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

Gabs Ricalde gsricalde at gmail.com
Tue Sep 8 11:46:02 UTC 2015


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
source 0 - assert 1441711931.000000004, sequence: 6248818 - clear
0.000000000, sequence: 0
source 0 - assert 1441711931.999999970, sequence: 6248819 - clear
0.000000000, sequence: 0
source 0 - assert 1441711932.999999929, sequence: 6248820 - clear
0.000000000, sequence: 0
source 0 - assert 1441711933.999999983, sequence: 6248821 - clear
0.000000000, sequence: 0
source 0 - assert 1441711934.999999966, sequence: 6248822 - clear
0.000000000, sequence: 0
source 0 - assert 1441711936.000000001, sequence: 6248823 - clear
0.000000000, sequence: 0
source 0 - assert 1441711936.999999960, sequence: 6248824 - clear
0.000000000, sequence: 0
source 0 - assert 1441711937.999999962, sequence: 6248825 - clear
0.000000000, sequence: 0

The 100 us interval is only for determining the coarse time.

> Joachim
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