Netduino Plus V1 Clock Accuracy vs. NTP

The Quakezure client application needs a reasonably accurate clock for time stamping the arrival of P-waves and S-waves (P waves travel at 2000-8000 m/sec so a couple of seconds can make a significant difference).  A GPS unit would provide a very accurate reference but it adds significant cost. For example the SeeedStudio GPS twig costs USD39.90 which is roughly 1/3 of the current BoM. The QuakeZure device (or separate GPS unit) would also need to be installed where it could receive information from the GPS satellite constellation. .

In New Zealand the Measurements Standards Laboratory which is part of Industrial Research Limited (a Crown Research Institute) offers public SNTP & NTP services. Using a tiered SNTP approach looked like a cost-effective alternative to GPS for internet connected devices. I started with a ping on my ADSL connection to see what the connectivity with MSL was like (will be much slower on cellular, have seen approaching 10x slower on 2G GPRS)

Ping statistics for     Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds:     Minimum = 33ms, Maximum = 33ms, Average = 33ms

I then wrote a quick ‘n dirty .NetMF console application which used a configurable (default 10mins) timer to regularly compare the current device time with the IRL time and display the difference. I wanted to see what the jitter was on the NTP request and how accurate the N+ on board clock was.

Current Time 12-12-16 06:45:00.09 Duration 43mSec Difference 0 mSec
Current Time 12-12-16 06:55:00.09 Duration 44mSec Difference 86 mSec
Current Time 12-12-16 07:05:00.09 Duration 44mSec Difference 174 mSec
Current Time 12-12-16 07:15:00.09 Duration 44mSec Difference 262 mSec
Current Time 12-12-16 07:25:00.09 Duration 44mSec Difference 349 mSec
Current Time 12-12-16 07:35:00.09 Duration 45mSec Difference 437 mSec
Current Time 12-12-16 07:45:00.09 Duration 44mSec Difference 524 mSec
Current Time 12-12-16 07:55:00.09 Duration 44mSec Difference 611 mSec

After running for an hour the N+ clock was just over half a second out, which equates to roughly 12.5 seconds a day so I then did some digging and found I was not alone.

Need to look at how often I should adjust the device time back into sync with the back office without generating to much NTP traffic.

One thought on “Netduino Plus V1 Clock Accuracy vs. NTP

  1. Pingback: HTTPS with NetMF HTTP Client managing certificates | devMobile's blog

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