Circuit Diagram
Description
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.
