrupt enabling.

2. Step 2: Enable interrupt by setting TICKINT bit

   Setting the TICKINT bit enables SysTick interrupts.

3. Final Answer:

   Set the TICKINT bit in the SysTick CTRL register -> Option B

4. Quick Check:

   Enable interrupt = set TICKINT bit [OK]

Hint: Enable SysTick interrupt by setting TICKINT bit [OK]

Common Mistakes:

• Clearing ENABLE disables timer, not interrupt
• Writing zero to RELOAD stops timer from counting
• Disabling NVIC stops all interrupts, not just SysTick

3. Given a CPU clock of 48 MHz and a desired RTOS tick interval of 1 ms, what reload value should be set in the SysTick timer?

medium

A. 47999

B. 480

C. 48000

D. 47

Solution

1. Step 1: Calculate ticks per 1 ms interval

   CPU clock = 48,000,000 Hz means 48,000 ticks per 1 ms (since 1 ms = 0.001 s).

2. Step 2: Determine reload value

   Reload value = ticks per interval - 1 = 48000 - 1 = 47999.

3. Final Answer:

   47999 -> Option A

4. Quick Check:

   Reload = (CPU clock * tick interval) - 1 = 47999 [OK]

Hint: Reload = (CPU clock x interval) - 1 [OK]

Common Mistakes:

• Using CPU clock directly without subtracting 1
• Confusing microseconds with milliseconds
• Using too small reload value causing faster ticks

4. A developer sets the SysTick reload value to 0 and enables the timer and interrupt. What issue will occur?

medium

A. System will reset immediately

B. SysTick will never generate interrupts

C. CPU clock speed will increase

D. SysTick will generate interrupts continuously without delay

Solution

1. Step 1: Understand reload value effect

   A reload value of 0 means the timer counts from 0 to 0, triggering interrupt immediately.

2. Step 2: Resulting behavior

   This causes continuous interrupts with no delay, overwhelming the system.

3. Final Answer:

   SysTick will generate interrupts continuously without delay -> Option D

4. Quick Check:

   Reload=0 causes continuous interrupts [OK]

Hint: Reload 0 means no delay, interrupts fire nonstop [OK]

Common Mistakes:

• Assuming no interrupts occur with reload 0
• Thinking CPU clock changes due to SysTick
• Expecting system reset from this setting

5. In an RTOS using SysTick for task switching, if the CPU clock changes dynamically (e.g., due to power saving), what must be done to maintain correct tick timing?

hard

A. Disable SysTick and rely on software delays

B. Keep the reload value fixed regardless of clock changes

C. Update the SysTick reload value to match the new CPU clock frequency

D. Increase the interrupt priority of SysTick

Solution

1. Step 1: Recognize effect of CPU clock changes

   SysTick timer counts CPU clock cycles, so changing clock speed affects tick timing.

2. Step 2: Adjust reload value accordingly

   To keep tick interval constant, reload value must be recalculated based on new CPU clock.

3. Final Answer:

   Update the SysTick reload value to match the new CPU clock frequency -> Option C

4. Quick Check:

   Adjust reload for clock changes to keep tick timing [OK]

Hint: Reload value must match CPU clock for correct ticks [OK]

Common Mistakes:

• Ignoring clock changes and keeping reload fixed
• Disabling SysTick breaks RTOS timing
• Changing interrupt priority doesn't fix timing