mirror of https://github.com/joan2937/pigpio
206 lines
8.0 KiB
HTML
206 lines
8.0 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<head>
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<meta name="description" content="Raspberry Pi Reg. C GPIO library and Python GPIO module and shell command utilities to control the GPIO, including SPI, I2C, and serial links." />
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<meta name="keywords" content="raspberry, pi, C, Python, GPIO, library, shell, command, utilities, module, SPI, I2C, serial" />
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<meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
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<title>pigpio library</title>
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<link rel="stylesheet" type="text/css" href="scripts/index.css">
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<link rel="icon" href="favicon.ico" type="image/x-icon">
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<link rel="shortcut icon" href="favicon.ico" type="image/x-icon">
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</head>
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<body>
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<table style="padding:0px; border:0px; margin:0px; width:780px; background-color:#e0e0e0;">
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<td style="background:#EAF2E6 url('images/sidebar.gif') repeat-y; width:35px; height:100%"></td>
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<td>
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<table>
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<div style="background:url('images/topbar.gif') repeat-x; height: 70px; font-size:1.5em; vertical-align: top;"><a href="index.html"><img src="images/pigpio-logo.gif" border="0" /></a>pigpio library</div>
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</table>
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<table><div>
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<td><img src="images/keypad.jpg" width="250"></td>
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<td><img src="images/ldr-cap.jpg" width="250"></td>
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<td><img src="images/meter.jpg" width="250"></td>
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</div></table>
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<table>
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<td style="vertical-align: top; background-color: #98bf21;"><a class="l1" href="index.html">pigpio</a>
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<a class="l1" href="cif.html">pigpio C I/F</a>
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<a class="l1" href="pigpiod.html">pigpiod</a>
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<a class="l1" href="pdif2.html">pigpiod C I/F</a>
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<a class="l1" href="python.html">Python</a>
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<a class="l1" href="pigs.html">pigs</a>
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<a class="l1" href="piscope.html">piscope</a>
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<a class="l1" href="misc.html">Misc</a>
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<a class="l1" href="examples.html">Examples</a>
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<a class="l1" href="download.html">Download</a>
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<a class="l1" href="faq.html">FAQ</a>
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<a class="l1" href="sitemap.html">Site Map</a>
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</td>
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<td><center><h2>LDR Example</h2></center>
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<p>The following code shows a method of reading analogue sensors on
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the digital input only Pi. A Light Dependent Resistor (LDR)
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varies its resistance according to the incident light
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intensisty.</p>
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<h3>SETUP</h3>
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<img src="images/LDR-fritz.png" alt="fritzing diagram" style=
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"width: 200px; height: 300px;" align="left" hspace="10">The LDR
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used is a Cadmium Sulphide device with a 1MOhm dark resistance and
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2-4KOhm at 100 lux. The capacitor is a 104
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ceramic.<span itemprop="name"><br>
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<br>
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One end of the capacitor is connected to Pi ground.<br>
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<br>
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One end of the LDR is connected to Pi 3V3.<br>
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<br>
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The other ends of the capacitor and LDR are connected to a spare
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gpio.</span><br>
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<p>Here P1-1 is used for 3V3, P1-20 is used for ground, and gpio 18
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(P1-12) is used for the gpio.<br clear="all"></p>
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<p><img src="images/LDR-photo.jpg" style=
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"width: 500px; height: 667px;" alt="photo of set-up"></p>
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<h3>CODE</h3>
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<code>#include <stdio.h><br>
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<br>
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#include <pigpio.h><br>
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<br>
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/*
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-----------------------------------------------------------------------<br>
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<br>
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3V3 ----- Light Dependent Resistor --+-- Capacitor
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----- Ground<br>
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|<br>
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+-- gpio<br>
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<br>
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<br>
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cc -o LDR LDR.c -lpigpio -lpthread -lrt<br>
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sudo ./LDR<br>
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<br>
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*/<br>
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<br>
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#define LDR 18<br>
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<br>
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/* forward declaration */<br>
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<br>
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void alert(int pin, int level, uint32_t tick);<br>
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<br>
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int main (int argc, char *argv[])<br>
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{<br>
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if (gpioInitialise()<0) return 1;<br>
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<br>
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gpioSetAlertFunc(LDR, alert); /* call alert when LDR
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changes state */<br>
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<br>
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while (1)<br>
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{<br>
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gpioSetMode(LDR, PI_OUTPUT); /*
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drain capacitor */<br>
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<br>
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gpioWrite(LDR, PI_OFF);<br>
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<br>
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gpioDelay(200); /* 50 micros is
