User's Manual

UART Configuration While RESET is Active

The UART interface for WT11 while the chip is being held in reset is tri-state. This will

allow the user to daisy chain devices onto the physical UART bus. The constraint on this

method is that any devices connected to this bus must tri-state when WT11reset is de-

asserted and the firmware begins to run.

7.2 USB Interface

WT11 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), WT11

only supports USB Slave operation.

7.2.1 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 WT11 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.

7.2.2 USB Pull-Up Resistor

WT11 features an internal USB pull-up resistor. This pulls the USB_DP pin weakly high

when

WT11 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.

7.2.3 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.

7.2.4 Detach and Wake-Up Signaling

WT11 can provide out-of-band signaling to a host controller by using the control lines

called ‘USB_DETACH’ and ‘USB_WAKE_UP’. These are outside the USB specification (no

wires exist for them inside the USB cable), but can be useful when embedding WT11 into a

circuit where no external USB is visible to the user. Both control lines are shared with PIO

pins and can be assigned to any PIO pin by setting the PS Keys PSKEY_USB_PIO_DETACH