I am trying to figure out how to program and use the timers in the Feather M0 using the Arduino IDE. I have seen a number of examples published, but I'm not one to just blindly copy some code and expect it to work. The issue that I'm having (and it seems to be ubiquitous in Arduino) is that there does not seem to be a concept of dependencies. In order to reference constants, they have to be defined somewhere.


How can I find the source of the definition of the constants that people keep referring to in their code? For example, in this code a Arduino Zero, M0 timer example, reference is made to TC_CTRLA_PRESCALER_DIV1 among other constants. Where is this constant defined?


I've searched generally and specifically for these constants. But, as you would imagine in C, it's difficult to find the actual source versus a simple reference to the constant.

I've also tried a search in the installed Arduino IDE with SAMD and Adafruit SAMD libraries installed. For example

~/Arduino$ grep -r --include=*.h --include=*.cpp "TC.*_PRESCALER_DIV"

Did not yield anything. I apologize if this is a naive question, but I haven't been able to find a simple, obvious answer. Actually, I haven't been able to find any answer.

  • in Eclipse I use 'go to definition' with F3 key
    – Juraj
    Aug 6, 2018 at 16:34

2 Answers 2


The name TC_CTRLA_PRESCALER_DIV1 indicates this is a macro. A trick for quickly finding the exact location of a macro definition is:

  1. File > Preferences > Compiler Warnings > All > OK
  2. In your sketch, add a #define directive for the macro name that definitely changes the definition:

    #define TC_CTRLA_PRESCALER_DIV1 foobar
  3. Sketch > Verify/Compile

  4. After the compilation finishes, examine the compiler output in the black console window at the bottom of the Arduino IDE window (you will need to scroll up to see it all) for a "redefined" warning. The warning will state the path of the original definition:
    C:\Users\per\AppData\Local\Temp\arduino_modified_sketch_804921\sketch_aug05a.ino:1:0: warning: "TC_CTRLA_PRESCALER_DIV1" redefined [enabled by default]

   #define TC_CTRLA_PRESCALER_DIV1 foobar


  In file included from C:\Users\per\AppData\Local\Arduino15\packages\arduino\tools\CMSIS-Atmel\1.1.0/CMSIS/Device/ATMEL/samd21/include/samd21g18a.h:268:0,

                   from C:\Users\per\AppData\Local\Arduino15\packages\arduino\tools\CMSIS-Atmel\1.1.0/CMSIS/Device/ATMEL/samd21/include/samd21.h:69,

                   from C:\Users\per\AppData\Local\Arduino15\packages\arduino\tools\CMSIS-Atmel\1.1.0/CMSIS/Device/ATMEL/samd.h:105,

                   from C:\Users\per\AppData\Local\Arduino15\packages\arduino\tools\CMSIS-Atmel\1.1.0/CMSIS/Device/ATMEL/sam.h:470,

                   from C:\Users\per\AppData\Local\Arduino15\packages\arduino\hardware\samd\1.6.19\cores\arduino/Arduino.h:48,

                   from C:\Users\per\AppData\Local\Temp\arduino_build_309126\sketch\sketch_aug05a.ino.cpp:1:

  C:\Users\per\AppData\Local\Arduino15\packages\arduino\tools\CMSIS-Atmel\1.1.0/CMSIS/Device/ATMEL/samd21/include/component/tc.h:113:0: note: this is the location of the previous definition


  • in Eclipse F3 shortcut key opens the definition source code on the value. end if it is a redefinition of some lower level macro, I drill down with next F3. And the value of a macro is displayed in tooltip bubble too. And same for functions.
    – Juraj
    Aug 6, 2018 at 16:33

First of all, the definitions of these constants won't tell you very much. They just represent the registers and bits used in the datasheet of the microcontroller.

If you do want to find their definitions, try looking in ~/.arduino15/.

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