[Coco] ArduinoCoCo

[Jayeson Lee-Steere]

@Allen, you are right, the Leonardo is much like the base Teensy 2.0 and if
they were around when I still felt I didn't have enough boards, I would
have purchased one for when I want that form factor.

I pulled out the datasheet for the 6809 and, without doing a careful
analysis, it looks like the 16MHz AVR could probably go straight onto the
Coco bus, sans the PIA. At least for low speed mode. It would be easier if
it ran on the Coco's system clock but I don't know of a simple way to do
that from the cartridge port. Dealing with interrupts within the Arduino
would be a bit of a pain though.

The other thought I have would be put an SRAM onto the bus also. At power
up, the Ardunio could HALT the CPU and fill the SRAM with the desired boot
code. Drivewire or other communication could be buffered directly through
the SRAM. Add an SD card for a ROM library or whatever.

Arduino + Dual ported SRAM would be even easier but large ones seem to be
expensive and perhaps not easy to find.

- Jayeson Lee-Steere