Hi,

This is my first hardware + software project with AT89C51 24PC. This is a simple timer project. Entire thing works very well, but the timing is a bit slow. I have observed that for every minute, it turns on the relay 3 sec later. e.g. For 5 min, it turns relay after 5 min 15 sec. I have tried every aspect of debugging but no luck. I am using 12Mhz crystal printed on it and now I doubt it not really 12Mhz
Any help is appreciated.

---------- Here is the source code -----------
#include <AT89X51.h>


// variables that keep track of the time. Note the "volatile" keyword
// should be used by all variables that are written by the interrupt
// service routines and may be read by the main program.


volatile int total_sec;
volatile int total_sec_fraction;
volatile int toggle_flag ;

int input_value ;

void timer0_isr(void) interrupt 1;
void delay_time(void);
void call_buzzer(int);
void find_input(void);


void main()
{


int i = 0 ;
total_sec = total_sec_fraction = toggle_flag = 0 ;

IE = 0;
// turn off all interrupts
total_sec = total_sec_fraction = 0 ;
// zero total seconds
TR0 = 0;
// make sure timer 0 is stopped
TF0 = 0;
// clear the overflow flag
TMOD =0x01 ;
// set Timer 0 mode 1
TL0 = 0xAF ;
TH0 = 0x3C ;
// Set the time to count for 50 milli seconds.
// Load counter to count for 50,000 counts
// Thus 0xFFFF - ( 50,000 decimal) = 0x3CAF ;

TR0 = 1;
// start the timing
IP = 0;
// set interrupt priorities (all low)

IE = 0x82;
// enable timer0 interrupt

ET0 = 1 ;
// Activate timer0

P2 = 0x01 ; // Turn off buzzer, turn off relay, turn off LED
P1 = 0xFF ;

P2_0 = 0 ; // Turn on LED indicating that input reading will be done.

while(P1_7); // Wait until the start button is pressed.

call_buzzer(1);

find_input();
TR0 = 0x0; // Stop timer to clear the count variables so that any sec used for buzzer dont get counted
total_sec_fraction = total_sec = 0; // Reset the timing here. Error will be max 0.05seconds.
TR0 = 0x1; // Start timer
P2_7 = 1; // Turn on relay

while(total_sec <= (input_value*60))
{

P2_0 = 1 ; // Flash LED
for ( i = 0 ; (total_sec <= (input_value*60)) && i < 15000 ; i++);

P2_0 = 0 ;
for ( i = 0 ; (total_sec <= (input_value*60)) && i < 15000 ; i++);


}
P2 = 0x0 ; // Turn off relay
call_buzzer(2);

P2 = 0x01 ; // Turn off entire port.

while(1);
}


void timer0_isr(void) interrupt 1
{
TR0 = 0x00 ;

TL0 = 0xAF ;
TH0 = 0x3C ;
if( total_sec_fraction >= 20)
{
total_sec_fraction = 0 ;
total_sec++ ;
}
else
total_sec_fraction++ ;

TR0 = 0x01 ;
}

void find_input(void)
{
if ( P1_0 == 0 ) input_value = 1 ;
if ( P1_1 == 0 ) input_value = 2 ;
if ( P1_2 == 0 ) input_value = 3 ;
if ( P1_3 == 0 ) input_value = 4 ;
if ( P1_4 == 0 ) input_value = 5 ;
if ( P1_5 == 0 ) input_value = 6 ;
if ( P1_6 == 0 ) input_value = 7 ;
}

void call_buzzer( int i)
{
int sec_count ;
int temp ;
sec_count = total_sec ;
if( i == 1)
{
temp = 2 ;
}
if( i == 2)
{
temp = 4 ;
}
while( (total_sec - sec_count) <= temp)
P2_1 = 1 ;
P2_1 = 0;
}