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enough, 200 is overkill */<br>
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<br>
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gpioSetMode(LDR, PI_INPUT); /* start
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capacitor recharge */<br>
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<br>
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gpioDelay(10000); /* nominal 100
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readings per second */<br>
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}<br>
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<br>
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gpioTerminate();<br>
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}<br>
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<br>
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void alert(int pin, int level, uint32_t tick)<br>
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{<br>
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static uint32_t inited = 0;<br>
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static uint32_t lastTick, firstTick;<br>
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<br>
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uint32_t diffTick;<br>
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<br>
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if (inited)<br>
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{<br>
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diffTick = tick - lastTick;<br>
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lastTick = tick;<br>
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<br>
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if (level == 1) printf("%u %d\ ",
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tick-firstTick, diffTick);<br>
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}<br>
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else<br>
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{<br>
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inited = 1;<br>
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firstTick = tick;<br>
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lastTick = firstTick;<br>
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}<br>
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}<br></code>
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<h3>BUILD</h3>
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<code>cc -o LDR LDR.c -lpigpio -lrt -lpthread<br></code>
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<h3>RUN</h3>
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<code>sudo ./LDR >LDR.dat &</code><br>
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<br>
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While the program is running you can capture the waveform using the
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notification feature built in to pigpio. Issue the following
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commands on the Pi.<br>
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<br>
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<code>pigs no<br>
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pig2vcd </dev/pigpio0 >LDR.vcd &<br>
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pigs nb 0 0x40000 # set bit for gpio 18<br></code>
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<p>Change the light falling on the LDR for a few seconds (e.g.
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shine a torch on it or shade it with your hands).<br></p>
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<code>pigs nc 0</code><br>
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<p>The file LDR.vcd will contain the captured waveform, which can
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be viewed using GTKWave.</p>
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<p>Overview</p>
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<img src="images/LDR-wave-1.png" style=
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"width: 600px; height: 100px;" alt="LDR waveform 1"><br>
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<p>Reading circa every 10ms<br></p>
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<img src="images/LDR-wave-2.png" style=
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"width: 600px; height: 100px;" alt="LDR waveform 2"><br>
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<p>One reading, circa 400us<br></p>
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<img src="images/LDR-wave-3.png" style=
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"width: 600px; height: 100px;" alt="LDR waveform 3"><br>
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<p>The file LDR.dat will contain pairs of timestamps and recharge
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time (in us). The following script will convert the
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timestamps into seconds.<span style=
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"font-style: italic;"><br></span></p>
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<p><code>awk '{print $1/1000000, $2}' LDR.dat
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>LDR-secs.dat</code></p>
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<p>Gnuplot is a useful tool to graph data.<br></p>
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plot [14:24] 'LDR-secs.dat' with lines title 'LDR'
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<p>Gnuplot readings 14-24 seconds<br></p>
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<p><img src="images/LDR-gnup-1.png" style=
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"width: 600px; height: 321px;" alt="gnuplot 1"></p>
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plot [18:21] 'LDR-secs.dat' with lines title 'LDR'<br>
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<br>
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Gnuplot readings 18-21 seconds
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<p><img src="images/LDR-gnup-2.png" style=
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"width: 600px; height: 321px;" alt="Gnuplot 2"></p>
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</td>
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</table>
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<div style="vertical-align: center; text-align: center; background-color:#98bf21; font-size:0.8em; height:30px"><a class="l2" href="index.html">[pigpio]</a>
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<a class="l2" href="cif.html">[pigpio C I/F]</a>
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<a class="l2" href="pigpiod.html">[pigpiod]</a>
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<a class="l2" href="pdif2.html">[pigpiod C I/F]</a>
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<a class="l2" href="python.html">[Python]</a>
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<a class="l2" href="pigs.html">[pigs]</a>
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<a class="l2" href="piscope.html">[piscope]</a>
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<a class="l2" href="misc.html">[Misc]</a>
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<a class="l2" href="examples.html">[Examples]</a>
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<a class="l2" href="download.html">[Download]</a>
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<a class="l2" href="faq.html">[FAQ]</a>
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<a class="l2" href="sitemap.html">[Site Map]</a>
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</div>
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<table><tr>
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<td style="width: 200px"><div style="text-align: left;"><small>© 2012-2020</small></div></td>
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<td style="width: 350px"><div style="text-align: center;">e-mail: pigpio @ abyz.me.uk</div></td>
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<td style="width: 200px"><div style="text-align: right;"><small>Updated: 29/04/2020</small></div></td>
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</tr></table>
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</td>
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</table>
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</body>
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</html>
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