Liquid Crystal Display (LCD) Pin Configuration & Interfacing

Liquid Crystal Display (LCD) Pin Configuration – A liquid crystal display is a thin, flat panel used for electronically displaying information such as text, images, and moving pictures. In LCD, each pixel consists of a layer of molecules aligned between two transparent electrodes and two polarizing filters, the axes of transmission of which are perpendicular to each other. The LCD is finding widespread use by replacing the LEDs.

In general, LCDs use much less power than their CRT counterparts. The size of LCDs is all small. In LCDs, there is no bulky picture tube. These factors made the LCDs practical where size and weight are important. The most common LCD controller is the HITACHI 44780, which provides a simple interface between the microprocessor or microcontroller and an LCD. The commonly used alphanumeric displays are 1 x 16 (single line & 16 characters), 2 x 16 (double line & 16 characters per line), and 4 x 20 (four lines & 20 characters per line). The figure below shows the 2 x 16 line LCD.

LCD Pin Configuration

The LCD requires 3 control lines (RS, R/W, and EN) and 8(or 4) data lines. The number of data lines depends on the mode of operation. If operated in 8-bit mode, then 8-bit data lines are required. And if the operation is in 4-bit mode, then 4-bit data lines are required. 8-bit mode is faster than the 4-bit mode. In 8-bit mode, LCD uses a total of 14 pins, including 8 data lines, 3 control lines, and 3 power supply lines (V_cc, V_ss, and V_EE).

Power supply

The LCD discussed here uses three power supply pins (V_cc, V_ss, and V_EE). V_cc and V_ss pins are used to provide +5V and ground, respectively. The pin V_EE is used for controlling LCD contrast.

Control Lines

There are three control lines in the LCD. These three are used to control the LCD operations. There are two very important registers inside the LCD: the command register and the data register. The RS (Register select) pin is used to select one of these two registers. If RS = 0, the command register is selected, and the user is allowed to send the command to the LCD. If RS = 1, the data register is selected and the data sent by the user is displayed on the LCD.

R/W (Read/Write) pin allows the user to read/write the information (data or code) to/from the LCD. R/W = 1 when reading and R/W = 0 when a writing operation is performed. Another control pin, EN (Enable), is used to latch the data present on the data pins. A high-low signal is required to latch the data. The LCD interprets and executes the command at the instant the EN line is brought low.

Data lines

The 8-bit data pins, D₀-D_7, are used to send the information to the LCD or read the contents of the LCD’s internal register. The table below shows the pin description for the LCD.

LCD Command Words

There are LCD commands that can be sent to the LCD to clear the display, shift the cursor from right to left or left to right, or blink the cursor. To write the LCD command word in the command register, RS must be zero (RS = 0) and R/W = 0. The LCD commands are given in the table below.

LCD Interfacing

The LCD can be interfaced to the microprocessor 8085 using the programmable peripheral interface (PPI-8255) IC. To display letters and numbers, the ASCII Code for the letters A to Z, a to z, and numbers 0 to 9 is sent to the data lines (D₀ – D₇). These codes may be sent to LCD data lines through one port of the 8255 (PPI), as shown in the figure below. In this figure, port A is used as the output port and sends the data to the LCD. The EN pin and RS pin are connected to port B of the 8255. Since it is used as a normal display, R/W is made low by connecting to ground directly. Power supply connections are provided to the V_cc and V_ss pins. The V_EE pin is connected to the moving node of the potentiometer, which is connected between the V_cc and V_ss pins. By moving the potentiometer, the contrast of the LCD can be changed.