ISR best practices in Arduino - Step-by-Step Execution

Concept Flow - ISR best practices in Arduino

Interrupt Occurs

↓

CPU Pauses Main Code

↓

Jump to ISR Function

↓

Execute ISR Quickly

↓

Clear Interrupt Flag

↓

Return to Main Code

↓

Resume Main Code Execution

When an interrupt happens, the main program pauses, runs the ISR quickly, then resumes the main code.

Execution Sample

Arduino

volatile int count = 0;

void ISR() {
  count++;
}

void setup() {
  attachInterrupt(digitalPinToInterrupt(2), ISR, RISING);
}

void loop() {
  // main code here
}

This code sets up an interrupt on pin 2 that increases count each time the pin signal rises.

Execution Table

Step	Event	Main Code State	ISR Action	Variable Changes	Return to Main Code
1	Main code running	count=0	No ISR	count=0	Continue main code
2	Interrupt on pin 2 rising	count=0	ISR called	count=1	Resume main code
3	Main code running	count=1	No ISR	count=1	Continue main code
4	Interrupt on pin 2 rising	count=1	ISR called	count=2	Resume main code
5	Main code running	count=2	No ISR	count=2	Continue main code
Exit	No more interrupts	count=2	No ISR	count=2	Program runs normally

💡 No more interrupts occur, main code runs continuously.

Variable Tracker

Variable	Start	After 1	After 2	Final
count	0	1	2	2

Key Moments - 3 Insights

Why must ISR functions be very short and fast?

Because during ISR execution, the main program is paused (see execution_table steps 2 and 4). Long ISRs delay the main code and can cause missed interrupts.

Why is the variable 'count' declared as volatile?

Volatile tells the compiler the variable can change anytime (like in ISR). Without volatile, the main code might not see updates made inside ISR (see variable_tracker).

Can we use delay() or Serial.print() inside ISR?

No, these functions are slow and can cause problems inside ISR. ISR should only do quick, simple tasks (see concept_flow).

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the value of 'count' after ISR runs?

A 1

B 0

C 2

D Undefined

💡 Hint

Check the 'Variable Changes' column at step 2 in execution_table.

At which step does the main code resume after the ISR finishes?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Return to Main Code' column in execution_table after ISR actions.

If 'count' was not declared volatile, what might happen?

A ISR would not run

B Program would crash

C Main code might not see updated 'count'

D Interrupts would be disabled

💡 Hint

Refer to variable_tracker and key_moments about volatile keyword.

Concept Snapshot

ISR best practices in Arduino:
- Keep ISR functions very short and fast.
- Use volatile for variables shared with ISR.
- Avoid delay(), Serial.print(), or heavy code inside ISR.
- Clear interrupt flags if needed.
- Main code pauses during ISR and resumes after.
- Use attachInterrupt() to link ISR to pin events.

Full Transcript

In Arduino, when an interrupt happens, the main program pauses and runs a special function called ISR (Interrupt Service Routine). The ISR should be very short and fast to avoid delaying the main program. Variables shared between ISR and main code must be declared volatile so the main code always sees the latest value. Functions like delay() or Serial.print() should not be used inside ISR because they are slow. After ISR finishes, the main program resumes where it left off. This example shows an interrupt on pin 2 that increases a count variable each time the pin signal rises. The execution table traces how the count changes and when the ISR runs. Remember, keeping ISRs quick and using volatile variables are key best practices.

Practice

1. What is the main reason to keep Interrupt Service Routines (ISRs) short and simple in Arduino?

easy

A. To use more memory

B. To avoid delaying other important tasks and interrupts

C. To make the code look cleaner

D. To allow ISRs to run slower

Solution

1. Step 1: Understand ISR impact on program flow

   ISRs interrupt the main program to handle urgent events, so they must finish quickly to avoid blocking other tasks.

2. Step 2: Consider consequences of long ISRs

   If ISRs are long, they delay other interrupts and slow down the main program, causing performance issues.

