HS_USB2.0 host
RM0082
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Doc ID 018672 Rev 1
Table 383.
HcLSThreshold register bit assignments
22.6.45 Root
hub
partition
All registers included in this partition are dedicated to the USB Root Hub which 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 interface. The HCD maintains many USB-
defined hub features which 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 which are better
suited to software than hardware.
The Root Hub register interface is otherwise developed to maintain similarity of bit
organization and operation to typical hubs which are found in the system. Below are four
register definitions: HcRhDescriptorA, HcRhDescriptorB, HcRhStatus, and
HcRhPortStatus[1:NDP]. Each register is read and written as a Dword. These registers are
only written during initialization to correspond with the system implementation. The
HcRhDescriptorA and HcRhDescriptorB registers should be implemented such that they are
writeable regardless of the HC USB state. HcRhStatus and HcRhPortStatus must be
writeable during the USBOPERATIONAL state.
Note:
IS denotes an implementation-specific reset value for that field.
22.6.46 HcRhDescriptorA
register
The HcRhDescriptorA register 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 HcRhDescriptorA and HcRhDescriptorB
registers.
Bits
Name
Reset
Read/Write
Description
HCD
HC
[31:12]
Reserved
[11:00]
LST
0628h
R/W
R
LSThreshold
This field contains a value which is compared to the
FrameRemaining field prior to initiating a Low Speed
transaction. The transaction is started only if
FrameRemaining ³ this field. The value is calculated by
HCD with the consideration of transmission and setup
overhead.