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<title>ChibiOS/RT: Designing an embedded application</title>
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<td style="width: 80px;"><img alt="ChibiOS/RT Logo" src="logo_small.png"></td>
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<td><big><big>ChibiOS/RT</big></big><br><br>Architecture - Reference Manual - Guides</td>
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<li><a href="main.html"><span>Main Page</span></a></li>
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<li><a class="el" href="main.html">ChibiOS/RT</a> </li>
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<h1>Designing an embedded application </h1> </div>
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<p>ChibiOS/RT offers a variety of mechanisms and primitives, often it is better to focus on a single approach for the system design and use only part of the available subsystems.<br/>
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When designing your application you may choose among several design alternatives:</p>
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<ul>
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<li><a class="el" href="article_design.html#nothreads">Single threaded superloop</a></li>
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<li><a class="el" href="article_design.html#messpass">Message Passing</a></li>
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<li><a class="el" href="article_design.html#thdshared">Threads sharing data</a></li>
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<li><a class="el" href="article_design.html#thdmixed">Mixed</a></li>
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</ul>
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<h2><a class="anchor" id="nothreads"></a>
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Single threaded superloop</h2>
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<p>Correct, single thread, it is not mandatory to use the multithreading features of the OS. You may choose to implements everything as a complex state machine handled in the main thread alone. In this scenario the OS still offers a variety of useful mechanisms:</p>
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<ul>
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<li>Interrupt handling.</li>
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<li>Virtual Timers, very useful in state machines in order to handle time triggered state transitions.</li>
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<li>Power management.</li>
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<li>Event Flags and/or Semaphores as communication mechanism between interrupt handlers and the main.</li>
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<li>I/O queues.</li>
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<li>Memory allocation.</li>
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<li>System time.</li>
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</ul>
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<p>In this configuration the kernel size is really minimal, everything else is disabled and takes no space. You always have the option to use more threads at a later time in order to perform separate tasks.</p>
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<h2><a class="anchor" id="messpass"></a>
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Message Passing</h2>
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<p>In this scenario there are multiple threads in the system that never share data, everything is done by exchanging messages. Each thread represents a service, the other threads can request the service by sending a message.<br/>
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In this scenario the following subsystems can be used:</p>
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<ul>
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<li>Synchronous Messages.</li>
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<li>Mailboxes (asynchronous message queues).</li>
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</ul>
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<p>The advantage of this approach is to not have to deal with mutual exclusion, each functionality is encapsulated into a server thread that sequentially serves all the requests. As example, you can have the following scenario:</p>
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<ul>
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<li>A buffers allocator server.</li>
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<li>A disk driver server.</li>
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<li>A file system server.</li>
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<li>One or more client threads.</li>
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</ul>
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<p>Example: <br/>
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<br/>
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</p>
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<p> <br/>
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<br/>
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Note that the threads should not exchange complex messages but just pointers to data structures in order to optimize the performance. Also note that a thread can be both client and server at the same time, the FS service in the previous scenario as example.</p>
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<h2><a class="anchor" id="thdshared"></a>
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Threads sharing data</h2>
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<p>This is the most common scenario, several threads have access to both their private data and shared data. Synchronization happens with one of the mechanisms described in the <a class="el" href="article_mutual_exclusion.html">Mutual Exclusion guide</a> article.<br/>
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</p>
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<h2><a class="anchor" id="thdmixed"></a>
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Mixed</h2>
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<p>All the above approaches can be freely mixed in a single application but usually I prefer to choose a way and consistently design the system around it. The OS is a toolbox that offers a lot of tools but you don't have to use them all necessarily. </p>
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<hr size="1"><address style="text-align: right;"><small>
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Generated on Sun Nov 28 2010 14:09:55 for ChibiOS/RT by <a href="http://www.doxygen.org/index.html"><img src="doxygen.png" alt="doxygen" align="middle" border="0"></a> 1.7.1</small></address>
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