Variant Subsystem

Template subsystem containing Subsystem blocks as variant choices

• 

Libraries:
Simulink / Ports & Subsystems
HDL Coder / Ports & Subsystems

Alternative Configurations of Variant Subsystem Block:
Variant Model | Variant Assembly Subsystem

Description

The Variant Subsystem block enables you to include multiple implementations of a component in a separate hierarchy.

For example, suppose you want to simulate a model that represents a vehicle with three possible engine configurations: 2-cylinder gas, 4-cylinder gas, and 8-cylinder gas. You could implement each engine model as a separate subsystem inside the Variant Subsystem block and then switch between the subsystems based on the Types of Variant Control Modes in Variant Blocks you select. For more information, see Introduction to Variant Controls.

Note

Only one implementation of a Variant Subsystem block is active during simulation.

The Variant Subsystem block is a template preconfigured to contain two or more blocks to use as variant choices. These choices represent multiple implementations of a system. Only one child block is active during model execution. The active child block is referred to as the active variant.

Each variant choice inside the Variant Subsystem block is associated with a Variant control mode and a Variant activation time. The variant control mode determines how to select the active variant. The variant activation time determines a time to activate the choices and to decide whether to include only the active choice or both active and inactive choices in the generated code.

A Variant Subsystem block can contain a mixture of Subsystem blocks, Model blocks, or Subsystem Reference blocks as variant choices. A Variant Subsystem block with only Model blocks as choices is called a Variant Model block. For more information, see Implement Variations in Separate Hierarchy Using Variant Subsystems. A Variant Subsystem block with Model or Subsystem Reference blocks as choices that can be added or removed only from external files and not within the block is called a Variant Assembly Subsystem block.

Apart from the variant choices, you can include Inport, control port (Enable, Trigger, Reset, and Function-Call Subsystem), Outport, or Connection Port (Simscape) blocks inside a Variant Subsystem block. There are no drawn connections between the blocks that are inside the Variant Subsystem blocks. The software automatically wires the active variant to the Inport and Outport blocks of the Variant Subsystem during model compilation. The blocks representing variant choices can have input and output ports that differ in number from the input and output ports in the parent Variant Subsystem block. For more information, see Map Input and Output Ports of Variant Choices in Variant Subsystem.

When you load a model with a Variant Subsystem block that has variant choices stored in subsystem files, by default, only the active choice is loaded to optimize the loading time for models. However, if you initialize a model programmatically using the find_system, get_param, or set_param function, inactive choices might get loaded.

Using Variant Subsystem block allows you to:

• Mix Subsystem, Model, and Subsystem Reference blocks as variant choices.

• Model components that do not have similar interface. You can have different numbers of inports and outports for multiple variant choices inside a Variant Subsystem. However, the conditions listed in Map Input and Output Ports of Variant Choices in Variant Subsystem must be met.

• Establish a hierarchical block diagram, where the Variant Subsystem block is on one layer and the variant choices are on another layer.

• Keeps functionally related blocks together.

• Reduce the complexity of your model.

• During model compilation, the software eliminates inactive blocks throughout the model, depending on the variant activation time you select.

The color and icon of the variant badge on the block icon changes depending on the values of the Variant activation time, Variant control mode, and Propagate conditions outside of variant subsystem parameters set on the block. For more information, see Variant Badges.

Examples

Implement Variations in Separate Hierarchy Using Variant Subsystems

Learn basic functionality of variant subsystems.

• Open Script

Variant Subsystem: Using Variant Subsystems for HDL Code Generation

2.4.3

(HDL Coder)

Ports

During simulation, the software disables the inactive ports in a Variant Subsystem block.

Input

In_1 — Input port corresponding to root-level Inport blocks in variant subsystem

same data types accepted by Inport blocks

Each Subsystem, Model, or Subsystem Reference block contained within a Variant Subsystem represents one variant choice. The variant choices can contain Inport, control port (Enable, Trigger, Reset, and Function-Call Subsystem), or Connection Port (Simscape) blocks at its input. The variant choices can have input ports that differ in number from the input ports in the parent Variant Subsystem block. However, the conditions described in Map Input and Output Ports of Variant Choices in Variant Subsystem must be met.

Output

Out_1 — Output port corresponding to root-level Outport blocks contained in variant subsystem

same data types accepted by Outport blocks

Each Subsystem, Model, or Subsystem Reference block contained within a Variant Subsystem represents one variant choice. The variant choices can contain Outport or Connection Port (Simscape) blocks at its output. The variant choices can have output ports that differ in number from the output ports in the parent Variant Subsystem block. However, the conditions described in Map Input and Output Ports of Variant Choices in Variant Subsystem must be met.

