Re: calibrating MIPS counter frequency

[Ralf Baechle <ralf@oss.sgi.com>]

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