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Arduino Time Clock Accuracy

I'm currently trying to create an Arduino time clock by using the PJRC Time library (http://www.pjrc.com/teensy/td_libs_Time.html). I know that since most of the Arduino boards are running with a 16MHz clock and a single resonator, the time can become "out-of-sync" after a certain period of time.

However, I was wondering if anyone has an idea about the accuracy of the Time library when used on the Arduino DUE with a 84MHz clock. I've been testing it out and so far, the clock has been kept in sync for a few hours. Thanks!

Answer*

Re-visiting an old question... as I found a very informative blog post
that sheds new light into it. But let me first provide some context
before giving the link.

When assessing the quality of a time base, be it a crystal, a ceramic
resonator or a lab-grade frequency standard, there are two notions that
should be distinguished:

* **accuracy**: how close is the frequency of the time base to its
 _nominal_ value
* **stability**: how much does that frequency drift over time

Accuracy is important if you want your clock to give correct time “out
of the box”. However, if you are willing to spend some time calibrating
your clock, then you do not really care because you are going to
calibrate out any inaccuracy you measure. jfpoilpret's answer provides
an example of a “manual” calibration protocol, which is by necessity
quite lengthy. If you can borrow a GPS module with a 1PPS output, the
calibration could be done in a few seconds.

Stability is a more serious issue. If the frequency of the time base
drifts randomly, this will defeat your calibration efforts. Essentially,
the calibration will tell you how fast or slow your clock is running
_right now_, but it will not allow you to predict how fast or slow it
will run _in the future_.

Here is the promised link: [Arduino clock frequency
accuracy][blog post], by Joris van Rantwijk.

What Joris did is measure the accuracy and stability of an Arduino Pro
Mini (clocked off a ceramic resonator) and an old Duemilianove (quartz
crystal). From my perspective, the main takeaways are:

* both clocks are _grossly_ inaccurate, thus both would need user
 calibration in order to be used as timepieces
* the quartz crystal of the Duemilianove has decent stability, better
 than 1.5e-8 at 6 h averaging time
* the stability of the Pro Mini’s ceramic resonator is pathetic, more
 than _two orders of magnitude_ worse than the crystal, which makes it
 essentially useless as a time piece

Here is his [Allan deviation][] plot, which measures clock instability
as a function of observation time:

[![Allan deviation of Arduino Clock Frequency][adev plot]][adev plot] 
<sub>(source: [jorisvr.nl](http://jorisvr.nl/images/arduinofreq/allandev_small.png))</sub> 

Although this study has some limitations (only two boards were tested,
and the observation time is too short), it is well thought and very
informative. I encourage you to read it in whole.

[blog post]: http://jorisvr.nl/article/arduino-frequency
[Allan deviation]: https://en.wikipedia.org/wiki/Allan_variance
[adev plot]: https://i.sstatic.net/Cy5Le.png

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1

Quartz crystals’ frequency changes slightly with temperature. If you can control the temperature it helps keep the frequency stable.

Duncan C
– Duncan C

2019年03月13日 00:36:17 +00:00
Commented Mar 13, 2019 at 0:36

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