quadrature_encoder.ino

// see http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1267553811/0
// and http://code.google.com/p/digitalwritefast/

// It turns out that the regular digitalRead() calls are too slow and bring the arduino down when
// I use them in the interrupt routines while the motor runs at full speed creating more than
// 40000 encoder ticks per second per motor.

// Tuning encoder
#define c_EncoderAInterrupt 18
#define c_EncoderPinA 18
#define c_EncoderPinB 19
#define EncoderIsReversed
volatile bool _EncoderBSet;
volatile long _EncoderTicks = 0;

void setup()
{
  Serial.begin(115200);

  // Quadrature encoders
  // Left encoder
  pinMode(c_EncoderPinA, INPUT);      // sets pin A as input
  digitalWrite(c_EncoderPinA, LOW);  // turn on pullup resistors
  pinMode(c_EncoderPinB, INPUT);      // sets pin B as input
  digitalWrite(c_EncoderPinB, LOW);  // turn on pullup resistors
  attachInterrupt(c_EncoderAInterrupt, HandleEncoderInterruptA, RISING);
}

void loop()
{
  Serial.print(_EncoderTicks);
  Serial.print("\n");
  delay(20);
}

// Interrupt service routines for the quadrature encoder
void HandleEncoderInterruptA()
{
  // Test transition; since the interrupt will only fire on 'rising' we don't need to read pin A
  _EncoderBSet = digitalReadFast(c_EncoderPinB);   // read the input pin

  // and adjust counter + if A leads B
#ifdef EncoderIsReversed
  _EncoderTicks -= _EncoderBSet ? -1 : +1;
#else
  _EncoderTicks += _EncoderBSet ? -1 : +1;
#endif
}

I have not tried this code with an Arduino, only the Teensy 3.6, though I suspect it would work well on the Teensy 3.2 or 4.0 as well.