Hy everybody.

I want to make a programm who accomplish the next tasks : generate a 
delay of 3 seconds.
The RD7 pin must stay ON 3 seconds, then must stay OFF 3 seonds, and so 
on.

freq. out = freq. clk / (Prescaler*(256 - TMR0)*count)
In my program I work with internal clk (T0CS=0) so the freq. clk is 
4Mhz/4 = 1Mhz. ( Period = 1 / freq; internal instruction cycle is 1 
microsec).

freq. out = 1 / T = 1 / 3 sec = 0.33Hz and replacing 0.33Hz = 1MHz / 
(256 * (256 - 0) * count)
count = 1MHz / 65536 * 0.33Hz
count = 46.

#include<pic.h>
unsigned char counter;
void interrupt Timer0_ISR(void)
{
     if ( T0IE && T0IF )                      // are TMR0 interrupts 
enabled and//is the TMR0 interrupt flag set?
        {
             T0IF = 0;                        // TMR0 interrupt flag 
must be cleared in software
             ++counter;                       // increment the counter 
variable by 1
             if(counter == 46)
             PORTDbits.RD7 ^= 1;              // toggle the RD7 pin
        }
}
Init(void)
{
      TMR0 = 0;                               // clear the TMR0 register
      OPTION_REG = 0B00000111;                // Use the internal 
instruction clock, choosing to work with a Prescaler (1 : 256)
      INTCONbits.T0IE = 1;                    // enable TMR0 overflow
      INTCONbits.GIE = 1;                     // enable Global 
interrupts
}
main(void)
{
      TRISD = 0B01111111;
      PORTDbits.RD7 = 0;
      Init();
      while(1)
      {
      }
}

I simulate this program with MPLAB SIM but timer0 dont want to 
increment.
Please help me.
I dont know if my computations is good.
All the best.