avr-rotary-encoder.cpp

#include <avr/interrupt.h>
#include <avr/io.h>

volatile uint8_t rotaryPosition = 0;

void __attribute__((constructor)) init_rotary()
{
  EICRA |= _BV(ISC00); // enable interrupt on change INT0 (for rotary encoder)
  EIMSK |= (1 << INT0); // enable INT0 (for rotary encoder)
  DDRD &= ~(_BV(PORTD3) | _BV(PORTD4) | _BV(PORTD5)); // input mode (rotary encoder clk, dt, sw)
  PORTD |= _BV(PORTD3) | _BV(PORTD4) | _BV(PORTD5); // enable pull-up (rotary encoder clk, dt, sw)
}

ISR(INT0_vect)
{
  static bool last_clk = 1;
  const uint8_t pind = PIND;
  bool clk = pind & _BV(PIND2);
  if (clk != last_clk) {
    bool dt = pind & _BV(PIND3);
    rotaryPosition += (dt == clk) ? -1 : 1;
    last_clk = clk;
  }
}

bool readSwitch()
{
  return PIND & _BV(PIND4);
}