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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

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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   [[
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