Newsgroups: comp.sys.mac.hardware

In comp.sys.mac.hardware, isr@rodan.acs.syr.edu (Michael S. Schechter - ISR group account) writes:

> they'll be more voltrage drop, and the CPU will draw still more amperage,

Ack!  Less voltage --> lower current

Actually, in the CMOS case, the output capacitances have to be charged to
lower voltages when the supply voltage decreases.  As a consequence the
average current goes down.  Unfortunately, the max speed of the logic gate
goes down since the current available to charge output capacitances is
quadratically (to a first order) related to the supply voltage.  This is why
low voltage circuits are so difficult to design for high speed.

As regards to the 25Mhz upgrade of the IIsi:  Power dissipation in CMOS
is dominated by the currents necessary to charge and discharge gate load
capacitances.  This form of power dissipation is given approximately by:

P = C(load)*[V(supply)^2]*frequency

Therefore, for lower V(supply) and constant C and f, power dissipation
goes down (quadratically).  Since power is a linear function of frequency,
25% increase in clock frequency generates a 25% increase in power
dissipation.  

In terms of reliability, heat is the enemy of chip reliability.  It was
mentioned earlier that diffusion of junctions would lead to failures but
that is not an issue at these temperatures.  Electromigration is
accelerated by heat and will most likely cause the failure.  For the
curious, electromigration is a slow migration of the metal conductors on
a chip due to a combination of heat and high current levels.  This
phenomenon can lead to open circuits.  In any case, adding a good sized
heat sink to the processor chip will improve the expected reliability
beyond that of an unmodified IIsi (if the chip runs at a lower
temperature than it did before).  Not adding the heat sink will reduce
the reliability.  The question is whether the reliability reduction is
noticeable, i.e. Does the MTBF go from 30 years to 20 years, or from 10
years to 2 years?  If it is reduced to 20 years, then you will be using
your wizbang third generation PowerPC with the neural interface by then
so who cares.  Since I don't know the answer, I would add the heat sink
to be conservative.

Another point in favor of the heat sink is the improved performance of
CMOS at lower temperatures.  For a given input drive voltage, the output
current of a MOS transistor decreases with increasing temperature.  This
is caused by the reduction in the carrier mobility as temperature
increases.  Again for the curious, lattice vibrations in the
semiconductor increase with increasing temperature so the current
carriers (electrons in an NMOS device) have a reduced 'mean free path'
i.e. they bang into things more often and take longer to get through the
device.   Could you get through a crowd of people slam-dancing faster
than a crowd of people standing perfectly still?   Probably not.  Of
course, this is a simplified explanation but you get the idea.  Since
chips are spec'd to run over a range of ambient air temperatures
(usually up to 85C), it stands to reason that a chip that will run at
20Mhz at 85C should run faster at room temperature (25C).  If 25Mhz is
at the outer edge of the chip speed margin, then running without a heat
sink can raise the temperature enough to degrade the performance so that
the chip will begin to have failures (i.e. dropped bits...) as it heats
up.  Again, your chances of success are improved if the temperature of
the chip is held lower.

All of this is focused on the main processor since we are working on the
assumption that the rest of the ASICs were designed originally for the
IIci and should be fine for 25Mhz operation.  This assumption seems to
be at least partially correct since most of the modifications are
working.

Bottom line:  Increasing your clock speed could work fine for several
weeks but you might crash in the middle of a big edit during a hot day
next August when the AC in your room isn't working so well.  Or you
might crash due to an init conflict tomorrow (hey, this is a Mac :-)).
As always, save early and save often.  Try it if you feel like it but be
aware of the risks.

I hope someone found some of this useful.

Travis Blalock
blalock@hpl.hp.com

p.s. My 25Mhz IIsi is working fine so far!

Newsgroups: comp.sys.mac.hardware