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Configuring the Data Interface

This example shows how to specify signals and parameters for inclusion in generated code.

Time: 45 minutes

Goals

Learn how to control the following attributes of signals and parameters in the generated code:

  • Name

  • Data type

  • Data storage class

Task: Open the model.Task: Open the model.

Background: Declaration of Data

Most programming languages require that you declare data and functions before using them. The declaration specifies:

  • Scope - The region of the program that has access to the data

  • Duration - The period during which the data is resident in memory

  • Data type - The amount of memory allocated for the data

  • Initialization - A value, a pointer to memory, or NULL

The combination of scope and duration is the storage class. If you do not provide an initial value, most compilers assign a zero value or a null pointer.

Supported data types include:

  • double - Double-precision floating point

  • single - Single-precision floating point

  • int8 - Signed 8-bit integer

  • uint8 - Unsigned 8-bit integer

  • int16 - Signed 16 bit integer

  • uint16 - Unsigned 16 bit integer

  • int32 - Signed 32 bit integer

  • uint32 - Unsigned 32 bit integer

  • Fixed Point - 8-, 16-, 32-bit word lengths

Supported storage classes include:

Name             Description                                 Parameters  Signals    Data
                                                             Supported   Supported  Types
Const            Use const type qualifier in declaration     Y           N          All
ConstVolatile    Use const volatile type qualifier in        Y           N          All
                 declaration
Volatile         Use volatile type qualifier in declaration  Y           Y          All
ExportToFile     Generate and include files, with            Y           Y          All
                 user-specified name, containing global
                 variable declarations and definitions
ImportFromFile   Include predefined header files containing  Y           Y          All
                 global variable declarations
Exported Global  Declare and define variables of global      Y           Y          All
                 scope
Imported Extern  Import a variable that is defined outside   Y           Y          All
                 of the scope of the model
BitField         Embed boolean data in a named bit field     Y           Y          Boolean
Define           Represent parameters with a #define macro   Y           N          All
Struct           Embed data in a named struct  to            Y           Y          All
                 encapsulate sets of data

Controlling Data in Simulink® and Stateflow®

Two methods are available for declaring data in Simulink® and Stateflow®: data objects and direct specification. This example uses the data object method. Both methods allow full control over the data type and storage class. You can mix the two methods in a single model.

You can use data objects in a variety of ways in the MATLAB® and Simulink® environment. The example focuses on three types of data objects.

  • Signal

  • Parameter

  • Bus

The code generator uses data objects from the MATLAB base workspace. You can create and inspect them by entering commands in the MATLAB Command Window or by using the Model Explorer.

The following example shows the definition of Simulink® signal object pos_cmd_one:

You can open the Model Explorer and display details about a specific data object in the example model.

Click each of the following object names:

(1) ThrottleCommands defines a Simulink® Bus object, ThrotComm is the instantiation of the bus. If the bus is a nonvirtual bus, the signal will generate a structure in the C code.

As in C, you can use a bus definition (ThrottleCommands) to instantiate multiple instances of the structure. In a model diagram, a bus object appears as a wide line with central dashes, as shown below.

The following figure shows the Model Explorer display if you click the signal name pos_rqst:

A data object has a mixture of active and descriptive fields. Active fields affect simulation or code generation. Descriptive fields do not affect simulation or code generation, but are used with data dictionaries and model-checking tools.

Active Fields

  • Data type

  • Storage class

  • Value (parameters)

  • Initial value (signals)

  • Alias (define a different name in the generated code)

  • Dimension (inherited for parameters)

  • Complexity (inherited for parameters)

Descriptive Fields

  • Minimum

  • Maximum

  • Units

  • Description

Adding New Data Objects

You can create data objects for named signals, states, and parameters. To associate a data object with a construct, the construct must have a name.

The Data Object Wizard is a tool that finds constructs for which you can create data objects, then creates the objects for you. The example model includes two signals that are not associated with data objects: fbk_1 and pos_cmd_two.

