USB Pin Multiplexing
14-50
There are at least three different types of signaling used by commercially-
available USB transceivers to interface to a USB function controller.
OMAP5910 is designed mainly for use with USB transceivers that use TXSE0
and TXD signaling. OMAP5910 is not designed to interface directly with USB
transceivers which use bidirectional signals between the transceiver and the
USB host controller, or with USB transceivers that use TXD+ and TXD– signal-
ing.
Figure 14–4 shows optional weak pulldown resistors R4 and R5. These can
be used to hold the D+ and D- signals at voltages below the USB transceiver
V
IL
level when there is no USB host connected to the USB Type B connector.
By keeping D+ and D- voltages below V
IL
, the USB transceiver I
DDQ
can be
reduced. Choice of value for R4 and R5 must be made carefully so that the
circuit meets the requirements of the USB Specification.
The OMAP5910 USB function controller only supports implementation as a full
speed USB device. As such, the pullup resistor must be connected to the D+
signal to indicate implementation of a full-speed USB device.
14.5.3 On-Board Transceiverless Connection Using OMAP5910 Transceiverless Link
Logic
The transceiverless link logic feature of the OMAP5910 USB signal multiplex-
ing enables connection of the OMAP5910 device to an external, on-board
USB host controller or external on–board USB function controller, without the
use of USB transceivers or associated circuitry. When the transceiverless lin
logic is used, both of the USB transceivers, the series resistors, pullup and pull-
down resistors, VBUS switching components, and USB connectors and
cables that normally are used between a USB host controller and the down-
stream USB function controller are removed.
The transceiverless link logic signaling system is not suitable for use across
a cable. It is intended only for use when the OMAP5910 device is used with
an external USB integrated circuit which is on the same board.
When using the transceiverless link logic, six of the external USB integrated
circuit pins that normally connect to a USB transceiver connect instead directly
to OMAP5910 device pins. Signaling on these pins use CMOS levels.