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>Writing a CAN hardware device driver</TITLE
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><DIV
CLASS="CHAPTER"
><H1
><A
NAME="IO-CAN-DEVICE-DRIVERS"
></A
>Chapter 34. Writing a CAN hardware device driver</H1
><P
>A CAN driver is nothing more than a named entity that supports the
basic I/O functions - read, write, get config, and set config. The
device driver uses and manages interrupts from the device. While the
interface is generic and device driver independent, the actual driver
implementation is completely up to the device driver designer.</P
><P
>That said, the reason for using a device driver is to provide access
to a CAN device from application code in as general purpose a fashion
as reasonable. Most driver writers are also concerned with making this
access as simple as possible while being as efficient as possible.</P
><P
>Like other device drivers the CAN device driver is concerned with the
movement of information - the CAN messages. In order to make the most
efficient use of system resources, interrupts are used. This will
allow other application processing to take place while the data
transfers are under way, with interrupts used to indicate when various
events have occurred. For example, a CAN device typically generates an
interrupt after a CAN message has been sent or a CAN message has been
received by a CAN hardware message buffer. It makes sense to allow
further application processing while the data is being sent since this
can take quite a long time. The interrupt can be used to allow the
driver to send a CAN message as soon as the current one is complete,
without any active participation by the application code.</P
><P
>The main building blocks for CAN device drivers are found in the
include files
<TT
CLASS="FILENAME"
><cyg/io/devtab.h></TT
> and <TT
CLASS="FILENAME"
><cyg/io/can.h></TT
></P
><P
>Like many other device drivers in eCos, CAN device drivers are described by a device
table entry, using the <SPAN
CLASS="TYPE"
>cyg_devtab_entry_t</SPAN
> type. The entry should be created using
the <CODE
CLASS="VARNAME"
>DEVTAB_ENTRY()</CODE
> macro.</P
><DIV
CLASS="SECTION"
><H1
CLASS="SECTION"
><A
NAME="IO-CAN-HOW-TO-WRITE-INTERFACE-DRIVER"
>How to Write a CAN Hardware Interface Driver</A
></H1
><P
>The standard CAN driver supplied with eCos is structured as a hardware
independent portion and a hardware dependent interface module. To add
support for a new CAN device, the user should be able to use the
existing hardware independent portion and just add their own interface
driver which handles the details of the actual device. The user should
have no need to change the hardware independent portion.</P
><P
>The interfaces used by the CAN driver and CAN implementation modules
are contained in the file <TT
CLASS="FILENAME"
><cyg/io/can.h></TT
>.</P
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN9656"
>DevTab Entry</A
></H2
><P
>The interface module contains the devtab entry (or entries if a single
module supports more than one interface). This entry should have the
form:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>
DEVTAB_ENTRY(<<module_name>>,
<<device_name>>,
0,
&can_devio,
<<module_init>>,
<<module_lookup>>,
&<<can_channel>>
);</PRE
></TD
></TR
></TABLE
><P
></P
><DIV
CLASS="VARIABLELIST"
><P
><B
>Arguments</B
></P
><DL
><DT
><CODE
CLASS="PARAMETER"
>module_name</CODE
></DT
><DD
><P
>The "C" label for this devtab entry</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>device_name</CODE
></DT
><DD
><P
>The "C" string for the
device. E.g. <TT
CLASS="FILENAME"
>/dev/can0</TT
>.</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>can_devio</CODE
></DT
><DD
><P
>The table of I/O functions. This set is defined in
the hardware independent CAN driver and should be used.</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>module_init</CODE
></DT
><DD
><P
>The hardware module initialization function.</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>module_lookup</CODE
></DT
><DD
><P
>The device lookup function. This function
typically sets up the CAN device for actual use, turning on
interrupts, configuring the message buffers, etc.</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>can_channel</CODE
></DT
><DD
><P
>This table (defined below) contains the interface
between the interface module and the CAN driver proper.</P
></DD
></DL
></DIV
><P
>Example devtab entry for Motorola FlexCAN device driver:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>DEVTAB_ENTRY(flexcan_devtab,
CYGDAT_DEVS_CAN_MCF52xx_FLEXCAN0_NAME,
0, // Does not depend on a lower level interface
&cyg_io_can_devio,
flexcan_init,
flexcan_lookup, // CAN driver may need initializing
&flexcan_can0_chan
);</PRE
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN9695"
>CAN Channel Structure</A
></H2
><P
>Each CAN device must have a “CAN channel”.
This is a set of data which describes all operations on the device.
