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Re: real time clocks & 60hz from power company
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I have a venerable old Heathkit digital clock that is driven off the
power line. It sometimes gains seconds and sometimes loses seconds, but
it's never exactly correct except for shortly after I set it to the NTP
time. It certainly doesn't drift by seconds a day, though.

No doubt the power line drifts and I don't know how accurate the power
company is trying to keep the frequency. In spite of their efforts there
are still the considerations of how your circuit is designed so that it
doesn't count extra pulses due to line noise or loose pulses.

Since you state that "approximate" is acceptable, then the power line
should work fine for elapsed times of reasonably short duration.

Oh heck, why not just include a GPS receiver and get a very accurate,
stable time reference?  :-)

...Tom
 
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Tom Sheppard
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Re: real time clocks & 60hz from power company
Tom Sheppard said for all posterity...

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I had one of those that I put together in 1975, I think.  It had 3
2-digit gas discharge displays. I noticed the same periodic
fluctuations in the seconds, but there never seemed to be any
significant accumulation of errors over time.

For a bit of nostalgia, this link shows a picture of the model I
had.

http://www.decodesystems.com/sp-352.html


Casey

Re: real time clocks & 60hz from power company

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I have the exact same model! I set it yesterday when I sent my message
and it seems to have gained half a second since then.

If you want to see a very interesting photo of the internals of the
Mostek MK-5017 clock chip used, then visit my web site at:
http://www.Surreal-Time.com/WarStories/Static.html

There's a little surprise on the chip.

...Tom

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Tom Sheppard
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Re: real time clocks & 60hz from power company
Tom Sheppard said for all posterity...
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That's downright funny.

One of the displays on my clock finally burned out years ago.  I
swapped the seconds one for the minutes display and ran it for
years after that (with no seconds).  I'm not sure if it's still in
a box around here somewhere or not.

Heathkit must have sold a lot of those particular clock kits.


Casey


Re: real time clocks & 60hz from power company
On Tue, 30 Sep 2003 16:26:41 GMT, Tom Sheppard

[...]
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More surprises at the Silicon Zoo.  See
http://micro.magnet.fsu.edu/creatures /

Regards,

                               -=Dave
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Re: real time clocks & 60hz from power company

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In the short term, the 60 Hz may drift somewhat.  But in the long term,
they will maintain a 60 Hz corrected average.

So it's accurate, not precise.  (Your Power Company May Vary.)

--
Ron Sharp.



Re: real time clocks & 60hz from power company
On Mon, 29 Sep 2003 13:37:40 +0000 (UTC), "Dave Sudolcan"

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You have to ask yourself, what is the time used for ?

If it is used just for human consumption as a nice feature ("nice to
have"), it should be fine. However, time stamps between your system
and some other systems based on crystal/GPS timing are used, then the
situation is much more complicated.

A long time ago, we had a PDP-11 as the central controller (which used
mains derived clock), communicating with various micros over half a
dozen serial lines, which counted their time from the local crystal
oscillators.  Some of these identical micros were in a moving open
frame construction with good ventilation, while some were in closed,
stationary boxes with poor ventilation. All the clocks in the micros
were synchronised over the serial line prior to start of operations.

The system was very critical about the drift between the various
micros and worst of all, the drift rate depended on the speed the open
frame devices moved and also if the doors to the hall were open (-30 C
outside), so the drift could be quite bad.

It was unthinkable to use a separate clock line to all devices (some
connected through slip rings).

Finally, we had to abandon the too unreliable crystal derived clocks
and use the mains as the common time base to all devices, counting
cycles since the serial line reset command, avoiding any clock
problems fora few decades.

It really depends what you are trying to achieve.

Paul
  

Re: real time clocks & 60hz from power company
Okay... I've heard enough on this topic.  Thanks for all the input.  I
think the 60 hz clock will do just fine for my elapsed run time timer.
+/- a few seconds occasionally should not be an issue for this system.
I've already got both an analog and a digital filter on the clamped
signal derived from my power supply transformer, so I think all will be
okay.  If it turns out otherwise, I'll post an OOOPS with details.

