One-shot vs auto-reload timers in FreeRTOS - Hands-On Comparison

One-shot vs Auto-reload Timers in FreeRTOS

📖 Scenario: You are developing a simple embedded application using FreeRTOS. You want to understand how to use one-shot and auto-reload timers to perform tasks after delays or periodically.

🎯 Goal: Build a FreeRTOS program that creates two timers: one one-shot timer and one auto-reload timer. The program will print messages when each timer expires, demonstrating the difference between the two timer types.

📋 What You'll Learn

Create a one-shot timer with a 2-second delay

Create an auto-reload timer with a 1-second period

Start both timers

Print messages when timers expire to show their behavior

💡 Why This Matters

🌍 Real World

Timers are used in embedded systems to perform actions after delays or periodically, such as blinking LEDs, reading sensors, or sending data.

💼 Career

Understanding FreeRTOS timers is essential for embedded software developers working on real-time applications and IoT devices.

1

Create the one-shot timer

Create a one-shot timer called oneShotTimer with a 2000 ms period and a callback function named vOneShotTimerCallback. Use xTimerCreate with pdFALSE for auto-reload parameter.

FreeRTOS

# Create the one-shot timer called oneShotTimer
# Your code here

Hint

Use xTimerCreate with the third argument pdFALSE to make a one-shot timer.

2

Create the auto-reload timer

Create an auto-reload timer called autoReloadTimer with a 1000 ms period and a callback function named vAutoReloadTimerCallback. Use xTimerCreate with pdTRUE for auto-reload parameter.

FreeRTOS

#include "FreeRTOS.h"
#include "timers.h"

void vOneShotTimerCallback(TimerHandle_t xTimer) {
    // Callback code will be added later
}

TimerHandle_t oneShotTimer = xTimerCreate("OneShotTimer", pdMS_TO_TICKS(2000), pdFALSE, NULL, vOneShotTimerCallback);

# Create the auto-reload timer called autoReloadTimer
# Your code here

Hint

Use xTimerCreate with the third argument pdTRUE to make an auto-reload timer.

3

Start both timers

Start both oneShotTimer and autoReloadTimer using xTimerStart with a block time of 0.

FreeRTOS

#include "FreeRTOS.h"
#include "timers.h"

void vOneShotTimerCallback(TimerHandle_t xTimer) {
    // Callback code will be added later
}

void vAutoReloadTimerCallback(TimerHandle_t xTimer) {
    // Callback code will be added later
}

TimerHandle_t oneShotTimer = xTimerCreate("OneShotTimer", pdMS_TO_TICKS(2000), pdFALSE, NULL, vOneShotTimerCallback);
TimerHandle_t autoReloadTimer = xTimerCreate("AutoReloadTimer", pdMS_TO_TICKS(1000), pdTRUE, NULL, vAutoReloadTimerCallback);

# Start both timers
# Your code here

Hint

Use xTimerStart with block time 0 to start timers immediately.

4

Print messages in timer callbacks

Add printf statements inside vOneShotTimerCallback and vAutoReloadTimerCallback to print "One-shot timer expired" and "Auto-reload timer expired" respectively. Then call startTimers() and run the FreeRTOS scheduler with vTaskStartScheduler().

FreeRTOS

#include "FreeRTOS.h"
#include "timers.h"
#include "stdio.h"

void vOneShotTimerCallback(TimerHandle_t xTimer) {
    // Add printf to show one-shot timer expired
    // Your code here
}

void vAutoReloadTimerCallback(TimerHandle_t xTimer) {
    // Add printf to show auto-reload timer expired
    // Your code here
}

TimerHandle_t oneShotTimer = xTimerCreate("OneShotTimer", pdMS_TO_TICKS(2000), pdFALSE, NULL, vOneShotTimerCallback);
TimerHandle_t autoReloadTimer = xTimerCreate("AutoReloadTimer", pdMS_TO_TICKS(1000), pdTRUE, NULL, vAutoReloadTimerCallback);

void startTimers() {
    xTimerStart(oneShotTimer, 0);
    xTimerStart(autoReloadTimer, 0);
}

int main(void) {
    // Start timers and scheduler
    // Your code here
}

Hint

Use printf inside each callback to show when timers expire. Call startTimers() and then vTaskStartScheduler() in main().

Practice

1. What is the main difference between a one-shot timer and an auto-reload timer in FreeRTOS?

easy

A. One-shot timers run once after a delay; auto-reload timers run repeatedly at fixed intervals.

B. One-shot timers run repeatedly; auto-reload timers run only once.

C. One-shot timers are faster than auto-reload timers.

D. Auto-reload timers cannot be stopped once started.

Solution

1. Step 1: Understand timer behavior

   One-shot timers execute their callback function only once after the specified delay.

2. Step 2: Compare with auto-reload timers

   Auto-reload timers automatically restart after each expiration, running repeatedly at fixed intervals.

