Timer for periodic status reporting in FreeRTOS - Mini Project: Build & Apply

Timer for periodic status reporting

📖 Scenario: You are building a simple embedded system that needs to report its status every few seconds. To do this, you will use a FreeRTOS timer that triggers a function periodically.

🎯 Goal: Create a FreeRTOS timer that calls a status reporting function every 2 seconds.

📋 What You'll Learn

Create a FreeRTOS software timer named statusTimer with a 2-second period

Create a callback function named vStatusReportCallback that prints "Status: OK"

Start the timer after creation

Print the status message each time the timer callback runs

💡 Why This Matters

🌍 Real World

Periodic status reporting is common in embedded systems to monitor device health or send heartbeat signals.

💼 Career

Understanding FreeRTOS timers is essential for embedded software engineers working on real-time systems.

1

Create the status reporting callback function

Write a function called vStatusReportCallback that takes a TimerHandle_t xTimer parameter and prints the text "Status: OK" using printf.

FreeRTOS

# Write the function vStatusReportCallback with parameter TimerHandle_t xTimer
# Inside the function, print "Status: OK"

Hint

Use void vStatusReportCallback(TimerHandle_t xTimer) as the function signature and printf("Status: OK\n") inside it.

2

Create the FreeRTOS timer

Create a FreeRTOS software timer named statusTimer using xTimerCreate with the following parameters: name "StatusTimer", period of 2000 milliseconds converted to ticks using pdMS_TO_TICKS(2000), auto-reload set to pdTRUE, timer ID set to NULL, and callback function vStatusReportCallback.

FreeRTOS

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

void vStatusReportCallback(TimerHandle_t xTimer) {
    printf("Status: OK\n");
}

// Create the timer statusTimer here

Hint

Use xTimerCreate with the exact parameters: name "StatusTimer", period pdMS_TO_TICKS(2000), auto-reload pdTRUE, ID NULL, and callback vStatusReportCallback.

3

Start the timer

Start the timer statusTimer by calling xTimerStart with the timer handle and a block time of 0.

FreeRTOS

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

void vStatusReportCallback(TimerHandle_t xTimer) {
    printf("Status: OK\n");
}

TimerHandle_t statusTimer = xTimerCreate(
    "StatusTimer",
    pdMS_TO_TICKS(2000),
    pdTRUE,
    NULL,
    vStatusReportCallback
);

// Start the timer statusTimer here

Hint

Call xTimerStart(statusTimer, 0); inside your main function or setup code.

4

Print status messages periodically

Add a simple infinite loop with vTaskDelay inside main to keep the program running so the timer can print "Status: OK" every 2 seconds. Use vTaskDelay(pdMS_TO_TICKS(1000)) inside the loop. Then print "Timer started" once before the loop.

FreeRTOS

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

void vStatusReportCallback(TimerHandle_t xTimer) {
    printf("Status: OK\n");
}

TimerHandle_t statusTimer = xTimerCreate(
    "StatusTimer",
    pdMS_TO_TICKS(2000),
    pdTRUE,
    NULL,
    vStatusReportCallback
);

int main(void) {
    xTimerStart(statusTimer, 0);
    // Add a print statement "Timer started" here
    // Add an infinite loop with vTaskDelay(pdMS_TO_TICKS(1000)) to keep running
    // Your code here
}

Hint

Print "Timer started" once, then keep the program alive with an infinite loop calling vTaskDelay(pdMS_TO_TICKS(1000)). The timer callback will print "Status: OK" every 2 seconds.

Practice

1. What is the main purpose of using a timer in FreeRTOS for periodic status reporting?

easy

A. To pause the entire system for a set time

B. To create a new task that runs forever

C. To run a function repeatedly at fixed time intervals

D. To allocate memory dynamically for tasks

Solution

1. Step 1: Understand timer functionality in FreeRTOS

   Timers in FreeRTOS are designed to call a function repeatedly or once after a delay.

2. Step 2: Relate to periodic status reporting

   Periodic status reporting requires running code repeatedly at fixed intervals, which timers enable.

3. Final Answer:

   To run a function repeatedly at fixed time intervals -> Option C

4. Quick Check:

   Timer = repeated function calls [OK]

Hint: Timers run code repeatedly at set intervals [OK]

Common Mistakes:

• Confusing timers with tasks
• Thinking timers pause the system
• Assuming timers allocate memory

2. Which of the following is the correct way to create a FreeRTOS timer named statusTimer with a 1000 ms period and auto-reload enabled?

easy

A. xTimerCreate("statusTimer", pdMS_TO_TICKS(1000), pdTRUE, NULL, TimerCallback);

B. xTimerCreate("statusTimer", 1000, pdFALSE, NULL, TimerCallback);

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

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

Solution

1. Step 1: Identify correct period conversion

   FreeRTOS timers use ticks, so pdMS_TO_TICKS(1000) converts 1000 ms to ticks correctly.

