What causes avrdude: stk500_getsync(): not in sync error (resp=0x4c)

Question

I am trying to program arduino pro mini (3.3v version) via hc-05 bluetooth module. I was getting "avrdude: stk500_getsync(): not in sync: resp=0x00 arduino" error. There are plenty of questions/answers for this error which if I am not mistaken is caused by the lack of connection between the programmer and arduino board.

Since then I made some changes, both hardware and software and am getting serial messages printed on arduino serial console meaning there is a connection and the error has changed to something that has barely a few mentions searching on google.

avrdude: stk500_getsync(): not in sync: resp=0x4c

What is this error exactly? Is there a list of possible errors thrown by avrdude anywhere?

Answer 1

I might be mistaken about the precise error, but recently I had a problem like that caused by enabling the watchdog timer - what wasn't being correctly handled by my bootloader - and it gave me a similar error message. On my case it caused my code to get stuck in some point making impossible to upload new code; in other cases - usually when the watchdog timer is configured to less than 2 seconds - it may cause the board to restart fast enough to do not allow the bootloader to fully initialize.

To solve that I've found two possible approaches:

1. Upload the bootloader again - I couldn't make it work, but some people do. I'm almost sure that I did something wrong.
2. Wire the Reset port to GND, upload a new code and as soon as it goes from compiling to uploading you take the wire off. This worked for me on an Arduino Mega 2560.