LCD INTERFACING

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LCD INTERFACING
8.1 LCD Features
LCD stands for Liquid Crystal Display are the most widely used output devices for 89c51 type microcontroller. Nowadays LCDs is finding widespread use compared to LEDs and 7-segment. This is due to following specific features:
1. The declining prices of LCDs make its use cost-effective.
2. The ability to display numbers, characters and graphics. This is contrast to LEDs, which has limited to numbers and few characters. Incorporation of a refreshing controller into LCD, thereby relieving the CPU of the task of refreshing the LCD. In contrast, the LED must be refreshed by the CPU to keep displaying data.


3. Ease of programming for characters and graphics.
8.2  LCD Pin Descriptions


The LCD which we have used in our TRAINER KIT is discussed in this section. It has 14 pins. Figure shows the position of various pins.

1. VCC, VSS, and VEE
While VCC and VSS provide +5V and ground, respectively, VEE is used for controlling LCD contrast.

2. RS (Register Select)
There are two very important registers inside the LCD. The RS pin is used for their selection as follows. If RS=0, the instruction command code register is selected, allowing the user to send a command such as clear display, cursor at home, etc. If RS=1, the data register is selected, allowing the user to send data to be displayed on LCD.

3. R/W (Read/Write)
R/W input allows the user to write information to LCD or read information from it. R/W=1 when reading; R/W=0 when writing.

4. E (Enable)
The enable pin is used by the LCD to latch information presented to its data pins. When data is supplied to data pins, a high-to-low pulse must be applied to this pin in order for the LCD to latch in the data present the data pins. This pulse must be a minimum of 450 ns wide.

5. D0-D7 (8-bit Data bus)
The 8-bit data pins, D0-D7, are used to send information to the LCD or read the contents of the LCD’s internal registers. To display letters and numbers, we send ASCII codes for the letters A-Z, a-z, and numbers 0-9 to these pins while making RS=1. There are also instruction command codes that can be sent to LCD to clear the cursor to the home position or blink the cursor. Table lists some of the instruction command codes.

8.3 Connection of LCD with 8255
The main purpose of LCD in our Trainer kit is to display the name of the practical which is going to perform by the user and so LCD is programmed according. Figure shows various connections required to interface an LCD to 89c51. Here data lines of LCD are connected to 8 port of 89c51 microcontroller.








Interfacing of LCD to Microcontroller
 On this data lines, various command codes for LCD settings and data codes which are required to display are sent. There are three controlling lines for LCD viz, RS, R/W and E. RS that is register select is connected to designated port pin of 89c51 and set or reset as required. Similarly remaining two lines R/W that is Read/Write and E that is Enable are connected to designated 89c51 port pins and set or reset as required.


 

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