USB Controller Registers
11-8
Intel
82801BA ICH2 Datasheet
11.2
USB I/O Registers
Some of the read/write register bits that deal with changing the state of the USB hub ports function
such that on read back they reflect the current state of the port, and not necessarily the state of the
last write to the register. This allows the software to poll the state of the port and wait until it is in
the proper state before proceeding. A Host Controller Reset, Global Reset, or Port Reset will
immediately terminate a transfer on the affected ports and disable the port. This affects the
USBCMD register, bit [4] and the PORTSC registers, bits [12,6,2]. See individual bit descriptions
for more detail.
NOTES:
1. These registers are Word writable only. Byte writes to these registers have unpredictable effects.
11.2.1
USBCMD—USB Command Register
I/O Offset:
Default Value:
Base + (00–01h)
0000h
Attribute:
Size:
R/W
16 bits
The Command Register indicates the command to be executed by the serial bus host controller.
Writing to the register causes a command to be executed. The table following the bit description
provides additional information on the operation of the Run/Stop and Debug bits.
Table 11-2. USB I/O Registers
Offset
Mnemonic
Register
Default
Type
00–01h
USBCMD
USB Command Register
0000h
R/W*
02–03h
USBSTS
USB Status Register
0020h
R/WC
04–05h
USBINTR
USB Interrupt Enable
0000h
R/W
06–07h
FRNUM
USB Frame Number
0000h
R/W (see Note 1)
08–0Bh
FRBASEADD
USB Frame List Base Address
Undefined
R/W
0Ch
SOFMOD
USB Start of Frame Modify
40h
R/W
0D–0Fh
—
Reserved
0
RO
10–11h
PORTSC0
Port 0 Status/Control
0080h
R/WC (see Note 1)
12–13h
PORTSC1
Port 1 Status/Control
0080h
R/WC (see Note 1)
14–17h
—
Reserved
0
RO
18h
LOOPDATA
Loop Back Test Data
00h
RO
Bit
Description
15:7
Reserved.
8
Loop Back Test Mode
—R/W.
1 = ICH2 is in loop back test mode. When both ports are connected together, a write to one port will
be seen on the other port and the data will be stored in I/O offset 18h.
0 = Disable loop back test mode.
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