🚀 go-pugleaf

On Thu, 18 Nov 1999 16:08:30 +0000, Oz <Oz@upthorpe.demon.co.uk> wrote:

>> *how* does one determine
>>whether it's speed that affects temperature *or* the other way round
>
>Very simple. See how fast you can run an 8MHz 8086 as you decrease the
>temperature. You will find there is a maximum rate and that this is very
>little affected by temperature simply because the switching speed of the
>components limits the maximum clock speed.

Well, yes and no.  In a way Dave Smith was right when he refuted my
assertion that a chip either works or doesn't work at a given clock speed.
The truth is that there are effects both ways round.  Let's try to summarize
them.

1. Dealing with what you've just said above.  The switching speed of the
components *is* affected by temperature, and I'm prepared to accept, on
Dave's authority as a semiconductor engineer and also because it accords with
my own hazy memory of it when hardware electronics was my job, that devices
tend to get slower rather than faster at higher temperatures.

2. Clocking any given device faster will generate more heat.  That's because
each transition charges or discharges capacitances and dissipates a fixed
amount of energy, so more transitions per second means more energy
dissipated per second (= power)

So when you increase the clock speed of a given chip, the first thing that
happens is you immediately reduce the time budget in the internal logic,
simply because the clock interval is shorter.  The second thing is that the
chip gets hotter, hence slower, which further reduces the time budget.  The
second effect can be cancelled by increasing the cooling sufficiently to
reduce the chip temperature to the same as it was before.  DS is suggesting
that the first effect can also be reduced by overcooling so the chip's
actually cooler that it was before despite higher power disspation.

Personally I suspect that the change in chip speed with temperature is
fairly small, and that short of using elaborate heat pump techniques (like
Intel's first 486-50) you can't make a chip go much faster that it naturally
wants to.  But since I don't have figures to support that, I think we'll
have to accept that to some extent everybody's right in this argument, but
nether side of it is the whole story.

>Really it's only been recent chips that have dissipated enough for the
>package dissipation to be a limiting factor in the permittable maximum
>clockspeed. Hence the use of lower core voltages in an effort to
>restrict dissipation at any given clockspeed (because dissipation is
>roughly proportional to supply voltage).
>
Don't know what you mean by recent, but cooling fans came in with the 486
DX2-66.  You are right about voltages; the other factor that reduces
dissipation is the size of the components on the chip.  Smaller transistors,
less capacitance, less charge shifted on each logic state transition, less
power.

-- 
Anahata
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