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188 lines
6.6 KiB
ReStructuredText
Discrete Driver
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###############
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The Discrete driver is responsible for providing an
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interface to Discrete Input/Outputs. The capabilities provided
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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
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bits of discrete I/O on a single card. This driver model is centered
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on the notion of reading bitfields from the card.
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There are currently no discrete I/O device drivers included in the
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RTEMS source tree. The information provided in this chapter
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is based on drivers developed for applications using RTEMS.
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It is hoped that this driver model information can form the
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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
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RTEMS Device Address Table.
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A *minor* number is associated with each device instance
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managed by a particular device driver. An RTEMS minor number
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is an ``unsigned32`` entity. Convention calls for
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dividing the bits in the minor number down into categories
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that specify a particular bitfield. This results in categories
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like the 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
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bitfield is 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
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can support multiple copies of the same board in a single system.
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The minor number is used to distinguish the devices.
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By providing a way to easily access a particular bitfield from
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the device driver, the application is insulated with knowing how
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to mask fields in and out 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
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fields are common across different drivers. The discrete I/O driver
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configuration table is typically an array of structures with each
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structure containing the information for a particular board.
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The following is a list of the type of information normally required
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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
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word on the board during initialization. This allows the driver
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to start with the 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
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whatever board initializatin is required and then set all
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outputs to their configured initial state.
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The discrete I/O driver may register a device name for bitfields of
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particular interest to the system. Normally this will be restricted
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to the names of each 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
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to do this 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
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where that 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
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number and arguments, this call reads the indicated bitfield. A
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discrete I/O devices may have to store the last value written to
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a discrete output. If the bitfield is output only, saving the last
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written value gives the appearance that it can be read from also.
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If the bitfield is input, then it is sampled.
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*NOTE:* Many discrete inputs have a tendency to bounce. The application
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may have to take account for bounces.
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The value returned is an ``unsigned32`` number
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representing the bitfield read. This value is stored in the``argument_block`` passed in to the call.
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*NOTE:* Some discrete I/O drivers have a special minor number
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used to access all discrete I/O bits on the board. If this special
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minor is used, then the area pointed to by ``argument_block`` must
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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
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number and arguments, this call writes the indicated device. If the
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specified device is an ADC, then an error is usually returned.
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The value written is an ``unsigned32`` number
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representing the value to be written to the specified
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bitfield. This value is stored in the``argument_block`` passed in to the call.
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*NOTE:* Some discrete I/O drivers have a special minor number
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used to access all discrete I/O bits on the board. If this special
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minor is used, then the area pointed to by ``argument_block`` must
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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
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device driver entry point. When this IOCTL function is invoked,
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the discrete outputs are disabled.
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*NOTE:* It may not be possible to disable/enable discrete output on all
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discrete I/O 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
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device driver entry point. When this IOCTL function is invoked,
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the discrete outputs are enabled.
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*NOTE:* It may not be possible to disable/enable discrete output on all
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discrete 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
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device driver entry point. When this IOCTL function is invoked,
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the discrete outputs are rewritten with the configured initial
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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
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device driver entry point. When this IOCTL function is invoked,
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the following information is 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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