Liquid-crystals are commonly used to create simple screens for electronic systems like digital clocks and
car audio faceplates.

In an LCD, the liquid-crystal controls whether the segments appear ON or OFF by interacting with the light traveling through the LCD. When a voltage is placed across a segment, the internal molecules align with the electrical field across it and allow light to pass through unobstructed. When no voltage has been placed across the screen, the light travels through the liquid-crystal and rotates 90˚. In both cases, the light travels through the front and rear polarized filters that block / pass the light depending on its polarization.

LCDs are unique such that their light sources can be either passive or active. For passive displays, external lights are used to illuminate and view the characters on the display. Active displays, on the other hand, are designed with an internal light source on the back of the screen. 

For a detailed description of how LCDs work, please refer to AN658, LCD Fundamentals, and the LCD Driver Module of 8-Bit PIC Microcontrollers, Application Note, Microchip. This article breaks down both the physical components and the scientific theory behind LCDs. It also describes the benefits and drawbacks of various LCD types including reflective, transmissive, and transflective displays.

This project improves upon the design described in the above-mentioned application note by offloading many of the processing and hardware requirements from the MCU onto a small and inexpensive GreenPAK IC.

Figure 2 shows the block diagram of this proposed solution. In addition to the LCD driving circuitry, the GreenPAK solution includes system monitoring features like hardware resets, watchdog timers, and RAM storage.

Below we described steps needed to understand how the system monitor has been programmed. However, if you just want to get the result of programming, download GreenPAK software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the device.

Creating Drive Signals for LCD Screens

LCDs are deceptively similar to LED screens in that they both require a DC voltage to turn on their segments, but the similarities end here. In fact, placing a sustained DC voltage across an LCD segment will damage the liquid-crystal. The following sections detail a few techniques used by designers to maintain an average of zero volts across the individual LCD segments.

LCD Driving Techniques: Static Drive and Multiplex Drive

When designing drive circuitry for a small LCD like a 7-segment display, there are generally two different categories: Static Drive and Multiplex Drive.

Static Drive Technique 

Static Drive LCD segments have two connections: a single common line shared amongst each of the segments and a unique control signal for each segment. To avoid creating a non-zero DC average voltage across a segment, the COM line and the SEGx lines are driven with square waves as shown in Figure 3. The difference between the SEGx lines and the COM line generates a DC voltage across each of the segments without changing the average DC voltage across the segment. To enable the LCD segment, simply invert the square wave driving the specific segment of interest. The average voltage across the LCD segment will still be 0V.

The Static Drive technique is simple and quick for small LCDs. It doesn’t require special voltage levels or complicated timing behavior, but the GPIO requirements impact the viability of this technique for high segment-count displays.

Multiplex Drive Technique

The Multiplex Drive technique reduces the GPIO requirements for driving larger LCDs. The basics of this technique centers around using multiple COM lines for specific SEG lines. Using time domain multiplexing, each segment can be individually enabled on one COM line.

Figure 4 shows the first Multiplex...

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