The unconnected output ports (inactive output ports) of a Variant Subsystem block output a ground value. To output a value other than ground, select the Specify output when source is unconnected parameter of the Outport block and specify a value.

Map Input and Output Ports of Variant Choices in Variant Subsystem

A Variant Subsystem block allows you to use a combination of Subsystem, Model, or Subsystem Reference blocks as variant choices. The inputs that the Variant Subsystem block receives from upstream model components map to the input and output ports of the variant choices.

The blocks that represent variant choices can have input and output ports that differ in number from the input and output ports in the parent Variant Subsystem block. However, the following conditions must be met:

• The variant choices must have same set of input ports as the Variant Subsystem container block or it must be a subset of ports on the container block.

• The variant choices must have same set of output ports as the Variant Subsystem container block or it must be a subset of ports on the container block.

• If the Variant Subsystem container block has control ports:

  • The type of control port blocks in all the variant choices must be same as the Variant Subsystem block. For example, you cannot use Enabled Subsystem and Function-Call Subsystem blocks as choices within a Variant Subsystem block.

  • The control port on the Variant Subsystem block and the corresponding control ports on its variant choices must have the same name. For example, if the name of the control port on the Variant Subsystem block is fcn, then the name of the corresponding control ports on all its variant choices must also be fcn.

Parameters

To access these parameters, right-click the variant badge on the block icon and select Block Parameters. For more information, see Variant Badges.

Variant control mode — Variant control mode

expression (default) | label | sim codegen switching

The variant control that determines the active variant choice can be any of these types.

• expression — The software chooses the active variant based on the evaluation of the variant conditions. When a condition expression evaluates to true, the corresponding variant choice becomes active. When a condition expression evaluates to false, the corresponding variant choice becomes inactive. See Switch Between Choices Using Condition Expressions in Variant Blocks.

• label — The software chooses the active variant based on the name of the variant. The variant control is a string and does not require you to create any variable in any workspaces. See Switch Between Choices Using Labels in Variant Blocks.

• sim codegen switching — Automatically switch between the variants for simulation and code generation workflows without creating any workspace variable. When you simulate a model, the software automatically chooses the sim branch as the active choice. Similarly, when you do a software-in-the-loop (SIL) or processor-In-Loop (PIL) simulation, generate code, or use external mode, the software automatically chooses the codegen branch. See Switch Between Choices for Simulation and Code Generation Workflows Without Using Control Variables in Variant Blocks.

For more information on variant control modes, see Introduction to Variant Controls. For a comparison between different types of variant control modes, see Compare Different Types of Variant Control Modes in Variant Blocks.

Dependencies

The availability of different variant activation times depends on the type of the Variant control mode that you specify. The Variant activation time parameter determines the time when the software sets the active choice. The parameter also determines which variability to include in the generated code. This table explains the variant activation time supported by each variant control mode.

Variant activation time
Variant control mode	update diagram	update diagram analyze all choices	code compile	startup	runtime (Variant Subsystem block only)
expression	✓	✓	✓	✓	✓
label	✓	x	x	x	x
sim codegen switching	✓	✓	x	x	x

Programmatic Use

Block Parameter: VariantControlMode

Type: character vector

Values: 'expression' | 'label' | 'sim codegen switching'

Default: 'expression'

Variant activation time — Time when software chooses active variant choice

update diagram (default) | update diagram analyze all choices | code compile | startup | runtime | inherit from Simulink.VariantControl

This parameter determines which variability to include in the simulation and code generation workflows. For more information, see Activate Variant During Different Stages of Simulation and Code Generation Workflow.

• update diagram — When you execute the model, only the active choice is included in the simulation and the code generation workflow. Generated code contains only the active choice.

• update diagram analyze all choices — When you execute the model, both active and inactive choices are analyzed for consistency across the model. However, only the active choice is included in the simulation and the code generation workflow. Generated code contains only the active choice.

• code compile — When you execute the model, both active and inactive choices are analyzed for consistency across the model, and all choices are included in the simulation and the code generation workflow. Generated code contains both the active and inactive choices enclosed in the preprocessor conditionals #if and #elif. However, the executable built from the generated code contains only active choice.

• startup — When you execute the model, both active and inactive choices are analyzed for consistency across the model, and all choices are included in the simulation and the code generation workflow. With this option, you can improve the speed of iterative simulations using fast restart. For more information, see Run Iterative Simulations Without Recompiling Model for Variant Systems Using Fast Restart. Code generated from the model contains both the active and inactive choices that are enclosed in regular if conditions. The executable built from the generated code also contains both active and inactive choices.

