Both of you are on the right track. There are a couple of ways this can be
done, depending on how many segments you want to drive.

Method #1
Connect all pins of the LCD to the PIC including the backplane. When
generating your code keep in mind that you must always keep the outputs
switching. LCD's must be driven by an AC voltage, DC will over time leave
the segment permanently on. The drive frequency should be 60 to 100 Hz, just
enough for persistence of vision and no noticeable flicker. Drive the
backplane at a constant frequency. If you want a segment on then drive it
with the same constant frequency out of phase with the backplane signal. If
you want the segment off drive the segment with an in phase signal to the
backplane. The way this works is that when both the backplane and segment
are in phase the voltage across the segment is zero. When they are out of
phase the voltage is 5V or whatever the supply voltage is.

Method #2
Use XOR gates. Drive the backplane and one XOR input of each gate with a 60
to 100Hz frequency. This can be easily generated from a 555 or a PIC.
Connect the outputs of each XOR gate to each segment pin on the LCD. Driving
the second input of the XOR gate with a simple high for 'on' and low for
'off' will turn the respective segment on or off. If you need to know how
this works draw it out as a timing diagram and work out each pulse with the
XOR truth table.

There have been some good app. notes written on the first method. One of the
best is from Microchip, AN563.

Hope this helps everyone out.

James


> netQ wrote:

> Hi. I want to use a 3 1/2 segment static drive LCD for data display with
> a PIC16F84. I've found a stable square wave to drive the segments and
> it's generated by a 4047 (1 KHZ square output, standard and inverted (Q &
'Q))
> I thought that in order to let the PIC control a segment, I should let it
> control AC current flowing to any pin (except for pins 1 & 40 which
correspond
> to the backplane).
> Apparently, if I want a segment off it must be supplied with the same AC
current
> of the backplane (let's say Q) and if I want to turn it on, inverted AC
> current ('Q) should be supplied.
> Could anybody suggest a way to let the pic switch a segment on or off
given the
> above?

> Do you people know of an IC that would let the PIC multiplex this type of
> signal?

>> Unless I am misunderstanding the problem a garden-variety 2-input XOR
ought to
>> provide what you want, yes?  Or am I way off base?

>> Michael