Thanks again,
Dave.


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Re: real time clocks & 60hz from power company

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I still don't understand why you can't use the crystal-driven
timer on your board.

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Re: real time clocks & 60hz from power company
-
-> Okay... I've heard enough on this topic.  Thanks for all the
-> input.  I think the 60 hz clock will do just fine for my
-> elapsed run time timer.
-
-I still don't understand why you can't use the crystal-driven
-timer on your board.

Drift. I have a sunrise/sunset light controller that uses a 32768 Hz crystal
that's in a unheated basement. It drifts a few minutes a month.

When I get around to redoing the project, I'll definitely take the power line
signal to stay on track.

BAJ

Re: real time clocks & 60hz from power company

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Ah, I see.  That could be annoying after a while.

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Re: real time clocks & 60hz from power company

Sudolcan wrote:
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   Try the DS32khz oscillator at maxim-ic.com. Worst-case drift should
be less than a second a month, and if it switches to battery power
when the 60 Hz power goes down, the clock won't need resetting (unless
the battery dies) and thus will be MORE accurate than the power line.

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   If you really want to be anal, use the power line normally for its
long-term accuracy, and switch to the oscillator-and-battery when AC
power is lost. :)

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Re: real time clocks & 60hz from power company
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The grid in the US and Canada, at least, is closely controlled
over a 24 hour period.  The same applies to most of Europe, and
other places.  However, it can go out (power loss), so that if
your system is not mains powered you should have an independant
crystal controlled clock.  You can resynchronize it at intervals
from the mains driven clock, and detect power losses by an
excessive change (at which time you resynchronize the mains
clock).

The system can easily tell the difference between 50 and 60 hz
power.  40 years ago I did it with a 10 hz multivibrator,
synchronized by the output of the zero-crossing detector.  This
gave the same result on either 50 or 60 hz input.

Your biggest problem will be immunity to line noise in the cycle
counting mechanism.  You need at least a low pass filter together
with a Schmidt trigger.  The alarm clock mechanisms are basically
synchronous motors, which do that filtering very well.

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Re: real time clocks & 60hz from power company
I checked with my power company about this and was told that their
frequency may vary around the 60 Hz nominal, but they strive to make
up or lose during each 24 hour period so that the total number of
cycles during each 24hr period is 60*86400.  Their reference us UTC,
which is referenced to the "atomic clock", cesium, and soon an even
better one, using mercury atoms.  UTC time makes sure the earth is
back at its starting point in its orbit every 365.24(whatever it is)
UTC days.  If needed, the UTC committe can add or subtract a "leap
second" once or twice a year to keep things right.  
It's interesting to compare a digital clock(referenced to 60 Hz, not
quartz xtal)to the time signals from WWV.  Sometimes you can actually
detect a change from the the digital clock gaining to losing during
the day.  If you really want to hurt your brain over the concept of
time, google for sundials or Equation of Time.

Practically, even a regular quartz controlled oscillator is just not
too hot for holding time.   At 50 ppm, that's 4.3 seconds/day.  I
built a clock using a Dalis temp-compensated 32KHZ chip together with
2 8-bit conters set to divide by 32768 and wound up with an accuracy
of about 0.7 ppm, or 2 seconds/month.  With more care given to the
supply voltage of the 32KHZ, I think I could have done better by a
factor of 3 or so.  Could your application use something like this?
It's 3 more chips and <$10.  The output pulse every second can be use
to update and LED/LCD displaly or provide an external interrupt to a
uP or microcontroller.

On Mon, 29 Sep 2003 13:37:40 +0000 (UTC), "Dave Sudolcan"

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He wouldn't have made them out of MEAT!                           - John Cleese

Re: real time clocks & 60hz from power company
On Mon, 29 Sep 2003 13:37:40 +0000 (UTC), "Dave Sudolcan"

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You might find this thread interesting, particularly the description
of how they deal with accumulated drift > 2 secs.

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&selm01%bc3051%249a876520%246d664ccf%40BAHN&rnum14 %

George




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