RE: [avr-chat] 32768 Hz clock

[Matt.VanDeWerken]

I've had problems in a past life with RTC crystals drifting due to mismatched 
capacitance. The purchasing guy bought a different crystal to the one designed 
in because it was cheaper, and we wondered why the clocks were drifting by 
large amounts. This was my first job after college, so I was a little green, 
and it took me a while to work out the cause. Check the data sheet for the 
crystal you have, and double-check you have loaded it with the correct 
capacitance. 

Cheers,
Matthew van de Werken - Electronics Engineer
CSIRO E&M - Rock Mass Characterisation - 1 Technology Court - Pullenvale - 4069
p: (07) 3327 4142 * f: (07) 3327 4455 * e: address@hidden
"We do not inherit the earth from our ancestors, we borrow it from our 
children." 
-- Native American Proverb

> -----Original Message-----
> From: address@hidden 
> [mailto:address@hidden
>  On Behalf Of Hugo González Monteverde
> Sent: Wednesday, 24 May 2006 2:46 PM
> To: address@hidden
> Cc: AVR-CHAT
> Subject: Re: [avr-chat] 32768 Hz clock
> 
> 
> 
>  > 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!  >
> 
> Totally.
> 
> This is realted to the capacitances you connect to the crystal.  I am 
> not an expert at this, but check application notes for crystal 
> considerations, such as the one from Dallas. They actually 
> explain a bit:
> 
> "The frequency accuracy of a crystal-based oscillator circuit 
> is mainly 
> dependent upon the accuracy of the crystal and the accuracy 
> of the match 
> between the crystal and the oscillator capacitive load."
> 
> Take a look at it :
> 
> http://www.maxim-ic.com/appnotes.cfm/appnote_number/58
> 
> 
> Hope that helps.
> 
> Hugo
> 
> 
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