[Icc-avr] Is there something wrong with this RTC interrupt?

[Scott]

My RTC is running slow.  I have a number of devices that are
running this code, and all of them are losing somewhere
around 3 or 4 minutes per month.  

Looking at my code, it seems that I must be missing ticks
while the isr is running.  If I did my math right, it seems
like I must be missing 3 or 4 ticks per second.

If this is right, I think what I need to do is to init the
timer as a countdown, with a preset value equal to 32,768
minus the number of lost ticks.  Am I right about this?
This is an ATMega64 running at 3.69 MHz.  Does it make sense
that it would lose this many ticks?  Seems like a lot when
there is more than 1,000:1 ratio between the main clock and
the RTC frequency...



void rtc_init(void) {  // init timer for 1 second interrupt
CLI(); //disable all interrupts - prevent errant interrupts
until set up
TCCR0 &= ~(1<<WGM00); TCCR0 &= ~(1<<WGM01); // Set timer for
normal operation  WGM00 = 0, WGM21 = 0
TCCR0 &= ~0x02; TCCR0 |= 0x05; // counter 0 prescaler = 128
CS02=1, CS01=0, CS00=1 (Timer Clock = 32,768 / TCCR2)
TIMSK &= ~((1<<TOIE0)|(1<<OCIE0)); // Disable Timer0
Interrupt  TOIE0 = 0, OCIE0 = 0 
ASSR |= (1<<AS0);  //set Timer/Counter0 for asynchronous
operation - 32,768 Hz crystal  AS0 = 1  
TCNT0 = 0x00;
TIMSK |= (1<<TOIE0); // Enable Timer0 overflow Interrupt
SEI(); //set the Global Interrupt Enable Bit  
}  //void rtc_init(void)


void timer0_overflow_isr(void) {  // Clock ISR - overflow
occured TCNT2=OCR2
  CLI(); // turn Off interrupts
  GeneralTimer++;
  CommunicationsTimeout ++;
  SecondFlag = 1;
  WDTimerFlag = 1;
  if (++second>=60) { //keep track of time, date, month, and
year
     second=0;
	 MinuteFlag = 1;  // Additional clock service is now
passed back to the main code 
    }
  SEI();
}