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22 Intel® PXA27x Processor Family Specification Update
Errata
E22. UDC: UDC does not correctly support alternate interfaces.
Problem: When a SET_INTERFACE command is sent from the host, the UDC reallocates the internal FIFO
memory.
When USB Client comes up after the USB Reset and Set Configuration is done other than
Configuration 0, it does not allow access to all the Interfaces available for that configuration.
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Some excerpts of USB Specification describing use of Interfaces
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As part of the configuration process, the host sets the device configuration and, where necessary,
selects the appropriate alternate settings for the interfaces.
Interfaces are numbered from zero to one less than the number of concurrent interfaces supported
by the configuration. Alternate settings range from zero to one less than the number of alternate
settings for a specific interface. The default setting when a device is initially configured is alternate
setting zero.
see section: Set Interface
This request allows the host to select an alternate setting for the specified interface. This request
cannot be used to change the set of configured interfaces (the SetConfiguration() request must be
used instead).
see section: Configuration
The descriptor describes the number of interfaces provided by the configuration. Each interface
may operate independently. For example, an ISDN device might be configured with two interfaces,
each providing 64 Kb/s bi-directional channels that have separate data sources or sinks on the host.
Another configuration might present the ISDN device as a single interface, bonding the two
channels into one 128 Kb/s bi-directional channel.
An endpoint is not shared among interfaces within a single configuration unless the endpoint is
used by alternate settings of the same interface. Endpoints may be shared among interfaces that are
part of different configurations without this restriction.
Implication: The processor USB Client does not follow the USB Specification when dealing with different
interfaces. External USB host connected to the processor USB Client cannot switch correctly
between alternate interfaces on the processor USB Client. This issue makes the processor USB
Client difficult to be a compound device.
Workaround: TBD
Status: No Fix