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[/] [openrisc/] [trunk/] [rtos/] [ecos-3.0/] [packages/] [io/] [usb/] [slave/] [current/] [doc/] [usbs-devtab.html] - Blame information for rev 786

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<!-- Copyright (C) 2002 Free Software Foundation, Inc.                                -->
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<HTML
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><HEAD
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><TITLE
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>Devtab Entries</TITLE
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><meta name="MSSmartTagsPreventParsing" content="TRUE">
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<META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.64
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"><LINK
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REL="HOME"
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TITLE="eCos USB Slave Support"
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HREF="io-usb-slave.html"><LINK
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REL="PREVIOUS"
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TITLE="Starting up a USB Device"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>eCos USB Slave Support</TH
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></TR
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><TR
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="usbs-start.html"
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>Prev</A
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HREF="usbs-start-rx.html"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><H1
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><A
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NAME="USBS-DEVTAB"
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>Devtab Entries</A
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></H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN200"
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></A
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><H2
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>Name</H2
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>Devtab Entries&nbsp;--&nbsp;Data endpoint data structure</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN203"
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></A
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><H2
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>Synopsis</H2
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="SYNOPSIS"
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>/dev/usb0c
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/dev/usb1r
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/dev/usb2w</PRE
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></TD
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></TR
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></TABLE
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="AEN205"
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></A
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><H2
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>Devtab Entries</H2
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><P
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>USB device drivers provide two ways of transferring data between host
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and peripheral. The first involves USB-specific functionality such as
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<A
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HREF="usbs-start-rx.html"
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><TT
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CLASS="FUNCTION"
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>usbs_start_rx_buffer</TT
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></A
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>.
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This provides non-blocking I/O: a transfer is started, and some time
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later the device driver will call a supplied completion function. The
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second uses the conventional I/O model: there are entries in the
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device table corresponding to the various endpoints. Standard calls
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such as <TT
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CLASS="FUNCTION"
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>open</TT
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> can then be used to get a suitable
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handle. Actual I/O happens via blocking <TT
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CLASS="FUNCTION"
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>read</TT
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> and
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<TT
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CLASS="FUNCTION"
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>write</TT
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> calls. In practice the blocking operations
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are simply implemented using the underlying non-blocking
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functionality.</P
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><P
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>Each endpoint will have its own devtab entry. The exact names are
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controlled by the device driver package, but typically the root will
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be <TT
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CLASS="LITERAL"
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>/dev/usb</TT
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>. This is followed by one or more
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decimal digits giving the endpoint number, followed by
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<TT
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CLASS="LITERAL"
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>c</TT
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> for a control endpoint, <TT
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CLASS="LITERAL"
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>r</TT
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> for
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a receive endpoint (host to peripheral), and <TT
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CLASS="LITERAL"
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>w</TT
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> for
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a transmit endpoint (peripheral to host). If the target hardware
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involves more than one USB device then different roots should be used,
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for example <TT
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CLASS="LITERAL"
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>/dev/usb0c</TT
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> and
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<TT
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CLASS="LITERAL"
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>/dev/usb1_0c</TT
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>. This may require explicit
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manipulation of device driver configuration options by the application
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developer.</P
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><P
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>At present the devtab entry for a control endpoint does not support
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any I/O operations. </P
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><DIV
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CLASS="REFSECT2"
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><A
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NAME="AEN221"
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></A
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><H3
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><TT
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CLASS="FUNCTION"
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>write</TT
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> operations</H3
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_io_write</TT
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> and similar functions in
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higher-level packages can be used to perform a transfer from
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peripheral to host. Successive write operations will not be coalesced.
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For example, when doing a 1000 byte write to an endpoint that uses the
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bulk transfer protocol this will involve 15 full-size 64-byte packets
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and a terminating 40-byte packet. USB device drivers are not expected
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to do any locking, and if higher-level code performs multiple
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concurrent write operations on a single endpoint then the resulting
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behaviour is undefined.</P
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><P
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>A USB <TT
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CLASS="FUNCTION"
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>write</TT
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> operation will never transfer less
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data than specified. It is the responsibility of higher-level code to
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ensure that the amount of data being transferred is acceptable to the
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host-side code. Usually this will be defined by a higher-level
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protocol. If an attempt is made to transfer more data than the host
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expects then the resulting behaviour is undefined.</P
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><P
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>There are two likely error conditions. <TT
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CLASS="LITERAL"
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>EPIPE</TT
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>
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indicates that the connection between host and target has been broken.
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<TT
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CLASS="LITERAL"
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>EAGAIN</TT
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> indicates that the endpoint has been
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stalled, either at the request of the host or by other activity
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inside the peripheral.</P
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></DIV
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><DIV
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CLASS="REFSECT2"
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><A
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NAME="AEN231"
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></A
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><H3
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><TT
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CLASS="FUNCTION"
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>read</TT
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> operations</H3
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_io_read</TT
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> and similar functions in higher-level
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packages can be used to perform a transfer from host to peripheral.
