(Part 1 looked at the theory of Digital Correlation and Density Modulation for LCD drive, click here to read it.)
Multiplexed LCDs have been used for over 30 years, and the methods for driving them are well-known and remained unchanged for the past 20 years. Advancements in LCD materials and a new understanding of density modulation, however, allow for new digital approaches which are scalable with silicon processes and allow for more cost-effective designs. This article will demonstrate a conventional implementation and show two alternative approaches, requiring only digital signals, to drive the LCD.
The first approach relies on the correlated and uncorrelated nature of signals of different frequencies, while the second method exploits the low-pass nature of the LCD glass, allowing it to be driven with a density-modulated signal. The advantages of each technique--such as discrimination ratio, on/off voltage thresholds as function of supply voltage, level of multiplexing, and multiplexing types--will be explained. A working demonstration of all three types will be shown as well .
The demonstration project was implemented in PSoC Creator and configured to run on a PSoC 3/5 part. The project was configured to implement both the Digital Correlation and the Density Modulated method with both ½ and ⅓ bias methods. By simply changing a few parameters in the project files though firmware, the project can demonstrate any LCD control technique.
There are two main sections of the LCD control. The first is the Sequencing and Drive Control, which can be observed in Figure 3 . The second section is the pin drive logic shown in Figure 4 , which takes the various control and drive signals and combines it with the appropriate logic to produce the required drive waveform.
Figure 3. Digital LCD Control Sequencing Control and Drive
The Sequencer is used to indicate which control waveform sub frame is currently being generated while also determining whether the