USB Engineering Change Notice USB ENGINEERING CHANGE NOTICE Title: Interface Association Descriptors Applies to: Universal Serial Bus Specification, Revision 2.0 Summary of ECN This ECN defines a new standard descriptor and interface numbering rules that allow a device to describe which interfaces are associated with the same device function. This allows the operating system to bind all of the appropriate interfaces to the same driver instance. Reasons for ECN The base configuration model assumed by the core USB framework was that there was always a 1:1 association between an interface and a function on a device. System software was designed to the intent of the core specification and assumes one driver per function (and one interface) (see figure below). System Software USB Device Configuration Function Device Driver A Device Interface 0 Interface 1 Function 0 Function Device Driver B Device Function 1 Several device class specifications have exceeded the core USB specification framework and defined device functions that utilize multiple interfaces (i.e. multiple interface descriptors). The model to support this still requires only one function driver per function, but also requires multiple interfaces getting bound to the same driver instance (see figure below). Unfortunately, there is no standard method to allow a device, via the device framework, to describe which interfaces in a configuration should be associated with the same function. System Software USB Device Configuration Interface 0 Function Device Driver A Device Function 0 Interface 1 USB EC