18. 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.

18.1. 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.

18.2. 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.

18.3. 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.

18.4. 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.

18.5. 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.

18.6. 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.

18.7. 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.

18.8. 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.

18.9. 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.

18.10. 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