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

void init_display();
void init_timer0();

volatile uint8_t lcdptr=0, lcdcmd=0xC0, lcdoff=0;  // 

unsigned char LCDline[32]={"               0               0"};
const char LookUp[16] ={"123*456*789**0**"};

int main(void)
{   
    int8_t i, carry,sum;
    unsigned char kbd;
    uint8_t kbdcnt=0;
    //_delay_ms(500);
    init_display();
    init_timer0();  // initialize timer0
    
    sei();  // enable global interrupts
    // loop forever
    while(1)
    {
        while((PINB|0xEF)==0xFF);
        while((PINB&0x10)==0x00);
        kbd = LookUp[PINB&0x0F];
        if(!(kbd=='*')) {
            if(kbdcnt==0) {
                if(!(kbd=='0')) {
                    LCDline[15]=kbd;
                    kbdcnt=1;
                }
            }
            else {
                for(i=1; i<15; i++)
                    LCDline[i]=LCDline[i+1];
                LCDline[15]=kbd;
                kbdcnt++;
            }
            
        } // if(!(kbd=='*'))
        if(kbdcnt>0) {
            carry = 0;
            for(i=0; i<kbdcnt; i++) {
                sum = 2*(LCDline[15-i]-'0')+carry;
                if(sum>=10) {
                    sum = sum-10;
                    carry = 1;
                }
                else {
                    carry = 0;
                }
                LCDline[31-i] = sum+'0';
            }
            if(carry==1) LCDline[31-i]='1';
        }
    } // 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_timer0()
{
    TCCR0B |= (1<<CS02)|(1<<CS00);	// prescaler 1024
    TCNT0 = 0;			            // initialize counter
    TIMSK0 |= (1 << TOIE0);	        // enable overflow interrupt 
}

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;
}