[time-nuts] RasberryPi, timing and GPS receivers

[xaos at darksmile.net]

Annoyance with small Linux boards:

My single biggest annoyance is that
all the I/O connectors are coming out from
all 4 directions.

What I mean is this:

It would be nice to have ethernet+LEDs+USB+VGA(HDMI)
come out from one side, and Power/I/O/RS232
from the opposite side.

This way if I make a case I only need
to worry about what is coming out of the
Ethernet side.

The RPI has ethernet+usb on one side and HDMI from another.
How do you make a decent case with only one side open?
It is impossible.

If they only had the HDMI on a header so I can break it out.
Same for audio/USB

-George


Quoting "shaun at impsec.net" <shaun at impsec.net>:

> Michael Tharp <gxti at partiallystapled.com> wrote
>
>
> [RE: raspberry pi ntp server]
>>
>> 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 been playing with a pair of Raspberry Pi ntp servers, one with  
> a $15 Sure Electronics GPS evaluation board, which was very easy to  
> interface to (3.3v uart and pps outputs) and the other connected to  
> a Lucent RFTG, which once I figured out that the Pi apparenty  
> tolerates the PPS out from the RFTG w/o a level shifter just fine  
> (mostly - I was getting false extra pulses until I shielded the  
> line, which I hadn't needed to do on the Sure GPS board).
>
> I'm a junior time-nut at best but it looks to me like jitter from  
> the USB Ethernet is acceptably low, based on ntpq -p anyway:
>
>    remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> oGPS_NMEA(0)     .GPS.            0 l    2   16  377    0.000     
> 0.003   0.004
> +pi2             .PPS.            1 s    7   16  377    0.945    
> -0.004   0.044
> +pool-test.ntp.o 216.218.192.202  2 u   43   64  377   73.855     
> 1.382   0.262
>
>
>    remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> oPPS(0)          .PPS.            0 l    1    8  377    0.000    
> -0.024   0.061
> +ntp             .GPS.            1 s   14    8  252    0.983     
> 0.217   0.615
> *tick.nullmodem. 128.252.19.1     2 u   60   64  377   35.066     
> 0.562   4.556
>
>
> Some of the offset between the two might be that I am waiting for a  
> 2nd GPS antenna to arrive for the RFTG and it's been undisciplined  
> for a few days, it might be from the ethernet jitter, the Sure GPS  
> might be off, or it might be from the fact that I may have a  
> configuration issue someplace - ntpq -c rl has them at different  
> precisions (-19 and -18), even though I think they're the same  
> kernel and ntpd compile.
>> 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.
> I haven't seen the issue yet but I haven't been running more than a  
> couple of weeks.  Part of the problem may be quality of power  
> supply, I've seen some people reporting issues if the 5v usb power  
> in isn't capable of providing enough current and a stable voltage.
>
>>
>> You will want to house the RPI and GPS receiver in a box where it  
>> will not be subject to wide temperature swings,
>
> I haven't been running either of mine in an enclosure, both are  
> sitting up in the unheated attic at the moment, typical offsets from  
> loopstats seem to be under 2us, but I should graph them.
>> 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.
> I am curious if I could replace the 19.2mhz system clock crystal  
> with a stable source generated from the 15mhz RFTG output via a  
> clockblock, ala what I have seen reported about the Soekris.
>> Personally I would recommend getting a more robust single-board computer
> I don't disagree that the Pi has its flaws but it has been fun to play with.
>
> -shaun
>
>
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