LIQUID CRYSTAL DISPLAY (LCD)

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7.    LIQUID CRYSTAL DISPLAY (LCD)

7.1 INTRODUCTION
Advances in the features, miniaturization, and cost of LCD (Liquid Crystal Display) controller chips have made LCDs usable not only in commercial products but also in hobbyist projects. By themselves, Liquid Crystal Displays can be difficult to drive because they require multiplexing, AC drive waveforms, and special voltages. LCD modules make this driving simpler by attaching hardware to the raw glass LCD to assist in some or all of these rudimentary driving tasks. LCD modules can be split into two groups: those that have built-in controller and driver chips, and those that have only driver chips. LCD displays that do not have controllers are typically used with powerful hardware, such as a laptop computer, where a video controller is available to generate the complex drive signals necessary to run the display.
Most color and large (greater than 320x240) monochrome displays are of this type. Other common sizes are 16x1, 20x1, 20x2, 20x4, 40x1, and 40x2 (characters x lines). Fortunately, all HD44780-based displays (of any size) use the same standard 14-wire interface.
Therefore, code and hardware made for one size/type display can be painlessly adapted to work for any HD44780 compatible. Information about these displays can be easily obtained on the web by including “HD44780” in our search keywords. Because of their widespread use, these displays can be purchased surp
lus with typical prices of $3 for small displays to $20 for large ones.

Interfacing our LCD module
The microcontroller/microprocessor interface to HD44780 LCD modules (hereafter generically Referred to as character LCD modules) is almost always 14 pins. We May find that some displays have additional pins for backlighting or other purposes, but the first 14 pins still serve as the interface.

The first three pins provide power to the LCD module. Pin 1 is GND and should be grounded to the power supply. Pin 2 is VCC and should be connected to +5V power. Pin 3 is the LCD Display Bias. By adjusting the voltage or duty cycle of pin 3, the contrast of the display can be adjusted. Most character LCDs can achieve good display contrast with a voltage between 5V and 0V on pin 3. Note that greater contrast comes with lower voltage and we should never apply a VLCD higher than VCC. Some displays, which are specially made to work over a large temperature range, may require a negative voltage to achieve readable contrast.
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