EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: MikeK on August 28, 2012, 10:35:31 pm
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Here was an eye opener for me. I recently bought a couple of 8x2 LCD modules on eBay. After wiring it up I found that updating any character position faster than about 2Hz makes it almost unreadable. I had no idea there could be such a difference. Isn't it all LCD technology the same?
Here's a video of it: Cheap 8x23 LCD with backlight (https://www.youtube.com/watch?v=Yc0N6lA_a60#)
Is it just backlit modules that are like this?
Mike
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What controller does it have?
Some have a programmable oscillator frequency (ST7032), which could have some effect.
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It's the pixel response time, that one looks pretty terrible.
You can see the pixels fade out/in
Is it a cheap china one?
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1. Are you sure you're not issuing a "clear lcd" before writing the number? The clear lcd takes quite a lot of time on these lcd screens.
2. Are you sure it's not a contrast issue? Are you giving it the proper voltage? That LCD looks really weak at contrast.
The 2x16 and 4x16 ones I have are much better at contrast and they refresh quite well, really no problems.
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Yeah, it's being fed 5V.
The contrast is the highest I can set it without the gray blocks showing up when there's nothing on.
I'm not issuing the "clear lcd" between digits (just at the beginning).
This is the datasheet: http://www.lcd-modules.com.tw/upload/product/profile_4.pdf (http://www.lcd-modules.com.tw/upload/product/profile_4.pdf), says it uses the KS0066.
The datasheet has one mistake, though, it shows the backlight having separate power pins, but they're integrated with the regular LCD power pins (1 and 2), and has a 5 ohm current limiting resistor.
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It looks like the oscillator is not running fast enough. There should be a resistor or some other passives on the board that sets its frequency, you could try changing the value lower/higher.
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I'm confused how the oscillator speed would have any effect. Like Psi pointed out, it has poor pixel response time. Probably a function of the on-chip FETs used to switch the pixels?
With so much being made in China, I wonder how we can tell which items to stay away from. Because I've seen Chinese LCDs that are fast and readable.
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I'm confused how the oscillator speed would have any effect. Like Psi pointed out, it has poor pixel response time. Probably a function of the on-chip FETs used to switch the pixels?
If you slow down the video you can almost see the individual pixels turning on at different times.
Another thing to check is the voltage/waveform at which the crystal is being driven, you might need to improvise something thin enough to slip under the zebra strips to the COM/SEG ITO. Your module might just have the crystal voltage tied to Vcc but the HD44780 allows a separate voltage for LCD drive, which can go up to 11V.
What is the ambient temperature? LCD response time is negatively affected by low temperatures.
Edit: KS0066 allows up to 13V for LCD drive voltage, but it must be below Vcc so the bias voltage should be connected to a negative supply.
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Of course you can see this. These are passive displays and they are beeing scanned scanning is in the prder of kilohertz with the backplane driven at a 30 to 50 hertz range. If they use a5 by 7 matrox and chop the 7 lines that means that a row refresh occurs in 25 milliseconds ... With an 1/7 duty cycle...
Of course you see that. Ir is inherent to the tech ology !
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Thanks for the advice. It's not worth my time trying to "fix" it - if even I could - but it sure would be nice to know how to stay away from these slow modules.
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My scientific calc watch project uses the LCD response time to generate "black and white" chess characters:
(http://calcwatch.com/chess3.gif)
Dave.
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That's a cool idea. I would guess it would only be feasible if the pixels switch on at the same time. I seem to have a display module that's not good for fast changes.
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These crappy LCDs have pixel responses in the hundreds of ms and clock speeds in kilohertz.
Required wait times often have to be ten times longer than what the datasheet says.
Can't even scroll the text because of ghosting...
Somebody obviously thinks it is good enough.
Funny they are often more expensive than orders of magnitude better displays from mobile phones.
Anyone knows where to get a better one? Mobile spare parts?
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Well ... the next option is a 5110 display
http://dx.com/p/replacement-1-6-lcd-screen-with-blue-backlight-for-nokia-5110-blue-145860?item=4 (http://dx.com/p/replacement-1-6-lcd-screen-with-blue-backlight-for-nokia-5110-blue-145860?item=4)
http://dx.com/p/arduino-1-6-lcd-display-screen-for-nokia-5110-red-silver-140226?item=3 (http://dx.com/p/arduino-1-6-lcd-display-screen-for-nokia-5110-red-silver-140226?item=3)
White or Blue, nice backlighting options
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Yeah, taking advantage of cellphone technology economies of scale is probably the wise choice. I got two more [different] LCDs from eBay recently. Chinese made. And they suffer from the same problem. One of them is an absolute pig on power.
Buying quality items from Mouser, or wherever, really isn't much more expensive. I just assumed it would be.
And finding datasheets for the Chinese LCDs can be difficult, and they're not that detailed. I just wish there was a way upfront to discern the good from the bad.