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.

Practice

1. What is the main reason to keep Interrupt Service Routine (ISR) code short in embedded C?

easy

A. To use more memory for ISR variables

B. To enable ISR to call blocking functions safely

C. To allow ISR to run complex calculations

D. To avoid delaying other interrupts and keep system responsive

Solution

1. Step 1: Understand ISR impact on system

   ISRs temporarily stop main code and other interrupts, so long ISRs delay system response.

2. Step 2: Reason why short ISR is important

   Short ISR avoids blocking other interrupts and keeps system fast and responsive.

3. Final Answer:

   To avoid delaying other interrupts and keep system responsive -> Option D

4. Quick Check:

   Keep ISR short = avoid delays [OK]

Hint: Short ISR avoids blocking other interrupts [OK]

Common Mistakes:

• Thinking ISR can run long calculations
• Using blocking calls inside ISR
• Assuming ISR uses more memory for variables

2. Which of the following is the correct way to signal the main program from an ISR?

easy

A. Call a heavy processing function directly inside ISR

B. Print debug messages inside ISR

C. Set a flag variable that main code checks

D. Use a blocking delay inside ISR

Solution

1. Step 1: Identify safe communication from ISR

   Heavy processing or blocking calls inside ISR are unsafe; printing can cause delays.

2. Step 2: Use flag to communicate

   Setting a flag variable is fast and lets main code handle processing safely.

3. Final Answer:

   Set a flag variable that main code checks -> Option C

4. Quick Check:

   Use flags to signal main code [OK]

Hint: Use flags, not heavy calls, inside ISR [OK]

Common Mistakes:

• Calling complex functions inside ISR
• Using delays or blocking calls in ISR
• Printing inside ISR causing slowdowns

3. Given this ISR code snippet, what will be the value of flag after the interrupt triggers once?

volatile int flag = 0;
void ISR() {
  flag = 1;
}

medium

A. 1

B. 0

C. Undefined

D. ISR causes error

Solution

1. Step 1: Understand ISR effect on flag

   ISR sets flag to 1 when triggered.

2. Step 2: Check initial and final flag value

   Initially flag is 0, after ISR runs once, flag becomes 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   ISR sets flag = 1 [OK]

Hint: ISR sets flag to 1 on trigger [OK]

Common Mistakes:

• Assuming flag stays 0
• Thinking ISR causes error
• Confusing volatile keyword effect

4. Identify the problem in this ISR code:

void ISR() {
  while(!data_ready) {
    // wait for data
  }
  process_data();
}

medium

A. Not setting a flag variable

B. Using a blocking loop inside ISR

C. ISR missing return statement

D. Using volatile variable incorrectly

Solution

1. Step 1: Analyze ISR code behavior

   ISR contains a while loop waiting for data_ready, which blocks ISR execution.

2. Step 2: Understand why blocking is bad in ISR

   Blocking inside ISR delays other interrupts and main code, causing system issues.

3. Final Answer:

   Using a blocking loop inside ISR -> Option B

4. Quick Check:

   Blocking loops inside ISR cause problems [OK]

Hint: Avoid loops that wait inside ISR [OK]

Common Mistakes:

• Ignoring blocking loops in ISR
• Thinking ISR needs return statement
• Confusing volatile usage

5. You want to handle a sensor interrupt by reading data and processing it. Which is the best approach following ISR best practices?

Options:
A) Read and process data fully inside ISR
B) Use delay inside ISR to wait for sensor
C) Set a flag in ISR and process data in main loop
D) Print sensor data inside ISR for debugging

hard

A. Set a flag in ISR and process data in main loop

B. Use delay inside ISR to wait for sensor

C. Read and process data fully inside ISR

D. Print sensor data inside ISR for debugging

Solution

1. Step 1: Evaluate ISR workload options

   Processing data fully or using delay inside ISR blocks system; printing slows ISR.

2. Step 2: Choose safe communication method

   Setting a flag in ISR and processing in main loop keeps ISR short and system responsive.

3. Final Answer:

   Set a flag in ISR and process data in main loop -> Option A

4. Quick Check:

   Flag in ISR, process later = best practice [OK]

Hint: Use flags in ISR, process outside [OK]

Common Mistakes:

• Doing heavy work inside ISR
• Using delays or prints in ISR
• Ignoring main loop processing