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📁 A tutorial on RT-Linux
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<html><head><title>EXAMPLE 1: PURE PERIODIC SCHEDULING OF A SINGLE TASK</title><link rel="stylesheet" type="text/css" href="style.css"></head><body><a href="./basics.htm">[previous]</a><a href="./tutorial.htm#index">[index]</a><a href="./ex02_twoper.htm">[next]</a><h1>Example 1: Pure Periodic Scheduling of a Single Task</h1><p>This example demonstrates a single fixed-period task that togglesthe speaker at 1 kHz, also demonstrating how to read and write I/Oaddresses. In this example we will introduce the basics of RT Linux,and refer to the <a href="../ex01_periodic/periodic_task.c">commentedsource code</a> of the example for the details.<h2>Principle of Operation</h2><ul><li>In RT Linux, the real-time scheduler is driven by timerinterrupts from the PC's 8254 Programmable Interval Timer (PIT) chip.<li>In <i>pure periodic mode</i>, the timer is programmed to expire ata fixed period. All tasks will run at a multiple of this period.<ul><li>The advantage is that timer reprogramming is avoided, which savesabout 2 microseconds.<li>The disadvantage is that tasks are forced to run at multiples ofthe fundamental period.</ul><li>In <i>one-shot mode</i>, the timer is reprogrammed at the end ofeach task cycle, so that it expires exactly when the next task isscheduled to run.<ul><li>The advantage is that tasks timing can vary dynamically, and newtasks can be added without regard to any fundamental period.<li>The disadvantage is that timer reprogramming overhead recurscontinually.</ul><li>In this example, we will show how a single task is set up to runin pure periodic mode.</ul><h2>Setting Up a Single Periodic Task</h2>The relevant RTAI functions are documented in the <a href="manual.html">RTAIdocumentation</a>. In this example we will do the following:<ul><li>Set up the timer with a fundamental period, in our case 1millisecond.<li>Define the task function, in our casetoggling the speaker port.<li>Set up the task structure and fill in fields for the task code,its priority, size of stack and other information.<li>Schedule the task to run at a specified period, a multiple of thetimer period.</ul><h2>Setting Up the Timer</h2><ul><li>We set up the timer to expire in pure periodic mode by calling thefunctions<pre>void rt_set_periodic_mode(void);RTIME start_rt_timer(RTIME period);</pre><li>'rt_set_periodic_mode()' establishes that the timer won't bereprogrammed after every task cycle. The value passed to'start_rt_timer()' is the period, of type RTIME "internal counts".<li>The function <pre>RTIME nano2counts(int nanoseconds);</pre>can be used to convert time in nanoseconds to these RTIME internalcount units.<li>The requested period is quantized to the resolution of the 8254 chipfrequency (1,193,180 Hz). Any timing request not an integer multipleof the period is satisfied at the closest period tick.</ul><h2>The Task Function</h2><ul><li>Each task is associated with a function that is called when the taskis scheduled to run.<li>This function is a usual C function that typically reads someinputs, computes some outputs and waits for the next cycle.<li>In this example, the task function will toggle the PC speaker portby alternately setting or clearing bit 2 of the speaker port address,61 hex (0x61 in C). A web search on "<ahref="http://www.google.com/search?q=PC+port+addresses">PC portaddresses</a>" is a good source for programming PC resources (parallel port, serial port, joystick, LEDs, etc.)<li>The task function should enter an endless loop, in which is doesits work, then calls<pre>void rt_task_wait_period(void);</pre>to wait for the its next scheduled cycle. Typical code looks likethis:<pre>void task_function(int arg){  while (1) {    /*      Do your thing here     */    rt_task_wait_period();  }  return;}</pre></ul><h2>Setting Up the Task Structure</h2><ul><li>An RT_TASK data structure is used to hold all the information about atask:<ul><li>the task function,<li> any initial argument passed to it,<li>the size of the stack allocated for its variables,<li>its priority,<li>whether or not it uses floating-point math,<li>and a "signal handler" that will be called when the task becomes active.</ul><li>The task structure is initialized by calling<pre>rt_task_init(RT_TASK *task,             void *rt_thread, int data,             int stack_size, int priority,	     int uses_fp, void *sig_handler);</pre><ul><li>'task' is a pointer to an RT_TASK type structure whose space mustbe provided by the application. It must be kept during the wholelifetime of the real time task and cannot be an automaticvariable.<li>'rt_thread' is the entry point of the taskfunction.<li>'data' is a single integer value passed to the new task.<li>'stack_size' is the size of the stack to be used bythe new task (see Example 9 for information on computing stack size).<li>'priority' is the priority to be given the task. The highestpriority is 0, while the lowest is RT_LOWEST_PRIORITY (1,073,741,823).<li>'uses_fp' is a flag. Nonzero value indicatesthat the task will use floating point, and the scheduler should makethe extra effort to save and restore the floating point registers.<li>'sig_handler' is afunction that is called, within the task environment and withinterrupts disabled, when the task becomes the current runningtask after a context switch.</ul><li>The newly created real time task is initially in a suspendedstate. It is can be made active either with'rt_task_make_periodic()', 'rt_task_make_periodic_relative_ns()' or'rt_task_resume()'.</ul><h2>Scheduling the Task</h2><ul><li>The task can now be started by passing its filled-in RT_TASKstructure to the function<pre>int rt_task_make_periodic(RT_TASK *task,                          RTIME start_time,			  RTIME period);</pre><ul><li>'task' is the address of the RT_TASK structure,<li>'start_time' is the absolute time, in RTIME units, when the taskshould begin execution. Typically this is "now" ('rt_get_time()').<li>'period' is the task's period, in RTIME units, which will berounded to the nearest multiple of the fundamental timer period.</ul><li>The task will now execute indefinitely every 'period' counts.</ul><h2>Running the Demo</h2>To run the demo, change to the 'ex01_periodic' subdirectory of thetop-level tutorial directory, and run the 'run' script by typing<pre>./run</pre>Alternatively, change to the top-level tutorial directory and run the'runall' script there by typing<pre>./runall</pre>and selecting the "Single Periodic Task" button.<p>You'll hear a tone corresponding to 1 KHz toggling (500 Hz for afull period) for about 10 seconds.<p><a href="../ex01_periodic/periodic_task.c">See the Code</a><hr><a href="./ex02_twoper.htm">Next: Example 2: Pure Periodic Scheduling of Two Tasks</a><p><a href="./basics.htm">Back: The Basics of Real-Time Linux</a></body></html>
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