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Fix 2002 iMac power problem with PMU reset System
Although not strictly OS X related, many readers will probably be using OS X on the new iMacs, so we felt this important to pass along...

I just received a new G4 flat panel iMac. After loading a beta driver for Kensington Mouseworks, I went to restart my iMac and and ended up with a hung-up system (spinning beach ball on blue screen).

I shut down via the power switch, but when I attempted to restart with the power switch, alas no power! I contacted Mac Technical Support, Mac's Tech department spoke with their engineering department and it seems that the new iMac uses a chip circuit for power management.

To remedy this problem you must open the bottom of your iMac. Next to the memory slot, there is a tiny rectangular button under a protective plastic covering. Press this just once, and power will be restored.

[Editor's warning: "A MacInTouch reader warned about gratuitous use of the flat panel iMac's hidden, special reset switch, something Apple Support sometimes has customers use when the computer fails to operate. This operation apparently resets the "PMU" (Power Management Unit) but can cause serious problems if not performed in precisely the correct way, with the power cord removed and with a waiting time after the push - and it should never be pushed twice in a row, because it could "crash" the PMU. (The operation also resets PRAM, so settings, such as time and date, must be re-entered.)" ]
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Fix 2002 iMac power problem with PMU reset | 3 comments | Create New Account
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PMU on Silver 867 ??
Authored by: cbin on Feb 28, '02 04:46:14AM

I have encountered a similar problem on a new G4 silver 867 mhz. Did it have a PMU with a reset possibility too?
"Désolé pour mon english", I'm french. Thanks
Claude Binovsky - Toulouse - France

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be careful
Authored by: serres on Feb 28, '02 09:05:20PM

there's an PMU reset on almost every mac, even the old 68k machines (at least the powerbooks, probably every mac which supports sleep mode). generally it works the way that you unplug ALL power, including batteries, than press the reset button for 10 seconds or so and than wait ... and finally you have to leave the computer or AC adaptor plugged in for at least 1 to 2 days! take a look in the apple knowledge base and do not play with this stuff.

the PMU ("Power Management Unit") is some sort of superchip running all the time, which can reset are start the CPU, switch on/off peripherals etc. you can imagine that this can be done with some wires :).

it's also responsible for the autowake and autopower feature (choose a time when you ibook wakes up!) -- sadly not enabled in current mac os x. but there is activity going on in the darwin source, including a command line utility to set the PMU features. so let's hope that ibook can be used as mp3 alarm clocks in the near future.

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Completely useless PMU bits
Authored by: Mikey-San on Mar 01, '02 04:42:08AM

First, a short set of tips on PMU reset switch (also known as the "CUDA" switch) antics:

What you've read about repeatedly pressing the PMU switch is true: DO NOT DO IT. The PMU is completely crashable, to the point of requiring replacement of the circuitry itself. On some machines, this is a logic board replacement, not on others. Good advice: Never press the reset button more than once in a ten-minute period.

Better advice: If you're not completely sure what the PMU and its reset button do, don't mess with 'em. There are a few Knowledge Base articles dealing with them, but as is the norm with computers, if you're unsure, leave it be and find someone who isn't. :-)

The PMU circuitry on modern Macs controls a TON of stuff. For those interested in operational theory, it's a branch from Apple's KeyLargo I/O architecture. It comes straight off the KeyLargo I/O bridge and manages power and certain data signals throughout a great deal of the computer. PowerBook owners, for example, might be interested to know their keyboard, trackpad, and trackpad button are all ADB devices routed through the PMU circuitry. (This includes all current PowerBooks and iBooks! ;-))

(For more useless Mac trivia, you'll be interested to know KeyLargo also encompasses the programmer's switch (for firmware updates), the backlight on your PowerBooks and iBooks, and your ATA bus, among a TON of other things! OS X users can hop into Terminal and type 'ioreg' (no hash marks) to see everything KeyLargo does.)

When a setting in the PMU becomes corrupted or set improperly, it can cause erratic power behaviour throughout the entire computer. No video, no power--all sorts of odd things can happen when the PMU goes to hell.

So, why do you have to reset certain preferences when you hit that button?

Well, from what I've come to understand--and anyone is completely open to point out any errors in this--think of it as a system flush. Resetting the power manager clears out any erroneous voltage sitting in circuitry it shouldn't be in, as well as--essentially--flushing the held settings (held by streams of power in the right places, governed by the PMU, which never really "turns off") that keep your time correct when your machine is off, for an example. (That's why Apple tells you to leave the computer sitting unplugged for a certain length of time after a PMU reset. Gotta make sure the voltages are cleared and reset properly, or it was a waste of time.) The "trickle power" is interrupted for a moment, and things like PRAM get reset back to their defaults.

Yeah, yeah, I know . . . Boring stuff you _probably_ already knew. But, there's gotta be someone out there looking for a little more Useless Information(tm)! :-)

Michael Watson
Apple Product Professional
Apple Certified Service Technician
Capitol Mac Consultants

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