122 lines
7.9 KiB
HTML
122 lines
7.9 KiB
HTML
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<?xml version="1.0" encoding="utf-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns:MadCap="http://www.madcapsoftware.com/Schemas/MadCap.xsd" MadCap:lastBlockDepth="6" MadCap:lastHeight="94" MadCap:lastWidth="853" MadCap:disableMasterStylesheet="true" MadCap:tocPath="" MadCap:InPreviewMode="false" MadCap:PreloadImages="false" MadCap:RuntimeFileType="Topic" MadCap:TargetType="WebHelp" lang="en-us" xml:lang="en-us" MadCap:PathToHelpSystem="../../../" MadCap:HelpSystemFileName="Default.xml" MadCap:SearchType="Stem">
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<head><title>POT</title>
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<link href="../../SkinSupport/MadCap.css" rel="stylesheet" />
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<link href="../../Resources/Stylesheets/BSE_Help.css" rel="stylesheet" />
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<script src="../../SkinSupport/MadCapAll.js" type="text/javascript">
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</script>
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</head>
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<body>
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<h1 class="code">POT</h1>
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<div class="ImagePlusCaption">
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<div class="Col2">
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<p>
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<img src="../../graphics/pgm_icon1.gif" border="0" alt="BS1 icon" title="BS1 icon" /><span class="code_in_text"> {PBASIC 1.0}</span>
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</p>
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</div>
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<p style="text-align: right;"><a href="../ExampleTopics/PotEx.htm" target="" title="" alt="" class="MCXref_0" xrefformat="{paratext}">POT Example</a>
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</p>
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<p> </p>
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</div>
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<p class="clear"> </p>
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<p> </p>
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<p class="PlainText">Syntax: <span class="keyword_in_text">POT</span> <![CDATA[ ]]><i>Pin</i>,<i> Scale</i>,<i> Variable</i></p>
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<h2>Function</h2>
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<p class="PlainText">Read a 5 kΩ to 50 kΩ potentiometer, thermistor, photocell, or other
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variable resistance.</p>
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<ul>
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<li value="1"><b><i>Pin</i></b> is a variable/constant (0 - 7) that specifies the I/O pin
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to use. This pin will be set to output mode initially, then to input mode. </li>
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<li value="2"><b><i>Scale</i></b> is a variable/constant (0 - 255) used to scale the
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command's internal 16-bit result. See Explanation below for steps to finding
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the scale value to use for your circuit.</li>
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<li value="3"><b><i>Variable</i></b> is a variable (usually a byte) where the final result
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of the reading will be stored. Internally, the <span class="keyword_in_text">POT</span> command calculates
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a 16-bit value, which is scaled down to an 8-bit value. </li>
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</ul>
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<p>Note: See <a href="RCTIME.htm" target="" title="" alt="" class="MCXref_0" xrefformat="{paratext}">RCTIME</a> for all BS2 models.</p>
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<h2>Explanation</h2>
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<p class="PlainText"><span class="keyword_in_text">POT</span> reads a variable resistance and returns a value (0 - 255) representing
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the amount of time it took to discharge the capacitor through the resistance. Pin
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must be connected to one side of the variable resistance, whose other side is
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connected through a capacitor to ground, as shown below. </p>
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<center>
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<img src="../../graphics/pot_sch.gif" border="0">
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</img>
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</center>
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<p> </p>
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<p class="PlainText"><span class="keyword_in_text">POT</span> works by first setting the specified I/O pin to an output and setting
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its state high. This step places +5 volts on one side of the capacitor and ground
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(0 volts) on the other side, which charges the capacitor. <span class="keyword_in_text">POT</span> will hold the
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pin high for 10 milliseconds to charge the capacitor. It is important to select
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component values that will allow the capacitor to charge in this period. If, for
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example, a 50 kΩ potentiometer is used with a 0.1 µF cap, the maximum
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charge time would be:</p>
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<p class="PlainText">Charge time: 5 x (50 x 10<sup>3</sup>) x (0.1 x 10<sup>-6</sup>)
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= 25 x 10<sup>-3</sup></p>
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<p class="PlainText">The full charge time (5 TC) is 25 x 10<sup>-3</sup> or 25 milliseconds.
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Obviously, the capacitor would never fully charge when the potentiometer is
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at its maximum position. In this case, changing the capacitor to 0.01 µF
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reduces the charge time to 2.5 milliseconds; well within the range of the
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<span class="keyword_in_text">POT</span> function.</p>
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<p class="PlainText">After the capacitor is charged the I/O pin to an input mode and <span class="keyword_in_text">POT</span>starts its timer. Initially the I/O pin will see a high (1) that will eventually
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drop to a low (0) when the capacitor discharges past the 1.4-volt threshold.
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The timer stops once the low is seen. The value of the variable resistor affects
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the time it takes to discharge the capacitor from 5 volts to approximately 1.4
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volts.</p>
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<p class="PlainText">The 16-bit reading is multiplied by (<i>Scale</i> ÷ 256), so a scale
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value of 128 would reduce the range by approximately 50%, a scale of 64 would
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reduce to 25%, and so on. The amount by which the internal value must be scaled
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varies with the size of the resistor being used.</p>
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<h3>Finding the best <i>Scale</i> value: </h3>
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<ol type="1">
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<li value="1">Build the circuit shown above and plug the BS1 into the PC. </li>
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<li value="2">Select Run | POT Scaling.<br></br> A special calibration window appears, allowing you to find the best value.</li>
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<li value="3">The window asks for the number of the I/O pin to which the variable resistor
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is connected. Select the appropriate pin (0 - 7).</li>
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<li value="4">The editor downloads a short program to the BS1 (this overwrites any program
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already stored in the BS1).</li>
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<li value="5">Two numbers will be displayed: scale and value. Adjust the resistor until
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the smallest number is shown for scale (assuming you can adjust the resistor,
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as with a potentiometer).</li>
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<li value="6">Once you've found the smallest number for scale, you're done. This number
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should be used for the <i>Scale</i> in the <span class="keyword_in_text">POT</span> command.</li>
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<li value="7">Optionally, you can verify the scale number found above by selecting the
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POT Value checkbox. This locks the scale and causes the BS1 to read the resistor
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continuously. The window displays the value. If the scale is good, you should
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be able to adjust the resistor, achieving a 0-255 reading for the value (or as
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close as possible). To change the scale value and repeat this step, just
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uncheck the box. Continue this process until you find the best scale.</li>
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</ol>
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<div class="Col2">
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<div class="MasterFoot">
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<p MadCap:conditions="BSEconditions.BSEWebHelp (Primary)-INCLUDE"><a href="../../HomeTopics/HomePage.htm">Go to Welcome page</a>
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</p>
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</div>
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</div>
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<div class="Col2">
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<div class="MasterFoot">
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<p style="text-align: right;"><span class="ContactInfoProjectName">BASIC Stamp Help</span> <![CDATA[ ]]><span class="ContactInfoVersion#">Version 2.5.4</span> <![CDATA[ ]]></p>
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<p style="text-align: right;">Copyright © <span class="ContactInfoCompanyName">Parallax Inc.</span></p>
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<p style="text-align: right;"><span class="SystemShortDate">8/8/2012</span>
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</p>
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</div>
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</div>
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