I made a mega2560 minimum system, with only the chip, resonator, i2c ports, serial ports and power pins out. When I uploaded a sketch using a usb-ttl adapter (the very ordinary one), it works fine, and the sketch is execuated. But when I let the system run alone by itself by powering it up with a 3.3v pins, without any serial pins connected, the chip seems to not bootable. I have the ordinary bootloader burned already and uploaded the sketch to the chip using a regular serial0 way.
Then I checked usb-ttl adapter and found that the rx and tx pins seem to have pull up resistors. I do remember that some old mcus don't have internal pull up resistors for their serial ports. Is this the case for 2560?
When the system runs, it dosen't need the rx0 or tx0. I run the system alone, with a pull up resistor on rx0 and it works...
I checked the arduino mega 2560 schematic and didn't find pull up resistors on their serial ports. Or maybe the usb-ttl chip has those inside? Does fuse bits do anything on this pull up behavior?
Or maybe, when I pull the rx0 up manually, it happened to screen the board better, so that the system started to run?