Michael,
Thank you for your most valuable information.
This is why time-nuts is so amazing!
My goal is actually two-fold.
First, I would like to have a NTP server that I can easily
hook up (well, as easily as possible) without
dedicating a Linux server each time.
My personal use would require 3 separate NTP servers
because I am a time-nut and stratum 2 is not an option.
The idea here is to create a smart piggy-back architecture
where the CPU/Ethernet sits on the bottom
with (perhaps) a few modules on top of that
with GPS right on top (let's call it the penthouse).
For obvious reasons I would put the Power supply
separately.
So we are looking for a very small footprint
NTP server.
Now, I thought the RasberryPi would be ok
even though I did not like the limitations you
mentioned as well. It is very popular.
You mentioned the Olimex olinuxino. Wow!
This is a thing of beauty indeed!
https://www.olimex.com/Products/OLinuXino/A13/A13-OLinuXino/
I really like this board.
My second goal is that if I go to the
trouble of creating a GPS-daughterboard and all that
I'd like to have other people use it as well
and maybe I would make a small batch.
Cost wise this should be in the 10-20 dollar range tops.
I think I will do some research on this board and maybe
order one. It looks very promising.
I wonder if I can replace the crystal with a better
high stability one.
-George
Quoting Michael Tharp <g...@partiallystapled.com>:
On 10/16/2012 05:06 PM, x...@darksmile.net wrote:
My goal is to design a custom board for the Pi and mount a GPS receiver
on it. With this combination, I should be able to configure NTP for the
Pi and thus have the Pi act as a Stratum 1 NTP server.
The new RasberryPI has 512MB memory so it should be fine for running
just ntpd.
Question: What GPS timing module should I go with? No more Motorola
Oncore so what's best right now? Who sell modules? What are the price
ranges?
It's not a terrible idea, but the RPI has a USB ethernet transceiver
so in addition to the latency/jitter of the ethernet it also has the
latency/jitter of the USB. I've also heard of stability problems
just keeping it running for weeks to months so you should integrate
some kind of watchdog timer if you can. The actual GPS module
doesn't matter much since NTP will smooth out even the worst GPS
jitter. I have heard second-hand (or third-hand or fourth-...) that
some have a significant persistent delay and that could be more of a
problem. If you want to go for a timing-oriented receiver you can
get a used Trimble Resolution T from ebay but they have a 2mm pitch
header.
You will want to house the RPI and GPS receiver in a box where it
will not be subject to wide temperature swings, insulated and
shielded from drafts. It would also be interesting to upgrade the
main oscillator to a temperature-compensated model so NTP doesn't
have to work as hard to keep the frequency locked.
Personally I would recommend getting a more robust single-board
computer, e.g. a PC Engines ALIX or Olimex olinuxino. RPI is cheap
but hard to source, not open-source, and does not have good
long-term prospects due to the microprocessor being used. Most of
the attention is due to deliberate publicity by the manufacturer and
not novelty or merit. If you must use the cheapest board then by all
means do so, but just know there is better available for not much
more.
Happy ticking,
-- m. tharp
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