Specifications

USB_D+

USB_D-

USB_ON

Voltage

regulator

WT12

VBUS

GND

WT32

Figure 11: 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 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

and PSKEY_USB_PIO_WAKEUP to the selected PIO number.

USB_DETACH is an input that, when asserted high, causes WT32 to put USB_D- and

USB_D+ in high impedance state and turns off the pull-up resistor on D+. This detaches

the device from the bus and is logically equivalent to unplugging the device. When

USB_DETACH is taken low, WT32 will connect back to USB and await enumeration by the

USB host.

USB_WAKE_UP is an active high output (used only when USB_DETACH is active) to wake

up the host and allow USB communication to recommence. It replaces the function of the

software USB WAKE_UP message (which runs over the USB cable), and cannot be sent

while WT32 is effectively disconnected from the bus.