[PIC] Delay routine drifting

Code C - [expand]
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#include <pic12f1822.h>
 
 
/* Setup chip configuration */
__code int __at(_CONFIG1) __CONFIG = _FOSC_INTOSC & _WDTE_OFF & _PWRTE_OFF & _MCLRE_ON & _CP_OFF & _CPD_OFF & _BOREN_OFF & _CLKOUTEN_OFF & _IESO_OFF & _FCMEN_OFF;
 
void setup(void);
void delay_ms(int milis);
volatile unsigned char done;
 
 
static void isr(void) __interrupt 0 {
 
    if(TMR0IE==1 && TMR0IF==1)//timer 1 interrupt overflow detect
    {
        done = 1; //latch enable as soon as interrupt detected
        TMR0IF = 0;//clear overflow flag
    }
 
}
 
 
void main(void) {
 
    setup();
 
 
    while(1) {
 
        delay_ms(1000);
        LATA |= (1<<2);
        delay_ms(1000);
        LATA &= ~(1<<2);
}
 
}
 
void setup(void) {
 
    OSCCON = 0b01011000; // 1MHZ internal clock
    LATA = 0;
    TRISA = 0x00;
    PORTA = 0x00;
 
 
//  Clock source selection (Fosc/4)
    TMR0CS = 0;
    PSA = 1;    // No prescaler
 
//  Enable Timer1 interrupt
    TMR0IE = 1;
    GIE = 1;
}
 
void delay_ms(int milis) {
 
    while (milis--) {
 
        done = 0; //disable delay latch
        TMR0 = 250;  //
        while (!done);  //latch till 1ms interrupt happens
    }
}
 
}

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#include <pic12f1822.h>
 
 
/* Setup chip configuration */
__code int __at(_CONFIG1) __CONFIG = _FOSC_INTOSC & _WDTE_OFF & _PWRTE_OFF & _MCLRE_ON & _CP_OFF & _CPD_OFF & _BOREN_OFF & _CLKOUTEN_OFF & _IESO_OFF & _FCMEN_OFF;
 
void setup(void);
void delay_ms(int milis);
 
 
 
void main(void) {
 
    setup();
 
 
    while(1) {
 
        delay_ms(1000);
        LATA |= (1<<2);
        delay_ms(1000);
        LATA &= ~(1<<2);
    }
 
}
 
void setup(void) {
 
    OSCCON = 0b01011000; // 1MHZ internal clock
    LATA = 0;
    TRISA = 0x00;
    PORTA = 0x00;
//  Clock source selection (Fosc/4)
    TMR0CS = 0;
    PSA = 1;    // No prescaler
 
//  Enable Timer1 interrupt
//  TMR0IE = 1;
//  GIE = 1;
}
 
void delay_ms(int milis) {
 
    while (milis--) {
 
        TMR0 = 6;  //
        while (!TMR0IF);  //latch till 1ms interrupt happens
        TMR0IF = 0;
    }
}