I can't upload a program that uses
Serial.print on my standalone Arduino, because I get this error:
avrdude: stk500_recv(): programmer is not responding avrdude: stk500_getsync() attempt 10 of 10: not in sync: resp=0xbf
However, I have no trouble uploading the blink program to it, which only uses
digitalWrite, so it has to do something with the methods mentioned at the start of this post.
You can see the diagram below.