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>Halted Endpoints</TITLE
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TITLE="eCos USB Slave Support"
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TITLE="Sending Data to the Host"
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>eCos Reference Manual</TH
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><H1
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><A
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NAME="USBS-HALT">Halted Endpoints</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN16457"
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></A
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><H2
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>Name</H2
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>Halted Endpoints&nbsp;--&nbsp;Support for Halting and Halted Endpoints</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN16460"><H2
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>Synopsis</H2
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><DIV
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CLASS="FUNCSYNOPSIS"
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><A
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NAME="AEN16461"><P
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></P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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>#include &lt;cyg/io/usb/usbs.h&gt;</PRE
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></TD
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></TR
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></TABLE
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_bool usbs_rx_endpoint_halted</CODE
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>(usbs_rx_endpoint* ep);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void usbs_set_rx_endpoint_halted</CODE
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>(usbs_rx_endpoint* ep, cyg_bool new_state);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void usbs_start_rx_endpoint_wait</CODE
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>(usbs_rx_endpoint* ep, void (*)(void*, int) complete_fn, void * complete_data);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_bool
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usbs_tx_endpoint_halted</CODE
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>(usbs_tx_endpoint* ep);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void usbs_set_tx_endpoint_halted</CODE
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>(usbs_tx_endpoint* ep, cyg_bool new_state);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void usbs_start_tx_endpoint_wait</CODE
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>(usbs_tx_endpoint* ep, void (*)(void*, int) complete_fn, void * complete_data);</CODE
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></P
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><P
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></P
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></DIV
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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="AEN16505"
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></A
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><H2
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><TT
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CLASS="FUNCTION"
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>Description</TT
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></H2
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><P
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>Normal USB traffic involves straightforward handshakes, with either an
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<TT
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CLASS="LITERAL"
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>ACK</TT
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> to indicate that a packet was transferred
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without errors, or a <TT
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CLASS="LITERAL"
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>NAK</TT
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> if an error occurred, or
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if a peripheral is currently unable to process another packet from the
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host, or has no packet to send to the host. There is a third form of
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handshake, a <TT
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CLASS="LITERAL"
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>STALL</TT
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>, which indicates that the
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endpoint is currently <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>halted</I
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></SPAN
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>.</P
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><P
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>When an endpoint is halted it means that the host-side code needs to
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take some sort of recovery action before communication over that
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endpoint can resume. The exact circumstances under which this can
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happen are not defined by the USB specification, but one example would
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be a protocol violation if say the peripheral attempted to transmit
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more data to the host than was permitted by the protocol in use. The
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host can use the standard control messages get-status, set-feature and
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clear-feature to examine and manipulate the halted status of a given
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endpoint. There are USB-specific functions which can be used inside
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the peripheral to achieve the same effect. Once an endpoint has been
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halted the host can then interact with the peripheral using class or
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vendor control messages to perform appropriate recovery, and then the
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halted condition can be cleared.</P
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><P
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>Halting an endpoint does not constitute a device state change, and
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there is no mechanism by which higher-level code can be informed
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immediately. However, any ongoing receive or transmit operations will
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be aborted with an <TT
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CLASS="LITERAL"
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>-EAGAIN</TT
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> error, and any new
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receives or transmits will fail immediately with the same error.</P
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><P
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>There are six functions to support halted endpoints, one set for
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receive endpoints and another for transmit endpoints, with both sets
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behaving in essentially the same way. The first,
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<TT
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CLASS="FUNCTION"
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>usbs_rx_endpoint_halted</TT
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>, can be used to determine
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whether or not an endpoint is currently halted: it takes a single
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argument that identifies the endpoint of interest. The second
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function, <TT
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CLASS="FUNCTION"
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>usbs_set_rx_endpoint_halted</TT
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>, can be
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used to change the halted condition of an endpoint: it takes two
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arguments; one to identify the endpoint and another to specify the new
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state. The last function
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<TT
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CLASS="FUNCTION"
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>usbs_start_rx_endpoint_wait</TT
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> operates in much the
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same way as <TT
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CLASS="FUNCTION"
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>usbs_start_rx_buffer</TT
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>: when the
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endpoint is no longer halted the device driver will invoke the
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supplied completion function with a status of 0. The completion
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function has the same signature as that for a transfer operation.
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Often it will be possible to use a single completion function and have
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the foreground code invoke either
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<TT
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CLASS="FUNCTION"
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>usbs_start_rx_buffer</TT
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> or
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<TT
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CLASS="FUNCTION"
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>usbs_start_rx_endpoint_wait</TT
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> depending on the
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current state of the endpoint.</P
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