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diff --git a/bsp-howto/discrete.rst b/bsp-howto/discrete.rst new file mode 100644 index 0000000..6fbc51e --- /dev/null +++ b/bsp-howto/discrete.rst @@ -0,0 +1,190 @@ +.. 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 |