CrossRoads:
I was directing you to the datasheet comparison area. 1284 is not officially supported in the IDE.
Skyjumper can help you out for sure.
Thanks.
I have found a file containing dozens of new board definitions including the 644p and 1284p which is now installed so it is possible to select the 1284p in the IDE.
Just waiting for the chips to arrive which should be on Monday. Will update with a complete step by step once I get it going and will talk to skywalker to see what help he can give (hopefully ge has already done a step by step for it).
I'm surprised it isn't included as it is a useful version of the chip. Not everyone has the equipment to solder 1280 chips onto their own boards!