User's Manual

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9.2.5 Bus Powered Mode
In bus powered mode the application circuit draws its current from the 5V VBUS supply on
the USB cable. WT12 negotiates with the PC during the USB enumeration stage about how
much current it is allowed to consume.
For WT12 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, WT12 requests 100mA during
enumeration.
When selecting a regulator, be aware that VBUS may go as low as 4.4V. The inrush
current (when charging 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 and 1.8V, applications should include careful filtering of the 5V line to
attenuate noise that is above the voltage regulator bandwidth. Excessive noise on the 1.8V
supply to the analogue supply pins of WT12 will result in reduced receive sensitivity and a
distorted RF transmit signal.
USB_D+
USB_D-
USB_ON
Voltage
regulator
WT12
VBUS
GND
Figure 13: USB in bus powered mode
In bus powered mode PSKEY_USB_PIO_PULLUP must be set to 16 for internal pull-up
(default configuration in WT12).
9.2.6 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 draw only 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 WT12. The entire circuit must be able to enter
the suspend mode. (For more details on USB Suspend, see separate CSR documentation).