Letux Kernel

PPS

You are looking at an old revision of the page PPS. This revision was created by Nikolaus Schaller.

Table of Content

GPS derived Pulse per Second for GTA04A5

Introduction

The GTA04A5 hardware uses an Wi2Wi W2SG0084 GPS module which provides a PPS (Pulse per Second) time reference impulse. This is connected to some GPIO (only on GTA04; GTA04A3 and GTA04A4 don't have this feature and most other Letux supported devices don't have GPS at all).

The latest letux-kernels starting with 4.13-rc2 have added Device Tree configs and defconfig so that it works.

How to use

open /dev/ttyGPS0 by e.g. gpsd or some other background process like

cat /dev/ttyGPS0 >/dev/null &

so that the GPS module becomes powered on.

Then run:

ppstest /dev/pps0

As long as there is no antenna available or too weak reception, there will be the message:

time_pps_fetch() error -1 (Connection timed out)

If GPS has good enough signals, we will see e.g.:

root@letux:~# ppstest /dev/pps0 </dev/ttyGPS0
trying PPS source "/dev/pps0"
found PPS source "/dev/pps0"
ok, found 1 source(s), now start fetching data...
source 0 - assert 1502827151.007645478, sequence: 69 - clear  0.000000000, sequence: 0
source 0 - assert 1502827152.007654824, sequence: 70 - clear  0.000000000, sequence: 0
source 0 - assert 1502827153.007420053, sequence: 71 - clear  0.000000000, sequence: 0
source 0 - assert 1502827154.007612516, sequence: 72 - clear  0.000000000, sequence: 0
source 0 - assert 1502827155.007621895, sequence: 73 - clear  0.000000000, sequence: 0
source 0 - assert 1502827156.007631285, sequence: 74 - clear  0.000000000, sequence: 0
source 0 - assert 1502827157.007610169, sequence: 75 - clear  0.000000000, sequence: 0
source 0 - assert 1502827158.007589063, sequence: 76 - clear  0.000000000, sequence: 0
^C
root@letux:~# date

As you can see, the precision is in sub-milliseconds.

What it is good for

Originally PPS is intended to provide precise timing for Servers (ntpd can be synchronized).

In a mobile device there are other use cases: * precise time measurements * AGPS * synchronize multiple mobile devices for acoustic events

Created: 6 years 6 months ago
by Nikolaus Schaller

Updated: 6 years 6 months ago
by Nikolaus Schaller

Old Revisions