➡ https://github.com/mathertel/RotaryEncoder

// -----
// InterruptRotator.ino - Example for the RotaryEncoder library.
// This class is implemented for use with the Arduino environment.
// Copyright (c) by Matthias Hertel, http://www.mathertel.de
// This work is licensed under a BSD style license. See http://www.mathertel.de/License.aspx
// More information on: http://www.mathertel.de/Arduino
// -----
// 18.01.2014 created by Matthias Hertel
// -----
 
// This example checks the state of the rotary encoder in the loop() function.
// The current position is printed on output when changed.
 
// Hardware setup:
// Attach a rotary encoder with output pins to A2 and A3.
// The common contact should be attached to ground.
 
#include <RotaryEncoder.h>
 
// Setup a RoraryEncoder for pins A2 and A3:
RotaryEncoder encoder(A2, A3);
 
void setup()
{
  Serial.begin(57600);
  Serial.println("SimplePollRotator example for the RotaryEncoder library.");
 
  // You may have to modify the next 2 lines if using other pins than A2 and A3
  PCICR |= (1 << PCIE1);    // This enables Pin Change Interrupt 1 that covers the Analog input pins or Port C.
  PCMSK1 |= (1 << PCINT10) | (1 << PCINT11);  // This enables the interrupt for pin 2 and 3 of Port C.
} // setup()
 
 
 
// The Interrupt Service Routine for Pin Change Interrupt 1
// This routine will only be called on any signal change on A2 and A3: exactly where we need to check.
ISR(PCINT1_vect) {
  encoder.tick(); // just call tick() to check the state.
}
 
 
// Read the current position of the encoder and print out when changed.
void loop()
{
  static int pos = 0;
 
  int newPos = encoder.getPosition();
  if (pos != newPos) {
    Serial.print(newPos);
    Serial.println();
    pos = newPos;
 
    // Just to show, that long lasting procedures don't break the rotary encoder:
    // When newPos is 66 the ouput will freeze, but the turned positions will be recognized even when not polled.
    // The interrupt still works.
    // The output is correct 6.6 seconds later.
    if (newPos == 66)
      delay(6600);
  } // if
} // loop ()
 
// The End

➡ https://github.com/PaulStoffregen/Encoder

/* Encoder Library - TwoKnobs Example
 * http://www.pjrc.com/teensy/td_libs_Encoder.html
 *
 * This example code is in the public domain.
 */
 
#include <Encoder.h>
 
// Change these pin numbers to the pins connected to your encoder.
//   Best Performance: both pins have interrupt capability
//   Good Performance: only the first pin has interrupt capability
//   Low Performance:  neither pin has interrupt capability
Encoder knobLeft(5, 6);
Encoder knobRight(7, 8);
//   avoid using pins with LEDs attached
 
void setup() {
  Serial.begin(9600);
  Serial.println("TwoKnobs Encoder Test:");
}
 
long positionLeft  = -999;
long positionRight = -999;
 
void loop() {
  long newLeft, newRight;
  newLeft = knobLeft.read();
  newRight = knobRight.read();
  if (newLeft != positionLeft || newRight != positionRight) {
    Serial.print("Left = ");
    Serial.print(newLeft);
    Serial.print(", Right = ");
    Serial.print(newRight);
    Serial.println();
    positionLeft = newLeft;
    positionRight = newRight;
  }
  // if a character is sent from the serial monitor,
  // reset both back to zero.
  if (Serial.available()) {
    Serial.read();
    Serial.println("Reset both knobs to zero");
    knobLeft.write(0);
    knobRight.write(0);
  }
}

/*     Arduino Rotary Encoder Tutorial
 *      
 *  by Dejan Nedelkovski, www.HowToMechatronics.com
 *  
 */
 
 #define outputA 6
 #define outputB 7
 int counter = 0; 
 int aState;
 int aLastState;  
 void setup() { 
   pinMode (outputA,INPUT);
   pinMode (outputB,INPUT);
 
   Serial.begin (9600);
   // Reads the initial state of the outputA
   aLastState = digitalRead(outputA);   
 } 
 void loop() { 
   aState = digitalRead(outputA); // Reads the "current" state of the outputA
   // If the previous and the current state of the outputA are different, that means a Pulse has occured
   if (aState != aLastState){     
     // If the outputB state is different to the outputA state, that means the encoder is rotating clockwise
     if (digitalRead(outputB) != aState) { 
       counter ++;
     } else {
       counter --;
     }
     Serial.print("Position: ");
     Serial.println(counter);
   } 
   aLastState = aState; // Updates the previous state of the outputA with the current state
 }