On 11/2/10 6:28 PM, Steve wrote:
Group;
I would love to hear your suggestions on the best way to keep a
microprocessor controlled clock accurate.
The micro platform I'm using is an Arduino; which has a crystal
bundled onto the board, but it's not near accurate enough for a clock.
From my reading of this group there are RTC chips, TXCO crystals and
everything in between. There's the Taylor Edge RTC board, the
ChronoDot, the Maxim RTC1307 IC; so many choices!
I only need accurate time keeping. This is for an HH:MM clock, no date
functions etc, so I'd like to keep it as simple (and cheap) as
possible.
Thank you in advance. Your advice is greatly appreciated.
Steve
Software can be used to keep a clock accurate. You can implement a speed
regulator by adding or subtracting N time units of perhaps 1/100 second
every day, where N is the speed regulation value.
So you need a way to adjust N via buttons or a knob. If you have an
extra A/D input, connect a potentiometer across the 5V supply and feed
its wiper to the A/D input. Read the A/D converter to obtain a number
and subtract half its range to make it vary over equally positive and
negative values.
Write the software to set N over a range of +/-128, then add it to your
time unit counter once a night at midnight perhaps.
This is just a basic idea; you can do the math any way that gives the
desired adjustment range of a couple seconds per day.
Expect a crystal to be accurate to about 1PPM once you adjust it, and
keep it at a constant temperature such as in your living room.
You can get fancier - the sky's the limit for time nuts. Buy three
cesium clocks at $60,000 each and compare them.
--
David Forbes, Tucson AZ
http://www.cathodecorner.com/
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