[Lcdproc] Should we document pre-requisit to try HD47780 at home.

[Robin Adams]

David GLAUDE wrote:

  > I feel like we frequently get electrical and electronic question.

Although I am no engineer, here a small list of problems I
encountered or have heard of. Perhaps we can add that to the
docs ? The electronic details of digital circuits and why they
are not working are sometimes not _that_ easy.

So here is my top ten gotcha list when connecting HD44780 or
other displays to the parallel port:

1: Check The Wiring
      Wiring errors can easily be made. If you are unexperienced
      with the soldering iron better have someone solder it for
      you. Display modules are sensitive to electro static
      discharges, so touch an earthed surface (computer case,
      water pipes...) before you handle these.

2: Power Source Unregulated / Noisy
      Make sure your power supply delivers steady 5 Volts with-
      out noise or interruptions. The bare wall plug in trans-
      former is not suitable, though you can make it stabilized
      by adding an 7805 and a few capacitors.
      Some noise induced in the supply lines my be tricky to
      track, even if you habe an oscilloscope.

3: Ground Lift
      The power supply wires and especially the GND wires should
      be a little thicker than the other wires. If GND is not
      thick enough (or not existent, see 1) the resistance of the
      wire may cause differing GND potentials in the circuit.
      This may lead to strange display behaviour. It may also be
      wise to solder a 100nF capacitor directly to the GND and
      VDD pins of the display.

2: Latchup
      Never let the supply voltage get much below the io signal
      voltage. It may lead to a latchup condition which will
      destroy the controller chip on the display.

3: Contrast
      If you don't see anything on your display it may be that
      your contrast voltage is set wrong. Turn your contrast
      potentiometer all the way to the end connected to GND.
      Contrast is highest then.
      Beware: The module you got so ultra cheap may be an
      enhanced temperature model which needs a negative
      contrast voltage for sufficient contrast - see chapter
      99 on how to make negative voltage.

4: Parallel Port Voltage
      Many modern mainboards and especially notebooks will not
      nearly output 5V for a logic H as the older parallel ports
      did, because the operating voltage of computers is lower then
      5V these days. I have measured voltages between 2.5V and 4V
      for logic H, which is barely within specification of the
      HD44780. If you account RCL of your cable, this may not be
      enough and can cause unreliable operation.

5: Enable Signal Rise Time
      If you ever read the HD44780 datasheet you will notice that
      somewhere in the 'signal timing' table is written: 'Enable
      Signal Rise Time  max. 20nS'. That means the Voltage on the
      HD44780 pin called 'Enable' has to rise from 0 Volts to 5
      Volts within 20 Nanoseconds and the other way round. They
      should better print that in big fat red letters, because
      most HD44780s are really picky about the enable signal rise
      time.
      That is a Problem: If you count together the bad driving
      characteristics of the parallel port combined with the
      capacitance of flat ribbon cable you may easily get an
      order of magnitude slower rise time.
      Therefore you should only use really short cable ( shorter
      than 50cm) for connecting the display to the parallel port.
      It may also be useful to use pull-up resistors on the display
      module or a schmitt-trigger.
      Note: The rise time of a digital output can (usually) not be
      altered by Software.

6: EMI
      The cable from the parallel port to the display may
      be sensible to electromagnetic interference and may emit
      electromagnetic radiation. If you place your cellphone near
      the cable, you may get unexpected display readings, on
      the other hand your house neighbour may not be able to listen
      to his/her favourite radio station any more - so better use
      shielded cable and put the display in a metallic case, perhaps
      a computer case.

7: One or Two Black Lines
      If you see one or two black lines on the display it means
      nothing more than that the display is powered and contrast
      voltage is present. If one or two black lines appear the
      controller has not been reset properly by the on chip power
      on reset generator. No need to worry - it will be reset by
      the LCDd software. But if the black line will not disappear
      although the wiring is working, the controller on the display
      may be defective.

8: Software Too Fast
      If you have a super GHz computer it may happen that the signal
      timing generated by LCDd is too fast. Adjust DELAYMULT in the
      source file to a bigger value. Parallel port wirings usually
      don't permit to read back the busy flag of the controller chip,
      so timing must be adjust so that the controller never is busy.

9: LED Backlight
      Check whether you need a resistor for your LED Backlight and
      which value it should have. If you forget the required resistor
      the backlighting LEDs might become hot and draw excessive
      current.

10: HD44780 Compatible
       The original HD44780 controller that we advertise to support
       has become the industry standard for alphanumeric character
       displays. The original HD44780 is out of production. It has
       many successors from many manufactures, which sometimes won't
       tell you that their chips are 'compatible'.
       To name a few: KS 0066, KS 0070, KS 0076, LC 7985, NT 3881,
       SED 1278, ST 7066 ...

11: Something To Add
       And I still have forgotten so many things...


bye,
Robin Adams
robin@adams-online.de