Multiple tasks running concurrently in FreeRTOS - Step-by-Step Execution

Concept Flow - Multiple tasks running concurrently

Start Scheduler

↓

Task 1 Ready

↓

Task 1 Running

↓

Task 1 Blocked

↓

Task 2 Ready

↓

Task 2 Running

↓

Task 2 Blocked

↩Back to Scheduler

The scheduler starts and switches between tasks that are ready to run, running one task at a time and switching when tasks block or yield.

Execution Sample

FreeRTOS

void Task1(void *pvParameters) {
  for(;;) {
    // Do work
    vTaskDelay(1000 / portTICK_PERIOD_MS);
  }
}

void Task2(void *pvParameters) {
  for(;;) {
    // Do work
    vTaskDelay(500 / portTICK_PERIOD_MS);
  }
}

Two tasks run concurrently, each delaying for different times to simulate work and allow switching.

Execution Table

Step	Running Task	Task1 State	Task2 State	Scheduler Action	Output
1	Task1	Running	Ready	Start Task1	Task1 runs first
2	Task1	Blocked (delay)	Ready	Switch to Task2	Task1 delays 1000ms
3	Task2	Blocked (delay)	Running	Run Task2	Task2 runs
4	Task2	Blocked (delay)	Blocked (delay)	Both tasks delayed	Task2 delays 500ms
5	Idle	Ready	Ready	Tasks ready, switch to Task1	Task1 delay over
6	Task1	Running	Ready	Run Task1	Task1 runs again
7	Task1	Blocked (delay)	Ready	Switch to Task2	Task1 delays again
8	Task2	Blocked (delay)	Running	Run Task2	Task2 runs again
9	Task2	Blocked (delay)	Blocked (delay)	Both delayed	Task2 delays again
10	Idle	Ready	Ready	Tasks ready, switch to Task1	Cycle repeats
Exit	-	-	-	-	Scheduler runs indefinitely, tasks keep switching

💡 Scheduler runs forever, switching tasks as they block and become ready again

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	After Step 7	After Step 9	Final
Task1 State	Ready	Running	Blocked (delay)	Ready	Blocked (delay)	Ready	Blocked (delay)
Task2 State	Ready	Ready	Running	Blocked (delay)	Ready	Running	Blocked (delay)

Key Moments - 3 Insights

Why does Task1 become blocked after running?

Because Task1 calls vTaskDelay, it tells the scheduler to pause it for 1000ms, so it moves to Blocked state (see execution_table step 2).

How does the scheduler decide which task to run next?

The scheduler runs the highest priority task that is Ready. When Task1 blocks, Task2 is Ready and runs next (see steps 2 and 3).

What happens when both tasks are delayed at the same time?

Both tasks are Blocked, so the scheduler runs the Idle task until one task's delay expires (see step 4 and 10).

Visual Quiz - 3 Questions

Look at the execution table, what is Task2's state at Step 3?

A Ready

B Blocked (delay)

C Running

D Suspended

💡 Hint

Check the 'Task2 State' column at Step 3 in the execution_table.

At which step does Task1 become Ready again after its delay?

A Step 5

B Step 7

C Step 2

D Step 9

💡 Hint

Look for when Task1 state changes from Blocked to Ready in the execution_table.

If Task2's delay was increased to 2000ms, how would the scheduler behavior change?

A Task2 would run more often

B Task2 would block longer, Task1 runs more

C Both tasks would block at the same time more often

D Scheduler would stop switching tasks

💡 Hint

Consider how longer delay affects Task2's Blocked state duration in variable_tracker.

Concept Snapshot

FreeRTOS runs multiple tasks by switching between them.
Each task runs until it blocks or yields.
vTaskDelay pauses a task, letting others run.
Scheduler picks the highest priority Ready task.
When all tasks block, Idle task runs.
This creates the illusion of tasks running at the same time.

Full Transcript

In FreeRTOS, multiple tasks run concurrently by the scheduler switching between them. When the scheduler starts, it runs the first ready task. If a task calls vTaskDelay, it becomes blocked for that time, and the scheduler switches to another ready task. Tasks alternate running and blocking, allowing multitasking. When all tasks are blocked, the Idle task runs until a task becomes ready again. This process repeats indefinitely, creating concurrent task execution. The execution table shows step-by-step how Task1 and Task2 states change and how the scheduler switches between them. The variable tracker highlights the state changes of each task after each step. Key moments clarify why tasks block and how the scheduler chooses which task to run next. The visual quiz tests understanding of task states and scheduler behavior. The concept snapshot summarizes the multitasking mechanism in FreeRTOS.