[PIC] multiplexing 7 Segment

//PIC16F627A
//4MHz Internal OSC
//MUX by the MUC itself with Interrupt
//TMR0 .. check the prescelar+delay in scan routine as they are related
//punkky@gmail.com
unsigned short number [10] = {0x5F, 0x06, 0x9b, 0x8f, 0xC6, 0xCd, 0xDD, 0x07, 0xDf, 0xCf};
unsigned short digit [6];
unsigned short counter;
unsigned short shift_register;
unsigned short x1;
unsigned short x2;
unsigned short x3;
unsigned short x4;
unsigned short x5;
unsigned short x6;
unsigned short tick;
void interrupt ()
{
    if (INTCON.T0IF)
    {
        //Scan digits with TMR0
        INTCON.T0IF = 0;
        if (counter == 5)
        {
            PORTA = number [digit [counter]];
            Delay_us (500);
            shift_register = 0x01;
            PORTB = ~shift_register;
            PORTA = 0x00;
            counter = 0;
        } else
        {
            PORTA = number [digit [counter]];
            Delay_us (500);
            shift_register = shift_register << 1;
            PORTB = ~shift_register;
            PORTA = 0x00;
            counter ++;
        }
    }
    if (PIR1.TMR1IF)
    {
        TMR1H = 0x80;
        PIR1.TMR1IF = 0;
        tick = 1;
        //update current time
        x6 ++;
        if (x6 > 9)
        {
            x6 = 0;
            x5 ++;
            if (x5 > 9)
            {
                x5 = 0;
                x4 ++;
                if (x4 > 9)
                {
                    x4 = 0;
                    x3 ++;
                    if (x3 > 9)
                    {
                        x3 = 0;
                        x2 ++;
                        if (x2 > 9)
                        {
                            x2 = 0;
                            x1 ++;
                            if (x1 > 9)
                            {
                                x1 = 0;
                            }
                        }
                    }
                }
            }
        }
    }
}
void main ()
{
    //Digital I/O for PORTA
    CMCON = 0x07;
    TRISA = 0x00;
    PORTA = 0x00;
    TRISB = 0x00;
    PORTB = 0x00;
    //Internal Clock 4MHz
    PCON.OSCF = 1;
    counter = 0;
    // Enable TMR0
    OPTION_REG.T0CS = 0;
    // Enable Prescaler
    OPTION_REG.PSA = 0;
    // PS0,1,2 = 010 = 3
    // 3 means 1:8 prescaler
    // 1:2, 1:4, 1:8, 1:16, 1:32, 1:64, 1:128, 1:256
    OPTION_REG.PS2 = 0;
    OPTION_REG.PS1 = 1;
    OPTION_REG.PS0 = 0;
    INTCON.T0IF = 0;
    INTCON.T0IE = 1;
    INTCON.GIE = 1;
    INTCON.PEIE = 1;
    T1CON = 0x0F;
    TMR1H = 0x80;
    TMR1L = 0x00;
    // Enable TMR1 interrupt
    PIE1.TMR1IE = 1;
    shift_register = 0x01;
    x1 = 0;
    x2 = 0;
    x3 = 0;
    x4 = 0;
    x5 = 0;
    x6 = 0;
    while (1)
    {
        if (tick)
        {
            tick = 0;
            //update digits
            digit [0] = x1;
            digit [1] = x2;
            digit [2] = x3;
            digit [3] = x4;
            digit [4] = x5;
            digit [5] = x6;
        }
    }
}