LM3S5651 Datasheet, PDF (869/1261 Pages) Texas Instruments – Stellaris® LM3S5651 Microcontroller

Stellaris^® LM3S5651 Microcontroller
18.3.2.8
USB RESET
If the RESET bit in the USBPOWER register is set, the USB Host controller generates USB RESET
signaling on the bus. The RESET bit must be set for at least 20 ms to ensure correct resetting of the
target Device. After the CPU has cleared the bit, the USB Host controller starts its frame counter
and transaction scheduler.
18.3.2.9
Connect/Disconnect
A session is started by setting the SESSION bit in the USB Device Control (USBDEVCTL) register,
enabling the USB controller to wait for a Device to be connected. When a Device is detected, a
connect interrupt is generated. The speed of the Device that has been connected can be determined
by reading the USBDEVCTL register where the FSDEV bit is set for a full-speed Device, and the
LSDEV bit is set for a low-speed Device. The USB controller must generate a RESET to the Device,
and then the USB Host controller can begin Device enumeration. If the Device is disconnected while
a session is in progress, a disconnect interrupt is generated.
18.3.3
OTG Mode
To conserve power, the USB On-The-Go (OTG) supplement allows VBUS to only be powered up
when required and to be turned off when the bus is not in use. VBUS is always supplied by the A
device on the bus. The USB OTG controller determines whether it is the A device or the B device
by sampling the ID input from the PHY. This signal is pulled Low when an A-type plug is sensed
(signifying that the USB OTG controller should act as the A device) but taken High when a B-type
plug is sensed (signifying that the USB controller is a B device). Note that when switching between
OTG A and OTG B, the USB controller retains all register contents.
18.3.3.1
Starting a Session
When the USB OTG controller is ready to start a session, the SESSION bit must be set in the
USBDEVCTL register. The USB OTG controller then enables ID pin sensing. The ID input is either
taken Low if an A-type connection is detected or High if a B-type connection is detected. The DEV
bit in the USBDEVCTL register is also set to indicate whether the USB OTG controller has adopted
the role of the A device or the B device. The USB OTG controller also provides an interrupt to
indicate that ID pin sensing has completed and the mode value in the USBDEVCTL register is valid.
This interrupt is enabled in the USBIDVIM register, and the status is checked in the USBIDVISC
register. As soon as the USB controller has detected that it is on the A side of the cable, it must
enable VBUS power within 100ms or the USB controller reverts to Device mode.
If the USB OTG controller is the A device, then the USB OTG controller enters Host mode (the A
device is always the default Host), turns on VBUS, and waits for VBUS to go above the VBUS Valid
threshold, as indicated by the VBUS bit in the USBDEVCTL register going to 0x3. The USB OTG
controller then waits for a peripheral to be connected. When a peripheral is detected, a Connect
interrupt is signaled and either the FSDEV or LSDEV bit in the USBDEVCTL register is set, depending
whether a full-speed or a low-speed peripheral is detected. The USB controller then issues a RESET
to the connected Device. The SESSION bit in the USBDEVCTL register can be cleared to end a
session. The USB OTG controller also automatically ends the session if babble is detected or if
VBUS drops below session valid.
Note:
The USB OTG controller may not remain in Host mode when connected to high-current
devices. Some devices draw enough current to momentarily drop VBUS below the
VBUS-valid level causing the controller to drop out of Host mode. The only way to get back
into Host mode is to allow VBUS to go below the Session End level. In this situation, the
device is causing VBUS to drop repeatedly and pull VBUS back low the next time VBUS is
enabled.
January 20, 2012
869
Texas Instruments-Production Data