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<p>Constants are predefined variables in the Arduino language. They are used to make the programs easier to read. We classify constants in groups.
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<pclass='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>.
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<pclass='vspace'></p><h4>false</h4>
<p>false is the easier of the two to define. false is defined as 0 (zero).
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<pclass='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.
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<pclass='vspace'></p><p>Note that the <em>true</em> and <em>false</em> constants are typed in lowercase unlike HIGH, LOW, INPUT, & OUTPUT.
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<pclass='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>.
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<pclass='vspace'></p><p><strong>HIGH</strong>
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<pclass='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.
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<pclass='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.
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<pclass='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.
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<pclass='vspace'></p><p><strong>LOW</strong>
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<pclass='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.
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<pclass='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.
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<pclass='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.
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<pclass='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.
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<pclass='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.
<pclass='vspace'></p><p><em>Corrections, suggestions, and new documentation should be posted to the <aclass='urllink'href='http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?board=swbugs'rel='nofollow'>Forum</a>.</em>
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<pclass='vspace'></p><p>The text of the Arduino reference is licensed under a
<aclass='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.
