Thanks,

I agree that SBBs suck and, like I said, I put a lot
of care into the PCB design (I know AN58 well by now)
but am hoping to resolve this wierd drift thing before
committing to solder.  I'd like to build it just once.
 I'm probably being just a little too AR about this
but you dont get perfect by accepting good enough. 

I wonder if humidity might be a culprit with the SBB
changing capacitance and thus affecting the crystal
frequency.  

This is part of a vehicle presence sensor monitor. 
I'll be using it to trigger a number of actions
including a camera and logging the event time so
reasonable accuracy is important.  I'll probably have
to regularly set the time in the RTC.

Thanks,
Phil

--- Paul Hutchinson <paullhutchinson@yahoo.com> wrote:
> The crystal inputs of Dallas RTC chips have an
> extremely high impedance
> (~10^9 ohms). IME, a solderless breadboard is
> definitely not suitable for
> this circuit.
> 
> Dallas Application Note 58 "Crystal Considerations
> with Dallas Real Time
> Clocks" has very good troubleshooting and PCB layout
> information.
> 
> Paul
> 
> > -----Original Message-----
> > From: piclist-bounces@mit.edu On Behalf Of phil B
> > Sent: Sunday, May 08, 2005 7:19 PM
> >
> > OK, I'll try it again.  Sync up == check atomic
> clock.
> >  PC clock is then *very* close to atomic clock
> (like
> > milliseconds).  sheesh, maybe some one else has an
> > idea.
> >
> > Restatement of problem with omission of anything
> to do
> > with PC:
> >
> > I have a Maxim/Dallas DS1302 RTC and supposed 20
> PPM
> > crystal (32768 hz).  It seems to work ok but I'm
> > seeing some fairly odd drift with it.  I compare
> it
> > against an atomic clock every day.
> >
> > What I'm seeing is that the DS1302 is not off by
> > a constant amount but drifts around by as much as
> 30
> > seconds + or - in a day.  That is to say one day
> the
> > RTC will be ahead by 20-30 seconds and the next
> day it
> > will be behind by some amount.  The room that its
> in
> > stays within 5 degrees F and I haven't gotten
> > physically near (2-3 ft) the circuit in question
> (I'm
> > using a PIC to send the time via serial I/O).  The
> > circuit is in a solderless breadboard so I know
> there
> > is extra capacitance probably slowing the crystal
> > down.
> >
> > So, I would have though under those circumstances,
> the
> > RTC would be off by a relatively fixed percentage.
>  Is
> > this just a case of the solderless BB causing
> problems
> > or is there something else going on?  something
> that I
> > might compensate for?
> >
> > The PCB its going into has ground rings on both
> sides
> > of the board for the crystal, I've routed signals
> away
> > from the RTC in general and the crystal in
> particular
> > and have lots of grounded copper areas.
> >
> > Phil Barrett
> 
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