The LCD backplane is typically driven with a 5 Vpp square wave
(alternating between 0V and 5V).  If a segment is driven with the same
voltage (in phase) the segment is the same as the background color.  If
the segment is driven out of phase, the segment is the opposite of the
background color.  The frequency is typically 30 Hz or so.
        The LCDs are VERY sensitive to DC.  If there is a DC component on
the drive signals (that is, a DC component between the segment and the
backplane), the life of the LCD will drop substantially.
        You can possibly drive the LCD directly with a PIC, using an
interrupt routine to generate the backplane and segment signals.  As I
recall, Microchip has a chip with an LCD driver in it.  You can also use
exclusive OR gates to do the phase inversion for driving segments, but
that takes a lot of chips to do many segments.  Finally, there is the
ICL7211, which is a 4 digit LCD driver in a 40 pin chip.  You feed it BCD
and digit select signals.  It generates backplane and segment drive
signals.


On Sat, 6 Jun 1998 21:53:45 -0700 Michael Ghormley
<mikeg@BLACKSAND.NS.COM> writes:

>I have a clock project that needs to display hh:mm:ss on a largish LCD
>display.  I have found a nice one at a good price (surplus -- no docs
>or
>markings of any kind) and got a sample to prototype.  It is a bare (no
>driver chips) LCD in a kind of huge DIP.  It would be perfect for my
>purposes.
>
>I sat down tonight to begin probing it for the pinouts when I realized
>that, while I have done any number of HD44780-based projects, I had no
>idea about driving a "raw" LCD.
>


>
>3)  What about series resistance?  I know these are *very* low
>amperage
>devices, but do I need a series resistor with each segment for some
>reason?  If so, what size?

        You can connect the segments and backplane directly to the driver
with no series resistor.

>
>4)  Is there going to be a chip-wide contrast pin on this thing, or is
>that up to the hardware that drives it?

        The contrast or viewing angle control is only used on multiplexed
LCDs.  A seven segment display is (typically) not multiplexed.  Further,
the contrast or viewing angle control drives the display driver chip of
muxed displays, not the display directly.

>
>I did a search with a couple of the search engines, but could not find
>any info on any 'raw" LCD's.  Any help or advice is appreciated.

        Check for application notes on the ICL7211.  Also, check LCD
manufacturers, such as AND, Hamlin, and others I can't think of right
now...


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