User's Manual

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9.2 USB Interface
WT12 USB devices contain a full speed (12Mbits/s) USB interface that is capable of driving
a USB cable directly. No external USB transceiver is required. The device operates as a
USB peripheral, responding to requests from a master host controller such as a PC. Both
the OHCI and the UHCI standards are supported. The set of USB endpoints implemented
can behave as specified in the USB section of the Bluetooth v2.0 + EDR specification or
alternatively can appear as a set of endpoint appropriate to USB audio devices such as
speakers.
As USB is a Master/Slave oriented system (in common with other USB peripherals), WT12
only supports USB Slave operation.
9.2.1 USB Data Connections
The USB data lines emerge as pins USB_DP and USB_DN. These terminals are connected
to the internal USB I/O buffers of the WT12 and therefore have low output impedance. To
match the connection to the characteristic impedance of the USB cable, resistors are
included with USB_DP / USB_DN and the cable.
9.2.2 USB Pull-Up Resistor
WT12 features an internal USB pull-up resistor. This pulls the USB_DP pin weakly high
when
WT12 is ready to enumerate. It signals to the PC that it is a full speed (12Mbit/s) USB
device. The USB internal pull-up is implemented as a current source, and is compliant with
Section 7.1.5 of the USB specification v1.2. The internal pull-up pulls USB_DP high to at
least 2.8V when loaded with a 15k: r5% pull-down resistor (in the hub/host) when VDD
=3.3V. This presents a Therein resistance to the host of at least 900Ohms. Alternatively,
an external 1.5k: pull-up resistor can be placed between a PIO line and D+ on the USB
cable. The firmware must be alerted to which mode is used by setting PS Key
PSKEY_USB_PIO_PULLUP appropriately. The default setting uses the internal pull-up
resistor.
9.2.3 Power Supply
The USB specification dictates that the minimum output high voltage for USB data lines is
2.8V. To safely meet the USB specification, the voltage on the VDD_USB supply terminals
must be an absolute minimum of 3.1V. CSR recommends 3.3V for optimal USB signal
quality.
9.2.4 Self Powered Mode
In self powered mode, the circuit is powered from its own power supply and not from the
VBUS (5V) line of the USB cable. It draws only a small leakage current (below 0.5mA)
from VBUS on the USB cable. This is the easier mode for which to design for, as the design
is not limited by the power that can be drawn from the USB hub or root port. However, it
requires that VBUS be connected to WT12 via a resistor network (Rvb1 and Rvb2), so
WT12 can detect when VBUS is powered up. WT12 will not pull USB_DP high when VBUS is
off.
Self powered USB designs (powered from a battery or PSU) must ensure that a PIO line is
allocated for USB pull-up purposes. A 1.5K 5% pull-up resistor between USB_DP and the
selected PIO line should be fitted to the design. Failure to fit this resistor may result in the
design failing to be USB compliant in self powered mode. The internal pull-up in WT12 is
only suitable for bus powered USB devices i.e. dongles.