LCD questions.

Discussion in 'General Electronics Chat' started by Gump, Jul 27, 2010.

  1. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Hello,

    I'm trying to get to grips with an alphanumeric LCD display, in particular this one: http://www.vishay.com/docs/37312/37312.pdf, although there are a few things I'm not quite understanding:

    1) I know there is a controller and driver, but what is the difference? It seems the most important part (from using the LCD POV) is the controller, correct?

    2) What exactly is the duty cycle?

    3) What is an N.V. (used in the phrase, N.V. optional for a +3V supply).


    Concerning the datasheet:

    4) Under the "Absolute Maximum Ratings", there is a note, Vss=0V, Vdd=5.0V, I'm not quite sure what that is meant to be saying as the valid ranges are given in the "Electrical Characteristics" table?

    5) "Electrical Characteristics" table seems to mix symbols quite a lot. For example, the input voltage is given the symbol Vdd, where as it had the symbol Vi in the "Asbolute Maximum Ratings" table, what's going on?

    6) Under the "Electrical Characteristics" table, what exactly is meant by the "Input Voltage", "Recommended LC Driving" and "LED Forward Voltage"?

    Thank you! :)
    Gump
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
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    1) Controller: remembers characters and sends signals with correct timing. Driver: often integrated into the controller, provides the right voltages and signals. Some LCDs require ±13 volts or so to work for the contrast setting. You sometimes have to provide this voltage! Check with your datasheet.

    2) Duty cycle, also called mark/space time, is the percentage average time the output of a signal is on. For example if a signal is on for 1ms with a total period of 2ms (on 1ms, off 1ms) the duty cycle is 50%. Duty cycle ranges from 0% (fully off) to 100% (fully on), and all the way in between.

    3) Maybe it's the negative supply.

    4) Just to clarify: Vss=ground, Vdd=supply voltage. Absolute maximum ratings are the ratings you must not exceed under any conditions, i.e. you risk frying the LCD or controller/driver chipset. Running a device at the absolute ratings is NOT recommended, you risk premature failure. It's like engine temperature, you can run a car with it overheating but it won't run for long, you risk failure. The typical ratings usually are the ranges at which the specifications are given, such as maximum clock speed, and the specifications at which the manufacturer mandates it will meet all the MTBF (mean time before failure) and data retention (if applicable) and possibly other figures.

    5) Vi=input voltage. This is different from Vdd=supply voltage. Input voltage is the maximum voltage you can put on any of the IO pins. This may be dependant upon the supply. A common rating is Vdd+0.3V, which means for a 3.3V supply putting any more than 3.6V could damage it, although for devices which have 5V tolerant IO it may be something like 5.5V, independent of supply.

    6)
    a) Input voltage: see above.
    b) Recommended LC driving: recommended liquid-crystal driving. Not sure, probably the recommended voltage for the display, discussed above.
    c) LED forward voltage: the voltage of the LED backlight at which approximately 20mA flows. Sometimes it's not 20mA, but whatever current is specified, usually 15mA to 30mA, although for some displays I have seen as high as 150mA, because a parallel configuration was used instead of a series configuration.
     
    Last edited: Jul 27, 2010
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  3. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Hi Tom,

    Thanks very much. Just a few follow-up questions if I may:

    1) What does the duty cycle mean in terms of an LCD? For example, the datasheet I linked to specifies a 1/16 duty cycle.

    2)
    That I understand, but the datasheet doesn't really give anything like that, unless it is something you have to work out.

    The datasheet specifies 2.7V <= Vi <= 5.3V providing Vdd is 5V. Would it be acceptable to expand this to say Vi is actually between Vdd-2.3V and Vdd+0.3V?

    3) From what you've said it seems that there are 3 unique voltages, the power supply, the input voltage (i.e. the voltage for the data and control pins) and the LC driving voltage. Could it be the power supply is for the chips etc and the LC driving voltage is for the actual LCD display?

    4) I don't actually have a backlight on this particular LCD, but do I still have to worry about contrast?

    Thanks,
    Gump
     
  4. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    1) I'm not so sure.

    2)
    a) You have to work it out. Sorry. It's usually something you pick up along the way, like me.
    b) Not necessarily true. On the datasheet, that variable is given at Vdd=+5V. It varies depending on supply voltage. Generally, it is a good idea to supply a HIGH voltage more than 70% of Vdd and a LOW voltage less than 30% of Vdd. In this case this LCD has been designed to interface with 3.3V IO so it will accept inputs as low as 2.7V as a logic HIGH.

    3) On your display, you're lucky, because the LC driving voltage is +5V, not something like -13V. Connect a potentiometer between Vdd (supply voltage for LCD) and Vss, wiper to Vf. The potentiometer allows you to adjust display contrast. If you don't have a pot just put a half supply divider on the input.

    4) Contrast and backlight are different. You still need contrast.
     
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  5. Gump

    Thread Starter Member

    Jun 7, 2010
    57
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    Just found this that explain the duty cycle in terms of LCDs brilliantly:
    http://www.crystalfontz.com/forum/showthread.php?t=11

    Hm, I see what you mean about the minimum and maximums, but it says that VDD has the absolute maximum ranges of -0.3V and 7.0V, so say I ran it at 3V, how would I know what the valid ranges are for the voltages for Iv, as I'm only given it for when Vdd=5V?

    It seems that there is a graph missing from the datasheet giving this information?
     
  6. tom66

    Senior Member

    May 9, 2009
    2,613
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    I said generally keep it above 70% of Vdd. This is usually safe logic high.

    Power it from the same supply as your microcontroller/ICs.
     
