Hey, wait a minute...

Aren't we getting this all backwards? Isn't the point of the PIC series
the price per chip? This whole discussion seems to be losing sight of
the objective, at least in my humble opinion.

Consider: what is the bandwidth needed for the switch scan? If it's for
a security system (detecting doors & windows & such) then the actual
bandwidth necessary is low; low bandwidth translates to long runs of
cheap wiring without special drivers.

Of course, you would need a custom protocol, but this is getting to
exactly what is attractive about the PIC family. Wouldn't this be an
ideal application for a distributed system, with local PIC's handling
short runs to a few switchs (I like the Dallas stuff, myself) and then
communicating the data to a central system? This would cheap up the
whole design, especially if you partitioned on the basis of chip count.
I think I would try to do it without any extra scanning logic, given the
lower limit of the price per chip is set by mechanical rather than
functional considerations. Low end PIC processors are awfully cheap, and
they can be made to consume such small amounts of power & run over such
a wide voltage range that distributed power for the whole system seems
possible.

-- Tom Rogers  VP - R&D  Time Tech, Inc.