mirror of
https://git.rtems.org/rtems-docs/
synced 2025-05-31 14:44:09 +08:00
191 lines
6.7 KiB
ReStructuredText
191 lines
6.7 KiB
ReStructuredText
.. comment SPDX-License-Identifier: CC-BY-SA-4.0
|
|
|
|
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:
|
|
|
|
- Initialize a Discrete I/O Board
|
|
|
|
- Open a Particular Discrete Bitfield
|
|
|
|
- Close a Particular Discrete Bitfield
|
|
|
|
- Read from a Particular Discrete Bitfield
|
|
|
|
- Write to a Particular Discrete Bitfield
|
|
|
|
- Reset DACs
|
|
|
|
- Reinitialize DACS
|
|
|
|
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
|
|
for applications using RTEMS. It is hoped that this driver model information
|
|
can form the discrete I/O driver model that can be supported in future RTEMS
|
|
distribution.
|
|
|
|
Major and Minor Numbers
|
|
=======================
|
|
|
|
The ``major`` number of a device driver is its index in the RTEMS Device
|
|
Address Table.
|
|
|
|
A ``minor`` number is associated with each device instance managed by a
|
|
particular device driver. An RTEMS minor number is an ``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:
|
|
|
|
- ``board`` - indicates the board a particular bitfield is located on
|
|
|
|
- ``word`` - indicates the particular word of discrete bits the bitfield is
|
|
located within
|
|
|
|
- ``start`` - indicates the starting bit of the bitfield
|
|
|
|
- ``width`` - indicates the width of the bitfield
|
|
|
|
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.
|
|
|
|
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:
|
|
|
|
``board_offset``
|
|
is the base address of a board.
|
|
|
|
``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.
|
|
|
|
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".
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
.. note::
|
|
|
|
Many discrete inputs have a tendency to bounce. The application may have to
|
|
take account for bounces.
|
|
|
|
The value returned is an ``unsigned32`` number representing the bitfield read.
|
|
This value is stored in the ``argument_block`` passed in to the call.
|
|
|
|
.. 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 ``argument_block`` must be the correct size.
|
|
|
|
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 ``unsigned32`` number representing the value to be
|
|
written to the specified bitfield. This value is stored in the
|
|
``argument_block`` passed in to the call.
|
|
|
|
.. 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 ``argument_block`` must be the correct size.
|
|
|
|
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.
|
|
|
|
.. note::
|
|
|
|
It may not be possible to disable/enable discrete output on all discrete I/O
|
|
boards.
|
|
|
|
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.
|
|
|
|
.. note::
|
|
|
|
It may not be possible to disable/enable discrete output on all discrete
|
|
I/O boards.
|
|
|
|
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.
|
|
|
|
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:
|
|
|
|
- last value written to the specified output word
|
|
|
|
- timestamp of when the last write was performed
|