3. Final Answer:

   One-shot timers run once after a delay; auto-reload timers run repeatedly at fixed intervals. -> Option A

4. Quick Check:

   One-shot = single run, Auto-reload = repeated runs [OK]

Hint: One-shot = once, Auto-reload = repeat [OK]

Common Mistakes:

• Confusing which timer runs repeatedly
• Thinking one-shot timers repeat automatically
• Assuming auto-reload timers cannot be stopped

2. Which of the following is the correct way to create an auto-reload timer in FreeRTOS using xTimerCreate?

easy

A. xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdFALSE, NULL, TimerCallback);

B. xTimerCreate("Timer", 1000, pdTRUE, NULL, TimerCallback);

C. xTimerCreate("Timer", 1000, pdFALSE, NULL, TimerCallback);

D. xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, TimerCallback);

Solution

1. Step 1: Identify auto-reload parameter

   The third parameter of xTimerCreate is uxAutoReload, which should be pdTRUE for auto-reload timers.

2. Step 2: Check time parameter format

   Use pdMS_TO_TICKS(1000) to convert milliseconds to ticks correctly.

3. Final Answer:

   xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, TimerCallback); -> Option D

4. Quick Check:

   Auto-reload needs pdTRUE and tick conversion [OK]

Hint: Auto-reload = pdTRUE, use pdMS_TO_TICKS for time [OK]

Common Mistakes:

• Using pdFALSE for auto-reload timers
• Passing raw milliseconds instead of ticks
• Confusing parameter order

3. Consider this FreeRTOS timer code snippet:

TimerHandle_t timer = xTimerCreate("MyTimer", pdMS_TO_TICKS(500), pdFALSE, NULL, TimerCallback);
xTimerStart(timer, 0);

What will happen when this timer expires?

medium

A. The TimerCallback function will be called once after 500 ms.

B. The TimerCallback function will be called repeatedly every 500 ms.

C. The timer will never start because pdFALSE is invalid.

D. The TimerCallback function will be called immediately and repeatedly.

Solution

1. Step 1: Analyze timer creation parameters

   The timer is created with pdFALSE for auto-reload, meaning it is a one-shot timer.

2. Step 2: Understand timer start and expiration

   After starting, the timer waits 500 ms, then calls TimerCallback once and stops.

3. Final Answer:

   The TimerCallback function will be called once after 500 ms. -> Option A

4. Quick Check:

   pdFALSE means one-shot = single callback [OK]

Hint: pdFALSE means one-shot, callback runs once [OK]

Common Mistakes:

• Thinking pdFALSE means repeated calls
• Assuming timer starts immediately
• Confusing pdFALSE with invalid parameter

4. You wrote this code to create an auto-reload timer but it only runs once:

TimerHandle_t timer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdFALSE, NULL, TimerCallback);
xTimerStart(timer, 0);

What is the likely error?

medium

A. The timer period is too short to see repeated calls.

B. xTimerStart was called with zero block time.

C. The auto-reload parameter is set to pdFALSE instead of pdTRUE.

D. The timer callback function is missing.

Solution

1. Step 1: Check auto-reload parameter

   The timer was created with pdFALSE, which means it is a one-shot timer.

2. Step 2: Understand effect on timer behavior

   Because of pdFALSE, the timer runs only once and does not auto-reload.

3. Final Answer:

   The auto-reload parameter is set to pdFALSE instead of pdTRUE. -> Option C

4. Quick Check:

   pdFALSE disables auto-reload, causing single run [OK]

Hint: Auto-reload needs pdTRUE, not pdFALSE [OK]

Common Mistakes:

• Ignoring auto-reload parameter
• Thinking block time affects reload
• Assuming callback absence causes single run

5. You want to toggle an LED every 2 seconds indefinitely using a FreeRTOS timer. Which timer type and setup should you use?

hard

A. Create a one-shot timer with 2000 ms period and restart it manually each time.

B. Create an auto-reload timer with 2000 ms period to toggle LED automatically.

C. Create a one-shot timer with 1000 ms period and toggle LED twice per cycle.

D. Create an auto-reload timer with 1000 ms period and toggle LED every 2 cycles.

Solution

1. Step 1: Identify requirement for repeated action

   Toggling LED every 2 seconds indefinitely means the action repeats automatically.

2. Step 2: Choose timer type and period

   An auto-reload timer with 2000 ms period will call the toggle function every 2 seconds without manual restart.

3. Final Answer:

   Create an auto-reload timer with 2000 ms period to toggle LED automatically. -> Option B

4. Quick Check:

   Repeated action = auto-reload timer with correct period [OK]

Hint: Repeated tasks use auto-reload timers with correct period [OK]

Common Mistakes:

• Using one-shot timer and forgetting to restart
• Setting wrong timer period
• Confusing toggle frequency with timer period