ISR best practices (keep it short) in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does ISR stand for in embedded programming?

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

beginner

Why should ISRs be kept short and fast?

Because ISRs block other interrupts and normal code, keeping them short avoids delays and keeps the system responsive.

beginner

Name one thing you should avoid doing inside an ISR.

Avoid long or blocking operations like waiting, heavy calculations, or calling slow functions inside an ISR.

intermediate

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

Use volatile variables and disable interrupts briefly when accessing shared data to avoid conflicts.

intermediate

What is a common technique to handle complex processing triggered by an ISR?

Set a flag or buffer data in the ISR, then process it later in the main loop outside the ISR.

What is the main goal when writing an ISR?

A Disable all interrupts permanently

B Include all processing inside the ISR

C Make it as short and fast as possible

D Use blocking calls to wait for events

ISRs should be short and fast to avoid blocking other interrupts and keep the system responsive.

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

A volatile

B static

C const

D register

The volatile keyword tells the compiler the variable can change unexpectedly, such as in an ISR.

What should you avoid doing inside an ISR?

A Calling a slow function

B Setting a flag

C Reading a hardware register

D Clearing the interrupt flag

Calling slow functions inside an ISR can delay other interrupts and hurt system performance.

How can you safely share data between ISR and main code?

A Use global variables without precautions

B Use local variables only

C Use dynamic memory allocation

D Use volatile variables and disable interrupts briefly when accessing

Volatile variables and briefly disabling interrupts prevent data corruption during access.

What is a good way to handle complex tasks triggered by an interrupt?

A Ignore the interrupt

B Set a flag in ISR and process later in main loop

C Do all processing inside the ISR

D Disable interrupts permanently

Setting a flag in the ISR and processing later keeps the ISR short and system responsive.

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

Think about how interrupts affect the whole system.

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

Consider how the compiler and CPU handle variable changes.