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@c  analog.t,v 1.5 2002/01/17 21:47:44 joel Exp

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@chapter 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:

@itemize @bullet

@item Initialize an Analog Board

@item Open a Particular Analog

@item Close a Particular Analog

@item Read from a Particular Analog

@item Write to a Particular Analog

@item Reset DACs

@item Reinitialize DACS

@end itemize

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 by OAR Corporation personnel

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.

@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

dividing the bits in the minor number down into categories

like the following:

@itemize @bullet

@item @b{board} - indicates the board a particular device is located on

@item @b{port} - indicates the particular device on a board.

@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.

@section 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:

@table @b

@item board_offset

is the base address of a board.

@item 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.

@end table

@section 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.

@section 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.

@section 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.

@section 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.

@b{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

@code{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.

@section 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 @code{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.

@section 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.

@section 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.

@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 DAC

@item timestamp of when the last write was performed

@end itemize