Boilerplate for AVR ATMega328p & KEL Shield

// lib.h
// Boilerplate for AVR ATMega328p and KEL shield
// Shield: http://www.elektronika.ftn.uns.ac.rs/images/MPE/Arduino%20KEL%20shield.pdf
// Timers & Interrupts: http://www.elektronika.ftn.uns.ac.rs/images/Elektronika(E2)/Prekidi%20i%20tajmeri.pdf
// BitWise: http://pastebin.com/4Y3vzZVx
// 7-segment Simulator: http://www.uize.com/examples/seven-segment-display.html

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

#define SCL_HI (PORTC |= (1<<5))
#define SCL_LO (PORTC &= ~(1<<5))
#define SDA (PINC & (1 << 4))
#define SHLD_HI (PORTB |= (1<<5))
#define SHLD_LO (PORTB &= ~(1<<5))

// A F B E D . C G
// 0 0 0 0 1 1 0 0 -> 0x0c -> A
#define SEG7_0 0x05
#define SEG7_1 0xdd
#define SEG7_2 0x46
#define SEG7_3 0x54
#define SEG7_4 0x9c
#define SEG7_5 0x34
#define SEG7_6 0x24
#define SEG7_7 0x5d
#define SEG7_8 0x04
#define SEG7_9 0x14
#define SEG7_A 0x0c
#define SEG7_B 0xa4
#define SEG7_C 0x27
#define SEG7_D 0xc4

#define SEG7_J 0x55
#define SEG7_E 0x26
#define SEG7_I 0xdd
#define SEG7_T 0xa6
#define SEG7_K 0x8c
#define SEG7_O 0x05
#define SEG7_Z 0x46
#define SEG7_S 0x34
#define SEG7_F 0x2e
#define SEG7_R 0xee
#define SEG7_L 0xa7
#define SEG7_V 0x85
#define SEG7_N 0xec
#define SEG7_P 0x0e
#define SEG7_LINE 0xfe

#define SEG7_NULL 0xff

void initTimer();
void initInterupt();
void initLEDs();
void init7seg();
void initButtons();
void initSwitches();
unsigned char readButton(unsigned char name);
void makeNumberWord(unsigned char *display_buffer, int num);
unsigned char readSwitches();
unsigned char readSwitch(int position);
void print7seg(unsigned char character, unsigned char position);
void print7segWord(unsigned char *array);

void initTimer() {
    TCCR0A = 0x02; //tajmer 0: CTC mod
    TCCR0B = 0x03; //tajmer 0: fclk = fosc/64
    OCR0A = 249; //perioda tajmera 0: 250 Tclk (OCR0A + 1 = 250)
    TIMSK0 = 0x02; //dozvola prekida tajmera 0 usled dostizanja vrednosti registra OCR0A
    sei(); // Globalna dozvola prekida
}

void initInterrupt() {
    PCICR = (1 << PCIE1); //dozvola prekida usled promene stanja
    PCMSK1 = 0x05; //pina PCINT10, ili pina PCINT8
    sei(); // Globalna dozvola prekida
}

void initLEDs() {
    DDRD = 0xff; // PORTD izlaz, postavljanje LEDova
    DDRB |= 1 << 4; // PORTB4 izlaz, tranzistor koji ukljucuje sve LEDove
    PORTB &= ~(1 << 4); // 0 na PORTB4 znaci 'omoguci'
}

void init7seg() {
    DDRD = 0xff; // PORTD izlaz, postavljanje segmenta
    DDRB |= 0xf; // PORTB[3-0] izlaz, za ukljucivanje jednog od cetiri sedmosegmentog
}

void initButtons() {
    DDRC &= 0xf0; // PORTC[3-0] ulaz
}

// * Prilicno sumnjiv dio, provjeriti na hardveru
void initSwitches() {
    DDRC |= 0x20; // PORTC5 izlaz, radi kao CLK za shift registar na ploci
    DDRC &= 0xef; // PORTC4 ulaz, preko njega se serijski ucitavaju stanja pinova
    DDRB |= 0x20; // PORTB5 izlaz, dozvola upisa u shift registar
}

unsigned char readButton(unsigned char name) {
    // 0 - Lijevo, 1 - Dole, 2 - Desno, 3 - Gore
    unsigned char position;
    switch (name) {
        case 'l':
            position = 0;
        break;
        case 'd':
            position = 1;
        break;
        case 'r':
            position = 2;
        break;
        case 'u':
            position = 3;
        break;
    }

