Timer0 PIC16F917

[Dragu Mircea]

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. (internal instruction cycle is 1 microsec). 
I want to generate a delay of 3 seconds. 
The RD7 pin must stay 3 seconds ON, then must stay 3 seonds OFF and so on. 
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. 
In previous programm I made some mistakes. I do not set up the interrupts :  INTCONbits.T0IE = 1;   // Timer 0 Interrupt Enable is set    
                                                                                                                            INTCONbits.GIE = 1;    // Global Interrupt Enable is set 
  

#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. 
All the best. 

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