configMAX_SYSCALL_INTERRUPT_PRIORITY in FreeRTOS - Step-by-Step Execution

Concept Flow - configMAX_SYSCALL_INTERRUPT_PRIORITY

Start

↓

Set configMAX_SYSCALL_INTERRUPT_PRIORITY

↓

Interrupt occurs

↓

Check interrupt priority

↓

Priority <= configMAX_SYSCALL_INTERRUPT_PRIORITY?

↓

Allow ISR to call FreeRTOS APIs

↓

ISR cannot call FreeRTOS APIs

↓

Continue execution

This flow shows how FreeRTOS uses configMAX_SYSCALL_INTERRUPT_PRIORITY to decide if an interrupt can safely call FreeRTOS functions.

Execution Sample

FreeRTOS

#define configMAX_SYSCALL_INTERRUPT_PRIORITY 5
void ISR_Handler(void) {
  if (interrupt_priority <= configMAX_SYSCALL_INTERRUPT_PRIORITY) {
    vTaskNotifyGiveFromISR(taskHandle, &xHigherPriorityTaskWoken);
  }
}

This code checks if the interrupt priority is allowed to call FreeRTOS API inside an ISR.

Execution Table

Step	Interrupt Priority	Condition (priority <= configMAX_SYSCALL_INTERRUPT_PRIORITY)	Action	Result
1	7	7 <= 5 (False)	Do NOT call FreeRTOS API	API call skipped
2	5	5 <= 5 (True)	Call FreeRTOS API from ISR	API called safely
3	3	3 <= 5 (True)	Call FreeRTOS API from ISR	API called safely
4	-	-	-	Execution continues

💡 Execution stops checking when interrupt priority is higher than configMAX_SYSCALL_INTERRUPT_PRIORITY, disallowing FreeRTOS API calls.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
interrupt_priority	-	7	5	3	-
API_call_allowed	False	False	True	True	-

Key Moments - 2 Insights

Why can't interrupts with priority higher than configMAX_SYSCALL_INTERRUPT_PRIORITY call FreeRTOS APIs?

Because FreeRTOS disables interrupts at or below configMAX_SYSCALL_INTERRUPT_PRIORITY to protect critical sections. Interrupts with higher priority run without this protection and calling FreeRTOS APIs could cause data corruption. See execution_table rows 1 and 4.

What happens if configMAX_SYSCALL_INTERRUPT_PRIORITY is set too low?

More interrupts can call FreeRTOS APIs safely, but this reduces system responsiveness because more interrupts are masked during critical sections. See execution_table rows 2 and 3.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the interrupt NOT call the FreeRTOS API?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Action' column in execution_table for step 1.

According to variable_tracker, what is the value of API_call_allowed after step 2?

A False

B Undefined

C True

D Error

💡 Hint

Look at the 'API_call_allowed' row under 'After Step 2' in variable_tracker.

If configMAX_SYSCALL_INTERRUPT_PRIORITY was set to 3, which interrupt priority from the table would be allowed to call FreeRTOS APIs?

A 5 and 7

B 3 and 5

C Only 3

D All priorities

💡 Hint

Compare interrupt priorities to configMAX_SYSCALL_INTERRUPT_PRIORITY in execution_table.

Concept Snapshot

#define configMAX_SYSCALL_INTERRUPT_PRIORITY <value>
- Sets max interrupt priority that can safely call FreeRTOS APIs
- Interrupts with priority <= this value can use FreeRTOS API in ISR
- Higher priority interrupts must NOT call FreeRTOS API
- Protects kernel critical sections from unsafe ISR calls

Full Transcript

configMAX_SYSCALL_INTERRUPT_PRIORITY is a FreeRTOS setting that controls which interrupt priorities can safely call FreeRTOS API functions from an ISR. When an interrupt occurs, its priority is compared to this setting. If the priority is less than or equal to configMAX_SYSCALL_INTERRUPT_PRIORITY, the ISR can call FreeRTOS APIs safely. If the priority is higher, the ISR must not call these APIs to avoid corrupting kernel data. This protects critical sections where FreeRTOS disables interrupts up to this priority level. Setting this value too high reduces system responsiveness, while setting it too low restricts which ISRs can interact with FreeRTOS. The execution table shows examples of priorities and whether API calls are allowed. The variable tracker shows how interrupt_priority and API_call_allowed change step by step. Understanding this helps write safe ISRs in FreeRTOS.

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