AVR-Chat

On Mon, 12 Dec 2005 15:12:52 +1030 you wrote:

>Hi All,
>
>I'm trying to get a RTC going on a Mega 8 using a 32 kc watch crystal
>as per Atmel app note AVR134.
>
>The code works well except that the time gains about 3 seconds per hour.
>
>The crystal is a surplus watch crystal  that I had and I have no idea of 
>the capacitance
>load that it requires.
>
>Anyone on the list used a watch crystal on an avr and how accurate can 
>you make them for
>time keeping ?
>
>Do I need a 6pf or 10pf crystal?
>
>Do I need any extra caps to gnd, not using any at the moment, would a 
>small trimmer help?

I would enable the CKOPT fuse which makes added capacitors unnecessary.
I would expect a few seconds a day error from a new crystal.
If you want something more accurate maxim give away free samples of the DS32KHz oscillator
which is good to about a second a week. 
--
Ralph Hilton
http://www.ralphhilton.org
C-Meter: http://www.cmeter.org
FZAOINT http://www.fzaoint.net