• runtime — When you execute the model, the active variant can switch during simulation or execution of the generated code by using the Parameter Writer block. Place a Parameter Writer block inside a conditionally executed subsystem or in an Initialize Function block, a Reinitialize Function block, or a Reset Function block. For more information on the blocks, see Event Functions. You can use the Parameter Writer block to write to block parameters, model workspace variables, and base workspace variables. Multiple variant control variables are not supported for runtime activation time. All modeling patterns supported by the Parameter Writer block are valid for run-time activation. For more information, see Common Uses of Parameter Writer Block.

  When you set the Variant activation time parameter of the Variant Subsystem block to runtime, the color of the inactive variant choices does not fade. In this case, the setting of the Variant Fading option in the Information Overlays menu on the Debug tab of the Simulink Editor is ignored.

• inherit from Simulink.VariantControl — When you execute the model, the block inherits the activation time from its variant control variables of type Simulink.VariantControl. If a variant block has multiple variant control variables of type Simulink.VariantControl, then all those variables must have the same activation time.

The table compares the different variant activation times based on conditional code, variant switching, handling of states, and fast restart support.

	update diagram or update diagram analyze all choices	code compile	startup	runtime
Generated conditional code	Inactive conditional code is not available.	Conditional code is present inside preprocessor macros. The inactive choice code is eliminated when generated code is compiled.	Conditional code is generated as part of an if-elseif-else statement. The variant control variable value cannot be modified during execution.	Conditional code is generated as part of an if-elseif-else statement. The variant control variable value can be modified during execution.
Variant switching during simulation or code execution	Not supported.	Not supported.	Supported at startup (before simulation start at t = 0).	Supported. The active variants are switched by changing the variant control variable using the Parameter Writer block.
Explicit setting of held or reset option for states	Not supported. States are reset for inactive blocks.	Not supported. States are reset for inactive blocks	States cannot be changed during simulation. In the generated code, if you attempt to change the variant, states are in held state.	States are held by default. Reset states explicitly through State Writer blocks. Read blocks with states by using the State Reader block and transfer to another choice by using the State Writer block.
Executable size	Smaller size. Code is not generated for the inactive variant.	Smaller size. The inactive variant code is removed during compilation.	Comparatively larger size. Both active and inactive variant code is available for startup switching.	Comparatively larger size. Both active and inactive variant code is available for run-time switching.
Fast restart support	Not supported.	Not supported.	Supported.	Supported.

Dependencies

• The availability of different variant activation times depends on the type of the variant control mode that you specify. The Variant activation time parameter determines when the software sets the active choice. The parameter also determines which variability to include in the generated code. This table explains the variant activation time supported by each variant control mode.

  Variant activation time
  Variant control mode	update diagram	update diagram analyze all choices	code compile	startup	runtime
  expression	✓	✓	✓	✓	✓
  label	✓	x	x	x	x
  sim codegen switching	✓	✓	x	x	x

• To enable switching of active variant choice of the Variant Subsystem block using the runtime activation time, you must use the Parameter Writer block. Set the Destination parameter of the Parameter Writer block based on the location of your variant control variable and select Validate parameter :

  • Block parameter — Control variable is a mask parameter in a subsystem.

  • Model workspace variable — Control variable is located in the model workspace.

  • Base workspace variable — Control variable is located in the base workspace or a data dictionary.

  For example, you can create a mask parameter V in a subsystem and use it in a variant condition expression for a variant control. The Parameter Writer block writes to the value of V in the variant control condition.

Programmatic Use

Block Parameter: VariantActivationTime

Type: character vector

Values: 'update diagram' | 'update diagram analyze all choices' | 'code compile' | 'startup'| 'runtime'

Default: 'update diagram'

Variant control variable — Variable to determine active variant choices

Simulink.VariantControl object | scalar variable | Simulink.Parameter object | struct

When you specify a variable of type listed in Types of Variant Control Variables (Operands) in Variant Blocks, the software generates variant control expressions for the variant choices of the Variant Assembly Subsystem block. The Variant control variable that you specify here is on the left side and the members of the Variant enumeration choice are on the right side of the generated expressions. Both the sides are related by ==.

The variant control variable acts as a switch to change the active variant choice. You specify a value for a variant control variable and run the simulation. During simulation, the software evaluates the variant control expressions to determine the active variant choice. When an expression evaluates to true, the corresponding variant choice is set active. To change the active variant, change the value of the variant control variable such that the corresponding variant control expression evaluates to true turns active.

Dependencies

This parameter is available only in Variant Assembly Subsystem block. To enable this parameter, set Variant control mode to expression.

