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USB Controller Registers (D31:F2)
10-8
82801AA and 82801AB Datasheet
10.2
USB I/O Registers
Some of the read/write register bits which 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
immediately terminates a transfer on the affected ports and disables 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.
10.2.1
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.
Table 10-3
provides additional
information on the operation of the Run/Stop and Debug bits.
Table 10-2. USB I/O Registers
Offset
Mnemonic
Register
Default
Type
00h
–
01h
02h
–
03h
04h
–
05h
06h
–
07h
08h
–
0Bh
USBCMD
USB Command Register
0000h
R/W
USBSTS
USB Status Register
0000h
R/WC
USBINTR
USB Interrupt Enable
0000h
R/W
FRNUM
USB Frame Number
0000h
R/W*
FRBASEADD
USB Frame List Base Address
Undefined
R/W
0Ch
SOFMOD
USB Start of Frame Modify
40h
R/W
0Dh
–
0Fh
10h
–
11h
12h
–
13h
14h
–
17h
—
Reserved
0
RO
PORTSC0
Port 0 Status/Control
0080h
R/WC
1
R/WC
1
PORTSC1
Port 1 Status/Control
0080h
—
Reserved
0
RO
18h
LOOPDATA
Loop Back Test Data
00h
RO
Bit
Description
15:7
Reserved.
8
Loop Back Test Mode.
1 = ICH is in loop back test mode. When both ports are connected together, a write to one port is
seen on the other port and the data stored in I/O offset 18h.
0 = Disable loop back test mode.
7
Max Packet (MAXP).
This bit selects the maximum packet size that can be used for full speed
bandwidth reclamation at the end of a frame. This value is used by the Host Controller to determine
whether it should initiate another transaction based on the time remaining in the SOF counter. Use of
reclamation packets larger than the programmed size will cause a Babble error if executed during
the critical window at frame end. The Babble error results in the offending endpoint being stalled.
Software is responsible for ensuring that any packet which could be executed under bandwidth
reclamation be within this size limit.
1 = 64 bytes
0 = 32 bytes