Alphanumeric Liquid Crystal Displays

Alphanumeric Liquid Crystal Displays

Copyright: © 2017 |Pages: 22
DOI: 10.4018/978-1-68318-000-5.ch008
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Introduction

Alphanumeric liquid-crystal displays (LCDs) play a crucial role in microcontroller-based systems for the following reasons:

  • They allow system designers to display text messages to monitor the status of the system.

  • LCDs help transform electronic gadgets into interactive and user friendly ones. The displayed messages guide the user to properly operate the device or gadget.

  • Since they allow users to display variables, LCDs are excellent debugging tools. Designers can trace the system and make decisions as to whether a hardware unit is functioning properly or not.

  • As opposed to 7-segment displays, LCDs display numbers as well as letters at a lower cost per bit of information. The Character Generator RAM (CGRAM) of the LCD can be programmed to display special characters such as Greek, Japanese, etc.

Figure 1.

4-bit vs. 8-bit mode; VEE is used for brightness control. It is usually connected to the output of a voltage divider to provide a voltage level between 0 and 5V. For maximum brightness, VEE is tied to ground.

Fortunately, all LCDs follow the protocol of the Hitachi HD44780 LCD controller chip. What differs from one LCD to another is the number of lines and the number of characters per line. Alphanumeric LCDs have three control signals E, RS and and 8 data lines. They can be operated in 4-bit or 8-bit mode.

This chapter emphasizes 4-bit mode for its pin-saving advantage. A specialized set of functions, intended to isolate the user from LCD peculiarities, will be developed.

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Lcd Interface And Functionality

LCD modules can operate in 4-bit mode or 8-bit mode as shown in Figure 1. In the former, data is transferred one nibble at a time starting with the most significant nibble. In 8-bit mode, data transfer is done in one shot. The advantage of 4-bit versus 8-bit mode is pin-saving. On this basis, 4-bit data transfer will be emphasized in this chapter and will be the mode of choice throughout this textbook. Note that the 4-bit mode appropriates only 7 microcontroller pins (4 data lines plus 3 control lines) whereas the 8-bit mode usurps 11 microcontroller pins.

All alphanumeric LCD modules have three control signals:

  • (Register Select Control):0 = LCD in command mode.1 = LCD in data mode.

Data mode allows the user to transfer ASCII characters to the LCD whereas command mode is in charge of transferring commands such as: clear the LCD, specify the position of the character to be displayed, etc.

  • (Read/Write Control):0 = MCU writes data to LCD.1 = MCU reads data from LCD.

When the MCU reads data from the LCD ( = 1), it is generally to read the busy flag (BF) which signals whether the LCD is busy or not. If an appropriate delay is inserted between characters being sent to the LCD, the pin may be tied to ground since there will no longer be any need to test the busy flag.

  • (Enable Signal):Rising Edge = Latches the control state (status of and ).Falling Edge = Latches Data.

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