User's Manual

Silicon Laboratories Finland Oy
Page 23 of 45
7 USB Interface
This is a full speed (12Mbits/s) USB interface for communicating with other compatible digital devices. WT41u
acts as a USB peripheral, responding to requests from a master host controller such as a PC.
The USB interface 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 v2.1 + EDR specification or alternatively can appear as a set of
endpoints appropriate to USB audio devices such as speakers.
As USB is a master/slave oriented system (in common with other USB peripherals), WT41u only supports
USB Slave operation.
7.1 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 the chipset, therefore, have a low output impedance. To match the connection to the
characteristic impedance of the USB cable, resistors must be placed in series with USB_DP/USB_DN and the
cable.
7.2 USB Pull-Up resistor
WT41u features an internal USB pull-up resistor. This pulls the USB_DP pin weakly high when WT41u is
ready to enumerate. It signals to the PC that it is a full speed (12Mbits/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 15kΩ 5% pull-
down resistor (in the hub/host) when VDD_PADS = 3.1V. This presents a Thevenin resistance to the host of
at least 900Ω. Alternatively, an external 1.5kΩ 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 PSKEY_USB_PIO_PULLUP
appropriately. The default setting uses the internal pull-up resistor.
7.3 USB 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 supply terminal must be an absolute minimum of 3.1V.
Silicon Labs recommends 3.3V for optimal USB signal quality.
7.4 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, 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 WT41u via a resistor network (R
vb1
and
R
vb2
), so WT41u can detect when VBUS is powered up. The chipset 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
pullup purposes. A 1.5kΩ 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 the chipset is only suitable for bus-powered USB devices, e.g., dongles.