Is it possible to create a Matlab System block with no inputs nor outputs?

I am working with the Simulink Coder Support package for STM Nucleo boards. I want to incorporate interrupt handling in my Simulink model. The External Interrupt block provided in this package triggers a Function-Call subsystem.

I wanted to place a simple Matlab System block in this subsystem.

The stepImpl method of the MatlabSystem block would call the interrup service routine to handle the interrupt.

function stepImpl(~)
    if isempty(coder.target)
        % Place simulation output code here
    else
        % Call C-function implementing device output
        coder.ceval('isr_encoder');
    end
end

However, when creating a Matlab System with no inputs nor outputs, build error saying

Reported by IRQ_encoder: System object that implements getSampleTimeImpl and has zero inputs and zero outputs is not supported.

pops up. How can I 'not-implement' the getSampleTimeImpl method? Or is there another approach?

The whole Matlab System is coded like this:

classdef IRQ_encoder < realtime.internal.SourceSampleTime ...
        & coder.ExternalDependency ...
        & matlab.system.mixin.Propagates ...
        & matlab.system.mixin.CustomIcon
    
    properties
        % Public, tunable properties.
    end
    
    properties (Nontunable)
        % Public, non-tunable properties.
    end
    
    properties (Access = private)
        % Pre-computed constants.
    end
    
    methods
        % Constructor
        function obj = IRQ_encoder(varargin)
            % ADC driver object
            setProperties(obj,nargin,varargin{:});
        end
    end
    
    methods (Access=protected)
        function setupImpl(~)
            if isempty(coder.target)
                fprintf("Encoder ISR initialized\n");
            else
                % Call C-function implementing device initialization
                coder.cinclude('IRQ_encoder.h');
                coder.ceval('init_irq_encoder');
            end
        end
        
        function stepImpl(~)
            if isempty(coder.target)
                % Place simulation output code here
            else
                % Call C-function implementing device output
                coder.ceval('isr_encoder');
            end
        end
        
        function releaseImpl(~)
            if isempty(coder.target)
                % Place simulation termination code here
            else
                % Call C-function implementing device termination
                %coder.ceval('source_terminate');
            end
        end
    end
    
    methods (Access=protected)
        %% Define output properties
        function num = getNumInputsImpl(~)
            num = 0;
        end
        
        function num = getNumOutputsImpl(~)
            num = 0;
        end
        
        function flag = isOutputSizeLockedImpl(~,~)
            flag = true;
        end
        
        function varargout = isOutputFixedSizeImpl(~,~)
            varargout{1} = true;
        end
        
        function flag = isOutputComplexityLockedImpl(~,~)
            flag = true;
        end
        
        function varargout = isOutputComplexImpl(~)
            varargout{1} = false;
        end
        
        function varargout = getOutputSizeImpl(~)
            varargout{1} = 0;
        end
        
        function varargout = getOutputDataTypeImpl(~)
            varargout{1} = [];
        end
        
%         function icon = getIconImpl(~)
%             Define a string as the icon for the System block in Simulink.
%             icon = 'Source';
%         end    
    end
    
    methods (Static, Access=protected)
        function simMode = getSimulateUsingImpl(~)
            simMode = 'Interpreted execution';
        end
        
        function isVisible = showSimulateUsingImpl
            isVisible = false;
        end
    end
    
    methods (Static)
        function name = getDescriptiveName()
            name = 'A block used to handle encoder button ISR';
        end
        
        function b = isSupportedContext(context)
            b = context.isCodeGenTarget('rtw');
        end
        
        function updateBuildInfo(buildInfo, context)
            if context.isCodeGenTarget('rtw')
                % Update buildInfo
                srcDir = fullfile(fileparts(mfilename('fullpath')),'src');
                includeDir = fullfile(fileparts(mfilename('fullpath')),'include');
                addIncludePaths(buildInfo,includeDir);
                addSourceFiles(buildInfo,'IRQ_encoder.c',srcDir);
                % 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
    end
end

[Sathvik]

It is unclear why you don't need an input to the MATLAB System Block, since the IRQ generated by the External Interrupt Block should drive the ISR call. You can use a function-call block that will give a boolean output when it is triggered by an IRQ from the External Interrupt block. You can use this boolean output as input to the MATLAB System block which will trigger the stepImpl function when the bool input is true.

[Ondrej]

Thank you very much for your comment @Sathvik. This is an interesting idea. So to conclude: Matlab System block cannot be implemented without an input anyhow, right?

To make my initial intention more clear...I wanted the Matlab System's stepImpl function to be called when the IRQ is generated. Thus I thought if I place it in the function-call block, it will execute when the interrupt occurs. And since I wanted only to reset a counter value in the ISR I didn't need any input.