FreeRTOS architecture overview - Step-by-Step Execution

Concept Flow - FreeRTOS architecture overview

Start FreeRTOS Kernel

↓

Initialize Hardware & Resources

↓

Create Tasks & Queues

↓

Start Scheduler

↓

Scheduler Chooses Task

↓

Task Runs

↓

Task Blocks or Yields

↓

Scheduler Chooses Next Task

↓

Idle Task Runs

↓

Repeat Scheduling Cycle

This flow shows how FreeRTOS starts, creates tasks, runs the scheduler, and switches between tasks.

Execution Sample

FreeRTOS

void main(void) {
  xTaskCreate(Task1, "Task1", 1000, NULL, 1, NULL);
  vTaskStartScheduler();
}

void Task1(void *pvParameters) {
  for(;;) { /* Task code */ }
}

This code creates one task and starts the FreeRTOS scheduler to run it.

Execution Table

Step	Action	Scheduler State	Running Task	Notes
1	System starts	Scheduler not running	None	Kernel initialized
2	Create Task1	Scheduler not running	None	Task1 added to ready list
3	Start Scheduler	Scheduler running	Task1	Scheduler picks highest priority task
4	Task1 runs	Scheduler running	Task1	Task1 executes its loop
5	Task1 blocks or yields	Scheduler running	Idle Task	No ready tasks, idle runs
6	Idle Task runs	Scheduler running	Idle Task	CPU idle until next event
7	Task1 ready again	Scheduler running	Task1	Scheduler switches back to Task1
8	Repeat cycle	Scheduler running	Task1 or Idle	Continuous scheduling
9	System shutdown	Scheduler stopped	None	Scheduler stops, system halts

💡 Scheduler runs indefinitely until system shutdown or reset

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	After Step 7	Final
Scheduler State	Not running	Not running	Running	Running	Running	Running
Running Task	None	None	Task1	Idle Task	Task1	Task1 or Idle
Task1 State	Not created	Ready	Running	Blocked/Yielded	Ready	Running or Blocked

Key Moments - 3 Insights

Why does the Idle Task run after Task1 blocks or yields?

When Task1 blocks or yields (see step 5 in execution_table), no other tasks are ready, so the scheduler runs the Idle Task to keep the CPU busy.

What happens if no tasks are created before starting the scheduler?

If no tasks exist, the scheduler runs only the Idle Task indefinitely, as shown in the scheduler state after step 3.

Does the scheduler stop automatically?

No, the scheduler runs indefinitely until the system is shut down or reset, as noted in the exit_note.

Visual Quiz - 3 Questions

Look at the execution_table, what is the running task immediately after starting the scheduler?

A Idle Task

B Task1

C None

D Scheduler

💡 Hint

Check step 3 in the execution_table where the scheduler starts and picks Task1.

At which step does the scheduler run the Idle Task?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look at the execution_table rows where the running task is Idle Task.

If Task1 never blocks or yields, which task runs continuously?

A Task1

B Scheduler

C Idle Task

D No task runs

💡 Hint

Refer to variable_tracker showing Task1 state during continuous running.

Concept Snapshot

FreeRTOS starts by initializing hardware and creating tasks.
The scheduler runs and picks the highest priority ready task.
Tasks run until they block, yield, or complete.
When no tasks are ready, the Idle Task runs.
The scheduler runs indefinitely managing task switching.

Full Transcript

FreeRTOS architecture begins with system start and hardware initialization. Tasks are created and added to the ready list. When the scheduler starts, it picks the highest priority ready task to run. Tasks execute their code until they block or yield, at which point the scheduler selects the next ready task. If no tasks are ready, the Idle Task runs to keep the CPU busy. This cycle repeats indefinitely until the system shuts down or resets.