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What is AN563?

This electronic component, produced by the manufacturer "Microchip", performs the same function as "Using PIC16C5X Microcontrollers as LCD Drivers".


AN563 Datasheet PDF - Microchip

Part Number AN563
Description Using PIC16C5X Microcontrollers as LCD Drivers
Manufacturers Microchip 
Logo Microchip Logo 


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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

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AN563 equivalent
FIGURE 9: EXAMPLE OF OUTPUT
WAVEFORMS FOR DIGIT 4
COM0
COM1
VDD
V1
V2
VDD
V1
V2
SEGn
VDD
V2
SEGn + 1
COM0 - SEGn
( selected waveform)
COM0 - SEGn + 1
(non selected waveform)
1 frame
FLCD
VDD
V2
V2
V1
0V
-V1
-V2
V1
0V
-V1
AN563
CONCLUSION
In this application note we have demonstrated the use
of PIC16C5X devices to implement a simple LCD
controller. As discussed earlier, it is important to keep
the generated DC voltage to a minimum to extend the
life of the LCD. Ideally one should switch all the I/O lines
simultaneously, however, implementation of software of
the LCD controller will necessarily introduce a delay
which is proportional to the instruction cycle of the
microcontroller, as shown in Figure 10. Therefore it is
necessary to keep the switching time to a minimum.
Our implementation introduced less than 50 mV of DC
voltage on the segment lines which is below the manu-
facturer’s recommended DC offset voltage of 60 mV.
REFERENCES
[1] Ocular Inc., Drawing number JH074.
FIGURE 10: MICROCONTROLLER GENERATED OUTPUT WAVEFORM
COM0
SEGX
t -4 µs
1/t -0.25 MHz
© 1997 Microchip Technology Inc.
DS00563C-page 5


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