Arduino Predefined Constants

There are a lot of #define-d constants in Arduino. Most of them are from avr-gcc compiler but some are also Arduino specific. Let’s take a look at some interesting ones.

Gcc constants

__DATE__and __TIME__

Date and time of compilation.

__FILE__and __LINE__

FILE is full path to current compiled file and LINE is current line number inside this file. However FILE is not *.pde (sketch file), it’s *.cpp. If you don’t know how Arduino build process works you will probably don’t need to use this constants. However it would be useful in libraries.

__func__

Name of current function.

void setup()
{
 Serial.println(__func__); // prints "setup"
}

__COUNTER__

Every time you use this one, it increases its value.

Serial.println(__COUNTER__); // 0
Serial.println(__COUNTER__); // 1
Serial.println(__COUNTER__); // 2

CPU Type and How to Determine Arduino Bord

According to selected target processor, compiler defined constant by processor name.

Constant	CPU	Board
__AVR_ATmega168__	ATmega 168	Arduino Decimilia and older
__AVR_ATmega328P__	ATmega 328P	Arduino Duemilanove and Uno
__AVR_ATmega1280__	ATmega 1280	Arduino Mega
__AVR_ATmega2560__	ATmega 2560	Arduino Mega 2560
__AVR_ATmega32U4__	ATmega 32U4	Arduino Leonardo
__SAM3X8E__	AT91SAM3X8E	Arduino Due

So by testing of these constants you can determine board type. For example:

#if not (defined(__AVR_ATmega2560__) || defined(__AVR_ATmega1280__))
    #error Sorry dude, this program works only on Arduino Mega 
#endif

Another example is in sketch in the end of article.

Arduino Specific Constant

Arduino compiler define these constants

ARDUINO

Version of Arduino SW. So if you use Arduino 0022 you wil get:

Serial.println(ARDUINO); // 22

F_CPU

Frequency of oscillator on the board (in hertz). For example on Arduino Uno you will get:

Serial.println(F_CPU); // 16000000

Try it

You can try example sketch on your board.