Alternative methods of LCD control (Part 1 of 2)

July 05, 2011 //By Robert Murphy
Alternative methods of LCD control (Part 1 of 2)
Robert Murphy of Cypress Semiconductor outlines how different LCD drive/control techniques can save power and simplify circuitry designs.

Abstract

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

Traditionally, LCD control in microcontrollers has been accomplished through two different but primarily analog implementations. These implementations consist of either a resistor ladder or a charge pump, with the resistive-ladder implementation being the more common in the industry.

With a resistor ladder, a series of resistors are stacked among each other to create a large voltage divider where various taps are placed to obtain the multiple voltage levels that are then multiplexed (muxed) to the proper GPIO at the proper time to generate the required LCD control waveform, depending on the MUX value of the LCD. The chosen resistor values for the resistor ladder, which are implemented in silicon, have an effect on both the power consumption and the quality of the display image.

 An LCD is ideally a capacitive load. By choosing a resistor value that is too high, this can create a distorted waveform due the constant charging and discharging of the capacitive load. This can result in improper generation of the LCD control waveforms which impact viewing of the LCD

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