Date: Sun, 25 Jan 2004 10:20:52 +0100 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: Bruce Evans <bde@zeta.org.au> Cc: current@freebsd.org Subject: Re: DragonflyBSD kernel clock improvements Message-ID: <51123.1075022452@critter.freebsd.dk> In-Reply-To: Your message of "Sun, 25 Jan 2004 19:53:26 %2B1100." <20040125183335.B30359@gamplex.bde.org>
next in thread | previous in thread | raw e-mail | index | archive | help
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.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?51123.1075022452>