BeagleBoard XM C2, Reference Manual

Page: 80 / 163

REF: BB_SRM_xM
BeagleBoard-xM System
Reference Manual
Revision C2
Page 80 of 163
The husb2_clk signal is an output only and is used to support a HS USB PHY that
supports an input clock mode. The SMSC PHY device supports this mode and is used on
the Beagle.
The PHY used in the design is a
USB3320 series device from SMSC. The USB3320 is a
highly integrated Hi-Speed USB2.0 Transceiver (PHY) that meets all of the electrical
requirements to be used as a Hi-Speed USB Host. In this design, only the host mode of
operation is being supported as it is used to connect to the HUB on the board.
In order to interface to the processor, the device must be used in the 60MHz clock mode.
This is done by tying the
CLKOUT signal on the USB PHY to VIO_1V8. On -XM Rev
A, a zero ohm series resistor was added. This is not required, but was added as a “just in
case” option if the CLKOUT signal was a source of noise in the PHY. It was proven not
to be the case. The clock for the PHY is derived from the 60MHz signal generated by the
processor. All of the signals and their functions align with the descriptions found in the
processor interface section.
The USB3322 device requires two voltages, the
USB_1V8 rail to power the I/O rails and
the HUB_3V3 to power the rest of the device. The 3.3V rail for the device is generated
internally and requires a filter and bypass cap to be connected externally. The USB_1V8
rail is derived from the
VAUX2 rail supplied by the TPS65950 PMIC.
The
RBIAS block in the PHY consists of an internal bandgap reference circuit used for
generating the driver current and the biasing of the analog circuits. This block requires an
external 8.06KΩ, 1% tolerance, reference resistor connected from
RBIAS to ground. The
nominal voltage at RBIAS is 0.8V and therefore the resistor will dissipate approximately
80μW of power.
As we are not using this device to support the OTG protocol but instead as a host device,
we ground the
ID pin to force it into a Host mode at all times. The USB3322 transceiver
fully integrates all of the USB termination resistors on both DP and DM . This includes
1.5kΩ pull-up resistors, 15kΩ pull-down resistors and the 45Ω high speed termination
resistors. These resistors require no tuning or trimming.
7.14.3 USB HUB
The key component in the HUB design is a SMSC LAN9514 USB HUB plus Ethernet
device.
Figure 40 is the HUB design.