GLCD Contrast Fluctuation !

musti463 · Joined: 19 Sep 2013 Posts: 66

I am using 240x128 GLCD with T6963C. when i am run the code glcd contrast has fluctuation. Why? Problem is the hardware or software?
_________________
M.Emir SADE

temtronic · Posted: Tue Dec 23, 2014 12:28 pm

'contrast' problems generally indicate a weak power supply...

possible causes include
1) minimal power supply
2) insufficient decoupling
3) excessive current demand
4) EMI/noise

Without knowing what your hardware is, I'd guess #1. I generally use a 5V 2 Amp 'switching wallwart' style supply for 99% of my projects, add 100mfd and .1 caps on the PCBs and do not have any LCD issues.

hth
jay

musti463 · Joined: 19 Sep 2013 Posts: 66

Thanks your helps.

I sent my hardware:

U4 is LM2576T-ADJ for GLCD Power Supply

http://i.imgur.com/7D3tAUx.jpg?1

http://i.imgur.com/Kgf7IBd.jpg?1

http://i.imgur.com/a3tB3BK.png?1
_________________
M.Emir SADE

dyeatman · Joined: 06 Sep 2003 Posts: 1934 Location: Norman, OK

...
_________________
Google and Forum Search are some of your best tools!!!!

temtronic · Posted: Wed Dec 24, 2014 8:08 am

Ok, had a look at the pngs....

Just where does Vee for the display get wired to ? You have the contrast pot getting power from 'Vee' of the GLCD connector. Does that LCD SUPPLY poewr to that pin? It's not usual for an LCD to supply power.

I suspect it needs to be connected to a power source. If you post a link to the display you're using, that will help.

having 3 switching power supplies on the PCB doesn't help ! Using linear regulators would really quiet down the EMI, be simpler to assemble,cheaper too !

also, traces seem very,very thin to me.I prefer 'thick/wide' traces and big donuts.make for more reliable/robust PCB.

hth
jay

musti463 · Joined: 19 Sep 2013 Posts: 66

I need 3.5V for GLCD Backlight supply. so i couldn't find lineer regulator for this.

GLCD datasheet:

http://www.newhavendisplay.com/specs/NHD-240128WG-ATFH-VZ.pdf
_________________
M.Emir SADE

gaugeguy · Joined: 05 Apr 2011 Posts: 303

That is the forward voltage drop of the backlight LED, not the supply voltage. You should be using either a current limited supply or a higher voltage with a dropping resistor to limit the current.

temtronic · Posted: Wed Dec 24, 2014 9:24 am

What's the voltage going to pin 4, VO of the LCD display ?
Also, voltage at VDD of the display ?

jay

Jerson · Joined: 31 Jul 2009 Posts: 125 Location: Bombay, India

I am making an experienced guess that the 12V power supply is not rated for adequate current supply to the circuit. So, as the GLCD gets written to/there is activity, you are getting contrast flicker.

The easy way to confirm this theory is to halt your code execution using a while(1); statement after writing some data to the GLCD. No/reduced flicker should be there in this case.

As for the V0 etc, it seems okay to me. I have used similar GLCDs based around the KS0108 driver chipset.