201 lines
13 KiB
HTML
201 lines
13 KiB
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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>PWM</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">PWM</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" />
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<img src="../../graphics/pgm_icon2.gif" border="0" alt="BS2 icon" title="BS2 icon" />
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<img src="../../graphics/pgm_icon2e.gif" border="0" alt="BS2e icon" title="BS2e icon" />
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<img src="../../graphics/pgm_icon2sx.gif" border="0" alt="BS2sx icon" title="BS2sx icon" />
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<img src="../../graphics/pgm_icon2p.gif" border="0" alt="BS2p icon" title="BS2p icon" />
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<img src="../../graphics/pgm_icon2pe.gif" border="0" alt="BS2pe icon" title="BS2pe icon" />
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<img src="../../graphics/pgm_icon2px.gif" border="0" alt="BS2px icon" title="BS2px icon" />
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</p>
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</div>
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<p style="text-align: right;"><a href="../ExampleTopics/PwmEx.htm" target="" title="" alt="" class="MCXref_0">PWM Examples</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">PWM</span> <![CDATA[ ]]><i>Pin</i>,<i> Duty</i>,<i> Duration</i></p>
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<h2>Function</h2>
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<p class="PlainText">Convert a digital value to analog output via pulse-width modulation.
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</p>
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<ul>
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<li value="1"><b><i>Pin</i></b> is a variable/constant/expression* (0 - 15) that specifies
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the I/O pin to use. This pin will be set to output mode initially then set to
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input mode when the command finishes.</li>
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<li value="2"><b><i>Duty</i></b> is a variable/constant/expression* (0 - 255) that specifies
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the analog output level as a number of 256<sup>ths</sup> of 5 V (0 to 4.98 V).</li>
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<li value="3"><b><i>Duration</i></b> is a variable/constant/expression* (0 - 255) that
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specifies the duration of the PWM output.</li>
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</ul>
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<p class="PlainText">*<img src="../../graphics/bs1note.gif" style="vertical-align: super;" /> Note: expressions are not allowed as arguments on the BS1. The range
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of the <i>Pin</i> argument on the BS1 is 0–7.</p>
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<h2>Quick Facts</h2>
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<table width="100%" cellpadding="4" cellspacing="0" border="1">
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<tr align="center" bgcolor="#CFCFCF">
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<td nowrap="" width="150"> </td>
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<td width="12%">BS1</td>
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<td width="12%">BS2 and BS2e</td>
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<td width="12%">BS2sx</td>
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<td width="12%">BS2p</td>
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<td width="12%">BS2pe</td>
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<td width="12%">BS2px</td>
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</tr>
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<tr align="center">
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<td bgcolor="#CFCFCF">Units in <i>Duration</i></td>
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<td>5 ms</td>
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<td>1 ms</td>
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<td>0.4 ms</td>
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<td>0.65 ms</td>
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<td>1.62 ms</td>
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<td>0.4 ms</td>
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</tr>
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<tr align="center">
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<td bgcolor="#CFCFCF">Average voltage equation</td>
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<td colspan="6">Average Voltage = (<i>Duty</i> ÷ 256) x 5 volts</td>
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</tr>
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<tr align="center">
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<td bgcolor="#CFCFCF">Require charge time (<i>Duration</i> ) equation</td>
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<td colspan="6">Charge time = 5 x R x C,
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<i>Duration</i> = Charge time ÷ Units in Duration</td>
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</tr>
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<tr align="center">
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<td bgcolor="#CFCFCF">Special Notes</td>
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<td colspan="6"><i>Pin</i> is set to output initially, and set to input at end</td>
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</tr>
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<tr>
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<td align="center" bgcolor="#CFCFCF">Related Commands</td>
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<td align="center" colspan="6">
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<p colspan="6" align="center"><a href="FREQOUT.htm" target="" title="" alt="" class="MCXref_0">FREQOUT</a>, <a href="DTMFOUT.htm" target="" title="" alt="" class="MCXref_0">DTMFOUT</a></p>
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</td>
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</tr>
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</table>
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<h2>Explanation</h2>
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<p class="PlainText">Pulse-width modulation (<span class="keyword_in_text">PWM</span>) allows the BASIC Stamp (a purely digital device)
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to generate an analog voltage. The basic idea is this: If you make a pin output
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high, the voltage at that pin will be close to 5 V. Output low is close to 0 V.