To find the signals and create data objects for them:

1. Task: Open the Data Object Wizard.Task: Open the Data Object Wizard.

2. Click Find to find candidate constructs.

3. Click Select All to select all candidates.

4. Click Apply Package to apply the Simulink® package for the data objects.

5. Click Create to create the data objects.

Configuring Data Objects

The next step is to set the data type and storage class.

1. Click the following object names to edit the data objects:

Clicking a name opens the Model Explorer to show the base workspace.

2. For each object listed in the preceding table, click the signal name in the Contents pane.

3. Change the field settings in the Data pane to match those in the table.

Note: If the Model Explorer does not open for either of the two signals, repeat the steps in Adding New Data Objects.

Controlling File Placement of Parameter Data

Embedded Coder® allows you to control the files in which the parameters and constants are defined. For the example model, all parameters were written to the file eval_data.c.

To change the placement of parameter and constant definitions, set the appropriate data placement options for the model configuration. Within the Model Explorer, you set the options at Configuration > Code Generation > Data Placement. For the example, the model has already been configured.

1. Task: Open the Data Placement pane of the Configuration Parameters dialog.Task: Open the Data Placement pane of the Configuration Parameters dialog.

2. Enter data in the Data Placement pane as shown in the following figure:

The generated code that results for eval_data.c is shown below.

Enabling Data Objects in Generated Code

The next step is to ensure that the data objects you have created appear in the generated code. To enable parameters in generated code, set the Inline parameters option for the model configuration. In the Model Explorer, this option is:

Configuration > Optimization > Signals and Parameters > Simulation and code generation > Inline parameters

Task: Set the Inline parameters option.Task: Set the Inline parameters option.

To enable a signal in generated code:

1. Right-click the signal line.

2. From the context menu, select Signal Properties. A Signal Properties dialog box appears.

3. Make sure the option Signal name must resolve to a Simulink signal object is selected.

You can enable signals associated with data objects individually, or you can enable all such signals in a model at once by entering disableimplicitsignalresolution in the MATLAB Command Window.

Task: Enable all signals with associated data objects.Task: Enable all signals with associated data objects.

Effects of Simulation on Data Typing

For the example model, all data types are set to double. Since Simulink® uses the double data type for simulation, no changes are expected in the model behavior when you run the generated code. To verify this, run the test harness model. The test harness model is automatically updated to include the rtwdemo_PCG_Eval_P2 model. That is the only change made to the test harness.

Task: Open the test harness.Task: Open the test harness.

Task: Run the test harness.Task: Run the test harness.

The resulting plot shows that the difference between the golden and simulated versions of the model remains zero.

Viewing Data Objects in Generated Code

Now view the file rtwdemo_PCG_Eval_P2.c to see how the use of data objects changed the generated code.

Task: Generate code for the model.Task: Generate code for the model.

Click the following file names to view generated code:

The following figure shows the code for function rtwdemo_PCG_Eval_P2_step as it appears in rtwdemo_PCG_Eval_P2.c before the use of data objects:

The figure below shows the code as it appears in rtwdemo_PCG_Eval_P2.c with data objects.

This figure shows that most of the Simulink® Coder™ data structures have been replaced with user-defined data objects. The local variable rtb_Sum2 and the state variable rtwdemo_PCG_Eval_P2_DWork.Discrete_Time_Integrator1_DSAT still use the Simulink® Coder™ data structures.

Data Management

Data objects exist in the MATLAB base workspace. They are saved in a separate file from the model. To save the data manually, enter save in the MATLAB Command Window.

The separation of data from the model provides many benefits.

One model, multiple data sets

  • Use of different data types to change the targeted hardware (for example, for floating-point and fixed-point targets)

  • Use of different parameter values to change the behavior of the control algorithm (for example, for reusable components with different calibration values)

Multiple models, one data set

  • Sharing of data between Simulink® models in a system

  • Sharing of data between projects (for example, transmission, engine, and wheel controllers might all use the same CAN message data set)

Further Study Topics

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