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Harprit
12-15-2009, 03:52 AM
I have had some weird observations for a 4 x 20 LCD display. The one I have has a standard Hitachi driver but needs a negative voltage on pin 3 to get the contrast to work. Essentially this interface is the same as for a standard 2 x 16 display specially for the first two lines. Now the weirdness.

If I put this display where a 16 x 2 display was, on start up and no negative voltage for contrast I can just barely see the display if the lighting is just right. But everything is working. If I provide the negative contrast voltage everything becomes visible just right. Now if I remove the negative voltage and lift the display off the breadboard and push it back down again, I get garbage scrolling on lines 1 and 3 (not 1 an 2) but I can see everything just fine without the contrast voltage. So apparently there is some way to have the contrast come alive without a negative voltage but this is interfered with by the initialization process. Does anyone have any experience with this? Is it possible to get around this? How do we explain this?

This is an Optrex uint

Harprit

Post Edited (Harprit) : 12/14/2009 9:10:53 PM GMT

Harley
12-15-2009, 04:22 AM
Harpit,

You wrote "...lift the display off the breadboard and push it back down again..."; do you mean the power was removed? Because all these LCD controller ICs need to be re-initialized after power is restored. Hope I didn't misunderstand what you described.

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Harley Shanko

Harprit
12-15-2009, 04:28 AM
Yes essentially the power is removed (and all other connections also) and then re-applied but the system is not reinitialized. When I reinitialize the contrast goes away.

Harprit

Peter Jakacki
12-15-2009, 05:46 AM
Are you running your display at 3.3V supply? Most displays require 5V and although the logic may work at 3.3V the LCD drive requires about -4.5V referenced to the supply. In a 5V system this equates to +0.5V but in a 3.3V system it is 3.3-4.5 = -1.2 which seems like a suitable explanation given that you haven't mentioned the supply voltage and knowing that any 5V character display does not require negative voltages while operated within normal temperature ranges.

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*Peter*

Harprit
12-15-2009, 05:54 AM
The 4 bit signals are coming directly from the Prop
The power to the LCD is 5 volts and ground
The contrast is at -3.0 (2 AAA cells)

Harprit

Harprit
12-15-2009, 06:10 AM
Peter

It works just fine if I add the contrast -AAAs so there is something else going on.
The specs do say it needs a negative voltage for contrast on this particular unit.

HSS

Peter Jakacki
12-15-2009, 06:19 AM
Ok, so you are saying that the 4 line display is connected and then power is applied and the display is initialized? I'm not sure how your contrast circuit is constructed so as a test just short the contrast pin to ground as this will always work although the contrast is off a bit. With the 4 line plugged in try starting it up. If that doesn't work then I suspect your init code as the only other possible culprit. A faulty display is always possible although unlikely.

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*Peter*

Harprit
12-15-2009, 01:06 PM
I'm going to leave it here for now and get another 4x20
and then see what happens.

HSS