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iMac G5 (20-inch iSight) Troubleshooting — General Info 146
SMU (System Management Unit)
The iMac G5 (iSight) uses an advanced system management unit (SMU) to manage the thermal
and wattage conditions, while keeping the acoustic noise to a minimum. The SMU controls the
fans and regulates the speeds to run each fan.
The SMU supports the following functions:
provides initial conguration of system clocks•
automatically initiates and sequences voltage and frequency slews when the OS •
requests a transition to a new run level
handles all power button and environmental user events•
controls RPM of the fans•
monitors processor temperature and power consumption•
he iMac G5 (20-inch iSight) enclosure is divided into two discrete thermal zones. The two zones
split the enclosure approximately in half. Temperature and power consumption are monitored by
the operating system which communicates with the SMU, which in turn controls and monitors
fan operation. If Mac OS X is not booted, thermal management must be provided by the alternate
development operating system.
Note: The SMU controls the fans and regulates the speeds to run each fan. If the SMU does not
receive an update from the operating system within two minutes, it begins to ramp up the speed
of the fans to full speed.
Resetting the SMU (System Management Unit)
Unplug all cables from the computer, including the power cord. 1.
Wait 10 seconds.2.
Plug in the power cord while simultaneously pressing and holding the power button on the 3.
back of the computer.
Let go of the power button.4.
Press the power button once more to start up your iMac G5 (20-inch iSight).5.