Programmatic Use

Block Parameter: VariantControlVariable

Type: character vector

Values: Simulink.VariantControl object, scalar variable, Simulink.Parameter object, struct

Default: ''

Variant choices enumeration — Enumeration class to add variant choices

enumeration class with model or subsystem filenames as its members

To add Model or Subsystem Reference blocks as variant choices to a Variant Assembly Subsystem block, specify an enumeration class name that contains the corresponding model or subsystem filenames as its members. You can then validate the enumeration class using the Refresh button in the Variant choices table. On a successful validation, the members of the enumeration class are added as variant choices to the Variant Assembly Subsystem block. Each choice has a variant control expression with Variant control variable on its left side and the members of the Variant choices enumeration that you specify here on its right side. Both the sides of the expression are related by ==. The newly added variant choices and their corresponding control expressions are displayed in the Variant choices table.

To remove variant choices from a Variant Assembly Subsystem block, remove the corresponding members from the enumeration class that you specify here. You cannot manage the variant choices from inside the Variant Assembly Subsystem block.

The enumeration class that you specify must be on the search path of your current MATLAB^® session. For information on how to add files or folders to the search path, see addpath.

For more information on how to add choices using the Variant choices enumeration parameter, see Add or Remove Variant Choices of Variant Assembly Subsystem Blocks Using External Files.

Dependencies

This parameter is available only in Variant Assembly Subsystem block. To enable this parameter, set Variant control mode to expression.

Programmatic Use

Block Parameter: VariantChoicesEnumeration

Type: character vector

Values: enumeration class defined using classdef or Simulink.defineIntEnumType

Default: ''

Variant choices specifier — MATLAB expression to add variant choices

any MATLAB expression that evaluates to a one-dimensional cell array of model or subsystem filenames, or filename patterns that match the model or subsystem filenames or an array of strings

To add Model or Subsystem Reference blocks as variant choices to a Variant Assembly Subsystem block, specify the corresponding model or subsystem filenames. The filenames must be specified as a MATLAB expression that returns a one-dimensional cell array of character vectors when evaluated or an array of strings. You can then validate the expression using the Refresh button in the Variant choices table. On a successful validation, the blocks that correspond to the specified files are added as variant choices to the Variant Assembly Subsystem block. For each choice, a variant control label is generated. The name of the variant control label is same as the name of the choice. The newly added variant choices and their corresponding control labels are displayed in the Variant choices table.

To remove variant choices from a Variant Assembly Subsystem block, modify the expression specified in this parameter. You cannot manage the variant choices of a Variant Assembly Subsystem block from inside the block.

The MATLAB expression that you specify must contain either of these:

• Model or subsystem filenames that are on the search path of your current MATLAB session. For example, if you specify {'model1.slx', 'subsystemRef1.slx'} as a specifier, the blocks that correspond to the model1.slx and subsystemRef1.slx files are added as variant choices of the Variant Assembly Subsystem block.

• File name patterns that match the model or subsystem filenames on the MATLAB path. The paths in the pattern can be absolute or relative to the parent folder of the model that contains the Variant Assembly Subsystem block. For example, if you specify {'folder_1/*.slx', 'folder_2/*.slx'} as a specifier, the software passes the character vectors folder_1/* and folder2/* to the dir function. The function then returns the model and the subsystem files that are located inside the folder1 and folder2 folders. The blocks that correspond to the returned files are added as variant choices of the Variant Assembly Subsystem block.

For information on how to add files or folders to the search path, see addpath.

For more information on how to specify choices in the Variant choices specifier parameter, see Add or Remove Variant Choices of Variant Assembly Subsystem Blocks Using External Files.

Dependencies

This parameter is available only in Variant Assembly Subsystem block. To enable this parameter, set Variant control mode to label.

Programmatic Use

Parameter: VariantChoicesSpecifier

Type: character vector

Value: Any MATLAB expression that evaluates to a one-dimensional cell array of filenames or filename patterns.

Default: ''

Variant choices — Table of variant choices, variant controls, and conditions

empty table (default)

The table has a row for each variant choice contained in the variant subsystem. If there are no variant choices, the table is empty.

You can use buttons to the left of the Variant choices table to modify the elements in the table.

To...	Click...
Create and add a new subsystem choice: Create a new Subsystem block as a variant choice and add an entry for the new choice in the table.
Create and add a new model variant choice: Create a new Model block as a variant choice and add an entry for the new choice in the table.
Delete selected variant choices: Delete variant choices permanently.
Create/Edit selected variant object: Create or edit a Simulink.VariantExpression object in the global workspace and specify the variant condition using the Simulink.VariantExpression object parameter dialog box. Note For a model that uses the base workspace, this operation creates the Simulink.VariantExpression object in the base workspace and the object is available only for the current MATLAB session. To permanently store the data, save the object in a MAT file or MATLAB script.
Open selected variant choice block: Open the Subsystem block for the selected row.
Refresh dialog information from Variant Subsystem contents: Update the Variant choices table according to the variant choices and values of the variant control in the global workspace.

