source: rtems-docs/bsp_howto/analog.rst @ 548f844

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

.. COMMENT: COPYRIGHT (c) 1988-2002.

.. COMMENT: On-Line Applications Research Corporation (OAR).

.. COMMENT: All rights reserved.