Don,
I don't think they do. I'm not sure why they recommend that configuration on their data sheets but many do.
Using ground seems to work just fine. In fact Michaal (orginal ks0108/glcd author) always wires his up that way.
But maybe it depends on the temperature and if the temperature is really cold or really warm perhaps
the voltage need changes enough to need a positive voltage.
Using ground vs +5v should also save a bit of power as well.
I got curious so I just went back and looked at several data sheets:
The ks0108 samsung data sheet says that the voltage should be
<= VDD and >= VEE
(between VDD and VEE,.... not that helpful)
So I looked at the 17 data sheets I had handy.
to see how they recommended wiring up the pot
- 10 of them used +5v
- 1 of them used ground
- 4 of them didn't specify
- 1 of them was totally bogus and their wiring wouldn't work.
From reading a few, it wasn't clear what the reference is.
Some appear to reference the voltage between VDD and VO as the voltage.
Most dont' specify.
Since most show needing voltages of about 9 to 10 volts, it wouldn't much matter if ground was used
on one leg since you need much more than 5 volts to make it work.
But then some show voltages as high as 14 volts at very high temperatures 50+ up to 70C.
and the only way to get that is if VDD were used, assuming VEE is -10v.
(but one data sheet showed VEE as -19 volts)
And one data sheet showed Vee as -5 volts (probabaly used a voltage doubler to get it).
So basically, I have no idea. It may actually depend on the glcd and the temperature.
I always just wired mine according the data sheet that came with it and it showed using +5v.
But I bet, that ground will probably work in nearly all if not all cases, at least at room
temperature.
--- bill