LCD driving techniques

Discussion in 'General Electronics Chat' started by casdsys, Jan 10, 2015.

  1. casdsys

    Thread Starter Member

    Mar 15, 2009
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    Hi all.
    I hear you need to apply a square wave when using a lcd display.
    Im using a 4543 type chip.
    Question is, do I need to apply and square wave to the
    4543
    the back plane of the lcd ( common/gnd )
    or both
    if both does the square wave need to be in sync or opposing/inverted to each other.

    Cheers.
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    The square wave is applied to both the phase pin and the back plane of the LCD.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    What joey said. Also, the phase between the two square waves is what makes a pixel on or off, typically opposite phase turns them on..
     
  4. JohnInTX

    Moderator

    Jun 26, 2012
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    What Joey and Ernie said plus its vital that there be no DC offset in the driving waveforms. That will cause plating from one electrode to the other and ruin the display.

    AN658 from Microchip is an excellent introduction to LCD technology. The few code examples are for PICs but the rest applies to any LCD application.
     
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  5. casdsys

    Thread Starter Member

    Mar 15, 2009
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    Thankyou for your input. I will apply a square wave to both, LCD common and the phase pin of the 4543 using a 741.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    I don't remember the exact details, but a popular scheme shown in many text books is to drive the segments via an XOR gate - if you feed a pulse train to one input, the logic state of the other input determines whether or not the XOR inverts the pulse train.

    Therefore; with one logic level, the pulse train is in phase with the backplane drive and the segment is not active. Change the logic level and the pulse train is inverted to out of phase with the backplane and the segment becomes visible.
     
  7. joeyd999

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    Jun 6, 2011
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    The 4543 has the xor function built into it.
     
  8. RichardO

    Well-Known Member

    May 4, 2013
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    A common op-amp (such as a 741) will not output proper CMOS logic levels. You need to use a CMOS inverter (sucha sa a 74C04), instead. The CMOS inverter will also take way less power than an op-amp.
     
  9. ian field

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    The most important thing is; not to leave steady DC across any LCD elements.
     
  10. joeyd999

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    Jun 6, 2011
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    I.e., your clock must be exactly 50% duty cycle.
     
  11. ian field

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    Oct 27, 2012
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    It gives the best contrast - but the sky isn't going to fall because its a few % off.

    Presumably, the greater the error - the more impact on life expectancy. But I wouldn't expect it to be catastrophic.
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Or, just add a flip-flop to your clock and you automatically have 50% duty cycle output albeit at half of clock speed.
     
  13. ian field

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    That's probably as close as you can get - all that's left is the propagation delay in the gate that controls the segment drive.
     
  14. RichardO

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    May 4, 2013
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    If I remember right, the displays I have used were allowed 50 mv of total offset. The total offset is a function of both the error from 50% duty cycle and actual DC offset on the waveforms.
     
  15. casdsys

    Thread Starter Member

    Mar 15, 2009
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    OK , so now I ask is I guess there would be no harm in using a simple low cost 8pin microprocessor dedicated to just generate the 50/50 ratio square wave??
    Does 30Khz sound fine?
     
  16. WBahn

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    Mar 31, 2012
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    You can do it any number of ways. What are you driving the 4543 from? If it's an MCU or anything similar, why not have it generate the necessary square wave?

    As to the frequency, that will depend on the LCD in question. Most direct-drive displays are run at much lower frequencies, like 30Hz to 100Hz. The faster you run them the more power they will consume and the more crosstalk you will have between segments.
     
  17. casdsys

    Thread Starter Member

    Mar 15, 2009
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    I have a 2 digit LCD display connected up to a couple of 4543.
    1). The main MCU will control the strobe ( 2 lines for the 2 x 4543, 1st digit and the 2nd digit ) and 4 lines for the ABCD lines, paralleled between the 2 x 4543. and 1 line to generate the squarewave.
    2). Or I can use a 8pin MCU just to produce the squarewave.
    3). Or as (2) plus 2 lines to sense UP/DOWN button and 4 lines to output the ABCD.
     
  18. WBahn

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    Mar 31, 2012
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    There are many ways to do most things. You need to decide what makes reasonable sense within the context of the overall project, your resources, experiences, and capabilities, and what it important and what is not.
     
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