FreeRTOS interrupt priority restrictions - Step-by-Step Execution

Concept Flow - FreeRTOS interrupt priority restrictions

Interrupt occurs

↓

Check interrupt priority

↓

Call ISR

↓

Return from ISR

When an interrupt happens, FreeRTOS checks its priority. If the priority is allowed, the ISR runs normally. If too high, the ISR must follow restrictions to keep the system safe.

Execution Sample

FreeRTOS

void ISR_Handler(void) {
  if (interrupt_priority <= configMAX_SYSCALL_INTERRUPT_PRIORITY) {
    // Safe to call FreeRTOS API
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xSemaphoreGiveFromISR(&xSemaphore, &xHigherPriorityTaskWoken);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
  } else {
    // Cannot call FreeRTOS API
    // Handle interrupt without FreeRTOS calls
  }
}

This ISR checks if its priority is allowed to call FreeRTOS API functions safely, otherwise it avoids those calls.

Execution Table

Step	Interrupt Priority	Condition	Action	FreeRTOS API Call Allowed	Output
1	12	12 >= configMAX_SYSCALL_INTERRUPT_PRIORITY (10)?	No	No	Do not call FreeRTOS API
2	5	5 >= configMAX_SYSCALL_INTERRUPT_PRIORITY (10)?	Yes	Yes	Call FreeRTOS API safely
3	5	ISR calls xSemaphoreGiveFromISR	Allowed	Yes	Semaphore given, possibly yield
4	12	ISR tries to call FreeRTOS API	Not allowed	No	Undefined behavior avoided by restriction
5	-	ISR returns	-	-	Scheduler may run if needed

💡 Execution stops after ISR returns and scheduler runs if a context switch is needed.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
interrupt_priority	N/A	12	5	5	12	N/A
xHigherPriorityTaskWoken	pdFALSE	pdFALSE	pdFALSE	pdTRUE or pdFALSE	pdFALSE	N/A
FreeRTOS API call allowed	N/A	No	Yes	Yes	No	N/A

Key Moments - 3 Insights

Why can't we call FreeRTOS API functions from all interrupt priorities?

Because calling FreeRTOS API from interrupts with priority higher than configMAX_SYSCALL_INTERRUPT_PRIORITY can cause system instability. See execution_table rows 1 and 4 where API calls are disallowed.

What happens if an ISR with too high priority calls FreeRTOS API anyway?

It can cause unpredictable behavior or crashes. The code must avoid such calls as shown in execution_table row 4.

How does FreeRTOS know when to switch tasks after an ISR?

The ISR sets xHigherPriorityTaskWoken to pdTRUE if a higher priority task was unblocked, then portYIELD_FROM_ISR triggers a context switch if needed (see execution_table row 3).

Visual Quiz - 3 Questions

Look at the execution_table, at step 2, what is the interrupt priority and is FreeRTOS API call allowed?

A Priority 5, API call not allowed

B Priority 5, API call allowed

C Priority 12, API call allowed

D Priority 12, API call not allowed

💡 Hint

Check execution_table row 2 for interrupt priority and API call permission.

According to variable_tracker, what is the value of xHigherPriorityTaskWoken after step 3?

A pdFALSE

B Always pdTRUE

C pdTRUE or pdFALSE

D Undefined

💡 Hint

Look at variable_tracker row for xHigherPriorityTaskWoken after step 3.

If interrupt_priority was changed to 8, how would the FreeRTOS API call allowed column change at step 1?

A It would change to Yes

B It would be undefined

C It would still be No

D It would cause a crash

💡 Hint

Compare interrupt_priority 8 with configMAX_SYSCALL_INTERRUPT_PRIORITY 10 in execution_table step 1.

Concept Snapshot

FreeRTOS interrupt priority restrictions:
- Only interrupts with priority <= configMAX_SYSCALL_INTERRUPT_PRIORITY can call FreeRTOS API.
- Higher priority interrupts must avoid FreeRTOS API calls.
- Use xHigherPriorityTaskWoken and portYIELD_FROM_ISR to request context switch.
- Violating priority rules causes instability.
- Always check priority before calling FreeRTOS API in ISR.

Full Transcript

When an interrupt happens in FreeRTOS, the system checks its priority. If the priority is less than or equal to configMAX_SYSCALL_INTERRUPT_PRIORITY, the interrupt service routine (ISR) can safely call FreeRTOS API functions like giving semaphores or notifying tasks. This is shown in the execution table where priority 5 allows API calls. If the priority is higher, like 12, the ISR must not call FreeRTOS API functions to avoid system crashes or unpredictable behavior. The variable tracker shows how the interrupt priority and the flag xHigherPriorityTaskWoken change during execution. The key moments explain why these restrictions exist and how FreeRTOS manages task switching after interrupts. The visual quiz tests understanding of priority checks and API call permissions. Remember, always keep ISR priorities and FreeRTOS API calls aligned to keep your system stable.

Practice

1.

What does configMAX_SYSCALL_INTERRUPT_PRIORITY control in FreeRTOS?

easy

A. The maximum number of interrupts allowed

B. The lowest interrupt priority allowed in the system

C. The highest interrupt priority that can safely call FreeRTOS API functions

D. The priority of the idle task

Solution

1. Step 1: Understand FreeRTOS interrupt priority limits

   FreeRTOS restricts API calls to interrupts at or below a certain priority level to maintain system stability.

2. Step 2: Role of configMAX_SYSCALL_INTERRUPT_PRIORITY

   This configuration sets the highest interrupt priority that can safely call FreeRTOS API functions.

