configMAX_SYSCALL_INTERRUPT_PRIORITY in FreeRTOS - Mini Project: Build & Apply

Understanding configMAX_SYSCALL_INTERRUPT_PRIORITY in FreeRTOS

📖 Scenario: You are working on a FreeRTOS-based embedded system. You need to configure interrupt priorities correctly to ensure that FreeRTOS API functions are called safely from interrupts.

🎯 Goal: Learn how to set configMAX_SYSCALL_INTERRUPT_PRIORITY and use it to protect critical sections in interrupt handlers.

📋 What You'll Learn

Create a macro configMAX_SYSCALL_INTERRUPT_PRIORITY with the value 5

Create a variable current_priority to simulate the current interrupt priority

Write a function can_call_freertos_api() that returns 1 if current_priority is equal or higher (numerically greater or equal) than configMAX_SYSCALL_INTERRUPT_PRIORITY, else 0

Print the result of can_call_freertos_api() for current_priority values 3 and 6

💡 Why This Matters

🌍 Real World

In embedded systems, setting <code>configMAX_SYSCALL_INTERRUPT_PRIORITY</code> correctly ensures that interrupts with higher priority do not call FreeRTOS API functions, preventing system crashes.

💼 Career

Embedded software engineers must configure interrupt priorities properly to write safe and reliable real-time applications using FreeRTOS.

1

Define configMAX_SYSCALL_INTERRUPT_PRIORITY

Define a macro called configMAX_SYSCALL_INTERRUPT_PRIORITY and set it to 5.

FreeRTOS

# Define configMAX_SYSCALL_INTERRUPT_PRIORITY as 5
# Your code here

Hint

This macro sets the highest interrupt priority from which FreeRTOS API calls can be made.

2

Create current_priority variable

Create an integer variable called current_priority and set it to 0.

FreeRTOS

#define configMAX_SYSCALL_INTERRUPT_PRIORITY 5

// Create current_priority variable and set to 0
# Your code here

Hint

This variable will simulate the priority of the current interrupt.

3

Write can_call_freertos_api() function

Write a function called can_call_freertos_api() that returns 1 if current_priority is greater than or equal to configMAX_SYSCALL_INTERRUPT_PRIORITY, otherwise returns 0.

FreeRTOS

#define configMAX_SYSCALL_INTERRUPT_PRIORITY 5

int current_priority = 0;

// Write can_call_freertos_api() function below
# Your code here

Hint

Use a simple if-else to compare current_priority with configMAX_SYSCALL_INTERRUPT_PRIORITY.

4

Test can_call_freertos_api() with different priorities

Set current_priority to 3 and print the result of can_call_freertos_api(). Then set current_priority to 6 and print the result again.

FreeRTOS

#define configMAX_SYSCALL_INTERRUPT_PRIORITY 5

int current_priority = 0;

int can_call_freertos_api() {
    if (current_priority >= configMAX_SYSCALL_INTERRUPT_PRIORITY) {
        return 1;
    } else {
        return 0;
    }
}

// Set current_priority to 3 and print can_call_freertos_api()
// Set current_priority to 6 and print can_call_freertos_api()
# Your code here

Hint

Remember to include stdio.h and use printf to show the results.

Practice

1.

What is the main purpose of configMAX_SYSCALL_INTERRUPT_PRIORITY in FreeRTOS?

easy

A. To set the highest interrupt priority that can safely call FreeRTOS API functions

B. To define the lowest priority for all interrupts

C. To disable all interrupts during task switching

D. To set the priority of the idle task

Solution

1. Step 1: Understand the role of interrupt priorities in FreeRTOS

   FreeRTOS requires controlling which interrupts can call its API to maintain system stability.

2. Step 2: Identify what configMAX_SYSCALL_INTERRUPT_PRIORITY controls

   This setting defines the highest priority an interrupt can have to safely call FreeRTOS API functions.

3. Final Answer:

   To set the highest interrupt priority that can safely call FreeRTOS API functions -> Option A

4. Quick Check:

   configMAX_SYSCALL_INTERRUPT_PRIORITY = highest safe interrupt priority [OK]

Hint: It limits which interrupts can use FreeRTOS API safely [OK]

Common Mistakes:

• Confusing it with lowest interrupt priority
• Thinking it disables interrupts
• Assuming it sets task priorities

2.

Which of the following is the correct way to define configMAX_SYSCALL_INTERRUPT_PRIORITY in FreeRTOSConfig.h for Cortex-M with 4 priority bits?

#define configMAX_SYSCALL_INTERRUPT_PRIORITY  ( ??? )

easy

A. 0x10

B. 0xF0

C. 0x01

D. 0xFF

Solution

1. Step 1: Understand Cortex-M priority encoding

   With 4 priority bits, priorities range from 0x00 (highest) to 0xF0 (lowest) in 8-bit registers.

