User's Manual

Silicon Laboratories Finland Oy
Page 25 of 45
Figure 15: USB Connections for Bus-Powered Mode
7.6 USB 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 2.5mA from USB VBUS (self-powered devices may
draw more than 2.5mA from their own supply). This current draw requirement prevents operation of the radio
by bus-powered devices during USB Suspend.
When computing suspend current, the current from VBUS through the bus pull-up and pull-down resistors
must be included. The pull-up resistor at the device is 1.5 kΩ. (nominal). The pull-down resistor at the hub is
14.25kΩ. to 24.80kΩ. The pull-up voltage is nominally 3.3V, which means that holding one of the signal lines
high takes approximately 200uA, leaving only 2.3mA available from a 2.5mA budget. Ensure that external
LEDs and/or amplifiers can be turned off by the chipset. The entire circuit must be able to enter the suspend
mode.
7.7 USB Detach and Wake-Up Signaling
WT41u 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 WT41u 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
PSKEY_USB_PIO_DETACH and PSKEY_USB_PIO_WAKEUP to the selected PIO number.
USB_DETACH is an input which, when asserted high, causes WT41u to put USB_DN and USB_DP in high
impedance state and turns off the pull-up resistor on DP. This detaches the device from the bus and is
logically equivalent to unplugging the device. When USB_DETACH is taken low, WT41u 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 the chipset is effectively disconnected from the
bus.