UART

The XBox 360 has an UART in the Southbridge.

Registers

To configure the UART, write to register 0x8000 0200 EA00 101C (real mode)

Register value	Configuration
0xE6010000	115200,8,N,1
0xB2010000	38400,8,N,1
0x63010000	19200,8,N,1

To send a byte, do a 32 bit write to 0x8000 0200 EA00 1014. After you sent something to the serial port, the status should be read from 0x8000 0200 EA00 1018.

Sample code

Init UART speed:

    li        %r7, 0x0200
    oris      %r7, %r7, 0x8000
    sldi      %r7, %r7, 32
    oris      %r7, %r7, 0xEA00
    #ori       %r7, %r7, 0x101C
    lis       %r8, 0
    oris       %r8, %r8, 0xE601       # 115200,8,N,1
    stw       %r8, 0x101C(%r7)

Send char '!' to the serial port (UART):

    li %r8, '!'
    slwi %r8, %r8, 24
    stw %r8, 0x1014(%r7)        # send the character
    sync
    isync
1:
    lwz %r8, 0x1018(%r7)        # wait until character is sent
    rlwinm. %r8, %r8, 0, 6, 6
    beq 1b

Get char from the serial port (UART):

     lis %r4, 0x8000
     ori %r4, %r4, 0x200
     rldicr %r4, %r4, 32,31
     oris %r4, %r4, 0xEA00
1:
     lwz %r8, 0x1018(%r4)
     rlwinm %r7, %r8, 0,8,5
     cmplwi %r7, 0             # check the status of the UART before input
     bne 1b
     rlwinm. %r7, %r8, 0,7,7
     beq 1b
     lwz %r3, 0x1010(%r4)      # input char from address 8000 0200 EA00 1010
     srwi %r3, %r3, 24

Serial Console

The Serial Console can be very useful when debugging the Linux Kernel or diagnosing the Boot Process.

Diagram FAT

Diagram ALL

You can either use the GND pin on the header (might be difficult to solder for the inexperienced) or some alternative GND pin like the one from the Stereo DAC nearby the J2B1 header.

Note that you will probably need a Level Shifter to connect the 3.3V RX/TX pins to your PC Serial Port (which usually uses +/-12V)

The Level Shifter can be either powered from the standby 3.3V on pin 7 (always on) or better from the active 3.3V on pin 8 or from an external power source.

It is also possible to use some cheap cell phone data cables as a PC interface.

Category: Hardware Category: Development Category: Homebrew Software