unsigned char data = 0x00;
unsigned char bitcount;

ISR(INT0_vect)
{

        if(bitcount)
        {
                if(bit_is_set(PIND, KBD_DATA_PIN)) // Button pressed
                {
                        data |= bitcount;
                }
        bitcount <<= 1;
        }else{
                char string[10];
                sprintf(string, "%02X", data);
                send_string(string);
                data = 0x00;
                bitcount = 0x01; // lsb first, shift left
        }
}