Date: Sun, 22 Oct 1995 22:52:09 +0930 (CST) From: Michael Smith <msmith@atrad.adelaide.edu.au> To: uhclem%nemesis@fw.ast.com (Frank Durda IV) Cc: wollman@lcs.mit.edu, joerg_wunsch@uriah.heep.sax.de, freebsd-current@FreeBSD.org Subject: Re: clock running faster? Message-ID: <199510221322.WAA00342@genesis.atrad.adelaide.edu.au> In-Reply-To: <m0t6W9B-000IxkC@nemesis.lonestar.org> from "Frank Durda IV" at Oct 21, 95 00:07:00 am
next in thread | previous in thread | raw e-mail | index | archive | help
Frank Durda IV stands accused of saying: > normal, but guess what? Despite having that nice TOD hardware down there, > the BSD 4.x system wall time clocks is now running fast, gaining a lot per > hour. (VMS used the TOD hardware so this didn't happen when running VMS.) If we have TOD hardware, use it. By all means, use a faster timer to calculate an offset _into_ the minimum resolution period, but be prepeared for it to be screwy. > Now I will be the first to admit that the 14.31818MHz (4 x NTSC color burst > frequency - what a choice) clock that the traditional PC system clock is > based from (although a multiple of this is used frequently in newer machines) > isn't that easy to work with, at least it is consistent on all PCs, and > even a 200ppm error in a given crystal will still be a pretty small drift > in the system clock, once you get down to ~18.2/sec or ~100/sec or whatever > you have it programmed to. (This clock has to be reasonably accurate or > else RS-232 operations will suffer from bit errors.) Minor quibble - the RS232 UARTs have their own reference oscialltors. Aside from this, your point is quite valid : the CMOS TOD clock isn't particularly brilliant (200ppm may be generous 8), but it's consistent, and if you want to be _really_ picky, offer an option to tune it. On a typical motherboard the TOD clock runs off a watch crystal, and whilst the absolute value of these suckers is somewhat of a lottery issue, they don't drift _that_ badly. > Frank Durda IV <uhclem@nemesis.lonestar.org>|"The Knights who say "LETNi" -- ]] Mike Smith, Software Engineer msmith@atrad.adelaide.edu.au [[ ]] Genesis Software genesis@atrad.adelaide.edu.au [[ ]] High-speed data acquisition and [[ ]] realtime instrument control (ph/fax) +61-8-267-3039 [[ ]] My car has "demand start" -Terry Lambert UNIX: live FreeBSD or die! [[
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199510221322.WAA00342>