System Management Controller (Xbox Original)
The Xbox versions 1.0 to 1.4 have a SMC. The 3.3V standby voltage powers the SMC, and after power is removed the capacitors keep the SMC powered for several hours.
The SMC isn't present on the v1.6 models. That model uses a custom Xyclops chip above the LPC port to replace the SMC.
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.
When the NVIDIA MCPX doesn’t correctly trigger the watchdog pin of the SMC, the SMC triggers a reset. This is where the FRAG (flashing red and green) come from.
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 3.3V rail is applied is the SMC listens for the power button to be pressed, and when it is it sends 3V3 on its 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).
After this the SMC listens for the Power OK (blue wire) from the PSU going to 3.3V to know those rails are stable, at which time it then enables and resets the CPU/GPU/MCPX and the system boots.