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What if you switched the pin rapidly between high and low so that it was high half
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the time and low half the time? The average voltage over time would be halfway
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between 0 and 5 V (2.5 V). <span class="keyword_in_text">PWM</span> emits a burst of 1s and 0s whose ratio is
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proportional to the duty value you specify.</p>
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<p class="PlainText">The proportion of 1s to 0s in <span class="keyword_in_text">PWM</span> is called the duty cycle. The duty
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cycle controls the analog voltage in a very direct way; the higher the duty cycle
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the higher the voltage. In the case of the BASIC Stamp, the duty cycle can range
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from 0 to 255. <i>Duty</i> is literally the proportion of 1s to 0s output by the
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<span class="keyword_in_text">PWM</span> command. To determine the proportional <span class="keyword_in_text">PWM</span> output voltage, use
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this formula: (<i>Duty</i> ÷ 256) x 5 V. For example, if <i>Duty</i>
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is 100, (100 ÷ 256) x 5 V = 1.953 V; <span class="keyword_in_text">PWM</span> outputs a train of pulses
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whose average voltage is 1.953 V.</p>
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<p class="PlainText">In order to convert <span class="keyword_in_text">PWM</span> into an analog voltage we have to filter out the
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pulses and store the average voltage. The resistor/capacitor combination shown
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below will do the job. The capacitor will hold the voltage set by <span class="keyword_in_text">PWM</span> even
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after the instruction has finished. How long it will hold the voltage depends on
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how much current is drawn from it by external circuitry, and the internal leakage
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of the capacitor. In order to hold the voltage relatively steady, a program must
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periodically repeat the <span class="keyword_in_text">PWM</span> instruction to give the capacitor a fresh
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charge. </p>
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<p class="PlainText">Just as it takes time to discharge a capacitor, it also takes time to charge
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it in the first place. The <span class="keyword_in_text">PWM</span> command lets you specify the charging time
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in terms of <span class="keyword_in_text">PWM</span> duration. The timing for the units in <i>Duration</i> is
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shown in in the table above. So, on the BS2, to charge a capacitor for five
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milliseconds, you would specify five units in <i>Duration</i>. </p>
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<p class="PlainText">How do you determine how long to charge a capacitor? Use this rule-of-thumb
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formula: Charge time = 5 x R x C. For instance, the circuit below uses a 10 kΩ
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(10 x 10<sup>3</sup> ohm) resistor and a 1 µF (1 x 10<sup>-6</sup> F) capacitor:</p>
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<p class="PlainText">Charge time = 5 x 10 x 10<sup>3</sup> x 1 x 10<sup>-6</sup> = 50 x 10<sup>-3</sup>
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seconds, or 50 milliseconds.</p>
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<center>
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<img src="../../graphics/pwm_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">Since, on the BS2, each unit in <i>Duration</i> is approximately a millisecond,
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it would take at least 50 units to charge the capacitor. Assuming the circuit is
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connected to P0, here's the complete <span class="keyword_in_text">PWM</span> instruction: </p>
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<p>
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<img src="../../graphics/bs2e_inline.gif" border="0">
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</img>
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</p><pre class="BScode" xml:space="preserve">
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PWM 0, 100, 50 ' Put a 1.96V charge on cap (BS2, BS2e)
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</pre>
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<p class="PlainText">To charge the same circuit to the same level using a BS2sx, the <i>Duration</i>would require adjustment as follows:</p>
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<p>
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<img src="../../graphics/bs2sx_inline.gif" border="0">
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</img>
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</p><pre class="BScode" xml:space="preserve">
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PWM 0, 100, 125 ' Put a 1.96V charge on cap (BS2sx)
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</pre>
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<p class="PlainText">After outputting the <span class="keyword_in_text">PWM</span> pulses, the BASIC Stamp leaves the pin in input
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mode (0 in the corresponding bit of DIRS). In input mode, the pin's output driver
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is effectively disconnected. If it were not, the steady output state of the pin
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would change the voltage on the capacitor and undo the voltage setting established
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by <span class="keyword_in_text">PWM</span>. Keep in mind that leakage currents of up to 1 µA can flow into
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or out of this "disconnected" pin. Over time, these small currents will cause the
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voltage on the capacitor to drift. The same applies for leakage current from an
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op-amp's input, as well as the capacitor's own internal leakage. Executing
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<span class="keyword_in_text">PWM</span> occasionally will reset the capacitor voltage to the intended value.</p>
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<p class="PlainText"><span class="keyword_in_text">PWM</span> charges the capacitor; the load presented by your circuit discharges
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it. How long the charge lasts (and therefore how often your program should repeat
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the <span class="keyword_in_text">PWM</span> command to refresh the charge) depends on how much current the
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circuit draws, and how stable the voltage must be. You may need to buffer <span class="keyword_in_text">PWM</span>output with a simple op-amp follower if your load or stability requirements are
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more than the passive circuit can handle. </p>
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<p class="PlainText">The term "PWM" applies only loosely to the action of the BASIC Stamp's <span class="keyword_in_text">PWM</span>
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command. Most systems that output <span class="keyword_in_text">PWM</span> do so by splitting a fixed period
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of time into an on time (1) and an off time (0). Suppose the interval is 1 ms
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and the duty cycle is 100 ÷ 256. Conventional <span class="keyword_in_text">PWM</span> would turn the
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output on for 0.39 ms and off for 0.61 ms, repeating this process each millisecond.
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The main advantage of this kind of <span class="keyword_in_text">PWM</span> is its predictability; you know the
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exact frequency of the pulses (in this case, 1 kHz), and their widths are
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controlled by the duty cycle. </p>
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<p class="PlainText">BASIC Stamp's <span class="keyword_in_text">PWM</span> <![CDATA[ ]]><u>does not work this way</u>. It outputs a rapid
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sequence of on/off pulses, as short as 1.6 µs in duration, whose overall proportion
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over the course of a full <span class="keyword_in_text">PWM</span> cycle of approximately a millisecond is equal
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to the duty cycle. This has the advantage of very quickly zeroing in on the desired
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output voltage, but it does not produce the neat, orderly pulses that you might
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expect. All BS2 modules also uses this high-speed <span class="keyword_in_text">PWM</span>technique to generate pseudo-sine wave tones with the
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<a href="DTMFOUT.htm" target="" title="" alt="" class="MCXref_0">DTMFOUT</a> and <a href="FREQOUT.htm" target="" title="" alt="" class="MCXref_0">FREQOUT</a>
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instructions. </p>
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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 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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