Agere Systems Inc.
177
Data Sheet
July 2001
T8302 Internet Protocol Telephone
Advanced RISC Machine (
ARM
)
13 USB Host Controller
(continued)
Table 162
.
Hc LS Threshold
Register
13.6 Root Hub Partition
All registers included in this partition are dedicated to the USB root hub, that is an integral part of the host controller
though still a functionally separate entity. The HCD emulates USBD accesses to the root hub via a register inter-
face. The HCD maintains many USB-defined hub features that are not required to be supported in hardware. For
example, the hub's device, configuration, interface, and endpoint descriptors are maintained only in the HCD as
well as some static fields of the class descriptor.
The HCD also maintains and decodes the root hub's device address as well as other trivial operations that are bet-
ter suited to software than hardware. The root hub register interface is otherwise developed to maintain similarity of
bit organization and operation to typical hubs that are found in the system.
The following four register definitions exist:
I
Hc Rh descriptor A register
.
I
Hc Rh descriptor B register
.
I
Hc Rh status register
.
I
Hc Rh port status
register [1:NDP]
, (NDP = number of data ports).
Each register is read and written as a DWORD. These registers are only written during initialization to correspond
with the system implementation.
13.6.1 Hc Rh Descriptor A Register
The
Hc Rh descriptor A registe
r is the first register of two describing the characteristics of the root hub. Reset
values are implementation-specific. The descriptor length (11), descriptor type (TBD), and hub controller
current (0) fields of the hub class descriptor are emulated by the HCD. All other fields are located in the
Hc Rh
descriptor A register
and
Hc Rh descriptor B register
.
Address 0xE000 7044
Bit #
Name
Bit #
31:12
RSVD
11:0
LST
Key
Reset
Read/Write
HCD
—
R/W
Description
HC
—
R
31:12
11:0
RSVD
LST
—
Reserved.
LS threshold. This field contains a value that is compared to
the FR field prior to initiating a low-speed transaction. The
transaction is started only if
FR
≥
this field. The value is cal-
culated by HCD with the consideration of transmission and
setup overhead.
0628