<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<title>Arduino Reference - Constants </title>
<link rel="shortcut icon" type="image/x-icon" href="http://arduino.cc/en/favicon.png">
<link rel='stylesheet' href='arduinoWideRender.css'
type='text/css' />
<!--HeaderText--><style type='text/css'><!--
ul, ol, pre, dl, p { margin-top:0px; margin-bottom:0px; }
code { white-space: nowrap; }
.vspace { margin-top:1.33em; }
.indent { margin-left:40px; }
.outdent { margin-left:40px; text-indent:-40px; }
a.createlinktext { text-decoration:none; border-bottom:1px dotted gray; }
a.createlink { text-decoration:none; position:relative; top:-0.5em;
font-weight:bold; font-size:smaller; border-bottom:none; }
img { border:0px; }
.editconflict { color:green;
font-style:italic; margin-top:1.33em; margin-bottom:1.33em; }
table.markup { border: 2px dotted #ccf; width:90%; }
td.markup1, td.markup2 { padding-left:10px; padding-right:10px; }
td.markup1 { border-bottom: 1px solid #ccf; }
div.faq { margin-left:2em; }
div.faq p.question { margin: 1em 0 0.75em -2em; font-weight:bold; }
div.faq hr { margin-left: -2em; }
.frame
{ border:1px solid #cccccc; padding:4px; background-color:#f9f9f9; }
.lfloat { float:left; margin-right:0.5em; }
.rfloat { float:right; margin-left:0.5em; }
a.varlink { text-decoration:none; }
--></style> <meta name='robots' content='index,follow' />
<meta name="verify-v1" content="TtxFIEJAB6zdJ509wLxjnapQzKAMNm9u0Wj4ho6wxIY=" />
</head>
<body>
<div id="page">
<!--PageHeaderFmt-->
<div id="pageheader">
<div class="title"><a href='http://arduino.cc/en'>Arduino</a></div>
</div>
<!--/PageHeaderFmt-->
<!--PageLeftFmt-->
<div id="pagenav">
<div id="navbar">
<p><a class='wikilink' href='http://arduino.cc/en/Main/Buy'>Buy</a>
<a class='wikilink' href='http://arduino.cc/en/Main/Software'>Download</a>
<a class='wikilink' href='Guide_index.html'>Getting Started</a>
<a class='wikilink' href='http://arduino.cc/en/Tutorial/HomePage'>Learning</a>
<a class='wikilink' href='index.html'>Reference</a>
<a class='wikilink' href='http://arduino.cc/en/Main/Hardware'>Hardware</a>
<a class='wikilink' href='FAQ.html'>FAQ</a>
</p>
<p class='vspace'></p>
</div>
</div>
<!--/PageLeftFmt-->
<div id="pagetext">
<!--PageText-->
<div id='wikitext'>
<p><strong>Reference</strong> <a class='wikilink' href='index.html'>Language</a> | <a class='wikilink' href='Libraries.html'>Libraries</a> | <a class='wikilink' href='Comparison.html'>Comparison</a> | <a class='wikilink' href='Changes.html'>Changes</a>
</p>
<p class='vspace'></p><h2>constants</h2>
<p>Constants are predefined variables in the Arduino language. They are used to make the programs easier to read. We classify constants in groups.
</p>
<p class='vspace'></p><h3>Defining Logical Levels, true and false (Boolean Constants)</h3>
<p>There are two constants used to represent truth and falsity in the Arduino language: <strong>true</strong>, and <strong>false</strong>.
</p>
<p class='vspace'></p><h4>false</h4>
<p>false is the easier of the two to define. false is defined as 0 (zero).
</p>
<p class='vspace'></p><h4>true</h4>
<p>true is often said to be defined as 1, which is correct, but true has a wider definition. Any integer which is <em>non-zero</em> is TRUE, in a Boolean sense. So -1, 2 and -200 are all defined as true, too, in a Boolean sense.
</p>
<p class='vspace'></p><p>Note that the <em>true</em> and <em>false</em> constants are typed in lowercase unlike HIGH, LOW, INPUT, & OUTPUT.
</p>
<p class='vspace'></p><h3>Defining Pin Levels, HIGH and LOW</h3>
<p>When reading or writing to a digital pin there are only two possible values a pin can take/be-set-to: <strong>HIGH</strong> and <strong>LOW</strong>.
