Author Topic: Atomic clock  (Read 4049 times)

Offline RJ

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Re: Atomic clock
« Reply #15 on: February 15, 2012, »
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My Aeon at work gains a second every 3 months. I reset it at the DST changes.

I haven't looked at the RTC chip pinouts but perhaps if you were THAT concerned about it, you might be able to change bump it forward or back by one second.

WOW! that inaccurate huh!

Read this I found to get an ideal of how accurate this is:

Crystal Clock accuracy is defined in terms of ppm or parts per million and it gives a convenient way of comparing accuracies of different crystal specifications.
The following headings give practical calculations showing the typical errors you will encounter when using a clock of a specific type with a specific accuracy.
How good is a 1% accurate clock ?
If you look at a day's worth of timekeeping then you have 24 x 60 x 60 = 86400 seconds in a day.  So the maximum error after a day has passed is 1% of 86400 = 864 seconds = 14 minutes!

How good is a typical crystal ?
A typical crystal has an error of 100ppm (ish) this translates as 100/10e6 or 0.001% (its the same number expressed as a percentage).  So the total error on a day is 86400 x 0.01%= 8.64 seconds per day so in a month you would loose 30x8.64 = 4.32 minutes per month.

How good is a watch crystal ?
A watch crystal has an error of 20ppm (ish), but you have to design the board layout well, this translates as 20/10e6 (0.002%) which  gives an error over a day of 86400 * 0.002% = 1.73 seconds per day so in a month it looses 30x1.72 = 51 seconds or 1 minute a month (approx). 

The modern pic chips have 1% clocks, this is why I use the $8 clock device on the Aeon. So you see 1/3rd of a second per month is fairly accurate as clocks goes.


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Offline rm357

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Atomic clock
« Reply #16 on: February 15, 2012, »
It also seems that a day is not exactly a day as the Earth doesn't quite spin perfectly, but rather wobbles on its axis. And even after taking that into account, the average day is not quite 24 hours, which results in the international standards people evaluating whether or not to add a leap second to UTC and GPS every 6 months. It looks like leap second #35 will be added on June 30th of this year... That makes 35 seconds in 50 years...

Warner Robins, Georgia, USA

Offline jnealand

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Re: Atomic clock
« Reply #17 on: February 15, 2012, »
I'm sticking with my Casio wristwatch which sets the time automatically every night.  I don't know how it does it I just know that I have not set the time on it in over 5 years.  Even for daylight savings time.  Better living thru science / technology I say.  And it was only about $35 years ago.
Jim Nealand
Kennesaw, GA

Offline JonB256

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Re: Atomic clock
« Reply #18 on: February 17, 2012, »
I use You are not allowed to view links. Register or Login for my checks.

And RJ - I'm perfectly happy with the accuracy. Putting a temperature compensated oscillator on the board is a great idea.

It is on my wall at work and many express an interest until they hear about soldering.