How can I program my Pro Mini using the native RS232 Serial port on my Mainboard (aka com1).

Google-ing did not really help and a quick search here for "pro mini programming com1" did not deliver any results.

I already have a MAX232 module connected between the Pro Mini and the PC.

I have already tested the com1 port for function by sending stuff to an Arduino mega via its usb-to-serial port and echo-ing that to com1.

Which succeeded (at least sort-of [receiving some random clutter in between characters or missing some characters at all])

I did try the obvious (i.e. selecting pro mini from the board menu and then selecting the com1 port and hit upload).

But that did just produce a timeout-error…

An Option to set the programmer to simple Serial Connection or something similar does not exist to my knowledge (well it does but only if we're talking about STM32 based boards like the blue pill).

  • please break the word "clump" into separate sentences
    – jsotola
    Jan 20, 2019 at 18:47
  • 3
    you must reset the board in the moment of Upload. the USB chip on Uno or Mega does the reset
    – Juraj
    Jan 20, 2019 at 18:48
  • Or connect the DTR signal (conditioned by the MAX232) through a 100nF capacitor to the RESET pin.
    – Majenko
    Jan 20, 2019 at 18:58
  • @Majenko the module does not offer dtr to the outside world and does not even offer the second channel of the ic …
    – der bender
    Jan 20, 2019 at 19:11
  • 1
    alternative is ISP flashing over the ICSP header with a programmer or other board as programmer.
    – Juraj
    Jan 20, 2019 at 20:49

2 Answers 2


To upload a hex file to Arduino over Serial interface, the bootloader must be active. The bootloader is active after reset. It waits a short time if a new application is uploaded. If not, it jumps to existing application.

To reset the MCU on Arduino boards with independent USB chip a circuit connected to DTR line of serial is used.

On a board like the Pro Mini there is no chip for USB neither an auto reset circuit. You must reset the board with the reset button right before avrdude sends the hex file. avrdude tries ten times, so you have 10 chances to time it right after you hit the Upload button.

Some USB-to-TTL-Serial adapters (or RS232 adapters) have the DTR line and it can be used to reset the board.

Or use ISP flashing over the ICSP header with a programmer or other board as programmer.

  • my module does not offer that line. damn that the ide does not print when avrdude does its attemps to make an estimation on when to press the button. i guess i simply asked for something that sounded great in theory and just not works in practise due to some minor detail...
    – der bender
    Jan 20, 2019 at 19:20
  • just found that it does print w/ some flag set in the preferences
    – der bender
    Jan 20, 2019 at 19:23
  • do not think i do get the Timing Right but it helps in the manner that i know i just wanted the impossible... gonna close this thread
    – der bender
    Jan 31, 2019 at 17:38

Open the IDE, select File:Preferences and turn on Verbose Outputs. Press & Hold Reset on the Promini. Start the download. At the end of compilation you will see something like

Sketch uses 960 bytes (2%) of program storage space. Maximum is 32256 bytes.

Then the IDE will try and download 3 times.

Release Reset when you see the line above - it may take a few tries to get the release timing correct.

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