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Re: calibrating MIPS counter frequency

To: "Maciej W. Rozycki" <>
Subject: Re: calibrating MIPS counter frequency
From: Ralf Baechle <>
Date: Fri, 30 Nov 2001 02:59:02 +1100
Cc: Jun Sun <>,
In-reply-to: <>; from on Thu, Nov 29, 2001 at 04:21:46PM +0100
References: <> <>
User-agent: Mutt/1.2.5i
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.


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