source: rtems/doc/bsp_howto/discrete.t @ 0660b4f8

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@c  COPYRIGHT (c) 1988-1999.
@c  On-Line Applications Research Corporation (OAR).
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@chapter Discrete Driver

The Discrete driver is responsible for providing an
interface to Discrete Input/Outputs.  The capabilities provided
by this class of device driver are:

@itemize @bullet
@item Initialize a Discrete I/O Board
@item Open a Particular Discrete Bitfield
@item Close a Particular Discrete Bitfield
@item Read from a Particular Discrete Bitfield
@item Write to a Particular Discrete Bitfield
@item Reset DACs
@item Reinitialize DACS
@end itemize

Most discrete I/O devices are found on I/O cards that support many
bits of discrete I/O on a single card.  This driver model is centered
on the notion of reading bitfields from the card.

There are currently no discrete I/O device drivers included in the 
RTEMS source tree.  The information provided in this chapter
is based on drivers developed by OAR Corporation personnel
for applications using RTEMS.   It is hoped that this
driver model information can form the basis for a standard
discrete I/O driver model that can be supported in future RTEMS
distribution.

@section Major and Minor Numbers

The @b{major} number of a device driver is its index in the 
RTEMS Device Address Table.

A @b{minor} number is associated with each device instance
managed by a particular device driver.  An RTEMS minor number 
is an @code{unsigned32} entity.  Convention calls for
dividing the bits in the minor number down into categories
that specify a particular bitfield.  This results in categories
like the following:

@itemize @bullet

@item @b{board} - indicates the board a particular bitfield is located on
@item @b{word} - indicates the particular word of discrete bits the
bitfield is located within
@item @b{start} - indicates the starting bit of the bitfield
@item @b{width} - indicates the width of the bitfield

@end itemize

From the above, it should be clear that a single device driver
can support multiple copies of the same board in a single system.
The minor number is used to distinguish the devices.

By providing a way to easily access a particular bitfield from
the device driver, the application is insulated with knowing how
to mask fields in and out of a discrete I/O.

@section Discrete I/O Driver Configuration

There is not a standard discrete I/O driver configuration table but some
fields are common across different drivers.  The discrete I/O driver
configuration table is typically an array of structures with each
structure containing the information for a particular board.
The following is a list of the type of information normally required
to configure an discrete I/O board:

@table @b
@item board_offset
is the base address of a board.

@item relay_initial_values
is an array of the values that should be written to each output
word on the board during initialization.  This allows the driver
to start with the board's output  in a known state.

@end table

@section Initialize a Discrete I/O Board

At system initialization, the discrete I/O driver's initialization entry point
will be invoked.  As part of initialization, the driver will perform
whatever board initializatin is required and then set all
outputs to their configured initial state.

The discrete I/O driver may register a device name for bitfields of 
particular interest to the system.  Normally this will be restricted
to the names of each word and, if the driver supports it, an "all words".

@section Open a Particular Discrete Bitfield

This is the driver open call.  Usually this call does nothing other than
validate the minor number.  

With some drivers, it may be necessary to allocate memory when a particular
device is opened.  If that is the case, then this is often the place
to do this operation.

@section Close a Particular Discrete Bitfield

This is the driver close call.  Usually this call does nothing.

With some drivers, it may be necessary to allocate memory when a particular
device is opened.  If that is the case, then this is the place
where that memory should be deallocated.

@section Read from a Particular Discrete Bitfield

This corresponds to the driver read call.  After validating the minor
number and arguments, this call reads the indicated bitfield.  A
discrete I/O devices may have to store the last value written to
a discrete output.  If the bitfield is output only, saving the last
written value gives the appearance that it can be read from also.  
If the bitfield is input, then it is sampled.

@b{NOTE:} Many discrete inputs have a tendency to bounce.  The application
may have to take account for bounces.

The value returned is an @code{unsigned32} number 
representing the bitfield read.  This value is stored in the
@code{argument_block} passed in to the call.

@b{NOTE:} Some discrete I/O drivers have a special minor number
used to access all discrete I/O bits on the board.  If this special
minor is used, then the area pointed to by @code{argument_block} must
be the correct size.

@section Write to a Particular Discrete Bitfield

This corresponds to the driver write call.  After validating the minor
number and arguments, this call writes the indicated device.  If the
specified device is an ADC, then an error is usually returned.  

The value written is an @code{unsigned32} number 
representing the value to be written to the specified
bitfield.  This value is stored in the
@code{argument_block} passed in to the call.

@b{NOTE:} Some discrete I/O drivers have a special minor number
used to access all discrete I/O bits on the board.  If this special
minor is used, then the area pointed to by @code{argument_block} must
be the correct size.

@section Disable Discrete Outputs

This is one of the IOCTL functions supported by the I/O control
device driver entry point.  When this IOCTL function is invoked,
the discrete outputs are disabled.

@b{NOTE:} It may not be possible to disable/enable discrete output on all 
discrete I/O boards.

@section Enable Discrete Outputs

This is one of the IOCTL functions supported by the I/O control
device driver entry point.  When this IOCTL function is invoked,
the discrete outputs are enabled.

@b{NOTE:} It may not be possible to disable/enable discrete output on all
discrete I/O boards.

@section Reinitialize Outputs

This is one of the IOCTL functions supported by the I/O control
device driver entry point.  When this IOCTL function is invoked,
the discrete outputs are rewritten with the configured initial
output values.

@section Get Last Written Values

This is one of the IOCTL functions supported by the I/O control
device driver entry point.  When this IOCTL function is invoked,
the following information is returned to the caller:

@itemize @bullet
@item last value written to the specified output word
@item timestamp of when the last write was performed
@end itemize