[ntp:questions] Atheros AR9331 w/GPS + PPS

Gabs Ricalde gsricalde at gmail.com
Thu Sep 10 10:46:45 UTC 2015


On Wed, Sep 9, 2015 at 2:47 PM, Joachim Fabini
<Joachim.Fabini at tuwien.ac.at> wrote:
> Comments inline
>
> On 08.09.2015 13:46, Gabs Ricalde wrote:
>> On Tue, Sep 8, 2015 at 5:17 PM, Joachim Fabini
>> <Joachim.Fabini at tuwien.ac.at> wrote:
>>> On 08.09.2015 09:44, Gabs Ricalde wrote:
>>>> I've been running two devices (TPLink WR703N, MR3020) for more than 2
>>>> years with almost no issue. Also impressed on what these devices could
>>>> do. Does the new OpenWRT versions have the PPS driver for AR9331? I had
>>>> to use this for Attitude Adjustment:
>>>> https://code.google.com/p/openwrt-stratum1/
>>>
>>> Thanks for the pointer, nice project. However, one side-note: the y-axis
>>> label in the diagram shown on the google code page can't be correct -
>>> the unit must be ms, NOT µs.
>>>
>>> An offset smaller than +-200ns is imho ways out of reach for such
>>> devices, 200µs is feasible. In particular when considering the
>>> mechanisms used according to the PpsGpioPollDriver description (100µs
>>> interval waiting/polling). Still the accuracy is good and should be
>>> sufficient for many tasks.
>>
>> The scale is correct, if I run ppstest I get this:
>>
>> root at OpenWrt:~# ppstest /dev/pps0
>> trying PPS source "/dev/pps0"
>> found PPS source "/dev/pps0"
>> ok, found 1 source(s), now start fetching data...
>> source 0 - assert 1441711928.999999999, sequence: 6248816 - clear
>> 0.000000000, sequence: 0
>> source 0 - assert 1441711929.999999949, sequence: 6248817 - clear
>> 0.000000000, sequence: 0
>> [...]
>> The 100 us interval is only for determining the coarse time.
>
> I'm really surprised, that's excellent. Do you have any figures how ntpd
> performs when using this PPS source? My earlier comment was relying on
> the assumption that the diagram displays the ntp log (loopstats)
> statistics. Until now, with Linux, desktop computers and rather
> expensive GPS equipment I couldn't get the system clock's offset better
> than +-5µs over some hours. Publications point out that the CPU
> performance has major influence on the ntp timesync accuracy, too. The
> PPS reporting is a different story but still a main prerequisite. Can
> you share some results on ntp loopstats based offset?

TL-WR703N, polling GPIO driver, ntpd maxpoll 2
https://imgur.com/a/ILF9Y

The offset depends on these things: the stability of the local clock,
PPS jitter, and loop time constant (controlled by the poll interval).
A low PPS jitter means you can use a faster time constant, which can
control the local clock faster when it drifts, which means lower
offsets.
https://www.eecis.udel.edu/~mills/ntp/html/discipline.html
https://www.eecis.udel.edu/~mills/database/brief/algor/algor.pdf

You can also improve the stability of the local clock by avoiding rapid
temperature changes. This is the loopstats of a TL-WR841N placed inside
a box where the temperature doesn't change that fast.
https://imgur.com/a/eNT0k


> Thanks,
> Joachim
>


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