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<!doctype html public "-//w3c//dtd html 4.0 transitional//en"><html><head> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <meta name="GENERATOR" content="Mozilla/4.78 [en] (X11; U; Linux 2.4.7-10 i686) [Netscape]"> <title>Lesson 3: Introducing Tasks for Application DataProcessing</title></head><body bgcolor="#F8F8FF" link="#005BB7" vlink="#005BB7"> <table BORDER=0 CELLSPACING=2 CELLPADDING=3 WIDTH="100%" hspace="4" ><tr BGCOLOR="#E0E0FF"><td WIDTH="100%"><b><font face="tahoma,arial,helvetica"><font size=-1>Lesson3: Introducing Tasks for Application Data Processing</font></font></b><p><font face="tahoma,arial,helvetica">Last updated 15 August 2003</font></td></tr></table><p>This lesson introduces the TinyOS notion of <b>tasks</b>, which canbe used to perform general-purpose "background" processing in an application.This lesson makes use of the <tt>SenseTask</tt> application, which is arevision of the <tt>Sense</tt> application from the previous lesson.<table BORDER=0 CELLSPACING=2 CELLPADDING=3 WIDTH="100%" hspace="4" ><tr BGCOLOR="#E0E0FF"><td WIDTH="100%"><b><nobr><font face="arial,helvetica">Task creation andscheduling</font></nobr></b></td></tr></table><p>TinyOS provides a two-level scheduling hierarchy consisting of<b>tasks</b> and <b>hardware event handlers</b>. Remember that the keyword<b>async</b> declares a command or event that can be executed by a hardwareevent handler. This means it could be executed at any time (preemptingother code), so <b>async</b> commands and events should do small amounts ofwork and complete quickly. Additionally, you should pay attention to thepossibility of data races on all shared data accessed by an <b>async</b>command or event. <b>Tasks</b> are used to perform longer processingoperations, such as background data processing, and can be preempted byhardware event handler.<p>A task is declared in your implementation module using the syntax<pre> task void taskname() { ... }</pre>where <tt>taskname()</tt> is whatever symbolic name you want to assignto the task. Tasks must return <tt>void</tt> and may not take any arguments.<p>To dispatch a task for (later) execution, use the syntax<pre> post taskname();</pre>A task can be posted from within a command, an event, or even another task.<p>The <tt>post</tt> operation places the task on an internal <b>task queue</b>which is processed in FIFO order. When a task is executed, it runs to completionbefore the next task is run; therefore, a task should not spin or blockfor long periods of time. Tasks do not preempt each other, but a task canbe preempted by a hardware event handler. If you need to run a series of long operations,you should dispatch a separate task for each operation, rather than usingone big task.<br> <table BORDER=0 CELLSPACING=2 CELLPADDING=3 WIDTH="100%" hspace="4" ><tr BGCOLOR="#E0E0FF"><td WIDTH="100%"><b><nobr><font face="arial,helvetica">The SenseTask Application</font></nobr></b></td></tr></table><p>To illustrate tasks, we have modified the Sense application from Lesson2, which is found in <tt><a href="../../apps/SenseTask">apps/SenseTask</a></tt>.The <tt>SenseTaskM</tt> component maintains a circular data buffer, <tt>rdata</tt>,that contains recent photo sensor samples; the <tt>putdata()</tt> functionis used to insert a new sample into the buffer. The <tt>dataReady()</tt>event simply deposits data into the buffer and posts a task, called<tt>processData()</tt>,for processing.<br> <center><table BORDER=0 CELLSPACING=2 CELLPADDING=3 WIDTH="80%" hspace="4" ><tr BGCOLOR="#E0E0E0"><td WIDTH="100%"><b>SenseTaskM.nc</b><pre> // ADC data ready event handler async event result_t ADC.dataReady(uint16_t data) { putdata(data); post processData(); return SUCCESS; }</pre></td></tr></table></center><p>Some time after the async event completes (there may be other taskspending for execution), the <tt>processData()</tt> task will run. It computesthe sum of the recent ADC samples and displays the upper three bits ofthe sum on the LEDs.<center><table BORDER=0 CELLSPACING=2 CELLPADDING=3 WIDTH="80%" hspace="4" ><tr BGCOLOR="#E0E0E0"><td WIDTH="100%"><b>SenseTaskM.nc, continued</b><pre> task void processData() { int16_t i, sum=0; atomic { for (i=0; i < size; i++) sum += (rdata[i] >> 7); } display(sum >> log2size); }</pre></td></tr></table></center><p>The keyword <b>atomic </b>in the task <tt>processData() </tt>illustratesthe use of nesC <i>atomic statements</i>. This means the code section withinthe <tt>atomic</tt> curly braces will execute without preemption. In thisexample, access to the shared buffer <tt>rdata</tt> is being protected.Where else should this be used ?<p>Atomic statements delay interrupt handling which makes the system lessresponsive. To minimize this effect, atomic statements in nesC should avoidcalling commands or signaling events when possible (the execution time ofan external command or event will depend on what the component is wiredto).<br> <table BORDER=0 CELLSPACING=2 CELLPADDING=3 WIDTH="100%" hspace="4" ><tr BGCOLOR="#E0E0FF"><td WIDTH="100%"><b><nobr><font face="arial,helvetica">Exercises</font></nobr></b></td></tr></table><p>Try to break up the <tt>processData()</tt> task so that each invocationof the task only adds one element of the <tt>rdata</tt> array to <tt>sum</tt>.<tt>processData()</tt> should then post itself to continue processing thecomplete <tt>sum</tt>, and display the LEDs when the final element of thearray has been processed. Be careful of concurrency issues - since<tt>processData()</tt>is also posted from <tt>ADC.dataReady()</tt>, you might want to add a flagto prevent a new instance of the task from being started before the previoussum has been computed!<p><hr><b><a href="lesson2.html">< Previous Lesson</a></b> |<b><a href="lesson4.html">NextLesson ></a></b> | <b><a href="index.html">Top</a></b></body></html><!-- LocalWords: TinyOS nesC nc async norace BlinkM FooM ncc SingleTimer Leds --><!-- LocalWords: LedsC StdControl tos init TimerC redOn redOff uint redToggle --><!-- LocalWords: metadata html nesdoc gcc exe avr objcopy srec uisp mib dprog --><!-- LocalWords: towards dapa xff ThinkPad dlpt Makelocal micasb Wshadow DDEF --><!-- LocalWords: finline fnesc cfile lm Atmel ATmega nesC's nesc SenseM rdata --><!-- LocalWords: ADCControl SounderControl dataReady getData rcombine someval --><!-- LocalWords: ADControl fooControl barControl MyTimer uniqueCount basicsb --><!-- LocalWords: sensorboard Makerules sensorboards SenseTask taskname --><!-- LocalWords: SenseTaskM putdata processData -->⌨️ 快捷键说明
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