Why Writing an ISR (Interrupt Service Routine) in Embedded C? - Purpose & Use Cases

The Big Idea

What if your program could jump into action the instant something important happens, without wasting a single moment?

The Scenario

Imagine you have a device that needs to react immediately when a button is pressed or a sensor triggers. Without interrupts, your program must constantly check (poll) the button or sensor in a loop, wasting time and missing quick events.

The Problem

Polling is slow and inefficient. Your program can miss events if it's busy doing other tasks. It also wastes power and CPU time checking repeatedly, even when nothing happens.

The Solution

An Interrupt Service Routine (ISR) lets your device instantly respond to events. When the event happens, the ISR runs automatically, handling it right away without waiting or wasting time.

Before vs After

✗ Before

while(1) { if(button_pressed()) { handle_button(); } }

✓ After

void ISR_button(void) { handle_button(); } // runs automatically on button press

What It Enables

ISRs enable your device to react instantly and efficiently to real-world events, making programs faster and more responsive.

Real Life Example

Think of a smoke detector that must sound an alarm immediately when smoke is detected. An ISR lets the detector respond right away, even if it was doing something else.

Key Takeaways

Polling wastes time and can miss events.

ISRs run automatically on events, improving speed and efficiency.

Using ISRs makes embedded devices responsive and reliable.