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

[BobGardner at aol.com]

My guess is the problem is the cli and sei in the interrupt handler. The  
first one does nothing, because the ints are off in the handler. The second one  
reenables ints BEFORE the regs are popped, which could be the problem.
 
 
In a message dated 1/3/2008 8:53:39 P.M. Eastern Standard Time,  
scottk at skelleyco.com writes:


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();
}   

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