From f2c1a053ceedec87aa355fafe60db1af27d7d830 Mon Sep 17 00:00:00 2001 From: Sanjeev Date: Sat, 24 Dec 2016 16:27:31 +0800 Subject: [PATCH] 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 Signed-off-by: Jonathan Corbet --- Documentation/pps/pps.txt | 7 +++++-- 1 file 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 -- 2.20.1