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

[Patrick Fletcher-Jones]

Ahh Johannes if only I had know that about 18 months ago...

Many thanks for pointing that out.

VBR,
Patrick


On 14/08/2008 20:30, "Johannes Assenbaum" <jassenbaum at htp-tel.de> wrote:

> 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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>>> 
>>> 
> 
> 
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