configMAX_SYSCALL_INTERRUPT_PRIORITY in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the maximum syscall interrupt priority.

FreeRTOS

#define configMAX_SYSCALL_INTERRUPT_PRIORITY [1]

Drag options to blanks, or click blank then click option'

A5

B10

C3

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using a priority value too high disables interrupts from calling FreeRTOS APIs.

Confusing priority numbering where lower numbers mean higher priority.

The value 3 is commonly used to set configMAX_SYSCALL_INTERRUPT_PRIORITY to allow interrupts with priority 3 and below to use FreeRTOS API calls.

2fill in blank

medium

Complete the code to define the interrupt priority mask using configMAX_SYSCALL_INTERRUPT_PRIORITY.

FreeRTOS

#define configKERNEL_INTERRUPT_PRIORITY ([1] << (8 - configPRIO_BITS))

Drag options to blanks, or click blank then click option'

AconfigMAX_SYSCALL_INTERRUPT_PRIORITY - 1

BconfigMAX_SYSCALL_INTERRUPT_PRIORITY

CconfigMAX_SYSCALL_INTERRUPT_PRIORITY + 1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using configMAX_SYSCALL_INTERRUPT_PRIORITY instead of 0 for kernel priority.

Incorrect bit shifting causing wrong priority masking.

The kernel interrupt priority is usually set to 0 (lowest priority) shifted by the number of priority bits.

3fill in blank

hard

Fix the error in setting configMAX_SYSCALL_INTERRUPT_PRIORITY to allow safe API calls from interrupts.

FreeRTOS

#define configMAX_SYSCALL_INTERRUPT_PRIORITY [1]

Drag options to blanks, or click blank then click option'

A5

B255

C0

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Setting priority to 0 which is too high and disables API calls.

Using 255 which is too low and disables interrupt handling.

Setting configMAX_SYSCALL_INTERRUPT_PRIORITY to 5 allows interrupts with priority 5 and lower to safely call FreeRTOS API functions.

4fill in blank

hard

Fill both blanks to correctly define kernel and max syscall interrupt priorities.

FreeRTOS

#define configKERNEL_INTERRUPT_PRIORITY ([1] << (8 - configPRIO_BITS))
#define configMAX_SYSCALL_INTERRUPT_PRIORITY ([2])

Drag options to blanks, or click blank then click option'

A0

B5

C3

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping kernel and max syscall priorities.

Using too high or too low values for priorities.

The kernel interrupt priority is set to 0 (lowest priority) shifted by priority bits, and max syscall interrupt priority is set to 3 to allow safe API calls.

5fill in blank

hard

Fill all three blanks to define priorities and shift correctly for FreeRTOS interrupt configuration.

FreeRTOS

#define configKERNEL_INTERRUPT_PRIORITY ([1] << (8 - configPRIO_BITS))
#define configMAX_SYSCALL_INTERRUPT_PRIORITY [2]
#define configPRIO_BITS [3]

Drag options to blanks, or click blank then click option'

A0

B3

C4

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect configPRIO_BITS value.

Mixing up kernel and syscall priorities.

The kernel interrupt priority is 0 shifted by (8 - 4) bits, max syscall interrupt priority is 3, and configPRIO_BITS is 4 for a typical Cortex-M microcontroller.

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