🚀 go-pugleaf

In article <7XRLoHAAGGN4EwWM@upthorpe.demon.co.uk>, Oz <Oz@upthorpe.demon.co.uk> writes:
|> In article <3833dc8c.0@nnrp1.news.uk.psi.net>, Andy Repton <andy+news@DE
|> LETE.pteron.demon.co.ME.uk> writes
|> Firstly the junction temperature of a normal spec chip with normal
|> cooling is probably about 100C or thereabouts. The junction temperature
|> of a clocked up chip which of course dissipates much more but with a
|> supercooling of the chip will STILL be about 100C or thereabouts. If not
|> and the process permits higher speed then clock it some more.
Ok, let's look at a scenario.

You have a chip, called the X400.  It's specced to run at 400 MHz, with normal
cooling.  Under these conditions, the die will be at 100 C.  However, you know 
that the semiconductor manufacturer has left a 25% safety margin on its timing,
so you know that a device will run at 500 MHz at 100 C die temperature.  So, you
crank up the clock speed to 500 MHz, and put on a couple of extra fans, which 
will keep the die temperature down to 100 C.  All you are doing here is eating
into the manufacturer's safety margin. 

It is true that most overclocked devices will run with a die temperature of 
about 100 C; this is because people are generally unadventurous with their
cooling systems.  They are not getting the benefit of cooler, faster devices 
because they simply do not add enough cooling.

However, you also know that, if you could get the die temperature down to 40 C,
the resistances in the device will be halved. Now, as the speed of the device
depends on the time constant, RC, halving the resistance will halve the delay
within the chip, allowing the chip to run at twice the speed.

So, you increase the clock speed to 1 GHz, and then strap on enough extra cooling
(fans, Peltier heat pump, refridgeration system, bathe in liquid nitrogen/helium/
etc.) to get the temperature of the die down to 40 C (this will probably mean 
that the case will be somewhere around 0 C or lower), then hey presto! You have a
1 GHz device.

Now, to tackle your point "If not and the process permits higher speed then clock 
it some more".  If you were to increase the clock speed beyond 1 GHz, this would 
break the device in two ways:

  o You are already at the maximum speed that the chip can run.  Data is expected
    to arrive ready for the next clock edge within 1 ns.  Increasing the clock 
    speed will mean an even tighter constraint, and the data simply won't get
    there in time.

  o Increasing the clock speed will cause the device to dissipate even more heat,
    warming it up.  This will slow down the transistors, and mean that the device
    isn't even able to meet 1 GHz any more.

Note: these numbers are made up.  They don't reflect reality, they just made the
maths easier.  They do, however, illustrate the point.

|> Your logic is thus totally faulty because the active region of the chip
|> is at the same temperature either way.

Like I said, this is true with a simple cooling mechanism like most people use.
They simply don't add enough cooling to get the die below 100 C.  They lack
imagination (and/or money).  They are simply eating up the safety margin, rather
than making the chip faster.

|> I could go on but I sincerely hope you don't design chips as you show
|> such profound ignorance of how they behave in a system.
|> 
|> You really should learn about junction to ambient heatflows then you
|> could understand what I am talking about. 

A few years ago, I did some characterisation experiments on an HCMOS device, using
a thermostream (a bit of kit that is able to squirt a high-speed stream of air on 
to a device at any temperature you like, between about -25 C and +125 C.  Looks
a bit like a cubic elephant, and runs on electricity and compressed air).
Unsurprisingly, when I cooled the device with -25 C air, it was able to run faster
without errors than when I blasted it with air at  +20 C or +70 C.  If, as you 
say, the die will always run at 100C, then the speed at which I get data errors 
should be the same no matter what temperature air I use.  I've probably got the
results lying around in a log book somewhere.  
I can only draw two possible conclusions from these results:

  1 CMOS devices run faster when they are colder, like I've been trying to say,

or

  2 A month or so of my life is just a figment of my imagination.

Given that questioning one's own sanity is a path I don't wish to go down, I
conclude that option 1 is correct, unless you would like to explain another 
reason why I got these results?

BTW:This will be my last post on the subject, as it has already got too out of 
    hand.

-- 
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