[time-nuts] Raspberry Pi tweaks and custom kernel, was RE: PPS for NTP Server - How Close Is "Good Enough"?

David J Taylor david-taylor at blueyonder.co.uk
Sun Jun 14 02:17:47 EDT 2015


At best, simply because of the way that USB interface works, you can go from 
the ~1/2 ms range into the ~1/4 ms range. Rewriting the low level drivers 
may be required.  This compares to an equivalent lag on a device with a 
built in ethernet of < 0.001 ms. The easy thing to do is to simply kill off 
everything else that is running on the device. That goes double for anything 
running on USB (keyboards / mice etc).

Since you can get boards with built in (integrated in the MCU)  ethernet for 
< $50, it’s not clear how much tweaking time this is worth. You don’t get 
any real use out of the Pi’s fancy graphics in this case. The board really 
is not optimized for doing this sort of thing. Also - check the threads on 
Pi heat sinking and boards burning out. If you plan to use this full time - 
invest the money in cooling now.



Thanks for your comments.  I made a simple comparison between the RPi and 
the BBB with help from the folks here, and wrote it up here:


Using NTP, the reported RMS offset on a client on the same LAN was 18 
microseconds (BBB) and 39 microseconds (RPi), and the mean RMS jitter was 7 
microseconds (BBB) and 29 microseconds (RPi).  So the USB/Ethernet does make 
the RPi less good as a server, but depending on your application it may 
still be more than adequate.  Neither system provides "microsecond" level, 
both are more than adequate for "tenth millisecond" level.

The BBB generated sufficient RF interference to stop a near-on-board GPS 
from working.

I have had no overheating troubles with the 9 RPi systems here, none have 
needed added cooling, and none have burnt out.  One is even running 
Windows-10 IoT.

SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor at blueyonder.co.uk 

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