2. Step 2: Identify correct value for configMAX_SYSCALL_INTERRUPT_PRIORITY

   This value must be set to a priority level that masks interrupts above it; commonly 0xF0 is used to allow lower priority interrupts to call FreeRTOS API.

3. Final Answer:

   0xF0 -> Option B

4. Quick Check:

   4 bits priority max syscall = 0xF0 [OK]

Hint: Use 0xF0 for 4-bit priority Cortex-M systems [OK]

Common Mistakes:

• Using 0xFF which is invalid for 4 bits
• Using 0x10 which is too high priority
• Confusing priority bits with interrupt numbers

3.

Given the following FreeRTOS configuration snippet for a Cortex-M processor with 3 priority bits:

#define configMAX_SYSCALL_INTERRUPT_PRIORITY 0x20

void ISR_Handler(void) {
    /* This interrupt calls FreeRTOS API */
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xSemaphoreGiveFromISR(&xSemaphore, &xHigherPriorityTaskWoken);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

What will happen if this ISR runs at priority 0x10?

medium

A. System may crash or behave unpredictably due to priority violation

B. ISR runs normally and FreeRTOS API calls succeed

C. ISR is masked and never runs

D. FreeRTOS disables all interrupts during ISR

Solution

1. Step 1: Compare ISR priority with configMAX_SYSCALL_INTERRUPT_PRIORITY

   ISR priority 0x10 is higher (numerically lower) than 0x20, which is the max allowed to call FreeRTOS API.

2. Step 2: Understand consequences of calling API at too high priority

   Calling FreeRTOS API from an interrupt with priority above configMAX_SYSCALL_INTERRUPT_PRIORITY can cause system instability or crashes.

3. Final Answer:

   System may crash or behave unpredictably due to priority violation -> Option A

4. Quick Check:

   ISR priority < max syscall priority = crash risk [OK]

Hint: ISR priority must be equal or lower than max syscall priority [OK]

Common Mistakes:

• Assuming ISR always runs safely
• Confusing higher priority with higher numeric value
• Thinking ISR is masked automatically

4.

In a FreeRTOS project, the system crashes intermittently. The configMAX_SYSCALL_INTERRUPT_PRIORITY is set to 0x40. An ISR running at priority 0x20 calls FreeRTOS API functions. What is the likely cause and how to fix it?

medium

A. ISR should not call FreeRTOS API; remove API calls from ISR

B. configMAX_SYSCALL_INTERRUPT_PRIORITY is too low; increase it to 0x10

C. ISR priority is too high; lower ISR priority to 0x40 or higher

D. No issue; crash caused by unrelated code

Solution

1. Step 1: Analyze priority levels

   ISR priority 0x20 is higher (numerically lower) than configMAX_SYSCALL_INTERRUPT_PRIORITY 0x40, violating the allowed priority range for API calls.

2. Step 2: Fix by adjusting ISR priority

   Lower ISR priority number to 0x40 or numerically higher (meaning lower priority) so it complies with configMAX_SYSCALL_INTERRUPT_PRIORITY.

3. Final Answer:

   ISR priority is too high; lower ISR priority to 0x40 or higher -> Option C

4. Quick Check:

   ISR priority must be >= configMAX_SYSCALL_INTERRUPT_PRIORITY [OK]

Hint: ISR priority must not exceed max syscall priority [OK]

Common Mistakes:

• Increasing max syscall priority instead of lowering ISR priority
• Removing API calls unnecessarily
• Ignoring priority number direction (lower number = higher priority)

5.

You have a FreeRTOS system on a Cortex-M7 with 5 priority bits. You want to allow interrupts with priority 16 and lower (numerically higher) to call FreeRTOS API safely. What should configMAX_SYSCALL_INTERRUPT_PRIORITY be set to?

Note: Priority values are shifted left to fill the 8-bit priority register.

hard

A. 0x40

B. 0x10

C. 0x20

D. 0x80

Solution

1. Step 1: Calculate shifted priority value

   With 5 priority bits, priorities are left-shifted by (8 - 5) = 3 bits. Priority 16 decimal = 0x10 hex. Shift left by 3 bits: 0x10 << 3 = 0x80.

2. Step 2: Match shifted value to options

   0x80 matches 0x80, confirming the correct shifted priority value.

3. Final Answer:

   0x80 -> Option D

4. Quick Check:

   Priority 16 << 3 bits = 0x80 [OK]

Hint: Shift priority left by (8 - priority bits) to get config value [OK]

Common Mistakes:

• Not shifting priority bits correctly
• Confusing decimal and hex values
• Choosing unshifted priority values