Event-driven architecture in FreeRTOS - Step-by-Step Execution

Concept Flow - Event-driven architecture

Event Occurs

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Event Detected by ISR

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Send Event to Queue

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Task Waits on Queue

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Task Receives Event

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Task Processes Event

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Task Waits for Next Event

Events happen and are caught by an interrupt. The event is sent to a queue. A task waits for events and processes them one by one.

Execution Sample

FreeRTOS

void ISR_Handler() {
  EventType event;
  BaseType_t xHigherPriorityTaskWoken = pdFALSE;
  xQueueSendFromISR(eventQueue, &event, &xHigherPriorityTaskWoken);
  portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

void EventTask(void *pvParameters) {
  EventType event;
  while(1) {
    if(xQueueReceive(eventQueue, &event, portMAX_DELAY)) {
      processEvent(event);
    }
  }
}

An ISR sends events to a queue, and a task waits on the queue to process events as they arrive.

Execution Table

Step	Action	Event Queue State	Task State	Output
1	ISR_Handler triggered by event	Event added to queue	Waiting on queue	No output yet
2	EventTask unblocked by event	Event removed from queue	Processing event	processEvent(event) called
3	EventTask finishes processing	Queue empty	Waiting on queue	Ready for next event
4	No new events	Queue empty	Waiting on queue	Idle, waiting

💡 Execution waits indefinitely for new events; no exit unless system stops

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
eventQueue	empty	1 event	empty	empty	empty
event	undefined	set by ISR	received by task	processed	ready for next
xHigherPriorityTaskWoken	pdFALSE	pdTRUE	N/A	N/A	pdFALSE

Key Moments - 3 Insights

Why does the task wait on the queue instead of polling?

The task uses xQueueReceive with portMAX_DELAY to block and wait efficiently, saving CPU instead of polling continuously. See execution_table step 4 where task waits idle.

What ensures the ISR does not block when sending to the queue?

ISR uses xQueueSendFromISR which is safe and non-blocking in interrupt context, as shown in execution_table step 1.

How does the system handle multiple events arriving quickly?

Events queue up in eventQueue. The task processes them one by one as it receives them from the queue, shown by eventQueue state changes in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what happens to the eventQueue?

A The event is removed from the queue

B The queue is cleared

C An event is added to the queue

D The queue is unchanged

💡 Hint

Check the 'Event Queue State' column at step 2 in execution_table

According to variable_tracker, what is the value of xHigherPriorityTaskWoken after step 1?

A pdFALSE

B undefined

C pdTRUE

D event

💡 Hint

Look at the 'xHigherPriorityTaskWoken' row after Step 1 in variable_tracker

If no new events occur, what is the task state at step 4 in execution_table?

A Processing event

B Waiting on queue

C Blocked indefinitely

D Terminated

💡 Hint

See the 'Task State' column at step 4 in execution_table

Concept Snapshot

Event-driven architecture in FreeRTOS:
- ISR detects event and sends it to a queue
- Task waits (blocks) on queue for events
- Task processes events one by one
- Uses xQueueSendFromISR and xQueueReceive
- Efficient CPU use by blocking, not polling

Full Transcript

In FreeRTOS event-driven architecture, when an event happens, an interrupt service routine (ISR) catches it and sends the event to a queue using xQueueSendFromISR. A dedicated task waits on this queue using xQueueReceive with an indefinite timeout, so it blocks and saves CPU. When the task receives an event, it processes it and then waits again for the next event. This design avoids polling and allows responsive, efficient handling of asynchronous events.