Interrupts

An interrupt in electronics is a signal that temporarily stops the normal flow of a program so that the system can respond immediately to an important event.

In simple terms:

An interrupt is like someone tapping you on the shoulder while you're doing something. You pause what you're doing, respond to the tap, then return to your task.

💡 What it's used for:

Interrupts are used to react quickly to events such as:

• A button press (like in your Arduino example)

• A sensor reaching a threshold

• Timers triggering at a set interval

• Communication events (like receiving data from a serial port)

Notes:

delay() depends on interrupts

The delay() function relies on the millis() timer, which is updated using a timer interrupt (usually Timer0). Here's what that means:

• Every millisecond, the Arduino fires an interrupt to increment the millis() count.

• delay() just waits until the required number of milliseconds has passed, by checking millis() in a loop.
  
  

❌ What happens in an ISR?

When you're inside an Interrupt Service Routine (ISR):

• Global interrupts are disabled by default.

• This means the timer interrupt that updates millis() won’t fire.

• As a result, delay() will never return — it just waits forever because millis() isn’t increasing.

So using delay() inside an ISR causes the program to hang (or freeze).

✅ What to do instead?

If you must use delays inside an ISR (which you generally shouldn't), you can use:

delayMicroseconds(us);  // Crude but works in ISRs

Or manually create a busy-wait:

for (volatile unsigned long i = 0; i < 100000UL; i++);

But again, ISRs should be short, fast, and non-blocking — typically you just set a flag and handle everything in loop().

Prompt: Write Arduino code that turns red, yellow, and blue LEDs on and off in succession, each one for 3 seconds. When a pushbuttlon is pushed, an interrupt is generated which turns off all the LEDs, then flashes the yellow LED on and off 5 times at half-second intervals, then returns to the main red-yelloe-blue loop

ChatGPT said:

Sure! Here's an Arduino sketch that does exactly what you described:

✅ Features:

• Red, Yellow, and Blue LEDs turn on in succession for 3 seconds each.

• When a pushbutton is pressed (using an interrupt), all LEDs turn off.

• Then the yellow LED blinks 5 times at 0.5-second intervals.

• After that, it resumes the red-yellow-blue loop.

// LED pins

const int greenPin = 7;

const int yellowPin = 8;

const int redPin = 9;

// Button pin

const int buttonPin = 3;

// Walk request flag

volatile bool walkRequested = false;

void setup() {

 pinMode(greenPin, OUTPUT);

 pinMode(yellowPin, OUTPUT);

 pinMode(redPin, OUTPUT);

 pinMode(buttonPin, INPUT_PULLUP); // Use internal pull-up resistor

 // Attach interrupt to button pin (INT1 on pin 9 for Uno)

 attachInterrupt(digitalPinToInterrupt(buttonPin), handleWalkRequest, FALLING);

}

void loop() {

 // Normal cycle

 digitalWrite(greenPin, HIGH);

 delay(2000);

 digitalWrite(greenPin, LOW);

 checkWalkRequest();

 digitalWrite(yellowPin, HIGH);

 delay(2000);

 digitalWrite(yellowPin, LOW);

 checkWalkRequest();

 digitalWrite(redPin, HIGH);

 delay(2000);

 digitalWrite(redPin, LOW);

 checkWalkRequest();

}

void handleWalkRequest() {

 walkRequested = true;

}

void checkWalkRequest() {

 if (walkRequested) {

   walkRequested = false; //reset flag

   // Flash yellow 3 times (0.5s on, 0.5s off)

   for (int i = 0; i < 5; i++) {

     digitalWrite(yellowPin, HIGH);

     delay(500);

     digitalWrite(yellowPin, LOW);

     delay(500);

   }

 }

}