6.3 LCD AND KEYPAD

HARDWARE INTERFACING

LCD AND KEYPAD

INTERFACING

LCD DISPLAY

� This component is specifically manufactured to be used with

microcontrollers, which means that it cannot be activated by

standard IC circuits.

� It is used for displaying different messages on a miniature

liquid crystal display.

� The model described here is for its low price and great

capabilities most frequently used in practice.

� It is based on the HD44780 microcontroller (Hitachi) and can

display messages in two lines with 16 characters each.

� It can display all the letters of alphabet, Greek letters,

punctuation marks, mathematical symbols etc.

� It is also possible to display symbols made up by the user.

� Other useful features include automatic message shift (left

and right), cursor appearance, LED backlight etc.

LCD CONNECTING

� Depending on how many lines are used for connecting an LCD to the

microcontroller, there are 8‐bit and 4‐bit LCD modes.

� The appropriate mode is selected at the beginning of the operation

in the process called 'initialization'.

� The 8‐bit LCD mode uses outputs D0‐ D7 to transfer data as

explained on the previous page.

� The main purpose of the 4‐bit LCD mode is to save valuable I/O pins

of the microcontroller.

� Only 4 higher bits (D4‐D7) are used for communication, while others

may be left unconnected.

� Each piece of data is sent to the LCD in two steps‐ four higher bits

are sent first (normally through the lines D4‐D7), then four lower

bits.

� Initialization enables the LCD to link and interpret received bits

correctly.

� Data is rarely read from the LCD (it is mainly transferred from

the microcontroller to the LCD) so it is often possible to save

an extra I/O pin by simple connecting the R/W pin to the

Ground.

� Such a saving has its price. Messages will be normally

displayed, but it will not be possible to read the busy flag

since it is not possible to read the display either.

� Fortunately, there is a simple solution. After sending a

character or a command it is important to give the LCD

enough time to do its job.

� Owing to the fact that the execution of a command may last

for approximately 1.64mS, it will be sufficient to wait about

2mS for the LCD.

LCD OPERATION

� In recent years, the LCD has been finding widespread use

replacing LEDs (seven‐segment LEDs or other multisegment

LEDs. This is due to the following reasons:

1. The declining prices of LCDs.

2. The ability to display numbers, characters, and graphics,

this is contrast to LEDs, which are limited to numbers

and a few characters.

3. Incorporation of a refreshing controller into the LCD,

thereby relieving the CPU of the task of refreshing the

LCD. In contrast, the LED must be refreshed by the CPU

(or in some other way) to keep displaying the data.

4. Ease of programming for character and graphics.

LCD PIN DESCRIPTIONS

Pin Symbol I/O Description

1 VSS ‐‐ Ground

2 VCC ‐‐ +5 V Power Supply

3 VEE ‐‐ Power supply to control contrast

4 RS I RS=0 to select command register,

RS=1 to select data register

5 R/W I R/W=0 for write, R/W=1 for read

6 E I/O Enable

7 DB0 I/O The 8‐bit data bus








LCD PIN DESCRIPTIONS (cont)

� VCC, VSS, VEE

� While VCC and VSS provide +5V and ground, respectively,

VEE is used for controlling LCD contrast

� RS, register select

� If RS=0, the instruction command code is selected, allowing

the user to send a command such as clear display, cursor at

home, and so on.

� 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 the En pin in order for the

LCD to latch in the data present at the data pins.

� This pulse must be a minimum of 450 ns wide.

� We call this delay the SDELAY (short delay) to

distinguish it from other delays.


� D0 – D7

� The 8‐bit data pins, D0 – D7, are used to send

information to the LCD or read 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 the LCD to clear the display or force the

cursor to the home position or blink the cursor.


� Table below lists the instruction command codes.

� To send any of the commands, make pin RS=0. For

data, make RS=1. Then send a high to low pulse to the

E pin to enable the internal latch of the LCD.

� There are two ways to send characters (command /

data) to the LCD

� Use a delay before sending the next one

� Use the busy flag to see if the LCD is ready for the

next one


Code

(Hex)

Command to LCD instruction

Register

1 Clear display screen

2 Return home

4 Decrement cursor

(shift cursor to left)

6 Increment cursor

(shift cursor to right)

5 Shift display right

7 Shift display left

8 Display off, cursor off

A Display off, cursor on

C Display on, cursor on

Code

(Hex)

Command to LCD instruction

Register

E Display on, cursor blinking

F Display on, cursor blinking

10 Shift cursor position to left

14 Shift cursor position to right

18 Shift the entire display to the left

1C Shift the entire display to the right

80 Force cursor to beginning of

1st line

C0 Force cursor to beginning of

2nd line

38 2 lines and 5x7 matrix

Sending commands and data to LCD with time delay

Sending commands and data to LCD busy flag

KEYBOARD INTERFACING

In this sections, we will discuss keyboard fundamental,

along with key press detection and key identification

mechanisms. Then we show how a keyboard is interfaced

to a PIC 18.

MATRIX KEYPAD BASIC OF OPERATION

� Matrix keypads are simply an extension to the simple tact switch

inputs.

� They consists of keys interconnected in the shape of a matrix.

� Each key is a simple mechanical switch located at the crossing

between the matrix rows and columns.

� When a key is pressed, its row and column form an electrical

contact. The rows and columns can be connected to the pins of

microcontroller ports.

� The big advantage of using a matrix keypad is that it allows to

interface a large number of keys with a relatively small number

of microcontroller pins.

� For example, a 16‐key keypad requires only 8 (instead of 16, if

interfaced individually) I/O pins of the microcontroller if

organized into a 4 rows and 4 columns matrix.

MATRIX KEYPAD BASIC OF OPERATION (CONT)

� The circuit diagram for this experimental tutorial is shown

below. It interconnects a 4×4 matrix keypad to PORTB of the

PIC16F1827 microcontroller.

SCANNING A MATRIX KEYPAD

There are many methods depending on how you connect your keypad with your controller, but the basic logic is same. We make the coloums as i/p and we drive the rows making them o/p, this whole procedure of reading the keyboard is called scanning.

In order to detect which key is pressed from the matrix, we make row lines low one by one and read the coloums. Lets say we first make Row1 low, then read the columns. If any of the key in row1 is pressed will make the corrospondingcolumn as low i.e if second key is pressed in Row1, then column2 will give low. So we come to know that key 2 of Row1 is pressed. This is how scanning is done.

So to scan the keypad completely, we need to make rows low one by one and read the columns. If any of the button is pressed in a row, it will take the corrosponding column to a low state which tells us that a key is pressed in that row. If button 1 of a row is pressed then Column 1 will become low, if button 2 then column2 and so on.

REVIEW QUESTIONS :

Jun 2012

1. Describe the function of pins R/W and E in the LCD.

(4 marks)

Jun 2013

1. Explain the command RS=0, RS=1, R/W=0 and R/W=1 for LCD

operation. (4 marks)

What Lies Behind Us

And What Lies Before Us

Are Tiny Matters

Compared To

What Lies Within Us.

Documents

6.3 LCD AND KEYPAD