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


--- Olin Lathrop <olin_piclist@embedinc.com> wrote:
> phil B wrote:
> > I should have been a bit clearer.  I am checking
> when
> > I sync up the time.  At that point the PC clock
> should
> > be  pretty close.
> 
> No, that was my point.  Take a look at the clock
> when you sync up.  It
> doesn't jump suddenly, unless maybe it's way off. 
> In some cases anyway, the
> system appears to try adjusting it smoothly.  It may
> take a while for it to
> get to the "correct" time.  You'll be much better
> off checking with a real
> reference.
> 
> 
>
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