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>Starting up a USB Device</TITLE
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><H1
><A
NAME="USBS-START">Starting up a USB Device</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN16210"
></A
><H2
>Name</H2
><TT
CLASS="FUNCTION"
>usbs_start</TT
> -- Starting up a USB Device</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
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>Synopsis</H2
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><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <cyg/io/usb/usbs.h></PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void usbs_start</CODE
>(usbs_control_endpoint* ep0);</CODE
></P
><P
></P
></DIV
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><DIV
CLASS="REFSECT1"
><A
NAME="AEN16222"
></A
><H2
>Description</H2
><P
>Initializing a USB device requires some support from higher-level
code, typically the application, in the form of enumeration data.
Hence it is not possible for the low-level USB driver to activate a
USB device itself. Instead the higher-level code has to take care of
this by invoking <TT
CLASS="FUNCTION"
>usbs_start</TT
>. This function takes
a pointer to a USB control endpoint data structure. USB device drivers
should provide exactly one such data structure for every USB device,
so the pointer uniquely identifies the device.</P
><TABLE
BORDER="5"
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>const usbs_enumeration_data usb_enum_data = {
…
};
int
main(int argc, char** argv)
{
usbs_sa11x0_ep0.enumeration_data = &usb_enum_data;
…
usbs_start(&usbs_sa11x0_ep0);
…
}</PRE
></TD
></TR
></TABLE
><P
>The exact behaviour of <TT
CLASS="FUNCTION"
>usbs_start</TT
> depends on the
USB hardware and the device driver. A typical implementation would
change the USB data pins from tristated to active. If the peripheral
is already plugged into a host then the latter should detect this
change and start interacting with the peripheral, including requesting
the enumeration data. Some of this may happen before
<TT
CLASS="FUNCTION"
>usbs_start</TT
> returns, but given that multiple
interactions between USB host and peripheral are required it is likely
that the function will return before the peripheral is fully
configured. Control endpoints provide a <A
HREF="usbs-control.html#AEN16552"
>mechanism</A
> for informing
higher-level code of USB state changes.
<TT
CLASS="FUNCTION"
>usbs_start</TT
> will return even if the peripheral is
not currently connected to a host: it will not block until the
connection is established.</P
><P
><TT
CLASS="FUNCTION"
>usbs_start</TT
> should only be called once for a given
USB device. There are no defined error conditions. Note that the
function affects the entire USB device and not just the control
endpoint: there is no need to start any data endpoints as well.</P
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