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----------------------------------------------
 TrueTime version 1.2
 Example: Deadline Overrun Handling
 Directory: $DIR/examples/overrun
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1. INTRODUCTION

This example will show how to use the TrueTime overrun handling
functionality. TrueTime provides two types of overrun handlers;
deadline and worst-case execution time overrun handlers. The example
considers PID-control of a DC-servo, with the controller task having a
stochastically varying execution time that occasionally will exceed
the sampling interval. Two approaches to deal with the period overruns
are evaluated in the simulation. The first allows the task to continue
into next sample (no overrun handler is attached), whereas the second
uses an overrun handler that terminates the current job if the
deadline is exceeded.


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 overrun.mdl to run the simulation. 

- The period of the controller task is 6 ms, and the execution time is
  modeled as C = 5 + U(0,2) ms. Consequently, the task will experience
  overruns. The bad control performance is due to the long delays and the
  sampling period jitter induced by the overruns.

- Uncomment the last two lines of the initialization file
  (overrun_init.m). This will create an interrupt handler and attach
  it to the controller task as a deadline handler. Study the code
  executed by the overrun handler (hdlcode.m).
  
- Run a simulation to evaluate the performance obtained by terminating
  jobs at the deadline (is this a good approach?). Studying the control 
  signal, one can notice that it often remains constant over several 
  samples.