#define F_CPU 16000000UL
#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>

void init_display();
void init_keypad();
void init_timer0();

volatile uint8_t lcdptr=0, lcdcmd=0xC0, lcdoff=0;  // 
volatile uint8_t buffcnt=0, buffstart=0, buffend=0, kbdbuff[4]; 
unsigned char LCDline[32]={"Keypad INT                      "};
//                          0123456789ABCDEF0123456789ABCDEF
const char LookUp[16] ={"0123456789ABCDEF"};

int main(void)
{   
    uint8_t kbdcnt=0;
    
    init_display();
    init_keypad();
    init_timer0();  // initialize timer0
    
    sei();  // enable global interrupts
    // loop forever
    while(1)
    {
        if(buffcnt!=0) {
            LCDline[kbdcnt+16] = kbdbuff[buffend];
            buffcnt--;
            buffend = (buffend+1)&0x03;
            kbdcnt = (kbdcnt+1) & 0x0F;
            LCDline[kbdcnt+16] = ' ';
        }
    } // while(1)
}

void init_display()
{
    DDRA = 0xFF;
    
    PORTC &= 0xAF;
    DDRC  |= 0x50;
    
    PORTC &= 0xBF;

    PORTA = 0x38;
    PORTC |= 0x10;
    PORTC &= 0xEF;
    PORTA = 0x0C;
    PORTC |= 0x10;
    PORTC &= 0xEF;
    PORTA = 0x01;
    PORTC |= 0x10;;
    PORTC &= 0xEF;  
    _delay_ms(2);
}

void init_keypad()
{
    EICRB |= (1<<ISC71)|(1<<ISC70); // INT7 on rising edge
    EIMSK |= (1<<INT7);             // enable INT7 - PORTE<7>
}

void init_timer0()
{
    TCCR0B |= (1<<CS02)|(1<<CS00);	// prescaler 1024
    TCNT0 = 0;			            // initialize counter
    TIMSK0 |= (1 << TOIE0);	        // enable overflow interrupt 
}

ISR(INT7_vect)
{
    if(buffcnt<4) {
        buffcnt++;
        kbdbuff[buffstart] = LookUp[PINB&0x0F];
        buffstart = (buffstart+1)&0x03;
    }
}

ISR(TIMER0_OVF_vect)
{
    if(lcdptr==16) {
        PORTC &= 0xBF;
        PORTA = lcdcmd;
        lcdptr = 0;
        lcdcmd ^= 0x40;
        lcdoff ^= 0x10;        
    }
    else {
        PORTC |= 0x40;
        PORTA = LCDline[lcdoff+lcdptr];
        PORTC |= 0x10;
        lcdptr++;
    }
    PORTC |= 0x10;
    PORTC &= 0xEF;
}