User's Manual

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.

PIO

USB_D+

USB_D-

USB_ON

R =1.5k

Rvb1

WT12

Rvb2

WT32

Figure 9: USB in self powered mode