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
