Rev. 3.0, 03/01, page 261 of 390
18.2.2
USB Host Controller
USB States
The Host Controller has four USB states visible to the Host Controller Driver via the Operational
Registers: U
SB
O
PERATIONAL
, U
SB
R
ESET
, U
SB
S
USPEND
, and U
SB
R
ESUME
. These states define the
Host Controller responsibilities relating to USB signaling and bus states.
The USB states are reflected in the HostControllerFunctionalState field of the HcControl
register. The Host Controller Driver is permitted to perform only the USB state transitions shown
in Figure 18-1. The Host Controller may only perform a single state transition. During a remote
wakeup event, the Host Controller may transition from U
SB
S
USPEND
to U
SB
R
ESUME
.
Hardware
Reset
Software
Reset
U
SB
O
PERATIONAL
U
SB
R
ESUME
U
SB
R
ESUME
write
or
Remote Wakeup
U
SB
S
USPEND
U
SB
R
ESET
U
SB
O
PERATIONAL
write
U
SB
S
USPEND
write
U
SB
R
ESET
write
U
SB
R
ESET
write
U
SB
R
ESET
write
U
SB
O
PERATIONAL
write
Figure 18.1 USB States
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