[time-nuts] RasberryPi, timing and GPS receivers

[xaos at darksmile.net]

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 <gxti at partiallystapled.com>:

> On 10/16/2012 05:06 PM, xaos at 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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