Biassed-mode is a mode which you can use via our API (or terminal interface) which places a downstream USB port into a state whereby it only senses a device attach or detach event, it does not allow charging.
Because an attached device may only draw a small amount of current (less than 1mA), the VBUS current sensing electronics requires a large-value sense (or ‘shunt’) resistor in series with the VBUS connection in order to generate a measurable voltage across it.
Voltage = Current x Resistance (Ohms Law).
The upside to using a large-value sense-resistor is that we can measure low currents (such as those associated with a device being attached to a port), the downside is that if the attached device attempts to draw more than 1mA or so, the VBUS voltage will drop below the USB specified lower limit.
In summary, biassed-mode is a mode which is used purely for determining whether a device is attached to a downstream port. It should not be used as a means to charge a device.
For details about how to switch into and out of biassed-mode, please take a look at our API Reference document by clicking here.