Using PIC16C5X microcontrollers as LCD drivers 

			   Al Lovrich
		    Logic Products Division
		     Microchip Technology

 Introduction 

 This application report describes an LCD controller implementation using a 
PIC16C55 microcontroller [1].  We start by an overview of LCD devices and their 
theory of operation followed by software implementation issues of the 
controller.  The source code for controlling a multiplexed LCD display is 
included in the appendix.  

 background 

 Liquid Crystal Displays (LCD) are modelled as a capacitor with one side 
connected to the ground (Common Plane) and the other side connected to the 
controller (Segment). The segment is then turned on by applying a voltage of
opposite polarity to the ground signal. To turn the segment off a voltage that
has the same polarity as the common plane is applied. Internally the LCD uses
an electrochemical process to display the segment.

 Driving a liquid crystal display at direct current (DC) will cause permanent 
damage the display unit.  In order to prevent irreversible electrochemical 
action from destroying the display, the voltage at all segment locations must 
be 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 driving method and multiplexed driving
method.  

 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.  To turn off a segment a voltage that is of the same polarity
to common plane signal is applied.  This drive method is universal to driving
LCD segments.  Figure-1 shows an example of this driving method.  

 Multiplexed LCDs maintain their liquid crystal characteristics. These are low
power consumption, high contrast ratio under high ambient light levels, and
reduce the number of external connections necessary for dot matrix and
alphanumeric displays.  The multiplex 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 25Hz. Since increasing the drive frequency significantly above this value
increases current demand by the CMOS circuitry, an upper drive frequency level
of 60Hz is recommended by most LCD manufacturers.  We have chosen a drive rate
of 50Hz for this application report 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.  

 LCDs are sensitive to Vrms (root mean squared voltage) levels.  Different LCDs 
have different characteristics; figure-2 shows a typical voltage vs relative 
contrast characteristics.  Notation on curve shows operating points for
multiplex operation with threshold voltage set to 1.7 Vrms.  The curve is 
normalized and assumes the viewing angle is 90 degrees to the plane of LCD.  It 
is possible to move the operating point of the LCD by applying voltage to the 
LCD, thus providing for contrast control.  Figure-3 shows a simple circuit to 
accomplish this.  

 To better understand multiplexed LCD control it is best to look at the general
case.  The segments in a multiplexed LCD are arranged in an X-Y grid form as 
shown in Figure-4.  The common plane signals maintain their relative shape at 
all times, as shown in Figure-5.  To turn on segment 1 (SEG1), we need to apply 
a voltage Vd, such that Vs+Vd turns the segment on and Vs-Vd turns the segment 
off.  Note that the segment signal Vd is symmetrical.  This is a consequence of 
the intervals that the common plane signal is not present at all times.  Use of 
nonsymmetrical waveform will result in a higher Vrms present on the unaddressed 
segments.  The symmetrical nature of the waveforms theoretically result in a 
zero DC voltage levels.  CMOS drivers (e.g.  microcontrollers) operate at 0 to 
+5V levels (rail voltage levels).  This would require driving voltages beyond 
the range of operation.  This constraint is addressed by a technique referred 
to as "level shifting" or "biasing".  Level shifting allows application of
voltages in the range of 0 to +2.5V, which is compatible with these drivers.
This would require an additional voltage level of +2.5V, which can be
implemented through a simple resistive voltage divider circuit.  

 Implementation

 The ideas presented in the previous section can be applied to any size
multiplexed LCD display.  In our implementation we used a 4-digit LCD from
Occular Inc.  The circuit diagram used in this application report is shown
in Figure-6.  Each I/O pin on the PIC16C55 device controls the state of two
segments (see Figure-6) which requires a total of 16 I/O pins.  The reference
voltages are generated through a simple resistive voltage divider circuit.  The 
0 and +V levels are generated by taking advantage of PIC16C5X I/O pin set to 
input, which tristates the voltage level seen on the pin.  This method uses 4 
I/O pins to generate the proper voltage levels.  Figure-7 shows the truth table 
for generating the voltage levels.  Figure-8 shows how to create a bitmap for 
different digits.  Figure-9 shows the waveforms generated for the accompanying
software which implements a hexadecimal counter.  

 Conclusion 

 In this application report 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 a software 
implementation of the LCD controller will necessarily introduce a delay which 
is proportional to the instruction cycle of the microcontroller.  Therefore it 
is necessary to keep the switching time to a minimum.  Our implementation 
introduced xx mV of DC voltage on the segment lines which is below the 
manufacturer's recommended DC offset voltage of 60 mV.  

 References

[1] PIC16C5X Data Sheet, DSxxxxx, Microchip Technology, 1993