82801DB Datasheet, PDF (408/587 Pages) Intel Corporation – Intel 82801DB I/O Controller Hub 4 (ICH4)

USB UHCI Controllers Registers
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.
Bit
Description
15:7 Reserved
Loop Back Test Mode — R/W.
8
0 = Disable loop back test mode.
1 = ICH4 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.
Max Packet (MAXP) — R/W. 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
7 ^{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.
0 = 32 bytes
1 = 64 bytes
Configure Flag (CF) — R/W. This bit has no effect on the hardware. It is provided only as a
semaphore service for software.
6
0 = Indicates that software has not completed host controller configuration.
1 = HCD software sets this bit as the last action in its process of configuring the Host controller.
Software Debug (SWDBG) — R/W. The SWDBG bit must only be manipulated when the controller
is in the stopped state. This can be determined by checking the HCHalted bit in the USBSTS
register.
5 0 = Normal Mode.
1 = Debug mode. In SW Debug mode, the Host controller clears the Run/Stop bit after the
completion of each USB transaction. The next transaction is executed when software sets the
Run/Stop bit back to 1.
Force Global Resume (FGR) — R/W.
0 = Software resets this bit to 0 after 20 ms has elapsed to stop sending the Global Resume signal.
4
At that time all USB devices should be ready for bus activity. The 1-to-0 transition causes the
port to send a low speed EOP signal. This bit will remain a 1 until the EOP has completed.
1 = Host controller sends the Global Resume signal on the USB, and sets this bit to 1 when a
resume event (connect, disconnect, or K-state) is detected while in global suspend mode.
Enter Global Suspend Mode (EGSM) — R/W.
0 = Software resets this bit to 0 to come out of Global Suspend mode. Software writes this bit to 0 at
3
the same time that Force Global Resume (bit 4) is written to 0 or after writing bit 4 to 0.
1 = Host controller enters the Global Suspend mode. No USB transactions occur during this time.
The Host controller is able to receive resume signals from USB and interrupt the system.
Software must ensure that the Run/Stop bit (bit 0) is cleared prior to setting this bit.
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Intel^® 82801DB ICH4 Datasheet