Writing an ISR (Interrupt Service Routine) in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an ISR (Interrupt Service Routine)?

An ISR is a special function in embedded systems that runs automatically when a specific hardware interrupt occurs. It handles urgent tasks immediately.

beginner

Why should an ISR be kept short and fast?

Because ISRs pause the main program, keeping them short avoids delaying other important tasks and prevents missing other interrupts.

intermediate

How do you declare an ISR in embedded C?

You declare an ISR using a special keyword or attribute depending on the compiler, for example:

void __interrupt() ISR_Name(void) { /* code */ }

intermediate

What should you avoid doing inside an ISR?

Avoid long loops, blocking calls, or heavy computations inside an ISR because they delay the main program and other interrupts.

intermediate

How do you clear an interrupt flag inside an ISR?

You clear the interrupt flag by writing to a specific hardware register or flag bit to tell the system the interrupt was handled.

What triggers an ISR in embedded systems?

A A hardware interrupt signal

B A function call from main()

C A timer delay

D A variable assignment

An ISR runs automatically when a hardware interrupt signal occurs.

Which of the following is best to do inside an ISR?

A Run a long calculation

B Set a flag variable

C Wait for user input

D Print to console

Setting a flag is quick and lets the main program handle the rest.

What happens if an ISR takes too long to execute?

A The ISR runs twice

B The program runs faster

C Other interrupts may be missed

D The main program ignores the ISR

Long ISRs can block other interrupts, causing missed events.

How do you typically clear an interrupt inside an ISR?

A By writing to an interrupt flag register

B By resetting the microcontroller

C By calling main() again

D By turning off the power

Clearing the interrupt flag tells the system the interrupt was handled.

Which keyword or attribute is commonly used to define an ISR in embedded C?

A static

B main

C volatile

D __interrupt

The __interrupt keyword marks a function as an ISR.

Explain what an ISR is and why it is important in embedded systems.

Think about how devices react immediately to events.

Describe best practices when writing an ISR in embedded C.

Focus on speed and minimal work inside the ISR.