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

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist