Thanks to all who responded. Maybe I did not make myself clear. I have
hd44780 compatible lcds which are no-name no-datasheet types ;-). They
work ok but there is not enough contrast even if the Vee is taken to GND,
with nominal 5V supply. These LCDs do not allow Vee to go negative. So I
increased the supply of the lcd alone to 5.5 then 5.8 V and the contrast
improved (with Vee tied to GND all the time). The controllers on the lcd
seem to be rated 6.5V absolute maximum so I don't see a problem. The
question was, do you see a problem with this, and is there another way to
achieve this contrast increase (besides by heating the lcd panel with an
ito heater ;-).

I have considered what happens when reading data from the LCD. It seems
that these LCDs are built with CMOS controllers whose data outputs consist
of a lower NMOS driven by data and an upper PMOS that is always on and
provides a weak pullup. I am relying on the internal PIC protection diodes
and on a zener on the PIC Vdd supply line to provide clamping of excessive
voltage coming from the LCD data lines. This is for a few-off device.

Peter

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