User's Manual

The terminal marked USB_ON can be any free PIO pin. The PIO pin selected must be registered by

setting PSKEY_USB_PIO_VBUS to the corresponding pin number. In self powered mode,

PSKEY_USB_PIO_PULLUP must be set to match with the PIO selected.

Note:

USB_ON is shared with WT32 PIO terminals (PIO2-PIO7).

6.2.3 Bus Powered Mode

In bus powered mode, the application circuit draws its current from the 5V VBUS supply on the USB

cable. WT32 negotiates with the PC during the USB enumeration stage about how much current it is

allowed to consume.

For WT32 Bluetooth applications, it is recommended that the regulator used to derive 3.3V from VBUS

is rated at 100mA average current and should be able to handle peaks of 120mA without fold back or

limiting. In bus powered mode, WT32 requests 100mA during enumeration.

When selecting a regulator, be aware that VBUS may go as low as 4.4V. The inrush current (when

charging the reservoir and supply decoupling capacitors) is limited by the USB specification (see USB

specification v1.1, Section 7.2.4.1). Some applications may require soft start circuitry to limit inrush

current if more than 10pF is present between VBUS and GND.

The 5V VBUS line emerging from a PC is often electrically noisy. As well as regulation down to 3.3V,

applications should include careful filtering of the 5V line to attenuate noise that is above the voltage

regulator bandwidth.

In bus powered mode, PSKEY_USB_PIO_PULLUP must be set to 16 for internal pull-up (default

configuration in WT32).

USB_D+

USB_D-

USB_ON

Voltage

regulator

WT12

VBUS

GND

WT32

Figure 10: USB in bus powered mode

6.2.4 Suspend Current

All USB devices must permit the USB controller to place them in a USB Suspend mode. While in USB

Suspend, bus powered devices must not draw more than 0.5mA from USB VBUS (self powered devices

may draw more than 0.5mA from their own supply). This current draw requirement prevents operation of

the radio by bus powered devices during USB Suspend.

The voltage regulator circuit itself should only draw a small quiescent current (typically less than 100uA)

to ensure adherence to the suspend current requirement of the USB specification. This is not normally a

problem with modern regulators. Ensure that external LEDs and/or amplifiers can be turned off by

WT32. The entire circuit must be able to enter the suspend mode. (For more details on USB Suspend,

see separate CSR documentation).

6.2.5 Detach and Wake-Up Signaling

WT32 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 WT32 into a circuit where no external