Driver Module. AN658 Datasheet

AN658 Module. Datasheet pdf. Equivalent


Microchip AN658
AN658
LCD Fundamentals and the LCD Driver Module of 8-Bit
PIC® Microcontrollers
Authors: Mary Tamar Tan
Rodger Richey
Microchip Technology Inc.
INTRODUCTION
The popularity of Liquid Crystal Displays (LCD) has
rapidly soared over the years due to the numerous
advantages it offers over other display technologies.
This application note provides an introduction to the
basics of LCD; its construction, physics behind its
operation, and the different factors affecting its
properties and performance. Moreover, 8-bit PIC®
microcontrollers with integrated LCD controllers are
also introduced. Prominent features of the LCD Driver
module of these MCU families are discussed, including
contrast control, drive waveforms, biasing methods,
power modes, and other LCD circuit design
considerations. Lastly, the code samples for a 1-Hour
Countdown Timer application for both the PIC16 and
PIC18 devices are presented. The application uses the
segmented and dot matrix LCD for the two families,
respectively.
LIQUID CRYSTALS
Liquid Crystals (LCs) exist in a state between isotropic
(liquid) and crystalline (solid), and exhibit the properties
of both as shown in Figure 1. Nematic phase, which is
the simplest of the LC phases, is the one employed in
the LCD technology.
LCs are affected by electric current and when a voltage
is applied, they react and may change order and
arrangement. This unique behavior of LCs allows them
to play a significant role in electro-optic devices, such
as the LCD.
FIGURE 1:
LIQUID CRYSTAL PHASES
BASIC COMPONENTS OF AN LCD
PANEL
An LCD panel, or more commonly known as a piece of
“glass”, is constructed of many layers. Figure 2 shows
all the layers that are typically present in LCD panels.
For this application, it is assumed that the LCD
employs a Twisted Nematic (TN) display, unless
otherwise stated. TN displays, as well as the other
display technologies are discussed in detail in the
Section “LCD Technologies”.
FIGURE 2:
BASIC LCD COMPONENTS
1997-2014 Microchip Technology Inc.
DS00000658B-page 1


AN658 Datasheet
Recommendation AN658 Datasheet
Part AN658
Description LCD Fundamentals and the LCD Driver Module
Feature AN658; AN658 LCD Fundamentals and the LCD Driver Module of 8-Bit PIC® Microcontrollers Authors: Mary Tama.
Manufacture Microchip
Datasheet
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Microchip AN658
AN658
Polarization is the process or state in which rays of light
exhibit different properties in different directions,
especially the state in which all the vibration takes
place in one plane. Essentially, a polarizer passes light
only in one plane. As shown in Figure 3, if light is
polarized in one plane, by passing through a polarizer,
it cannot pass through a second polarizer if its plane is
90° out of phase to the first.
The front polarizer is applied to the outside surface of
the top piece of glass. The top piece of glass also
provides structural support for the LCD panel.
FIGURE 3:
OUT OF PHASE AND
IN-PHASE POLARIZERS
LC molecules are long and cylindrical. On any plane
within the LC fluid, the molecules align themselves
such that the major axis of each molecule is parallel to
all others, as shown in Figure 4. The outermost planes
of the LC molecules will align themselves on the same
axis that the polyimide is “rubbed”. The direction of
“rubbing” of the polyimide on the bottom glass is 90°
apart from that of the polyimide on the top glass. This
orientation creates the twist in the LC fluid.
FIGURE 4:
LC MOLECULES IN
ALIGNMENT
On the bottom of the top glass, a transparent coating of
Indium Tin Oxide (ITO) is applied to the glass. ITO is
conductive and forms the backplane or the common
electrodes of the LCD panel. The patterns of the
backplane and segment ITO form the numbers, letters,
symbols, icons, etc.
After the ITO has been applied to the glass, a thin
polyimide coating is applied to the ITO. The polyimide
is “rubbed” in a single direction that matches the
polarization plane of the front polarizer. The action of
“rubbing” the polyimide causes the Liquid Crystal (LC)
molecules in the outermost plane to align themselves in
the same direction.
The next layer is a reservoir of LC. The LC fluid has
many planes of molecules.
The next layer is the polyimide coating on the bottom
glass followed by the ITO segment electrodes. The
bottom glass also supplies structural integrity for the
LCD panel as well as mounting surface for the
electrode connections. Applied to the external surface
of the bottom glass is the rear polarizer. Depending on
the type of viewing mode employed by the LCD panel,
the axis of polarization is the same or 90° apart from
the front polarizer.
A consequence of this alignment is that each
intermediate plane of LC molecules will have a slightly
different orientation from the plane above or below as
seen in Figure 5.
The twisting of the planes causes the polarization of the
light to twist as it passes through the LC fluid. The
twisting of the LC planes is critical to the operation of
the LCD panel as will be shown in the next section.
FIGURE 5:
LC MOLECULES PLANE
ORIENTATION
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1997-2014 Microchip Technology Inc.



