Troubleshooting The PMU Chip - 9
The PMU Chip The PMU (Power Management Unit) is a microcontroller chip that
controls all power functions for the computer. The PMU is a
computer within a computer. It has memory, software, firmware,
I/O, two crystals, and a CPU. 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.
Important:
Be
very
careful when handling the main logic board.
Remove the battery when handling the logic board so the PMU is
not affected. The PMU is
very
sensitive and touching the circuitry
on the logic board can cause the PMU to crash. If the PMU crashes,
the battery life goes from about five years to about two days if the
PMU is not reset. Refer to the next topic, “Resetting the PMU on
the Logic Board” for the procedure. Many system problems can be
resolved by resetting the PMU chip. The PMU reset button is
located on the right side of the board, refer to the logic board dia-
gram below.
iMac (350 MHz) Logic Board Diagram