Tick timer and scheduler in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Tick Count Increment Behavior

What will be the output of the following FreeRTOS tick count code snippet after 3 ticks?

FreeRTOS

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

TickType_t tickCount = 0;

void vApplicationTickHook(void) {
    tickCount = xTaskGetTickCount();
}

int main() {
    // Simulate 3 ticks
    for (int i = 0; i < 3; i++) {
        vApplicationTickHook();
    }
    printf("Tick count: %u\n", (unsigned int)tickCount);
    return 0;
}

A Tick count: 1

B Tick count: 3

C Tick count: 0

D Tick count: 2

💡 Hint

Observe that the scheduler is not started in the code, so xTaskGetTickCount() returns 0.

The code attempts to simulate 3 ticks by calling vApplicationTickHook() in a loop. However, since vTaskStartScheduler() is never called, the FreeRTOS scheduler and tick interrupt are not running. Therefore, xTaskGetTickCount() always returns 0, and the output is 'Tick count: 0'.

🧠 Conceptual

intermediate

Role of the Tick Timer in FreeRTOS

What is the primary role of the tick timer in FreeRTOS?

A To provide a periodic interrupt that drives the scheduler and time-related functions

B To handle all hardware interrupts in the system

C To manage memory allocation for tasks

D To initialize the CPU clock frequency

💡 Hint

Think about what triggers task switching and delays.

The tick timer generates periodic interrupts that allow the scheduler to switch tasks and manage delays/timeouts.

🔧 Debug

advanced

Why Does the Scheduler Not Switch Tasks?

Given the following FreeRTOS configuration snippet, why does the scheduler never switch tasks?

#define configUSE_PREEMPTION 0
#define configTICK_RATE_HZ 1000

void vTask1(void *pvParameters) {
    while(1) {
        // Task code
    }
}

void vTask2(void *pvParameters) {
    while(1) {
        // Task code
    }
}

int main() {
    xTaskCreate(vTask1, "Task1", 1000, NULL, 1, NULL);
    xTaskCreate(vTask2, "Task2", 1000, NULL, 1, NULL);
    vTaskStartScheduler();
    return 0;
}

A Because preemption is disabled, so tasks only switch on yield or blocking calls

B Because the tick rate is too high causing scheduler overload

C Because tasks have the same priority and scheduler cannot decide

D Because tasks are missing calls to vTaskDelay()

💡 Hint

Check the configUSE_PREEMPTION setting.

With preemption disabled, the scheduler switches tasks only when tasks yield or block. Without explicit yield, the first task runs forever.

📝 Syntax

advanced

Identify the Syntax Error in Tick Hook Implementation

Which option contains the correct syntax for implementing the FreeRTOS tick hook function?

A void vApplicationTickHook() { /* user code */ }

B void vApplicationTickHook(void) { /* user code */ }

C void vApplicationTickHook(void) -> void { /* user code */ }

D void vApplicationTickHook(void); { /* user code */ }

💡 Hint

Check the correct C function declaration syntax.

Option B correctly declares the tick hook function with void parameter and no return value.

🚀 Application

expert

Calculating Tick Overflow Time

Given a FreeRTOS tick rate of 1000 Hz and a 32-bit tick counter, how long in days will it take for the tick count to overflow?

A Approximately 24 hours

B Approximately 365 days

C Approximately 100 days

D Approximately 49.7 days

💡 Hint

Calculate total ticks before overflow and convert to time.

The 32-bit counter max value is 2^32 - 1 ticks. At 1000 ticks per second, overflow time = (2^32 / 1000) seconds ≈ 4294967 seconds ≈ 49.7 days.

Practice

1. What is the main role of the tick timer in FreeRTOS?

easy

A. To handle input/output operations

B. To store task priorities

C. To allocate memory for tasks

D. To count time in small steps called ticks

Solution

1. Step 1: Understand tick timer function

   The tick timer counts time in small units called ticks, which are the basic time measurement in FreeRTOS.

2. Step 2: Differentiate from other functions

   Other options like storing priorities or handling I/O are not related to the tick timer's role.

