Starting and stopping timers in FreeRTOS

Introduction

Timers help you run code after some time or repeatedly. Starting and stopping timers lets you control when the timer runs or pauses.

You want to blink an LED every second.

You need to check a sensor every 5 seconds.

You want to stop a task if it runs too long.

You want to delay an action but be able to cancel it.

Syntax

FreeRTOS

TimerHandle_t xTimerCreate(
    const char * const pcTimerName,
    const TickType_t xTimerPeriodInTicks,
    const UBaseType_t uxAutoReload,
    void * const pvTimerID,
    TimerCallbackFunction_t pxCallbackFunction
);

BaseType_t xTimerStart(TimerHandle_t xTimer, TickType_t xTicksToWait);

BaseType_t xTimerStop(TimerHandle_t xTimer, TickType_t xTicksToWait);

xTimerCreate makes a new timer but does not start it.

xTimerStart begins the timer counting.

xTimerStop stops the timer if it is running.

Examples

This creates a timer that repeats every 1000 milliseconds and starts it immediately.

FreeRTOS

TimerHandle_t myTimer = xTimerCreate("MyTimer", pdMS_TO_TICKS(1000), pdTRUE, NULL, myCallback);
xTimerStart(myTimer, 0);

This creates a timer that runs once after 5 seconds, starts it, then stops it before it fires.

FreeRTOS

TimerHandle_t oneShotTimer = xTimerCreate("OneShot", pdMS_TO_TICKS(5000), pdFALSE, NULL, oneShotCallback);
xTimerStart(oneShotTimer, 0);
xTimerStop(oneShotTimer, 0);

Sample Program

This program creates a repeating timer that calls a function every second. It starts the timer, waits for 3 seconds (simulated), then stops the timer.

FreeRTOS

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

void vTimerCallback(TimerHandle_t xTimer) {
    printf("Timer callback called!\n");
}

// Dummy vTaskDelay for simulation
void vTaskDelay(const TickType_t xTicksToDelay) {
    // Just print delay for demo
    printf("Delaying for %lu ticks\n", xTicksToDelay);
}

int main(void) {
    TimerHandle_t xTimer = xTimerCreate("TestTimer", pdMS_TO_TICKS(1000), pdTRUE, NULL, vTimerCallback);
    if (xTimer == NULL) {
        printf("Timer creation failed!\n");
        return 1;
    }

    if (xTimerStart(xTimer, 0) != pdPASS) {
        printf("Timer start failed!\n");
        return 1;
    }

    // Simulate waiting for 3 timer periods
    for (int i = 0; i < 3; i++) {
        vTaskDelay(pdMS_TO_TICKS(1000));
    }

    if (xTimerStop(xTimer, 0) != pdPASS) {
        printf("Timer stop failed!\n");
        return 1;
    }

    printf("Timer stopped successfully.\n");
    return 0;
}

Important Notes

Timers run their callback functions in the timer service task, so keep callbacks short.

Always check if timer creation and start/stop calls succeed.

Summary

Use xTimerCreate to make a timer but it does not start automatically.

Use xTimerStart to begin the timer counting.

Use xTimerStop to stop a running timer.

Practice

1. What does the function xTimerCreate do in FreeRTOS?

easy

A. Creates a timer but does not start it automatically

B. Starts a timer immediately after creation

C. Stops a running timer

D. Deletes a timer permanently

Solution

1. Step 1: Understand the purpose of xTimerCreate

   This function is used to create a software timer in FreeRTOS but it does not start the timer automatically.

2. Step 2: Differentiate from other timer functions

   Starting a timer requires xTimerStart, stopping requires xTimerStop, and deleting requires xTimerDelete.

3. Final Answer:

   Creates a timer but does not start it automatically -> Option A

4. Quick Check:

   xTimerCreate only creates timer [OK]

Hint: Remember: create ≠ start; start needs separate call [OK]

Common Mistakes:

• Thinking xTimerCreate starts the timer automatically
• Confusing xTimerStart with creation
• Assuming timer is running immediately after creation

2. Which of the following is the correct way to start a FreeRTOS timer named xTimer?

easy

A. xTimerDelete(xTimer, 0);

B. xTimerCreate(xTimer, 0);

C. xTimerStop(xTimer, 0);

D. xTimerStart(xTimer, 0);

Solution

1. Step 1: Identify the function to start a timer

   The function xTimerStart is used to start a timer in FreeRTOS.

