Multitasking Pins

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It’s entirely logical
that low-cost miniature microcontrollers have fewer ‘legs’ than their
bigger brothers and sisters – some-times too few. The author has given
some consideration to how to economise on pins, making them do the work
of several. It occurred that one could exploit the high-impedance
feature of a tri-state output. In this way the signal produced by the
high-impedance state could be used for example as a CS signal of two ICs
or else as a RD/WR signal. All we need are two op-amps or comparators
sharing a single operating voltage of 5 V and outputs capable of
reaching full Low and High levels in 5-V operation (preferably types
with rail-to-rail outputs).

Suitable examples to use are the LM393 or LM311. The resistances in
the voltage dividers in this circuit are uniformly 10k. Consequently
input A lies at half the operating voltage (2.5V), assuming nothing is
connected to the input – or the microcontroller pin connected is at high
impedance. The non-inverting input of IC1A lies at two-thirds and the
inverting input of IC1B at one third of the operating voltage, so that
in both cases the outputs are set at High state. If the microcontroller
pin at input A becomes Low, the output of IC1B becomes Low and that of
IC1A goes High. If A is High, everything is reversed.

Multitasking Pins Circuit

Multitasking Pins Circuit Diagram

Roland Plisch
Elektor Electronics 2008