avrdude: programmer is not responding (Pro Mini)

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C:\Program Files (x86)\Arduino\hardware\tools\avr/bin/avrdude -CC:\Program Files (x86)\Arduino\hardware\tools\avr/etc/avrdude.conf -v -patmega328p -carduino -PCOM3 [color=red]-b39600[/color] -D -Uflash:w:C:\Users\AppData\Local\Temp\arduino_build_325512/Blink.ino.hex:i

:
        Using Port                    : COM3
        Using Programmer              : arduino
        Overriding Baud Rate          : 57600
avrdude: serial_baud_lookup(): Using non-standard baud rate: 39600avrdude: stk500_recv(): programmer is not responding

OK; "39600" is a very weird (and wrong) number to be seeing in the avrdude command. It SHOULD be 57600.
Your Arduino install could be broken, or it could be a bug the version you are using (sometime around then, they added the "cpu" sub-menu, and perhaps there's a code path that leaves the bitrate uninitialized or something.
Here are a couple of suggestions:

  • Best idea: upgrade to the latest IDE version (1.8.5) and see if that works better.
  • (immediate results #1) Try switching the "Tools/Procesor" menu to a different value, and then back again.
  • (immediate results #2) Try using one of the other "board" types that should be "mostly compatible" with the pro-mini. Nano, Duemilanove w m328, or Lilypad should probably work. (although, they could have the same bug.)
  • (advanced debugging?) Check the "boards.txt" file that your IDE uses, and see if it contains the 39600 number.

(Check the output to see if the speed has changed. (Well, I guess if one of these works, you don't need to do that!))