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

[Bengt Ragnemalm]

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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