PIC18F4580 timer 0 question - [SOLVED]

#include <xc.h>
#include "timer_header.h"
#include "stdint.h"
#include "stdbool.h"

#define LED1 LATCbits.LATC0 // Green LED
//#define LED2 LATCbits.LATC1 // Yellow LED

//#define BUTTON PORTBbits.RB0 // Button

uint32_t blink_count = 0;
//uint16_t button_count = 0;
//bool button_press = false;

// HIGH priority ISR
void __interrupt(high_priority) HPbuttonISR(void){ 
    // Check The Timer 0 Flag Bit
   if (INTCONbits.TMR0IF)
   {
      blink_count++;
      if(blink_count==50) // with 8 bit it is 10000 timer overflows!
      {
        // Toggle green LED
        LED1 = ~LED1;
        // Clear The Global Counter
        blink_count = 0;
      }
      // Preload The Value To TMR1 Register Every Overflow Interrupt
      TMR0L += 25538;
      INTCONbits.TMR0IF = 0; // Clear The Flag Bit
   }
}


void main(void) {
    // Configure the pins: OUT = 0, IN = 1
    TRISBbits.RB0 = 1;
    TRISCbits.RC0 = 0;
    TRISCbits.RC1 = 0;
    
    // 8MHz internal oscillator
    OSCCONbits.IRCF = 0x07;
    // Internal oscillator selected
    OSCCONbits.SCS = 0x03;
    // I wait until the oscillator's freq is stable
    while(OSCCONbits.IOFS!=1);
    
    // Timer 0 OFF
    T0CONbits.TMR0ON=0; 
    // Timer 0: 16 bit
    T0CONbits.T08BIT=0;
    // Internal source clock
    T0CONbits.T0CS = 0;
    // Prescaler is bypassed --> 1:1
    T0CONbits.PSA = 1;
        
    // Preload The Value Which We've Calculated To The TMR0 16-Bit Register!
    TMR0L += 25538;
    
    // Enable Timer 0 interrupt
    INTCONbits.TMR0IE=1;
    // High priority for timer 0 interrupt
    INTCON2bits.TMR0IP=1;
        
    ei(); // This is like flipping the master switch to enable
          // all interrupts: it's like GIE = 1 
    // Timer 0 ON
    T0CONbits.TMR0ON=1;
    
    LED1 = 0; // Switch OFF Green LED
    //LED2 = 0; // Switch OFF Yellow LED
        
    // Infinite loop
    while(1){
        
    }
            
    return;
}

Attached Image(s)

Lucky Luka
Using the formula
TMR0L += 256 - 200 + 2;
The situation improved a lot: I obtain 1.002 sec ON and and 1.002 sec OFF.

TMR0L += 256 - 200 + 2 + 1;

Lucky Luka
Substituting 8 to 2 in the formula as I suggested the result is much worse.
I still don't understand why tough.

1and0
I forgot to account for one more instruction cycle for the write to the TMR0 register; that is,

TMR0L += 256 - 200 + 2 + 1;

#include <xc.h>
#include "timer_header.h"
#include "stdint.h"
#include "stdbool.h"

#define LED1 LATCbits.LATC0 // Green LED
//#define LED2 LATCbits.LATC1 // Yellow LED

//#define BUTTON PORTBbits.RB0 // Button

uint16_t blink_count = 0;
//uint16_t button_count = 0;
//bool button_press = false;

// HIGH priority ISR
void __interrupt(high_priority) HPbuttonISR(void){ 
    // Check The Timer 0 Flag Bit
   if (INTCONbits.TMR0IF)
   {
      blink_count++;
      if(blink_count==500) // with 8 bit 8MHz it was 10000 timer overflows!
      {
        // Toggle green LED
        LED1 = ~LED1;
        // Clear The Global Counter
        blink_count = 0;
      }
      // Preload The Value To TMR1 Register Every Overflow Interrupt
      TMR0L += 8;
      INTCONbits.TMR0IF = 0; // Clear The Flag Bit
   }
}


void main(void) {
    // Configure the pins: OUT = 0, IN = 1
    TRISBbits.RB0 = 1;
    TRISCbits.RC0 = 0;
    TRISCbits.RC1 = 0;
    
