AVR-Chat

Hi All. Look at my source code, i couldn't find my fault there.
I want to get 40Khz, but in VMLAB it shows 29Khz.here is my source code.

/*****************************************************
This program was produced by the
CodeWizardAVR V1.24.8b Professional
Automatic Program Generator
© Copyright 1998-2006 Pavel Haiduc, HP InfoTech s.r.l.
http://www.hpinfotech.com

Project : 
Version : 
Date    : 13.02.2008
Author  : Khahramon                            
Company : TUIT                           
Comments: 


Chip type           : ATmega16
Program type        : Application
Clock frequency     : 4,000000 MHz
Memory model        : Small
External SRAM size  : 0
Data Stack size     : 256
*****************************************************/

#include <mega16.h>
#define xtal 4000000

#define hz 40000


interrupt [TIM1_OVF] void timer1_ovf_isr(void)
{
// i need 40Khz signal in PORTA.0, that's way i took 80kHz interrupt 
TCNT1=0x10000-(xtal/1/(2*hz));
PORTA.0^=0x01;


}

void main(void)
{

PORTA=0x00;
DDRA=0x01;
TCCR1A=0x00;
TCCR1B=0x01;
// i need 40Khz signal in PORTA.0, that's way i took 80kHz interrupt 
TCNT1=0x10000-(xtal/1/(2*hz));

ICR1H=0x00;
ICR1L=0x00;
OCR1AH=0x00;
OCR1AL=0x00;
OCR1BH=0x00;
OCR1BL=0x00;



// External Interrupt(s) initialization
// INT0: Off
// INT1: Off
// INT2: Off
MCUCR=0x00;
MCUCSR=0x00;

// Timer(s)/Counter(s) Interrupt(s) initialization
TIMSK=0x04;

// Analog Comparator initialization
// Analog Comparator: Off
// Analog Comparator Input Capture by Timer/Counter 1: Off
ACSR=0x80;
SFIOR=0x00;

// Global enable interrupts
#asm("sei")

while (1)
      {
         
      };
}
/***********************************
Can you help me where's my mistake