Re: RTC 32.768Khz 6pf -v- 12pf

On Sun, 22 Apr 2007 00:16:48 +0100, "Aly"
<,shfskfjsf@xxxxxxxxxxxxxxxxxx> 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 data*** 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
.

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