OMAP5910 USB Hardware Considerations
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14.10 OMAP5910 USB Hardware Considerations
14.10.1
VBUS Power Switching For USB Type A Host Receptacles
The USB specification places several VBUS requirements on USB hosts,
including current capability, droop, and other important characteristics.
Circuits that meet the USB specification requirements can be implemented
using Texas Instruments devices such as the TPS2014 and TSP2015 power
distribution switch devices. Further information, including data sheets and
application notes, can be found at the Texas Instruments web site.
14.10.2
Transient Suppression for USB Connectors
It is important to provide transient suppression for USB connectors. Electro-
static discharge that occurs when a user connects or disconnects a USB cable
can have a dramatic effect on a system if not suppressed. Texas Instruments
offers several devices for transient suppression on USB connections, such as
the SN65220, SN65240, and SN75240 universal serial bus port transient
suppressor devices. Further information, including data sheets and applica-
tion notes, can be found at the Texas Instruments web site.
14.10.3
VBUS Monitoring for USB Function Controller
A USB function controller must be capable of monitoring the VBUS voltage
provided by the upstream USB host controller. The OMAP5910 device pro-
vides the input pin USB_VBUSI, which is provided to the OMAP5910 USB
function controller. This input is a CMOS input that is not rated for the full VBUS
range specified by the USB specification. An external signal level converter is
required to convert the VBUS signal range to a range suitable for the
USB_VBUSI pin.
14.10.4
USB D+ Pullup Enable for USB Function Controller
When using a USB signal multiplexing mode that provides USB function con-
troller signals to OMAP5910 pins, the OMAP5910 top-level pin multiplexing
options lead to several possible USB pullup implementations.
When the USB.PUEN pin is set for top-level multiplexing configuration 0, the
USB.PUEN pin is driven low when the pullup should be active and is driven
high when the pullup should be inactive. In this mode of operation, an external
inverter or an external 3-statable device can be used to provide a nominal
3.3-V signal to the supply end of the USB D+ pullup.
When the USB.PUEN pin is set for top-level multiplexing configuration 1, the
USB.PUEN pin provides a clock output and cannot be used to control the USB
pullup.