Power Management Unit
Encyclopedia
The Power Management Unit (PMU) is a microcontroller
that governs power
functions of digital platforms. This microchip
has many similar components to the average computer
, including firmware
and software, memory
, a CPU, input/output functions, timers to measure intervals of time, as well as analog
to digital
converters to measure the voltages of the main battery or power source of the computer. The PMU is one of the few items to remain active even when the computer is completely shut down, powered by the backup battery.
For portable computers, the PMU is responsible for coordinating many functions, including:
The PMU controls almost every power-consuming function in an Apple computer. It is constantly running diagnostics on the various power-related operations and checking them against the current Energy-Saver settings, allowing the PMU to actively manage power consumption for optimum user performance.
The PMUs functions may become corrupt over time. If this happens, it may become unresponsive and stop performing tasks. The user may not notice the PMUs malfunctions so much as the side effects of the corruption, including but not limited to:
Resetting the PMU in these circumstances can be a relatively quick and easy fix to some of these issues. There is a keyboard shortcut on newer Apple laptops with an internal battery, nicknamed "SCOP". This stands for Shift Control Option Power. This "reboots" the PMU software in order to get it working as it should. For Apple laptops with a removable battery, resetting the PMU involves unplugging the power adapter, disconnecting the battery, then holding down the power button for five seconds. Another PMU-related fix would be to reset the logic board, where one removes the backup battery on the board for a few minutes, then reinstalls it, causing the PMU to reset itself with clean, fresh parameters (that need to be corrected, if desired, to its previous state) during the next Mac OS boot (for typical PC users, this is similar to "resetting the CMOS
").
The PMU is very sensitive and a reset may be necessary if a backup battery dies. Even plugging in one's laptop in the wrong order can cause issues (power into the outlet first, THEN power to the computer). Never turn off an attached UPS without first unplugging the AC adapter.
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
that governs power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
functions of digital platforms. This microchip
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
has many similar components to the average computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
, including firmware
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...
and software, memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
, a CPU, input/output functions, timers to measure intervals of time, as well as analog
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
to digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
converters to measure the voltages of the main battery or power source of the computer. The PMU is one of the few items to remain active even when the computer is completely shut down, powered by the backup battery.
For portable computers, the PMU is responsible for coordinating many functions, including:
- Monitoring power connections and battery charges
- Charging batteriesBattery (electricity)An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
when necessary - Controlling power to other integrated circuitIntegrated circuitAn integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s - Shutting down unnecessary system components when they are left idle
- Controlling sleep and power functions (on and off)
- Managing the interface for built in keypad and trackpads on portable computers
- Regulating the real-time clockReal-time clockA real-time clock is a computer clock that keeps track of the current time. Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time.-Terminology:The term is used to avoid...
(RTC)
The PMU controls almost every power-consuming function in an Apple computer. It is constantly running diagnostics on the various power-related operations and checking them against the current Energy-Saver settings, allowing the PMU to actively manage power consumption for optimum user performance.
The PMUs functions may become corrupt over time. If this happens, it may become unresponsive and stop performing tasks. The user may not notice the PMUs malfunctions so much as the side effects of the corruption, including but not limited to:
- Failure to turn on
- Failure to restore from sleep mode
- Failure to recognize connected devices (FireWireIEEE 1394 interfaceThe IEEE 1394 interface is a serial bus interface standard for high-speed communications and isochronous real-time data transfer, frequently used by personal computers, as well as in digital audio, digital video, automotive, and aeronautics applications. The interface is also known by the brand...
, USBUniversal Serial BusUSB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....
, etc.)
Resetting the PMU in these circumstances can be a relatively quick and easy fix to some of these issues. There is a keyboard shortcut on newer Apple laptops with an internal battery, nicknamed "SCOP". This stands for Shift Control Option Power. This "reboots" the PMU software in order to get it working as it should. For Apple laptops with a removable battery, resetting the PMU involves unplugging the power adapter, disconnecting the battery, then holding down the power button for five seconds. Another PMU-related fix would be to reset the logic board, where one removes the backup battery on the board for a few minutes, then reinstalls it, causing the PMU to reset itself with clean, fresh parameters (that need to be corrected, if desired, to its previous state) during the next Mac OS boot (for typical PC users, this is similar to "resetting the CMOS
Nonvolatile BIOS memory
Nonvolatile BIOS memory refers to a small memory on PC motherboards that is used to store BIOS settings. It was traditionally called CMOS RAM because it used a low-power Complementary metal-oxide-semiconductor SRAM powered by a small battery when system power was off...
").
The PMU is very sensitive and a reset may be necessary if a backup battery dies. Even plugging in one's laptop in the wrong order can cause issues (power into the outlet first, THEN power to the computer). Never turn off an attached UPS without first unplugging the AC adapter.
External links
- Resetting the PMU on Macbook and MacbookPro computers - contains general information on the PMU