Subject: Re: PReP RTC vs Decrementer accuracy...

From: Corey Minyard <[email-masked]>
Date: 09 Dec 1998 17:52:04 -0600

Cort Dougan <[email-masked]> writes:

> No, this would tend to make it slower if anything.
> 
> }Okay, would this cause the clock to be fast? (mine was about 45 minutes
> }fast) (And, is it feasable to make it 'impossible' to miss a decr?)
> 
> Rebooting a lot would affect it.  Are you running the code that sync's the
> rtc with the system clock every 11 minutes?  If so, turning it off and
> seeing which clock was accurate after a while would be a useful
> experiment.
> 
> }I have also rebooted the machine quite a bit.. This may also be the
> }reason.
> 

I'm having this same problem on an MVME2700 (233MHZ 750).  With the
default configuration the clock gains about 1.5 seconds every 30
minutes.  LinuxPPC on my PowerMac keeps great time.

I removed the hardcoding of the decrementer setting but the computed
values stayed pretty much the same.  I have modified the hardcoded
frequency value to be 999950000 (after some experimentation with
xntpd) from the old value of 998700000.  This keeps time pretty well
(<16ppm error) and the stability is very good.

Also, I'm not seeing the time being written to the RTC at all.  I just
left my card up all day and it made no difference in the time in the
RTC, it is about 600 seconds off (and that value is growing quite
rapidly, probably 30 seconds a day, which might explain why the
calculated value is so far off).  But why isn't the RTC being written?
Maybe I'll look at this tomorrow.

-- 
Corey Minyard               Internet:  [email-masked]
  Work: [email-masked]       UUCP:  [email-masked]

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