When programming the Pro Mini using this USB-to-serial adapter the boot loader only works once. After the first programming the programmer is out of sync or not responding.

To be able to program the Pro Mini again I have to burn the bootloader using my Uno as an ISP.

Programming the Pro Mini through an Uno does not present the same problem.

Are the lock bits not being set properly or somehow overridden?

Usb to serial module

  • 1
    Does that adapter really have a DTR signal? Dec 22, 2015 at 20:20
  • Please show your full wiring.
    – Majenko
    Dec 22, 2015 at 20:59
  • I actually have the same adapter and it does not have the DTR signal. Therefore it cannot be used. A reset signal cannot be generated. The adapter has 3.3V, RST, TXD, RXD, GND and 5V. The RTS is not the same as DTR. Dec 22, 2015 at 21:10
  • 1
    Make sure you add the inline 100nF capacitor in the DTR line or the newer versions of avrdude will fail miserably.
    – Majenko
    Dec 24, 2015 at 12:59
  • 1
    @mikepatel. It is not true that RTS will not work. Using it is mentioned in official arduino guides: arduino.cc/en/Guide/ArduinoMini this sounds more like a fuses problem with the bootloader memory not being preserved. Dec 26, 2015 at 5:47

2 Answers 2


The module I had does not have a DTR signal. I mistook the RTS for DTR. Ordering correct module.

  • 1
    You have received faulty advice. The RTS signal is usable, and in fact used in official guides. Your problem is likely something else. Dec 26, 2015 at 5:49

It sounds likely that the OP (long since gone...) was resetting the board manually to get the bootloader to run even that once; and that the bootloader allowed itself to be overwritten, a critical fuse got changed, or that one successful upload otherwise left itself or the board in an un-bootable state.

  • no. the problem is OP didn't reset (see his answer), but a fresh bootloader doesn't need reset to bootloader
    – Juraj
    Aug 7, 2021 at 14:56

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