STV0676 datasheet(8/21 Pages) STMICROELECTRONICS

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External interfaces

STV0676

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Revision C

External interfaces

2.1

USB interface

The USB interface is designed to be compliant with version 1.1 of the USB specification. The

STV0676 is a low power device and is therefore suitable for connection to any USB port on a PC,

self-powered hub or when connected to a bus-powered hub.

The device complies with the device framework specified in Chapter 9 of the USB specification as

follows:

The device supports a single high power configuration (

Configuration 1).

Endpoint 0 is the default control endpoint and is always supported.

Endpoint 0 supports all of the USB commands required by the device framework.

Vendor specific commands on Endpoint 0 are used for all device control.

Configuration 1 supports a single interface (interface 0).

Interface 0 supports 8 alternate settings (alternates 0-7).

The alternate settings support between 0 and 2 additional endpoints.

Endpoint 1 is used for isochronous transfer of image data.

Endpoint 3 is used for transferring status information, e.g. state of a hardware button.

The endpoints are configured as follows (

Table 2) in the alternate settings:

The best and most consistent performance in terms of image quality is always obtained in the

highest bandwidth setting (alternate 7). Under some circumstances it may not be possible for the

host to allocate this amount of USB bandwidth to the device.

The isochronous settings reserve varying quantities of bandwidth - from 10% to 85% of USB

bandwidth. The lower settings result in poor image quality due to heavy compression applied to

maintain a high framerate streaming of image data, but at the same time leaving more bandwidth

free for other USB devices. This is desirable if more than one camera is to be used, or if there are

other isochronous peripherals connected. The device driver allows the user to specify the maximum

bandwidth they wish to allocate to transfer data from the device. If the maximum specified by the

user is not available, perhaps because another isochronous device has already reserved that

bandwidth, then lower alternates are tried until one succeeds.

Table 2: Endpoint alternate settings

Alternate setting

Endpoint1 (isochronous)

Endpoint3 (interrupt)

Not present