#include    <pic.h>

__CONFIG(WDTDIS & XT & UNPROTECT & LVPDIS);

#define XTAL    4000000         // crystal frequency - 4MHz


long tics = 0l;
int flag = 1;

void interrupt prv_int(void)
{
    //****************************************************************
    // To calculate when will enter here:
    //
    // Freq = clock cycle / 4 / 256 = 4000000Hz / 4 / 256 = 3906Hz
    // Period = 1 / 3906Hz = 0.256msec
    //****************************************************************
    if (!T0IF)
       return;

    //****************************************************************
    // T0IF is 1 when the counter TMR0 is arrived to the
    // overflow = 0xFF = 256 so to calculate when will enter here:
    //
    // Freq = (clock cycle / 4 / 256) / 256 = 3906Hz / 256 = 15Hz
    // Period = 1 / 15Hz = 65msec
    //****************************************************************

    T0IF = 0;

    tics++;

    if (tics == 15l)
    {
        tics = 0l;

        //***********************************************************
        // To calculate when will enter here:
	// Freq = 15Hz / 15 = 1Hz and periord is 1 second
        //******************************+****************************
       
        if (flag == 1)
        {
            RB5 = 1;
            flag = 0;
        }
        else
        {
            RB5 = 0;
            flag = 1;
        }
    }
}

main()
{
    TRISB  = 0b11000000;
    CMCON  = 0b00000111;    // disable comparator on ra0,1,2
    TRISA  = 0b11111111;


    TMR0   = 0;	 // Reset the TMR0 counter
    T0CS   = 0;  // Enable timer to get the input from internal counter XTAL/4
    PSA    = 0;  // Enable prescaler to divide the frequency
    PS2    = 1;  //
    PS1    = 1;  // 111 mean prescaler set to 1:256
    PS0    = 1;  //

    INTCON = 0;  // reset all the flag bits for all the interrupts
    GIE    = 1;  // global interrupt enable bit (1: enable all)
    T0IE   = 1;  // TMR0 overflow interrupt enable bit (1: enable TMR0)
   
    while(1)
        continue;    // while forewer
}