Structure of Device Driver System Object

A device driver block is a specialized MATLAB System block that generates custom C/C++ code when deployed to a Raspberry Pi^® hardware board. Device driver blocks provide easy access to features of hardware boards, such as communication protocols or hardware libraries, not included in the default Simulink Support Package for Raspberry Pi Hardware.

A Simulink^® device driver block can be generalized to one of two groups based on their port types:

Source blocks have output ports only. During simulation, a source block outputs a predetermined signal. This signal can be created from a random number generator, a stored array, or a constant value. When deployed to hardware, a source block outputs data captured from the I/O device that the block represents. For example, when deployed to hardware, the digital read block outputs the logical state of the digital I/O pin.
Sink blocks have input ports only. During simulation, a sink block acts as a Terminator block. When deployed to hardware, the sink block sets and updates the I/O device according to the input port signals. For example, when deployed to hardware, the digital write block sets and updates the logical state of the digital I/O pin equal to the input port signal.

To define the behavior of the device driver block, you use a System object™. Then you use a MATLAB System block to reference the object and include it in a model. Each System object uses the setupImpl, stepImpl, and releaseImpl methods to define the code initialization, pin output behavior, and code termination for the device driver block. Through conditional statements in the stepImpl, the device driver block operates in simulation mode. This mode enables the entire model to run on the host computer either when testing or when you do not have access to a Raspberry Pi. System objects also provide services for adding build artifacts. Such artifacts include source files, include paths, shared libraries, and preprocessor defines, to the Simulink generated code. These artifacts automatically define the port properties of a block and generate a block mask.

Class Definition

At the top of the System object code, you define the name of your System object and the classes it inherits from.

classdef classname < matlab.System & coder.ExternalDependency

	...

end

All System objects must inherit from matlab.System. In addition, device driver System objects inherit from coder.ExternalDependency that provides API’s to add build artifacts to generated code.

Constructor Method

The template System object class, used in the Create a Digital Write Block and Create a Digital Read Block guides, uses a standard constructor method:

methods
   % Constructor
   function obj = classname(varargin)
      coder.allowpcode('plain');
      setProperties(obj,nargin,varargin);
   end

   ...

end

Initialization, Output, and Termination Methods

These methods define what happens at initialization, output, and termination. Use setupImpl to initialize the hardware peripheral. Use stepImpl to read from or write to the hardware peripheral. Use releaseImpl to release hardware resources used. These three methods are the backbone of defining the behavior of a device driver block.

methods (Access=protected)
   function setupImpl(obj)
      % Implement tasks that need to be performed only once
   end

   function stepImpl(obj,u)
      % Device driver output
   end

   function releaseImpl(obj)
      % Termination code
   end
end

Input and Output Signal Properties

This code section defines the number of inputs or outputs of a block and the data types and sizes. For example, the getNumInputsImpl method in a sink block, sets the number of input ports. Similarly, the getNumOutputsImpl method in a source block sets the number of outputs ports.

methods (Access=protected)
   % Simulink functions
   function isInputSizeMutableImpl(~,~)
      flag = false;
   end

   ...

   function num = getNumInputsImpl(~)
      num = 1;
   end

   function num = getNumOutputsImpl(~)
      num = 0;
   end
end

Build Artifacts

The build artifacts define the source file locations, include paths, shared libraries, library search paths, and preprocessor definitions required to compile the device driver code. Use the getDescriptiveName method to define an identification string to the System object. The code generation engine uses this character vector to report errors. Use the isSupportedContext method to specify the code generation context. In device driver blocks, only the real-time workshop (rtw) code generation context applies, so this function always specifies 'rtw'. Use the updateBuildInfo method to specify source and header files, include paths, libraries, and defines required to build the System object.

methods (Static)
   
   function name = getDescriptiveName()
      name = 'decriptive name';
   end

   function tf = isSupportedContext(context)
      tf = context.isCodeGenTarget('rtw');
   end

   function updateBuildInfo(buildInfo, context)
        if context.isCodeGenTarget('rtw')
            % Update buildinfo
            srcDir = fullfile(fileparts(mfilename('fullpath')),'src'); %#ok<NASU>
            includeDir = fullfile(fileparts(mfilename('fullpath')),'include');
            addIncludePaths(buildInfo,includeDir);
            % Use the following API's to add include files, sources and
            % linker flags
            %addIncludeFiles(buildInfo,'source.h',includeDir);
            %addSourceFiles(buildInfo,'source.c',srcDir);
            %addLinkFlags(buildInfo,{'-lSource'});
            %addLinkObjects(buildInfo,'sourcelib.a',srcDir);
            %addCompileFlags(buildInfo,{'-D_DEBUG=1'});
            %addDefines(buildInfo,'MY_DEFINE_1')
   end
end