LCDInfo.com

Hello.

I have buyed a 128x64 LCD (RG12864B-BIW-V: http://zefiryn.tme.pl/dok/a06/rg12864b-biw-v.pdf).

I have all connected like in schematic from datasheet. The LCD have a negative voltage supply.

My problem is as follow:
I can't set the contrast rate like i want. If i set the contrast for one line i have to low contrast for fast all pixels turn on. When i set contrast when more pixels are on, the when i display one line i have too much contrast.

Is it normal or i have something wrong.

It's look like:


And sorry for my english.

It might be just a feature of this particular LCD. Not all displays are created with equal quality...
Can you measure the negative voltage supply with pixels turned on and off?

I have also an another LCD. The same size and controller LCD-AG-128064H-FHW K/W-E6 PBF http://www.artronic.pl/o_produkcie.php?id=668?. It has the same symptoms.

The negative voltage is floating in mV range (~25mV) from -3.102 to -3.077 (from one line to full lines with "X")

Is something strange when two different LCDs behave the same.

I have tried:
1. Bigger, and lower pot resistances (from 1k to 20k)
2. Another negative voltage from MAX232
3. Separate the negative voltage with OPAMP (LM243)

The voltage float is lover (2 and 3) but contrast is not stable.

Have you solved the issue with the changing contrast?

As it is happening with two different displays its unlikely a product problem.

It seems the LCD drive voltages (V0 thru V5) on the module are being affected and these are generated from VDD and VEE.

If the difference between VDD and VEE reduces then so does the contrast.

As you have tried many negative voltage supplies with low potentiometer values I suggest you check the main +5V supply to see if it changes with more displayed on the LCD. If this is also driving the backlight it could be overloaded such that it is affecting the LCD drive voltages.

Also confirm the +5V to ground measurement on the module in case a missing ground is causing the module to float.

It would be good to report the final solution here.

All the best.