[avr-chat] 32768 Hz clock

[Royce Pereira]

Hi all,

I built an experimental digital clock using Mega8. The output is 7 seg  
HH:MM:SS.
System clock is internal 1MHz RC oscillator.
A 32768 Hz watch crystal is connected to TOSC1 & TOSC2.
Timer2 is used with the following setup upon reset-

        ASSR |= 1<<AS2; //select external asynchronus clock.

      TCCR2=0x9;    //prescaler= CK/1
      OCR2=64;    //512 interrupts per sec
      TCNT2=0;

      while(ASSR & 7){ } //wait for busy flags.

      TIMSK=0x80;    //Enable Timer2 comp interrupt.

      sei();      //enable global interrupt

In my ISR I have:

ISR(TIMER2_COMP_vect)
   {
      if(!--sec)
         {
            sec=512;
                //update time every sec..
          }
        return;
   }

However, comparing to my wristwatch,  I find my clock falls behind about 1 
minute every hour, so after 10 hrs, it's behind by 10 mins! My code above seems 
OK. Are there any hardware issues I need to address? Thanks for any help!

Regards,
-Royce



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