pigpio/DOC/tmp/body/ex_rotary_encoder.body

<p>The following code shows one way to read an incremental
mechanical rotary enoder (the sort used for volume control in audio
systems).&nbsp; These rotary encoders have two switches A and B
which return a quadrature output, i.e. they are 90 degrees out of
phase.</p>
<h3>SETUP</h3>
<img alt="fritzing diagram" style="width: 200px; height: 300px;"
src="images/re-fritz.png" align="left" hspace="10">The common
(centre) terminal should be connected to a Pi ground.
<p>The A and B terminals may be connected to any spare gpios.</p>
<p>Here A to gpio18 (P1-12), common to ground (P1-20), B to gpio7
(P1-26).<br clear="all"></p>
<p><img src="images/re-photo.jpg" style=
"width: 400px; height: 300px;" alt="photo of set-up"></p>
<h3>CODE</h3>
<code>#include &lt;stdio.h&gt;<br>
<br>
#include &lt;pigpio.h&gt;<br>
<br>
/*<br>
&nbsp;&nbsp; Rotary encoder connections:<br>
<br>
&nbsp;&nbsp; Encoder A&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; - gpio
18&nbsp;&nbsp; (pin P1-12)<br>
&nbsp;&nbsp; Encoder B&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; - gpio
7&nbsp;&nbsp;&nbsp; (pin P1-26)<br>
&nbsp;&nbsp; Encoder Common - Pi ground (pin P1-20)<br>
*/<br>
<br>
#define ENCODER_A 18<br>
#define ENCODER_B&nbsp; 7<br>
<br>
static volatile int encoderPos;<br>
<br>
/* forward declaration */<br>
<br>
void encoderPulse(int gpio, int lev, uint32_t tick);<br>
<br>
int main(int argc, char * argv[])<br>
{<br>
&nbsp;&nbsp; int pos=0;<br>
<br>
&nbsp;&nbsp; if (gpioInitialise()&lt;0) return 1;<br>
<br>
&nbsp;&nbsp; gpioSetMode(ENCODER_A, PI_INPUT);<br>
&nbsp;&nbsp; gpioSetMode(ENCODER_B, PI_INPUT);<br>
<br>
&nbsp;&nbsp; /* pull up is needed as encoder common is grounded
*/<br>
<br>
&nbsp;&nbsp; gpioSetPullUpDown(ENCODER_A, PI_PUD_UP);<br>
&nbsp;&nbsp; gpioSetPullUpDown(ENCODER_B, PI_PUD_UP);<br>
<br>
&nbsp;&nbsp; encoderPos = pos;<br>
<br>
&nbsp;&nbsp; /* monitor encoder level changes */<br>
<br>
&nbsp;&nbsp; gpioSetAlertFunc(ENCODER_A, encoderPulse);<br>
&nbsp;&nbsp; gpioSetAlertFunc(ENCODER_B, encoderPulse);<br>
<br>
&nbsp;&nbsp; while (1)<br>
&nbsp;&nbsp; {<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; if (pos != encoderPos)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; {<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; pos =
encoderPos;<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; printf("pos=%d\ ",
pos);<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; gpioDelay(20000); /* check pos 50
times per second */<br>
&nbsp;&nbsp; }<br>
<br>
&nbsp;&nbsp; gpioTerminate();<br>
}<br>
<br>
void encoderPulse(int gpio, int level, uint32_t tick)<br>
{<br>
&nbsp;&nbsp; /*<br>
<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+---------+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+---------+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 0<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |<br>
&nbsp;&nbsp; A&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |<br>
&nbsp;&nbsp;
+---------+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+---------+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +-----
1<br>
<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+---------+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+---------+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
0<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |<br>
&nbsp;&nbsp; B&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |<br>
&nbsp;&nbsp; ----+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+---------+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+---------+&nbsp; 1<br>
<br>
&nbsp;&nbsp; */<br>
<br>
&nbsp;&nbsp; static int levA=0, levB=0, lastGpio = -1;<br>
<br>
&nbsp;&nbsp; if (gpio == ENCODER_A) levA = level; else levB =
level;<br>
<br>
&nbsp;&nbsp; if (gpio != lastGpio) /* debounce */<br>
&nbsp;&nbsp; {<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; lastGpio = gpio;<br>
<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; if ((gpio == ENCODER_A) &amp;&amp;
(level == 0))<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; {<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; if (!levB)
++encoderPos;<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; else if ((gpio == ENCODER_B)
&amp;&amp; (level == 1))<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; {<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; if (levA)
--encoderPos;<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }<br>
&nbsp;&nbsp; }<br>
}<br></code>
<h3>BUILD</h3>
<code>cc -o rotary_encoder rotary_encoder.c -lpigpio -lrt
-lpthread<br></code>
<h3>RUN</h3>
<code>sudo ./rotary_encoder</code><br>
<p>While the program is running you can capture the waveform using
the notification feature built in to pigpio.&nbsp; Issue the
following commands on the Pi.</p>
<code>pigs no<br>
pig2vcd&nbsp; &lt;/dev/pigpio0 &gt;re.vcd &amp;<br>
pigs nb 0 0x40080 # set bits for gpios 7 (0x80) and 18
(0x40000)<br></code>
<p>Twiddle the rotary encoder forwards and backwards for a few
seconds.&nbsp; Then enter<br></p>
<code>pigs nc 0</code><br>
<p>The file re.vcd will contain the captured waveform, which can be
viewed using GTKWave.</p>
<br>
Overview<br>
<br>
<img src="images/re-wave-1.png" style=
"width: 600px; height: 100px;" alt=
"rotary encoder waveform overview"><br>
<br>
Detail of switch bounce.&nbsp; Contact A bounces for circa 700 us
before completing the level transition<br>
<br>
<img src="images/re-wave-2.png" style=
"width: 600px; height: 100px;" alt=
"rotary encoder waveform detail"><br>