1 sec timer with PIC16F877A

volatile int tenMillSecCounter=0;               // incremented every 10mSec
static volatile int tenMillSecCounter2=0;       // incremented every 10mSec - used for delays
static volatile int revolutionTimer=0;          // used for counting revs/min or motor
volatile int tenthsSecond=0;                    // incremented every tenth of a second
volatile int second=0;                          // incremeneted every second

// Timer 0 is programmed to interrupt every 10mSeconds
//  T0 clock is FOSC/4 = 200000
//  with 256 prescaler decrements 2000000/256 times / second
//  to get an interrupt every T mSec the calculation for TMR0 overflow from 0xFF to 0 is
//    count = 255 - T * 2000/256
//    i.e. for 10mSec counter is
#define tenMillSec 177
static void interrupt isr(void)
{
    if (T0IF)                           // A TMR0 interupt occurred
    {
        TMR0=tenMillSec;                // increment counters every 10 mSec
        tenMillSecCounter2++;
        revolutionTimer++;
        if((++tenMillSecCounter % 10)==0) tenthsSecond++;
        if((tenMillSecCounter % 100)==0) { second++; tenMillSecCounter=0; }
    }
    T0IF=0;                             // clear TMR0 interrupt flag
}

// delay for count tenths of a second
void tenthsSecondDelay(int count)
{
    tenMillSecCounter2=0;                         // zero counter
    while( tenMillSecCounter2  < (count*10)) ;    // wait
}
        
// delay for count seconds
void secondDelay(int count)
{
    tenMillSecCounter2=0;                         // zero counter
    while( tenMillSecCounter2  < (count*100)) ;   // wait
}
        
// initalise clock, set port A for digital input, set up timer 0, etc
void systemInitialise(void)
{
        OSCCON=0x70;             // Fosc use oscillator 8MHz
        ANSEL=0;                 // set digital inputs
        // PORTB PULL-UP DISABLED, TMR0 Timer mode Clock Source Select bit FOSC/4,  
        // Prescaler is assigned to the Timer0 module & TMR0 RATE 1:256
        OPTION=0x87;             
        TMR0=tenMillSec;         // initialise timer 0 counter
        T0IE = 1;                // enable interrupt on TMR0 overflow from 0xFF to 0
        GIE = 1;                 // Global interrupt enable
 }