The Xbox SMBus

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xbox

 · 20 Feb 2024

written by Michael Steil, 15 June 2002 (Updated 26 June 2002)

All non-PC components of the Xbox console are connected through an I2C/SMbus interface. I2C/SMBus is a slow low-cost serial bus with each device having a unique 7-bit ID.

There are only two operations of an SMBus controller: write and read. Writing means that an 8 bit command code and an 8 or 16 bit operand are sent to an SMBus device. Reading means that an 8 bit command code is sent to the device and an 8 or 16 bit answer is expected.

The controller for the SMBus interface in the Xbox is a PCI device with the DevID 01B4 and it is built into MPCX southbridge.

SMBus Controller Port Layout

The port layout is as follows:

This is very similar (but not identical) to the AMD756/AMD766/AMD768 SMBus controllers. The lm_sensors project includes GPLed Linux kernel code for it since version 2.6.4 (kernel/busses/i2c-amd756.c).

SMBus Controller Programming

The following two routines illustrate how to read and write data from and to an SMBus device:

int SMBusWriteCommand(unsigned char slave, unsigned char command, int isWord, unsigned short data) { 
again: 
    _outp(0xc004, (slave<<1)&0xfe); 
    _outp(0xc008, command); 
    _outpw(0xc006, data); 
    _outpw(0xc000, _inpw(0xc000)); 
    _outp(0xc002, (isWord) ? 0x0b : 0x0a); 
    while ((_inp(0xc000) & 8)); /* wait while busy */ 
    if (_inp(0xc000) & 0x02) goto again; /* retry transmission */ 
    if (_inp(0xc000) & 0x34) return 0;  /* fatal error */ 
    return 1; 
} 

int SMBusReadCommand(unsigned char slave, unsigned char command, int isWord, unsigned short *data) { 
again: 
    _outp(0xc004, (slave<<1)|0x01); 
    _outp(0xc008, command); 
    _outpw(0xc000, _inpw(0xc000)); 
    _outp(0xc002, (isWord) ? 0x0b : 0x0a); 
    while ((_inp(0xc000) & 8)); /* wait while busy */ 
    if (_inp(0xc000) & 0x02) goto again; /* retry transmission */ 
    if (_inp(0xc000) & 0x34) return 0;  /* fatal error */ 
    *data = _inpw(0xc006); 
    return 1; 
}

To avoid busy waiting of the CPU, the SMBus controller can also issue an interrupt when the operation is complete, by setting bit #4 in the control port when initiating the transfer.

Xbox SMBus Devices

The following four devices are connected to the Xbox SMBus:

Do not confuse the hardware with the software addresses: The software ID is the hardware ID shifted by one bit left. The code above expects the hardware ID.