3.3 V Pro Micro clones flashing LEDs after 1200 bit/s touch

Question

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:}

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?

Answer 1

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"

Answer 2

Arduino Pro micro will enter bootloader if you connect it with 1200 baudrate. See this pos Upload code on Arduino Leonardo using command line