Lpc2000

arm7dude wrote:

>Tom,
>
>In production environments, you would like to "tune" the oscillator
>circuit so that the rtc runs on time. Having a 32768 hz output makes
>this really easy. National Semi had some rtc chips that could output
>the clock for "tuning." This is the only time I ever used it. 
>
>To do tuning, you need a calibrated crystal from the crystal
>manufacturer. You can then change the load caps until the frequency
>come out right. You have to do this as the stray capacitance on the
>board can only be approximated.
>  
>
Now that brings up a thought.  You don't use surface mount caps in 
production, or do you? Do you tune each crystal or only  a few crystals 
within a Lot Number and then run the rest off with that value?

In the past, clock circuits on most boards used a higher frequency xtal 
to divide down to 32KHz.  We had set the 2.45MHz (example) using freq 
counter.  I've not a lot of experience with how the 32KHz is going to 
behave.  This board of mine is the first real 32KHz clock design so I'd 
like to avoid some problems that you've seen?

TIA,

TomW



-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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