...The PMU is a microcontroller chip that controls power functions for the computer. The PMU is a computer within a computer, as it has its own memory, software, firmware, input/output, a CPU, and more. Its function is to:
• 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.
Over time, the settings in the Power Manager may become unusable, which can result in operational anomalies with the computer. Examples include not turning on, not waking from sleep, or not seeing devices plugged into the USB and FireWire ports, among others.
Resetting the PMU is not intended for resolution of a stall or situation in which the computer is unresponsive. A PMU reset should not be necessary except as a last resort in cases where a hardware failure of the power management system is suspected. Performing a PMU reset returns the computer hardware, including NVRAM, to default settings and forces the computer to shut down. ...
