sfundsc2.m

数字通信第四版原书的例程

function  [sys, x0] = sfundsc2(t,x,u,flag)
%SFUNDSC2 An example S-function for discrete systems with a sample time.
%	This M-file is designed to be used as a template for other
%	S-functions. Right now it acts as a zero order hold. This template
%	is an example of a discrete system with an explicit update time.
%	That is, it makes use of FLAG==4, so the states get updated only at
%	the specified time intervals.
%
%	Copyright (c) 1990-94 by The MathWorks, Inc.
%	Ned Gulley 8-11-92

% Uncomment the following two lines to show how the flags are called.
% dispString=['flag = ' num2str(flag) ' at time = ' num2str(t)];
% disp(dispString)

% Set the sample time to 0.1 seconds.
Tsamp = 0.1;

if abs(flag) == 2,
  % The FLAG==2 branch is at the top of the file for efficiency, since
  % the S-function will be called with FLAG==2 very frequently.
  % If FLAG==2, then SIMULINK is looking for the next point in real time. 
  % **** In this template system, the current state x gets the input u ****
  % Now we must explicitly test to see if this is the appropriate update
  % time. If it is, update the discrete state. Otherwise, leave it at its
  % current value.
  % There has to be an explicit test here for the correctness of the time
  % because a FLAG==2 call is made every time the clock advances, not
  % simply at the time specified as TNEXT under FLAG==4.
  if abs(round(t/Tsamp)-(t/Tsamp)) < 1e+8*eps,
    % (SYS =) XNEW = U
    sys = u;
  else
    % (SYS =) XNEW = X
    sys = x;
  end;

elseif flag == 3,
  % If FLAG==3, then SIMULINK wants to know what the next output is.
  % **** In this template system, y gets the current state x ****
  % (SYS =) Y = X
  sys = x;

elseif flag == 4,
  % If FLAG==4, then SIMULINK wants to know what TNEXT is.
  % TNEXT is the time when the integrator should be constrained to
  % call the system with FLAG==2.
  % First compute the number of samples so far
  numSamp = floor(t/Tsamp + 1e+8*eps);
  % Now sys returns TNEXT, the next time the discrete state should
  % be updated.
  % (SYS =) TNEXT = ...
  sys = (numSamp + 1)*Tsamp;
  
elseif flag  == 0,
  % This part takes care of all initialization; it is used only once.
  % **** In this template system, there is one discrete state, one ****
  % **** input and one output.                                     ****
  % The sizes vector is six elements long, and it looks like this:
  % sizes(1) = number of continuous states
  sizes(1) = 0;
  % sizes(2) = number of discrete states
  sizes(2) = 1;
  % sizes(3) = number of system outputs (length of output y)
  sizes(3) = 1;
  % sizes(4) = number of system inputs (length of input u)
  sizes(4) = 1;
  % sizes(5) = number of discontinuous roots; unused feature, set to zero
  sizes(5) = 0;
  % sizes(6) = direct feedthrough flag; used to detect algebraic loops.
  % Set sizes(6) to 1 if the output y depends directly on the input u.
  % Otherwise, it should be set to 0.
  sizes(6) = 0;
  % Set the initial conditions on the states
  x0 = 0;
  % (SYS =) SIZES = [0 1 1 1 0 0]';
  sys = sizes';

else 
  % Flags not considered here are treated as unimportant.
  % Notice that since there are no continuous states in this system,
  % there is no need to deal with FLAG==1.
  % Output is set to [].
  sys = [];

end     % if abs(flag) == ...