[Icc-avr] 32 KHZ Crystal problem, solved I think

[Paul Mateer]

Whenever we use a crystal we always include 22pF caps from the xtal leads to
ground.  Never had a problem.

73 de Paul, AA9GG


On Thu, Aug 14, 2008 at 1:45 AM, Bengt Ragnemalm <benra at imt.liu.se> wrote:

> Sorry for lat answer but I couldn't resist.
>
> Loading capacitors to crystal is a never ending story on AVR freaks among
> others. But actually there is nothing like "trying with xxx values". Every
> crystal you buy has a predefined value of the loading capacitors and this
> is
> the one and only value you should use. There are exceptions though. First,
> sometimes the datasheet of for example the AVR may say that loading caps
> are
> not necessary. Ok, fine, I guess that must be correct so do not use any in
> this case. Sometimes the board (if not using a PCB) may introduce so much
> extra capacitance that you need to reduce the typical value.
>
> This should be no big source for problems if you just follow the datasheet
> of the crystal and the AVR.
>
> /Bengt
>
> > -----Ursprungligt meddelande-----
> > Från: icc-avr-bounces at imagecraft.com [mailto:icc-avr-
> > bounces at imagecraft.com] För Patrick Fletcher-Jones
> > Skickat: den 31 juli 2008 11:05
> > Till: Discussion list for ICCAVR and ICCtiny Users. You do NOT need
> > tosubscribe to icc-announce if you are a member of this.
> > Kopia: Paul Mooney; Tom Byrne; Martin Pearce
> > Ämne: [Icc-avr] 32 KHZ Crystal problem, solved I think
> >
> > Morning all,
> >
> > As some of you are aware I've had a nightmare with the 32KHz crystal
> > oscillator connected to timer 2 on a mega 1280 (running at either 3.3V or
> > 5V)
> >
> > There is / was a bug in the apps builder which set the prescaler bits
> > wrong
> > which I fixed by manually. Even after that my nice little 32 kHz crystal
> > was
> > running at 192KHz, not good when trying to run a timer.
> >
> > After tons of playing around, trawling the AVRfreaks news groups, trying
> > different crystals etc.. I have found that you really need to add a 22pF
> > cap
> > on each side of the crystal to ground. Once I did this it runs fine, take
> > the caps off it stops working.
> >
> > I've now changed my circuit diagrams to include these caps.
> >
> > I am going to try a 6pF crystal, because years ago I had a similar
> problem
> > with a Dallas RTCC that only worked with a 6pF crystal and crystals
> > (correct
> > me if I'm wrong) are normally 12.5pF
> >
> > Anyway its taken me ages to fix this problem and a lot of stress (unhappy
> > customer, well they are being ok really, its just very late) so I just
> > wanted to share this with you guys and hope it saves someone else a lot
> of
> > trouble.
> >
> > Patrick
> >
> >
> > _______________________________________________
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>
>
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-- 
Paul Mateer, AA9GG
Elan Engineering Corp.
www.elanengr.com
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