#include <Arduino.h>
/************************************************************************/
/*                                                                      */
/*                      Reading rotary encoder                          */
/*                      one, two and four step encoders supported       */
/*                                                                      */
/*              Author: Peter Dannegger                                 */
/*              target: ATmega16                                        */
/************************************************************************/
#include <avr/io.h>
#include <avr/interrupt.h>

//#define XTAL        8e6         // 8MHz

#define F_TIMER1        100          // Timer 1 frequency /Hz

#define ENCODER_PIN PINC
#define PHASE_A     (1<<PC2)
#define PHASE_B     (1<<PC4)

#define LEDS_DDR     DDRF
#define LEDS        PORTF           // LEDs against VCC

volatile int8_t enc_delta;          // -128 ... 127
volatile int8_t last;

struct Counter
{
  int32_t neu {0};
  int32_t alt {0};
} count;


void encode_init( void ) {
  int8_t neu, tmp;

  // init encoder
  tmp = ENCODER_PIN;
  neu = 0;
  if ( tmp & PHASE_A ) neu  = 3;
  if ( tmp & PHASE_B ) neu ^= 1;      // convert gray to binary
  last = neu;                         // power on state
  enc_delta = 0;

  // put your setup code here, to run once:
  // Timer 1, CTC mode 4, prescaler  64
  TCCR1A  = 0;
  TCCR1B  = (1 << WGM12) | (1 << CS11) | (1 << CS10);
  OCR1A   = (F_CPU / (256L * F_TIMER1)) - 1;
  TIMSK1 |= (1 << OCIE1A);
}

ISR( TIMER1_COMPA_vect ) {           // 1ms for manual movement
  int8_t neu, diff, tmp;

  tmp = ENCODER_PIN;
  neu = 0;
  if ( tmp & PHASE_A ) neu  = 3;
  if ( tmp & PHASE_B ) neu ^= 1;   // convert gray to binary
  diff = last - neu;               // difference last - new
  if ( diff & 1 ) {                // bit 0 = value (1)
    last = neu;                    // store new as next last
    enc_delta += (diff & 2) - 1;   // bit 1 = direction (+/-)
  }
}

// read 1, 2, or 4 step encoders
int8_t encode_read( uint8_t klicks )  {
  int8_t val;

  // atomic access to enc_delta
  const uint8_t saveSREG {SREG};
  cli();
  val = enc_delta;
  switch (klicks) {
    case  2: enc_delta = val & 1; val >>= 1; break;
    case  4: enc_delta = val & 3; val >>= 2; break;
    default: enc_delta = 0; break;
  }
  SREG = saveSREG;
  return val;                   // counts since last call
}

////////////////////////////////////////////////////////////////////////////////
// void setup()
//
void setup()
{
  Serial.begin( 9600 );

  PORTC |= PHASE_A | PHASE_B;       // activate internal pull up resistors
  LEDS_DDR = 0xFF;
  encode_init();
  sei();
} // Ende void setup()


void loop()
{
  count.neu += encode_read(4);          // read a single step encoder

  if (count.alt != count.neu)
  {
    Serial.print(F("count = "));
    Serial.println(count.neu);
    LEDS = count.neu;
    count.alt = count.neu;
  }

} // Ende void loop()