    // 500 kHz internal oscillator
    OSCCONbits.IRCF = 0x03;
    // Internal oscillator selected
    OSCCONbits.SCS = 0x03;
    // I wait until the oscillator's freq is stable
    while(OSCCONbits.IOFS!=1);
    
    // Timer 0 OFF
    T0CONbits.TMR0ON=0; 
    // Timer 0: 8 bit
    T0CONbits.T08BIT=1;
    // Internal source clock
    T0CONbits.T0CS = 0;
    // Prescaler is bypassed --> 1:1
    T0CONbits.PSA = 1;
        
    // Preload The Value Which We've Calculated To The TMR0 8-Bit Register!
    TMR0L += 8;
    
    // Enable Timer 0 interrupt
    INTCONbits.TMR0IE=1;
    // High priority for timer 0 interrupt
    INTCON2bits.TMR0IP=1;
        
    ei(); // This is like flipping the master switch to enable
          // all interrupts: it's like GIE = 1 
    // Timer 0 ON
    T0CONbits.TMR0ON=1;
    
    LED1 = 0; // Switch OFF Green LED
    //LED2 = 0; // Switch OFF Yellow LED
        
    // Infinite loop
    while(1){
        
    }
            
    return;
}

Attached Image(s)

#include <xc.h>
#include "timer_header.h"
#include "stdint.h"
#include "stdbool.h"

#define LED1 LATCbits.LATC0 // Green LED
//#define LED2 LATCbits.LATC1 // Yellow LED

//#define BUTTON PORTBbits.RB0 // Button

uint16_t blink_count = 0;
//uint16_t button_count = 0;
//bool button_press = false;

// HIGH priority ISR
void __interrupt(high_priority) HPbuttonISR(void){ 
    // Check The Timer 0 Flag Bit
   if (INTCONbits.TMR0IF)
   {
      blink_count++;
      if(blink_count==500) // with 8 bit 8MHz it was 10000 timer overflows!
      {
        // Toggle green LED
        LED1 ^= 1;
        // Clear The Global Counter
        blink_count = 0;
      }
      // Preload The Value To TMR1 Register Every Overflow Interrupt
      TMR0L += 9;
      INTCONbits.TMR0IF = 0; // Clear The Flag Bit
   }
}


void main(void) {
    // Configure the pins: OUT = 0, IN = 1
    TRISBbits.RB0 = 1;
    TRISCbits.RC0 = 0;
    TRISCbits.RC1 = 0;
    
    // 500 kHz internal oscillator
    OSCCONbits.IRCF = 0x03;
    // Internal oscillator selected
    OSCCONbits.SCS = 0x03;
    // I wait until the oscillator's freq is stable
    while(OSCCONbits.IOFS!=1);
    
    // Timer 0 OFF
    T0CONbits.TMR0ON=0; 
    // Timer 0: 8 bit
    T0CONbits.T08BIT=1;
    // Internal source clock
    T0CONbits.T0CS = 0;
    // Prescaler is bypassed --> 1:1
    T0CONbits.PSA = 1;
        
    // Preload The Value Which We've Calculated To The TMR0 8-Bit Register!
    TMR0L += 9;
    
    // Enable Timer 0 interrupt
    INTCONbits.TMR0IE=1;
    // High priority for timer 0 interrupt
    INTCON2bits.TMR0IP=1;
        
    ei(); // This is like flipping the master switch to enable
          // all interrupts: it's like GIE = 1 
    // Timer 0 ON
    T0CONbits.TMR0ON=1;
    
    LED1 = 0; // Switch OFF Green LED
    //LED2 = 0; // Switch OFF Yellow LED
        
    // Infinite loop
    while(1){
        
    }
            
    return;
}

Attached Image(s)

dan1138
The specific issue I raised is that in the real device any write to the TIMER0 count register results in a two instruction cycle delay before TIMER0 starts to count, but there are some opcodes that can be used to write the TIMER0 count register where the simulator starts the TIMER0 count without a delay.