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  Microchip Technology Semiconductor Electronic Components Datasheet  

AN563 Datasheet

Using PIC16C5X Microcontrollers as LCD Drivers

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AN563 pdf
AN563
Using PIC16C5X Microcontrollers as LCD Drivers
Author:
Al Lovrich
Microchip Technology Inc.
INTRODUCTION
This application note describes an LCD controller
implementation using a PIC16C55 microcontroller. This
technique offers display capabilities for applications
that require a small display at a low cost, together with
the capabilities of the standard PIC16C55 microcontrol-
ler. We start by an overview of LCD devices and their
theory of operation followed by software implementa-
tion issues of the controller. The source code for con-
trolling a multiplexed LCD display is included in
Appendix A.
LIQUID CRYSTAL DISPLAYS
The Liquid Crystal Display (LCD) is a thin layer of
“Liquid Crystal Material” deposited between two plates
of glass. The raw LCD is often referred to as “glass”.
Electrodes are attached to both sides of the glass. One
side is referred to as common or backplane, while the
other side is referred to as segment.
An LCD is modeled as a capacitor, with one side
connected to the common plane and the other side
connected to the segment, as shown in Figure 1. LCDs
are sensitive to Root Mean Square Voltage (VRMS) lev-
els. When a VRMS level of zero volts is applied to the
LCD, the LCD is practically transparent.
To turn an LCD segment “on,” which makes the
segment turn dark or opaque, an LCD RMS voltage
that is greater than the LCD threshold voltage is applied
to the LCD. The RMS LCD voltage is the RMS voltage
across the capacitor C in Figure 1, which is equal to the
potential difference between SEG and COM values.
Different LCDs have different characteristics; Figure 2
shows typical voltage vs relative contrast
characteristics. Notations on the curve show operating
points for multiplex operation with the threshold voltage
set to 1.7 VRMS. This voltage is often used as the
measure of voltage for the LCD to be “off” or transpar-
ent. The curve is normalized and assumes a viewing
angle of 90° to the plane of the LCD.
Contrast control, the process of turning on a segment,
is achieved by moving the operating point of the LCD.
This is achieved by applying a voltage to the LCD that
is greater than the LCDs threshold voltage. A typical cir-
cuit to accomplish this task is shown in Figure 3.
Driving a liquid crystal display at direct current (DC) will
cause permanent damage to the display unit. In order
to prevent irreversible electrochemical action from
destroying the display, the voltage at all segment
locations must reverse polarity periodically so that a
zero net voltage is applied to the device. This process
is referred to as AC voltage application. There are two
LCD driving methods available: Static and multiplexed.
FIGURE 1: ELECTRICAL MODEL OF AN LCD SEGMENT WITH DRIVING VOLTAGES
SEG
COM
SEG
COM - SEG
C SEG
Segment Off
COM
COM - SEG
VON RMS = VDD
VOFF RMS = 0
Segment On
© 1997 Microchip Technology Inc.
DS00563C-page 1


  Microchip Technology Semiconductor Electronic Components Datasheet  

AN563 Datasheet

Using PIC16C5X Microcontrollers as LCD Drivers

No Preview Available !

AN563 pdf
AN563
Conventional LCDs have separate external
connections for each and every segment plus a
common plane. This is the most basic method that
results in good display quality. The main disadvantage
of this driving method is that each segment requires
one liquid crystal driver. The static driving method uses
the frame frequency, defined as a period of the
common plane signal, of several tens to several
hundred Hz. A lower frequency would result in blinking
effects and higher frequencies would increase power
requirements. To turn a segment on, a voltage that has
an opposite polarity to the common plane signal must
be applied resulting in a large RMS voltage across the
plates. To turn off a segment, a voltage that is of the
same polarity to the common plane signal is applied.
This drive method is universal to driving LCD seg-
ments. Figure 1 shows an example of this driving
method.
The LCD frequency is defined as the rate of output
changes of the common plane and segment signals,
whereas the frame rate is defined as
FFRAME
FFRAME =
N
where N is the multiplex rate or number of backplanes.
Typically, FFRAME ranges from 25 Hz to 300 Hz. The
most commonly used frame frequency is 40-70 Hz. A
lower frequency would result in flicker effects and
higher frequency would increase power requirements.
FIGURE 2: TYPICAL LCD
CHARATERISTICS
100%
VRMSOFF
1.7 VRMS
Two Common Planes
V = 4.1 VRMS
0
12 345 6 7
Drive Voltage VRMS
Multiplexed LCDs maintain their liquid crystal
characteristics. They have a low power consumption, a
high contrast ratio under high ambient light levels, and
they reduce the number of external connections neces-
sary for dot matrix and alphanumeric displays. The mul-
tiplex driving method reduces the number of driver
circuits, or microcontroller I/O pins if a software method
is used. The method of drive for multiplexed displays is
Time Division Multiplex (TDM) with the number of time
divisions equal to twice the number of common planes
used in a given format. In order to prevent permanent
damage to the LCD display, the voltage at all segment
locations must reverse polarity periodically so that zero
net voltage is applied. This is the reason for the
doubling in time divisions; each common plane must be
alternately driven with a voltage pulse of opposite
polarity. The drive frequency should be greater than the
flicker rate of 25 Hz. Since increasing the drive
frequency significantly above this value increases cur-
rent demand by the CMOS circuitry, an upper drive
frequency level of 60 Hz is recommended by most LCD
manufacturers. We have chosen a drive rate of 50 Hz
for this application note which results in a frame period
of 20 ms. The most commonly available formats are
2x4, 3x3, and 5x7. In this report we use a 2x4 format
LCD to display hexadecimal digits.
FIGURE 3: CONTRAST CONTROL CIRCUIT
VDD
VLCD
VDD
Contrast
Control
V1
R
V2
VSS R
DS00563C-page 2
© 1997 Microchip Technology Inc.


Part Number AN563
Description Using PIC16C5X Microcontrollers as LCD Drivers
Maker Microchip
Total Page 15 Pages
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