Greg,
This is for a product, so I am not interested in one-off or
selected-component solutions. I have a rubidium Nixie clock at home that
I use to set and check my Nixie watches. I find that the typical 20PPM
watch crystal can be adjusted to keep time to 1PPM if it's kept at room
temperature on your wrist.
It sounds like the DS3231 is well liked among clock makers. I am just
perturbed that their $1 DS1307 can't be made to run as well as their $5
product, all because they wanted to save a penny for volume
manufacturers by adding internal capacitors to its oscillator circuit.
On 11/2/2016 8:50 AM, gregebert wrote:
Keep the DS3231; it's very accurate. One of my clocks uses it, and it
has drifted about 10 seconds since daylight savings started more than 6
months ago. My wristwatch demo board has been running 1.5 years and it's
also within a few seconds. If you want anything better, you'll probably
need GPS or a rubiduim time standard.
I recall most crystals are accurate to around 10ppm, which is about 1
second per day. My interpretation of that is the manufacturing
tolerances result in multiple batches of crystals that will fall within
10ppm of their rated frequency; I suppose you can pay more for
cherry-picked units that are more accurate. I doubt individual crystals
drift back-n-forth, so you could add some offset to your software and
fine-tune it to each crystal if you dont want to use RTC/GPS/Internet time.
--
David Forbes, Tucson AZ
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