Difference between revisions of "USB"
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While it _is_ possible for a host to switch configuration on a device offering more than one, it is very rare for devices to offer more than one configuration. | While it _is_ possible for a host to switch configuration on a device offering more than one, it is very rare for devices to offer more than one configuration. | ||
| + | |||
| + | All descriptors, device, configuration or interface, follow the same common format: | ||
| + | |||
| + | {| class=wikitable | ||
| + | |- | ||
| + | ! Offset | ||
| + | ! Field | ||
| + | ! Size | ||
| + | ! Value | ||
| + | ! Description | ||
| + | |- | ||
| + | | 0 | ||
| + | | bLength | ||
| + | | 1 | ||
| + | | Number | ||
| + | | Size of the complete descriptor in bytes | ||
| + | |} | ||
=== Device Descriptors === | === Device Descriptors === | ||
Revision as of 05:51, 31 August 2021
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Descriptors
One of the most complex things when developing USB is the descriptors. Descriptors are used by a USB Gadgets to inform the USB Host about it's capabilities.
Overall, USB Descriptors is a hierarchy of descriptors organised like this:
A USB device (or gadget) can only have one device descriptor.
When a USB host enumerates a USB device, the device and configuration descriptors are analysed and one configuration is enabled. Only one configuration can be active at any time.
While it _is_ possible for a host to switch configuration on a device offering more than one, it is very rare for devices to offer more than one configuration.
All descriptors, device, configuration or interface, follow the same common format:
| Offset | Field | Size | Value | Description |
|---|---|---|---|---|
| 0 | bLength | 1 | Number | Size of the complete descriptor in bytes |
Device Descriptors
Configuration Descriptors
