How to Create Custom Block in Simulink: Step-by-Step Guide

To create a custom block in Simulink, use the MATLAB Function block for custom code or create a masked subsystem to group blocks with a custom interface. Both methods allow you to define inputs, outputs, and behavior tailored to your needs.

Syntax

There are two main ways to create a custom block in Simulink:

• MATLAB Function block: Write MATLAB code inside the block to define custom behavior.
• Masked Subsystem: Group existing blocks into a subsystem and create a mask to customize its interface.

Basic syntax for MATLAB Function block code:

function y = fcn(u)
%#codegen
% Your custom code here
 y = u * 2; % Example operation

For masked subsystems, you create a subsystem and then apply a mask via the Simulink UI to add parameters and custom dialog.

matlab

function y = fcn(u)
%#codegen
% Example: multiply input by 2
 y = u * 2;

Example

This example shows how to create a custom MATLAB Function block that doubles the input signal.

matlab

function y = fcn(u)
%#codegen
% Double the input value
 y = u * 2;

Output

If input u = 3, output y = 6

Common Pitfalls

• Not enabling code generation with %#codegen in MATLAB Function blocks can cause simulation errors.
• For masked subsystems, forgetting to define mask parameters leads to unclear block interfaces.
• Using unsupported MATLAB functions inside MATLAB Function blocks causes errors; only supported functions work.
• Not connecting input/output ports properly in subsystems results in simulation failures.

matlab

%% Wrong: Missing %#codegen
function y = fcn(u)
 y = u * 2;
end

%% Right: Include %#codegen
function y = fcn(u)
%#codegen
 y = u * 2;
end

Quick Reference

Summary tips for creating custom blocks in Simulink:

• Use MATLAB Function blocks for custom code logic.
• Use Masked Subsystems to create reusable blocks with custom parameters.
• Always define input and output ports clearly.
• Test your block with simple inputs before complex models.
• Check MATLAB Function block compatibility with supported functions.

Key Takeaways

Use MATLAB Function blocks to write custom code inside Simulink blocks.

Create masked subsystems to group blocks and customize their interface.

Always include %#codegen in MATLAB Function blocks for simulation compatibility.

Define clear input and output ports for your custom blocks.

Test custom blocks with simple inputs to verify behavior before complex use.