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Remove Discrete Driver chapter
There exists no standard Discrete Driver in the RTEMS code base. Close #2851.
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Sebastian Huber
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.. comment SPDX-License-Identifier: CC-BY-SA-4.0
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Discrete Driver
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***************
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The Discrete driver is responsible for providing an interface to Discrete
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Input/Outputs. The capabilities provided by this class of device driver are:
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- Initialize a Discrete I/O Board
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- Open a Particular Discrete Bitfield
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- Close a Particular Discrete Bitfield
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- Read from a Particular Discrete Bitfield
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- Write to a Particular Discrete Bitfield
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- Reset DACs
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- Reinitialize DACS
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Most discrete I/O devices are found on I/O cards that support many bits of
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discrete I/O on a single card. This driver model is centered on the notion of
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reading bitfields from the card.
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There are currently no discrete I/O device drivers included in the RTEMS source
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tree. The information provided in this chapter is based on drivers developed
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for applications using RTEMS. It is hoped that this driver model information
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can form the discrete I/O driver model that can be supported in future RTEMS
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distribution.
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Major and Minor Numbers
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=======================
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The ``major`` number of a device driver is its index in the RTEMS Device
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Address Table.
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A ``minor`` number is associated with each device instance managed by a
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particular device driver. An RTEMS minor number is an ``unsigned32`` entity.
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Convention calls for dividing the bits in the minor number down into categories
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that specify a particular bitfield. This results in categories like the
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following:
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- ``board`` - indicates the board a particular bitfield is located on
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- ``word`` - indicates the particular word of discrete bits the bitfield is
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located within
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- ``start`` - indicates the starting bit of the bitfield
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- ``width`` - indicates the width of the bitfield
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From the above, it should be clear that a single device driver can support
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multiple copies of the same board in a single system. The minor number is used
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to distinguish the devices.
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By providing a way to easily access a particular bitfield from the device
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driver, the application is insulated with knowing how to mask fields in and out
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of a discrete I/O.
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Discrete I/O Driver Configuration
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=================================
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There is not a standard discrete I/O driver configuration table but some fields
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are common across different drivers. The discrete I/O driver configuration
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table is typically an array of structures with each structure containing the
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information for a particular board. The following is a list of the type of
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information normally required to configure an discrete I/O board:
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``board_offset``
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is the base address of a board.
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``relay_initial_values``
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is an array of the values that should be written to each output word on the
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board during initialization. This allows the driver to start with the
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board's output in a known state.
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Initialize a Discrete I/O Board
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===============================
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At system initialization, the discrete I/O driver's initialization entry point
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will be invoked. As part of initialization, the driver will perform whatever
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board initializatin is required and then set all outputs to their configured
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initial state.
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The discrete I/O driver may register a device name for bitfields of particular
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interest to the system. Normally this will be restricted to the names of each
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word and, if the driver supports it, an "all words".
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Open a Particular Discrete Bitfield
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===================================
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This is the driver open call. Usually this call does nothing other than
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validate the minor number.
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With some drivers, it may be necessary to allocate memory when a particular
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device is opened. If that is the case, then this is often the place to do this
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operation.
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Close a Particular Discrete Bitfield
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====================================
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This is the driver close call. Usually this call does nothing.
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With some drivers, it may be necessary to allocate memory when a particular
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device is opened. If that is the case, then this is the place where that
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memory should be deallocated.
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Read from a Particular Discrete Bitfield
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========================================
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This corresponds to the driver read call. After validating the minor number
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and arguments, this call reads the indicated bitfield. A discrete I/O devices
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may have to store the last value written to a discrete output. If the bitfield
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is output only, saving the last written value gives the appearance that it can
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be read from also. If the bitfield is input, then it is sampled.
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.. note::
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Many discrete inputs have a tendency to bounce. The application may have to
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take account for bounces.
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The value returned is an ``unsigned32`` number representing the bitfield read.
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This value is stored in the ``argument_block`` passed in to the call.
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.. note::
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Some discrete I/O drivers have a special minor number used to access all
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discrete I/O bits on the board. If this special minor is used, then the
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area pointed to by ``argument_block`` must be the correct size.
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Write to a Particular Discrete Bitfield
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=======================================
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This corresponds to the driver write call. After validating the minor number
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and arguments, this call writes the indicated device. If the specified device
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is an ADC, then an error is usually returned.
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The value written is an ``unsigned32`` number representing the value to be
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written to the specified bitfield. This value is stored in the
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``argument_block`` passed in to the call.
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.. note::
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Some discrete I/O drivers have a special minor number used to access all
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discrete I/O bits on the board. If this special minor is used, then the
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area pointed to by ``argument_block`` must be the correct size.
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Disable Discrete Outputs
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========================
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This is one of the IOCTL functions supported by the I/O control device driver
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entry point. When this IOCTL function is invoked, the discrete outputs are
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disabled.
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.. note::
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It may not be possible to disable/enable discrete output on all discrete I/O
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boards.
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Enable Discrete Outputs
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=======================
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This is one of the IOCTL functions supported by the I/O control device driver
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entry point. When this IOCTL function is invoked, the discrete outputs are
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enabled.
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.. note::
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It may not be possible to disable/enable discrete output on all discrete
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I/O boards.
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Reinitialize Outputs
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====================
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This is one of the IOCTL functions supported by the I/O control device driver
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entry point. When this IOCTL function is invoked, the discrete outputs are
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rewritten with the configured initial output values.
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Get Last Written Values
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=======================
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This is one of the IOCTL functions supported by the I/O control device driver
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entry point. When this IOCTL function is invoked, the following information is
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returned to the caller:
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- last value written to the specified output word
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- timestamp of when the last write was performed
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@ -57,7 +57,6 @@ to the Community Project hosted at http://www.rtems.org.
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networking
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shared_memory_support
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frame_buffer
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discrete
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command
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* :ref:`genindex`
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