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Cannot upload to ATtiny85 using bootloader

I have flashed the Optiboot bootloader to an ATTiny85 using the Arduino IDE. Through this IDE I can also upload sketches. Everything works as expected (besides the “tuning of the bootloader”).

Now that I have the bootloader working I wanted to switch to Platformio (via VSCode). However, flashing doesn’t work.


platform = atmelavr
board = attiny85
board_build.f_cpu = 1000000L ;8MHz
framework = arduino
upload_protocol = stk500v1
; each flag in a new line
upload_flags =
board_bootloader.file = ./bootloader/optiboot_attiny85_8000000L.hex
board_bootloader.lfuse = 0xE2
board_bootloader.hfuse = 0xD5
board_bootloader.efuse = 0xFE
board_bootloader.lock_bits = 0xFF
board_bootloader.unlock_bits = 0xFF
board_fuses.lfuse = 0xE2
board_fuses.hfuse = 0xD5
board_fuses.efuse = 0xFE

; edit these lines
upload_port = COM3
upload_speed = 19200

Now Pio doesn’t seem to be able to connect to the chip:

avrdude -PCOM3 -b19200 -v -p attiny85 -C path\to\avrdude.conf -c stk500v1 -b 19200 -e -D -U flash:w:.pio\build\attiny85\firmware.hex:i

avrdude: Version 6.3, compiled on Sep 12 2016 at 17:24:16
         Copyright (c) 2000-2005 Brian Dean,
         Copyright (c) 2007-2014 Joerg Wunsch

         System wide configuration file is "path\to\tool-avrdude\avrdude.conf"

         Using Port                    : COM3
         Using Programmer              : stk500v1
         Overriding Baud Rate          : 19200
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x03

FYI: Arduino IDE’s avrdude’s command is

C:\Program Files (x86)\Arduino\hardware\tools\avr/bin/avrdude -CPATH\TO\avrdude.conf -v -pattiny85 -carduino -PCOM3 -b19200 -D -Uflash:w:PATH\TO\attiny85.ino.hex:i 

Anyone has an idea?

So the working command uses

and the nonworking uses

Then change your


upload_protocol = arduino


Or what programmer device have you attached to your ATTiny for ISP programming?


After burning the bootloader I connect the ATtiny85 via a CH340 usb-to-serial chip.

Changing the upload_protocol did the trick. An extra pair of eyes… as often.

The timing seems to be off however, gotta look into the fuses.

1 Like

You can burn the new fuse settings via pio run -t fuses -v. Does that invoke avrdude with the right fuse parameters? (You might have to change the upload_protocol again after burning your UART bootloader)

Again a stupid mistake. Fuses were allright, it was the platformio.ini file that had an error:

board_build.f_cpu = 1000000L

changed to

board_build.f_cpu = 8000000L

All is working now. FYI: my goal was to be able to update my 3.3V target without removing the battery. As my only ISP-device, the Arduino Uno works on 5V, it was too complicated to go via the ICSP route. Now I only have to attach a my 3.3V USB-to-serial without connecting the adapter’s Vcc.

1 Like

Fixed! the code that worked for me is this:

platform = atmelavr
board = attiny85
board_build.f_cpu = 16000000L
framework = arduino
upload_protocol = stk500v1
upload_flags =
upload_port = COM4
upload_speed = 19200

Make sure to change your PORT if it’s not correct. I couldn’t get it to find the port automatically.

You can change the CPU speed from 16000000L to 8000000L in order to change the speed from 16mhz to 8mhz or pick another lower value to change it to 1mhz with 1000000L for example.