/*

convert two reed contacts into two UP/DOWN clock pulses for bidirectional counters
ATtiny13 @ 1.2 MHz

*/

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

#define F_CPU 1200000UL
#define F_TIMER 2000

#define RELOAD (F_CPU / (8L*F_TIMER) - 1)

#if RELOAD >255
#error RELOAD value >255
#endif

#define ENCODER_PIN PINB
#define PHASE_A     (1<<PB3)
#define PHASE_B     (1<<PB4)

#define UP    PORTB |= (1<<PB0)
#define DOWN  PORTB |= (1<<PB1)
#define NONE  PORTB &= ~((1<<PB1) | (1<<PB0))

int8_t last;

int main(void) {
	int8_t tmp, new;

    //  UP/DOWN outputs
    DDRB  = (1<<PB1) | (1<<PB0);
    // reed contact inputs, internal pull up 
    PORTB = (1<<PB4) | (1<<PB3);

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

    // timer 0 init
    // mode 2, CTC, prescaler 8

    TCCR0A  = (1<<WGM01);
    TCCR0B  = (1<<CS01);
    TIMSK0 |= (1<<OCIE0A);
    OCR0A   = RELOAD;

    sei();

    while (1) {
        // sleep and save energy
        set_sleep_mode(SLEEP_MODE_IDLE);
        sleep_mode();
    }

}

// timer isr @ F_TIMER Hz

ISR (TIM0_COMPA_vect) {

	int8_t tmp, new, diff, enc_delta=0;
	
	tmp = ENCODER_PIN;
	new = 0;
	if ( tmp & PHASE_A ) new  = 3;
	if ( tmp & PHASE_B ) new ^= 1;   	// convert gray to binary
	diff = last - new;               	// difference last - new
	if( diff & 1 ) {                 	// bit 0 = value (1)
    	last = new;                    	// store new as next last
    	enc_delta += (diff & 2) - 1;   	// bit 1 = direction (+/-)
  	}

	// update outputs
    
	if (enc_delta > 0) {
		UP;
	} else if (enc_delta < 0) {
		DOWN;
	} else {
		NONE;
	}	
}