Arduino

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## constants

Constants are predefined expressions 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`, and `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 Arduino (Atmega) will report `HIGH` if:

- a voltage greater than 3 volts is present at the pin (5V boards);
- a voltage greater than 2 volts is present at the pin (3.3V boards);

A pin may also be configured as an `INPUT` with `pinMode()`, and subsequently made `HIGH` with `digitalWrite()`. This will enable the internal 20K pullup resistors, which will pull up the input pin to a `HIGH` reading unless it is pulled `LOW` by external circuitry. This is how `INPUT_PULLUP` works and is described below in more detail.

When a pin is configured to `OUTPUT` with `pinMode()`, and set to `HIGH` with `digitalWrite()`, the pin is at:

- 5 volts (5V boards);
- 3.3 volts (3.3V boards);

In this state it can source current, e.g. light an LED that is connected through a series resistor to ground.

#### 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 Arduino (Atmega) will report `LOW` if:

- a voltage less than 3 volts is present at the pin (5V boards);
- a voltage less than 2 volts is present at the pin (3.3V boards);

When a pin is configured to `OUTPUT` with `pinMode()`, and set to `LOW` with `digitalWrite()`, the pin is at 0 volts (both 5V and 3.3V boards). In this state it can sink current, e.g. light an LED that is connected through a series resistor to +5 volts (or +3.3 volts).

### Defining Digital Pins modes: `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.

If you have your pin configured as an `INPUT`, and are reading a switch, when the switch is in the open state the input pin will be "floating", resulting in unpredictable results. In order to assure a proper reading when the switch is open, a pull-up or pull-down resistor must be used. The purpose of this resistor is to pull the pin to a known state when the switch is open. A 10 K ohm resistor is usually chosen, as it is a low enough value to reliably prevent a floating input, and at the same time a high enough value to not not draw too much current when the switch is closed. See the Digital Read Serial tutorial for more information.

If a pull-down resistor is used, the input pin will be `LOW` when the switch is open and `HIGH` when the switch is closed.

If a pull-up resistor is used, the input pin will be `HIGH` when the switch is open and `LOW` when the switch is closed.

#### Pins Configured as INPUT_PULLUP

The Atmega microcontroller 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-up resistors, you can use the INPUT_PULLUP argument in `pinMode()`.

See the Input Pullup Serial tutorial for an example of this in use.

Pins configured as inputs with either `INPUT` or `INPUT_PULLUP` can be damaged or destroyed if they are connected to voltages below ground (negative voltages) or above the positive power rail (5V or 3V).

#### 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 current) or sink (absorb current) up to 40 mA (milliamps) of current to other devices/circuits. This makes them useful for powering LEDs because LEDs typically use less than 40 mA. Loads greater than 40 mA (e.g. motors) will require a transistor or other interface circuitry.

Pins configured as outputs can be damaged or destroyed if they are connected to either the ground or positive power rails.

### Defining built-ins: LED_BUILTIN

Most Arduino boards have a pin connected to an on-board LED in series with a resistor. The constant `LED_BUILTIN` is the number of the pin to which the on-board LED is connected. Most boards have this LED connected to digital pin 13.

#### See also

- pinMode()
- Integer Constants
- boolean variables

Reference Home

Corrections, suggestions, and new documentation should be posted to the Forum.

The text of the Arduino reference is licensed under a Creative Commons Attribution-ShareAlike 3.0 License. Code samples in the reference are released into the public domain.

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