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USB System Architecture (USB 2.0).pdf
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USB System Architecture

Table 6-6: Full-Speed Interrupt Bandwidth

 

Percentage

Max

 

Data

of Frame

Maximum

Xfers/

Payload

Bandwidth/

Bandwidth

Frame

 

Transfer

 

 

 

 

 

 

 

 

 

 

 

 

1

1%

107

107KB/s

 

 

 

 

2

1%

100

200KB/s

 

 

 

 

4

1%

88

352KB/s

 

 

 

 

8

1%

71

568KB/s

 

 

 

 

16

2%

51

816KB/s

 

 

 

 

32

3%

33

1.056MB/s

 

 

 

 

64

5%

19

1.216MB/s

 

 

 

 

Control Transfers

Control transfers provide a way to configure a USB device and control certain aspects of its operation. Each device must implement a default control endpoint (always endpoint zero) used for configuring the device, controlling device states, and other aspects of the device’s operation. The control endpoint responds to a variety of USB specific requests that are delivered via control transfers. For example, when a device is detected on the USB, system software must access the device’s descriptors to determine the device type and operational characteristics. The USB specification also defines a variety of USB device requests for hubs, as well as other types of USB devices. These requests may be standard requests defined for all USB devices, requests defined for a particular device class, or vendor-specific requests. Only the device driver and device have knowledge of such requests. See “Standard Device Requests” on page 436 for a detailed list of standard requests supported by USB.

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