ISR-safe API functions (FromISR suffix) in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'FromISR' suffix in FreeRTOS API functions indicate?

It indicates that the function is safe to call from an Interrupt Service Routine (ISR). These functions are designed to be used in interrupt context without causing issues like blocking or priority inversion.

beginner

Why can't regular FreeRTOS API functions be called directly from an ISR?

Regular FreeRTOS API functions may block or use operations that are not safe in interrupt context, such as waiting for a resource. ISRs must be fast and non-blocking, so special 'FromISR' versions are provided to be safe in ISRs.

intermediate

How do 'FromISR' functions handle task switching differently?

They use a pointer parameter to indicate if a context switch should happen after the ISR completes. This allows the kernel to defer the switch safely until after the interrupt, maintaining system stability.

beginner

Give an example of a FreeRTOS API function and its ISR-safe counterpart.

Example: xQueueSend() is the regular function to send to a queue. Its ISR-safe counterpart is xQueueSendFromISR(), which can be safely called inside an ISR.

intermediate

What is the purpose of the 'pxHigherPriorityTaskWoken' parameter in 'FromISR' functions?

It is a pointer to a variable that the function sets to pdTRUE if calling the function caused a higher priority task to become ready to run. This informs the ISR that a context switch should be requested after the ISR finishes.

What is the main reason to use 'FromISR' functions in FreeRTOS?

A They are safe to call inside Interrupt Service Routines.

B They run faster than regular functions.

C They allocate more memory.

D They block until a resource is available.

FromISR functions are specifically designed to be safe when called inside ISRs, avoiding blocking and unsafe operations.

Which parameter in 'FromISR' functions helps decide if a context switch is needed?

A pxHigherPriorityTaskWoken

B xQueueHandle

C xTicksToWait

D pvParameters

The pxHigherPriorityTaskWoken pointer is set by the function to indicate if a higher priority task was woken.

Can you call xQueueSend() directly inside an ISR?

A Yes, always.

B Only if interrupts are disabled.

C No, use xQueueSendFromISR() instead.

D Only if the queue is empty.

xQueueSend() is not safe in ISRs; the FromISR version must be used.

What happens if a 'FromISR' function sets pxHigherPriorityTaskWoken to pdTRUE?

A Nothing happens.

B The ISR must request a context switch after it finishes.

C The ISR disables interrupts.

D The system crashes.

Setting pxHigherPriorityTaskWoken to pdTRUE signals that a higher priority task is ready and a context switch should occur.

Which of these is NOT true about 'FromISR' functions?

A They never block.

B They can be called from normal tasks.

C They use special parameters for context switching.

D They are unsafe in ISRs.

'FromISR' functions are specifically designed to be safe in ISRs.

Explain why FreeRTOS provides 'FromISR' versions of API functions and how they differ from regular API calls.

Think about what makes interrupt routines special and how these functions avoid problems.

Describe the role of the pxHigherPriorityTaskWoken parameter in ISR-safe FreeRTOS API functions.

Consider how FreeRTOS decides when to switch tasks after an interrupt.