Sunday, January 2, 2011

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.


Name of current function.

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

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 

Another example is in sketch in the end of article.

Arduino Specific Constant

Arduino compiler define these constants


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

Serial.println(ARDUINO); // 22

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.


  1. Thanks for posting this, I keep coming back here when I forget the exact format of the CPU names!

    One to add might be the (__AVR_ATmega32U4__), comes in very handy for dealing with the new pin assignments!

    1. Thanks for suggestion, Tom. I added processor type for Arduino Leonardo and Due.

  2. Adam, you can add the Arduino Yun under ATmega32U4. Thanks!

  3. No doubt it will be very useful for my future projects. Would like to see some other posts on the same subject!
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