Re: Pismo status

On Wed, 17 May 2000, Timothy A. Seufert wrote:

No, it will not.

The PowerPC 750 (G3) and 7400 (G4) can both fire off an interrupt
when the on-die temperature sensor reading rises above a trigger
value (or falls below a second trigger value).  This feature *could*
be used by an operating system to slow down the CPU (through the
instruction cache throttling feature) or halt it to prevent
overheating.  However, there is no hardware feature which can halt
the CPU without software control.

So in other words, without a little bit of help from the OS, the fan won't
come on when it the temperature hits this trigger value.  The temperature
would of course, continue to rise.

The normal mode is that MacOS controls the fan through the PMU.  The
PMU is connected to a thermistor located somewhere on the
motherboard.  MacOS scans the value of this thermistor every so often
and uses the fan to cool the machine if it's getting warm.  MacOS
takes advantage of the speed control feature of the fan, so it won't
crank the fan up to full speed if it doesn't have to.

Has any code been written for LinuxPPC that will let us monitor this
sensor?  I know there's a whole lm_sensors package for i386 and the like,
but is there something like this for us?

If MacOS fails to turn the fan on for some reason, the PMU acts as a
backup controller -- it is also monitoring the thermistor and will
force the fan to turn on at full speed if a moderately high (but not
yet unsafe) temperature is reached.

If the PMU's fan override fails to stop the computer's temperature
from rising, and the temperature begins to approach dangerous levels,
the PMU will simply shut the machine down without asking.  (Most Macs
since the Mac II have had a thermal shutdown feature like this,
including the desktops.)

So, at least for 101, the fan should still come on without operating
system support, followed by an abrupt shutdown if the computer gets
way too hot.

Yes, my fan does come on Linux.  But it takes a long time, and the case
gets very warm before it does.  I assume this means the PMU has finally
decided it's too hot and turned on the fan.  I know it's most likely not
Linux doing it.

The current PowerBook G3, 102 (the one with FireWire, popularly and
in this case correctly known as "Pismo"), does have a different PMU
than 101.  Apple did a major upgrade to the PMU used with all Core99
chipset machines, 102 included.  But it's fairly unlikely that Apple
changed the fan control technique very much.

Yes, I had heard they upgraded the PMU, but I wasn't aware of how much
they had changed it.  Is it just a matter of knowing some extra bits we
can switch on and off in the PMU, or is it more than that?

Tim


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