I offer '1284P cards in several form factors, such as...Would be a lot cleaner than trying to glom a 1284 onto a 328 board and it also gives you access to the extra IO for future use.
Check out what john1993 says about making a small SMD board for cheap (extremely cheap), I thought it was interesting: https://forum.arduino.cc/index.php?topic=390148.msg2690277#msg2690277
...also a .1mfd similarly tacked between power pins but too bad no photos..
unfortunately the only clear assembly instruction that I found in the above post is about the placement of the oscillator.. I doubt that I'll manage to get a working "ProMini328 replacement" on that sketchy info alone.. an additional .1mfd capacitor also seems to be needed (but what specific type: ceramic, smd 0805, 0402, 0603, etc?, and where to put it exactly?) on the photos there is some additional wire/led/resistor.. (is there anything else that needs to be added?)
Advantage of going with ATmega2560 instead of ATmega128 is the ATmega2560 is more mainstream and will work with the Arduino IDE without adding anything. From a software perspective you can treat the board as a Mega2560. Fewer people will know answers to questions about ATmega128 on this forum. The price of the ATmega128 is cheaper, so that is something to consider.
Shipping to Europe from US has gotten ridiculous for sure.Cheapest I can do for shipping is a padded envelope for $13.50 with shipping time that "varies by destination":....Meanwhile China has free shipping to the US to dump their products here.'1284P is great chip with 16K SRAM, 128K flash, SPI, I2C, dual hardware serial ports, and the other stuff you'd find in a '328P.
It has been discussed on the forum before it is a good idea to connect all the GND pins together and apply power to all of the VCC pins. Do not connect power to AREF.
extInterrupts SPI328: 24 2128: 8 1
- ATMEGA 128A-AU (US $0.79)- QFP TQFP SMD DIP Adaptor PCB Board (US $0.32)- DC-DC Step Down Power Supply 3A (US $0.41)- 16 MHz cristal oscillator (US $0.17)- several pin headers, 2 resistors, 4 capacitors and a LED (some cents)
below my preliminary experience report.