Liquid Crystal Display The LCD is used for the purpose of displaying the words which we are given in the program code. This code will be executed on microcontroller chip. By following the instructions in code the LCD display the related words. Fig.5.6 shows the LCD display.

5.3.1 Introduction
[pic]
Fig. 5.6: LCD Display The LCD display consists of two lines, 20 characters per line that is interfaced with the PIC16F73.The protocol (handshaking) for the display is as shown in Fig. The display contains two internal byte-wide registers, one for commands (RS=0) and the second for characters to be displayed (RS=1). It also contains a user-programmed RAM area (the character RAM) that can be programmed to generate any desired character that can be formed using a dot matrix. To distinguish between these two data areas, the hex command byte 80 will be used to signify that the display RAM address 00h will be chosen Port1 is used to furnish the command or data type, and ports 3.2 to 3.4 furnish register select and read/write levels.
Theory

A liquid crystal is a material (normally organic for LCDs) that will flow like a liquid but whose molecular structure has some properties normally associated with solids. The Liquid Crystal Display (LCD) is a low power device. The power requirement is typically in the order of microwatts for the LCD. However, an LCD requires an external or internal light source. It is limited to a temperature range of about 0°C to 60°C and lifetime is an area of concern, because LCDs can chemically degrade.

There are two major types of LCDs which are:

1. Dynamic-scattering LCDs and 2. Field-effect LCDs The turn-on and turn-off time is an important consideration in all displays. The response time of LCDs is in the range of 100 to 300ms.The lifetime of LCDs is steadily increasing beyond 10,000+hours limit. Since the color generated by LCD units is dependent on the source of illumination, there is a wide range of color choice.

5.3.2 Pin Diagram The Pin diagram for LCD is shown in the following fig 5.7 and the pin description is also explained in Table 5.1.

Fig. 5.7: Pin Diagram of LCD

Table 5.1: Pin Description of LCD

|Pins |Description |
|1 | "Vss" - Ground |
|2 | "Vcc" - +5v power supply |
|3 | "Vee" - Contrast Voltage |
|4 | "R/S" - Instruction/Register Select |
|5 | "R/W" - Read/Write LCD Registers |
|6 | "E" - Enable |
|7 - 14 | Data I/O Pins |

Pin Description
Vcc, Vss, VEE While Vcc and Vss provide +5v and ground, respectively, Vee is used for controlling LCD contrast. 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 as 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 the LCD.

R/W, Read/Write

R/W input allows the user to write information to the LCD or read information from it.

R/W = 1 when reading;

R/W = 0 when writing.

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 latch in the data pins. This pulse must be a minimum of 450 ns wide.

D0 - D7

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, send ASCII codes for the letters A - Z, a - z, and numbers 0 - 9 to these pins while RS = 1. When RS = 0 to check the busy flag bit to see if the LCD is ready to receive information. The busy flag is D7 and can be read when R/W = 1 and RS = 0, as follows: if R/W = 1, RS = 0. When D7 =1 (busy flag = 1), the LCD is busy taking care of internal operation and will not accept any new information. The description of the pins is shown in Table 5.1.

5.3.3 LCD Commands Description

Table 5.2: LCD Commands

|RS |R/W |D7 |D6 |D5 |D4 |
|0 |0 |1 | Display data address |Write to display RAM address after this |
|0 |1 |BF | Current addresses |BF=1/0:busy/Not busy |
|0 |1 | Character type |Write byte to last RAM chosen |

Handling the EN Control Line

As mentioned above, the EN line is used to tell the LCD that it is ready to execute an instruction that prepared on the data bus and on the other control lines. Note that the EN line must be raised/lowered before/after each instruction sent to the LCD regardless of whether that instruction is read or write, text or instruction. In short, someone must always manipulate EN when communicating with the LCD. EN is the LCD's way of knowing that someone is talking to it. If one doesn’t raise/lower EN, the LCD doesn't know that someone is talking to it on the other lines.

Checking the Busy Status of the LCD

As previously mentioned, it takes a certain amount of time for each instruction to be executed by the LCD. The delay varies depending on the frequency of the crystal attached to the oscillator input of the controller as well as the instruction which is being executed. While it is possible to write code that waits for a specific amount of time to allow the LCD to execute instructions, this method of "waiting" is not very flexible. If the crystal frequency is changed, the software will need to be modified. Additionally, if the LCD itself is changed for another LCD which, although 44780 compatible, requires more time to perform its operations, the program will not work until it is properly modified. A more robust method of programming is to use the "Get LCD Status" command to determine whether the LCD is still busy executing the last instruction received. The "Get LCD Status" command will return to the user two bits of information; the information that is useful to the user right now is found in DB7. In summary, when user issue the "Get LCD Status" command the LCD will immediately raise DB7 if it's still busy executing a command or lower DB7 to indicate that the LCD is no longer occupied. Thus the program can query the LCD until DB7 goes low, indicating the LCD is no longer busy. At that point user is free to continue and send the next command.

Initializing the LCD

LCD must be initialized and configured before using. This is accomplished by sending a number of initialization instructions to the LCD. The first instruction send must tell the LCD whether it is to be communicated with an 8-bit or 4-bit data bus. 5x8 dot character font should also be selected. These two options are selected by sending the command 38h to the LCD as a command.
a) Clearing the Display

When the LCD is first initialized, the screen should automatically be cleared by the controller.

b) Writing Text into the LCD

The data to be displayed is send to the LCD through data bus.

c) Cursor Positioning

The cursor positioning in a LCD can be done in the right entry mode or left entry mode. As left entry mode is flexible it is implemented. The circuit diagram of Check post Control System using RFID Module and its components description is described in next chapter.

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GND

+5v

VDD

A

K

1 2 3 15 16

4 5 6 7 8 9 10 11 12 13 14

2x16 Liquid Crystal Display

RS

R/wwwwow

En

D0 0D6

0

D2

D3

D5

D7

D6

D4

D1