After getting an Arduino Decimila for my repstrap, Zach at RRRF developed the bigger-brained (RAM&ROM) & additionally-fingered (digital input/output) Sanguano.
Oh, and Zach has done a lot of clever work on the software side of things too.
Anyway, I assumed I could get one of these larger Atmega microcontrollers and stuff it into the Arduino so it uses all the PSU, USB etc. that are already there. This would need a new board with a header to go into the old brain's socket, and pins to carry the extra IO lines.
I use 'EasyPC' which produced the 3D. The 'old' brain is shown as a DIL package but the pins would be soldered in to project out of the bottom of the PCB. The grey square is a 44 pin surface mount version of the 644P microcontroller. The 3 sets of four pins are the extra 12 I/O.
Then it got a bit more complicated. The big hole in the board is there to clear the Decimmila's in-circuit-programming connector. Unfortunately, it is connected to different pins on the new brain, so I have added a new connector on my PCB.
Also, I am unsure of the performance of the crystal oscillator - the PCB track length between the original XTAL and up to the new brain may cause problems. I have added pads for a new XTAL and a pair of capacitors close to the '644P and a pair of vias in the tracks so they can be drilled to disconnect. This isn't shown above 'cos I only thought of it last night.
The schematic is a bit tricky to read - I don't like tags that go places that I cant find, but when I connected all the points by lines on this one it is just as tricky to trace. It was originally a quick 'n' dirty hack anyway. I also haven't put the effort into finding out how to make EasyPC schematics look good. Anyway, this is a 44 pin chip so there is a lot going on anyway.
The aim of course is to make a USB sanguano by reusing as much of what I have already got. I shouldn't really have bothered because of high parts cost in small quantities and all my time (I am better then most lawyers I have met, so that makes my hours for inventing very expensive indeed!) (I do seem to enjoy myself much more than many lawyers I have met though!!).
I could make these rebrain boards for under ten pounds if I get several tens of them. Anyone interested ?