[time-nuts] Re: Poor's man NTP

[Rob Seaman]

A couple of Meinberg SyncMon plots for the past month are attached of a Raspberry Pi 4B with a TimeHat (https://millerjs.org/timehat <https://millerjs.org/timehat>) against a Meinberg M1000 (midrange OXCO), both close to original configuration. The plot with the 1ms scale is for unloaded NTP (meaning Chrony for the Debian variant running on the Pi) with both remaining within about a 0.1 ms envelope.

The plot with the 100ns scale has zoomed in on the 1PPS signals, both likely direct from their u-blox chips. (At least, I think this particular Meinberg relies on u-blox. I’m not at the office to pull the card.) SyncMon can track all sorts of other information, not just offsets, which would allow analyzing the behavior with different numbers of NTP clients, etc.

Our interest is in TTL time capture, and u-blox supports these directly (their TimeMark feature), though this is not implemented for the TimeHats. Both our Meinberg reference clocks and our Meinberg PCIe IRIG-B cards (no u-blox) implement time capture, described here: https://arxiv.org/abs/1807.01370 <https://arxiv.org/abs/1807.01370>

The Raspberry Pi Compute Module 4 has a hardware timestamping NIC, though it appears no kernel support yet. There are CM4 carrier boards that support multiple gigabyte ethernet connections that could serve as PTP master clocks or boundary clocks (if somebody implements kernel support). There are a lot of variations of Raspberry Pi at this point and the 4Bs and CM4s are significantly more performant than earlier versions.

One could imagine designing a purpose-built CM4 carrier board that would incorporate various features from the OCP TAP design (https://www.opencompute.org/wiki/Time_Appliances_Project <https://www.opencompute.org/wiki/Time_Appliances_Project>).

Which is to say that the Raspberry Pi itself is not the limiting factor here, and indeed such an affordable single board computer is required to provide inexpensive ubiquitous precision timekeeping.

Rob Seaman
Lunar and Planetary Laboratory
University of Arizona

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> On Dec 17, 2021, at 4:07 AM, David Taylor via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> On 16/12/2021 23:37, giuseppe at marullo.it wrote:
>> Hi,
>> just wondering if a PI4 could be a suitable NTP server for a small lab (and
>> maybe with some other NTP servers for my company, about 2000 clients).
>> Main use is for correct timestamp on logs/computer time sync.
>> I setup the NTP using Adafruit Ultimate GPS shield(with battery), and the
>> GPS/GLONASS Antenna(cheap one, not a timing antenna). Antenna is on a roof
>> window with a small metal base, outside.
>> What is the accuracy I could expect from it?
> 
> You can see the offsets reported by a number of Raspberry Pi NTP servers here:
> 
>  https://www.satsignal.eu/mrtg/performance_ntp.php
> 
> As a very rough guide I would expect a PPS-based RPi to be within 100 us easily, and perhaps considerably better (~10 us) in a constant temperature environment, and with no other load.  I don't know about load capacity as I've not tested that, but here it's serving perhaps 50 devices.  With classic NTP which is what I use there will be a period before the offset becomes near-zero so 24 x 7 is the way to operate.
> 
> Although there are some devices on that page with LAN, 2.4 GHz or 5 GHz connections, for beat accuracy you must use the PPS signal from the GPS as you know.  The "o" tally code suggests that's OK.
> 
> I suggest using the "pool" directive rather than multiple pool servers:
> 
>  https://www.satsignal.eu/ntp/setup.html#pool
> 
> Cheers,
> David
> -- 
> SatSignal Software - Quality software for you
> Web: https://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk
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