</p>
<p class='vspace'></p><p><strong>HIGH</strong>
</p>
<p class='vspace'></p><p>The meaning of HIGH (in reference to a pin) is somewhat different depending on whether a pin is set to an INPUT or OUTPUT.
When a pin is configured as an INPUT with pinMode, and read with digitalRead, the microcontroller will report HIGH if a voltage of 3 volts or more is present at the pin.
</p>
<p class='vspace'></p><p>A pin may also be configured as an INPUT with pinMode, and subsequently made HIGH with digitalWrite, this will set the internal 20K pullup resistors, which will <em>steer</em> the input pin to a HIGH reading unless it is pulled LOW by external circuitry.
</p>
<p class='vspace'></p><p>When a pin is configured to OUTPUT with pinMode, and set to HIGH with digitalWrite, the pin is at 5 volts. In this state it can <em>source</em> current, e.g. light an LED that is connected through a series resistor to ground, or to another pin configured as an output, and set to LOW.
</p>
<p class='vspace'></p><p><strong>LOW</strong>
</p>
<p class='vspace'></p><p>The meaning of LOW also has a different meaning depending on whether a pin is set to INPUT or OUTPUT. When a pin is configured as an INPUT with pinMode, and read with digitalRead, the microcontroller will report LOW if a voltage of 2 volts or less is present at the pin.
</p>
<p class='vspace'></p><p>When a pin is configured to OUTPUT with pinMode, and set to LOW with digitalWrite, the pin is at 0 volts. In this state it can <em>sink</em> current, e.g. light an LED that is connected through a series resistor to, +5 volts, or to another pin configured as an output, and set to HIGH.
</p>
<p class='vspace'></p><h3>Defining Digital Pins, INPUT and OUTPUT</h3>
<p>Digital pins can be used either as <strong>INPUT</strong> or <strong>OUTPUT</strong>. Changing a pin from INPUT TO OUTPUT with pinMode() drastically changes the electrical behavior of the pin.
</p>
<p class='vspace'></p><h4>Pins Configured as Inputs</h4>
<p>Arduino (Atmega) pins configured as <strong>INPUT</strong> with pinMode() are said to be in a high-impedance state. One way of explaining this is that pins configured as INPUT make extremely small demands on the circuit that they are sampling, say equivalent to a series resistor of 100 Megohms in front of the pin. This makes them useful for reading a sensor, but not powering an LED.
</p>
<p class='vspace'></p><h4>Pins Configured as Outputs</h4>
<p>Pins configured as <strong>OUTPUT</strong> with pinMode() are said to be in a low-impedance state. This means that they can provide a substantial amount of current to other circuits. Atmega pins can source (provide positive current) or sink (provide negative current) up to 40 mA (milliamps) of current to other devices/circuits. This makes them useful for powering LED's but useless for reading sensors. Pins configured as outputs can also be damaged or destroyed if short circuited to either ground or 5 volt power rails. The amount of current provided by an Atmega pin is also not enough to power most relays or motors, and some interface circuitry will be required.
</p>
<p class='vspace'></p><h4>See also</h4>
<ul><li><a class='wikilink' href='PinMode.html'>pinMode()</a>
</li><li><a class='wikilink' href='IntegerConstants.html'>Integer Constants</a>
</li><li><a class='wikilink' href='BooleanVariables.html'>boolean variables</a>
</li></ul><p class='vspace'></p><p><a class='wikilink' href='index.html'>Reference Home</a>
</p>
<p class='vspace'></p><p><em>Corrections, suggestions, and new documentation should be posted to the <a class='urllink' href='http://arduino.cc/forum/index.php/board,23.0.html' rel='nofollow'>Forum</a>.</em>
</p>
<p class='vspace'></p><p>The text of the Arduino reference is licensed under a
<a class='urllink' href='http://creativecommons.org/licenses/by-sa/3.0/' rel='nofollow'>Creative Commons Attribution-ShareAlike 3.0 License</a>. Code samples in the reference are released into the public domain.
</p>
</div>
</div>
<!--PageFooterFmt-->
<div id="pagefooter">
©Arduino |
<a href='#'>Edit Page</a> | <a href='#'>Page History</a> | <a href='#' target='_blank'>Printable View</a> | <a href='http://arduino.cc/en/Site/AllRecentChanges'>All Recent Site Changes</a>
</div>
<!--/PageFooterFmt-->
</div>
</body>
</html>
