Tick timer and scheduler in FreeRTOS - Step-by-Step Execution

Concept Flow - Tick timer and scheduler

System starts

↓

Tick Timer Interrupt

↓

Increment Tick Count

↓

Check if Scheduler Needed

↓

Call Scheduler

↓

Select Next Task

↓

Switch Context

↓

Run Next Task

↩Back to Tick Timer Interrupt

The tick timer triggers an interrupt regularly, increments the tick count, and calls the scheduler to decide if a task switch is needed.

Execution Sample

FreeRTOS

void vTickISR() {
  tick_count++;
  if (scheduler_needed()) {
    vTaskSwitchContext();
  }
}

This code runs on each tick interrupt, updates the tick count, and switches tasks if needed.

Execution Table

Step	Tick Count	Scheduler Needed?	Action	Current Task	Next Task
1	0	No	Increment tick_count to 1	Task A	Task A
2	1	Yes	Call scheduler and switch	Task A	Task B
3	2	No	Increment tick_count to 3	Task B	Task B
4	3	Yes	Call scheduler and switch	Task B	Task C
5	4	No	Increment tick_count to 5	Task C	Task C
6	5	Yes	Call scheduler and switch	Task C	Task A
Exit	6	-	Stop simulation	-	-

💡 Simulation stops after 6 ticks for demonstration.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
tick_count	0	1	2	3	4	5	6
current_task	Task A	Task A	Task B	Task B	Task C	Task C	Task A
next_task	Task A	Task A	Task B	Task B	Task C	Task C	Task A

Key Moments - 2 Insights

Why does the scheduler sometimes not switch tasks even though the tick count increases?

Because the scheduler only switches tasks when 'scheduler_needed()' returns Yes, as shown in steps 2, 4, and 6 in the execution_table.

What happens if the scheduler switches tasks?

The current task changes to the next task selected by the scheduler, as seen in steps 2, 4, and 6 where current_task updates.

Visual Quiz - 3 Questions

Look at the execution_table, what is the tick_count at step 4?

A 3

B 4

C 2

D 5

💡 Hint

Check the Tick Count column at step 4 in the execution_table.

At which step does the scheduler NOT switch tasks despite the tick increment?

A Step 2

B Step 3

C Step 4

D Step 6

💡 Hint

Look for steps where 'Scheduler Needed?' is No in the execution_table.

If the scheduler always returned No, what would happen to current_task over time?

A It would keep switching tasks.

B It would cause an error.

C It would stay the same task.

D It would reset to Task A every tick.

💡 Hint

Refer to variable_tracker to see how current_task changes only when scheduler_needed() is Yes.

Concept Snapshot

Tick timer triggers an interrupt regularly.
Each tick increments a global tick count.
Scheduler checks if a task switch is needed.
If yes, it switches context to the next task.
If no, current task continues running.
This cycle repeats continuously.

Full Transcript

In FreeRTOS, the tick timer is a hardware timer that generates interrupts at fixed intervals. Each interrupt runs the tick ISR, which increments the system tick count. After incrementing, the scheduler checks if a higher priority task is ready to run. If so, it switches context to that task. Otherwise, the current task continues. This process repeats, allowing multitasking by switching tasks on each tick when needed.

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