The PMU (Power Management Unit) or the System Management Controller is an integrated circuit (computer chip) that is on the logic board of the computer. As the name implies, it is responsible for power management of the computer. It controls backlighting, hard disk spin down, sleep and wake, some charging aspects, trackpad control, and some input/output as it relates to the computer sleeping.
Over time, the settings in the System Management Controller may become unusable, which can result in operational anomalies with the computer. Examples include not turning on, not waking from sleep, not charging the battery, or not recognizing the AC Adapter, among others.
Resetting the SMC is not intended for resolution of a stall or situation in which the computer is unresponsive. An SMC reset should not be necessary except as a last resort in cases where a hardware issue with the power management system is suspected. Performing an SMC reset resets the hardware power management functions.
For most situations, a restart is sufficient. If the computer has stopped responding, try each one of these steps, one by one. Test between each step to see if it has worked. If one step works, don't worry about the next one. You should only reset the SMC if you've tried all of the steps listed here and the computer still isn't working.
To reset the PMU on a Mac mini:
- Unplug all cables from the computer, including the power cord.
- Wait 10 seconds.
- Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.
- Let go of the power button.
- Press the power button once more to start up your Mac mini.
Apple Portable Computers
Resetting the PMU on these computers varies by model. Please refer to the Apple Knowledge Base article on this at http://support.apple.com/kb/HT1411