    unsigned char buttons = PINC & 0x0f;
    if(!(buttons & (1 << position))) {
        return 1;
    }
    return 0;
}

void makeNumberWord(unsigned char *display_buffer, int num) {
    unsigned char numbers[] = { SEG7_0, SEG7_1, SEG7_2, SEG7_3, SEG7_4, SEG7_5,
    SEG7_6, SEG7_7, SEG7_8, SEG7_9 };
    unsigned char negative = 0;

    if (num < 0) {
        num *= -1;
        negative = 1;
    }

    display_buffer[3] = numbers[num % 10];
    display_buffer[2] = numbers[num / 10 % 10];
    display_buffer[1] = numbers[num / 100 % 10];
    display_buffer[0] = numbers[num / 1000 % 10];

    if (num < 10) {
        display_buffer[0] = 0xff;
        display_buffer[1] = 0xff;
        display_buffer[2] = 0xff;
        if (negative == 1) {
            display_buffer[2] = 0xfe;
        }
    } else if (num < 100) {
        display_buffer[0] = 0xff;
        display_buffer[1] = 0xff;
        if (negative == 1) {
            display_buffer[1] = 0xfe;
        }
    } else if (num < 1000) {
        display_buffer[0] = 0xff;
        if (negative == 1) {
            display_buffer[0] = 0xfe;
        }
    }
}

unsigned char readSwitches() {
    unsigned char i, tmp = 0, mask = 0x80;
    SHLD_HI;
    SHLD_LO;
    SHLD_HI;
    for (i=0; i<8; i++) {
        SCL_LO;
        SCL_HI;
        if (SDA) {
            tmp |= mask;
        }
        mask >>= 1;
    }
    return tmp;
}

unsigned char readSwitch(int position) {
    unsigned char switches = readSwitches();
    if (switches & (1 << position)) {
        return 1;
    }
    return 0;
}


void print7seg(unsigned char character, unsigned char position) {
    PORTB = ~(0x01 << (3 - position));
    PORTD = character;
    _delay_ms(2);
}

void print7segWord(unsigned char *array) {
    int i;
    for (i = 0; i < 4; i++) {
        print7seg(array[i], i);
    }
}


// main.c
#include "lib.h"

const unsigned char words_length[2] = { 15, 24 };
const unsigned char words[2][30] = { { SEG7_NULL, SEG7_NULL, SEG7_NULL, SEG7_NULL,
SEG7_J, SEG7_E, SEG7_D, SEG7_I, SEG7_T, SEG7_E, SEG7_NULL, SEG7_K, SEG7_O,
        SEG7_D, SEG7_NULL, SEG7_D, SEG7_Z, SEG7_O, SEG7_A },

        { SEG7_NULL, SEG7_NULL, SEG7_NULL, SEG7_NULL, SEG7_A, SEG7_F, SEG7_R, SEG7_I, SEG7_C, SEG7_K, SEG7_A, SEG7_NULL, SEG7_S, SEG7_L, SEG7_J, SEG7_I, SEG7_V, SEG7_A, SEG7_NULL, SEG7_LINE, SEG7_NULL, SEG7_N, SEG7_E, SEG7_NULL, SEG7_P, SEG7_A, SEG7_D, SEG7_A }
};
unsigned char display[5] = { SEG7_NULL, SEG7_NULL, SEG7_NULL, SEG7_NULL, SEG7_0 };

int timer = 0;
int cursorStart = 0;
int activeWord = 0;
int direction = 1;

ISR(PCINT1_vect) {
    // Interrupt
}

ISR(TIMER0_COMPA_vect) {
    timer++;
    if (timer == 500) {
        cursorStart += direction;

        if (direction == 1 && cursorStart == words_length[activeWord] + 1) {
            cursorStart = 0;
        }

        if (direction == -1 && cursorStart == 0) {
            cursorStart = words_length[activeWord];
        }

        for (int i = 0; i < 4; i++) {
            display[i] = words[activeWord][i + cursorStart];
        }

        timer = 0;
    }
}

int main() {
    init7seg();
    initTimer();
    initButtons();

    while (1) {
        print7segWord(display);

        if (readButton('u') == 1) {
            activeWord = 0;
            cursorStart = (direction == 1) ? 0 : words_length[activeWord] + 1;

        }
        if (readButton('d') == 1) {
            activeWord = 1;
            cursorStart = (direction == 1) ? 0 : words_length[activeWord] + 1;

        }

        if (readButton('l') == 1) {
            direction = 1;
        }

        if (readButton('r') == 1) {
            direction = -1;
        }
    }

    return 0;
}