Writing S-Functions

Example - Variable Sample Time S-Function

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-Function

Writing S-Functions in C