FreeRTOS interrupt priority restrictions - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Interrupt Priority and API Call Safety

Consider a FreeRTOS system where configMAX_SYSCALL_INTERRUPT_PRIORITY is set to 0x20. An interrupt with priority 0x10 calls a FreeRTOS API function that is not safe to call from that priority level. What will most likely happen?

A The FreeRTOS kernel may behave unpredictably or crash due to priority violation.

B The system will run normally without any issues.

C The interrupt will be automatically lowered to a safe priority by FreeRTOS.

D The API call will be ignored silently by FreeRTOS.

💡 Hint

Think about what happens if an interrupt calls a FreeRTOS API function from a priority above the allowed maximum.

FreeRTOS requires that interrupts calling certain API functions must have a priority equal to or numerically higher (lower urgency) than configMAX_SYSCALL_INTERRUPT_PRIORITY. Violating this causes unpredictable behavior or crashes.

🧠 Conceptual

intermediate

Understanding configMAX_SYSCALL_INTERRUPT_PRIORITY

What does the FreeRTOS configuration constant configMAX_SYSCALL_INTERRUPT_PRIORITY control?

A It disables all interrupts above a certain priority level.

B It defines the lowest interrupt priority allowed in the system.

C It sets the highest interrupt priority from which FreeRTOS API calls can be safely made.

D It sets the priority of the FreeRTOS kernel task.

💡 Hint

Think about which interrupts can safely call FreeRTOS API functions.

configMAX_SYSCALL_INTERRUPT_PRIORITY defines the highest interrupt priority (numerically lowest) from which FreeRTOS API functions that end in "FromISR" can be called safely.

🔧 Debug

advanced

Debugging Priority Violation in ISR

A developer reports that their FreeRTOS application crashes when an ISR calls xQueueSendFromISR(). The ISR priority is set to 0x10, and configMAX_SYSCALL_INTERRUPT_PRIORITY is 0x20. What is the most likely cause?

A The ISR is missing a call to <code>portYIELD_FROM_ISR()</code> after sending to the queue.

B The queue handle passed to <code>xQueueSendFromISR()</code> is NULL.

C The FreeRTOS scheduler was not started before the ISR executed.

D The ISR priority is too high (numerically lower) and violates the maximum syscall interrupt priority restriction.

💡 Hint

Check the relationship between ISR priority and configMAX_SYSCALL_INTERRUPT_PRIORITY.

Calling FreeRTOS API functions from an ISR with priority higher than configMAX_SYSCALL_INTERRUPT_PRIORITY causes crashes or undefined behavior.

📝 Syntax

advanced

Correct Priority Setting for ISR

Which of the following correctly sets an ISR priority to be safe for calling FreeRTOS API functions if configMAX_SYSCALL_INTERRUPT_PRIORITY is 0x30?

A NVIC_SetPriority(IRQn, 0x40);

B NVIC_SetPriority(IRQn, 0x00);

C NVIC_SetPriority(IRQn, 0x10);

D NVIC_SetPriority(IRQn, 0x20);

💡 Hint

Remember that lower numerical values mean higher priority.

Interrupt priorities numerically equal or higher (less urgent) than configMAX_SYSCALL_INTERRUPT_PRIORITY are safe to call FreeRTOS API functions. 0x40 is numerically higher than 0x30, so it is safe.

🚀 Application

expert

Designing ISR Priorities with FreeRTOS Restrictions

You are designing a system with three interrupts: Timer (priority 0x10), UART (priority 0x40), and ADC (priority 0x50). configMAX_SYSCALL_INTERRUPT_PRIORITY is set to 0x40. Which interrupts can safely call FreeRTOS API functions?

A Only Timer interrupt can safely call FreeRTOS API functions.

B Only UART and ADC interrupts can safely call FreeRTOS API functions.

C All three interrupts can safely call FreeRTOS API functions.

D None of the interrupts can safely call FreeRTOS API functions.

💡 Hint

Compare each interrupt priority to configMAX_SYSCALL_INTERRUPT_PRIORITY.

Only interrupts with priority numerically equal or higher (less urgent) than configMAX_SYSCALL_INTERRUPT_PRIORITY (0x40) can safely call FreeRTOS API functions. Timer (0x10) is higher priority and cannot.