Texas Instruments OMAP5912 Reference Manual page 1525

USB OTG Controller
The effects of On-The-Go functionality on OMAP5912 USB device controller
operation are documented in the USB device controller flowcharts. Because
USB host controller operations are mainly defined in the open host controller
for USB specification, some On-The-Go functionality issues are discussed
here.
Most OMAP5912 USB host controller software is not affected by USB
On-The-Go functionality. A host controller driver written for non-OTG
functionality needs few modifications to support On-The-Go functionality. In
general, On-The-Go events precede USB host controller driver operations, so
the On-The-Go event can cause the OTG driver software to pass the
appropriate message to the USB host controller driver. It is never necessary
for the USB host controller driver to access OTG controller registers.
For an On-The-Go connection, the SRP and HNP operations can be
considered to replace the attach, detach, suspend, resume, and remote wake
functions. This means that it can be beneficial for system software to control
the USB host controller driver differently for an OTG link than for a typical USB
host port.
The USB host controller driver must implement a mechanism where the
application can specify that the OTG port is no longer needed. The host
controller driver software must respond to this message by suspending the
port associated with the OTG link and cancelling all EDs and TDs associated
with the OTG connection. These ED and TD removal activities are ones
usually associated with a disconnect event, so the host controller driver
already has these functions available. The application typically sends this
message and then informs the OTG controller driver that its use of the host is
complete and that the OTG controller can now release the bus for a possible
HNP.
When the USB device controller owns the USB OTG link, the USB host
controller port need not remain active. If no other USB host controller ports are
being used, it is possible to disable the USB host controller clock for the
duration while the USB device controller owns the OTG link. System software
must re-enable the USB host controller clock and re-initialize the USB host on
an OTG driver switch interrupt to USB host controller control of the OTG link
if the host clock is dynamically disabled.
Software to support communication between the USB OTG controller and the
2
external OTG transceiver via I C is described in Figure 52. This software must
handle control information from the OTG controller to the external OTG. These
software operations apply directly to the OTG controller OPRT_CHG interrupt.
2
Because I C operations are slow (relative to the MPU instruction speed), it is
best if these functions are not implemented as part of an interrupt handler;
SPRU761A Universal Serial Bus (USB) 233