Understanding Interrupts and Real-Time Execution in Microcontrollers

In real-time embedded systems, the ability of a microcontroller to respond to both internal and external events promptly and efficiently is critical. Interrupts provide a mechanism for microcontrollers to respond immediately to important signals, while real-time execution ensures that operations occur within predictable timing constraints. Without effective interrupt handling, systems would waste time continuously polling for events, leading to inefficiencies and missed deadlines in time-critical applications.

This post explores:

The types of interrupts in microcontrollers.
Interrupt handling mechanisms and how they work.
Priority management for nested and simultaneous interrupts.
The design principles of real-time systems, including RTOS integration.
Advanced techniques for optimizing real-time performance.

1. What Are Interrupts?

An interrupt is a signal that temporarily halts the normal execution flow of a program, allowing the microcontroller to respond to an event before resuming previous operations. Interrupts ensure real-time responsiveness without the need for continuous polling, making them indispensable in applications such as medical devices, automotive systems, and industrial control.

1.1 Why Interrupts Matter

Enable real-time responsiveness.
Improve power efficiency by allowing the CPU to sleep until an event occurs.
Allow the CPU to handle multiple tasks asynchronously.
Enhance system reliability by promptly responding to critical events.
Essential for low-latency communication in networking and control systems.

1.2 Interrupt Characteristics

Asynchronous Nature: Can occur at any time during program execution.
Edge vs. Level Triggering: Defines whether an interrupt triggers on a signal change (edge) or a sustained signal level.
Maskable vs. Non-Maskable Interrupts: Some interrupts can be disabled (maskable), while others cannot (non-maskable), ensuring critical events always receive attention.

2. Types of Interrupts in Microcontrollers

2.1 External Interrupts

Triggered by external events such as:

Button presses
Sensor signals
Communication requests

Example (C code for AVR):

ISR(INT0_vect) {
    // Handle external interrupt on INT0 pin
}

2.2 Internal Interrupts

Triggered by internal microcontroller events, including:

Timer overflows
ADC conversions
UART data received

Example (Timer Interrupt in AVR):

ISR(TIMER1_OVF_vect) {
    // Handle Timer1 overflow interrupt
}

2.3 Software Interrupts

Triggered programmatically, often used for:

Context switching in RTOS
Debugging and exception handling

Example:

__asm("SWI #0");  // Software interrupt

2.4 Watchdog Timer Interrupts

Ensure system reliability by resetting the microcontroller if software fails to operate as expected.
Useful in safety-critical applications where continuous operation is essential.

2.5 Peripheral Interrupts

Triggered by peripheral events such as SPI transfers, I2C communications, and PWM updates.

3. Interrupt Handling Mechanism

3.1 Interrupt Service Routine (ISR)

The ISR is a special function executed when an interrupt occurs. It should be concise and fast to minimize latency.

3.2 Interrupt Vector Table (IVT)

The IVT holds the addresses of all ISRs. When an interrupt occurs, the microcontroller references the IVT to determine the appropriate ISR to execute.

3.3 Steps in Interrupt Handling

Interrupt Request (IRQ): Event triggers an interrupt request.
Context Saving: CPU saves current register values.
ISR Execution: The relevant ISR is executed.
Context Restoration: CPU restores saved registers.
Resume Execution: Program resumes from where it was interrupted.

3.4 Best Practices for Writing ISRs

Keep ISRs short and efficient.
Avoid complex logic and long loops.
Use volatile variables for shared data.
Prioritize critical operations within ISRs.

4. Interrupt Priority and Nesting

4.1 Priority Levels

Fixed Priority: Each interrupt has a predefined priority level.
Configurable Priority: Developers can set priority levels based on application needs.

4.2 Nested Interrupts

Higher-priority interrupts can preempt lower-priority ISRs.
Requires careful design to prevent stack overflows and resource conflicts.

Example (Nested Interrupt Handling in ARM Cortex-M):

NVIC_SetPriority(TIM2_IRQn, 1);
NVIC_SetPriority(USART1_IRQn, 0); // Higher priority

4.3 Handling Priority Inversion

Priority Inversion occurs when a lower-priority task holds a resource needed by a higher-priority task.
Solutions: Priority inheritance protocols or avoiding long critical sections.

5. Real-Time Execution in Microcontrollers

5.1 What is Real-Time Execution?

Real-time execution means that the microcontroller completes tasks within defined timing constraints. Systems can be:

Hard Real-Time: Missing a deadline causes system failure.
Soft Real-Time: Occasional deadline misses are tolerable.

5.2 Designing Real-Time Systems

Deterministic Behavior: Ensure that execution times are predictable.
Minimal Latency: Reduce the delay between event occurrence and ISR execution.
Efficient Context Switching: Optimize ISR length and complexity.
Task Prioritization: Use priority-based scheduling to meet deadlines.

5.3 Real-Time Operating Systems (RTOS)

An RTOS provides:

Task scheduling based on priority.
Preemptive multitasking for responsive execution.
Inter-task communication and synchronization.

Example (FreeRTOS Task Creation):

xTaskCreate(Task1, "Task1", 128, NULL, 1, NULL);
xTaskCreate(Task2, "Task2", 128, NULL, 2, NULL);
vTaskStartScheduler();

5.4 Real-Time Constraints

Jitter: Variability in task execution times.
Deadline: Time by which a task must complete.
Response Time: Time taken from event occurrence to task execution.

6. Advanced Techniques for Real-Time Performance

6.1 Interrupt Latency Optimization

Use fast ISRs to minimize latency.
Prioritize critical interrupts.
Optimize context saving and restoration routines.

6.2 Critical Section Management

Temporarily disable interrupts during critical operations.
Ensure atomicity using cli() and sei() in AVR.