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authorSanjeev <ghane0@gmail.com>2016-12-24 16:27:31 +0800
committerJonathan Corbet <corbet@lwn.net>2017-01-04 14:40:52 -0700
commitf2c1a053ceedec87aa355fafe60db1af27d7d830 (patch)
tree82bbd99ae7e22efbff187a0ee12d5183d62ed51b /Documentation/pps
parentfe4c56c98c1338c0b8b69d08ccf5694bf8e1dcaf (diff)
downloadlinux-f2c1a053ceedec87aa355fafe60db1af27d7d830.tar.bz2
Doc: clarify source of jitter in USB1.1, and USB2.0
Even though the jitter due to USB1.1 may be 1ms, NTP can reduce its effect significantly. And USB2.0 reduces this anyway. Signed-off-by: Sanjeev Gupta <ghane0@gmail.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Diffstat (limited to 'Documentation/pps')
-rw-r--r--Documentation/pps/pps.txt7
1 files changed, 5 insertions, 2 deletions
diff --git a/Documentation/pps/pps.txt b/Documentation/pps/pps.txt
index a9f53bba910d..1fdbd5447216 100644
--- a/Documentation/pps/pps.txt
+++ b/Documentation/pps/pps.txt
@@ -72,8 +72,11 @@ 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 have reported clock instability around +-1ms when
-synchronized with PPS through USB. This isn't suited for time server
-synchronization.
+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