Interrupts, EEPROM, I2C

Interrupts: 
The processor responds to important events. Interrupts interrupt whatever the processor is doing and executes code designer to react to external stimulus. 

EEPROM (Electrically Erasable Programmable Read-Only Memory):

Solid state computer memory. Permanent storage similar to a hard drive in computers. Can be read, erased and re-written electronically. 

Microcontroller	EEPROM
ATmega328 (Arduino Uno, Nano, Mini)	1024 bytes
ATmega168 (Arduino Nano)	512 bytes
ATmega2560 (Arduino Mega)	4096 bytes

Last State: The EEPROM keeps data even when the Arduino resets or when power is removed.

Remembers the last state of a variable/remembers how many times an appliance was activated.

Ex Scenario. LAMP (on) connected to Arduino -- Arduino loses power -- When power is back on, LAMP stays off (does not keep it's last change).

Solution: Save LAMP state in EEPROM and add condition to sketch to check whether the state of the lamp corresponds to the previous state. 

Schematic:

1x Arduino Uno
1x LED
1x Resistor
1x Button
1x 1k Resistor 
1x Breadboard
5x Jumpers

Code: 
#include <EEPROM.h>

const int buttonPin = 8;    // pushbutton pin
const int ledPin = 4;       // LED pin

int ledState;                // variable to hold the led state
int buttonState;             // the current reading from the input pin
int lastButtonState = LOW;   // the previous reading from the input pin


// the following variables are long's because the time, measured in miliseconds,
// will quickly become a bigger number than can be stored in an int.
long lastDebounceTime = 0;  // the last time the output pin was toggled
long debounceDelay = 50;    // the debounce time; increase if the output flickers

void setup() {

  pinMode(buttonPin, INPUT);
  pinMode(ledPin, OUTPUT);

  digitalWrite(ledPin, ledState);

  Serial.begin (9600);

  checkLedState(); 
}

void loop() {
 
  int reading = digitalRead(buttonPin);

  if(reading != lastButtonState) {

    lastDebounceTime = millis();
  }

  if((millis() - lastDebounceTime) > debounceDelay) {
  

    if(reading != buttonState) {
      buttonState = reading;
    
      if(buttonState == HIGH) {
        ledState = !ledState;
      }
    }
  }


  digitalWrite(ledPin, ledState);
  EEPROM.update(0, ledState);
  lastButtonState = reading;
}

void checkLedState() {
   Serial.println("LED status after restart: ");
   ledState = EEPROM.read(0);
   if(ledState == 1) {
    Serial.println ("ON");
    digitalWrite(ledPin, HIGH);
   } 
   if(ledState == 0) {
    Serial.println ("OFF");
    digitalWrite(ledPin, LOW);
   }
}