Date: Fri, 9 Jan 1998 17:32:44 +0100 (MET) From: Dirk-Willem van Gulik <Dirk.vanGulik@jrc.it> To: "Louis A. Mamakos" <louie@TransSys.COM> Cc: Tim Tsai <tim@futuresouth.com>, Greg Lehey <grog@lemis.com>, David Kelly <dkelly@hiwaay.net>, FreeBSD Hackers <Hackers@FreeBSD.ORG> Subject: Re: GPS for xntpd Stratum 1 servers Message-ID: <Pine.SOL.3.96.980109172751.8962J-100000@elect6.jrc.it> In-Reply-To: <199801091506.KAA22405@whizzo.TransSys.COM>
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On Fri, 9 Jan 1998, Louis A. Mamakos wrote: > The point of all this is that it's very important that the corrections > used by the local clock algorithm have as low jitter as possible so > you can get the PLL to "tighten up" it's control loop. The preferred > way of doing this is to arrange that the 1-PPS (pulse per second) > signal from the external reference clock capture the current offset > when it fires; typically the 1PPS signal is connected to a control > line (like DCD or CTS) which generates an interrupt when it transitions; > a line discipline or other kernel-level interrupt handler captures the > current system timestamp, and this is queued to be handled by the > daemon process leisurely. As an aside, whilst using cheap receivers such as the Garmin and the Tripmate I found that both seriously lack an extra 1PPS to the DCD, as both the NMEA string start/stop points on the TxD data stream jitter by as much as 0.2 second; even when switching off all other strings, etc, etc. (Although you can solder a wire to the 1PPS pin on either unit; it is just a sall SMD contact). Dw.
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