[time-nuts] RasberryPi, timing and GPS receivers

Michael Tharp gxti at partiallystapled.com
Tue Oct 16 21:47:37 UTC 2012


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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