[ntp:questions] Have Pi, have GPS = low powered NTP server?

Ron Frazier (NTP) timekeepingntplist at techstarship.com
Wed May 30 15:40:57 UTC 2012

Hi Dave,

I cannot speak to using the PI specifically. I have been experimenting with GPS + NTP quite a bit and now have a cross platform, Windows and Linux, stratum 1 (non public) time server running on my LAN. Depending on your requirements, you may not need PPS through the UART. I have successfully used PPS through a USB Serial converter and have achieved + / - 1 ms accuracy that way. I was using Windows and the new NTP binaries from Dave Hart. The USB serial converter I was using was the Trendnet TU-S9. This passes the handshake signals as USB messages. I connected the PPS signal from the GPS to pin 1 of the serial connector, which is DCD. I don't think the older version of NTPD that I have access to in the Ubuntu repositories can accept PPS directly. However, you might be able to use GPSD in conjunction with NTPD to read the PPS signal. I'm doing that on the Linux side of my dual boot fence with the GPS connected to a serial port. I don't think I've tried PPS over USB with GPSD and NTPD. On Ubuntu, at least, I think that signal would appear on /ttyUSB0. Debian is probably similar. Good luck. I'll be glad to answer any questions about my setup if I can.




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Ron Frazier
timekeepingdude AT techstarship.com

Dave Baxter <spam at goes.nowhere.com> wrote:


>From the title, you might (maybe) guess this is about the Raspberry Pi, 
and NTP.

I've only had the thing a few days, but been experimenting (playing) 
with the default NTP behaviour as seen with ntpq -p on the command line.

The Fedora remix distro' is a bit of a disaster, unless I've screwed up 
creating the SD card, as it drifts all over the shop, to many 1000's of 
(whatever units) the delay and jitter are listed in. If left to it's 
own devices for a few hours, several 10's of 1000's of units jitter at 
times were listed.

There again, that OS would pause and stutter anyway about every 10 to 15 
seconds, while it's local console output (video monitor) reported timer 
timout errors at irregular intervals. Googling the issue, I see other's 
also see much the same symptoms. (The timer errors.)

Trying another option of Debian, it's much better. This is prety 
typical, and similar to what a more powerful machine would see under the 
same circumstances, from past experience. 

pi at raspberrypi:~$ ntpq -p
remote refid st t when poll reach delay offset jitter

*ntp.websters-co 2 u 11 128 377 37.918 -7.068 20.879
+ns1.luns.net.uk 2 u 65 128 377 34.099 -5.716 197.916
+ns0.luns.net.uk 2 u 64 128 377 33.914 -6.204 181.881
-time.xilo.net 3 u 101 128 377 25.476 -29.662 4.126

(A little bit of reformatting to fit here has been done)

All I've done so far, is edit ntp.conf so that it uses 4 servers from 
uk.pool.ntp.org And the above is as it's seen looking out from my desk 
at work, via our very thin bit of internet damp string.

That was all done over a SSH remote session.

Our ADSL here is slow and creaky, and does suffer from buffer bloat.
As determined by running netalyzr. http://netalyzr.icsi.berkeley.edu/
(Quite how accurate that is, running on US servers, from the UK, I dont 
know, but...) From anecdotal experience, whenever anyone is "doing 
anything" significant with the 'net, then we all suffer.


The RasPi: It has (it is said) a UART on board, and some uncommitted 
I/O, so what thoughts from the assembled estemed collective, re the 
possibility of "doing it by GPS" with PPS etc.

This is found within dmesg's output, after boot...

Serial: AMBA PL011 UART driver
dev:f1: ttyAMA0 at MMIO 0x20201000 (irq = 83) is a PL011 rev3
console [ttyAMA0] enabled

And there's always the option of a USB<>RS232 device, for the NMEA data 
at least.

I'm not asking about how to sort the hardware out, I can handle that I 
think. Just the opinion from anyone who knows more about Linux(Debian 
or??) on the ARM cpu, and it's ability in this respect.

It's said, that the RasPi, has about the same cpu "grunt" as a 300MHz 
Pentium, but I have no way to qualify that statement.

I do realise some software cookery is needed, in particular to get use 
of the UART and one of the uncommitted I/O pins for the PPS to raise an 
interrupt of course.

At present, this is all at the "it'd be good if it could be done" state. 
As if I don't have too many other projects on the go at this time.

Comments, brickbats, bouquet's etc.


Dave B (G0WBX)


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