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<html><head><title>EXAMPLE 2: PURE PERIODIC SCHEDULING OF TWO TASKS</title><link rel="stylesheet" type="text/css" href="style.css"></head><body><a href="./ex01_periodic.htm">[previous]</a><a href="./tutorial.htm#index">[index]</a><a href="./ex03_variable.htm">[next]</a><h1>Example 2: Pure Periodic Scheduling of Two Tasks</h1><p>This example demonstrates two periodic tasks, one that toggles thespeaker port and another that varies the frequency of toggling.In this example we will expand the number of tasks and use a simpleshared variable. Refer to the <ahref="../ex02_twoper/twoper_task.c">commented source code</a> of the example for the details.<h2>Principle of Operation</h2><ul><li>Any number of tasks may be scheduled, up to memory limits (thereis no predefined maximum)<li>Tasks may share variables, since they share the same address space<ul><li>In general, some care must be taken to ensure "mutual exclusion,"so that one task's actions on shared data aren't interrupted byanother's<li>For small data types (char, int) that are acted on "atomically,"i.e., whose reads and writes can't be interrupted, no worries<li>For large types (structures, arrays), you need to use semaphoresor other mechanisms, described in <a href="ex06_shm.htm#MUTEX">Example 6: Shared Memory</a></ul><li>For this example, one task runs at a fixed 100 microsecond period,and toggles the speaker port at intervals determined by a delaycount. The larger the count, the lower the audible frequency.<li>A second task runs at a fixed 1 second period, and changes thedelay count each time, so you will hear the frequency change each second.<li>The delay count is a shared variable, an 'int', so no worriesabout interrupted reads/writes<li>Since the task timing need not vary, the tasks both run in pureperiodic mode, using the shortest period of the two tasks as the base<li>The frequency will continue to drop indefinitely as the delaycount increases. The 'run' script in the example directory lets it gofor 10 seconds.</ul><h2>Setting up the Tasks</h2><ul><li>With two or more periodic tasks, you'll need to determine the baseperiod, multiples of which will be allowable for each task's period.<ul><li>In our case, we have a 100 microsecond task and a 1 second task,so the base period is 100 microseconds.<li>In general, you'll need to find the greatest common divisor of allyour desired task periods.<li>Practically speaking, the lower bound on the base period is about10 microseconds.</ul><li>The tasks are scheduled according to priority. If a task is readyto run (its period has transpired), it can only interrupt tasks oflower priority. A higher priority task will run its cycle tocompletion. <ul><li>Recall from Example 1 that the lowest priority isRT_LOWEST_PRIORITY, a large positive number, and that priorities arehigher as the numbers get lower, down to 0 as the highest-prioritytask.<li>In our case, we'll run the slowest task atRT_LOWEST_PRIORITY, and the fastest task at <nobr>RT_LOWEST_PRIORITY-1</nobr>.</ul></ul><h2>Running the Demo</h2>To run the demo, change to the 'ex02_twoper' 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 "Two Periodic Tasks" button.<p>You'll hear a series of tones for about 10 seconds, starting at 5KHz acoustically and dropping in frequency as the second taskincreases the delay time used by the first.<p><a href="../ex02_twoper/twoper_task.c">See the Code</a><hr><a href="./ex03_variable.htm">Next: Example 3, Variable Task Timing</a><p><a href="./ex01_periodic.htm">Back: Example 1, A Single Periodic Task</a></body></html>⌨️ 快捷键说明
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