2. Step 2: Check auto-reload parameter

   pdTRUE enables auto-reload for periodic timers, which is needed here.

3. Final Answer:

   xTimerCreate("statusTimer", pdMS_TO_TICKS(1000), pdTRUE, NULL, TimerCallback); -> Option A

4. Quick Check:

   Use pdMS_TO_TICKS and pdTRUE for periodic timers [OK]

Hint: Use pdMS_TO_TICKS for ms and pdTRUE for auto-reload [OK]

Common Mistakes:

• Using raw milliseconds instead of ticks
• Setting auto-reload to pdFALSE for periodic timer
• Confusing pdTRUE and pdFALSE values

3. Given the code snippet below, what will be printed every second?

void TimerCallback(TimerHandle_t xTimer) {
    printf("Status: OK\n");
}

int main() {
    TimerHandle_t statusTimer = xTimerCreate("statusTimer", pdMS_TO_TICKS(1000), pdTRUE, NULL, TimerCallback);
    xTimerStart(statusTimer, 0);
    vTaskStartScheduler();
    return 0;
}

medium

A. Status: OK printed every second repeatedly

B. Status: OK printed once after 1 second

C. No output because timer is not started

D. Compilation error due to missing parameters

Solution

1. Step 1: Analyze timer creation and start

   The timer is created with pdTRUE for auto-reload and started immediately, so it runs repeatedly.

2. Step 2: Understand callback behavior

   The callback prints "Status: OK" each time the timer expires, which is every 1000 ms (1 second).

3. Final Answer:

   Status: OK printed every second repeatedly -> Option A

4. Quick Check:

   Auto-reload timer prints every second [OK]

Hint: Auto-reload timers call callback repeatedly [OK]

Common Mistakes:

• Assuming timer runs only once
• Forgetting to start the timer
• Confusing callback parameters

4. Identify the error in the following FreeRTOS timer code snippet:

TimerHandle_t statusTimer = xTimerCreate("statusTimer", 1000, pdTRUE, NULL, TimerCallback);
xTimerStart(statusTimer, 0);

medium

A. Timer callback function is missing

B. Timer name must be NULL, not a string

C. Auto-reload parameter must be pdFALSE for periodic timers

D. Timer period should be converted to ticks using pdMS_TO_TICKS

Solution

1. Step 1: Check timer period parameter

   The period is given as 1000 (milliseconds) but FreeRTOS expects ticks, so pdMS_TO_TICKS is needed.

2. Step 2: Verify other parameters

   The timer name as string is allowed, auto-reload pdTRUE is correct, and callback is assumed defined.

3. Final Answer:

   Timer period should be converted to ticks using pdMS_TO_TICKS -> Option D

4. Quick Check:

   Always convert ms to ticks with pdMS_TO_TICKS [OK]

Hint: Always convert ms to ticks with pdMS_TO_TICKS [OK]

Common Mistakes:

• Passing raw milliseconds instead of ticks
• Confusing auto-reload true/false
• Assuming timer name must be NULL

5. You want to implement a FreeRTOS timer that reports system status every 2 seconds but only for 5 times, then stops automatically. Which approach correctly achieves this?

hard

A. Create a one-shot timer with 2-second period and restart it manually 5 times in callback

B. Create an auto-reload timer with 2-second period and count executions in callback, then stop timer after 5 times

C. Create an auto-reload timer with 10-second period and print status every 2 seconds inside callback

D. Create a one-shot timer with 10-second period and start it 5 times in a loop

Solution

1. Step 1: Understand timer types and requirements

   Auto-reload timers run repeatedly; one-shot timers run once. We need repeated calls every 2 seconds.

2. Step 2: Implement counting and stopping logic

   Use an auto-reload timer with 2-second period, count callback calls, and stop timer after 5 executions.

3. Final Answer:

   Create an auto-reload timer with 2-second period and count executions in callback, then stop timer after 5 times -> Option B

4. Quick Check:

   Auto-reload + count + stop after 5 [OK]

Hint: Use auto-reload timer and stop after count in callback [OK]

Common Mistakes:

• Using one-shot timer without manual restarts
• Incorrect timer period causing wrong intervals
• Not stopping timer after desired count