rtems-docs/bsp_howto/analog.rst

.. comment SPDX-License-Identifier: CC-BY-SA-4.0

.. COMMENT: COPYRIGHT (c) 1988-2002.
.. COMMENT: On-Line Applications Research Corporation (OAR).
.. COMMENT: All rights reserved.

Analog Driver
#############

The Analog driver is responsible for providing an interface to Digital to
Analog Converters (DACs) and Analog to Digital Converters (ADCs).  The
capabilities provided by this class of device driver are:

- Initialize an Analog Board

- Open a Particular Analog

- Close a Particular Analog

- Read from a Particular Analog

- Write to a Particular Analog

- Reset DACs

- Reinitialize DACS

Most analog devices are found on I/O cards that support multiple DACs or ADCs
on a single card.

There are currently no analog 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 basis for a standard analog 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
like the following:

- ``board`` - indicates the board a particular device is located on

- ``port`` - indicates the particular device on a board.

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.

Analog Driver Configuration
===========================

There is not a standard analog driver configuration table but some fields are
common across different drivers.  The analog 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 analog board:

``board_offset``
    is the base address of a board.

``DAC_initial_values``
    is an array of the voltages that should be written to each DAC during
    initialization.  This allows the driver to start the board in a known
    state.

Initialize an Analog Board
==========================

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

The analog driver may register a device name for each DAC and ADC in the
system.

Open a Particular Analog
========================

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 Analog
=========================

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 Analog
=============================

This corresponds to the driver read call.  After validating the minor number
and arguments, this call reads the indicated device.  Most analog devices store
the last value written to a DAC.  Since DACs are output only devices, saving
the last written value gives the appearance that DACs can be read from also.
If the device is an ADC, then it is sampled.

.. note::

   Many boards have multiple analog inputs but only one ADC.  On these boards,
   it will be necessary to provide some type of mutual exclusion during reads.
   On these boards, there is a MUX which must be switched before sampling the
   ADC.  After the MUX is switched, the driver must delay some short period of
   time (usually microseconds) before the signal is stable and can be sampled.
   To make matters worse, some ADCs cannot respond to wide voltage swings in a
   single sample.  On these ADCs, one must do two samples when the voltage
   swing is too large.  On a practical basis, this means that the driver
   usually ends up double sampling the ADC on these systems.

The value returned is a single precision floating point number representing the
voltage read.  This value is stored in the ``argument_block`` passed in to the
call.  By returning the voltage, the caller is freed from having to know the
number of bits in the analog and board dependent conversion algorithm.

Write to a Particular Analog
============================

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 a single precision floating point number representing the
voltage to be written to the specified DAC.  This value is stored in the
``argument_block`` passed in to the call.  By passing the voltage to the device
driver, the caller is freed from having to know the number of bits in the
analog and board dependent conversion algorithm.

Reset DACs
==========

This is one of the IOCTL functions supported by the I/O control device driver
entry point.  When this IOCTL function is invoked, all of the DACs are written
to 0.0 volts.

Reinitialize DACS
=================

This is one of the IOCTL functions supported by the I/O control device driver
entry point.  When this IOCTL function is invoked, all of the DACs are written
with the initial value configured for this device.

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 DAC

- timestamp of when the last write was performed