Posted by Jerry Stuckle on 06/28/07 03:18

Moe Trin wrote:
> On Tue, 26 Jun 2007, in the Usenet newsgroup comp.os.linux, in article
> <DOGdnZ68FraQCxzbnZ2dnUVZ_vOlnZ2d@comcast.com>, Jerry Stuckle wrote:
>
>> First of all, uncompensated clocks may or may not be 100ppm. I've got
>> one server, for instance, which is off more than 500 ppm. Several
>> others run in the 200-400ppm range.
>
> Not enough detail - the original PC design had (as noted) a simple
> crystal controlled oscillator on the motherboard. To reduce costs
> (someone has to adjust the trimmer on the motherboard). manufacturers
> went to oscillators in a can - a hermetically sealed can shaped like
> a 14 or 16 pin DIP - the oscillator vendor sets the internal trimmer
> in manufacturing, and the motherboard manufacturer merely plus in this
> device. These devices, available from vendors like Dale, Raltron,
> Vectron, and US Crystal are quite cheap (under US$4 in unit quantities.
> less than a third that when buying a thousand at a time) and nasty, but
> are normally spec'ed at +/-100 ppm over 0-70C. Finally, to reduce
> costs still more, the oscillator may be part of the "real time clock"
> such as the Dallas Semiconductor family. I don't have spec sheets on
> these, but expect their accuracy to be comparable.
>

I haven't seen many in that price range (even in large quantities and
sealed) which are spec'd to within 100ppm. And those in the RTC are
definitely not within that accurate.

In fact, I haven't seen many PC's at all which drift less than 8.6
seconds/day. Sure, there are a few. But most I've seen drift at least
twice that.

>> You've come up with a lot of arguments. So you are saying you can
>> guarantee - WITHOUT QUALIFICATION - that the clock will NEVER be set
>> back while things are running?
>
> Of course not - I have absolutely no control over what your users may
> be doing to your systems. One would hope that if you are subject to
> various legal requirements with respect to dinking with the clock, that
> you have trained your users appropriately. If not, it really is your
> problem, not anyone else's.
>
> Old guy

No, it's not my problem at all. I don't write code which is dependent
on the clock always going forward (which is what started this whole thread).

--
==================
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Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
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