====== 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. This is how INPUT_PULLUP works as well

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, INPUT_PULLUP, and OUTPUT   =====

Digital pins can be used as **INPUT**, **INPUT_PULLUP**, or **OUTPUT**. Changing a pin with pinMode() changes the electrical behavior of the pin.

==== Pins Configured as INPUT   ====

Arduino (Atmega) pins configured as **INPUT** with pinMode() are said to be in a high-impedance state. Pins configured as INPUT make extremely small demands on the circuit that they are sampling, 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 INPUT_PULLUP   ====

The Atmega chip on the Arduino has internal pull-up resistors (resistors that connect to power internally) that you can access. If you prefer to use these instead of external pull-down resistors, you can use the **INPUT_PULLUP** argument in pinMode(). This effectively inverts the behavior, where HIGH means the sensor is off, and LOW means the sensor is on.

==== 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   ====

* [[arduino:digital io:PinMode |pinMode()]]
* [[arduino:constants:IntegerConstants| Integer Constants]]
* [[arduino:boolean_operators:boolean | boolean variables]]


Source: arduino.cc