How to detect frequency of a sinusoidual signal with s-function?

I want to detect the frequency of grid voltage(outside of an PV inverter) in Matlab. The following is the m-file program, however what i want is to implement it in Simulink with s-function. does anyone can help? how to transfer it to the form of s-function. thank you in advance.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
clear;
clc;
f1=50.05;
fs=400;
det=1/400;
t=0:det:2-det;
N=length(t);

y1=sin(2*pi*f1*t+pi/3);

a=0;
  for i=1:1:N-1
     if (y1(i)>0)&&(y1(i+1)<=0)
          a=a+1;
          if a==1
          t1=t(i)+(y1(i)*det)/(y1(i)-y1(i+1));
          end
          t2=t(i)+(y1(i)*det)/(y1(i)-y1(i+1));
     end

    end
  f=(a-1)/(t2-t1)
  e=(f-f1)/f1*100
  end
  %%%%%%%%%%%%%%%%%%%

[Rick Rosson]

Please format your code properly. Thanks.

Answer 1

[Nirmal Gunaseelan]

Provided you are not interested in generating code from the resulting Simulink model, Level 2 MATLAB-file S-functions would be the way to go. I have not looked at your code (follow instructions in the help page).