Microchip AN658
AN658
HOW AN LCD WORKS
As explained in the previous section, the twist created
in the LC fluid is the basis of how the panel operates.
An LCD basically produces an output display by the
switching of segments or pixels between ON or OFF. A
pixel is considered to be ON when enough electric
potential is applied between the segment and common
electrodes, resulting to a dark pixel on the display. On
the contrary, a pixel is considered to be in the OFF
state when insufficient electric potential is applied
between the electrodes, creating a clear pixel on the
display.
OFF Pixel
Figure 6 shows how an LCD panel creates a pixel that
is OFF. For this example, the LC fluid is not energized
(i.e., there is 0 VRMS potential between the common
and the segment electrodes). The following is a
step-by-step description of the path light takes through
the LCD panel. The illustrative representation of the
process is shown in Figure 8.
FIGURE 6:
LC ORIENTATION WITH NO
ELECTRIC FIELD
Aligned with upper
polymer coating
Aligned with lower
polymer coating
90° twist for
Twisted Nematic (TN)
• Light enters the panel through the rear polarizer.
At this point, light becomes polarized to the
vertical plane.
• The polarized light passes unobstructed through
the transparent common electrode.
• As the polarized light passes through the LC fluid,
it gets twisted into the horizontal plane.
• The polarized light passes unobstructed through
the transparent segment electrode.
• Since the light is now polarized in the horizontal
plane, it passes unobstructed through the front
polarizer which has a horizontal polarization.
• The observer does not detect that the pixel is ON
because the light has not been obstructed.
ON Pixel
If a potential is applied across the common and
segment electrodes, the LC fluid becomes energized.
The LC molecule planes will now align themselves
such that they are parallel to the electrical field
generated by the potential difference. This removes the
twisting effect of the LC fluid. Figure 7 shows how a
pixel that is ON, or more specifically energized, is
created. The following is a step-by-step description of
the path that the light takes through this LCD panel.
Refer to Figure 9 for the illustrative representation.
FIGURE 7:
LC ORIENTATION WITH
ELECTRIC FIELD
- - --
Indium Tin Oxide
electrodes
~V
All planes aligned with
electric field
+ + ++
• Light enters the panel through the rear polarizer.
At this point, the light becomes polarized to the
vertical plane.
• The polarized light passes unobstructed through
the transparent common electrode.
• As the polarized light passes through the LC fluid,
it does not twist and remains in the vertical plane.
• The polarized light passes unobstructed through
the transparent segment electrode.
• Since the light is still polarized in the vertical
plane, it is obstructed by the front polarizer which
has a horizontal polarization.
• The observer detects that the pixel is ON because
the light has been obstructed and creates a dark
image on the panel.
1997-2014 Microchip Technology Inc.
DS00000658B-page 3







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