diff options
Diffstat (limited to 'Documentation/pps/pps.txt')
| -rw-r--r-- | Documentation/pps/pps.txt | 18 |
1 files changed, 11 insertions, 7 deletions
diff --git a/Documentation/pps/pps.txt b/Documentation/pps/pps.txt index 50022b3c8ebf..1fdbd5447216 100644 --- a/Documentation/pps/pps.txt +++ b/Documentation/pps/pps.txt @@ -63,7 +63,7 @@ for instance) is a PPS source too, and if not they should provide the possibility to open another device as PPS source. In LinuxPPS the PPS sources are simply char devices usually mapped -into files /dev/pps0, /dev/pps1, etc.. +into files /dev/pps0, /dev/pps1, etc. PPS with USB to serial devices @@ -71,9 +71,12 @@ PPS with USB to serial devices It is possible to grab the PPS from an USB to serial device. However, you should take into account the latencies and jitter introduced by -the USB stack. Users has reported clock instability around +-1ms when -synchronized with PPS through USB. This isn't suited for time server -synchronization. +the USB stack. Users have reported clock instability around +-1ms when +synchronized with PPS through USB. With USB 2.0, jitter may decrease +down to the order of 125 microseconds. + +This may be suitable for time server synchronization with NTP because +of its undersampling and algorithms. If your device doesn't report PPS, you can check that the feature is supported by its driver. Most of the time, you only need to add a call @@ -166,7 +169,8 @@ Testing the PPS support In order to test the PPS support even without specific hardware you can use the ktimer driver (see the client subsection in the PPS configuration menu) -and the userland tools provided in the Documentation/pps/ directory. +and the userland tools available in your distribution's pps-tools package, +http://linuxpps.org , or https://github.com/ago/pps-tools . Once you have enabled the compilation of ktimer just modprobe it (if not statically compiled): @@ -183,8 +187,8 @@ and the run ppstest as follow: source 0 - assert 1186592700.388931295, sequence: 365 - clear 0.000000000, sequence: 0 source 0 - assert 1186592701.389032765, sequence: 366 - clear 0.000000000, sequence: 0 -Please, note that to compile userland programs you need the file timepps.h -(see Documentation/pps/). +Please, note that to compile userland programs you need the file timepps.h . +This is available in the pps-tools repository mentioned above. Generators |