3. Final Answer:

   The highest interrupt priority that can safely call FreeRTOS API functions -> Option C

4. Quick Check:

   configMAX_SYSCALL_INTERRUPT_PRIORITY = highest safe ISR priority [OK]

Hint: Remember: configMAX_SYSCALL_INTERRUPT_PRIORITY limits safe ISR API calls [OK]

Common Mistakes:

• Confusing highest with lowest priority
• Thinking it limits number of interrupts
• Assuming it sets task priorities

2.

Which of the following is the correct way to define configMAX_SYSCALL_INTERRUPT_PRIORITY in FreeRTOSConfig.h?

#define configMAX_SYSCALL_INTERRUPT_PRIORITY  ( ??? )

easy

A. 0x80 // Mid-level priority

B. 0x00 // Highest priority

C. 0xFF // Lowest priority

D. "5" // Priority as string

Solution

1. Step 1: Understand priority values in FreeRTOS

   Lower numeric values represent higher priorities; 0x00 is highest, 0xFF is lowest.

2. Step 2: Correct format for configMAX_SYSCALL_INTERRUPT_PRIORITY

   This macro must be a numeric value representing the highest interrupt priority allowed to call FreeRTOS API, typically a mid-level priority like 0x80.

3. Final Answer:

   0x80 // Mid-level priority -> Option A

4. Quick Check:

   Priority value must be numeric, not string [OK]

Hint: Use numeric hex values for priorities, not strings [OK]

Common Mistakes:

• Using string instead of numeric value
• Setting priority to 0x00 (too high)
• Using lowest priority 0xFF which disables API calls

3.

Given configMAX_SYSCALL_INTERRUPT_PRIORITY is set to 0x50, which ISR priority can safely call FreeRTOS API functions?

ISR Priority Options:
A) 0x40
B) 0x50
C) 0x60
D) 0x30

medium

A. 0x40

B. 0x30

C. 0x50

D. 0x60

Solution

1. Step 1: Compare ISR priorities with configMAX_SYSCALL_INTERRUPT_PRIORITY

   Only ISRs with priority equal or numerically higher (lower urgency) than 0x50 can call FreeRTOS API.

2. Step 2: Evaluate each option

   0x40 and 0x30 are higher priority (lower number) than 0x50, so cannot call API. 0x50 equals limit, allowed. 0x60 is lower priority (higher number), also allowed.

3. Final Answer:

   0x50 and 0x60 -> Option D

4. Quick Check:

   ISR priority must be >= configMAX_SYSCALL_INTERRUPT_PRIORITY [OK]

Hint: ISR priority must be equal or lower urgency than configMAX_SYSCALL_INTERRUPT_PRIORITY [OK]

Common Mistakes:

• Confusing numeric priority order
• Allowing higher urgency ISRs to call API
• Ignoring equal priority as allowed

4.

Identify the error in this ISR code snippet if configMAX_SYSCALL_INTERRUPT_PRIORITY is set to 0x60:

void ISR_Handler(void) {
    NVIC_SetPriority(IRQn, 0x40);
    vTaskNotifyGiveFromISR(taskHandle, NULL);
}

medium

A. ISR priority 0x40 is higher than allowed to call FreeRTOS API

B. vTaskNotifyGiveFromISR cannot be called from ISR

C. NVIC_SetPriority should not be called inside ISR

D. No error, code is correct

Solution

1. Step 1: Check ISR priority against configMAX_SYSCALL_INTERRUPT_PRIORITY

   Priority 0x40 is higher urgency (lower number) than 0x60, so ISR cannot safely call FreeRTOS API.

2. Step 2: Analyze API call in ISR

   Calling vTaskNotifyGiveFromISR requires ISR priority at or below 0x60, which is violated here.

3. Final Answer:

   ISR priority 0x40 is higher than allowed to call FreeRTOS API -> Option A

4. Quick Check:

   ISR priority must not exceed configMAX_SYSCALL_INTERRUPT_PRIORITY [OK]

Hint: ISR priority must be equal or lower urgency than configMAX_SYSCALL_INTERRUPT_PRIORITY [OK]

Common Mistakes:

• Ignoring priority set inside ISR
• Thinking API calls are always allowed in ISR
• Confusing priority numeric order

5.

You have two ISRs: ISR_A with priority 0x70 and ISR_B with priority 0x40. configMAX_SYSCALL_INTERRUPT_PRIORITY is set to 0x60. Which statement is true?

hard

A. Neither ISR can call FreeRTOS API

B. ISR_A can call FreeRTOS API, ISR_B cannot

C. Both ISRs can call FreeRTOS API safely

D. ISR_B can call FreeRTOS API, ISR_A cannot

Solution

1. Step 1: Compare ISR priorities with configMAX_SYSCALL_INTERRUPT_PRIORITY

   Only ISRs with priority numerically equal or higher (lower urgency) than 0x60 can call FreeRTOS API.

2. Step 2: Evaluate ISR_A and ISR_B priorities

   ISR_A priority 0x70 is lower urgency than 0x60, so it can call API. ISR_B priority 0x40 is higher urgency, so it cannot.

3. Final Answer:

   ISR_A can call FreeRTOS API, ISR_B cannot -> Option B

4. Quick Check:

   Only ISRs with priority >= configMAX_SYSCALL_INTERRUPT_PRIORITY call API [OK]

Hint: Only ISRs with priority numerically >= configMAX_SYSCALL_INTERRUPT_PRIORITY call API [OK]

Common Mistakes:

• Assuming higher urgency ISRs can call API
• Confusing numeric priority order
• Thinking all ISRs can call API regardless of priority