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+.. _arduino-constants:
+
+constants
+=========
+
+Constants are predefined variables in the Arduino language. They
+are used to make the programs easier to read. We classify constants
+in groups.
+
+
+
+Defining Logical Levels, true and false (Boolean Constants)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are two constants used to represent truth and falsity in the
+Arduino language: **true**, and **false**.
+
+
+
+false
+-----
+
+false is the easier of the two to define. false is defined as 0
+(zero).
+
+
+
+true
+----
+
+true is often said to be defined as 1, which is correct, but true
+has a wider definition. Any integer which is *non-zero* is TRUE, in
+a Boolean sense. So -1, 2 and -200 are all defined as true, too, in
+a Boolean sense.
+
+
+
+Note that the *true* and *false* constants are typed in lowercase
+unlike HIGH, LOW, INPUT, & OUTPUT.
+
+
+
+Defining Pin Levels, HIGH and LOW
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When reading or writing to a digital pin there are only two
+possible values a pin can take/be-set-to: **HIGH** and **LOW**.
+
+
+
+**HIGH**
+
+
+
+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.
+
+
+
+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 *steer* the input pin to
+a HIGH reading unless it is pulled LOW by external circuitry.
+
+
+
+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
+*source* 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.
+
+
+
+**LOW**
+
+
+
+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.
+
+
+
+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
+*sink* 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.
+
+
+
+Defining Digital Pins, INPUT and OUTPUT
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Digital pins can be used either as **INPUT** or **OUTPUT**.
+Changing a pin from INPUT TO OUTPUT with pinMode() drastically
+changes the electrical behavior of the pin.
+
+
+
+Pins Configured as Inputs
+-------------------------
+
+Arduino (Atmega) pins configured as **INPUT** 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.
+
+
+
+Pins Configured as Outputs
+--------------------------
+
+Pins configured as **OUTPUT** 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.
+
+
+
+See also
+--------
+
+
+-  `pinMode() <http://arduino.cc/en/Reference/PinMode>`_
+-  `Integer Constants <http://arduino.cc/en/Reference/IntegerConstants>`_
+-  `boolean variables <http://arduino.cc/en/Reference/BooleanVariables>`_
+
+