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simulink function block error: Function output 'y' cannot be an mxArray in this context.

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i am dealing with a function block in simulink, trying to calculate the interval of a signal with variable frequency to get the real time frequency. My thought is to use tic/toc get the interval between two peaks.
So confused by this error. The diagnostic says:
Function output 'y' cannot be an mxArray in this context. Consider preinitializing the output variable with a known type. Function 'MATLAB Function1' (#312.9.10), line 1, column 10: "y" Launch diagnostic report.
Simulink cannot determine sizes and/or types of the outputs for block 'signalTimer/MATLAB Function1' due to errors in the block body, or limitations of the underlying analysis. The errors might be inaccurate. Fix the indicated errors, or explicitly specify sizes and/or types for all block outputs.
function y = timer(u)
coder.inline('never');
coder.extrinsic('toc')
coder.extrinsic('tic')
persistent x1 x2
% x1=0;
% x2=0;
if isempty(x1)
x1 = 0;
x2 = 0;
else
x2=x1;
end
x1=u;
if x1<x2
y=toc;
y=double(y);
tic
end
end
  2 Comments
Jiang Ping
Jiang Ping on 9 Jan 2019
Thank you, but this problem can be addressed if I execute tic in the command line before run the script. Actually what I am really want to do is to measure the frequency of sqaure wave whose frequency is variable in real time .

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