I want the simplest way. i don't want to include more circuitry just to connect the mega2560 to exist arduino board (arduino mega) that doesn't have the microcontroller on it (desoldered from the board). how can i connect it?
Ok, lets assume you have a arduino Mega board with it's micro chip removed, and on your 'target' PCB you have a ATmega2560 chip that has the proper arduino bootloader already installed in it. The hook-up would be as follows:
Arduino shield pin name ATmega2560 chip pin number on custom PCB
D0 serial rec 2
D1 serial tx 3
GND (any ground pin) PCB ground
RST (reset pin) 30
This also assumes your custom PCB supplies the needed +5vdc for it's chip, if not you could also jumper shield 5V pin to the Vcc bus of your PCB if the current requirements are within the USB current limit.