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Topic: millis (crystal) accuracy & thoughts (Read 3 times) previous topic - next topic


pushing data on spi and putting things on the serial will need interrupts.
interrupts are "robbed time".
are you sure your timekeeping is not bothered by these
interrupts ?



I can tell you that here in the future, we expect proper data, just as they did back in 2010 (and 1910 and 1810, etc.)
Proper data has an explanation for each line. Does your list represent readings at 1min, 2min, etc?  Then say so. Even back in 2010, not doing so was called "unlabeled".
Proper data is concise. It does not include the same number represented as seconds and milliseconds. Even back in 2010 repeating values was called "redundant".

Your responses have become redundantly useless.

I'll try the same test with an LCD hooked up via parallel (eliminating spi) and report back.

I promise to post concise labeled unredundant proper data in the future.



Dec 20, 2010, 09:33 am Last Edit: Dec 20, 2010, 09:33 am by graynomad Reason: 1
As I see it the bottom line is that even the crappiest crystal is good for an accuracy of about 100ppm and most are a lot better.  

Surely this is accurate enough, and if so the problem must be in the code.

I'm thinking of using a DS3231 (like a chronodot) to create an accurate 1Hz interrupt on the Arduino.

I gather then that you're timing a long event, not a drag race.

The RTCs with inbuilt TXCOs can be calibrated to about 2ppm and even uncalibrated are very good.

A couple of questions.

Is this a start-time-stop application?
How many things are being timed?
Do you need to display the time while the timer is running?
How long is the event being timed?
What accuracy do you need?


Rob Gray aka the GRAYnomad www.robgray.com


Dec 20, 2010, 09:38 am Last Edit: Dec 20, 2010, 09:42 am by westfw Reason: 1
The methodology looks OK to me.  The internal millis() timer shouldn't be subject to drift because of other code, but it seems that that his arduino are drifting about 1.3 s over 10 minutes (600s.)  That's only 0.2% error, not completely "unreasonable" for a resonator, though better would be ... better.

I'm seeing unacceptable timing errors for anything more accurate than an egg timer.

0.2% error is unacceptable?  I guess it would be nicer if it looked more regular...

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