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diff --git a/bsp-howto/analog.rst b/bsp-howto/analog.rst new file mode 100644 index 0000000..ac8ddb0 --- /dev/null +++ b/bsp-howto/analog.rst @@ -0,0 +1,165 @@ +.. 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 |