3. Final Answer:

   To avoid delaying other important tasks and interrupts -> Option B

4. Quick Check:

   Keep ISRs short = avoid delays [OK]

Hint: Short ISR means faster response and less delay [OK]

Common Mistakes:

• Thinking ISR length only affects code readability
• Assuming ISRs can run as long as needed
• Ignoring impact on other interrupts

2. Which of the following is the correct way to declare a variable shared between an ISR and the main program?

easy

A. volatile int counter = 0;

B. int counter = 0;

C. static int counter = 0;

D. const int counter = 0;

Solution

1. Step 1: Identify variable usage in ISR and main code

   Variables shared between ISR and main code must be declared volatile to prevent compiler optimizations that cause wrong values.

2. Step 2: Check each declaration

   Only 'volatile int counter = 0;' tells the compiler the variable can change anytime, ensuring correct behavior.

3. Final Answer:

   volatile int counter = 0; -> Option A

4. Quick Check:

   Shared variable = volatile [OK]

Hint: Use volatile for variables changed in ISR [OK]

Common Mistakes:

• Forgetting volatile keyword
• Using const for variables that change
• Using static without volatile

3. What will be the output of this Arduino code snippet?

volatile int count = 0;
void setup() {
  Serial.begin(9600);
  attachInterrupt(digitalPinToInterrupt(2), increment, RISING);
}
void loop() {
  Serial.println(count);
  count = 0;
  delay(1000);
}
void increment() {
  count++;
}

Assuming pin 2 receives 3 rising signals per second, what will Serial print each second?

medium

A. 1

B. 0

C. 3

D. Random values

Solution

1. Step 1: Understand ISR increments

   Each rising signal on pin 2 triggers the ISR 'increment' which increases 'count' by 1.

2. Step 2: Calculate increments per second

   3 rising signals per second means 'count' increases by 3 every second.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   3 signals/sec = count increments 3 times [OK]

Hint: ISR increments count per event; count matches event rate [OK]

Common Mistakes:

• Ignoring volatile keyword effect
• Assuming count resets automatically
• Thinking ISR runs slower than events

4. Identify the error in this ISR code snippet:

volatile int flag = 0;
void setup() {
  attachInterrupt(digitalPinToInterrupt(3), toggleFlag, FALLING);
}
void loop() {
  if(flag) {
    // do something
    flag = 0;
  }
}
void toggleFlag() {
  flag = !flag;
}

medium

A. Not declaring 'flag' as volatile

B. Missing Serial.begin() in setup

C. Incorrect interrupt mode FALLING

D. Using non-atomic operation on 'flag' inside ISR

Solution

1. Step 1: Analyze ISR operation on 'flag'

   The ISR toggles 'flag' using '!flag', which is a read-modify-write operation and not atomic.

2. Step 2: Understand atomicity importance

   Non-atomic operations on shared variables can cause race conditions and unpredictable behavior.

3. Final Answer:

   Using non-atomic operation on 'flag' inside ISR -> Option D

4. Quick Check:

   ISR must avoid non-atomic variable changes [OK]

Hint: Avoid read-modify-write on shared vars in ISR [OK]

Common Mistakes:

• Ignoring atomicity of operations
• Thinking volatile alone fixes all issues
• Confusing interrupt mode with logic error

5. You want to count button presses using an ISR on pin 2. Which approach best follows ISR best practices in Arduino?

hard

A. Increment a volatile counter in ISR; read and reset it safely in loop

B. Print a message directly inside the ISR using Serial.println()

C. Use delay() inside ISR to debounce the button

D. Declare the counter as static inside ISR without volatile

Solution

1. Step 1: Review ISR best practices for counting events

   ISRs should be short and only update volatile variables; printing or delays inside ISR are bad.

2. Step 2: Evaluate each option

   Increment a volatile counter in ISR; read and reset it safely in loop uses a volatile counter incremented in ISR and safely handled in loop, which is correct.

3. Final Answer:

   Increment a volatile counter in ISR; read and reset it safely in loop -> Option A

4. Quick Check:

   Keep ISR short; update volatile vars only [OK]

Hint: Update volatile counter in ISR; handle logic outside [OK]

Common Mistakes:

• Using Serial.print inside ISR causing delays
• Using delay() inside ISR causing blocking
• Not declaring shared variables volatile