[Day 8] Interrupts – Stop Polling, Start Responding

In previous lessons, we used polling to check conditions—like whether a button is pressed or a timer has elapsed. But polling wastes CPU time and power.

That’s where Interrupts shine:
🧠 They let your microcontroller react instantly to events — without constantly checking.

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🧠 What is an Interrupt?

An Interrupt is a hardware-triggered signal that pauses your main program and immediately runs a special function called an Interrupt Service Routine (ISR).

After the ISR finishes, your main code resumes as if nothing happened.

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🖱️ Real-Life Analogy

Polling:
👀 You constantly check your phone for messages.

Interrupts:
📩 Your phone notifies you when a message arrives — and you respond immediately.

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🔁 Polling vs Interrupt Example

❌ Polling (inefficient):

while (1) {
    if (BUTTON_PIN & (1 << 2)) {
        toggle_led();
    }
}

✅ Interrupt-Based (efficient):

void button_isr() {
    toggle_led();  // Respond only when pressed
}

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🛠️ How Interrupts Work (Basic Flow)

1. External event occurs (e.g., button press)
   
2. MCU detects the signal on a configured interrupt pin
   
3. It pauses the current task
   
4. Executes the ISR
   
5. Returns to the main code
   

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🧪 Register-Level Example (Simplified for STM32/AVR-like MCU)

// Pseudocode
#define INTERRUPT_ENABLE_REG (*((volatile uint32_t*)0x40000010))
#define ISR_VECTOR_TABLE     (*((void (**)())0x00000000))

void init_interrupts() {
    INTERRUPT_ENABLE_REG |= (1 << 2);  // Enable interrupt for button pin
    ISR_VECTOR_TABLE[2] = &button_isr; // Point vector table to our ISR
}

void button_isr() {
    toggle_led();  // This is your response
}

📌 Actual syntax will vary by microcontroller vendor.

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🧩 Types of Interrupts

Type	Example
External	GPIO button, sensor pulse
Timer	Delayed events, PWM refresh
UART/Serial	Data received
ADC Conversion	Analog read completed

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⚠️ Tips for Writing ISRs

• Keep them short and fast
  
• Don’t use delay() or heavy logic
  
• Use flags to signal the main loop
  
• Avoid using malloc(), printf(), or any blocking function
  

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🧠 Common Pattern: Flag Set in ISR

volatile bool button_pressed = false;

void button_isr() {
    button_pressed = true;
}

int main() {
    while (1) {
        if (button_pressed) {
            button_pressed = false;
            toggle_led();
        }
    }
}

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💬 Pro Tip

Every interrupt has a priority. Some are more critical (e.g., system faults, timers). Learn how to configure interrupt priority levels for real-time systems.

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🧰 Debugging Interrupts

• Use LED toggles inside ISRs for quick visual checks.
  
• Use a logic analyzer or breakpoints to trace ISR triggers.
  
• Make sure to clear the interrupt flag after handling.
  

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🔍 Up Next (Day 9):

🧱 Digital Input – Reading Sensors, Buttons, and Switches
How do you reliably read buttons, avoid bouncing, and handle digital inputs with confidence?