Dependencies

The Variant Assembly Subsystem block allows you to only open the selected variant choice and refresh the list of variant choices using the buttons in this section. To add new variant choices to the Variant Assembly Subsystem block, see Add or Remove Variant Choices of Variant Assembly Subsystem Blocks Using External Files.

Name (read-only) — Variant choice name

Read-only: Subsystem and Subsystem1 (default) | name of choices contained in Variant Subsystem

This parameter is read-only.

This parameter displays the name of the choices. Each choice has a variant control in the form of labels, expressions, or switches. For Variant Assembly Subsystem in the label mode, no separate variant control is available. The name of the choice is used as a variant control to set the active choice.

Variant control label — Name of choice in label mode

true or false (default) | string

A name for a choice, specified as a string.

Dependencies

To enable this parameter, set Variant control mode to label.

Programmatic Use

To set the block parameter value programmatically, use the set_param function.

To get the block parameter value programmatically, use the get_param function.

Parameter:	VariantControl
Values:	variant control that is associated with the variant choice
Data Types:	char

Example: set_param(gcb, 'VariantControl', 'V == 3'), where gcb is the variant choice of the Variant Subsystem block.

Example: get_param(gcb, 'VariantControl'), where gcb is the variant choice of the Variant Subsystem block.

Variant control switch — Variant choices for simulation and code generation workflows

(sim) | (codegen)

Specify (sim) to activate the corresponding variant choice during simulation. Specify (codegen) to activate the corresponding variant choice while performing code generation workflows such as SIL, PIL, external mode, and so on.

Dependencies

To enable this parameter, set Variant control mode to sim codegen switching.

Programmatic Use

To set the block parameter value programmatically, use the set_param function.

To get the block parameter value programmatically, use the get_param function.

Parameter:	VariantControl
Values:	variant control that is associated with the variant choice
Data Types:	char

Example: set_param(gcb, 'VariantControl', '(sim)'), where gcb is the variant choice of the Variant Subsystem block.

Example: get_param(gcb, 'VariantControl'), where gcb is the variant choice of the Variant Subsystem block.

Variant control expression — Condition expression that determines the active choice

true or false (default) | boolean condition expression | a Simulink.VariantExpression object representing a boolean condition expression | (default)

Specify the condition expression to determine the active choice. When a condition expression evaluates to true, the software activates the corresponding variant choice. When a condition expression evaluates to false, the software deactivates the corresponding variant choice.

The variant controls can be:

• Boolean condition expression for rapid prototyping. For example, A == 1, A ~= B, A && B == 1, and so on.

• A Simulink.VariantExpression object that contains a condition expression for condition reuse. See Simulink.VariantExpression Objects for Variant Condition Reuse of Variant Blocks.

• Default Variant Choice if none of the choices evaluates to true.

Here, A and B are operands called as variant control variables. ==, ~=, and && are operators in the condition expression. The condition expression can contain one or more such variant control variables and operators. For information on supported types and storage location of variant control variables, see Types of Variant Control Variables (Operands) in Variant Blocks and Storage Locations for Variant Control Variables (Operands) in Variant Blocks. For information on operators, see Types of Operators in Variant Blocks for Different Activation Times.

For more information, see Switch Between Choices Using Condition Expressions in Variant Blocks.

In Variant Assembly Subsystem block, this parameter is a list of auto-generated boolean expressions with Variant control variable on the left-hand side and the members of the Variant enumeration choice are on the right-hand side of the expressions. Both the sides of the expressions are connected with ==. This parameter is read-only.

Programmatic Use

To set the block parameter value programmatically, use the set_param function.

To get the block parameter value programmatically, use the get_param function.

Variant Subsystem block:

Parameter:	VariantControl
Values:	variant control that is associated with the variant choice
Data Types:	char

Example: set_param(gcb, 'VariantControl', 'V == 3'), where gcb is the variant choice of the Variant Subsystem block.

Example: get_param(gcb, 'VariantControl'), where gcb is the variant choice of the Variant Subsystem block.

Variant Source and Variant Sink blocks:

Parameter:	VariantControls
Values:	variant controls that are associated with variant choices
Data Types:	char

Example: set_param(gcb, 'VariantControls', {'A == 1','A == 2'}), where gcb is the Variant Sink or Variant Source block.

