LCD varying contrast / garbled characters - Page 1
Forum section: electronics / beginners

author: Emil
author_rank: Regular Contributor
timestamp: August 26, 2012, 12:56:50 pm
content: I'm using a Displaytech 161A with an (HD44780 compatible) KS0066U controller. I'm controlling it from a PIC16F628A.I have a strange problem with the lcd, the characters have varying contrast. Parts of each character is drawn normally, parts are very faint. I can see a similar effect on the squares that show up on the display before I initialise it. The effect on the squares is more subtle, so it was hard to get a picture of it.I don't think this can be a software issue because it shows up before any data is sent to the display and because it actually shows the correct character at the correct position etc. Because of that I didn't include my code.I have tried googling the it, but I have not found anything about this problem. I will appreciate any help.My circuit:1 Vss <--> Gnd2 Vdd <--> +5V3 Vo <--> Gnd4 RS <--> a35 R/W <--> Gnd6 E <--> a211 DB4 <--> b012 DB5 <--> b113 DB6 <--> b214 DB7 <--> b37,8,9,10,15,16 not connectedDatasheet:http://docs-europe.electrocomponents.com/webdocs/06dd/0900766b806dda14.pdf

author: madires
author_rank: Super Contributor
timestamp: August 26, 2012, 02:10:46 pm
content: Quote from: Emil on August 26, 2012, 12:56:50 pmYou might want to connect Vo to the wiper of a 10k potentiometer (between Vdd and Gnd). For example:http://www.robotroom.com/LCD-Nanox-Sanyo-3.html

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 26, 2012, 02:30:19 pm
content: And it looks like the frame of the display is connected to pin 2 (first picture), that's not a good idea either.

author: Emil
author_rank: Regular Contributor
timestamp: August 26, 2012, 03:17:38 pm
content: Quote from: PA0PBZ on August 26, 2012, 02:30:19 pmIt's not connected, it just looks that way because of the angle of the picture.

author: Emil
author_rank: Regular Contributor
timestamp: August 26, 2012, 03:45:57 pm
content: Quote from: madires on August 26, 2012, 02:10:46 pmIs the value critical? I can't find a 10K pot now (should be here somewhere..) but I have tried with a 22K potentiometer. I have to adjust the resistance between Gnd and Vo all the way down to see anything on the display. It then looks the same as when connecting it diretly to Gnd.I have also tried various fixed resistors (one from Vo to Gnd and one from Vo to +5V), the resistor to Gnd needs to be very low before I can see anything on the display. Looks the same then too.

author: madires
author_rank: Super Contributor
timestamp: August 26, 2012, 04:35:40 pm
content: Quote from: Emil on August 26, 2012, 03:45:57 pm22k should work too. Have you checked Vdd (5V)? Have you tried another LCD module?

author: AndyC_772
author_rank: Super Contributor
timestamp: August 26, 2012, 04:54:48 pm
content: I'm struggling a bit to see what the problem might be here. Have you tried another display?What voltage is the PIC running at? What are the logic 0 and 1 levels?

author: Emil
author_rank: Regular Contributor
timestamp: August 26, 2012, 05:38:22 pm
content: I have not tried another LCD yet, but I will order a few displays later today, so I will try that in a few days.The PIC and LCD Vdd are both at 4.92V. The datasheet for the LCD says 4.5 to 5.5 volts, so I think that should be fine.The output pins on the pic are at 4.92V when on and 0.01V when off. This was measured when turning on and off at a slow rate. I can't measure the actual signal to the LCD because I don't have an oscilloscope, only a multimeter.

author: amyk
author_rank: Super Contributor
timestamp: August 27, 2012, 09:36:06 am
content: I agree that this is not a software issue. There is no way to control individual pixel's value using that controller.Is it always the same pixels which are lighter/darker?Check the zebra strips between the glass and the PCB to ensure they're making good contact. They canget shifted out of placeand produce those symptoms.Also check the resistors forming the bias voltage divider, ensure that they are all neither shorted nor open.The other possibility is electrolytic damage from DC bias, although if you bought this display new that shouldn't occur.

author: maxiq4
author_rank: Contributor
timestamp: August 27, 2012, 01:32:41 pm
content: Quote from: Emil on August 26, 2012, 12:56:50 pmthis is wrongyou must use a potentiometer

author: DavidDLC
author_rank: Frequent Contributor
timestamp: August 27, 2012, 09:14:07 pm
content: Bad display, put it on the trash can and buy another one from ebay, very cheap there.David.

author: Emil
author_rank: Regular Contributor
timestamp: August 28, 2012, 06:45:32 pm
content: Quote from: DavidDLC on August 27, 2012, 09:14:07 pmI'm starting to think that too. Hopefully the new displays will be here tomorrow.

author: Emil
author_rank: Regular Contributor
timestamp: August 28, 2012, 06:55:09 pm
content: Quote from: amyk on August 27, 2012, 09:36:06 amI had a look at the zebra strip. It didn't look bent like in that picture, but not easy to see if everything is lined up correctly.If I write the same characters in the same way it will be the same pixels. If I write one character at a time instead of all in sequence it can be different. Writing a character at on position can change the pixels of another character.

author: Emil
author_rank: Regular Contributor
timestamp: August 28, 2012, 07:21:56 pm
content: Quote from: maxiq4 on August 27, 2012, 01:32:41 pmWhy?I I understand it correctly, the contrast of the display is set by the voltage difference between Vdd and Vo. According to the datasheet this voltage should be Vdd - 4.8 to 5.1 volts. I can see three practical ways to achieve this:1. A voltage divider using either potentiometer or fixed resistors.2. Connect it to the output of a DAC.3. Simply connect it to Gnd, as that is withing the acceptable voltage range.If I'm wrong about this I hope someone can explain why.

author: Balaur
author_rank: Unknown
timestamp: August 28, 2012, 08:17:58 pm
content: Quote from: Emil on August 28, 2012, 07:21:56 pmThe display contrast is extremely finicky. Some displays happily accept 0V and have a good contrast, while others exhibit a wide interval of behaviors.Please try the pot and find the value that works best.

author: Emil
author_rank: Regular Contributor
timestamp: September 01, 2012, 08:28:50 am
content: I received the new LCDs a few days ago.When I connected one of the new ones to the test circuit on my breadboard it worked fine. So the problem was that the first LCD panel was damaged.It gets more interesting.. I then connected a second LCD to the circuit I was working on in the first place and got the same problem again. My circuit broke another LCD. I'm sure I'll learn something from troubleshooting thatThank you for all the replies.

author: amyk
author_rank: Super Contributor
timestamp: September 01, 2012, 08:39:21 am
content: If you can, try exchanging the glass LCD proper between the different modules to see if the fault is with the controller or the crystal/electrodes.QuoteAre you taking proper ESD precautions? These symptoms suggest a damaged controller.