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This should be a complete transfer: higher-level protocols should
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define an upper bound on the amount of data being transferred, and the
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<TT
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CLASS="FUNCTION"
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>read</TT
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> operation should involve at least this
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amount of data. The return value will indicate the actual transfer
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size, which may be less than requested.</P
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><P
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>Some device drivers may support partial reads, but USB device drivers
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are not expected to perform any buffering because that involves both
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memory and code overheads. One technique that may work for bulk
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transfers is to exploit the fact that such transfers happen in 64-byte
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packets. It is possible to <TT
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CLASS="FUNCTION"
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>read</TT
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> an initial 64
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bytes, corresponding to the first packet in the transfer. These 64
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bytes can then be examined to determine the total transfer size, and
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the remaining data can be transferred in another
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<TT
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CLASS="FUNCTION"
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>read</TT
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> operation. This technique is not guaranteed
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to work with all USB hardware. Also, if the delay between accepting
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the first packet and the remainder of the transfer is excessive then
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this could cause timeout problems for the host-side software. For
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these reasons the use of partial reads should be avoided.</P
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><P
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>There are two likely error conditions. <TT
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CLASS="LITERAL"
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>EPIPE</TT
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>
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indicates that the connection between host and target has been broken.
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<TT
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CLASS="LITERAL"
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>EAGAIN</TT
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> indicates that the endpoint has been
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stalled, either at the request of the host or by other activity
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inside the peripheral.</P
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><P
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>USB device drivers are not expected to do any locking. If higher-level
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code performs multiple concurrent read operations on a single endpoint
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then the resulting behaviour is undefined.</P
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></DIV
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><DIV
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CLASS="REFSECT2"
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><A
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NAME="AEN244"
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></A
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><H3
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><TT
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CLASS="FUNCTION"
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>select</TT
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> operations</H3
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><P
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>Typical USB device drivers will not provide any support for
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<TT
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CLASS="FUNCTION"
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>select</TT
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>. Consider bulk transfers from the host to
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the peripheral. At the USB device driver level there is no way of
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knowing in advance how large a transfer will be, so it is not feasible
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for the device driver to buffer the entire transfer. It may be
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possible to buffer part of the transfer, for example the first 64-byte
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packet, and copy this into application space at the start of a
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<TT
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CLASS="FUNCTION"
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>read</TT
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>, but this adds code and memory overheads.
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Worse, it means that there is an unknown but potentially long delay
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between a peripheral accepting the first packet of a transfer and the
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remaining packets, which could confuse or upset the host-side
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software.</P
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><P
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>With some USB hardware it may be possible for the device driver to
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detect OUT tokens from the host without actually accepting the data,
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and this would indicate that a  <TT
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CLASS="FUNCTION"
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>read</TT
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> is likely to
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succeed. However, it would not be reliable since the host-side I/O
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operation could time out. A similar mechanism could be used to
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implement <TT
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CLASS="FUNCTION"
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>select</TT
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> for outgoing data, but again
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this would not be reliable.</P
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><P
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>Some device drivers may provide partial support for
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<TT
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CLASS="FUNCTION"
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>select</TT
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> anyway, possibly under the control of a
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configuration option. The device driver's documentation should be
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consulted for further information. It is also worth noting that the
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USB-specific non-blocking API can often be used as an alternative to
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<TT
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CLASS="FUNCTION"
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>select</TT
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>.</P
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></DIV
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><DIV
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CLASS="REFSECT2"
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><A
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NAME="AEN256"
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></A
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><H3
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><TT
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CLASS="FUNCTION"
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>get_config</TT
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> and
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<TT
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CLASS="FUNCTION"
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>set_config</TT
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> operations</H3
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><P
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>There are no <TT
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CLASS="FUNCTION"
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>set_config</TT
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> or
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<TT
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CLASS="FUNCTION"
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>get_config</TT
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> (also known as
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<TT
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CLASS="FUNCTION"
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>ioctl</TT
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>) operations defined for USB devices.
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Some device drivers may provide hardware-specific facilities this way. </P
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><DIV
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CLASS="NOTE"
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><BLOCKQUOTE
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CLASS="NOTE"
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><P
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><B
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>Note: </B
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>Currently the USB-specific functions related to <A
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HREF="usbs-halt.html"
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>halted endpoints</A
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> cannot be accessed readily
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via devtab entries. This functionality should probably be made
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available via <TT
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CLASS="FUNCTION"
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>set_config</TT
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> and
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<TT
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CLASS="FUNCTION"
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>get_config</TT
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>. It may also prove useful to provide
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a <TT
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CLASS="FUNCTION"
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>get_config</TT
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> operation that maps from the
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devtab entries to the underlying endpoint data structures.</P
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></BLOCKQUOTE
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></DIV
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></DIV
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><DIV
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CLASS="REFSECT2"
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><A
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NAME="AEN270"
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></A
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><H3
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>Presence</H3
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><P
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>The devtab entries are optional. If the USB device is accessed
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primarily by class-specific code such as the USB-ethernet package and
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that package uses the USB-specific API directly, the devtab entries
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are redundant. Even if application code does need to access the USB
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device, the non-blocking API may be more convenient than the blocking
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I/O provided via the devtab entries. In these cases the devtab entries
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serve no useful purpose, but they still impose a memory overhead. It
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is possible to suppress the presence of these entries by disabling the
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configuration option
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<TT
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CLASS="LITERAL"
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>CYGGLO_IO_USB_SLAVE_PROVIDE_DEVTAB_ENTRIES</TT
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>.</P
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></DIV
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></DIV
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><DIV
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><HR
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WIDTH="100%"><TABLE
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><TD
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WIDTH="33%"
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VALIGN="top"
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><A
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HREF="usbs-start.html"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="io-usb-slave.html"
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>Home</A
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></TD
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WIDTH="33%"
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ALIGN="right"
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HREF="usbs-start-rx.html"
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><TD
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ALIGN="left"
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>Starting up a USB Device</TD
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><TD
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ALIGN="center"
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VALIGN="top"
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>&nbsp;</TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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>Receiving Data from the Host</TD
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