I just connected the socket to Serial line without any IC. However, I am concern one thing that is I drew a wrong reset circuit to Reset pin, +5V -> push button parallel with 10uF capacitor -> reset pin parallel a resistor -> ground. I measure the voltage of the reset pin is around 2.65V. Is it that part to make a fault?
But I found my board is quit unstable because after I connected AREF to 3.3V, then upload the program to the Mega board, it would fail to upload too. The error is ser_send(): write error: sorry no info avail. P.S. in this case, I had not insert my external board to Mega. Is it normal?