Event-driven architecture in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Event Group Bits Setting and Clearing

What is the output of this FreeRTOS code snippet that uses event groups to set and clear bits?

FreeRTOS

EventGroupHandle_t eventGroup = xEventGroupCreate();

xEventGroupSetBits(eventGroup, (1 << 0));
xEventGroupSetBits(eventGroup, (1 << 1));

EventBits_t bits = xEventGroupClearBits(eventGroup, (1 << 0));

printf("Bits before clear: 0x%X\n", bits);
printf("Bits after clear: 0x%X\n", xEventGroupGetBits(eventGroup));

A

Bits before clear: 0x2
Bits after clear: 0x1

B

Bits before clear: 0x1
Bits after clear: 0x3

C

Bits before clear: 0x3
Bits after clear: 0x2

D

Bits before clear: 0x0
Bits after clear: 0x0

💡 Hint

Remember that xEventGroupClearBits returns the bits before clearing.

The code sets bits 0 and 1 (0x1 and 0x2), so before clearing, bits are 0x3. Clearing bit 0 removes 0x1, leaving 0x2.

🧠 Conceptual

intermediate

Understanding Event-Driven Task Notification

In FreeRTOS, which statement best describes the behavior of task notifications used as lightweight event flags?

A Task notifications require a separate queue to send data between tasks.

B Task notifications can only be used to unblock a task and cannot carry data.

C Task notifications automatically queue multiple events if sent rapidly.

D Task notifications can be used both to unblock a task and to send 32-bit data atomically.

💡 Hint

Think about how task notifications combine signaling and data transfer.

Task notifications in FreeRTOS are lightweight and can both unblock a task and carry a 32-bit value atomically without extra queues.

🔧 Debug

advanced

Debugging Event Group Wait Timeout

A task waits for bits 0x01 and 0x02 using xEventGroupWaitBits with the wait option to clear bits on exit. The task never proceeds even though another task sets bit 0x01. What is the most likely cause?

FreeRTOS

xEventGroupWaitBits(eventGroup, 0x03, pdTRUE, pdTRUE, portMAX_DELAY);

A The wait condition requires both bits 0x01 and 0x02 to be set, but only 0x01 is set.

B The bits are cleared before the wait, causing the function to return immediately.

C The event group handle is NULL, so the wait never succeeds.

D The task priority is too low to run after the bits are set.

💡 Hint

Check the 'wait for all bits' parameter and which bits are set.

The function waits for both bits 0x01 and 0x02 (0x03) to be set. If only 0x01 is set, the wait never completes.

📝 Syntax

advanced

Identify the Syntax Error in Event Group Creation

Which option contains the correct syntax to create an event group in FreeRTOS?

A EventGroupHandle_t eg = xEventGroupCreate();

B EventGroupHandle_t eg = xEventGroupCreate;

C EventGroupHandle_t eg = xEventGroupCreate[];

D EventGroupHandle_t eg = xEventGroupCreate();;

💡 Hint

Remember that xEventGroupCreate is a function that returns a handle.

Option A correctly calls the function with parentheses. Option A misses parentheses, C uses invalid brackets, A has an extra semicolon but compiles (though stylistically odd).

🚀 Application

expert

Designing an Event-Driven Task Synchronization

You want to synchronize three tasks so that each waits until all three have reached a certain point before continuing. Which FreeRTOS event-driven mechanism is best suited for this?

A Use a counting semaphore initialized to zero and each task gives once, then all tasks take once.

B Use an event group where each task sets its own bit and all tasks wait for all bits to be set.

C Use a binary semaphore that each task takes and gives to signal readiness.

D Use task notifications with each task notifying the others directly.

💡 Hint

Think about a mechanism that can track multiple flags and wait for all.

An event group allows each task to set a unique bit and wait until all bits are set, effectively synchronizing all tasks at a barrier.