Tom=E1s =D3 h=C9ilidhe wrote:
> I've already got a 40-pin chip and I'm out of pins.

There are plenty of 64 and 80 pin PICs.

> Please elaborate on "external hardware".

I don't want to get into this too much since I'm not convinced this whole
idea makes sense.  However, imagine a resistor divider from a PIC output to
the base of a transistor to ground (for NPN) or to Vdd (for PNP).  You can
arrange the divider so that the transistor is off when 2.5V is applied but
on when 5V (NPN case) or 0V (PNP case) is applied.  Put both the NPN and PNP
circuits driven from the same PIC pin, and you've got three states.  One
transistor is on, the other on, or neither.  How to turn that into LED drive
is left as a exercise to the reader (hint: it's not as trivial as you
probably think).

There are other possibilities, like a both a NPN and PNP emitter follower to
a 1/2 Vdd voltage divider.

You could use a I/O expander.


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