It also contains buffers, etc. The CAN channel is created by the macro: </P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>CAN_CHANNEL_USING_INTERRUPTS(l, funs, dev_priv, baud,
out_buf, out_buflen,
in_buf, in_buflen)</PRE
></TD
></TR
></TABLE
><P
></P
><DIV
CLASS="VARIABLELIST"
><P
><B
>Arguments</B
></P
><DL
><DT
><CODE
CLASS="PARAMETER"
>l</CODE
></DT
><DD
><P
>The "C" label for this structure.</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>funs</CODE
></DT
><DD
><P
>The set of interface functions (see below).</P
></DD
><DT
><CODE
CLASS="STRUCTFIELD"
>dev_priv</CODE
></DT
><DD
><P
>A placeholder for any device specific data for
this channel.</P
></DD
><DT
><CODE
CLASS="STRUCTFIELD"
>baud</CODE
></DT
><DD
><P
>The initial baud rate value
(<SPAN
CLASS="TYPE"
>cyg_can_baud_rate_t</SPAN
>).</P
></DD
><DT
><CODE
CLASS="STRUCTFIELD"
>out_buf</CODE
></DT
><DD
><P
>Pointer to the output buffer</P
></DD
><DT
><CODE
CLASS="STRUCTFIELD"
>out_buflen</CODE
></DT
><DD
><P
>The length of the output buffer.</P
></DD
><DT
><CODE
CLASS="STRUCTFIELD"
>in_buf</CODE
></DT
><DD
><P
>pointer to the input buffer.</P
></DD
><DT
><CODE
CLASS="STRUCTFIELD"
>in_buflen</CODE
></DT
><DD
><P
>The length of the input buffer.</P
></DD
></DL
></DIV
><P
>Example CAN channel implementation for Motorola FlexCAN device driver:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>CAN_CHANNEL_USING_INTERRUPTS(
flexcan_can0_chan,
flexcan_lowlevel_funs,
flexcan_can0_info,
CYG_CAN_BAUD_RATE(CYGNUM_DEVS_CAN_MCF52xx_FLEXCAN0_KBAUD),
flexcan_can0_txbuf, CYGNUM_DEVS_CAN_MCF52xx_FLEXCAN0_QUEUESIZE_TX,
flexcan_can0_rxbuf, CYGNUM_DEVS_CAN_MCF52xx_FLEXCAN0_QUEUESIZE_RX
);</PRE
></TD
></TR
></TABLE
><P
>The interface from the hardware independent driver into the hardware
interface module is contained in the <CODE
CLASS="STRUCTFIELD"
>funs</CODE
>
table. This is defined by the macro:</P
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN9746"
>CAN Lowlevel Functions Structure</A
></H2
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>CAN_LOWLEVEL_FUNS(l, putmsg, getevent, get_config, set_config, start_xmit, stop_xmit)</PRE
></TD
></TR
></TABLE
><P
></P
><DIV
CLASS="VARIABLELIST"
><P
><B
>Arguments</B
></P
><DL
><DT
><CODE
CLASS="STRUCTFIELD"
>l</CODE
></DT
><DD
><P
>The "C" label for this structure.</P
></DD
><DT
><CODE
CLASS="STRUCTFIELD"
>putmsg</CODE
></DT
><DD
><P
><TT
CLASS="LITERAL"
> bool (*putmsg)(can_channel *priv, cyg_can_message *pmsg, void *pdata)
</TT
></P
><P
> This function sends one CAN message to the interface. It should
return <TT
CLASS="LITERAL"
>true</TT
> if the message is actually
consumed. It should return <TT
CLASS="LITERAL"
>false</TT
> if there is
no space in the interface
</P
></DD
><DT
><CODE
CLASS="STRUCTFIELD"
>getevent</CODE
></DT
><DD
><P
><TT
CLASS="LITERAL"
> bool (*getevent)(can_channel *priv, cyg_can_event *pevent, void *pdata)
</TT
></P
><P
> This function fetches one event from the interface.
</P
></DD
><DT
><CODE
CLASS="STRUCTFIELD"
>get_config</CODE
></DT
><DD
><P
><TT
CLASS="LITERAL"
>
Cyg_ErrNo (*get_config)(can_channel *priv, cyg_uint32 key, const void *xbuf, cyg_uint32 *len)
</TT
></P
><P
> This function is used to query the configuration of a CAN channel.
</P
></DD
><DT
><CODE
CLASS="STRUCTFIELD"
>set_config</CODE
></DT
><DD
><P
><TT
CLASS="LITERAL"
>
Cyg_ErrNo (*set_config)(can_channel *priv, cyg_uint32 key, const void *xbuf, cyg_uint32 *len)
</TT
></P
><P
> This function is used to change configuration of a CAN channel.
</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>start_xmit</CODE
></DT
><DD
><P
><TT
CLASS="LITERAL"
>void (*start_xmit)(can_channel *priv)</TT
></P
><P
> Enable the transmit channel and turn on transmit interrupts.
</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>stop_xmit</CODE
></DT
><DD
><P
><TT
CLASS="LITERAL"
>void (*stop_xmit)(can_channel *priv)</TT
></P
><P
>Disable the transmit channel and turn transmit interrupts off.</P
></DD
></DL
></DIV
><P
>Example implementation of low level function structure for Motorola FlexCAN
device driver:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>CAN_LOWLEVEL_FUNS(flexcan_lowlevel_funs,
flexcan_putmsg,
flexcan_getevent,
flexcan_get_config,
flexcan_set_config,
flexcan_start_xmit,
flexcan_stop_xmit
);</PRE
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN9802"
>Callbacks</A
></H2
><P
>The device interface module can execute functions in the
hardware independent driver via <TT
CLASS="LITERAL"
>chan->callbacks</TT
>.
These functions are available:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>void (*can_init)(can_channel *chan)</PRE
></TD
></TR
></TABLE
><P
>This function is used to initialize the CAN channel.</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>void (*xmt_msg)(can_channel *chan, void *pdata)</PRE
></TD
></TR
></TABLE
><P
>This function would be called from an interrupt handler after a
transmit interrupt indicating that additional messages may be
sent. The upper driver will call the <CODE
CLASS="FUNCTION"
>putmsg</CODE
>
function as appropriate to send more data to the device.</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>void (*rcv_event)(can_channel *chan, void *pdata)</PRE
></TD
></TR
></TABLE
><P
>This function is used to tell the driver that a message has arrived
at the interface or that an event has occurred. This function is typically
called from the interrupt handler. </P
></DIV
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