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.