Comtrol eCos - Manual (Page 686)

USB Enumeration Data
calculate this: the first argument to the macros specify the number of interfaces, the second the number of end-
points. The
number_interfaces field is self-explanatory. If the peripheral involves multiple configurations
then each one must have a unique id, and this will be used in the set-configuration control message. The id 0 is
reserved, and a set-configuration control message that uses this id indicates that the peripheral should be inactive.
Configurations can have a string description if required. The
attributes field must have the REQUIRED bit set;
the SELF_POWERED bit informs the host that the peripheral has its own power supply and will not draw any power
over the bus, leaving more bus power available to other peripherals; the REMOTE_WAKEUP bit is used if the periph-
eral can interrupt the host when the latter is in power-saving mode. For peripherals that are not self-powered, the
max_power field specifies the power requirements in units of 2mA.
usb_interface_descriptor
A USB configuration involves one or more interfaces, typically corresponding to different streams of data. For
example, one interface might involve video data while another interface is for audio. Multiple interfaces in a single
configuration will be active at the same time.
const usb_interface_descriptor usb_interface = {
length: USB_INTERFACE_DESCRIPTOR_LENGTH,
type: USB_INTERFACE_DESCRIPTOR_TYPE,
interface_id: 0,
alternate_setting: 0,
number_endpoints: 2,
interface_class: USB_INTERFACE_DESCRIPTOR_CLASS_VENDOR,
interface_subclass: USB_INTERFACE_DESCRIPTOR_SUBCLASS_VENDOR,
interface_protocol: USB_INTERFACE_DESCRIPTOR_PROTOCOL_VENDOR,
interface_str: 0
};
const usbs_enumeration_data usb_enum_data = {
...
total_number_interfaces: 1,
interfaces: &usb_interface,
...
};
Again, the length and type fields are specified by the standard. Each interface within a configuration requires
its own id. However, a given interface may have several alternate settings, in other words entries in the interfaces
array with the same id but different
alternate_setting fields. For example, there might be one setting which
requires a bandwidth of 100K/s and another setting that only needs 50K/s. The host can use the standard set-
interface control message to choose the most appropriate setting. The handling of this request is the responsibility
of higher-level code, so the application may have to
install its own handler.
The number of endpoints used by an interface is specified in the number_endpoints field. Exact details of
which endpoints are used is held in a separate array of endpoint descriptors. The class, subclass and protocol fields
are used by host-side code to determine which host-side device driver should handle this specific interface. Usually
this is determined on a per-peripheral basis in the usb_device_descriptor structure, but that can defer the details to
individual interfaces. A per-interface string is allowed as well.
For USB peripherals involving multiple configurations, the array of usb_interface_descriptor structures should first
contain all the interfaces for the first configuration, then all the interfaces for the second configuration, and so on.
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