gistfile1.c

// include the XC and PIC18 libraries
#include <xc.h>
#include <pic18.h>

#pragma config FOSC=HS1

// timer that will increment 4 times per second
unsigned long int timer;
 
//  main loop of the program
void main() {
    // enable Timer 0, part of the T0CON register (datasheet page 211)
    TMR0ON = 1;
    
    // enable prescaler for Timer 0
    T0CONbits.PSA = 0;
    
    // prescale TMR0 to 1:128
    T0CONbits.T0PS = 0b110;
    
    // alternatively, you can set the 3 previous commands in one line, like this:
    // T0CON = 0b10000110;
    
    // enable Timer 0's interrupt
    TMR0IE = 1;
    
    // enable general interrupts
    GIE = 1;
    
    // and you can set those last 2 in one line, like this:
    // INTCON = 0b10100000;
    
    // main loop
    while (1) { }
}
 
// interrupt service routine to handle all interrupts
void interrupt main_isr(void) {
    // if Timer 0's interrupt flag has been raised
    if (TMR0IF) {
        // increment the seconds counter
        timer++;
        // adjust the TMR0 value to 65536 - 15625 = 50271
        TMR0 = 50271;
        // lower the interrupt flag
        TMR0IF = 0;
    }
}