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Using Generic USB Controllers

class interface. However, the value isn’t used if the device doesn’t handle call

management.

If the communication interface has an interrupt or bulk endpoint for event

notifications, that endpoint has a standard endpoint descriptor. The example

descriptors include an endpoint descriptor for an interrupt IN endpoint. The

wMaxPacketSize field specifies the number of bytes the device sends in each

transaction. The bInterval field specifies the requested maximum period

between IN token packets from the host. At full speed, bInterval equals a num-

ber of milliseconds. At high speed, bInterval can range from 1 to 16, and the

requested maximum interval in milliseconds equals the following value:

2 bInterval-1 * 0.125

Thus, for a 1-ms maximum interval at full speed, bInterval =1, and for a maxi-

mum interval of 1 ms at high speed, bInterval = 4.

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In addition to the communication class interface, a communication device typ-

ically has an interface to carry application data. A CDC data class interface can

perform this function. If bInterfaceSubClass in the communication class inter-

face specifies the abstract control model, the CDC specification requires a data

class interface.

In the descriptor for a data class interface, bInterfaceClass = 0Ah. The interface

can have bulk or isochronous endpoints for carrying application data. Each

endpoint has a standard endpoint descriptor. Instead of a data class interface,

some CDC devices use other class or vendor-specific interfaces to carry applica-

Table 16-4: In the interface descriptor for a communication device, the

bInterfaceProtocol field can indicate a protocol the communications model

supports.

        

00h No class-specific protocol required

01h AT commands (specified in ITU V.25ter)

02h–06h AT commands for WMC devices

07h–FDh Reserved for future use

FEh External protocol for WMC devices

FFh Vendor specific