LCD Module in 4-bit Mode

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In many projects use is made of alphanumeric LCDs
that are driven internally by Hitachi’s industry-standard HD44780
controller. These displays can be driven either in 4-bit or 8-bit mode.
In the first case only the high nibble (D4 to D7) of the display’s data
bus is used. The four unused connections still deserve some closer
attention. The data lines can be used as either inputs or outputs for
the display. It is well known that an unloaded output is fine, but that a
floating high-impedance input can cause problems. So what should you do
with the four unused data lines when the display is used in 4-bit mode?
This question arose when a circuit was submitted to us where D0-D3 where
tied directly to GND (the same applies if it was to +5 V) to stop the problem of floating inputs.

The LCD module was driven directly by a
microcontroller, which was on a development board for testing various
programs and I/O functions. There was a switch present for turning off
the enable of the display when it wasn’t being used, but this could be
forgotten during some experiments. When the R/Wline of the display is
permanently tied to GND (data only goes from
the microcontroller to the display) then the remaining lines can safely
be connected to the supply (+ve or GND). In
this application however, the R/Wline was also controlled by the
microcontroller. When the display is initialised correctly then nothing
much should go wrong. The data sheet for the HD44780 is not very clear
as to what happens with the low nibble during initialisation.

LCD Module in 4 bit Mode Circuit

LCD Module in 4-bit Mode Circuit Diagram

After the power-on reset the display will always be in 8-bit mode. A
simple experiment (see the accompanying circuit) reveals that it is
safer to use pull-down resistors to GND for
the four low data lines. The data lines of the display are configured as
outputs in this circuit (R/Wis high) and the ‘enable’ is toggled (which
can still happen, even though it is not the intention to communicate
with the display). Note that in practice the RS line will also be driven
by an I/O pin, and in our circuit the R/W line as well. All data lines
become high and it’s not certain if (and if so, for how long) the
display can survive with four shorted data lines. The moral of the story
is: in 4-bit mode you should always tie D0-D3 via resistors to ground
or positive.
Author: L. Lemmens
Copyright: Elektor Electronics