Starting and stopping timers in FreeRTOS - Mini Project: Build & Apply

Starting and stopping timers in FreeRTOS

📖 Scenario: You are building a simple embedded system that needs to perform an action repeatedly after a delay. To do this, you will use FreeRTOS software timers to start and stop timed events.

🎯 Goal: Create a FreeRTOS software timer, start it, stop it, and print messages to show the timer's state changes.

📋 What You'll Learn

Create a FreeRTOS software timer named xTimer with a period of 1000 milliseconds

Create a variable timerStarted to track if the timer is running

Start the timer using xTimerStart

Stop the timer using xTimerStop

Print messages to show when the timer starts and stops

💡 Why This Matters

🌍 Real World

Embedded systems often need to perform tasks repeatedly or after delays, such as blinking LEDs or reading sensors. FreeRTOS timers help manage these timed events efficiently.

💼 Career

Understanding how to start and stop timers in FreeRTOS is essential for embedded software developers working on real-time applications like IoT devices, automotive systems, and industrial controllers.

1

Create the FreeRTOS software timer

Create a FreeRTOS software timer called xTimer with a period of 1000 milliseconds (1 second). Use xTimerCreate with the name "Timer1", auto-reload set to pdTRUE, and no timer ID (NULL).

FreeRTOS

# Create the FreeRTOS software timer xTimer with 1000 ms period
# Your code here

Hint

Use xTimerCreate with the exact parameters: name "Timer1", period 1000 ms converted with pdMS_TO_TICKS, auto-reload pdTRUE, and NULL for timer ID and callback.

2

Create a variable to track timer state

Create a variable called timerStarted of type BaseType_t and set it to pdFALSE to track if the timer is running.

FreeRTOS

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

TimerHandle_t xTimer;
BaseType_t timerStarted = pdFALSE;  // Your code here

void setupTimer() {
    xTimer = xTimerCreate("Timer1", pdMS_TO_TICKS(1000), pdTRUE, NULL, NULL);
}

Hint

Declare timerStarted as BaseType_t and initialize it to pdFALSE.

3

Start the timer and update the state variable

Use xTimerStart to start xTimer with a block time of 0. If the timer starts successfully (return value == pdPASS), set timerStarted to pdTRUE.

FreeRTOS

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

TimerHandle_t xTimer;
BaseType_t timerStarted = pdFALSE;

void setupTimer() {
    xTimer = xTimerCreate("Timer1", pdMS_TO_TICKS(1000), pdTRUE, NULL, NULL);
    // Start the timer and update timerStarted
    // Your code here
}

Hint

Call xTimerStart(xTimer, 0) and check if it returns pdPASS. If yes, set timerStarted to pdTRUE.

4

Stop the timer and print status messages

Use xTimerStop to stop xTimer with a block time of 0. If the timer stops successfully (return value == pdPASS), set timerStarted to pdFALSE. Then print "Timer started" if timerStarted is pdTRUE, else print "Timer stopped".

FreeRTOS

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

TimerHandle_t xTimer;
BaseType_t timerStarted = pdFALSE;

void setupTimer() {
    xTimer = xTimerCreate("Timer1", pdMS_TO_TICKS(1000), pdTRUE, NULL, NULL);
    if (xTimerStart(xTimer, 0) == pdPASS) {
        timerStarted = pdTRUE;
    }
    // Stop the timer and update timerStarted
    // Print timer status message
    // Your code here
}

Hint

Call xTimerStop(xTimer, 0) and check if it returns pdPASS. If yes, set timerStarted to pdFALSE. Then print "Timer started" or "Timer stopped" based on timerStarted.

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