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Fig. 2: LCD clock generation
Fig. 3: Type A versus type B waveforms
shows both types of waveforms for
1/3 MUX and 1/3 bias.
The voltage applied across a particular
pixel is the voltage on the COM pin
minus the voltage on the SEG pin. If
the resulting voltage is at or above the
Von threshold then the pixel is visible.
If it is at or below the Voff threshold
then the pixel is not visible.
The contrast of an LCD can be
determined by calculating the
discrimination ratio, which is the
ratio between the RMS voltage of
an on-pixel and the RMS voltage
of an off-pixel. Segment mapping
provides a simple and organised way
to determine which pixels should be
on or off.
External resistor biasing
The resistor ladder method is
most commonly used for higher
VDD voltages. This method uses
inexpensive resistors to create the
multi-level LCD voltages. Regardless
of the number of pixels that are
energised, the current remains
constant.
The resistance values are determined
by the display quality and power
consumption. Display quality is a
function of the LCD drive waveform.
Since the LCD panel is a capacitive
load, the waveform is distorted due to
the charging and discharging currents.
This distortion can be reduced by
decreasing the resistance value.
However, this change increases
the power consumption due to the
increased current flowing through the
resistors. As the LCD panel increases
in size, the resistance value must
be decreased to maintain the image
quality.
Sometimes adding capacitors in
parallel to the resistance can reduce
the distortion caused by charging and
discharging currents. This effect is
limited since, at some point, a large
resistor and large capacitor cause
a voltage level shift that negatively
impacts the display quality. Adding
a potentiometer allows external
contrast control.
Internal resistor biasing
To avoid the trouble of adding external
components and to save the use of up
to three pins for voltage generation,
PIC microcontrollers provide internal
resistor biasing and internal contrast
control. This mode does not use
resistors but internal resistor ladders
that are configured to generate the
bias voltage.
The internal resistor ladder can be
used to divide the LCD bias voltage
to two or three equally spaced
voltages that will be supplied to the
LCD segment pins. To create this,
the reference ladder consists of three
matched resistors.
When in 1/2 bias mode, the middle
resistor of the ladder is shorted out so
that only two voltages are generated.
This mode reduces the ladder
resistance thus increasing current
consumption.
Conclusion
Various MCUs with LCD controllers
provide design flexibility and
straightforward ways to drive the LCD
glass. The internal biasing, contrast
control and power-saving features in
the LCD module eliminate the need
for extra hardware. They let designers
make the most of these features while
maintaining display quality.
40 l New-Tech Magazine Europe