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16x2 LCD — Parallax Forums

16x2 LCD

Robot FreakRobot Freak Posts: 168
edited 2008-04-23 21:06 in Propeller 1
Hello,

Just bought a 16x2 LCD screen, the datasheet can be found here.
I connected the Vss to ground
Vdd to 5v
Vo to 5v
RS to pin16
R/W to pin 17
E to pin18
DB4..7 to pin11..8
LED+ to 5v
LED- to ground

After using this object, the 5v voltage regulator was getting warm, the LCD didn't show anything. (backlight went on)

So I decided it would be a good idea to ask here for some help.
Am I connecting it wrong or do I have to use an other object?

Kind regards,
Robot Freak

Comments

  • RaymanRayman Posts: 14,813
    edited 2008-04-23 15:47
    Just a guess, but you probably need ~1k resistors between the Prop pins the device since it's at 5V and the prop is 3.3V...
  • Robot FreakRobot Freak Posts: 168
    edited 2008-04-23 15:52
    It is working!
    I connected Vo to ground and added a resistor for the LED backlight.
    Regulators are cold now.

    Is it recommended to use resistors on the data pins?
    For what are they necessary?

    Kind regards,
    Robot Freak
  • Mike GreenMike Green Posts: 23,101
    edited 2008-04-23 16:02
    For a start, the LCD is a 5V device and the Propeller is a 3.3V device. The DBxx pins can become outputs and the higher logic supply voltage can damage the Propeller. You need at least 100 Ohm resistors in series with DBxx pins from the LCD to protect the Propeller.

    Double check your connections and set up a potentiometer to provide an adjustable contrast voltage. The 5V may be too high. A 10K pot between 5V and ground with the wiper connected to the Vo pin would work.

    The Propeller runs off 3.3V, not 5V. If you connect an I/O pin to a source of more than 3.3V (actually 3.9V), there are protective diodes to the Propeller's Vdd and Vss supplies that will conduct. If the amount of current conducted is too large, the Vdd supply will actually shift due to on-chip resistances and the Propeller can be damaged if Vdd rises too much. In addition, the diodes and possibly the conductors on the chip and to the chip can melt if there's too much current. The resistors limit that current to something the chip can handle (<50mA typically).

    Post Edited (Mike Green) : 4/23/2008 4:09:53 PM GMT
  • RaymanRayman Posts: 14,813
    edited 2008-04-23 16:32
    Now that I think about it, you probably only need resistors for those Prop pins being used as inputs... Maybe for your LCD there are no inputs to the Prop, all the pins are outputs to the LCD. In that case, you don't need the resistors...
  • Robot FreakRobot Freak Posts: 168
    edited 2008-04-23 18:38
    The LCD object only uses the outa register, so it isn't supposed to use the pins as inputs.
    So the resistors are not necessary.

    Thanks for the replies!
  • Erik FriesenErik Friesen Posts: 1,071
    edited 2008-04-23 19:55
    Some 150 to 300 ohm resistors can't hurt. The ks0108 chip for example doesn't seem to work right without some resistors inline.
  • ColeyColey Posts: 1,110
    edited 2008-04-23 21:06
    IMHO it is always a good idea to add resistors for a 5V device.
    The LCD object does indeed use OUTA but if you don't issue the correct command and/or suffer a corrupted data stream it is possible for the LCD to switch to 'output' mode.
    I speak from experience blush.gif

    Coley

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