🚀 go-pugleaf

> I have a 486DX/33MHz system, and I have been told by the manufacturer 
> that the motherboard is upgradeable to 50MHz by plugging in a new
> crystal and CPU.
> 
> Question:
> Is it possible to plug in (say) a 40MHz or even a 50MHz crystal
> into the board *without* replacing the old (33MHz) CPU? 

Integrated circuits can WEAR OUT.

There are two types of mechanisms to worry about - heat induced degradation
and hot carrier injection.  The first problem results in hard failures,
and can be avoided by keeping the chip cool.  Hot carrier injection occurs
at all temperatures - and is enhanced a bit by cooling the chip - and acts
to make the chip slower with time.  

What this means to you is that a chip that works up to, say, 55 MHz this
year may only work up to 45 MHz a year from now.  Intel may have marked 
that chip as 33 MHz, so that the part would keep working for a decade at
its rated speed.  Intel has a powerful incentive to sell those chips at
as fast a speed rating as they can get away with.  If a batch will safely
go at 50 MHz over its lifetime, they gain no advantage by marking them as
33 MHz.  If they mark them as too fast, then have to recall them, they
lose a lot of money.

Heat also slows chips down, by about 0.5% per degree C.  If Intel rated
the chip at 55C, and you can guarantee the chip stays below 35C at all
times, you've bought an extra 10% speed margin.   However, if you tweek
the machine to go as fast as it can go this winter at 18C ambient, and
this summer it gets up to 38C *outside the case*, you are going to have
a miserable summer.  Most chips run too hot anyway.  Put a fan on it.

Lastly, consider how you test the modified system .  I don't know if
anybody has developed a test program for PCs that checks all the
potential data pattern sensitivities for all the different peripheral
combinations and chipsets - it would be a difficult problem.  I'm
pretty sure Checkit and QAPlus find many potential errors, but I
doubt they find all the speed related ones.

The bottom line?  Don't do it if you count on the machine for anything.
Go ahead and do it if you enjoy testing the hell out of it and will
report the results of your extensive tests to the rest of us.  
Please, no "I increased my clock xx% and it hasn't crashed for days"
stories - I can make a cursor blink with unreliable hardware, too.

Keith
-- 
Keith Lofstrom         keithl@klic.rain.com       Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Power ICs