Character LCD problems?  Read this......

Testing/Troubleshooting
1

Hey all!

Sorry I have not had the time to respond to these LCD issues yet, but I’ll fix that with one of my usual extended play posts now.  ;)

On the BG Micro displays:

I purchased a 40x2 LCD with LED backlight from BG Micro and built the display according to the diagram above. When I plugged the display into the computer and powered it up it displayed nothing. Most displays, when powered, will display all block characters across the first line until they receive some sort of signal. I saw no blocks, no smoke, and had no funny smells. The LED backlight was working which made me feel a little better. I checked and re-checked my connections a dozen times before I caught a slight glimpse of characters. When I held the display at an angle I could see faint characters on the display. I was relieved that it was working but confused why the characters were so dim.

BG Micro supplied a technical sheet with the LCD which identified the LCD as being manufactured by Tianma. After searching through their website I discovered they offered their LCD’s in both standard and extended temperature models. I don’t really know what the extended temperature means but the documentation stated that the contrast controls require negative voltage for control rather than the simple ground used in the diagram above.

Jonathan Hamilton

Another quote about contrast/blank display:

The main problem tends to be that the display contrast is not set correctly, or that they have used an extended temperature range display without realising that it needs a negative contrast voltage.

Steve Lawther

If you can sweep the contrast control pot from one end to the other and you don’t see any change, (even if there is no text on the display, you should be able to see the pattern of pixels with the contrast full up or full down)

And you are sure the wiring is correct,

Then you -probably- have a display that needs a negative contrast voltage (between 0 and -5 volts, not 0 and +5 like the CORE gives).

The link in the first quote shows a decent example of the easiest way to create a negative contrast voltage without a negative rail on your power supply.  This works because the current draw/power requirement of the contrast supply is very very small, and there is a wide range of easy to use IC’s besides the example given on the site for this.

<edit>

On backlights:

Does your LCD backlight flash then go dark when you apply power to the core?  If yes you probably have the “A” and “K” connections reversed, And this condition might be pulling several times the usual power requirement from the power supply. (IE regulator hot enough to burn it’s number into your fingertip!)

</edit>

Sorry I don’t have better news…:-/

Best!

Smash

2

Hi Tim,

could you please add this info to the HowToConnectLCD page of the Wiki?

Best Regards, Thorsten.

3

smash, thanks so much for the info.  the links are extremely helpful!  i’m going to try the -5V fix soon.  i really think that’s my problem.  i’ll let everyone know how it goes.

thanks!

james

4

Another issue on this:

<edit>

On backlights:

Does your LCD backlight flash then go dark when you apply power to the core?  If yes you probably have the “A” and “K” connections reversed, And this condition might be pulling several times the usual power requirement from the power supply. (IE regulator hot enough to burn it’s number into your fingertip!)

</edit>

If you have connected A and K right and your backlight is still flashing AND your regulator is very hot…then you can be sure that your powersupply isn’t strong enough for the whole box (especially if you have more than one display).

Solution: Build a seperate powersupply for the backlight(s).

greets

Doc

5

Yes!  I knew there was more… :wink:

I need to re-word this to say “flash once” and add your post to the wiki.

Thanks Doc!

Best

Smash

6

so i tried out the -5v thing on my breadboard and i saw blocks across the screen!  i’m really hoping this will solve the problem.  just finished placing a mouser order for the parts i need to build the -5v mod that smash posted.  as for the backlight issue: i’m using the original c64 power source.  surely this is providing enough to power the whole thing, including a 40x2 lcd backlight, right?  the regulator does heat up, but i don’t think it’s a problem.  if i were to decrease the brightness of the contrast, that would probably do the trick, right?

p.s. just read doc’s post again and it mentioned that it’s only a problem if the backlight “flashes”.   mine stays lit, so i think i’m in the clear.

7

Hi!

Maybe I’m asking a strange question, but where do you recommend to insert thisâ€converter†circuit? Could I just put it in parallel with the core power supply in order to make -5, or do I need to soldier out the contrast pot and insert it here?( Obviously if I hook it up the other way the core contrast pot will not have any function.) or could I just insert it after J15 (like on the lcd cable)? Sorry that my formulation of this question is a bit clumsy, it’s a bit late in the evening for these issues.

:wink:

Nikolai

8

Hi,

Converter input can be taken from +5v supply. The negative output voltage goes to the contrast circuit - replacing GND contact of the contrast trimmer.

Probably the easiest way is taking the whole contrast trimmer out and putting it in the same board where converted is?!

Bye, Moebius

9

ah! thank you for this detail,.

Best, Nikolai

10

Hi all!

I made the voltageconverter now, it works, like a dream.

Only one small problem, the chip, icl7662(witch should be equal to 7660) only gave me ca. -2v out when i gave it 5 in. (?) i wired Lv to ground and now i had -5. it didn’t say anything about this anywhere around here, so I just wanted to ask you people if this was a very stupid thing to do, or a smart thing do do(?) here is the spec pdf:

http://www.elfa.se/pdf/73/731/07314222.pdf

thank you for all help

Best Nikolai

11

..so I just wanted to ask you people if this was a very stupid thing to do, or a smart thing do do(?)

Very smart thing to do!?! :wink: Basicly you solved your low voltage problem by doing just as it’s supposed to do! :wink:

Bye, Moebius

12

Jolly Good! :smiley: