For some applications I need access to the registers that control the output/input and high/low state of pins. For example pin 10 on a adalogger is PORTB with a mask of _BV(6). This information can be found in pins_arduino.h. For some boards many variants get installed. I turned on verbose compiling and looked through the output there as well as looking through boards.txt and platform.txt. In that output I see an include to a directory with a pins_arduino.h file but there is nothing in that file but a definition for BUILTIN_LED.

When I run a function like digitalWrite(13,HIGH); how does it know which registers to use for pin 13? Since my board has this pin, and digitalWrite(13, HIGH); makes an LED light up ... it's working.

1 Answer 1


In the boards.txt file for the core in question you have a line for each board:


where yyyy is the name of the folder in variants for the board.

For instance, the Uno, has:


which relates to:


with the pins_arduino.h file in it for that board.

  • Thanks, I found that line and it agrees with the output from the compiler. The problem is that particular file is basically empty. Does that mean there is a default somewhere that it's using as well?
    – Matt
    Commented Oct 30, 2017 at 19:53
  • It's far from empty on my installation. What version are you running? And what board / core is it?
    – Majenko
    Commented Oct 30, 2017 at 19:54
  • I have hundreds of pins_arduino.h files throughout the system. My problem is with the adafruit huzzah ESP8266. The pins_arduino.h file is basically empty.
    – Matt
    Commented Oct 30, 2017 at 19:55
  • Well, that is down to the core. The core most likely has defaults that don't need overriding. Most ESP8266 boards just map the GPIOs 1:1 and don't need anything else. Remember: an ESP8266 is not an AVR Arduino and does not follow the same rules.
    – Majenko
    Commented Oct 30, 2017 at 19:57

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