There has been some discussion on this list concerning
ways to generate the negative voltage required to bias
certain LCD modules.

I have a method that I have been using with great
success that I would like to share. This method 'steals'
the required signal from the pin 15 osc2/clkout pin
of the PIC 16C84. So as not to load down the pin I
connect immediately from pin 15 to any CMOS logic
gate I happen to have available. It does not matter
what type. I have used AND NAND OR NOR BUFFER and
INVERTER types with equal success. The output of this
gate then drives the usual negative generator circuit:

Pin 15 -----> CMOS Gate-----> Neg. Gen Ckt---> 10K pot

If I find that an LCD circuit requires the negative bias,
then I do NOT provide any positive bias at all to the pot.
In other words, one side of pot to ground, one side to
negative generator ckt, and output of pot to LCD.


                      +5v
                      |
     (CMOS)           |
      |\              e
(15)--| >O--10K-*---b(PNP)
      |/        |     c
                |     |     ||
                |     *-----||---*---|<----*------
                |     |     ||   |         |     |
                |     c          V        ---    \ (10K)
                ----b(NPN)       -        ---    /<---
                      e          |         |     \
                      |          |         |     |
                     GND        GND       GND   GND

I generally use .1 ufd caps and 1N914 diodes.
Any general purpose PNP and NPN transistors
can be used.

Sometimes you can dispense with the transistor section
and its associated base resistor altogether if your
CMOS gate can supply a reasonable current. You can always
parallel CMOS gates to get more current capability.

Just another variation on a vasic theme, but I hope that
perhaps it helps someone out there keep their cost or
parts count just a little bit lower.

Fr. Tom McGahee