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  7. Gump

    Thread Starter Member

    Jun 7, 2010
    57
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    Hmm, this particular LCD screen states the absolute maximum for Vi is Vdd, yet the standard maximum for Vi when Vdd is 5V, is 5.3V, this doesn't really make sense to me?
     
  8. tom66

    Senior Member

    May 9, 2009
    2,613
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    Yeah, that's Vdd+0.3V.

    So at 3.3V you can give it Vdd+0.3V, or 3.6V.

    Or at 5V you can give it Vdd+0.3V, or 5.3V.

    These are absolute maximum ratings. Avoid getting too close to them.
     
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  9. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Hi Tom,

    Thanks for your patience here, what I'm not quite getting though is that they say at 5V, Vi has an absolute maximum of 5.3V, but just above that in the "absolute maximum ratings" it specifically states the maximum for Vi is Vdd, meaning that Vdd at 5V Vi cannot be more than 5V. For Vi, the "absolute maximum ratings" doesn't make any mention of Vdd+0.3V, just Vdd.

    Thanks,
    Gump.
     
  10. tom66

    Senior Member

    May 9, 2009
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    Hmm. It seems that this may not be protected from excessive voltages! So you have to be careful. The writers of the datasheet may have just assumed this, apparently this is wrong!
     
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  11. Gump

    Thread Starter Member

    Jun 7, 2010
    57
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    Ah okay, although is it a safe bet to say for a HIGH on Vi, it can be >= 70% of Vdd and say < 90% of Vdd, in your opinion does that give enough room from the absolute maximum of Vi?
     
  12. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    Vo is the contrast voltage. Note how it is defined, as a voltage below Vdd, not as a voltage above ground. For normal temperatures and a 5V supply, a pot set up as a voltage divider from Vdd to ground will work; I'd add a resistor between the top of the pot and Vdd since otherwise the usable range will be squeezed to the very bottom of the pot. For really cold temperatures or a 3.3v supply, you will need to supply a negative voltage (with respect to ground) on the contrast pin; if you have a MAX232 or similar serial port level shifter chip, you can steal the negative voltage from there.

    /mike
     
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  13. tom66

    Senior Member

    May 9, 2009
    2,613
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    Yes. I don't know what microcontroller you're using, but here's an example using a PIC microcontroller. The Rds(on) of a PIC output is 20 ohms. So you can be fairly safe in assuming a few millivolts are likely to be dropped across the output FET and the wires you're using. It's not like you can't go at Vdd+1mV.
     
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  14. Gump

    Thread Starter Member

    Jun 7, 2010
    57
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    Hi, as you say, I can see Vo is the contrast from the pin table, but on the electrical ratings table on the page above it, what would Vo be?

    Sorry, I'm not quite seeing exactly where this is on the datasheet?
     
  15. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Hi both,

    I've been reading the datasheet a few times and from what I gather this datasheet is also for the same model but with a backlight, which would explain why entries such as "LED Forward Voltage" are there, even though the particular LCD I'm looking at doesn't have a backlight.

    Things make a little more sense now. :)

    So:

    1) Vdd can be between -0.3V and 7V, but to keep things simpler, say 0V to 5V.

    2) Iv can be between 70% of Vdd and Vdd, so if Vdd is 3.0V, the range of Iv would be 2.1V and 3V.

    3) Vo (LC Driving Voltage) isn't relative to ground, but is relative to Vdd, so if Vdd is 3.0V, then Vo (assuming 25^{o}c), would be between -1.1V (3.0-4.1) and -1.7V (3.0-4.7).

    Have I just about got it?

    Thanks,
    G.
     
  16. tom66

    Senior Member

    May 9, 2009
    2,613
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    1) Yes, but it will only be operational from 2.5V - 7V or so.

    2) Correct.

    3) That is true. But it is rare to measure voltages relative to the Vdd pin. You would measure it relative to the Vss pin.
     
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  17. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Do you mind me asking how you come up with 2.5V, as I'm not seeing that on the datasheet, yet obviously Vdd cannot be 0V, otherwise it ain't going to do anything, although there doesn't seem to be a specified minimum?

    If I measured it against the Vss pin (which is ground), then wouldn't I have to calculate all the values again? Or are you simply saying that the datasheet is presenting information in a bit of an unconventional manner?

    Thanks,
    G.
     
  18. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    The key term is "or so". I didn't check the datasheet, this was an example.

    No, the datasheet is presenting values relative to ground. If they are negative, it means the ground is more positive than those voltages. Generating a negative voltage involves a charge pump and can be tricky, so many LCDs include one. Check if yours does.

    Checking the datasheet, it seems Vo must be 4.5V (or so) less than Vdd. Hence the symbol "Vdd to Vo". If you put a multimeter between here with positive terminal on Vdd and negative on Vo you should get 4.5V (or so).
     
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  19. Gump

    Thread Starter Member

    Jun 7, 2010
    57
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    Ok, got you.

    I'm not seeing how you find this information from the datasheet. :( For the LC Driving Voltage it is using the symbol "Vdd to Vo", which IIRC is saying Vdd is relative to Vo by XYZ amounts, which are all positive, so Vo is XYZ lower than Vdd?

    Thank you!
     
  20. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    Sorry I'm not being very clear.

    I'm not finding this in the datasheet because it is the standard to assume, unless told otherwise, that the values are relative to ground. In this case, the value is above ground, but it is below Vdd. So that's where the confusion comes from I suppose. Do the math: say your Vdd is 5V, then your Vdd-Vo should be 4.5V, or 0.5V (relative to ground). Do you understand? I had another look over the datasheet and I think I should have explained it better. Making Vo relative to Vdd allows the use of a potentiometer. As mentioned in my previous post:

     
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