Example: get_param(gcb, 'VariantControls'), where gcb is the Variant Sink or Variant Source block.

Condition (read-only) — Condition expression in Simulink.VariantExpression object

Read-only: N/A (default)

This parameter is read-only.

This parameter displays the condition expression specified as Simulink.VariantExpression object. To change or edit the condition expression, use the Simulink.VariantExpression parameter dialog box that appears when you double-click the object in the workspace.

Note

The operands that you specify in a condition expression of type Simulink.VariantExpression must be defined in the base workspace or a data dictionary. Specifying operands that are defined in the mask or model workspace is not supported.

Label mode active choice — Name of active choice in label mode

true (Subsystem) (default) | false (Subsystem1)

This list contains the labels of all the variant choices. To set an active choice, select a label from the list. The corresponding choice becomes active. Alternatively, to change the active choice in label mode, you can follow the approaches described in Set Active Choices Using Variant Control Labels.

Dependencies

To enable this parameter, set Variant control mode to label.

Programmatic Use

Parameter: LabelModeActiveChoice

Type: character vector

Value: If no label mode active choice is specified, the value is empty. If a label mode active choice is specified, the value is the variant control label for the active choice.

Default: ''

Built-in empty choice — Output zero if no variant choice is active

off (default) | on

• off — Simulink^® generates an error when there is no active variant choice.

• on — When you select this option, Simulink adds a nonmodifiable variant choice entry to the Variant choices table with its variant control set to (default). This entry does not appear as a visible variant choice in the Variant Subsystem block. During simulation, if there is no active variant choice, Simulink disconnects the variant choices of the Variant Subsystem block, thus removing the variant regions completely from the model. The inactive outports of the subsystem output zero. To output a value other than zero, select the Specify output when source is unconnected parameter of the Outport block and specify a value of your choice.

For more information, see Manage Variant Components to Pass Specified Values from Inactive Variant Subsystems with No Active Choice (Simulink Coder).

Dependencies

This parameter is available only if there are no default variant choices in the Variant Subsystem and if you set Variant control mode to expression.

Programmatic Use

Parameter: EmptyChoice

Type: character vector

Value: 'off' | 'on'

Default: 'off'

Built-in passthrough choice — Assign input port values to output ports if no variant choice is active

off (default) | on

• off — Simulink generates an error when there is no active variant choice.

• on — When you select this option, Simulink adds a nonmodifiable variant choice entry to the Variant choices table with its variant control set to (default). This entry does not appear as a visible variant choice in the Variant Subsystem block. During simulation, if there is no active variant choice, Simulink bypasses the variant choices of the Variant Subsystem block. Only the input and output ports remain active, and the input port pass values directly to the output ports.

  For values to be mapped from input ports to output ports when no choice is active, the Variant Subsystem block must have:

  • Only data ports, with no other types of input and output ports

  • An equal number of data input and output ports

  • Input and output ports that exactly match the ports of variant choices

For more information, see Manage Variant Components to Pass Specified Values from Inactive Variant Subsystems with No Active Choice (Simulink Coder).

Dependencies

This parameter is available only if there are no default variant choices in the Variant Subsystem and if you set Variant control mode to expression.

Programmatic Use

Parameter: PassThrough

Type: character vector

Value: 'off' | 'on'

Default: 'off'

Propagate conditions outside of variant subsystem — Option to choose if components outside of the variant subsystem must be informed of the active and inactive state of underlying blocks

off (default) | on

When you select the Propagate conditions outside of variant subsystem parameter, the software propagates the variant condition of the underlying blocks to the Variant Subsystem container block so the subsystem can adapt its interface to the state of the underlying blocks. Ports that are mapped to the ports on the active choice becomes active. Ports that are not mapped to the ports on the inactive choice becomes inactive. Selecting this option ensures that the components outside of the Variant Subsystem are aware of the active and inactive state of blocks within the Variant Subsystem block. For more information, see Propagate Variant Conditions to Define Variant Regions Outside Variant Subsystems to Promote Consistency and Reduce Errors and Propagate Variant Conditions to Control Execution of Conditional Subsystems

Variant conditions do not propagate outside a Variant Subsystem block if all variant choices within the block have the same interface.

Programmatic Use

Parameter: PropagateVariantConditions

Type: character vector

Value: 'off' | 'on'

Default: 'off'

Allow flexible interface — Option to choose to make interface of the variant subsystem flexible

on (default) | off

Specify whether to make the interface of a variant subsystem flexible.

• on — Simulate with variant choices that have input and output ports that differ in number from the input and output ports of the Variant Subsystem block.

