Idle task and idle hook in FreeRTOS - Step-by-Step Execution

Concept Flow - Idle task and idle hook

System starts FreeRTOS scheduler

↓

All tasks blocked or suspended?

No→Run highest priority ready task

Yes↓

Run Idle Task

↓

Idle Hook called?

Yes→Execute user Idle Hook function

No↓

Idle Task loops back waiting for tasks

The FreeRTOS scheduler runs tasks by priority. When no tasks are ready, the Idle Task runs. If an Idle Hook is defined, it runs inside the Idle Task loop.

Execution Sample

FreeRTOS

void vApplicationIdleHook(void) {
    // User code here
    // e.g., enter low power mode
}

// FreeRTOS runs idle task when no other task is ready

This code shows the Idle Hook function that runs inside the Idle Task when the system is idle.

Execution Table

Step	Condition	Action	Output
1	Scheduler started	Run highest priority ready task	Task1 runs
2	Task1 blocked or suspended	Check other tasks	No ready tasks
3	No ready tasks	Run Idle Task	Idle Task runs
4	Idle Hook defined?	Yes	Idle Hook function called
5	Idle Hook executes user code	User code runs	e.g., low power mode entered
6	Idle Hook ends	Idle Task loops	Wait for tasks to become ready
7	New task ready	Scheduler switches to new task	New task runs

💡 Idle Task runs continuously when no other tasks are ready; Idle Hook runs inside Idle Task if defined

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	After Step 7
Ready Tasks	Task1 ready	No ready tasks	No ready tasks	No ready tasks	New task ready
Idle Task State	Not running	Not running	Running	Running	Not running
Idle Hook State	Not running	Not running	Not running	Running	Not running

Key Moments - 3 Insights

Why does the Idle Task run only when no other tasks are ready?

Because FreeRTOS always runs the highest priority ready task. The Idle Task has the lowest priority and runs only when no other tasks are ready, as shown in execution_table row 3.

What happens if the Idle Hook is not defined?

The Idle Task runs an empty loop doing nothing. The Idle Hook function is only called if defined, as shown in execution_table row 4.

Can the Idle Hook be used to save power?

Yes, the Idle Hook is a good place to put code to enter low power mode, as shown in execution_table row 5.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the Idle Hook function run?

A Step 7

B Step 2

C Step 4

D Step 1

💡 Hint

Check the 'Action' column in execution_table row 4 where Idle Hook is called.

According to variable_tracker, what is the state of 'Ready Tasks' after Step 3?

A No ready tasks

B New task ready

C Task1 ready

D Idle Task running

💡 Hint

Look at the 'Ready Tasks' row and the 'After Step 3' column in variable_tracker.

If a new task becomes ready at Step 7, what happens to the Idle Task?

A Idle Task continues running

B Idle Task suspends and new task runs

C Idle Hook runs again

D System resets

💡 Hint

See execution_table row 7 describing scheduler switching to new task.

Concept Snapshot

Idle Task runs when no other tasks are ready.
It has the lowest priority.
Idle Hook is a user function called inside Idle Task loop.
Use Idle Hook for low power or background work.
If no Idle Hook, Idle Task loops doing nothing.

Full Transcript

FreeRTOS runs tasks based on priority. When no tasks are ready, the Idle Task runs. The Idle Task has the lowest priority and runs continuously until a task becomes ready. If the user defines an Idle Hook function, FreeRTOS calls it inside the Idle Task loop. This hook is useful for running background code or entering low power mode. The execution table shows the scheduler starting, running tasks, then running the Idle Task and Idle Hook when no tasks are ready. Variables track which tasks are ready and the state of the Idle Task and Idle Hook. Key moments clarify why the Idle Task runs only when no other tasks are ready, what happens if the Idle Hook is not defined, and how the Idle Hook can save power. The visual quiz tests understanding of when the Idle Hook runs, the state of ready tasks, and what happens when a new task becomes ready.