RTC 32.768Khz 6pf -v- 12pf

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

"Aly" <,shfskfjsf@sliuflky4iuhdf.erl> wrote in message
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.
>
<SNIP>

Got one!!

Rapid;

90-3313
90-3050

In article <TNmdnVfkKKuHBrfbnZ2dnUVZ8vidnZ2d@bt.com>, 
,shfskfjsf@sliuflky4iuhdf.erl says...
> 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

When I used a 12 pf crytal with a DS1202, it gained a minute a DAY.

--Gene

On Apr 21, 7:16 pm, "Aly" <,shfskf...@sliuflky4iuhdf.erl> wrote:

> 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

As a point of interest, note that PC motherboards used to have a
trimcap on the 32768Hz RTC xtal to tweak the timebase. Several minutes
per day were available through a small (<20pF) trimcap.

These trimmers have disappeared completely, and I don't think it's
because the circuits or components are vastly more stable/better
matched; I think it's because most people are now running OSes that
connect to an NTP server frequently to correct the RTC.

larwe wrote:


> As a point of interest, note that PC motherboards used to have a
> trimcap on the 32768Hz RTC xtal to tweak the timebase. Several minutes
> per day were available through a small (<20pF) trimcap.

Never seen that on the PC AT and higher. XT didn't have the RTC at all.
Will you provide a reference?

> These trimmers have disappeared completely, and I don't think it's
> because the circuits or components are vastly more stable/better
> matched; I think it's because most people are now running OSes that
> connect to an NTP server frequently to correct the RTC.

I think this is because nobody really cares about the RTC accuracy. 
Besides, it is lot easier to correct the time count digitally, rather 
then tweak the trimmers.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

On Apr 21, 9:15 pm, Vladimir Vassilevsky <antispam_bo...@hotmail.com>
wrote:

> > As a point of interest, note that PC motherboards used to have a
> > trimcap on the 32768Hz RTC xtal to tweak the timebase. Several minutes
> > per day were available through a small (<20pF) trimcap.
>
> Never seen that on the PC AT and higher. XT didn't have the RTC at all.
> Will you provide a reference?

Take a look at any 486-class or earlier Pentium motherboard. I am much
too lazy to take photos right now!

> Besides, it is lot easier to correct the time count digitally, rather
> then tweak the trimmers.

I'm not aware that the RTC macrocells embedded in super I/O chips
include such tweaking functionality.

larwe wrote:

> On Apr 21, 9:15 pm, Vladimir Vassilevsky <antispam_bo...@hotmail.com>
> wrote:
> 
> 
>>>As a point of interest, note that PC motherboards used to have a
>>>trimcap on the 32768Hz RTC xtal to tweak the timebase. Several minutes
>>>per day were available through a small (<20pF) trimcap.
>>
>>Never seen that on the PC AT and higher. XT didn't have the RTC at all.
>>Will you provide a reference?
> 

I just depopulated an 8MHz XT-clone M/B which had the trimmer; I was
earlier going to suggest specifying a rock high in freq at 12pf and
add the trimmer.

Most XT-clones with RTC and AT clones had the trimmer.

Regards,

Michael

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
to 12 pF capacitors in series to get 6 pF nominally, but again the
strays may cause problems.

If there are some free PCB space, making a 6 pF capacitance from just
the stray capacitance between the PCB tracks is not hard. This could
even be adjusted by cutting out or adding sections, which can be
useful if the required load capacitance varies from one crystal to an
other. 

If the capacitance is from crystal terminal to ground, a multilayer
PCB with a ground plane, the capacitance can be formed between a PCB
track and the ground plane through the PCB dielectric, but
unfortunately some PCB materials are sensitive to humidity and the
capacitance may vary.

A DC blocking capacitor and a capacitance diode could also be used if
voltage control is required, but there are temperature coefficient
issues with capacitance diodes.

Paul

Gene S. Berkowitz wrote:

> When I used a 12 pf crytal with a DS1202, it gained a minute a DAY.

That's off allot more than a few pF of capacitance is going to make.  These 
things are prone to picking up stray AC noise and counting it.  At 60Hz, it 
could add as much as 158 seconds per day, 132 seconds at 50Hz.  When I was 
tinkering with some on a breadboard, I had to ground the crystal case to get 
it to keep good time.  Using smaller caps probably increased the amplitude 
of the oscillator perhaps drowning out the AC noise. 

"Anthony Fremont" <spam-not@nowhere.com> wrote in message
news:132mcb1jhspiq58@news.supernews.com...
> Gene S. Berkowitz wrote:
>
> > When I used a 12 pf crytal with a DS1202, it gained a minute a DAY.
>
> That's off allot more than a few pF of capacitance is going to make.
These
> things are prone to picking up stray AC noise and counting it.  At 60Hz,
it

<SNIP>

Also picked up off the net is that grounding the can case can limit odd
behaviour.