Interrupt priority levels in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this interrupt priority code?

Consider the following embedded C code snippet that sets interrupt priorities and triggers interrupts. What will be the printed output?

Embedded C

#include <stdio.h>

volatile int interrupt_flag = 0;

void ISR_high_priority() {
    interrupt_flag = 1;
    printf("High priority interrupt handled\n");
}

void ISR_low_priority() {
    interrupt_flag = 2;
    printf("Low priority interrupt handled\n");
}

int main() {
    int priority_high = 1; // Higher priority
    int priority_low = 2;  // Lower priority

    // Simulate interrupts
    if (priority_low > priority_high) {
        ISR_high_priority();
        ISR_low_priority();
    } else {
        ISR_low_priority();
        ISR_high_priority();
    }

    printf("Interrupt flag value: %d\n", interrupt_flag);
    return 0;
}

A

High priority interrupt handled
Low priority interrupt handled
Interrupt flag value: 2

B

Low priority interrupt handled
High priority interrupt handled
Interrupt flag value: 1

C

High priority interrupt handled
Interrupt flag value: 1

D

Low priority interrupt handled
Interrupt flag value: 2

💡 Hint

Think about the order in which the interrupt service routines are called and how the interrupt_flag variable changes.

The code checks if the low priority number is greater than the high priority number. Since 2 > 1, it calls the high priority ISR first, then the low priority ISR. Each ISR sets the interrupt_flag. The last ISR called is the low priority one, so the flag ends with value 2.

🧠 Conceptual

intermediate

Which statement correctly describes interrupt priority levels?

In embedded systems, what does a lower numerical interrupt priority level usually mean?

A It means the interrupt priority is dynamic and changes at runtime.

B It means the interrupt has a lower priority and will be serviced last.

C It means the interrupt is disabled and will never be serviced.

D It means the interrupt has a higher priority and will be serviced first.

💡 Hint

Think about how priority numbers are assigned in many microcontrollers.

In many embedded systems, a lower numerical value for interrupt priority means higher priority. For example, priority level 0 is often the highest priority.

🔧 Debug

advanced

What error does this interrupt priority code cause?

Examine the following code snippet. What error will occur when compiling or running it?

Embedded C

void ISR() {
    // Interrupt service routine
    int priority = 1;
    if(priority < 0) {
        printf("Invalid priority\n");
    }
}

int main() {
    ISR();
    return 0;
}

A Compilation error: missing #include <stdio.h>.

B Compilation error: 'printf' not declared.

C Runtime error: priority variable uninitialized.

D No error, code runs and prints nothing.

💡 Hint

Check if all required headers are included for used functions.

The code uses printf but does not include , so the compiler will raise an error about printf being undeclared.

📝 Syntax

advanced

Which option correctly sets interrupt priority in this embedded C code?

Given the following code snippet, which option correctly sets the interrupt priority to 3?

Embedded C

void set_interrupt_priority(int priority);

int main() {
    // Set priority here
    
    return 0;
}

A set_interrupt_priority = 3;

B set_interrupt_priority(3);

C set_interrupt_priority( priority = 3 );

D set_interrupt_priority[3];

💡 Hint

Remember how to call functions with arguments in C.

Option B correctly calls the function with argument 3. Option B tries to assign a value to a function name, which is invalid. Option B uses an assignment inside the argument which is unnecessary and confusing. Option B uses brackets which is invalid syntax for function calls.

🚀 Application

expert

How many interrupts will be serviced given these priorities and mask settings?

In an embedded system, three interrupts have priorities 1, 2, and 3 (1 is highest). The system mask level is set to 2, which blocks interrupts with priority 2 and lower. How many interrupts will be serviced?

A No interrupts will be serviced.

B Interrupts with priorities 1 and 2 will be serviced.

C Only the interrupt with priority 1 will be serviced.

D All three interrupts will be serviced.

💡 Hint

Consider which priorities are blocked by the mask level.

The mask level blocks interrupts with priority 2 and lower (meaning priority 2 and 3). Only priority 1 is higher than the mask and will be serviced.