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I have several Pro Micro clones.

This morning I uploaded a sketch into one of them (that was previously unprogrammed, so it stayed in bootloader mode) and when I wanted to upload the sketch a second time, the problem began:

When I now run the reset procedure for Arduinos with integrated USB (open the COM port of the Pro Micro for example in PuTTY at 1200 bit/s and close it again), the LEDs start flashing rapidly and the Arduino disappears from the Windows Device Manager.

Hover for a (slightly annoying) image:

LEDs flashing

So then I grabbed a second, identical Pro Micro and uploaded the Blink sketch into it. It was also previously unprogrammed. The sketch executed, but when I now do the reset procedure, it exhibits the same behavior now (flashing LEDs).

I can still upload sketches just fine, by setting the COM port to a different (arbitrary) COM port to prevent the reset procedure from reaching the Pro Micro and resetting the Pro Micro manually after starting the upload procedure.

This only happens with the 3.3 V/8 MHz variant, not with the 5 V/16 MHz variant. I have selected the appropriate variant in the IDE.

How can I repair the automatic reset function?

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  • It's not really clear what you're asking. If you open and close the CDC @1200 baud, it will reset to receive new firmware over USB. This is what the Arduino IDE uses to upload new sketches. Link 1 Link 2 What are you trying to do? Why do you need to open it at a baud rate of 1200 baud?
    – tttapa
    Dec 2, 2017 at 16:03
  • That's the point - it should reset and go to bootloader mode but instead it starts flashing and disappears completely from the device manager.
    – AndreKR
    Dec 2, 2017 at 17:27

2 Answers 2

Reset to default
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I was finally able to solve this by using one of the Pro Micros to re-burn the bootloader of the other one.

For reference, the process is like this:

  1. Flash the example sketch "ArduinoISP" into the first Pro Micro

  2. Connect the two Pro Micros like this:

    Programmer (on USB)   Target
GND ----------------- GND
VCC ----------------- VCC
15 ------------------ 15
14 ------------------ 14
16 ------------------ 16
10 ------------------ RST

  3. Choose "Arduino as ISP" as Programmer

  4. Because both are Pro Micros, selecting a different board is not necessary.
  5. Click "Burn Bootloader"
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  • Striking out is not a good idea. An answer should only have the current best information. The historical information belongs in the revision history (do provide a comprehensive summary, though, as the intent is not always in the diff). If there should be information for earlier versions of the IDE then split it in two, e.g. "For Arduino IDE v. y.y.y and later: ABC" and "For Arduino IDE v. x.x.x and earlier: XYZ". Put the information that benefits more readers first (there is no requirement to have it in chronological ascending order) - that is probably for the newer versions of the IDE.
    – Peter Mortensen
    Apr 22, 2019 at 10:58
  • I removed the step, will (should) long be using a newer version anyway.
    – AndreKR
    Apr 22, 2019 at 17:14
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Arduino Pro micro will enter bootloader if you connect it with 1200 baudrate. See this pos Upload code on Arduino Leonardo using command line

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  • I like your other post. Maybe you could expand this post a little, as it is a little short and terse.
    – Greenonline
    Jul 11, 2018 at 7:32
  • And it doesn't answer the question. The question is about why it does not enter the bootloader. (The bootloader would be visible on the USB bus.)
    – AndreKR
    Jul 11, 2018 at 9:01
  • @vanbwodonk, AndreKR will not receive a notification in his inbox and may see your reply, if you don't address your comment to him using an@ as I have just done to you. Repost your comment and add their name with the preceding @
    – Greenonline
    Jul 11, 2018 at 11:51
  • @AndreKR Actually i not really understand what are you asking about. Led flashing indicate that's Arduino Pro Micro successfully jump into bootloader. Of course it will change COM port under windows (I assume you using windows).
    – vanbwodonk
    Jul 12, 2018 at 4:58
  • @vanbwodonk Are you sure that it should flash when in bootloader mode? Because mine doesn't, so this might indicate something. Can you make a video of the flashing and the device manager/lsusb so I can see the timing?
    – AndreKR
    Jul 12, 2018 at 6:40

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