2. Step 2: Check the syntax

   The correct syntax is xTimerStart(xTimer, 0); where the second parameter is the block time.

3. Final Answer:

   xTimerStart(xTimer, 0); -> Option D

4. Quick Check:

   Start timer = xTimerStart call [OK]

Hint: Start timers with xTimerStart, not create or stop [OK]

Common Mistakes:

• Using xTimerCreate to start a timer
• Confusing stop and delete functions with start
• Missing the block time parameter

3. Given the following code snippet, what will be the output if the timer callback increments a counter each time it runs and the timer is started and stopped immediately?

int counter = 0;
TimerHandle_t xTimer = xTimerCreate("Timer", pdMS_TO_TICKS(1000), pdTRUE, NULL, timerCallback);
xTimerStart(xTimer, 0);
xTimerStop(xTimer, 0);
// Assume timerCallback increments counter

medium

A. counter will remain 0

B. counter will be incremented at least once

C. Compilation error due to missing parameters

D. Runtime error due to invalid timer handle

Solution

1. Step 1: Analyze timer start and stop sequence

   The timer is started and then immediately stopped before it can expire and call the callback.

2. Step 2: Understand timer callback execution

   Since the timer period is 1000 ms and it is stopped immediately, the callback does not get a chance to run, so the counter stays 0.

3. Final Answer:

   counter will remain 0 -> Option A

4. Quick Check:

   Immediate stop prevents callback = counter 0 [OK]

Hint: Timer callback runs only after timer expires, not immediately [OK]

Common Mistakes:

• Assuming callback runs immediately on start
• Ignoring timer period delay
• Confusing start and stop timing effects

4. Identify the error in the following code snippet that attempts to stop a timer:

TimerHandle_t xTimer = xTimerCreate("Timer", pdMS_TO_TICKS(500), pdFALSE, NULL, timerCallback);
xTimerStop(xTimer, 1000);

medium

A. The block time parameter in xTimerStop is too large

B. The timer was never started before stopping

C. The timer handle is NULL

D. The timer period is set incorrectly

Solution

1. Step 1: Check timer lifecycle

   The timer is created but never started before calling xTimerStop.

2. Step 2: Understand stopping a timer

   Stopping a timer that is not running has no effect and may cause unexpected behavior.

3. Final Answer:

   The timer was never started before stopping -> Option B

4. Quick Check:

   Stop requires running timer first [OK]

Hint: Always start timer before stopping it [OK]

Common Mistakes:

• Stopping timer without starting it
• Assuming block time causes error
• Misunderstanding timer handle validity

5. You want to create a periodic timer in FreeRTOS that starts automatically and stops after 5 executions. Which approach correctly achieves this behavior?

hard

A. Create the timer with pdTRUE, start it automatically by xTimerCreate, and delete after 5 executions

B. Create the timer with pdFALSE, start it manually, and stop it after 5 seconds

C. Create the timer with pdTRUE for auto-reload, start it manually, and in the callback count executions and stop after 5

D. Create the timer with pdFALSE, start it automatically, and restart it 5 times

Solution

1. Step 1: Understand timer creation parameters

   Setting pdTRUE makes the timer periodic (auto-reload), but it does not start automatically; you must call xTimerStart.

2. Step 2: Implement stopping after 5 executions

   Use a counter inside the callback function to track executions and call xTimerStop after the 5th execution.

3. Final Answer:

   Create the timer with pdTRUE for auto-reload, start it manually, and in the callback count executions and stop after 5 -> Option C

4. Quick Check:

   Periodic timer + manual start + callback stop after 5 [OK]

Hint: Use callback counter to stop periodic timer after N runs [OK]

Common Mistakes:

• Assuming xTimerCreate starts timer automatically
• Using one-shot timer for periodic behavior
• Not counting executions in callback