ISR best practices in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does ISR stand for in Arduino programming?

ISR stands for Interrupt Service Routine. It is a special function that runs automatically when an interrupt occurs.

beginner

Why should ISRs be kept short and fast?

ISRs should be short and fast because they pause the main program. Long ISRs can cause delays and missed interrupts, making the program less responsive.

intermediate

Can you use functions like delay() inside an ISR? Why or why not?

No, you should not use delay() inside an ISR because delay() relies on interrupts to count time. Since interrupts are disabled during an ISR, delay() will not work and can cause the program to freeze.

intermediate

How should variables shared between an ISR and the main program be handled?

Variables shared between ISR and main code should be declared as volatile. This tells the compiler the variable can change anytime, preventing wrong optimizations.

beginner

What is the best way to communicate data from an ISR to the main program?

The best way is to set a flag or update a simple variable inside the ISR, then check and handle it in the main loop. Avoid complex processing inside the ISR.

What happens to the main program when an ISR runs?

A It pauses until the ISR finishes

B It continues running normally

C It restarts from the beginning

D It skips the ISR

When an ISR runs, the main program pauses until the ISR completes.

Which keyword should you use for variables shared between ISR and main code?

A static

B volatile

C const

D register

The volatile keyword tells the compiler the variable can change unexpectedly, which is important for ISR shared variables.

Which of these is safe to do inside an ISR?

A Set a flag variable

B Print to Serial

C Use delay() to wait

D Run a long loop

Setting a simple flag variable is safe and recommended inside an ISR.

Why should ISRs avoid long or complex code?

A Because ISRs run slower than main code

B Because ISRs only run once

C Because ISRs cannot access variables

D Because long ISRs block other interrupts and delay the main program

Long ISRs block other interrupts and delay the main program, reducing responsiveness.

What happens if you use delay() inside an ISR?

A The delay works normally

B The program crashes immediately

C The delay does not work and can freeze the program

D The ISR runs faster

delay() relies on interrupts, which are disabled during an ISR, so it will not work and can freeze the program.

Explain why ISRs should be short and what you should avoid doing inside them.

Think about how ISRs affect the main program's flow and timing.

Describe how to safely share data between an ISR and the main Arduino program.

Focus on variable declaration and communication method.