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

volatile uint8_t buffcnt=0, buffstart=0, buffend=0, kbdbuff[4]; 
const char LookUp[16] ={"0123456789ABCDEF"};

void init_keypad()
{
    EICRB |= (1<<ISC71)|(1<<ISC70); // INT7 on rising edge
    EIMSK |= (1<<INT7);             // enable INT7 - PORTE<7>
    DDRE |= 0x02;		    // buzzer
    TCCR1B |= (1<<CS12)|(1<<CS10);	// TIMER1 prescaler 1024 - buzzer turn off
}


uint8_t getkey(uint8_t* key)
{
    if(buffcnt!=0) {
        *key = kbdbuff[buffend];
        buffcnt--;
        buffend = (buffend+1)&0x03;
        return(1);
    }
    else {
        return(0);
    }
}

ISR(INT7_vect)
{
    if(buffcnt<4) {
        buffcnt++;
        kbdbuff[buffstart] = LookUp[PINB&0x0F];
        buffstart = (buffstart+1)&0x03;
    }
    else
    {
    	PORTE |= 0x02;
    	TCNT1 = 61536;
    	TIMSK1 |= (1 << TOIE1);
    }
}

ISR(TIMER1_OVF_vect)
{
	PORTE &= 0xFD;
    TIMSK1 &= ~(1 << TOIE1);
}