BeagleBoard XM C2, Reference Manual

Page: 73 / 163

REF: BB_SRM_xM
BeagleBoard-xM System
Reference Manual
Revision C2
Page 73 of 163
7.12.3 McBSP_CLKS
An additional clock is also provided by the
TPS65950 called McBSP_CL KS. This clock
is provided to the PROCESSOR in order to insure synchronization of the I2S interface
between the processor and the
TPS65950 .
7.13 USB OTG Port
The BeagleBoard has a USB OTG (On-the-Go) port. It can be used as an OTG port,
Client port, or Host port. The main use is as a client port, as that is the mode that will
supply the power needed to power the BeagleBoard. With the addition of the USB Host
ports, the need to use three OTG port as a Host, is not really needed.
7.13.1 USB OTG Overview
USB OTG is a supplement to the USB 2.0 specification. The standard USB uses a
master/slave architecture, a USB host acting as a master and a USB peripheral acting as a
slave. Only the USB host can schedule the configuration and data transfers over the link.
The USB peripherals cannot initiate data transfers, they only respond to instructions
given by a host.
USB OTG works differently in that gadgets don't need to be pure peripherals because
they can sometimes act as hosts. An example might be connecting a USB keyboard or
printer to BeagleBoard or a USB printer that knows how to grab documents from certain
peripherals and print them. The USB OTG compatible devices are able to initiate the
session, control the connection and exchange Host/Peripheral roles between each other.
The USB OTG supplement does not prevent the use of a hub, but it describes role
swapping only in the case of a one-to-one connection where two OTG devices are
directly connected. If a standard hub is used, the supplement notes that using it will lead
to losing USB OTG role-swap capabilities making one device as the Default-Host and the
other as the Default-Peripheral until the hub is disconnected.
The combination of the processor and the
TPS65950 allows the BeagleBoard to work as
an OTG device if desired. The primary mode of operation however, is intended to be a
client mode in order to pull power from the USB host which is typically a PC. As the Rev
B does not have a Host USB port, this port will be used as a Host port in many
applications.
NOTE: In order to use the OTG in the Host mode, the
BeagleBoard must be powered from the DC supply.