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<html><head><title>EXAMPLE 9: DISPLAYING MESSAGES ON AN LED WAND CLOCK</title><link rel="stylesheet" type="text/css" href="style.css"></head><body><a href="./ex08_rcservo.htm">[previous]</a><a href="./tutorial.htm#index">[index]</a><a href="./ex10_stack.htm">[next]</a><h1>Example 9: Displaying Messages on an LED Wand Clock</h2><p>This example demonstrates the application of tasking, FIFOs andinterrupt service routines to the display of messages on aFantazein® LED wand clock. <ul><li>The stock clock has a built-in microcontroller that lets you setmessages via a keypad.<li>A wand with 8 LEDs waves back and forth. As it waves, the LEDschange to display dots for the current vertical segment of the messagecharacter. Persistence of vision makes the message appear suspended inspace.</ul><table><tr><td><img src="./ledclock.jpg"></td><td><img src="./ledwand.jpg"></td><td>Information on the stock clock is at <ahref="http://www.fantazein.com">www.fantazein.com</a><p>You'll need tohack into it and do a bunch of rewiring to connect it to the parallelport.<p><a href="./ledclock.txt">Read these instructions</a> to hack theclock and void your warrantee.</td></tr></table><h2>Principle of Operation</h2><ul><li>Hardware hacks to the clock redirect the 8 LEDs from the built-inmicrocontroller to the output data bits of the parallel port.<li>A Hall effect sensor generates an interrupt when the wand passesinto and out of one of the ends. This is also redirected to theparallel port.<li>The main task that writes the LEDs suspends itself immediatelyupon execution. It never runs periodically.<li>An interrupt service routine resumes the main task, with a flagthat signifies which direction the wand is waving.<li>Depending on the wand direction, the main task outputs a bitpattern appropriate to the current vertical segment of the characterto be displayed, and reschedules itself to run sometime later.<li>These segments are computed from a font definition file, stored inan array in the Linux task, and sent using a FIFO to the RT task.<li>No new RT Linux concepts are introduced here. We're puttingtogether what we've learned in the previous examples. For the curious,the Linux application code shows how to get immediate keyboard inputby setting the non-blocking option.</ul><h2>Running the Demo</h2>To run the demo, change to the 'ex09_ledclock' 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 "LED Clock" button.<p>Type in messages, which are then displayed banner-like on the wandclock. RETURN clears the messages, CTRL-C stops the demo.<p><a href="../ex09_ledclock/ledclock_task.c">See the Real-Time Task Code</a><p><a href="../ex09_ledclock/ledclock_app.c">See the Linux Application Code</a><hr><a href="./ex10_stack.htm">Next: Example 10, Determining Stack Size</a><p><a href="./ex08_rcservo.htm">Back: Example 8, RC Airplane Servomotor Control</a></body></html>⌨️ 快捷键说明
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