3.3v pro mini can only load sketch once

was working with a 3.3v pro mini (clone,) damaged the cpu and replaced it.

burned the boot loader from the arduino ide using a usbtiny. i can upload
a sketch only once. additional attempts to upload a sketch result in
"programmer not responding".

as long as i reload the bootloader, i can upload any sketch on the first attempt.

my first guess was fuse settings. so i captured the avrdude bootloader
output and the fuses are as listed in the boards.txt file.

unlock = 0x3f
lock = 0x0f
low = 0xff
high = 0xda
ext = 0x05

using arduino v1.6.9 on an older linux system.

i have run out of ideas. any suggestions?


Once you have successfully burned the bootloader onto the pro mini using the USBtiny, what are you then using to upload your sketch to the pro mini ?

Typically “only can upload once” means the auto-reset is not happening. When the serial connection to the Pro Mini is made it should automatically reset, which puts it temporarily in a state of listening for a sketch to be uploaded. After burning the bootloader it is constantly in that state so a reset is not needed to do a reset on that first upload.

using the same 3.3v ftdi serial board to download the sketches before i blew up the cpu. killed the cpu by fumbling around and allowing a 12v jumper touch something on the cpu (i was working on a 3.3v logic input high-side driver.) this might have damaged the ftdi, i just assumed it was ok as the first upload works.

i'm confused about the auto-reset initial handshake. after the bootloader is flashed, the board goes into the default pin blink that i assumed was included as part of the bootloader. (the chip has been erased with avrdude command line. the only file on the bootloader log is ATmegaBoot_168_atmega328_pro_8MHZ.hex.)

so how is the auto-reset initiated after a first upload? is this done by the ftdi dtr pin?

so how is the auto-reset initiated after a first upload? is this done by the ftdi dtr pin?


You should be able to manually reset the board just as the reset is happening but you have to get the timing right because the bootloader is only active for a short amount of time before the sketch starts running.

knowing that the bootloader is in a listening state after flashing was the key. a cold solder joint existed on the reset line coupling cap. once i figured out which one it was, pushing down on it a scope probe and uploading at the same time would allow the upload. don't know if this is due to being a lowest cost board or a result of my hot air pencil removing the cpu.

thanks for the suggestions!