I still remember some member thought about building a Arduino compatible using a 64-pin ATmega2561. Recently I obtained a ATmega128 in TQFP-64 and I want to roll an Arduino compatible out of it. Now I have a few questions:
ATmega128 have a PEN# pin that have to be pulled low upon reset to enter ICSP mode. Does this mean I have to add some logic on the board to properly reset via ICSP reset pin, or should I go with the 10-pin ICSP header that have 4 GND pins tied together in the programmer, while marking the 6-pin header, like Arduino Due, as SPI only?
I want to break all pins out, and that resulted in a "sub-Mega-plus" format, that is, 18x1 instead of 18x2 pins at the right of the board, and analog pins A8-12 on Mega 2560 became digital pins 38-42 and analog pins 13-15 became N/C. (I also had a sub-Mega format with the entire 36 pins on the right missing, for breaking a 44-pin PIC out into Arduino-compatible format) Is that a good idea?
I chose to use CH340 as the USB to Serial adapter. Is this a good idea?
What if I want to sell the design?