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[Lcdproc] HD44780 40x4 with Backlight and winamp wiring

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  • From: bsdfan at (Markus Dolze)
  • Subject: [Lcdproc] HD44780 40x4 with Backlight and winamp wiring
  • Date: Thu, 22 Jan 2009 20:24:31 +0100

Ethan Dicks wrote:
> On Thu, Jan 22, 2009 at 12:23 PM, Sascha <sascha.zielinski at> wrote:
>> Hi,
>> I bought a 40x4 LCD with LED Backlight...
>> The Documentation describes to connect the Backlight pin through the BC327
>> Transistor to Pin 17 of the Printer Port.
> Yes. That is the "standard" way to hook up a backlight.
>> Should I then connect the EN2 to GND?
> If you do that, then you will only be able to use half of your display.
> The "problem" is that there aren't enough pins for an 8-bit interface,
> plus 2xHD44780 (EN1 + EN2), *plus* backlight. AFAIK, all 40x4
> displays are implemented as two HD44780s with all pins tied together
> except enables. This is because there isn't enough onboard RAM (or
> column/row drivers, too?) to handle 160 on-screen chars with a regular
> HD44780, and I don't think anyone extended the chip that way - it's
> easy enough to add a second controller chip to the back of the LCD,
> and there's plenty of room.
It is not only RAM. The display controller has to address each pixel by
a row and column line. There would be just too many lines for a single
chip. Additionally there are limitation regarding the current flowing
through the chip.

On displays like 40x4 you will not only find two controllers but also
some extension driver chips which are conntected through an internal
interface to the controller.
>> I use Debian Unstable/Experimental. The Version 0.5.2 compiles fine and
>> runs
>> now for about a day without a problem. I tried also the the version from
>> cvs
>> server (checked out on 01.22.09 at 5.00 pm CET). If I start LCDd with my
>> LCDd.conf from stable version, the Lines 3+4 stays blank.
> If you read C, you could try browsing the hd44780*.c code and see if
> there were any changes made since 0.5.2-stable that might come into
> play. I'm wondering how long tying two displays into one virtual
> screen has been around, or if perhaps the default line for EN2 may
> have changed in the past couple of years.
It is true that you cannot have a switchable backlight and a multi
controller display like a 40x4. As there has been no change between
0.5.2 and HEAD regarding this, something other must be wrong.

If you do like to have both you could just change the one or two lines
of code, but you will have to keep track of this change.
> I don't know that lots of the people on this list have 40x4 displays,
> but perhaps someone with a parallel-attached LCD can comment. I
> mostly work from laptops now, and few laptops provide parallel ports
> anymore. Even serial ports are getting scarce. :-( I'm doing more
> and more work with USB-attached displays even though I miss the
> directness of driving ones own I/O ports without an intervening
> software layer to debug.
I have one 40x4 I use with lcd2usb, but I currently don't have a spare
connector to hook it up to the parallel port directly. I may give it a
try this weekend.


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