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<html><head><title>EXAMPLE 3: VARIABLE-PERIOD SCHEDULING OF A SINGLE TASK</title><link rel="stylesheet" type="text/css" href="style.css"></head><body><a href="./ex02_twoper.htm">[previous]</a><a href="./tutorial.htm#index">[index]</a><a href="./ex04_fifo.htm">[next]</a><h1>Example 3: Variable-Period Scheduling of a Single Task</h1><p>This example demonstrates a single task that toggles thespeaker port with a continuously varying frequency. Since there is noidentifiable base period for the timer, we'll put it in one-shot modeand change the task period each time it runs. This adds some minoroverhead (about 2 microseconds) each cycle for timer reprogramming,but it avoids quantization of the speaker frequency to some base timerfrequency. Refer to the <ahref="../ex03_variable/variable_task.c">commented source code</a> of the example for the details.<h2>Principle of Operation</h2><ul><li>In the previous examples, we established each pure periodic task'speriod as an argument to 'rt_task_make_periodic()'. This was done onceat the beginning, in 'init_module()'. <li>Then, in the task code, we called 'rt_task_wait_period()' at theend of each cycle, to put it to sleep and await the next cycle.<li>In the task code here, we'll call 'rt_sleep(RTIME delay)' with anexplicit delay, which can vary each cycle.<li>We still call 'rt_task_make_periodic()' to start the task, but wecan use a dummy value as the base period, since there is none.</ul><h2>Setting up the Timer</h2><ul><li>Set up the timer to expire in one-shot mode by calling<pre>void rt_set_oneshot_mode(void);</pre>This sets the one-shot mode for the timer. The 8254 timer chipwill be reprogrammed each task cycle, at a cost of about 2 microseconds.<li>Start the one-shot timer by calling<pre>RTIME start_rt_timer(RTIME count);</pre>with 'count' set to 1 as a dummy nonzero value provided for the periodsince we don't have one. We habitually use a nonzero value since 0could mean disable the timer to some.</ul><h2>Setting up the Task</h2><ul><li>The task is set up as before, calling 'rt_task_init()' and'rt_task_make_periodic()'.<li>In the task function, however, we need to call 'rt_sleep(delay)'with 'delay' set to the time, in counts, that we want to delay thetask. Calling 'rt_task_wait_period()', as before, will cause it tosleep for the dummy period provided earlier, which is not what wewant.<li>The sound will now sweep continuously up and down as the delay ischanged.</ul><h2>The Task Function and Timing</h2><ul><li>Each cycle, the task function computes the amount of delay untilthe next cycle,following a relatively simple algorithm that adds or subtracts someincrement tuned to sound good for this example.<li>In order to improve the timing, the task function reads its actualstart time to use as a basis for computing the delay. While this isnot really necessary for this example, it introduces the function<pre>RTIME rt_get_time(void);</pre>which returns the number of counts since RTAI booted, as an RTIME,which is a long long int, rolling over into negative territory at 63bits.<li>In periodic mode, a 'count' is one tick of the 8254 timer chip, or1,193,180 counts per second. This gives rollover at<nobr>2<sup>63</sup>/1,193,180</nobr> seconds, or over 200,000years. No need to worry about rollover here.<li>In one-shot mode, a 'count' is one tick of the CPU clock, say1 billion counts per second on a 1 GHz machine. This gives rollover at<nobr>2<sup>63</sup>/1,000,000,000</nobr> seconds, or about 70years. No need to worry about rollover here either.<li>On an embedded system powered up for many years, you'd need toworry. This means checking if a new timestamp was less than a previousone, and subtracting the difference from the range to get the truedifference.</ul><h2>Running the Demo</h2>To run the demo, change to the 'ex03_variable' 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 Variable-Period Task" button.<p>You'll hear a continuously varying tone for about 10 seconds.<p><a href="../ex03_variable/variable_task.c">See the Code</a><hr><a href="./ex04_fifo.htm">Next: Example 4, FIFOs</a><p><a href="./ex02_twoper.htm">Back: Example 2, Pure PeriodicScheduling of Two Tasks</a></body></html>⌨️ 快捷键说明
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