From owner-freebsd-hackers Fri Jan 9 08:34:40 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.7/8.8.7) id IAA15783 for hackers-outgoing; Fri, 9 Jan 1998 08:34:40 -0800 (PST) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from mrelay.jrc.it (mrelay.jrc.it [139.191.1.65]) by hub.freebsd.org (8.8.7/8.8.7) with SMTP id IAA15745 for ; Fri, 9 Jan 1998 08:34:26 -0800 (PST) (envelope-from Dirk.vanGulik@jrc.it) Received: from elec.isei.jrc.it (elec.jrc.it) by mrelay.jrc.it (4.1/EB-950131-C) id AA15112; Fri, 9 Jan 98 17:35:39 +0100 Received: from elect6.jrc.it by elec.isei.jrc.it (4.1/EI-3.0m) id AA12766; Fri, 9 Jan 98 17:33:34 +0100 Posted-Date: Fri, 9 Jan 1998 17:32:44 +0100 (MET) Date: Fri, 9 Jan 1998 17:32:44 +0100 (MET) From: Dirk-Willem van Gulik X-Sender: dirkx@elect6.jrc.it Reply-To: Dirk-Willem van Gulik To: "Louis A. Mamakos" Cc: Tim Tsai , Greg Lehey , David Kelly , FreeBSD Hackers Subject: Re: GPS for xntpd Stratum 1 servers In-Reply-To: <199801091506.KAA22405@whizzo.TransSys.COM> Message-Id: Reply-Path: Dirk.vanGulik@jrc.it Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk 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.