Constants are like predefined variables, whose values can’t change. They are used to make the programs easier to read and modify. This page describes the most commonly used constants.
Contents
There are two constants used to represent truth and falsity: true, and false.
true is the true bool value. As an integer, true is often said to be 1. This is correct in the sense that true evaluates to 1 as an integer. However, any integer which is non-zero is true as a bool. So -1, 2 and -200 are all “true”, in the sense that these numbers are treated the same as true in a boolean context.
Note that the true and false constants are typed in lowercase; unlike e.g. HIGH, LOW, INPUT, and OUTPUT (which are described below).
When reading or writing to a digital pin there are only two possible values a pin can be set to: HIGH and LOW.
The meaning of HIGH (in reference to a pin) is somewhat different depending on whether the pin is set to INPUT or OUTPUT. When a pin is configured as an INPUT (using pinMode()), and read with digitalRead(), the microcontroller will report HIGH if a voltage of 3 volts or more is present at the pin.
When a pin is configured to OUTPUT with pinMode(), and set to HIGH with digitalWrite(), the pin is at 3.3 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.
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 microcontroller will attempt to keep that pin’s voltage at 0 V. In this state it can sink current, e.g. light an LED that is connected through a series resistor to +3.3 V, or to another pin configured as an output, and set to HIGH.
Digital pins can be used in a variety of modes. The basic modes, INPUT and OUTPUT, have been introduced above. Changing a pin from INPUT TO OUTPUT with pinMode() drastically changes the electrical behavior of the pin.
This section describes the basic digital pin modes (INPUT and OUTPUT) only. For a detailed description of all possible pin modes, see the pinMode() reference page.
Pins configured as INPUT are said to be in a high-impedance state. One way of explaining this is that pins configured as INPUT make very few demans on circuit that they are connected to. This makes them useful for reading a sensor, but not powering an LED.
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. Pins can source (provide positive current) or sink (provide negative current) up to 50 mA (milliamps) of current to other devices/circuits. This makes them useful for powering LEDs, but useless for reading sensors.
Pins configured as outputs can also be damaged or destroyed if short circuited to either ground or power supplies. The amount of current provided by a pin is also not enough to power most relays or motors, and some interface circuitry will be required.
Integer constants are numbers used directly in a sketch, like 123. By default, an integer constant is treated as a (signed) int, but you can change this with the U and L modifiers (see below). You can specify negative numbers by putting a minus sign in front, like -123.
Normally, integer constants are treated as base 10 (decimal) integers, but special notation (formatters) may be used to enter numbers in other bases. These are explained in the following table:
| Base | Example | Formatter | Comment |
|---|---|---|---|
| 10 (decimal) | 123 | None | |
| 2 (binary) | 0b1111011 | Leading “0b” | GCC extension; not standard C++ |
| 8 (octal) | 0173 | Leading “0” | Characters 0-7 valid |
| 16 (hexadecimal) | 0x7B | Leading “0x” | Characters 0-9, A-F (or a-f) valid |
Binary constants (like B1111011) for values between 0 and 255 are supported for compatibility with Arduino only. You shouldn’t use them in new programs.
Decimal is base 10. This is the common number system we learn in school. Integer literals without other prefixes are assumed to be in decimal format.
For example, the decimal literal 101 is one hundred and one: 1×102 + 0×101 + 1×100 = 101.
Binary is base two. Only characters 0 and 1 are valid. Binary literals are indicated by the prefix 0b.
For example, the binary literal 0b101 is five: 1×22 + 0×21 + 1×20 = 5.
Octal is base eight. Only characters 0 through 7 are valid. Octal literals are indicated by the prefix 0.
For example, the octal literal 0101 is sixty five: 1×82 + 0×81 + 1×80 = 65.
Warning
Bugs sometimes result by (unintentionally) including a leading “0” before an integer literal, which makes the compiler treat it as an octal number.
Hexadecimal (or “hex”) is base sixteen. Valid characters are 0 through 9 and letters A through F; A has the value 10, B is 11, up to F, which is 15. Hex values are indicated by the prefix 0x. A-F can be typed in upper or lower case (a-f).
For example, the hexadecimal constant 0x101 is two hundred fifty seven: 1×162 + 0×161 + 1×160 = 257.
The hexadecimal constant 0xCF2 is three thousand, three hundred fourteen: 12×162 + 15×161 + 2×160 = 3314.
(Remember that in hex, A means 10, and counting up, B=11, so C=12 and F=15).
By default, an integer constant is treated as an int (and must be in the int type’s range limits). To specify an integer constant with another data type, follow it with:
A floating point constant is any number which includes a decimal point. For instance, 3.0 is a floating-point constant for the number 3. By default, a floating-point constant is a double. In order for the constant to be interpreted as a float, you can write f directly after it. For example, 3.0f is a floating-point constant with type float.
Floating point constants can also be expressed in a variety of scientific notation. E and e are both accepted as valid exponent indicators. Some examples are given in the following table:
| Floating-point constant | Evaluates to | Alternate expression |
|---|---|---|
| 10.0 | 10 | |
| 2.34E5 | 2.34×105 | 234000.0 |
| 67e-12 | 67.0×10-12 | 0.000000000067 |
There are several board-specific constants whose value depends on which LeafLabs board you have. If you use them, it will help make sure that your code will work well on all LeafLabs boards, not just the one you have. This will make it easier to share your code with others.
For example, the pin number connected to the board’s built-in LED is different on the different boards, but the board-specific constant BOARD_LED_PIN will always be the correct value for each board.
License and Attribution
Portions of this page were adapted from the Arduino Reference Documentation, which is released under a Creative Commons Attribution-ShareAlike 3.0 License.
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