RTC 32.768Khz 6pf -v- 12pf

"Paul Keinanen" <keinanen@sci.fi> wrote in message
news:of3m23tkai4ukpruljifrfklfeo4qd90le@4ax.com...
> On Sun, 22 Apr 2007 00:16:48 +0100, "Aly"
> <,shfskfjsf@sliuflky4iuhdf.erl> wrote:
>
> >I am working with the Maxim/Dallas RTCs which specify a requirement of
6pf
> >load capacitance crystals.  I can only source 12pf.
>
> One reason for low availability of very small capacitors is that the
> stray capacitances will dominate anyway. If everything else fails, put
<SNIP>

Thanks Paul :-)  Acknowledged.

"Aly" <,shfskfjsf@sliuflky4iuhdf.erl> wrote in message
news:a8WdncTMytRfBrfbnZ2dnUVZ8sGvnZ2d@bt.com...
> "Aly" <,shfskfjsf@sliuflky4iuhdf.erl> wrote in message
> news:TNmdnVfkKKuHBrfbnZ2dnUVZ8vidnZ2d@bt.com...
>
<SNIP>
>
> Rapid;
>
> 90-3313
> 90-3050
>

For anyone reading this thread in future, also search on similar Maxim part
numbers;

DS1305
DS1306
DS1307

Each will bring back different hits but are sufficiant for sourcing useful
information from a personal DIY point of view.

On Sun, 22 Apr 2007 00:16:48 +0100, "Aly"
<,shfskfjsf@sliuflky4iuhdf.erl> wrote:

>Hello,
>
>I am working with the Maxim/Dallas RTCs which specify a requirement of 6pf
>load capacitance crystals.  I can only source 12pf.
>
>It is mentioned in the datasheets that a load (12pf) which is greater than
>what the RTC is designed for (6pf); "If the capacitive load is less than the
>crystal was designed for, the oscillator runs fast. If the capacitive load
>is greater than what the crystal was designed for, the oscillator runs
>slow."
>
>Would anyone have any idea how much difference this will make in practice?
>I think I've read on the net in the past that this is only runs into a
>minute or so over each month but can't find that information now.
>
>Just wondering if anyone has experience of this?  A few minutes a month
>isn't a huge problem, it's really just for date stamping.
>
>Many thanks,
>
>Alison
>
Are you sure the datasheet says "what the RTC is designed for"? I
would expect not. The crystal has been specified to run on its
frequency with a certain load, not the RTC.

Taking the first hit I find with google, the motional capacitance of a
watch crystal is 2fF. Putting 12pF in series wil give about 7 seconds
change per day. Putting 6pF in series gives 14. So the difference
between 12p and 6p will be about 7 seconds per day.

If your crystals are specified for 12p, just place 2 times 22p (or
better 22p and 27p) at the crystal. I would be really surprised if the
RTC would not work. You might have to substract the port capacitance
of the RTC itself (order of 3p?).

Cheers,

Joop

Aly" <,shfskfjsf@sliuflky4iuhdf.erl> a &#4294967295;crit dans le message de news: 
TNmdnVfkKKuHBrfbnZ2dnUVZ8vidnZ2d@bt.com...
> Hello,
>
> I am working with the Maxim/Dallas RTCs which specify a requirement of 6pf
> load capacitance crystals.  I can only source 12pf.
>
> It is mentioned in the datasheets that a load (12pf) which is greater than
> what the RTC is designed for (6pf); "If the capacitive load is less than 
> the
> crystal was designed for, the oscillator runs fast. If the capacitive load
> is greater than what the crystal was designed for, the oscillator runs
> slow."
>
> Would anyone have any idea how much difference this will make in practice?
> I think I've read on the net in the past that this is only runs into a
> minute or so over each month but can't find that information now.
>
> Just wondering if anyone has experience of this?  A few minutes a month
> isn't a huge problem, it's really just for date stamping.
>
> Many thanks,
>
> Alison





Hi,

Have a look at DALLAS app note :
"Application Note 58 - Crystal Considerations for Dallas Real-Time Clocks"

Yvan