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Topic: Why are real-time clocks always 32.768khz? (Read 3 times) previous topic - next topic

afremont


I have a question, 2 infact.

What is the best oscilator circuit for a "clock crystal". circuits for 8+MHz crystals abund, can the same topologies be used for clock crystals ? I'm designing a clock circuit to drive an old firestation stncro clock but I'm having trouble keep my clock source stable.

Can the arduino be burned with a "watch crystal" bootloader ? it would be great for clock and timing applications where speed is not problem and low power is needed,


What frequency do you need to generate?  Yes, you can use a micro such as the arduino to generate a much more accurate signal than what you are working with right now.  It will be as accurate as the crystal that clocks the micro.

Do you have the master clock too, or just slave(s)?
Experience, it's what you get when you were expecting something else.

sparkylabs

I'm working with a 32.768 KHz clock crystal

I'm asking for the arduino from an academic point of view. Of course having a secondary clock oscillator on the board that connects to an input pin with interrupt would be a better way of doing it so that the arduino can do plenty between clocks if needed.

BillO


Right, well we did but mine was a bit unstable so I'm avoiding it.


Well then, it did not work in that situation did it?  My guess is, if it had worked adequately, you'd still be using it.  Perhaps Dr. Dave was right after all.

What you could try is constructing an external crystal oscillator circuit, optimized for the type of crystal you want to use, or use a pre-built oscillator, then feed the output of that into the Rs input of the MC14541B.
Facts just don't care if you ignore them.

sparkylabs

Well I am messing about on a breadboard and with what looks like "transmission central" even using an external clock the timer chip does some funny stuff so I'm wondering if my instability is due to the mess of wiring or the part not being suitable. The chip will oscillate with the crystal but it was unstable, the lower frequency sections are also unstable though and I'm guessing that the higher in frequency I go the more unstable it becomes more quickly, that's all.

I can try wirig it up nice on some vero board to try and eliminate the long wires being a problem. Yes I could do with a diagram suited for a 32.768 KHz crystal and someone previously described one.

retrolefty


Quote
Wristwatch  is Wrist watch.  The temperature range should be ~37°C and not 0°C to +40°C.

The 32kHz tuning fork crystal for watches has been chosen as a compromise of size and the fact it has a most stable region at 25-28degC, what is a typical temperature at your wrist. The ratio of wearing the watches on the wrist, afaik, is 16h/8h. So the watches crystal frequency is set. I have got a cheap kitchen clock with 4.194M crystal and it does a few seconds a year (!), what is a matter of luck, of course. Pls consider the capacitance used with the crystal as well, as it has a huge effect. For example a 1-2 pF change with a 32kHz watch crystal makes few seconds a day.
PS: pls mind the terms "stability", "precision" and "accuracy" are tricky when talking timekeeping  :)
A clock might be perfectly precise but inaccurate..
http://literature.agilent.com/litweb/pdf/5965-7984E.pdf


Nice article. I particularly liked this paragraph:

Quote

The most accurate measurement known to humanity is the measure-
ment of the duration of the second. The peak of the pyramid for accu-
rate time and frequency is the international reference, UTC. The current
best accuracy for the determination of the second results in a time error
of ±0.3 nanoseconds (billionths of a second) per day. This is equivalent
to ±1 second in 10 million years.


As the article may be somewhat dated the current time error may be even less?

Lefty

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