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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?


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