3. Final Answer:

   To count time in small steps called ticks -> Option D

4. Quick Check:

   Tick timer = counts ticks [OK]

Hint: Tick timer counts time steps, not priorities or memory [OK]

Common Mistakes:

• Confusing tick timer with task priority management
• Thinking tick timer handles memory or I/O
• Mixing tick timer with scheduler directly

2. Which of the following is the correct way to delay a task for 100 ticks in FreeRTOS?

easy

A. xTaskDelay(100);

B. vTaskDelay(1000);

C. vTaskDelay(100);

D. delayTask(100);

Solution

1. Step 1: Identify correct function name

   The correct FreeRTOS function to delay a task is vTaskDelay().

2. Step 2: Check parameter correctness

   The parameter is the number of ticks to delay. For 100 ticks, vTaskDelay(100); is correct.

3. Final Answer:

   vTaskDelay(100); -> Option C

4. Quick Check:

   Delay task = vTaskDelay(ticks) [OK]

Hint: Use vTaskDelay with tick count to pause tasks [OK]

Common Mistakes:

• Using wrong function name like xTaskDelay or delayTask
• Passing milliseconds instead of ticks
• Using too large or incorrect delay values

3. Consider this code snippet in FreeRTOS:

TickType_t start = xTaskGetTickCount();
vTaskDelay(50);
TickType_t end = xTaskGetTickCount();
TickType_t diff = end - start;

What is the expected value of diff?

medium

A. 0

B. 50

C. 100

D. Depends on task priority

Solution

1. Step 1: Understand xTaskGetTickCount()

   This function returns the current tick count since the scheduler started.

2. Step 2: Analyze vTaskDelay(50)

   The task delays itself for 50 ticks, so the tick count advances by 50 during the delay.

3. Step 3: Calculate difference

   Subtracting start from end gives the delay duration, which should be 50 ticks.

4. Final Answer:

   50 -> Option B

5. Quick Check:

   Delay ticks = 50 [OK]

Hint: Delay of 50 ticks means tick count difference is 50 [OK]

Common Mistakes:

• Assuming delay is zero or doubled
• Thinking task priority affects tick count difference
• Confusing ticks with milliseconds

4. What is wrong with this FreeRTOS code snippet?

vTaskDelay(0);
TickType_t ticks = xTaskGetTickCount();

medium

A. vTaskDelay(0) does not delay the task

B. xTaskGetTickCount() returns an error

C. vTaskDelay requires a pointer argument

D. TickType_t is not a valid type

Solution

1. Step 1: Check vTaskDelay parameter

   Passing 0 to vTaskDelay means no delay; the task yields but continues immediately.

2. Step 2: Verify xTaskGetTickCount()

   This function correctly returns the current tick count; no error here.

3. Final Answer:

   vTaskDelay(0) does not delay the task -> Option A

4. Quick Check:

   Delay(0) means no wait [OK]

Hint: vTaskDelay(0) yields but does not pause task [OK]

Common Mistakes:

• Expecting delay with zero ticks
• Thinking xTaskGetTickCount returns error
• Misunderstanding TickType_t type

5. You want a task to run every 200 ticks precisely using FreeRTOS. Which approach ensures this timing despite task execution time?

hard

A. Use vTaskDelayUntil() with a previous wake time variable

B. Use vTaskDelay(200) at the end of the task loop

C. Use xTaskGetTickCount() inside the task and delay manually

D. Use vTaskDelay(100) twice in the task loop

Solution

1. Step 1: Understand vTaskDelay() limitation

   Using vTaskDelay(200) delays from current time, so task execution time adds up causing drift.

2. Step 2: Use vTaskDelayUntil() for fixed periodic timing

   vTaskDelayUntil() uses a previous wake time to maintain exact periodic intervals regardless of execution time.

3. Step 3: Evaluate other options

   Manual delay or multiple delays do not guarantee precise timing; only vTaskDelayUntil() does.

4. Final Answer:

   Use vTaskDelayUntil() with a previous wake time variable -> Option A

5. Quick Check:

   Precise periodic task = vTaskDelayUntil() [OK]

Hint: Use vTaskDelayUntil() for exact periodic task timing [OK]

Common Mistakes:

• Using vTaskDelay causes timing drift
• Trying manual delay calculations
• Ignoring task execution time in timing