System Management Controller (Xbox Original)
The Xbox versions 1.0 to 1.4 have a SMC. The 5V standby voltage powers the SMC, and after power is removed the capacitors keep the SMC powered for several hours.
The System Management Controller (SMC) is a PIC16LC63A-04/SO microcontroller. The PIC chip handles rebooting the system, telling the Xbox the type of video cable attached, the DVD tray state, controlling the fan, LED control and sensing temperature. It is also the hardware which is connected to the Power and Eject buttons.
The PIC is running at 20 MHz with its own ROM, RAM and I/O lines.
The PIC is always running, even if the Xbox is turned off. When the power cable is unplugged, it gets its energy from a capacitor for some hours. It is connected via I²C and located on address 0x10.
The next step after the 5V rail is applied is the SMC listens for the power button to be pressed, and when it is it sends 3V3 on it's Power On wire on the power supply connector telling the power supply to leave the standby mode and enable all the other voltage rails (12V, 5V and 3.3V).