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----------------------------------------------
 TrueTime version 1.2
 Example: Task Synchronization using Monitors
 Directory: $DIR/examples/synch
----------------------------------------------

1. INTRODUCTION

This example shows how to use monitors to obtain mutual exclusion in
TrueTime. A cascaded controller for a ball-and-beam process (3rd order
system) is implemented using separate tasks for the two loops in the
cascade. The output from the outer controller is used as input to the
inner controller and is communicated using a global variable. This
variable is a shared resource, and mutual exclusion is achieved by a
TrueTime monitor.


2. COMPILING THE SIMULATION FILES

2.1 Configuring the TrueTime Environment

    Before starting Matlab, you must set the environment variable
    TTKERNEL to point to the directory with the TrueTime kernel files:

        Unix/Linux: > export TTKERNEL=$DIR/kernel
        Windows:  Control Panel / System / Advanced / Environment Variables

    Then add the following lines to your MATLAB startup script. This
    will set up all necessary paths to the TrueTime kernel files.

        addpath(getenv('TTKERNEL'))
        init_truetime;

2.2 Compilation

    As described in the reference manual it is possible to write a
    TrueTime simulation (i.e. the code functions for the tasks and the
    initialization commands) either as m-files or as C++ functions. For
    this example only the m-file approach is provided.

    TrueTime in the MATLAB version is compiled once and for all by
    running the command make_truetime from the command prompt. This
    should be done the first time you are using TrueTime and no
    further compilation is then required.


3. SIMULATIONS

Open the model synch.mdl to run the simulation. 

- Study the initialization function (synch_init.m). This creates two
  tasks for the outer and inner loop, respectively. A global variable,
  outerU, is used for task communication. This variable is the output
  from the outer controller (thus its name) and is used as reference
  for the inner controller. Finally, a TrueTime monitor is created.

- Study the code functions for the controller tasks (outercode.m and
  innercode.m). To ensure that no further instructions are executed in
  the case that ttEnterMonitor fails, this primitive needs to be called
  from its own segment (since all code of a TrueTime segment is executed
  at once before scheduling decisions are made). The same holds for 
  ttExitMonitor to make sure that no further code is executed in the
  case a context switch will occur when the monitor is released.

- Study the monitor schedule of the simulation. This graph displays when 
  the various tasks have been holding the monitor during the simulation.

- Try modifying the periods of the tasks to change the phasing and to see 
  which loop that is most sensitive to slower sampling.