Real time clock delay problem

Real time clock delay problem

Post by Florian Lorenze » Wed, 01 Nov 2000 18:26:58



Hi there,

probably a little bit off-topic. My Real time clock in my Linux-box
(older 486) is delaying in one month about half an hour. Battery seems
to be alright (about 4 V, specified with 3.6 V). Also the computer has
been on all the time. Someone got an idea how to get the clock alright?

Florian

 
 
 

Real time clock delay problem

Post by Gene Hesket » Wed, 01 Nov 2000 22:12:57


Unrot13 this;

Gene Heskett sends Greetings to Florian Lorenzen;

 FL> Hi there,

 FL> probably a little bit off-topic. My Real time clock in my
 FL> Linux-box
 FL> (older 486) is delaying in one month about half an hour. Battery
 FL> seems to be alright (about 4 V, specified with 3.6 V). Also the
 FL> computer has been on all the time. Someone got an idea how to get
 FL> the clock alright?

 FL> Florian

Since you said its an older 486, a couple of thngs come to mind.

1. That someone has found and turned the capacitor (if there is one)
that calibrates many of the older clock's crystal frequency.

2. That, due to its age, the crystal, being an el-cheapo in the first
place and not in a nitrogen filled, sealed can, has by now sufficiently
oxidized the silver plating that is its terminal contacts.  This silver
plating's weight is part of the mechanical weight of what is basicly a
moving mass centered by a spring.  Yes, these things actually do
vibrate!  But at frequencies much to high to hear.

Anyway, the oxidation adds the weight of the oxygen absorbed to the
mass part of the formula, without increasing the hardness of the spring.
The end result is that it slows down.

There may be other items in the chemical soup besides oxygen to
consider, in some locations the airborne sulfer, which will make whats
called 'liver of sulpher' on a silver surface might also come into play
over that length of time.

If the crystal is a seperate item, and not cast right into the epoxy of
the clock chip, it can be replaced by a competant tech.  They are
normally in a tin tube sort of a can, 2 leads out one end, and the other
formed closed, usually somewhat less than 1/8" in diameter, half inch or
so long.

Of course the clock chip could also be replaced, but those things tend
to be held on the shelf, and an exact replacement is probably going to
be within a year of your old slow ones age, and not much of an
improvement.  A modern chipset would be the ideal, but that would
require actual cut and paste wire hacking to install, not worth the
effort/risk.

You should set up a script that fires off rdate to go get the time from
some time server everytime it goes online.  I've been doing that for
years, even on this Amiga so I have no idea how slow my 10 year old
clock actually is, its always right +- a second.

Cheers, Gene
--

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Real time clock delay problem

Post by Andrey Vlaso » Fri, 03 Nov 2000 06:01:33


Hi Florian,

I agree with previous post about physical nature why you timer slow down.
But in very end of that message was exactly what I would recommend - NTP.
And now question - "Do you connected to internet on permanent base?"
(Sorry I do, I am working for ISP and my ADSL connection is free for me
24/7). If your answer yes, I'd recommend to turn you computer in time
server. Procedure is very simple and you will never ever to adjust time on
your computer - precision is very high 128 milliseconds - probably higher
precision require only in military and science. If you need help I will do
it for you. And now URL for NTP

http://www.eecis.udel.edu/~ntp/

But I believe that this software installed on your system. Just check for
ntpq on your system. On website you will find all you need.

Andrey


> Hi there,

> probably a little bit off-topic. My Real time clock in my Linux-box
> (older 486) is delaying in one month about half an hour. Battery seems
> to be alright (about 4 V, specified with 3.6 V). Also the computer has
> been on all the time. Someone got an idea how to get the clock alright?

> Florian

 
 
 

1. Delay with no real time clock

I am working with a driver that must wait for a specified period of
time (usually less than one second) after issuing a command before
assuming that a device is not present.  This must happend during
device initialization time.

There are no real time clock interrupts during the device initialization
phase of our kernel.  The system time counter (lbolt in our case) does
not get incremented HZ times per second until after all devices
are initialized.  So,

        How do you determine how much time has expired
        when you have no real time clock interrupt?

One would suppose the answer would be to set up a loop with a specific
number of instructions and then have some macro that tells you how
many of these loops occur per second.  But that would entail changing
the macro everytime you change the clock rate of the processor
(or changing the contents of the loop).  This is also somewhat
non-deterministic due the fact that the loop may or may not be in
an instruction cache.

I am curious to find our how others have dealt with this problem.

Thanx,
jeff jones

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