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Author Topic: Why are real-time clocks always 32.768khz?  (Read 3326 times)
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... is $1,700 USD, we have $550 USD gaps.

Well, $450, but close enough.

The 1969 watch was more than the 1966 car.  According to Wikipedia: "The price of Toyota Corolla E10 was 432,000 yen (US$1,200) when it was released in 1966"

from yesterday my math has problem...

Wiki here use apple  v.s. orange comparison, 1966  v.s. 1969.  Correct way should be apple  v.s. apple. The author of wiki might face same problem as I did, Can not find official 1969 Toyota Corolla price, such press release...

I figure out how to find out official 1969 Toyota Corolla price, have a trip to NYC central library by look up Microfilm of 1969 New York Times. but I run out of time lately...

« Last Edit: April 12, 2013, 09:20:03 pm by sonnyyu » Logged

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First quartz controlled wrist watch at 8192Hz in 1969:
http://en.wikipedia.org/wiki/Astron_%28wristwatch%29
Analog dial and hands.

Quote from: Wikipedia
The Astron was accurate to ±5 seconds per month, or one minute per year.

2ppm, better than the average 32.768kHz crystal-controlled RTC smiley-wink

if you need 2ppm RTC, You could try Temperature-Compensated one.

sample;-

DS32kHz 32.768kHz Temperature-Compensated Crystal Oscillator  0°C to +40°C  -/+2.0 ppm

http://datasheets.maximintegrated.com/en/ds/DS32kHz.pdf

It will cost far less than US$1,250.   smiley-razz


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First quartz controlled wrist watch at 8192Hz in 1969:
http://en.wikipedia.org/wiki/Astron_%28wristwatch%29
Analog dial and hands.

Quote from: Wikipedia
The Astron was accurate to ±5 seconds per month, or one minute per year.

2ppm, better than the average 32.768kHz crystal-controlled RTC smiley-wink

Railroad grade pocket watches of the late 1800's easily kept time within 40ppm as a bare minimum, many were much more accurate than this.  This being a mechanical watch that someone carries around in their pocket and constantly exposes it to violent temperature shifts, besides getting generally knocked about.  I love mechanical timepieces, they are truly the most amazing machines in terms of raw precision and reliability.  smiley
« Last Edit: April 12, 2013, 09:33:51 pm by afremont » Logged

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if you need 2ppm RTC, You could try Temperature-Compensated one.
sample;-
DS32kHz 32.768kHz Temperature-Compensated Crystal Oscillator  0°C to +40°C  -/+2.0 ppm

I like the DS3231 and DS3232 I2C RTCs, they include a TCXO with the same spec for almost the same price. DS3234 is an SPI version.
« Last Edit: April 12, 2013, 09:45:18 pm by Jack Christensen » Logged

MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

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Let's guess how Seiko did 2 ppm Crystal Oscillator for watch at 1969, but we can not even do it  for RTC at today.

my guess is;

1. Seiko produce a lot of  +/- 10 ppm Crystal Oscillators, by measure only use +/- 2 ppm one and drop the rest.
2. temperature-compensated circuit.
3. Wristwatch  is Wrist watch.  The temperature range should be ~37°C and not 0°C to +40°C.
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I have a TAG Heuer that is a 'certified chronometer'.  I don't know what that means these days, but it is noticeably more accurate if it is worn than if I leave it a the cottage all winter.
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Let's guess how Seiko did 2 ppm Crystal Oscillator for watch at 1969, but we can not even do it  for RTC at today.

It used to be common to include a trimmer capacitor in the crystal oscillator circuit, allowing the frequency to be set to exactly 32768Hz at a particular temperature; but you don't see that very often these days.
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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,
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For 8MHz crystals, you normally connect the crystal between the output and input of an un-buffered inverter, with a high-value resistor (1M or greater) in parallel with it, and capacitors between both ends of the crystal and ground.

For 32768Hz crystals, this circuit needs to be modified by connecting a series resistor of a few hundred K between the inverter output and the crystal/capacitor. When I developed a RTC oscillator for a commercial computer many years ago, I found that 560K for the series resistor and 33M for the parallel one worked well. The reason I chose such a high value for the parallel resistor was to keep the current consumption as low as possible.

The atmega processors don't need either of these resistors if the fuses are set correctly, because they provide equivalent functionality in the chip. There is a separate fuse setting for "Low frequency crystal oscillator", see the datasheet.

You don't need a bootloader on the Arduino if you program it via ICSP.
« Last Edit: April 13, 2013, 03:26:52 am by dc42 » Logged

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Thank you. I'm using a counter IC with a built in oscillator but the datasheet only showed RC methods. I did follow a circuit like the one you describe but it was unstable, so I tried using a gate and the clock was stable but the counter still acted weird so maybe it's all the flying wires on my breadboard as running a "ground" wire from one end of the board to the other through the "mess" calmed it slightly.
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Post a link to the datasheet, then we can see whether the oscillator is suitable for use with a crystal. Alternatively, get a 74HCU04 hex inverter (note the U in the part number), and use one of the inverters in that, unless that's what you are already using.

On a breadboard, use the power and ground buses normally provided, and keep the wires between the chips and those buses short. Connect a decoupling capacitor directly across each chip, not between the buses.
« Last Edit: April 13, 2013, 03:37:58 am by dc42 » Logged

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it's the MC14541B programmable timer IC
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OK, I don't think you'll have much luck getting the oscillator in that chip to work with a crystal.
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Right, well we did but mine was a bit unstable so I'm avoiding it.
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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  smiley
A clock might be perfectly precise but inaccurate..
http://literature.agilent.com/litweb/pdf/5965-7984E.pdf
« Last Edit: April 13, 2013, 04:36:17 am by pito » Logged

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