Specifications

6.2 USB Interface

WT32 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. To match the connection to

the characteristic impedance of the USB cable, series resistors must be included to both of

the signal lines. These should be of 1% tolerance and the value required may vary

between 0 and 20 ohm with 10 ohm being nominal. The resistors should be placed close to

the USB pins of the module in order to avoid reflections. Internally, the module has 22-

ohm resistors in series. The total input impedance seen by the cable is affected by the IC

characteristics, track layout and the connector. The cable impedance is approximately 40

ohms.

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 endpoints appropriate to

USB audio devices such as speakers.

As USB is a Master/Slave oriented system (in common with other USB peripherals), WT32

only supports the USB Slave operation.

6.2.1 USB Pull-Up Resistor

WT32 features an internal USB pull-up resistor. This pulls the USB_DP pin weakly high

when WT32 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_D+ high to at least

2.8V when loaded with a 15k +/-5% pull-down resistor (in the hub/host). This presents a

Thevenin resistance to the host of at least 900. 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 as to which mode is used by setting PS Key PSKEY_USB_PIO_PULLUP

appropriately. The default setting uses the internal pull-up resistor.

6.2.2 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 only draws a small leakage current (below 0.5mA)

from VBUS on the USB cable. This is the easier mode 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 WT32 via a voltage divider (Rvb1 and Rvb2), so that

WT32 can detect when VBUS is powered up. The voltage divider is essential to drop the 5V

voltage at the VBUS to 3.3V expected at the USB interface of WT32. WT32 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 the self powered mode. The internal pull-up in WT32

is only suitable for bus powered USB devices, such as dongles.