All About The PMU

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Keith of Knoxville, Tennessee wrote in with an interesting story about his Quicksilver failing to shut down, restart, and perform other basic functions:

_Do you happen to have an old article in which you address the problems/solutions involving the Power Management Unit (PMU) on the motherboard? I was told by an Apple store Genius that if the computer is left unplugged for about 4 hours that the PMU will reset itself.

Just had an interesting episode involving the PMU on our tower G4 Quicksilver. Our APC battery backup was 5 years old and even though the charge and run time looked as good as new, it could be heard doing a lot of clicking sounds every once in a while that was not correlated with bad weather conditions that could actually give rise to voltage fluctuations on the 120VAC incoming line.

It turns out (according to the Apple Genius) that this clicking was probably faulty operation of the APC unit which got my PMU screwed up. The G4 was really sick, having trouble shutting down, restarting and other problems. By the time I got it to an Apple store and they started it, it worked perfectly and all the diagnostics they ran found nothing wrong. So I got a new APC unit, and the clicking noises are gone and the computer is now running fine.

Anyway, I am curious to learn more about the PMU and how it can impact the way the computer runs.

Best, Keith_

Keith, I have little experience with Uninterruptible Power Supplies (UPS), but I do know from extensive diagnostic experience that the power source can greatly affect a computer’s performance, and sometimes, cause irreversible damage. The clicking noise coming from your UPS is an indication of improper function. It’s normal and expected to hear clicking noises at the very moment your electricity goes out. This is the UPS “kicking in.” If it’s making those noises haphazardly without any power fluctuation, something is awry. For what it’s worth, five years is a really good run on one of these!

Because power supplies inside computers contain capacitors that take a little while to dissipate electricity once removed from power, very brief and minor brownouts may not affect the operation of your computer. You can see this in action by connecting a power adapter to a PowerBook or iBook (the MagSafe adapters don’t do this quite as well), plugging it into the wall power to light the adapter, and unplugging it from the wall. You’ll notice the amber or green indicator slowly turn dark. This is the power dissipating from the capacitors in the power brick.

The Power Management Unit (PMU) is basically a computer within your computer. On Intel-based Macs, this is now called the System Management Controller (SMC) and serves basically the same functions. The PMU/SMC has its own memory, software, firmware, central processing unit (CPU), I/O architecture (in/out) for sleep control, some charging functions, trackpad control, and signals to the other components of the computer that they are to shut down, restart, sleep, and wake up. It serves to regulate the amount of power (electricity) going to each part of the main logic board. If excessive and repetitive power fluctuations course through the power supply and to the PMU or SMC, these components can be damaged, causing failure to shut down, restart, go to sleep, or wake from sleep. Resetting the PMU or SMC can sometimes revive it, but not always in these situations.

Resetting the PMU or SMC is the first step in troubleshooting many issues. So much is controlled by these units, and resetting them is often (and surprisingly) the solution to many problems. There are many different methods to perform the reset–depending on your model–so I searched through Apple’s excellent and extensive knowledge base (www.apple.com/support) and realized there are too many articles on the PMU to even list here. If you’d like to check out a wide-ranging set of articles on the PMU, go to the Apple support site and simply search for PMU. If you need to find the specific method for resetting the PMU or SMC on your computer, I’d suggest search terms like: iMac G4 PMU reset for an iMac G4, or Power Mac G5 PMU reset for a Power Mac G5, etc.