Writing S-Functions

Example - Variable Sample Time S-Functions

This M-file is an example of an S-function that uses a variable sample time. This example, in an M-file called vsfunc.m, calls mdlGetTimeOfNextVarHit when flag = 4. Because the calculation of a next sample time depends on the input u, this block has direct feedthrough. Generally, all blocks that use the input to calculate the next sample time (flag = 4) require direct feedthrough. Here is the code for the M-file S-function.

function [sys,x0,str,ts] = vsfunc(t,x,u,flag)
% This example S-function illustrates how to create a variable 
% step block in Simulink.  This block implements a variable step 
% delay in which the first input is delayed by an amount of time 
% determined by the second input.
%
%     dt      = u(2)
%     y(t+dt) = u(t)
%
switch flag,

  case 0
    [sys,x0,str,ts] = mdlInitializeSizes; % Initialization 

  case 2
    sys = mdlUpdate(t,x,u); % Update Discrete states

  case 3
    sys = mdlOutputs(t,x,u); % Calculate outputs

  case 4
    sys = mdlGetTimeOfNextVarHit(t,x,u); % Get next sample time

  case { 1, 9 } 
    sys = []; % Unused flags
  otherwise
    error(['Unhandled flag = ',num2str(flag)]); % Error handling
end
% End of vsfunc.
%==============================================================
% mdlInitializeSizes
% Return the sizes, initial conditions, and sample times for the 
% S-function.
%==============================================================
%
function [sys,x0,str,ts] = mdlInitializeSizes
%
% Call simsizes for a sizes structure, fill it in and convert it 
% to a sizes array.
%
sizes = simsizes;
sizes.NumContStates  = 0;
sizes.NumDiscStates  = 1;
sizes.NumOutputs     = 1;
sizes.NumInputs      = 2;
sizes.DirFeedthrough = 1;  % flag=4 requires direct feedthrough
                           % if input u is involved in 
                           % calculating the next sample time 
                           % hit.
sizes.NumSampleTimes = 1;  
sys = simsizes(sizes);
%
% Initialize the initial conditions.
%
x0 = [0];
%
% Set str to an empty matrix.
%
str = [];
%
% Initialize the array of sample times.
%
ts = [-2 0];      % variable sample time
% End of mdlInitializeSizes.
%
%==============================================================
% mdlUpdate
% Handle discrete state updates, sample time hits, and major time 
% step requirements.
%==============================================================
%
function sys = mdlUpdate(t,x,u)
sys = u(1);
% End of mdlUpdate.
%
%==============================================================
% mdlOutputs
% Return the block outputs.
%==============================================================
%
function sys = mdlOutputs(t,x,u)
sys = x(1);
% end mdlOutputs
%
%==============================================================
% mdlGetTimeOfNextVarHit
% Return the time of the next hit for this block.  Note that the 
% result is absolute time.
%==============================================================
%
function sys = mdlGetTimeOfNextVarHit(t,x,u)
sys = t + u(2);
% End of mdlGetTimeOfNextVarHit.

mdlGetTimeOfNextVarHit returns the "time of the next hit," the time in the simulation when vsfunc is next called. This means that there is no output from this S-function until the time of the next hit. In vsfunc, the time of the next hit is set to t + u(2), which means that the second input, u(2), sets the time when the next call to vsfunc occurs.

Example - Hybrid System S-Functions

Processing S-Function Parameters