Table of Contents
Advertisement
www.ti.com
20.2.3 VBUS Voltage Sourcing Control
When any of the USB controllers assumes the role of a host, the USB is required to supply a 5V power
source to an attached device through its VBUS line. In order to achieve this task, the USB controller
requires the use of an external power logic (or charge pump) capable of sourcing 5V power. A
USB_DRVVBUS is used as a control signal to enable/disable this external power logic to either source
or disable power on the VBUS line. The control on the USB_DRVVBUS is automatic and is handled by
the USB controller. The control should be transparent to the user so long as the proper hardware
connection and software initialization are in place. The USB controller drives the USB_DRVVBUS
signal high when it assumes the role of a host while the controller is in session. When assuming the
role of a device, the controller drives the USB_DRVVBUS signal low disabling the external charge
pump/power logic; hence, no power is driven on the VBUS line (in this case, power is expected to be
sourced by the external host).
Note that both USBs are self-powered and the device does not rely on the voltage on the VBUS line
sourced by an external host for controller operation when assuming the role of a device. The power on
the VBUS is used to identify the presence of a Host. It is also used to power up the pull-up on the D+
line. The USB PHY would continually monitor the voltage on the VBUS and report the status to USB
controller.
20.2.4 Pull-up/Pull-Down Resistors
Since the USB controllers are dual role controllers, capable of assuming a role of a host or device, the
necessary required pull-up/pull-down resistors can not exist external to the device. These
pull-up/pull-down resistors exist internal to the device, within the PHY to be more specific, and are
enabled and disabled based on the role the controller assumes allowing dynamic hardware
configuration.
When assuming the role of a host, the data lines are pulled low by the PHY enabling the internal
15KOhms resistors. When assuming the role of a device the required 1.5Kohm pull-up resistor on the
D+ line is enabled automatically to signify the USB capability to the external host as a FS device (HS
operation is negotiated during reset bus condition).
20.2.5 Role Assuming Method
For an OTG controller, the usual method followed by the controller to assume the role of a host or a
device is governed by the state of the ID pin which in turn is controlled by the USB cable connector
type. Since no ID pin signal has been bonded out on the device, there exists a register field within the
USB subsystem register file that is programmed by the user indirectly controlling the state of the ID pin
and ultimately selecting the role of the USB controller.
Two registers, USB0 Mode Register at offset 10E8h and USB1 Mode Register at offset 18E8h, are
used for a user to select the role the USB controller assumes. The user is required to program the
corresponding register prior to the USB controller is in session.
20.2.6 USB Signal Conditioning
The device supports a built in logic to condition the signal quality of the transmit signal by adjusting
transmit parameters including transmit level, pre-emphasis, etc using the USB_CTRL0/1 Registers.
20.2.7 USB PHY Initialization
Prior to configuring the USB Module Registers, release the USB SubSystem and PHY from reset,
enable the interconnect and controller clocks, and as configure PHY values. Registers related for this
task are captured within this doc for ease of reference. Only related information for this task is included
within this document. The actual program values are shown at the end.
The USB registers within Device Configuration register space are used to control the PHY. The
Cortex™-A8 CPU must be in Supervisory Mode when accessing the USB related registers within the
Device Configuration Spaces. The registers used to control the PHY are USB_CTRL,
USBPHY_CTRL0, and USBPHY_CTRL1. See
Consult the device manual for more information.
*0x48180B10 &= 0xFFFFFF9F; // RSTCTRL (Release USB Module from Reset)
SPRUGX9 – 15 April 2011
Submit Documentation Feedback
Preliminary
Section 20.9.8
© 2011, Texas Instruments Incorporated
for the definitions of these registers.
Universal Serial Bus (USB)
Architecture
1757
Advertisement
Table of Contents
