Specifications

34
PIO
USB_D+
USB_D-
USB_ON
R =1.5k
Rvb1
WT12
Rvb2
WT32
Figure 10: USB in self powered mode
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).