利用液晶的MAXQ微控制器-Using an LCD wit

文章摘要：Abstract: This application note discusses issues that should be considered when designing a system using an LCD with a MAXQ Microcontroller. The note covers LCD basics, how to choose one of the available LCD types, and how to connect an LCD to a MAXQ device. IntroductionLiquid crystal displays (LCDs

利用液晶的MAXQ微控制器-Using an LCD wit,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com
Abstract: This application note discusses issues that should be considered when designing a system using an LCD with a MAXQ Microcontroller. The note covers LCD basics, how to choose one of the available LCD types, and how to connect an LCD to a MAXQ device.

Introduction

Liquid crystal displays (LCDs) are a great way for embedded systems to provide information to users. LCDs are typically available in two forms: LCD glass and LCD modules, which use integrated controllers to drive the various segments of the attached glass. LCD glass is relatively inexpensive, but requires many I/O lines and significant processing power to control the lines. Complete LCD modules are, moreover, really too costly for small, embedded applications.

The MAXQ® family of microcontrollers with its on-board LCD driver solves the problem of controlling an LCD, allowing information to be displayed simply, quickly, and inexpensively.

When designing a system using a MAXQ with an LCD, the following questions should be considered:

• What kind of data should be displayed?
• What type of LCD glass should be used?
• How should the LCD be connected to the MAXQ?

LCD Fundamentals

LCDs do not produce light; they either block it, or allow it to pass through. This is accomplished with the combined use of polarizers and twisted nematic (TN) material stacked together. Polarizers are specially treated glass that block light of a certain orientation. TN material consists of many similarly oriented, large organic molecules which realign when a voltage is applied. This new alignment bends the light passing through it by 90°. The modified light is then diffused by the polarizer and does not pass back to the observer.

LCDs must be driven with AC voltage; DC operation causes the TN material to permanently stop twisting. Displays are normally driven at a frequency close to 100Hz, fast enough to prevent perceptible flicker. Many LCDs use multiplexing to allow more segments to be controlled by fewer pins. Multiplexed displays are subject to crosstalk between segments. If an individual segment is turned on too quickly, adjacent segments may activate as well. To avoid this problem, instantaneous voltage change for each segment must be limited. The more a display is multiplexed, the lower the RMS voltage required to turn on a segment. Higher order multiplexing reduces the difference between active and inactive voltages, limiting the number of multiplexed layers that can be used with normal TN material.

Choosing a Glass

LCD glass comes in all shapes, sizes, and display formats. Displays are available in positive (dark characters with a light background) and negative (light characters with a dark background). The simplest LCDs are reflective, using a mirror-like material applied to the backside of the display to bounce ambient light back to the observer. Reflective LCDs should be used in well-lit situations. The TN material and the polarizers found in LCDs absorb a significant amount of light passing through them, causing them to be somewhat dark. To overcome this issue, many displays include a backlight. These are referred to as transmissive LCDs and work best in low-light applications. Transflective LCDs use both reflection and backlighting to be effective in all lighting conditions.

Displays are also available in a variety of data formats, with some of the more common formats listed below. See Figures 1 and 2 for examples.

• Numeric: Consisting of 7 segments per digit (0-9)
• Alphanumeric: Consisting of 14 or 16 segments per digit (0-9, A-Z)
• Symbol/Character: Consisting of single-segment pictures or characters, i.e., a bell
• Combination: Any combination of the above

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