From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (from majordomo@localhost) by oss.sgi.com (8.11.2/8.11.3) id fAU1vUM20567 for linux-mips-outgoing; Thu, 29 Nov 2001 17:57:30 -0800 Received: from dea.linux-mips.net (localhost [127.0.0.1]) by oss.sgi.com (8.11.2/8.11.3) with ESMTP id fAU1vRo20562 for ; Thu, 29 Nov 2001 17:57:27 -0800 Received: (from ralf@localhost) by dea.linux-mips.net (8.11.1/8.11.1) id fATFx2M09359; Fri, 30 Nov 2001 02:59:02 +1100 Date: Fri, 30 Nov 2001 02:59:02 +1100 From: Ralf Baechle To: "Maciej W. Rozycki" Cc: Jun Sun , linux-mips@oss.sgi.com Subject: Re: calibrating MIPS counter frequency Message-ID: <20011130025902.A8456@dea.linux-mips.net> References: <20011130020240.F638@dea.linux-mips.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.2.5i In-Reply-To: ; from macro@ds2.pg.gda.pl on Thu, Nov 29, 2001 at 04:21:46PM +0100 X-Accept-Language: de,en,fr Sender: owner-linux-mips@oss.sgi.com Precedence: bulk On Thu, Nov 29, 2001 at 04:21:46PM +0100, Maciej W. Rozycki wrote: > > Better stick with the calibration procedure. The crystal oscilators used > > in most computer systems don't provide the high accuracy of frequency > > that is required to keep the clock accurately over long time. An RTC > > chip which you'd probably use as reference clock is better at that so > > should be used as the ultimate reference time in the kernel. > > On the contrary, the DECstation's undocumented I/O ASIC FCR (free running > counter) driven by the TURBOchannel bus clock is said to be much more > accurate than the DS1287 used there. And it's David L. Mills who says so, > thus we may safely assume it's true. :-) Whooa, Master Secundus Minutius Hora himself ;-) Actually the accuracy of crystal oscillators may differ wildly depending on the use in a system and environmental conditions. In case of the SGI Indy the DS1387 RTC (which is rated at +/- 25ppm ~= 1 minute / month) is actually more accurate. Ralf