Timer for periodic status reporting in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this FreeRTOS timer callback?

Consider a FreeRTOS software timer set to trigger every 1000 ms. The callback function increments a counter and prints it. What will be printed after the timer triggers 3 times?

FreeRTOS

static int count = 0;

void vTimerCallback(TimerHandle_t xTimer) {
    count++;
    printf("Count: %d\n", count);
}

// Timer created with xTimerCreate("StatusTimer", pdMS_TO_TICKS(1000), pdTRUE, NULL, vTimerCallback);
// Timer started with xTimerStart(timer, 0);

A Count: 0\nCount: 1\nCount: 2\n

B No output because timer callback is not called

C Count: 3\nCount: 3\nCount: 3\n

D Count: 1\nCount: 2\nCount: 3\n

💡 Hint

Remember the timer callback increments the count each time it runs.

The timer callback runs every 1000 ms, incrementing the count from 0 to 1, then 2, then 3, printing each value.

🧠 Conceptual

intermediate

Which FreeRTOS timer type is best for periodic status reporting?

You want to report system status every 5 seconds automatically. Which timer type should you use in FreeRTOS?

A Task delay with vTaskDelay(5000)

B Periodic timer that triggers every 5 seconds

C Hardware timer interrupt

D One-shot timer that triggers once after 5 seconds

💡 Hint

Think about repeating actions without manual restart.

A periodic timer automatically triggers repeatedly at the set interval, ideal for periodic status reporting.

🔧 Debug

advanced

Why does this FreeRTOS timer callback not print anything?

Given this code snippet, why is there no output from the timer callback?

FreeRTOS

void vTimerCallback(TimerHandle_t xTimer) {
    printf("Timer triggered\n");
}

void main() {
    TimerHandle_t timer = xTimerCreate("MyTimer", pdMS_TO_TICKS(1000), pdTRUE, NULL, vTimerCallback);
    // Missing xTimerStart call
    vTaskStartScheduler();
}

A The timer was never started with xTimerStart, so callback never runs

B The callback function has a syntax error and does not compile

C The timer period is zero, so it never triggers

D The scheduler was not started, so no tasks run

💡 Hint

Check if the timer was activated after creation.

Creating a timer does not start it. Without calling xTimerStart, the timer callback will never be called.

📝 Syntax

advanced

Identify the syntax error in this FreeRTOS timer callback declaration

Which option contains the correct syntax for a FreeRTOS timer callback function?

A int vTimerCallback(TimerHandle_t xTimer) { /* code */ }

B void vTimerCallback(void xTimer) { /* code */ }

C void vTimerCallback(TimerHandle_t xTimer) { /* code */ }

D void vTimerCallback(TimerHandle xTimer) { /* code */ }

💡 Hint

Check the parameter type and return type required by FreeRTOS.

The callback must return void and accept a TimerHandle_t parameter. Option C matches this exactly.

🚀 Application

expert

How to safely update a shared variable in a FreeRTOS timer callback?

You have a global variable 'statusCount' updated in a timer callback and read in a task. How should you protect 'statusCount' to avoid data corruption?

FreeRTOS

volatile int statusCount = 0;

void vTimerCallback(TimerHandle_t xTimer) {
    statusCount++;
}

void vStatusTask(void *pvParameters) {
    while(1) {
        printf("Status count: %d\n", statusCount);
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

A Use a mutex or critical section around accesses to 'statusCount'

B Declare 'statusCount' as volatile and do nothing else

C Disable interrupts globally when reading or writing 'statusCount'

D Use a local copy of 'statusCount' in the task without synchronization

💡 Hint

Think about how to prevent race conditions when multiple contexts access the same variable.

Using a mutex or critical section ensures atomic access to 'statusCount', preventing data corruption.

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