• off — When you simulate with this option, if there is inconsistency in the interface, the software will generate errors with fix-it actions to match the ports of the variant choices to the Variant Subsystem block.

Programmatic Use

Parameter: AllowFlexibleInterface

Type: character vector

Value: 'off' | 'on'

Default: 'on'

Block Characteristics

Data Types	Booleana | busa | doublea | enumerateda | fixed pointa | halfa | integera | singlea | stringa
Direct Feedthrough	no
Multidimensional Signals	yesa
Variable-Size Signals	yesa
Zero-Crossing Detection	no
a Actual data type or capability support depends on block implementation.

Alternative Configurations

Variant Model — Template subsystem containing Model blocks as variant choices

The Variant Model block is a template that contains Model blocks.

Libraries:
Simulink / Ports & Subsystems

Variant Assembly Subsystem — Template subsystem that supports adding or removing variant choices from external sources without model modification

Since R2022b

A Variant Assembly Subsystem block enables you to add or remove its variant choices from external sources without modifying the model. A Variant Assembly Subsystem block can contain Model blocks, Subsystem Reference blocks, or both as its variant choices. You specify the source of variant choices in the block parameter dialog box once and then modify the source to add or remove the variant choices from the block. You can also move or resize the Model or Subsystem Reference blocks inside the Variant Assembly Subsystem block. Unlike the Variant Subsystem block, you cannot manage the variant choices from inside the Variant Assembly Subsystem block. Any modifications to add or remove variant choices must be made through the specified source. The attempts to modify variant choices, such as adding or deleting choices, renaming them, or changing the variant controls associated with them, by alternative methods result in an error. For more information, see Add or Remove Variant Choices of Variant Assembly Subsystem Blocks Using External Files.

To convert a Variant Subsystem block to a Variant Assembly Subsystem block, follow the steps in Convert Variant Subsystem to Variant Assembly Subsystem.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using Simulink® Coder™.

Actual data type or capability support depends on block implementation.

HDL Code Generation

Generate VHDL, Verilog and SystemVerilog code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic. Actual data type or capability support depends on block implementation.

HDL Architecture

Architecture	Description
Module (default)	Generate code for the subsystem and the blocks within the subsystem. HDL Coder generates code for all the variant choices.
BlackBox	Generate a black-box interface. That is, the generated HDL code includes only the input/output port definitions for the subsystem. In this way, you can use a subsystem in your model to generate an interface to existing manually written HDL code. The black-box interface generated for subsystems is similar to the interface generated for Model blocks, but without generation of clock signals.
No HDL	Remove the subsystem from the generated code. You can use the subsystem in simulation but treat it as a “no-op” in the HDL code.

Black Box Interface Customization

For the BlackBox architecture, you can customize port names and set attributes of the external component interface. See Customize Black Box or HDL Cosimulation Interface (HDL Coder).

HDL Block Properties

General
AdaptivePipelining	Automatic pipeline insertion based on the synthesis tool, target frequency, and multiplier word-lengths. The default is inherit. See also AdaptivePipelining (HDL Coder).
BalanceDelays	Detects introduction of new delays along one path and inserts matching delays on the other paths. The default is inherit. See also BalanceDelays (HDL Coder).
ClockRatePipelining	Insert pipeline registers at a faster clock rate instead of the slower data rate. The default is inherit. See also ClockRatePipelining (HDL Coder).
ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline (HDL Coder).
DistributedPipelining	Pipeline register distribution, or register retiming. The default is inherit. See also DistributedPipelining (HDL Coder).
DSPStyle	Synthesis attributes for multiplier mapping. The default is none. See also DSPStyle (HDL Coder).
FlattenHierarchy	Remove subsystem hierarchy from generated HDL code. The default is inherit. See also FlattenHierarchy (HDL Coder).
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline (HDL Coder).
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline (HDL Coder).
SharingFactor	Number of functionally equivalent resources to map to a single shared resource. The default is 0. See also Resource Sharing (HDL Coder).
StreamingFactor	Number of parallel data paths, or vectors, that are time multiplexed to transform into serial, scalar data paths. The default is 0, which implements fully parallel data paths. See also Streaming (HDL Coder).

Target Specification

This block cannot be the DUT, so the block property settings in the Target Specification tab are ignored.

Restrictions

• The Variant Subsystem cannot be a design-under-test (DUT).

• Variant activation time parameter must be set to update diagram, update diagram analyze all choices, or startup.

• For the Outport block of the Variant Subsystem, Specify output when source is unconnected parameter must be disabled.

Fixed-Point Conversion

Design and simulate fixed-point systems using Fixed-Point Designer™.

