>> Get a 74HC86 (a quad 2-input XOR chip) and tie one input of each gate
>> to common.  Tie the other input of each gate to a separate output pin
>> on the PIC.  Then connect the output of each gate to a display segment.
>
>       What aboat those special driving voltages that are needed in
>       LCD? I haven«t yet done much research to know how LCD«s actually
>       work, but I think the driver sends some kind of A.C. voltage.
>       Wether it«s sine- or square wave, I don«t know..

The simple LCD that you are wanting to use is "static" (as distinct from
"multiplexed").  That is, you have a single 'common' or 'backplane' signal,
and one signal for each segment on the display.

For a static LCD you only need two drive voltages (also called "bias"
voltages).

Unless you have an unusual display, you will find that drive voltages of
+5/-5 volts gives very adequate performance.  Therefore, you can simply
use your standard logic power rail to drive the LCD (don't worry, you'll
see where the -5V comes from soon).

There is then the question of the voltage WAVEFORM to use.  As you correctly
state, you must use an AC waveform to drive the display.  If you apply a DC
voltage across a liquid crystal you very quickly destroy the crystal.  To
avoid this, the applied voltage must be reversed frequently.

Let's look at a _single_ segment on the LCD.  The liquid crystal is sand-
wiched between two sheets of glass, and on each sheet of glass is a trans-
parent metal electrode.  The bottom electrode is connected to 'backplane'
and the top electrode is connected to the 'segment' pin.

Apply the following waveforms to each pin:

            ----      ----      ----      ----
         ____|    |____|    |____|    |____|    |____   common

       ----      ----      ----      ----      ----
        |    |____|    |____|    |____|    |____|               segment

                   ^    ^
                   |    |
                  one  two

Assume that the high voltage is +5V, and the low voltage is 0V.  At point
'one' the segment electrode is +5V relative to the common electrode.  At
point 'two' the segment electrode is -5V relative to the common electrode.
This causes the liquid crystal at this segment to appear opaque.

Note that the voltage continuously reverses.  This prevents the crystal
>from breaking down.  The _average_ DC voltage across the segment is very
nearly zero.  For 'typical' static LCD's the waveform frequency should be
between 60Hz and 120Hz approximately.

To turn the segment off (transparent) simply invert the segment signal.
The voltage across the liquid crystal is now always zero.

Now presumably you already have an LCD driver chip, but it doesn't have
enough segment drive pins for your purpose.  The driver chip will produce
the 'backplane' signal for you, and this signal is simply the square wave
shown above (look at it with an oscilloscope).

To drive an _additional_ segment pin you only need to connect to it the
backplane signal (to turn the segment off), or the backplane signal
inverted (to turn the segment on).  That is, you want a logic element
which either GATES its input to its output, or INVERTS its input to its
output, based on the state of a CONTROL pin.

Such a logic element is simply an XOR gate.

Just in case you _don't_ have an LCD driver chip, you will find the
MM145453 from Motorola is very easy to use.  Phillips also have some
I2C interfaced LCD driver chips.

___Bob