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.
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.
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
Copyright: Elektor Electronics
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