Actual data type or capability support depends on block implementation.

Version History

Introduced in R2010b

R2025a: Reduce overhead and irrelevant diagnostic messages by suppressing callbacks for inactive variant choices and commented blocks

The callbacks LoadFcn, ContinueFcn, InitFcn, ModelCloseFcn, PauseFcn, PostSaveFcn, PreSaveFcn, StartFcn, and StopFcn do not execute for inactive variant choices in Variant Subsystem blocks with the update diagram activation time and for commented blocks. This suppression reduces computational overhead and prevents unnecessary diagnostic messages from the blocks that do not affect the current model iteration. However, these callbacks are not suppressed for blocks within models or subsystem files inside a commented or an inactive Model block or Subsystem Reference block. Also, the mask initialization and mask parameter callbacks are not suppressed. For the list of applicable callbacks, see Author Mask Initialization and Callbacks and Mask Parameters.

R2025a: Parameter to output zero when no variant choices exist renamed from Allow zero active variant controls

The Allow zero active variant controls parameter is now renamed to Built-in empty choice. This new parameter maintains the previous functionality to remove the variant components and output zero when no variant choice is active.

R2025a: Assign input values to output ports when no variant choices exist

Use the Built-in passthrough choice parameter to bypass variant region and assign input values to the output ports when no variant choice is active.

R2024b: runtime activation time : Change the active choice during simulation or code generation

Starting in R2024b, use the runtime variant activation time in a Variant Subsystem block to change the active variant during simulation or code generation by using the Parameter Writer block. You can model all scenarios where the Parameter Writer block can write to the variant control variable in the global workspace, mask workspace, or model workspace.

R2024a: Automated port synchronization

Simulink provides options to add missing ports and delete unused ports in Variant Subsystem and Variant Assembly Subsystem blocks to match the ports of the underlying variant choices. During simulation, if Simulink encounters a port mismatch between a Variant Subsystem or Variant Assembly Subsystem block and its variant choices, the resulting error message provides the option of adding missing ports and deleting unused ports to resolve the issue.

R2024a: Use adapted rate-based model blocks as choices

You can use adapted rate-based model blocks as variant choices in a Variant Subsystem block. When you add adapted rate-based models as variant choices, the inports in the variant subsystem are matched to the port names of the respective rate-based models.

R2024a: Access the mask of the active variant choice

You can access the mask of the active variant choice of the Variant Subsystem directly without having to navigate inside the Variant Subsystem block.

• If the Variant Subsystem block is unmasked, double-clicking on the block opens the mask of its active choice. Clicking on the Look under mask badge or selecting the corresponding option from the context menu of the Variant Subsystem block takes you directly inside the active choice.

• If the Variant Subsystem block is masked and the active choice also has a mask, then double-clicking on the block opens the Variant Subsystem mask.

• If the Variant Subsystem block is unmasked and its active choice is masked, then double-clicking on the Look under mask badge takes you to the variant layer. You can see similar behavior in a masked Variant Subsystem block with an unmasked active choice.

When you click on the Look under mask badge of a linked Variant Subsystem block with its active choice as a linked and masked Model Reference block, the Model Reference opens in a separate editor. Additionally, you must perform set_param operations on the mask parameters of the active choice block on the active choice, and not on the Variant Subsystem block.

Promotion of the mask of the active choice is limited to its immediate parent. Nested Variant Subsystem mask promotion is not supported.

R2024a: Specify variant choices as an array of strings

You can specify Variant choices specifier as an array of strings in Variant Assembly Subsystem block.

R2023b: Promote content preview of active choice

Simulink promotes the content preview of the active choice of the Variant Subsystem block automatically. When the active choice is a Model Reference that is not loaded, and when the active choice is a Stateflow chart, the Variant Subsystem is shown as the content preview.

R2023b: Navigate directly into active choice

You can navigate directly inside the active variant by double clicking on the Variant Subsystem block. When you select Open in New Tab or Open in New Window from the context menu, the active choice is opened in a new tab or a new window. You can also use the open_system function to access the variant layer and the active choice.

open_system(vssBlock) 
open_system(vssBlock,'ActiveChoice','window')

When the active choice has an OpenFcn callback, double-clicking on the Variant Subsystem block does not execute that callback. Additionally, double-clicking on the Variant Subsystem does not navigate into the active choice if the active choice is a referenced model that is not loaded.

R2021a: Promote mask icon of active variant automatically

Simulink automatically displays the mask icon of the active variant block on the parent Variant Subsystem. Prior to R2021a, you had to provide icon drawing commands on the mask of the Variant Subsystem to display the mask icon of the underlying active variant block.