[求助]MEGA128定时器不工作的问题！！！万分火急！！！

fanmj

弄了好几天，发现定时器不工作。然后遍了这么个程序来看看它是否真的不工作，下载到片子上，发现灯一直都是亮的，没有产生循环效果。由于是刚接触128这个片子，所以有的寄存器不是很熟悉。
希望哪位好心人能帮我看看，问题到底出在哪里？？？
//ICC-AVR application builder : 2005-5-10 16:21:30
// Target : M128
// Crystal: 4.0000Mhz
#include <iom128v.h>
#include <macros.h>



unsigned char an_time_count=0;
unsigned char an_time_1s=0;

#pragma interrupt_handler timer0_ovf_isr:17
void timer0_ovf_isr(void)
{

TCNT0 = 0x64; //reload counter value

an_time_count++;

if (an_time_count>100) {an_time_count=0;an_time_1s++;}

}
void port_init(void)
{
PORTA = 0xFF;
DDRA  = 0xFF;
PORTB = 0xFF;
DDRB  = 0xF8;
PORTC = 0xFF; //m103 output only
DDRC  = 0xFF;
PORTD = 0xFF;
DDRD  = 0xF7;
PORTE = 0xFF;
DDRE  = 0x00;
PORTF = 0xFF;
DDRF  = 0xFF;
PORTG = 0x1F;
DDRG  = 0x06;
}
//TIMER0 initialize - prescale:256
// WGM: Normal
// desired value: 10mSec
// actual value:  9.984mSec (0.2%)
void timer0_init(void)
{
TCCR0 = 0x00; //stop
ASSR  = 0x00; //set async mode
TCNT0 = 0x64; //set count
OCR0  = 0x9C;
TCCR0 = 0x06; //start timer
}
//call this routine to initialize all peripherals
void init_devices(void)
{
//stop errant interrupts until set up
CLI(); //disable all interrupts
XDIV  = 0x00; //xtal divider
XMCRA = 0x00; //external memory
port_init();
timer0_init();
MCUCR = 0x00;
EICRA = 0x00; //extended ext ints
EICRB = 0x00; //extended ext ints
EIMSK = 0x00;
TIMSK = 0x01; //timer interrupt sources
ETIMSK = 0x00; //extended timer interrupt sources
SEI(); //re-enable interrupts
//all peripherals are now initialized
}
void main (void)
{
init_devices();
while(1)
  {
  PORTD=0xff; PORTB=0xff;
if(an_time_1s>=3)
  {PORTD=0x00; PORTB=0x00;
an_time_1s=0;}
}
}