An Exploration into power management issues

A recent submission by MacFixIt reader Jon Brown coupled with several related issues appearing in Late-Breakers over the past few weeks have prompted further investigation into power management issues - primarily with external USB and FireWire devices, but also with standard internal components.

The interesting element of this situation is not the problem itself (kernel panics, problems waking form sleep, device failure have all been reported), but the workaround that has prevailed in a striking number of cases - resetting the PMU (power management unit).

As we have previously reported, the PMU performs the following functions:

• Tell the computer to turn on, turn off, sleep, wake, idle, etc.
• Manage system resets from various commands.
• Maintain parameter RAM (PRAM).
• Manage the real-time clock.

Jon Brown describes two separate issues that at first seem unrelated, but may be linked to an underlying power management problem - somehow solved by resetting the PMU.

"I use a Sawtooth G4/400/AGP. If put to sleep overnight, most often it will wake up with the keyboard frozen or an immediate kernel panic. When it occasionally comes to, applications immediately quit or crash.

"A seemingly unrelated observation: a FireWire burner continually failed to verify disks in Toast, yet the same data backed up perfectly when I hooked up the FireWire drive to a Pismo PowerBook, also running Panther. Further, after pulling an IOGear USB 2.0 card, a PMU reset, a battery voltage check, and multiple restarts, the FireWire drive copied and verified correctly. However, if I started up with the FireWire drive on, problems ensued.

"Therefore I wonder if several issues that seem unrelated come together as a power management issue. Is it possible that peripherals or cards connected to certain computers under Panther (or Jaguar) draw too much power, perhaps with the OS implicated in the problem, and that's behind the kernel panics, crashes, and freezes?as I understand it, the PMU handles not only sleep but ports."

Brown's submission bears striking resemblance to previous reports regarding the widespread FireWire device failure updating to Mac OS X 10.3, where users were successful in restoring FireWire device communication by resetting the PMU.

There are also confirmed cases of PMU resets resolving PowerBook charging problems, a black display on startup issue, and various flat-panel iMac problems.

So why are power management problems cropping up after major OS updates, when new devices are introduced, or seemingly randomly? We'd love to hear your thoughts.

In the meantime, you can reset your PMU if you think you're experiencing a related problem, but be careful. In order to reset the PMU, you will need to disconnect power (and all other external devices for good measure), open the case (on some PowerBooks resetting the PMU can be accomplished by simultaneously pressing and then releasing Shift-Control-Option-power on the keyboard while the system is off), and press the PMU reset once. Pressing the rest button more than one time could have bad consequences, like causing the PMU to no longer operate properly. [Additional instruction on resetting the PMU can be found in Knowledge Base article #95037. A separate Knowledge Base article describes the process on a flat-panel iMac.

Resources