From owner-freebsd-current@FreeBSD.ORG Sun Jan 25 01:21:02 2004 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id E80B116A4CE for ; Sun, 25 Jan 2004 01:21:02 -0800 (PST) Received: from critter.freebsd.dk (critter.freebsd.dk [212.242.86.163]) by mx1.FreeBSD.org (Postfix) with ESMTP id A04A143D49 for ; Sun, 25 Jan 2004 01:20:57 -0800 (PST) (envelope-from phk@phk.freebsd.dk) Received: from critter.freebsd.dk (localhost [127.0.0.1]) by critter.freebsd.dk (8.12.10/8.12.10) with ESMTP id i0P9KquO051124; Sun, 25 Jan 2004 10:20:53 +0100 (CET) (envelope-from phk@phk.freebsd.dk) To: Bruce Evans From: "Poul-Henning Kamp" In-Reply-To: Your message of "Sun, 25 Jan 2004 19:53:26 +1100." <20040125183335.B30359@gamplex.bde.org> Date: Sun, 25 Jan 2004 10:20:52 +0100 Message-ID: <51123.1075022452@critter.freebsd.dk> cc: 'Bill Moran' cc: current@freebsd.org Subject: Re: DragonflyBSD kernel clock improvements X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 25 Jan 2004 09:21:03 -0000 In message <20040125183335.B30359@gamplex.bde.org>, Bruce Evans writes: >relevant sysctl so that they are all as accurate as possible. For the >i8254 clock, the best possible is 0.5 / 1193182 seconds or about 0.5 >ppm, since fractional frequencies can't be set. 0.5 ppm is a good >practical accuracy to aim for since the frequencies of all clocks on >all my machines drift with the temperature by about +- 2 ppm every >day. It is indeed worth noting that while quartz crystals has just the right combination of properties to be our only affordable timekeeping device of any precision (in fact: no other crystals have the right combination of properties), it is even better at measuring temperature. For anybody venturing into timekeeping I would really urge you to start by reading John. R Vig's tutorial "Introduction to Quartz Frequency Standards" before you attempt anything: http://www.ieee-uffc.org/fcmain.asp?view=review Also, you have to realize that you need _three_ identically good oscillators for measuring anything, unless you have a GPS receiver with precision PPS output. It's an expensive hobby, but very rewarding, for people with an inclination to physics: http://www.leapsecond.com/ -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.