Finite State Machines - 2022.1 English

This example shows how to create a finite state machine using the MCode block with internal state variables. The state machine illustrated below detects the pattern 1011 in an input stream of bits.

The M-function that is used by the MCode block contains a transition function, which computes the next state based on the current state and the current input. Unlike example 3 though, the M-function in this example defines persistent state variables to store the state of the finite state machine in the MCode block. The following M-code, which defines function detect1011_w_state is contained in file detect1011_w_state.m:

function matched = detect1011_w_state(din)
% This is the detect1011 function with states for detecting a
% pattern of 1011.

  seen_none = 0; % initial state, if input is 1, switch to seen_1
  seen_1 = 1;    % first 1 has been seen, if input is 0, switch
                 % seen_10
  seen_10 = 2;   % 10 has been detected, if input is 1, switch to
                 % seen_1011
  seen_101 = 3;  % now 101 is detected, is input is 1, 1011 is
                 % detected and the FSM switches to seen_1

  % the state is a 2-bit register
  persistent state, state = xl_state(seen_none, {xlUnsigned, 2, 0});

  % the default value of matched is false
  matched = false;

  switch state
    case seen_none
      if din==1
        state = seen_1;
      else
        state = seen_none;
      end
    case seen_1 % seen first 1
      if din==1
        state = seen_1;
      else
        state = seen_10;
      end
    case seen_10 % seen 10
      if din==1
        state = seen_101;
      else
      % no part of sequence seen, go to seen_none
      state = seen_none;
      end
    case seen_101
      if din==1
        state = seen_1;
        matched = true;
      else
        state = seen_10;
        matched = false;
      end
  end

The following diagram shows a state machine Subsystem containing a MCode block after compilation; the MCode block uses M-function detect1101_w_state.