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

[Johannes Assenbaum]

It's just because Mega8/103/128 have on-chip caps for 32kHz xtal which Mega1280 etc. don't have.

On Atmel website there is app note "AVR097: Migration between ATmega128 and
ATmega1281/ATmega2561" addressing this.

Best regards,
Johannes
 

> Hi, Patrick Again the guy who had the crystal problems.

> I¹ve used the 32 KHz crystal connected in async mode on many of the Atmel
> processors, mainly Mega 8, 103 & 128 (all PCBs). Never having  a problem at
> all.

> It was only when we moved to using the mega 1280, upgrade in design from a
> 128 that these problems arose.

> We have produced many pcbs using the the mega 1280 and they have all had the
> same problem, with the 32 khz crystal not working so great.

> Basically some boards would not oscillate at all, other would be the wrong
> frequency and a few would work ok.

> On all the boards the crystals have been at max 2mm from the processor,
> protected with ground planes etc..

> Since we have fitted 22pF caps to ground on the 32 KHz crystal we¹ve not had
> a single problem. I¹ve read all the Atmel data sheets and not one of them
> that I can find says you need to add caps to the 32 KHz crystal

> We have spent months on and off with this problem, so I¹m just hoping our
> experiences may well save someone else from from pulling their hair out just
> as I ended up doing many times.

> VBR,
> Patrick


> On 14/08/2008 14:28, "Paul Mateer" <paul.aa9gg at gmail.com> wrote:

>> 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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