From: "Larry Bradley"


Note that some LCD displays (I have a couple of Seiko units that are
this
way) actually require a small negative voltage for the contrast - my

Are these Hitachi 44780 compatible, or are they graphical displays?

Seiko's require about 1 volt negative. I didn't have a -ve supply
available, so I used a PIC pin to generate a  square wave at about 50 Hz
(that was the interrupt rate I was using), then a couple of diodes and
10uf
electrolytic capacitors  to make a charge pump -ve voltage source.

If one is already present in a circuit, a max232 is a possible source of
a negative voltage.


At 05:34 PM 3/5/2005 -0500, you wrote:
>In the book "Programming and Customizing PICmicro=AE Microcontrollers
>(2nd edition)", it shows a circuit for connecting a PIC to a 44780 LCD
>(Figure 2-25 on page 278 if you are interested).
>
>What I don't understand is it shows a 10 Ohm pot connected in a simple
>voltage divider fasion (One pin to VCC (5v), one pin ground, and the
>wiper pin to the contrast control pin on the LCD).  My understanding
>is that .5A will be wasted though the pot (I =3D E / R =3D 5 / 10 =3D .5=
).
>It will also be dissipating 2.5 Watts. My spec sheet shows that the
>max current drain of the LCD is 4 mA.
>
>So my question is doesn't 10 Ohm seem way low?  Maybe 10k or 100k?
>
>I checked the errata for the book at http://www.myke.com/pic-book.htm
>and it doesn't list this issue.
>
>Thanks.
>
>-
>Mark Bellamy
>
>--
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Larry Bradley
Orleans (Ottawa), Ontario, CANADA
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