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I'm working on a project with a Digispark ATTiny85, that performs keystrokes using the DigisparkKeyboard library (https://github.com/digistump/DigisparkArduinoIntegration/tree/master/libraries/DigisparkKeyboard). Besides the DigiKeyboard, I also import EEPROM.h.

My own code is only about 150 lines, I've cut down the size following a number of online guides, trying to avoid larger data types and so on. The Digispark is limited to ~6kB and the imported DigiKeyboard library takes up a little more that 5.4kB. In total the compiled project takes up about 7kB, which exceeds the boards capacity.

From the DigiKeyboard library I only use DigiKeyboard.sendKeyStroke() and DigiKeyboard.print() (with about only ten different characters). So I assume there is a lot of unused code I could remove, but I don't know how to approach this. I use Visual Studio Code and PlatformIO.

How could I reduce the size of the used DigisparkKeyboard library?

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    Are you printing numbers or only fixed messages? In the later case, does it help if you use write() instead of print()? Commented Jul 3, 2022 at 9:51
  • Also, I have no experience with PlatformIO, but I guess you should be able to enable a “verbose” mode that shows you the full command used to call the compiler. Could you please check whether this command contains the option -flto? Commented Jul 3, 2022 at 10:02
  • Thanks @EdgarBonet! The compiler flag -O3 -flto didn't change a byte for me and I couldn't find any documentation on write(). However, I switched to .sendKeyStroke for all write operations from .print(), which shaved off about 3kB! and the code fits the board! If you formulate an answer, I'll gladly accept it! Commented Jul 3, 2022 at 14:15
  • I just noticed that the big issue here wasn't the inclusion of print(), but rather one single instance of print, where I did: DigiKeyboard.print("Time: "+String(round(runtime/1000)));. So I believe the large code was due to the inclusion of String(). Commented Jul 3, 2022 at 15:07
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    Re “If you formulate an answer, I'll gladly accept it!”: Well, actually you found your solution yourself! In such a case, the recommended practice it to post and accept your own answer. Commented Jul 3, 2022 at 16:13

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With input from @EdgarBonet, I looked into the functions used and it turned out that the String() function, I used once in my code, takes up about 3kB of space in the compiled program.

I ended up removing the line with String() and kept the imported DigiKeyboard library as it is.

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The compiler/linker will remove unused functions/methods automatically, so I doubt you can save memory by removing unused functions/methods yourself.

However, you could possibly gain by:

  • Removing the calls of check functions/methods when you know the result will be always false, true, or always the same number, string or data type in general.
  • Removing (parts of) functions/methods which are related to physical or logical functionality you never use. Like e.g. if, case statements that will never apply).
  • Prevent using an entire class (e.g. String) (see second comment of Emma Makes below).
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    Thank you for the feedback! I wasn't aware that unused functions and methods aren't included anyways! I was able to shave off about 3kB by switching from .print() to .sendKeyStroke! I'll keep the other suggestions in mind for future issues! Commented Jul 3, 2022 at 14:16
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    I would need to edit the comment, but it's too late to do so. The reduced size was not due to the change from print() to sendKeyStroke(), rather the fact that I didn't use String(), which I previously used in the print() function call. Commented Jul 5, 2022 at 16:03
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    That's also indeed a good way to reduce size, to preventing using a complete class (String in your case). I will add it to my answer Commented Jul 5, 2022 at 19:04

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