Hi Geoff,
That's interesting enough but using a microcontroller and a "daughter-board" is cheating a bit I reckon.
The "hands-on purists" who're into micro-electronics, would probably suggest developing some kind of programmable logic device or mixed-mode programmable array using Hardware Descriptor Language, either digital or mixed-mode.
You could then get get a like-for-like chip fabricated or programmed from the design, to the same, or very similar, electrical/mechanical specification and form factor.
Saying that the Microchip Scenix processor, for example, was specifically meant to be a microcontroller device that was fast enough to be able to emulate digital hardware functions thus avoid having dedicated digital logic, e.g. registers, counters and I/O ports, fabricated on-chip.
It had very few dedicated hardware functions built-in - making it really flexible in theory (coding and raw speed was meant to do the rest). Saying that I don't see many about in commercial medical equipment (too low spec).
Microcontrollers are Ok for emulating the functions of a combinational/sequential I.C, I guess, but getting critical I/O timing spot-on must be a bit of a nightmare.
