[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
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
SatSignal Software - Quality software written to your requirements
Email: david-taylor at blueyonder.co.uk
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